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NOTICE 

This document is disseminated under the sponsorship of the Department of Transportation in the 

interest of information exchange. This publication does not constitute a standard, specification, or 

regulation. 

The United States Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear herein only because they are considered essential to the object of 

this document. 

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. Report No. 

FHWA-SA-95-037 

2. Government Accession No. 

Technical Report Documentation Page 

3. Recipient’s Catakg No. 

5. -pon Oat0 March, 1995 

Geotechnical Engineering Circular No. 1 

DYNAMIC COMPACTION I 6. Performing Organization Code 

Author(s) 

I 6. Performing Organization Report No. 

Robert G. Lukas 

I Pwforming Organirauon Name and Address 10. Work Unit No. (TRAIS) 

Ground Engineering Consultants, Inc. 

350 Pfingsten Road, Suite 106 

Northbrook, Illinois 60062 

ll.ContractorGI pant --’ NO. 

DTFH6 l-9. 3-C-00072 

13. TWO of Rerxxt ana PenW coverod 

2 Sponsoring Agency Name and Address 

I 

. . r 

Federal Highway Administration 

Office of Technology Applications Final Manual 

400 Seventh Street, SW 

Washington, D.C. 20590 14. Sponsoring Agency Code 

5. Supplementary Notes 

FHWA Project Manager: Chien-Tan Chang, HTA-22 

FHWA Technical Consultant: Jerry DiMaggio, HNG-3 1 

6. Abstract 

This manual provides state-of-the-practice methods and techniques to assist the highway 

engineer in the planning, design, and construction monitoring of dynamic compaction to improve 

the load supporting capacity of weak foundation soils. Guidelines are presented for: 

* completing a preliminary evaluation to determine if dynamic compaction is appropriate 

for the site and subsurface conditions 

* detailed design for site improvement 

* preparation of a specification 

* construction monitoring 

Two case histories of actual projects are presented to demonstrate the use of the guidelines. 

17. Key Words 

Dynamic Compaction 

Soil Improvement 

18. DiwWtlon Statement 

No restrictions. This document is 

available to the poublic from the 

National Technical Information Servce, 

19. Sacwity Cbmsil. (of thlJ repon) 

unchssified 

form DOT f 1700.7 (8-72) 

Springfield, Virginia 22 16 1. 

20. secufity Classif. (of thb pago) 21.FJo.ofPage# 22Pda 

unclassified 105 

Roproducdon of completed peg. authofttad 

- 

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DYNAMIC COMPACTION CIRCULAR 

TABLE OF CONTENTS 

CHAPTER 1 INTRODUCTION 

PURPOSE 

BACKGROUND 

ORGANIZATION 

APPLICATIONS 

CHAPTER 2 PRELIMINARY EVALUATION 

PRELIMINARY EVALUATION STEPS 

CATEGORIZE SOIL TYPE 

Site Investigation 

General Description of Soil Categories 

Most Favorable Soil Deposits - Zone 1 

Unfavorable Soil Deposits - Zone 3 

Intermediate Soil Deposits - Zone 2 

ASSESS SITE RESTRAINTS 

Ground Vibrations 

Lateral Ground Displacements 

High Water Table 

Presence of Hard or Soft Layers 

EVALUATE DESIGN REQUIREMENTS 

Tolerable Settlement 

Minimum Soil Property 

Depth of Improvement Limitation 

CONSIDER COSTS AND ALTERNATIVES 

Dynamic Compaction Costs 

Alternate Site Improvement Techniques 

CHAPTER 3 DESIGN CONSIDERATIONS 

DEVELOPMENT OF DESIGN PLAN 

SELECTION OF TAMPER AND DROP HEIGHT 

APPLIED ENERGY REQUIREMENTS 

AREA TO DENSIFY 

GRID SPACING AND NUMBER OF DROPS 

MULTIPLE PASSES 

SURFACE STABILIZING LAYER 

CHAPTER 4 CONTRACTING PROCEDURES 

INTRODUCTION 

METHOD SPECIFICATION 

PERFORMANCE SPECIFICATION 

7 

7 

9 

9 

10 

11 

11 

11 

12 

12 

14 

14 

16 

16 

16 

21 

21 

23 

23 

26 

27 

27 

27 

33 

35 

35 

36 

38 

39 

39 

39 

43 

. . . 

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COORDINATION BETWEEN DESIGNER AND 47 

SPECIFICATION WRITER 

CHAPTER 5 CONSTRUCTION MONITORING 49 

INTRODUCTION 49 

GROUND HEAVE AND PORE WATER PRESSURE 49 

INDUCED SETTLEMENT 51 

GROUND VIBRATIONS 51 

VERIFICATION TESTING 52 

OWNER AND CONTRACTOR RESPONSIBILITIES 54 

CHAPTER 6 ADJUSTMENTS DURING DYNAMIC COMPACTION 57 

MULTIPLE PHASES AND PASSES 57 

THICKNESS OF GRANULAR WORKING MAT 57 

GROUND WATER CONTROL 58 

GROUND VIBRATIONS 58 

BURIED ENERGY ABSORBING LAYERS 58 

CHAPTER 7 CASE HISTORIES AND DESIGN EXAMPLES 59 

INTRODUCTION 59 

DENSIFICATION OF A LANDFILL 59 

DENSIFICATION OF LOOSE POCKETS AND VOIDS 65 

REFERENCES 71 

GLOSSARY 77 

APPENDIX A -TYPICAL METHOD SPECIFICATION -MISCELLANEOUS FILL 79 

APPENDIX B -TYPICAL METHOD SPECIFICATION - LANDFILL 85 

APPENDIX C -TYPICAL PERFORMANCE SPECIFICATION 89 

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LIST OF FIGURES 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

Dynamic compaction. 

Typical dynamic compaction grid pattern with 13.6 Mg tamper in background. 

Square tamper with low contact pressure used for ironing pass. 

U.S. highway dynamic compaction projects. 

Grouping of soils for dynamic compaction. 

Bureau of Mines safe levels of blasting vibration for houses. 

Scaled energy factor versus particle velocity. 

Lateral movements at 3 m from drop point. 

Lateral movements at 6 m from drop point. 

Variations in improvements with depth during dynamic compaction. 

Increase in cone resistance in granular soils with applied energy. 

Observed trend between limit pressure and applied energy for granular soils. 

Observed trend between limit pressure and applied energy for cohesive soils. 

Observed trend between SPT Value and applied energy level. 

Housing development; comparison of improvements in a fill consisting of clay, 

shale, limestone, ash, sand , and pottery fragments. 

Effects of time on the relative improvements in CPT tests values in sandy soil 

in depth range of 2 to 8 m. 

Criteria for evaluating looseness and probability of soil collapse. 

Trend between apparent maximum depth of influence and energy per blow. 

Depth of improvements as measured by lateral deflection obtained at 

inclinometer located 3.0m from center of drop point. 

2 

2 

3 

4 

10 

12 

13 

15 

15 

18 

19 

19 

20 

20 

22 

22 

23 

29 

31 

V 

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20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

Depth of improvements as measured by lateral deflection obtained at 

inclinometer located 6.1 m from center of drop point. 

Relationship between size of tamper and drop height. 

Ground heave pattern due to volumetric displacement. 

Ground heave induced by dynamic compaction. 

Load test. 

Results of a load test. 

Sand boil formed from dissipation of pore water pressure in a silty sand. 

Cross-section of highway embankment over landfill, Indiana site. 

Increase in SPT values with energy application. 

Increase in PMT values with energy application. 

Gradation of landfill deposits, Indiana site. 

Aerial view of dynamic compaction operation. 

Soil boring log - Florida site. 

SPT values before and after dynamic compaction. 

PMT values before and after dynamic compaction. 

Induced settlement contours. 

31 

33 

38 

51 

52 

53 

57 

59 

60 

61 

62 

64 

66 

68 

68 

70 

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LIST OF TABLES 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

Preliminary evaluation guidelines. 

Upper bound test values after dynamic compaction. 

Equipment requirements for different size tampers. 

Dynamic compaction costs. 

Comparative costs of ground improvement methods. 

Design guidelines. 

Recommended n value for different soil types. 

Applied energy guidelines. 

Contracting for dynamic compaction. 

Construction monitoring. 

Velocity of tamper prior to impact. 

Work pattern at Indiana landfill site. 

Florida project. 

8 

17 

23 

24 

25 

28 

30 

34 

40 

50 

54 

63 

67 

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DYNAMIC COMPACTION CIRCULAR 

ABBREVIATIONS AND SYMBOLS 

AE = Applied energy 

CPT = Cone penetration test 

D = Depth of improvement in meters 

g = Acceleration due to gravity 

H = Height of tamper drop in meters 

n = An empirical coefficient used in the depth of improvement prediction 

PMT = Pressuremeter test 

SPT = Standard penetration test 

W = Mass of tamper 

V = Velocity of tamper 

SI CONVERSION UNITS 

1 t (metric ton) = 1 Mg (Megagram) 

1 tm (ton meter) = 9.807 kN/m = 9.807 kJ (kiloJoule) 

1 tm/m* (ton meter/mete?) = 9.807 kJ/m* (kiloJoule/meter squared) 

Approximate conversion: 

100&n/m* = 1 MJ/m* 

. . . 

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CHAPTER 1 

INTRODUCTION 

PURPOSE 

This document provides guidelines for evaluation, design and contracting procedures for the use 

of dynamic compaction to produce ground improvement in unstable or compressible soil deposits. 

Although guidelines are useful in planning and implementing dynamic compaction, it cannot 

replace good judgment. Frequently during site densification, adjustments must be made to the 

planned program based on how the ground responds to impact. The design engineer must always 

use good judgment to supplement or alter the guidelines. 

BACKGROUND 

Highways and interchanges are frequently required to be constructed on land with poor support 

conditions. This is particularly true in or near urban areas where land with good ground support 

conditions has already been developed for commercial or other purposes, and the remaining space 

is undeveloped land due to deficient subsoils. 

In the past, poor support areas have been avoided or structures with deep foundations such as a 

bridge supported roadway have been constructed over the top of the loose deposits. Many types 

of site improvement techniques are now available that allow embankments and interchanges to be 

constructed directly on densified ground. One form of site improvement is dynamic compaction. 

Dynamic compaction consists of using a heavy tamper that is repeatedly raised and dropped with 

a single cable from varying heights to impact the ground. The mass of the tampers generally 

ranges from 5.4 to 27.2 Mg, and drop heights range from 12.2 to 30.5 m. The energy is generally 

applied in phases on a grid pattern over the entire area using either single or multiple passes. 

Following each pass, the craters are either levelled with a dozer or filled with granular fill material 

before the next pass of energy is applied. 

All of the energy is applied from existing grade and the degree of improvement is a function of the 

energy applied: i.e., the mass of the tamper, the drop height, the grid spacing, and the number of 

drops at each grid point location. Lighter tampers and smaller drop heights result in depths of 

improvement on the order of 3.0 to 4.6 m. Heavier tampers and greater drop heights result in 

improvements on the order of 6.1 to 9.1 m. Figures 1 to 3 illustrate the dynamic compaction 

process and the equipment that has been used on a regular basis. References 7,26,32,35,38, 

39,46,54, and 60 describe the dynamic compaction procedure. 

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APPLICATIONS 

Dynamic compaction has been successfblly used to improve many types of weak ground deposits 

including: 

0 Loose naturally occurring soils such as alluvial, flood plain, or hydraulic fill 

deposits. (See references 2,9,10,20,3’7,47,55,61.) 

0 Landfill deposits both recent and old. (See references 5,17,25,33,34.) 

0 Building rubble and construction debris deposits.(3’) 

0 Strip mine spoil. (3s6) 

l Partially saturated clay fill deposits that are elevated above the water table.(42) 

0 Collapsible soils including loess. (See references 4,36,48,49,50,53.) 

0 Formations where large voids are present such as karst topography or sinkholes 

that are located close to grade. 

0 Loose sands and silts to reduce liquefaction potential. (See references 

8,12,15,21,58.) 

0 Special wastes.’ (See references 27 to 30.) 

An estimated 500 dynamic compaction projects have been completed in the U.S. Most were for 

commercial purposes. The actual number may be much greater because many projects are not 

reported in the literature. 

A list of highway-related dynamic compaction projects completed in the U.S. was compiled in 

1992.(13) Twenty-five projects were identified where dynamic compaction was used on at least a 

part of the project site. Figure 4 indicates where these projects are located and the type of deposit 

that was densified. 

TYPES OF DEPOSlTS DENSIFIED BY 

DYNAMIC COMPACTI’ON AT 

TRANSPORTATION PROJECTS 

Landfills 14 

Collapsible Soils 5 

Mine Spoil 3 

Loose Fills 3 

Figure 4. U.S. highway dynamic compaction projects. (13) 

The greatest use of dynamic compaction has been to stabilize former landfills. This is attributed 

to the need for routing highways through or adjacent to urban sites where the land is at a premium 

and frequently the only spaces available are sites such as former landfills that have been bypassed 

for commercial development. 

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Dynamic compaction has been frequently used to densifl collapsible soils present in the western 

part of the United States. The purpose of densitication is to reduce settlement of the pavements 

that occurs as the soils become wetted tier the highways are constructed. 

Mine spoil deposits consisting of reworked shales and sandstones plus soil overburden have also 

been densified by dynamic compaction. The soil and rock mixture is usually in a medium-dense 

condition, but o&n there are pockets of very loose deposits within an otherwise more stable 

formation. Dynamic compaction has been found to be effective in making the subgrade more 

uniform. 

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CHAPTER 2 

PRELIMINARY EVALUATION 

PRELIMINARY EVALUATION STEPS 

The following steps are suggested for conducting a preliminary evaluation to determine the 

suitability of using dynamic compaction at a specific project site. These steps are listed in the 

order in which they should be evaluated. However, some steps may be eliminated while other 

steps may require additional information or be expanded for a more thorough evaluation 

depending upon the site and soil conditions. 

1. Categorize soil type: The properties, thicknesses, and extent of the weak ground must be 

known. This is usually determined by soil borings with Standard Penetration Tests 

(SPT), Cone Penetrometer Tests (CPT), or Pressuremeter tests (PMT).(32*62) Other tests 

such as the dilatometer, Becker Hammer, geophysical measurements, or deceleration 

measurements have also been used. (See references 6,11,18,45.) The site history such as 

when fill deposits were placed or the geologic origin of natural soils is also important. 

Test pits might be necessary to further explore erratic deposits. 

Based upon the types of soils that are in need of improvement at the site, the deposits can 

be rated as favorable, unfavorable, or intermediate for dynamic compaction. 

2. Assess site restraints The project site should be examined to determine if the ground 

vibrations or lateral ground displacement could have an effect on adjacent properties. 

This would be especially important in urban areas where roadways or buildings might be 

situated in very close proximity to the area to be densified. 

3. Determine design requirements If reduction in settlement is desired, a settlement estimate 

should be made before and after dynamic compaction and then compared with the 

requirements of the new embankment or facility. If the settlement is still larger than the 

new facility can tolerate, an alternate form of site improvement or support should be 

considered. Fortunately, roadway embankments can tolerate settlements of 0.3 to 0.6 

m.(44) A properly des’g 1 ned dynamic compaction procedure usually results in settlement 

predictions less than this amount. 

4. Estimate costs A preliminary estimate of costs for dynamic compaction should be made. 

The cost estimate can be refined later, but a quick cost estimate is necessary to compare 

with alternate site improvement techniques. 

Table 1 lists parameters for rating each of these factors. This table can be used as a decision 

tree.If an unfavorable rating is obtained for any of the evaluation steps, other forms of site 

improvement should be considered. To permit the use of dynamic compaction, alterations or 

adjustments can sometimes be made to the item that produced an unfavorable rating. In this case, 

the additional cost for these adjustments or alterations needs to be considered. The next sections 

discuss the evaluation steps in more detail. 

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Table 1. Preliminary evaluation guidelines. 

Steps Favorable for Dynamic Favorable with Restrictions* Unfavorable for Dynamic 

Compaction Compaction 

1. Categorize Soil Type 

Zone 1: Pervious Best deposit for dynamic compaction ----m-__-mm--_ ------- 

Zone 2: Semipervious ---s-e- Apply energy in phases to allow for dissipation of --_--__- 

Zone 3: Impervious ------- Partially saturated impervious soils with deep water Saturated or nearly saturated impervious soils 

2. Assess Site Restraints 

Vibrations Adjacent to: modem construction, < 19 mm per 19 to 51mm per set allowable if adjacent to Adjacent to: modem construction, > 19 mm per 

Lateral Ground Displacements DY namic compaction > 7.6 m from buried utilities Most buried utilities can tolerate. 76 to 127 mm per Immediately adjacent to easily damaged 

00 Water Table > 2 m below grade < 2 m below grade, with drainage provided to lower < 2 m below grade 

Presence of Hard or No hard or soft layers 1. Hard surface layer: loosen prior to dynamic Energy absorbing layer that limits depth of 

Energy-Absorbing Layer compaction 2. Energy-absorbing surface layer: improvement, such as Zone 3 soil of lm or more 

remove or stabilize with aggregate in thickness at a depth that is impractical to 

3. Determine Design 

Requirements 

Settlement < 0.3 to 0.6 m for embankments > 0.3 to 0.6 m if site conditions preclude large Settlement > design engineer can tolerate. 

Minimum Soil Property Can usually achieve relatively high SPT, CPT, and May need wick drains in saturated Zone 2 soils to ___-I-- 

Depth of Improvement Limitation Deposit <9 m thick Special equipment required for deposits in range of Soils cannot be signifiwrtly improved below 

4. Estimate Costs 

Dynamic Compaction Generally least expensive form of site improvement Multiple phases could slightly increase cost If costs exceed alternate forms of site 

Surface Stabilization Frequently not required se-- lm layer could cost more than dynamic 

*Judgment must be used in assessing the applicability of dynamic compaction for these cases. For further explanation, see text. Also, consult a dynamic compaction specialist. 

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CATEGORIZE SOIL TYPE 

Site Investigation 

Before soils can be grouped into categories ranging from suitable to unsuitable for dynamic 

compaction, it will be necessary to evaluate the subsurface ground and water table conditions. 

Ordinarily this is accomplished by a site investigation consisting of borings with SPT, CPT, or 

PMT tests. 

The type of field exploration undertaken is dependent upon the characteristics of the soil at the 

site as well as local practice. In formations containing large boulders or broken concrete CPT 

testing would not be appropriate because of the chance of not being able to penetrate these 

obstructions. The SPT results also can be affected in a deposit where a large obstruction that 

influences the driving record is encountered. If the proper size borehole can be formed, PMT 

testing is appropriate in these formations. In heterogeneous deposits that are smaller in size than 

gravel, CPT testing is appropriate because of the near continuous record of penetration resistance 

in the vertical direction. In addition, the speed of CPT testing allows a larger number of tests to 

be done and provides some additional information on lateral variation in properties. 

The type of testing chosen will also depend on what is currently in use in the area, the 

availability of equipment, and the experience of the designer with that type of in situ testing. 

Samples of the various soils should also be obtained for the performance of laboratory index tests 

consisting of water content, grain size distribution, and Atterberg limits. This is especially 

important for fine-grained soil deposits. Organic content tests may also be appropriate, 

depending on the soil deposit. 

Extended water level readings should be obtained in all the boreholes. If necessary, an 

observation well can be installed to obtain variations in water levels with time. The position of 

the water table affects the dynamic compaction operations. Soils below the water table are 

considered fully saturated, and excessive pore water pressures developed during dynamic 

compaction could influence the grid spacing and number of drops that can be made at each 

specific drop point location. In addition, if the water table is close to the ground surface, 

dewatering wells might be required to temporarily lower the water table to at least 2 m below the 

working surface. 

The site history at man-made fill sites is also important. The age of the fill and the source of the 

fill are important considerations in planning and designing dynamic compaction and for 

estimating settlements. Older fill deposits have usually consolidated under their own weight, 

while newer fill deposits can still have a significant amount of voids present. 

In extremely variable deposits such as landfills, examination of soil samples from either SPT 

testing or auger cuttings can be misleading. For this reason, test pits are frequently dug to obtain 

a better understanding of the composition and relative state of packing of the landfill deposits. 

Some idea of the age of the landfill can be obtained from visual observations, methane gas 

readings, and ground temperature readings. In newer landfills, a significant amount of methane 

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is generally emitted, and the ground temperature is generally elevated above the prevailing 

average ground temperature for the region.“*) 

The site history can also be established by talking with adjacent land owners, reviewing 

permitting records at local government agencies, and reviewing topographic maps and air photos 

taken at different times. In natural soil deposits, the available geologic mapping should be 

reviewed to provide further insight into the origin of the soil deposits. 

General Description of Soil Categories 

During dynamic compaction, the soils are densified at the prevailing water content. At many 

sites, the soils being densified are fully saturated, being below the water table. For densification 

to be effective, the deposit should be relatively permeable so excess pore water pressures that 

develop during densification can dissipate relatively quickly thereby allowing the soil particles to 

move into a denser state of packing. Following this reasoning, the most favorable soil deposits 

for dynamic compaction would be those where the permeability of the soil mass is high and 

drainage is good. Likewise, deposits with a very low permeability and poor drainage would be 

unfavorable for dynamic compaction. Figure 5 shows the range of soil gradation over which 

dynamic compaction is appropriate. 

U.S. STANDARD SIEVE NUMBERS 

4 40 200 

100 

90 

80 

70 

60 

: 

0 

IO 

ZONE.3 

lMPErlvIou6 gem P.1. > I) 

PERMEABn.ITYL&PST *O 

1x10’” mlsec 

sot 

e 

40 s 

ZONE11 

PERVIOUS SOIL9 

PLASI’ICIIY INDEX (PI.) = 0 

PE~ABILITY OREArn 

THAN 1x10" idsec \ 

SAND 

uen,,,u I ClUr I SILT OR CLAY 

Figure 5. Grouping of soils for dynamic compaction!“) 

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Most Favorable Soil Deposits - Zone 1 

Dynamic compaction works best on deposits where the degree of saturation is low, the 

permeability of the soil mass is high, and drainage is good. Deposits considered most 

appropriate for dynamic compaction include pervious granular soils. If these deposits are 

situated above the water table, densification is immediate as the soil particles are forced into a 

denser state of packing. If these deposits are situated below the water table, the permeability is 

sufficiently high, excess pore water pressures generated by the impact of the tamper dissipate 

almost immediately, and densification is nearly immediate. Pervious granular deposits include 

not only natural sands and gravels but also fill deposits consisting of building rubble, some mine 

spoil, some industrial waste fill such as slag, and decomposed refuse deposits. 

Dynamic compaction extends the range of compactable soils beyond that which is ordinarily 

undertaken by conventional compaction. Ordinary roller compaction would be very difficult on 

some of the coarser grained pervious deposits such as boulders and cobbles, building rubble, or 

slag deposits. 

Unfavorable Soil Deposits - Zone 3 

Deposits in which dynamic compaction is not appropriate would clayey soils, either natural or 

fill, that are saturated. In saturated deposits, improvements cannot occur unless the water content 

of the deposit is lowered. Generally, clayey soils have permeabilities of less than lOen to 10s9 m/s, 

so dissipation of excess pore water pressures generated during dynamic compaction cannot 

occur, except perhaps over a lengthy period of time. This makes dynamic compaction 

impractical for these deposits. Furthermore, the degree of improvement is generally minor. 

Some improvements have been achieved in clayey fill deposits that are only partially saturated. 

This includes fills elevated well above the water level and with good surface drainage. In this 

case, improvement occurs as the particles are compacted before the deposits become fully 

saturated. After saturation occurs, no further improvement will be realized regardless of the 

amount of energy applied. Generally, the water content of the clayey soils prior to dynamic 

compaction should be less than the plastic limit of the deposit. 

Intermediate Soil Deposits - Zone 2 

There is a third zone of soils, labeled Zone 2 on figure 5, that is intermediate between the most 

favorable soils and the unfavorable soils for dynamic compaction. Silts, clayey silts, and sandy 

silts fall into this category. Normally, the soils in Zone 2 have a permeability on the order of 10e5 

to 1 Om8 m/s. Dynamic compaction works in these deposits, but because of the lower than desired 

permeability, the energy must be applied using multiple phases or multiple passes. Sufficient 

time should be allowed between the phases or passes to allow excess pore water pressures to 

dissipate. Sometimes, the excess pore water pressure takes days to weeks to dissipate. On some 

projects, wick drains have been installed in these formations to facilitate drainage.(‘2*58) 

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ASSESS SITE RESTRAINTS 

Site restraints may necessitate an alteration in the dynamic compaction procedure or 

supplemental construction activity to compensate for a site’s deficiency. These site restraints 

should be evaluated in the preliminary study to determine what effect they might have on the 

project cost and timing. 

Ground Vibrations 

When a tamper strikes the ground, vibrations are transmitted off site. The vibrations are largest 

when heavier tampers and higher drop heights are used. If dynamic compaction is undertaken in 

a congested area, some off-site structures could be affected by the ground vibrations. 

The U . . S Bureau of Mines(52) has studied the effect of ground vibrations on structures and has 

established threshold particle velocities beyond which cracking in walls of homes may occur. 

These limits are shown in figure 6. Numerous measurements from dynamic compaction projects 

have indicated that the frequency of ground vibrations from dynamic compaction is in the range 

of 6 to 10 hz. At this frequency, the U. S. Bureau of Mines criteria indicates that the particle 

velocities should be less than 13 and 19 mm/set for older and more modern construction to 

prevent cracks in the walls. Structural damage does not occur until the particle velocities exceed 

about 50 mm/set, although the tolerance to vibrations depends upon the condition of the 

structure. 

00 mmkec 

---..-----;------' ,-----~---t--t--l--l--I- 

____ -- _-__ T- ---_- r ----,--- T--T -.,-- )_ _,_ 

____ -__---l---__-L--_~---L--L--I--I--I- I I / IIll, 

J-... __-__; _--__ ‘-----.---,--,‘-:--‘.-I--I-- ____ -____I_-_..- __---_ ;---;--L--r-;---l 

10 

Frequency, Hz 

Figure 6. Bureau of Mines safe levels of blasting vibration for houses.@‘) 

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Particle velocities can be measured with a portable field seismograph and compared with the 

criteria shown in figure 6. Readings should be taken on the ground adjacent to the concerned 

facility. 

The particle velocities that will develop as a result of dynamic compaction should be predicted in 

advance of construction to determine if threshold vibration levels will be exceeded. Figure 7 has 

been developed from measurements taken on numerous projects and can be used to predict 

particle velocities. (32) The scaled energy factor incorporates the energy imparted into the ground 

from a single drop plus the distance from the point of impact to the point of concern. The chart is 

entered with the calculated scaled energy factor and a line projected vertically to the most 

appropriate soil type. A horizontal line is then extended laterally and the predicted particle 

velocity read off the vertical axis. This chart is based on records taken from many sites and 

rovides a good estimation of ground vibration levels for planning purposes. 

1000 

100 

10 

1 

0.1 

f- I I 

: 1 4 

0 Very Stiff Silty Clay I I .I I 

‘. a Medium Dense Micaceous Sand I 

‘. Q) Clayey Mine Spoil 

.---@BuiidiwRubble _______ _ 

.! 0 Collapsible Silt 

@Loose Sand 

_. a Loose Sand Fill 

. . @Loose Decomposed Garbage 

; Slightly 

; Perceptible I I 1 1 i+:: I I a’+ 

0.001 0.01 0.1 1 10 100 

SCALED ENERGY FACTOR, (” ENERGY (kJ)/ 9.8 

DISTANCE (m) 

> 

Figure 7. Scaled energy factor versus particle velocity.(32) 

If dynamic compaction must be performed near an existing facility and the ground vibrations 

need to be minimized, some success has been obtained with digging a trench to a depth of 

approximately 3.0 m between the point of impact and the structure of concern. The trench 

should be installed at a location where it will not undermine the foundations of the structure or 

lateral support of a buried utility. An open trench is the most effective in reducing vibrations. 

However, open trenches which could cause undermining or other concerns should be filled with 

13 

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some loosely placed soil or compressible material. The purpose of the trench is to cut off the 

Rayleigh wave, which is a surface wave that travels off site from the point of impact. At some 

sites, off-site ground vibrations have been reduced by reducing the thickness of the loose deposit 

by excavation and then using a lighter tamper and smaller drop height to density the remaining 

soils. Afterwards, the upper portion of the excavated soil can be replaced and densitied in a 

similar manner. 

Lateral Ground Displacements 

Some lateral displacements occur in the ground following the impact. Unfortunately an 

established procedure has not been developed to predict lateral ground movements. Reliance is 

placed on experience and measured data reported in the literature. As part of the FHWA study 

on dynamic compaction (32) , three project sites were instrumented with inclinometers located at 

distances of 3.0 m and 6.1 m from the point of impact. Lateral ground displacements were 

measured at both of these locations, and the results are shown in figures 8 and 9. At a distance of 

3 .O m from the point of impact, lateral displacements ranging from 152 to 3 18 mm were 

measured within the zone of 6.1 m below grade. At 6.1 m from the point of impact, the lateral 

ground displacements were only on the order of 19 to 76 mm within the upper 6.1 m of the soil 

mass. Less displacement would occur for sites where a smaller tamper and reduced drop height 

were used. 

If there are roadways or buried utilities located close to the point of impact, the likelihood of 

permanent ground displacements should be considered. Field measurements of lateral 

displacement or ground vibrations can be used to assess potential damage at structure locations. 

Particle velocity measurements have been made with a seismograph on the ground over buried 

utilities.‘63) Particle velocities of 76 mm/set have not damaged pipes and mains. Pressure 

pipelines have withstood 250 to 500 mm/set without distress. 

High Water Table 

Water table levels within approximately 2 m below the level of dynamic compaction often cause 

problems. During impacting, crater depths are frequently on the order of 0.6 to 1.2 m, and high 

pore water pressures generated in the soil mass generally cause the ground water table to rise. 

This could result in water filling into the craters. Additional drops could cause intermixing of the 

soil and water with subsequent softening of the upper portion of the soil mass. 

If the water table is within 2 m of ground surface, consider: 

l Lowering the ground water table by dewatering ditches or dewatering wells. 

l Raising the ground surface by placing fill. 

14 

Archived
DEFLECTION (mm) 

400 -350-300 -250 -200 -150 -100 -50 0 

I I I I I. I I I I 

4-lSdropsof16.3Mgat21.3m 

on cohesive mine spoil 

3 - 12 drops of27.2 Mg at 30.5 m 

on silty tine sand 

Z-15dropsof15.0Mgat18.3m 

on loose dumped sand 

D- (4 

- 0.0 

- 05 

- I.0 

- I.5 

- 2.0 

- 2.5 

- 1.0 

- 3.5 

- 4.0 

- 4.5 

- 5.0 

- 5.5 

- LO 

-6-l 

- 7.0 

- 1.5 1 

- *Al 

- u 

- 9.0 

- 9.5 

- IOA 

- IO.5 

- 11.0 

- ii.5 

- Ia. 

- Iz.5 

- I3.0 

- 133 

- ,.A 

- 14.5 

- 15.0 

Figure 8. Lateral movements 3 m from drop point.(“) 

DEFLECTION (mm) 

-80 -70 -60 -50 -40 -30 -20 -10 0 

I I I I I I I I I I I I I 

on cohesive mine spoil 

B-- 12dropsof27.2Mgat30.5m 

on silty fine sand 

C-15dropsof15.0Mgat18.3m 

on loose dumped sand 

Dm Cm) 

- il.0 

- 0.5 

- 1.0 

- I.5 

- LO 

- 2.5 

- 5.0 

- 5.5 

- Id 

- 4.5 

- 5.0 

- 5.5 

- 6.0 

- 6.5 

- 7.0 2 

- %5 

- 8.0 !8 

Figure 9. Lateral movements 6 m from drop poirko2) 

15 

Archived
Presence of Hard or Soft Layers 

The depth of improvement from dynamic compaction can be affected by the presence of a hard 

surface layer overlying a weak deposit or the presence of a soft and compressible layer within an 

otherwise stiffer deposit. 

Hard surface layers can form as a result of aging, cementation, or compaction from surface 

traffic. If this hardened layer is relatively thick (1 to 2 m), the energy from impact can be 

distributed throughout this layer and transmitted at a much lower intensity to the underlying 

weak deposits thereby resulting in less depth and degree of improvement. Thick hardened layers 

either have been removed or loosened prior to dynamic compaction so that the energy is 

transmitted to the deeper formations. 

If this hardened layer is relatively thin the tamper will likely penetrate through the hardened crust 

and still deliver the proper energy to the underlying layers. 

Soft energy absorbing soils at grade can be excavated or stabilized by adding granular soil that is 

driven into the soft soil during impact. Soft clays or organic deposits at depth within the 

formations can absorb the energy from dynamic compaction. In this case, very little energy will 

be transmitted below these layers so the lower lying layers will not be improved as much as 

desired. The effect that the soft layer will have on the densification is dependent to a large extent 

upon the thickness of the layer and its position below the ground surface. Test sections will be 

required to evaluate the depth and degree of improvement that can be attained. 

EVALUATE DESIGN REQUIREMENTS 

Tolerable Settlement 

When planning a new embankment or other facility, the settlement under new loading should be 

estimated. This includes: 

l Settlement prediction under loading without site improvement. This helps to justify the 

need for the site improvement. 

l The estimated settlement under loading after dynamic compaction. 

l Establishing the tolerable settlement of the embankment or other facility. 

Predicting settlement before and after dynamic compaction can be done using the test results of 

conventional procedures such as the SPT, CPT, or PMT tests. In very loose deposits such as 

recent landfills, SPT, CPT, or PMT test procedures for estimating settlement can be misleadingly 

low. Settlement predictions in recent and mid-age landfills based upon SPT and PMT tests have 

been found to underestimate the settlement that was measured by actual load tests on landfills 

before dynamic compaction. (24) Large objects within the loose fill matrix cause misleadingly 

high SPT values that result in low settlement predictions. The pressuremeter is inappropriate in 

deposits that are still consolidating under their own weight. Except for these recent-age landfills, 

16 

Archived
conventional settlement predictions made for other sites provide reasonable estimates of 

settlement and differential settlement. 

The value of making a settlement prediction in advance of site improvement is to compare the 

estimated movement with the tolerable movement. Excessive movement is justification for site 

improvement. 

The amount of tolerable movement depends on the sensitivity of the new facility to total and 

differential settlement. Post-construction settlements during the economic life of a roadway of as 

much as 0.3 to 0.6 m are generally considered tolerable provided:(44) 

l The settlements are reasonably uniform. 

l The settlements do not occur adjacent to a pile-supported structure. 

l The settlements occur slowly over a long period of time. 

If a building, bridge, or more sensitive facility is to be constructed on the loose deposit, other 

guidelines have been presented.(43’59) 

The amount of settlement following dynamic compaction is difficult to predict in advance of the 

actual work since the improvement depends to a large degree upon the amount of energy applied. 

Table 2 shows the maximum amount of improvement that can generally be achieved following 

dynamic compaction in terms of SPT, CPT, and PMT tests. 

Table 2. Upper bound test values after dynamic compaction.(32) 

Maximum Test Value 

Soil Type 

Standard Penetration Static Cone Tip Pressuremeter 

Resistance Resistance Limit Pressure 

(blows / 300 mm) (Mpa) (MW 

Pervious coarse-grained soil: 

sands & gravels 40 - 50 

Semipervious soil: 

sandy silts 34-45 

silts & clayey silts 25 - 35 

Partially saturated impervious 

deposits: 

clay fill & mine spoil 30 - 40* 

Landfills 20 - 40* 

‘Higher test values may occur due to large particles in the soil mass. 

19-29 1.9 - 2.4 

13 - 17 1.4 - 1.9 

lo- 13 1.0 - 1.4 

N/A 1.4 - 1.9 

N/A 0.5 - 1.0 

17 

Archived
The average improvement will be less than the maximum amount. The maximum improvement 

generally occurs at a depth of 112 to l/3 of the maximum depth of improvement as shown in 

figure 10. Figures 11 to 14 also show typical SPT, CPT, and PMT values that were measured 

following dynamic compaction as related to soil type and applied energy. 

SURFACE DEPOSITS 

LOOSENED TO 

DEPTH OF CRATER 

PENETRATION 

INCREASING IMPROVEMENT--c 

0 

\ I NOTE 

I ; 

INITIAL - \ I 

0 = MAXIMUM DEPTH OF 

IMPROVEMENT PREDICTED 

BY EQUATION I ,Chapter 3 

M = MAXIMUM IMPROVEMENTUSUALLY OCCURS 

AROUND D/3 TO D/2. 

A-INITIAL STAGES OF TAMPING 

INCREASING IMPROVEtENT - 

SURFACE DEPOSITS 

DENSIFIED BY 

L MAXIMUM 

IMPROVEMENT 

AT ABOUT 

b/3 TO D/2 

B- AFTER DEi4SlFlCATlON INCLUDING 

IRQbublc PASS 

.u 

Figure 10. Variations in improvements with depth during dynamic compaction.@‘) 

18 

Archived
2oMPa + 1 I I 

sit. Na bG8tb8 0 / 

0 3 3mIN / . 

0 I8 5lJMATRA 

0 13 8ORNCO /’ 

E 

m I5 AnAm l /O 

&ISMPa - 8 0 3 INOIANA 

0 

18 

ILLINOIS 

0 /‘/. 

3 0 

A I3 ILLGlUM <I + 

F 

0 I5 WANCC 

z 0 41 VlI)GINIA 

0 I SWCOCN 0 Ll 

w IOhm - 

8 

A I7 IUIACL o /; 

9 

V I6 SCOTLAND 

0 34 ON0 I/. 0 

I4 

f 

I l 91 BAcrGLAOLfM .O d 0 

0 22 SWGAPOW l 

6 II5 S.C. 0 / // a No70: Qoon lo silty, sand* 

5MP4 - 0 # qm s3.5MPa 

I I 

0.5 I 2 5 

U = APPLIED ENERGY (MJ/m2) 

Figure 11. Increase in cone resistance in grauular soils with applied energy.““) 

@ SANDWQRANULAR FILLS 

fl MISCELLANEOUS FILLS 

0.5 1 2 3 5 8 

u l APPLIED ENEMY (MJ/m2) 

Figure 12. Observed trend between limit pressure and applied energy for granular soils.P* 

(See refmnce 38 for details of the numbers included in this figure.) 

19 

Archived
Y V ORGANIC SILT v40 

$ 0.8 _ M7vlos 

a 

ii!g VIS 027 

5 % 0.6 

w A32 

‘j 

A= 

‘la 

k’F 

= ti 

30 0.4 - 

AS2 ‘05 

vss 

1.2 MPa’ I , I I L 

A SOFT CLAYS 

1.0 MPa V CLAYEY SILTS 

A CLAY /SILT FILLS 

” 

a? 

0.2 MPa _ 

0 MPa 

c t I I I I 

0.5 1 2 3 5 8 

U l APPLIED ENERGY c”Jirnz > 

Figure 13. Observed trend between limit pressure and applied energy for cohesive 

soikg8’ (See reference 38 for details of the numbers included in this figure.) 

- . 

@ SANDS,GRANULAR FILLS @ ‘@ 

m MISCELLANEOUS FILLS 

$ 

ia 

A* CLAYS,CLAY FILLS 

is4. E, 3o - 

@@ 0 

0 

Figure 14. Observed trend between SPT-N Valye and applied energy level.(38) (See 

reference 38 for details of the numbers included in this figure.) 

20 

A’ 

o- ’ I I I go.5 1 2 5 10 

U = APPLIED ENERGY @‘fJ / m*) 

Archived
The estimated post-densitication settlement can be made using table 2 and figures 11 to 14 as a 

guide. Some judgement is required. 

In landfill deposits secondary compression should be estimated.(24) In young-age landfills, 

secondary compression will be large but may still be tolerable provided higher than normal 

maintenance with periodic overlays is provided in the budget. 

Borings with SPT, CPT, or PMT tests should be completed when dynamic compaction is 

underway and after completion to determine if the required properties of the soils are being met 

as a result of the site densification. If the desired improvement has not been achieved, additional 

energy could be applied to result in greater ground improvement. 

In most deposits, the engineering properties of the soil improve with additional time following 

dissipation of the excess pore water pressures. (See references 3 1,32,40,5 1.) Figure 15 

illustrates the gain in PMT values with time in a fill deposit consisting of a mixture of clay, 

shale, limestone, ash, sand, and pottery fragments. Figure 16 illustrates the gain in strength in 

CPT tests in a sandy soil deposit. Thus, borings with tests made immediately after dynamic 

compaction should take into account that the soils will gain in strength with time, so something 

less than the desired SPT, CPT, or PMT test results could suffice if the borings are made shortly 

after dynamic compaction is completed in that area. 

Minimum Soil Property 

In some cases, the goal of densification is to reach a minimum soil property that will satisfy a 

criteria other than settlement. An example would be a site where earthquakes could cause 

liquefaction of the soil deposit. An initial engineering analysis must be undertaken to determine 

what minimum value of SPT would be required to render the soils nonliquefiable for a design 

magnitude earthquake. Dynamic compaction would then be planned to impart enough energy to 

reach this minimum desired SPT value. 

In areas where loessial soils are present, the goal of densitication might be to achieve a minimum 

in place unit weight that renders the soil noncollapsible. Figure 17 shows the relationship 

between the collapse potential and unit weight for loess deposits. (19) Sufficient energy should be 

applied during dynamic compaction to achieve the minimum desired unit weight. 

In collapsible alluvial soils, the single oedemeter-collapse test is the primary method for 

characterizing the collapse potential of these deposits. (4) Other index tests such as changes in 

void ratio and unit weight as a result of dynamic compaction have also been proposed to evaluate 

the effectiveness of stabilizing these deposits.(36) 

Depth Of Improvement Limitation 

The depth of improvement is primarily a function of the mass of the tamper and the drop height. 

Other factors which enter into the predicted depth of improvement are discussed in more detail in 

21 

Archived
LIm PRESSURE akF% PRESSUROETER MDDULUS &Pa 

400 800 L200 0 4,000 Rm 1COOO ldOO0 

0 

1 

c 

-2 

f 

33 

4 

s 

6 

BEFORE TREATMENT 

--- AFTER 1st PASS 

--m--e 

--m z: 

4th 

-.-.a 4th + extended recovery period 

r lgure 13. Houslng aevelopment; comparison ot mlprovements ln a till consisting of clay, shale, 

limestone, ash, sand, and pottery fragments. (42) 

-. r I I 

Tend line for 6 drops 

2.2 I 

5 

Denotes data point with number of 32 m or - 

s equivalent 32 m dynamic compaction drops.) 

0 10 20 30 40 50 60 70 80 

Time after dynamic compaction (days) 

-. -_ -^^ ^ . _ _. rlgure 16. Etkcts of time on the relative improvements in CPT test values in sandy soil 

in depth range of 2 to 8 m. (“) 

22 

Archived
LIQUIO LIMIT - % 

IO 20 30 40 50 60 10 60 SO 

10 

12 

14 

16 

Natural 

Dry 

Density - 

kN/m’ 

Figure 17. Criteria for evaluating looseness and probability of soil collapse. (19) 

chapter 3. Using tampers in the range of 18.1 to 22.7 Mg and drop heights on the orde of 22.9 

to 30.5 m, the maximum predicted depth of improvement would range from about 9.1 to 12.2 m. 

For most projects, this is an adequate depth of improvement. Even if loose deposits extend 

below these levels, the pressure increase relative to the existing overburden pressure is generally 

very small, so the contribution of settlement from these deeper unimproved deposits may not be 

large. 

If ground improvement must be attained at depths greater than 9.1 to 12.2 m, dynamic 

compaction in combination with other systems is discussed in the next section. 

CONSIDER COSTS AND ALTERNATIVES 

Dynamic Compaction Costs 

The costs for dynamic compaction depend upon the type of equipment required to complete the 

work. Lighter tampers and smaller drop heights require a smaller crane size, and dynamic 

compaction can be done at a lower cost per unit area than for the heavier tampers that require a 

much larger crane. Equipment requirements for different size tampers are listed in table 3. 

Table 3. Equipment requirements for different size tampers.(32) 

Tamper Mass Crawler Crane Size 

(Mg) Qw 

Cable Size 

(mm) 

5.4 - 7.3 36.3 - 45.4 19-22 

7.3 - 12.7 45.4 - 90.7 22 - 25 

12.7 - 16.3 90.7 : 113.4 25-29 

16.3 - 22.7 136.1 - 158.8 32 38 - 

23 

Archived
For projects where a method specification (see chapter 4) is used, an estimate of costs for 

dynamic compaction can be obtained by using table 4 and equation 1 of chapter 3. For 

preliminary cost estimates a value of IZ = 0.5 should be used in equation 1. The prices include 

mobilization but do not include: 

l A cost for the owner’s quality assurance. For projects using a method specification, the 

owner is responsible for providing the monitoring during construction plus any borings or 

tests taken after dynamic compaction. 

l A cost for granular fill, if required, to fill craters or provide a surface stabilizing layer. At 

sites where there is a weak surface layer such as a landfill, granular fill is imported to 

construct a working platform to allow for travel of the equipment across the site and to 

fill craters. The aggregate particle size generally ranges from 10 to 150 mm and the 

thickness ranges from 0.3 to 1 .O m. Local aggregate prices should be obtained to estimate 

costs. 

Table 4. Dynamic compaction costs.(32) 

Size of Tamper Required Unit Cost 

o&9 Dollars / m* 

4 to 7 5.50 to 8.00 

7to 15 8.00 to 10.75 

15 to23 10.75 to 16.25 

23 to 32 16.25 to 32.25 

32 to 91 Negotiated for each job. 

Note: Prices based on projects undertaken during 1985 to 1993 

At sites where stable materials are present at grade, granular materials are not needed. At these 

sites the ground is levelled following dynamic compaction by blading the soil from between the 

centers into the crater depressions. The loosened surface layer is then compacted with either 

normal compaction equipment or an ironing pass. 

For projects undertaken using a performance specification (see chapter 4), the contractor will 

plan and engineer the job, provide field control, and assume more risk. The cost for dynamic 

compaction will increase depending upon the complexity of the job and the risk level. High-risk 

jobs will include projects where the specifications require a performance that is difficult to 

achieve. An example would be an unrealistic maximum allowable deflection under load. A 

lower risk job would be one where the contractor only has to meet a reasonable minimum value 

of SPT, CPT or PMT. The additional costs will vary for each project depending upon the factors 

24 

Archived
listed above. A dynamic compaction contractor could provide an estimate for a project after 

provided with project details. 

The costs for dynamic compaction relative to other forms of site improvement are shown in 

table 5. As can be seen, the cost for dynamic compaction is generally significantly less than for 

other forms of site improvement. 

Table 5. Comparative costs of ground improvement methods. 

Cost Basis 

Volume of 

Treated Soil Surface Length 

Treatment Method ($/m3) ( Wm’) w4 

Dynamic compaction 0.7 - 3 4.3 - 22 

Vibro-replacement 4- 12 30 - 52 

Vibro-compaction 1-7 16 - 39 

Excavate-replace 10-20 

Slurry grouting 40 - 80 

Chemical grouting 160 - 525 

Compaction grouting 30 - 200 

Jet grouting 100 -400 82 - 325 

Freezing 275 - 650 110 - 160* 

Plus $2 to $10.75 per sq. meter/week to maintain frozen zones. 

A more refined cost estimate can be prepared if dynamic compaction is selected as being suitable 

for the site and a dynamic compaction plan is developed based upon considerations given in 

chapters 3 and 4. The weekly or monthly rate for rental of equipment to repeatedly raise and 

drop the tamper selected for the project can be obtained from local suppliers. 

In the case of the lighter tampers such as 3.6 to 9.1 Mg, the operator should be able to achieve 

500 to 600 drops per day depending, of course, upon the number of moves or standby time 

involved. For the tampers in the range of 9.1 to 18.1 Mg, the tamper can be repeatedly raised and 

dropped approximately 300 to 400 times per day. Based upon an initial mobilization charge plus 

an estimated length of time for the project, a better idea of the costs for dynamic compaction can 

be obtained. If there are other costs such as earth moving equipment for leveling of the ground 

or for importation of granular materials, this would have to be added into the estimate. The costs 

for monitoring can be calculated based upon the estimated duration of dynamic compaction and 

the unit rate for the person. 

25 

Archived
Alternate Ground Improvement Techniques 

Alternate methods of ground improvement could be considered either as a replacement for or in 

combination with dynamic compaction. This could include: 

l Removal and replacement. 

l Vibro-compaction, vibro-replacement (stone columns). 

l Grouting. 

l Surcharging with or without prefabricated vertical drains. 

An overview describing various methods of site improvement except for surcharging is presented 

in summarized form in the AGC-ARTBA Joint Committee Task Force 27 report.(‘) Dynamic 

compaction has been used in combination with some of these alternate methods of ground 

improvement. Dynamic compaction was used at a building sitec2’) to densify the upper soil 

deposits and compaction grouting used to improve the deep-seated deposits. A similar technique 

was used for an electric power plant.(51) Dynamic camp action in combination with stone 

columns was used to improve a loose clayey sand deposit beneath a dam.(8’41) 

The removal of soft surface deposits and replacement with more suitable material has been 

undertaken either prior to or during dynamic compaction at numerous project sites. Unsuitable 

material generally consisted of soft clay or an organic deposit overlying a more favorable deposit 

for improvement by dynamic compaction. 

Wick drains have been used in combination with dynamic compaction to allow for more rapid 

pore water pressure dissipation. (12,58) The wick drains were installed in silty soil deposits. If the 

wick drains had not been installed, a significantly longer period of time would have been 

required between successive passes for the pore water pressures to dissipate and ground 

improvement to occur. 

A combination of wick drains and surcharge plus dynamic compaction have been used on some 

projects.@‘) 

Variations in the normal dynamic compaction procedures have been used with success for special 

situations. In fine-grained soils a process called “select fill displacement” was used to form short 

granular columns that penetrated 5 to 7 m into the loose deposits.(‘4) This was accomplished by 

imparting as many as 70 to 150 high energy drops at each drop point location using multiple 

phases and passes and filling the craters with a select granular material. The granular columns 

improved the transmission of energy to greater depths than would otherwise occur with 

conventional dynamic compaction. In silty deposits with a high water table, the granular 

columns aided in dissipation of excess pore water pressures.(“) 

Granular columns have also been driven into soft saturated cohesive soils and organic 

deposits . (9,23) The term “Dynamic Replacement and Mixing” has been applied to this method of 

ground treatment. 

26 

Archived
CHAPTER3 

DESIGN CONSIDERATIONS 

DEVELOPMENT OF DESIGN PLAN 

If the preliminary evaluation discussed in chapter 2 indicates that dynamic compaction will be 

appropriate, a more detailed dynamic compaction plan must be prepared. Items that need to be 

addressed include: 

1. Selection of the tamper mass and drop height to correspond to the required depth of 

improvement. 

2. Determination of the applied energy to be used over the project site to result in the 

desired improvement. 

3. Selection of the area to densify. 

4. Determination of the grid spacing and number of phases. 

5. Establishing the number of passes. 

6. The need for a surface stabilizing layer. 

These six steps should be addressed regardless of whether the project will be completed with a 

method or a performance specification (see chapter 4). If the project will be undertaken with a 

method specification, the design agency or their consultant will determine the dynamic 

compaction procedure incorporating an evaluation of these six items. If the project will be 

undertaken with a performance specification, the specialty contractor will address these items 

based upon the level of improvement required. However, the design agency or their consultant 

should review the specialty contractor’s plan to determine if these items have been adequately 

considered. 

The design evaluation process is summarized in table 6. A detailed discussion of each step is 

presented in the remainder of this chapter. 

SELECTION OF TAMPER AND DROP HEIGHT 

Usually the thickness of the loose deposit and hence the required depth of improvement is known 

from the subsurface exploration. The relationship between the depth of improvement and the 

tamper mass and drop height is as follows: 

D=n(WEI))” (1) 

Where: D = depth of improvement in meters 

W = mass of tamper in Megagrams 

H = drop height in meters 

n = empirical coefficient that is less than 1 .O 

27 

Archived
Table 6. Design guidelines. 

Parameters to be Determined Evaluation Process 

Step 1: Selection of tamper and drop height A. Determine thickness of loose deposit from subsurface exploration 

for required depth of improvement or the portion of the deposit that needs densitication to satisfy 

design requirements. 

Equation 1: D = FZ(WH)“~~ B. Use Equation 1 and select n value from Table 7 for soil type. 

C. Use Figure 21 as a guide in selecting tamper mass and drop height 

for dynamic compaction equipment currently in use. 

Step 2: Determine applied energy to achieve A. Use Table 8 to select the unit energy for the proper deposit 

required depth of improvement classification. 

B. Multiply the unit energy by the deposit thickness to obtain the 

average energy to apply at ground surface. 

ltep 3: Project area to density A. For level sites, use a grid spacing throughout the area in need of 

improvement plus a distance beyond the project boundaries equal 

to the depth of improvement. 

B. If slope stability is a concern, improvement over a wider plan area 

may be required. 

C. At load concentration areas, apply additional energy as needed. 

ltep 4: Grid spacing and drops A. Select a grid spacing ranging from 1.5 to 2.5 times the diameter of 

the tamper. 

Equation 2: AE = /Vwj(I B. Enter Wand H from step I and applied energy from step 2 into 

(grid spacing)2 Equation 2. 

Where: N = number of drops C. Use Equation 2 to calculate the product of N and P. Generally 7 to 

P = number of passes 

15 drops are made at each grid point. If the calculations indicate 

significantly more than 15 or less than 7 drops, adjust the grid 

W = mass of tamper spacing. 

H = drop height 

Step 5: Multiple Passes 

A. Crater depths should be limited to the height of the tamper plus 

0.3 m. 

Prediction of crater depths or ground 

heave in advance of dynamic B. Energy application should stop if ground heave occurs. 

compaction is difficult. The contract C. If items A or B occur before the required number of drops are 

should provide for multiple passes 

applied, multiple passes should be used to: 

* permit ground leveling if item A occurs 

where very loose deposits like landfills 

are present or where silty deposits are 

nearly saturated. 

* allow pore pressure dissipation if item B occurs 

itep 6: Surface stabilizing layer A. Not needed for Zone I soils. May be required for Zone 2 soils if 

nearly saturated. Usually required for landfills. 

B. When surface stabilizing layer is used, the thickness generally 

ranges from 0.3 to 0.9 m. 

28 

Archived
The empirical coeffkient n attempts to account for factors that affect the depth of improvement 

other than the mass of the tamper and the drop height. As shown in figure 18; n has been found 

at project sites to range from 0.3 to 0.8. The variation in n is attributed to the: 

a Efficiency of the drop mechanism of the crane. 

a Total amount of energy applied. 

0 Type of soil deposit being densified. 

0 Presence of energy absorbing layers. 

l Presence of a hard layer above or below the deposit being densified. 

a Contact pressure of the tamper. 

l Maximum obsrrvod depth of influww 

l lnflurnca grsatrr than depth tasted 

/ 

ENERGY PER BLOW * WH (kJ) 

Figure 18. Trend between apparent maximum depth of influence and energy per blow.8*) 

(See reference 3 8 for details of the numbers included in this figure.) 

The first three variables listed above have been investigated previously(32) The efficiency of the 

drop mechanism using a single cable for lifting and dropping of the tamper was found to be 

approximately 80 percent of the maximum potential energy of the mass of the tamper times the 

drop height. This efficiency was found to be the same for different pieces of lifting equipment 

and for raising and dropping tampers in the range of 5.4 to 18.1 Mg. Thus, even though there is 

some energy loss in using the single cable for raising and dropping the tamper, the variable is 

29 

Archived
approximately the same for different pieces of equipment thereby resulting in approximately the 

same percentage of energy delivered. 

There is less energy loss when the tamper is raised and then allowed to free fall. However, the 

cycle time for 1 impact is approximately 5 to 10 times longer than for a tamper with a single 

cable attached. For this reason, the free fall method of dynamic compaction is rarely used. 

The total amount of applied energy at a site has some influence on the depth of improvement. 

Figures 19 and 20 illustrate the measured depth of improvement for the number of drops of the 

tamper. In the case of the sandy deposits, approximately 90 percent of the maximum depth of 

improvement is achieved after only 2 to 4 drops at one location. In the case of the clayey soils, 

there is still an increasing depth of improvement even after 14 drops at one location. On most 

projects, the tamper is dropped on the order of 7 to 15 times at one specific grid point location. In 

the sandy soils, the maximum depth of improvement would be reached, but in the case of the 

clayey soils, some additional depth of improvement could occur with additional applied energy. 

For the conditions where the energy is applied with a tamper that is raised and dropped with a 

single cable and where the average applied energy is in the range of 1 to 3 MJ/m*, the coefficient, 

n, was found to be related to soil type as shown in table 7. These values can be used in equation 

1 as a first step in estimating the depth of improvement. For most projects, this is all that is 

needed. 

Table 7. Recommended n value for different soil types. 

Soil Type Degree of Saturation Recommended n Value* 

Pervious Soil Deposits - High 0.5 

Granular soils Low 0.5 - 0.6 

Semipervious Soil Deposits - 

Primarily silts with plasticity 

index of < 8 

High 0.35 - 0.4 

Low 0.4 - 0.5 

Impervious Deposits - 

Primarily clayey soils with 

plasiticity index of > 8 

High 

Low 

Not recommended 

0.35 - 0.40 

Soils should be at water 

content less than the 

plastic limit. 

*For an applied energy of 1 to 3 M?m2 and for a tamper drop using a single cable with a free spool drum. 

30 

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Lenend 

0 Fine to Medium Sand 

Lb Silty Fine Sand 

0 Clayey Mine Spoil 

id I I 1 I I 1 I 

“0 2 4 6 8 10 12 14 

Number of Drops 

Figure 19. Depth of improvements as measured by lateral deflection obtained at 

inclinometer located 3.0 m from center of drop point.“‘) 

12 

0 Clayey Mine Spoil 

01 1 1 t I t I I 

0 2 4 6 8 10 12 14 

Number of Drops 

Figure 20. Depth of improvements as measured by lateral deflection obtained at 

inclinometer located 6.1 m from center of drop point.02) 

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The other three variables previously listed could have an effect on the depth of improvement, but 

there is no quantitative method of taking these variables into account. Some judgment needs to 

be exercised on a case-by-case basis. This includes: 

l If there is an energy absorbing layer such as a weaker saturated clay within the soil mass, 

the depth of improvement will be reduced to an extent that is dependent upon the 

thickness of the layer and the position within the soil deposit. If the energy absorbing 

layer is relatively thick and located within the center of the loose deposit, the depth of 

improvement will not extend below the depth of the weak layer. If the weak layer is near 

the surface of the deposit and is not very thick, it is possible that the tamper will penetrate 

through the layer and deliver the energy to the underlying loose deposits wherein 

equation 1 would be a relatively valid prediction of depth of improvement. Borings 

should be made after completion of dynamic compaction to determine the influence of the 

weak layer on the depth and degree of improvement. 

l A hard layer present at ground surface could restrict the amount of energy transferred to 

the deeper layers. On projects where a thick crust of densitied material is present, it will 

be necessary to loosen the surface layer to allow the energy to be transmitted to greater 

depths. A hard layer located below the loose deposit has a favorable effect in reflecting 

energy back upward into the deposit resulting in either a greater degree of improvement 

in the lower portion or a greater depth of improvement. 

l Most tampers have a flat bottom with a contact pressure on the order of 40 to 75 kN/m2. 

If the tamper falls within this range, there is no need to consider adjusting the depth of 

improvement by equation 1. However, it has been found by experience that if the contact 

pressure is significantly less than the lower bound value, the energy is distributed over 

too wide an area and a hard surface layer develops without the depth of improvement. 

Contact pressure significantly higher than the typical values could result in a tamper 

plunging into the ground. 

After selecting the required depth of improvement and the most appropriate n value for the 

deposit, the product of WH is calculated from equation 1. Figure 21 shows the relationship 

between the tamper mass and drop height for various dynamic compaction equipment currently 

in use. This figure can be used to select values of Wand H. 

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w Pw A fGi@ 

loo - 

I 

0 STAmno CaANL 

A 1n1PGo CRANE AA 

l UNDLI)WATCR CASE l 8 A O 

w LAGORATORY ¶VSILY 

IO - 

0 

00 

00 

00 

0 00, 

w H (meters) 

0 

0.11 I I , t I I 

0.1 t 16 loo 

Figure 2 1. Relationship between size of tamper and drop height. OS ) 

APPLIED ENERGY REQUIREMENTS 

A sufficient amount of energy must be applied during dynamic compaction to cause ground 

compression to result in property improvements that are necessary for design. The applied 

energy is generally given as the average energy applied over the entire area. It can be calculated 

as follows: 

where: 

AE =fl~(kf?~ 

(grid spacing)2 

AE = applied energy 

N = number of drops at each specific drop point location 

W= tamper mass 

H= drop height 

P = number of passes 

If different size tampers and drop heights are used, the total applied energy would be the sum of 

both levels of effort. The high level energy is applied first with a heavy tamper and a higher drop 

height. High energy application could result in craters of 1 to 1.5 m. After ground levelling this 

will result in’s loosened surface layer. This loosened layer is densified, by an ironing pass using 

33 

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a smaller size tamper and a lesser drop height. The total applied energy would be the sum of the 

energy applied during the high energy pass plus the ironing pass. Where crater depths are 

shallow, the ironing pass can be omitted and surface densification is attained with conventional 

compaction equipment. 

On typical projects, the average applied energy ranges from about 1 to 3 MJ/m2. However, the 

amount of energy for any specific project should be varied taking into account the: 

0 Classification of the deposit being densified. 

0 Initial relative density of the deposit. 

0 Thickness of the deposit being densified. 

l Required degree of improvement. 

Table 8 can be used as a starting point to calculate the required average applied energy. This 

table takes into account the initial three factors listed above. The soil types are grouped into 

three broad categories in table 8. The range in applied energy accounts for the initial relative 

density of the deposit. More energy should be applied to the looser deposits and less to the 

denser deposits. The thickness of the deposit being densified is incorporated into table 8 by 

listing the applied energy in terms of a unit volume. The average energy to be applied at the 

surface of the deposit can be obtained by multiplying the suggested values by the thickness of the 

deposit being densified. i 

Table 8. Applied energy guidelines.02) 

Unit Percent 

Type of Deposit 

Applied Energy Standard 

(kJ/m3) Proctor Energy 

Pervious coarse-grained soil - Zone 1 of Figure 5 200- 250 33 - 41 

Semipervious fine-grained soils - Zone 2 and clay fills 

above the water table - Zone 3 of Figure 5 250 - 350 41 - 60 

Landfills 600 - 1100 100 - 180 

Note: Standard Proctor energy equals 600 kJ/m3 . 

In table 8, the recommended unit energies range from about one-third standard proctor energy for 

the pervious coarse grain soils to about one-half this energy for the semipervious fine grain soils 

and clay fills above the water table. Less than full proctor energy is sufficient to densify these 

deposits as long as they have been in place for more than 3 to 5 years. Older fills have 

experienced compression under their own weight and are at least normally consolidated. If these 

soils have just been recently placed, a higher unit applied energy would be appropriate. 

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Landfills are usually in an extremely loose condition because of the low unit weight of the debris 

at the time of placement plus the creation of additional void spaces due to decomposition of the 

organic components. These deposits are usually underconsolidated. Applied unit energies of 1 

to 1.8 standard proctor are needed to dens@ these deposits. 

To illustrate the use of table 8, consider the case of a building rubble fill deposit that is on the 

order of 4 m thick in one area and 8 m thick in another. This deposit has been in place for 10 

years and it is considered to be in a medium dense condition. Building rubble would fall into the 

category of a pervious coarse grain soil. Because the deposit is in a medium dense condition and 

apparently consolidated under its own weight, the appropriate unit applied energy would be 200 

kJ/m3. For the 4 m thick deposit, this would require an average applied energy of 800 kJ/m*, 

while the 8 m thick deposit would require an average applied energy of 1.6 MJ/m’. If soil 

borings indicate the fill to be in a loose condition or if voids are present within the fill, the higher 

unit energy of 250 kJ/m3 should be used for determining the energy application. In this case, the 

average applied energy at the ground surface would be on the order of lMJ/m* for the 4 m thick 

deposit and 2 MJ/m* for the 8 m thick deposit. 

The guidelines given in table 8 are to be used as a starting point and adjustments may be 

necessary in the field to attain the minimum desired property values. If densitication is taking 

place to reduce liquefaction, the minimum SPT or CPT value will govern when sufficient energy 

has been applied. If densification is undertaken to reduce settlement, the design might call for a 

minimum pressuremeter modulus or minimum SPT value, and sufficient energy will need to be 

applied to reach these values. The manner in which the energy will be applied, whether in single 

or multiple passes, will be discussed in the following sections. 

AREA TO DENSIFY 

Dynamic compaction is generally completed over an area larger than the plan area of the 

embankment or the loaded area. This is to induce densification of the below ground area that 

will be subjected to stress increase due to the pressure distribution resulting from the new 

loading. 

On many projects, dynamic compaction is undertaken beyond the edge of the loaded area for a 

distance equal to the depth of the weak deposit. This would include projects where heavy loads 

are applied near the edges of the plan area such as retaining walls or building footings. In the 

case of an embankment constructed over weak ground where slope stability is a concern, it might 

be necessary to dens@ the entire zone of soil beyond the toe that would lie within the predicted 

deep-seated failure zone. 

GRID SPACING AND NUMBER OF DROPS 

The energy is generally applied at a relatively tight grid spacing over the entire area to be 

densified. The high energy drop point locations do not have to be contiguous since some of the 

35 

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energy distributes laterally into the soil mass. A drop point spacing of 1% to 2% times the 

diameter or width of the tamper is common. In the fme grain soils where there is a concern with 

pore water pressures developing in the soil, the work plan should provide for two or more phases. 

The first phase would involve dropping the tamper at every second or third drop point location. 

After a period of time to allow dissipation of pore pressures, the intermediate drop point 

locations could be densified as part of the second or third phase. 

The number of drops at each grid point location can be calculated using equation 2. The input 

includes: 

0 The applied energy calculated as per section 3.3. 

0 The tamper mass and drop height calculated as per section 3.2. 

l A grid spacing ranging from 1.5 to 2.5 times the diameter of the tamper. 

0 An assumption that all the energy will be applied in one pass. 

Normally, 7 to 15 drops of high level energy are applied at each drop point. If significantly less 

than 7 or more than 15 drops are calculated, consider adjusting the grid spacing. 

If there are concentrated loads at isolated locations, such as from a retaining wall or building 

footing, an additional phase of energy could be applied at these locations. 

The upper surface of the soil mass is generally loosened to a depth equal to the crater depth 

following the high energy level application. The loosened zone should be densified by a low 

level energy pass called an ironing pass. A square tamper, figure 3, with a low contact pressure 

is frequently used for this purpose. The area is densified on a contiguous or even overlapping 

grid. Generally a low drop height and only a few drops are needed to densify the surface soils. 

If the depth of craters is less than 0.5 m, the upper loosened soils could be densified by 

conventional compactors after levelling. 

MULTIPLE PASSES 

The number of drops that can be applied at a grid point location at one time could be limited by 

the depth of the crater. In extremely loose deposits, the initial drops may result in crater depths 

greater than the height of the tamper. This is undesirable for a number of reasons including: 

l Extracting the tamper from a deep crater is difficult and could result in cable breakage. 

Sometimes a suction force develops as the tamper is lifted from the deep crater, and at 

other times loose debris falls in on top of the tamper, increasing the extraction force. 

l After the tamper is extracted from a deep crater, the sides may cave into the crater, 

providing a cushioning effect for the next impact. In addition, the caving that occurs 

could cause the tamper to strike the base irregularly with some of the energy being 

absorbed as the tamper strikes the side walls of the crater. 

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l Applying the energy at a fairly deep level below ground surface could result in the tamper 

becoming closer to the ground’water table and generating high pore water pressures. 

l The loosened zone of soil above the base of the crater presents a problem for densifying 

the upper layer. A higher than normal level of energy may be required for the ironing 

pass to densify this relatively thick, loose deposit. 

The crater depth should be limited to about the height of the tamper plus 0.3 m. If the full 

amount of energy has not been delivered at this time, either fill the craters with good material or 

level the ground and then apply the remaining energy during a subsequent pass. 

The number of drops that can be applied at a grid point location at one time would also be 

limited if excess pore water pressures develop during impacting. In the finer grain deposits such 

as Zone 2 of figure 5, excess pore pressures may require days to weeks to dissipate following 

impact with the tamper. When high pore water pressures develop, the energy does not result in 

densification but rather in volumetric displacement of the soil mass. In this case, apply the 

energy in multiple passes to allow the excess pore water pressure to dissipate between passes. In 

the highly pervious deposits, pore water pressures generated in the soil mass as a result of 

tamping will dissipate between impacts. In this case, grain-to-grain contact is established very 

rapidly between the soil particles, and the energy can be applied all in one pass. In deposits 

consisting of building rubble, coarse sands, and gravels, or in some of the partially saturated 

deposits, the energy can usually be applied in one pass. It is more efficient for the contractor to 

apply the energy in one pass because there are fewer moves with the equipment. 

If more than one pass is required to apply the energy, the number of drops per pass decreases 

proportionally. In equation 2, the product of number of drops and number of passes must remain 

the same. For example, if 12 drops are required at each grid point location (as per equation 2) 

but only 6 drops can be completed before the crater depth becomes excessive or excessive pore 

water pressures develop, two passes of 6 drops per pass will be required. 

The required number of passes is very difficult to determine in advance of the actual site work. 

In fully saturated soils, more passes will be required than for partially saturated soils. Ideally, a 

pore water pressure measuring device should be installed in the ground at the start of 

construction to measure the rise and decay in pore water pressure following each drop of the 

tamper. The initial few drops might not cause a significant increase in pore water pressure, but 

repeated drops could result in very high values that could take a long time to dissipate. The 

information generated from the field readings would be helpful in planning both the proper 

number of drops at each location and the waiting period before additional energy can be applied. 

When writing the specification, it is preferable to specify multiple passes or phases for deposits 

classified as Zone 2 or Zone 3 soils. The contractor can then plan accordingly. 

Ground heave measurements represent an indirect measurement of excess pore water pressure. 

Figure 22 illustrates ground heaving. Ground heaving is an indication that the soils are 

displacing plastically at no volume change rather than compacting. The energy is being 

transmitted through the pore water and, at this time, dynamic compaction is ineffective in 

37 

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L 

Figure 22. Ground heave pattern due to volumetric displacement. 

causing densification. Ground heave measurements can be obtained at occasional drop point 

locations by installing settlement monitoring devices adjacent to the point of impact and 

measuring the change in elevation following each drop of the tamper. If enough observation 

points are established, the ground heave adjacent to the crater can be calculated. This can then be 

compared to the volumetric displacement within the crater itself, which is also determined from 

elevation readings. When the peripheral ground heave equals the change in crater volume, 

plastic deformation without densification is occurring. No additional energy should be applied if 

this condition occurs until there is a rest period to allow excess pore water pressures to dissipate. 

SURFACE STABILIZING LAYER 

At sites where the surface is in an extremely loose condition, such as an old landfill, it might be 

necessary to add a stabilizing surface material of granular soil to form a working mat. The 

purpose of the working mat is to provide a stable platform for the dynamic compaction 

equipment as well as to limit crater depth penetration. Working mats ranging from 0.3 to 1.2 m 

in thickness have been used at some project sites. 

The most favorable type of material to use for a working mat is a coarse-grain granular deposit 

such as gravel, crushed rock, or building rubble. 

In most cases, thick working mats should be avoided because a stiffened surface deposit tends to 

limit the energy penetration to greater depths. For very heavy tampers, working mats of 0.5 to 

lm do not appear to be a limiting factor since there is so much energy applied that the tamper 

easily penetrates through these relatively thick deposits. However, if lighter tampers are used, 

the tamper may not penetrate the working mat and the depth of improvement is limited. 

Unfortunately, the cost of importing a granular working mat adds significantly to the expense of 

dynamic compaction. For this reason, a granular stabilizing blanket may not be specified. 

However, granular material can be imported as dynamic compaction is underway to fill the 

craters. 

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CHAPTER 4 

CONTRACTING PROCEDURES 

INTRODUCTION 

Two basic types of specifications are used for dynamic compaction projects: method 

specifications and performance specifications. The decision as to which type of specification to 

use will depend on the experience of the design agency and their consultants with dynamic 

compaction, the complexity of the job, the proximity of specialty and non-specialty contractors 

to the site, the time available for test sections, and experimentation plus the department or agency 

philosophy. 

Table 9 highlights the differences between these two types of specifications. Details are 

provided in the next two sections. 

METHOD SPECIFICATION 

Preparation of a method specification requires knowledge of the various facets of dynamic 

compaction either within the design agency or by their consultants. In the method specification, 

the owner and/or their consultants do all of the engineering calculations regarding the size of 

tamper; drop height; energy that needs to be applied; area that is needed to be densitied; number 

of passes to be made plus the delay time, if any, between passes; plus consideration of off-site 

vibration or displacement as a result of dynamic compaction. Basically, the owner or their 

consultant provides all the engineering services needed to obtain the desired improvement. Test 

sections completed just prior to or at the start of production work could be specified to adjust or 

confirm the proposed dynamic compaction procedure. 

In this type of contract, the contractor assumes very little, if any, risk related to the improvement 

that occurs as a result of dynamic compaction. The contractor’s primary duties under this type of 

contract include: 

Providing a tamper of the prescribed size and with the proper contact pressure at the base. 

Providin 

repeated % 

the proper equipment with a single cable to raise and drop the tamper on a 

asis through the specified drop height. 

Providing sufficient cables, swivels, and other equipment to keep the operation 

progressmg on a continuous operation. 

.paintaining safety at the job site includin 

tmpact for working personnel; barriers, w % 

a safe working distance from the point of 

ere necessary, to prevent debris from going off 

site; and maintenance of the swivels that connect the cable and the tamper. 

:Keeping records of the number of drops, the number of passes, and any imported material 

that is required for a working mat. 

Maintaining production to meet the schedule assigned for the project. 

39 

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Table 9. Contracting for dynamic compaction. 

Method Specification Performance Specification 

Lgency should have in-house experience or hire a 

onsultant with experience to prepare detailed 

pecitications for contractors. Specifications should 

wlude: 

Owner or designer prepares specification outlining 

desired end product. This could include: 

Tamper mass and size 

Drop height 

Grid spacing 

Applied energy 

Number of phases or passes 

Site preparation requirements 

8 Surface compaction after dynamic compaction 

Drawings of work area 

- Minimum property values 

- Maximum permissible settlement 

- Other objectives of site improvement 

Owner provides initial subsurface data and lateral extent of 

project site. 

Owner or designer provides: 

. Subsurface investigation data 

s Monitoring during construction 

e Borings and tests after dynamic compaction 

The contractor is required to meet the minimum 

specified end product and is responsible for: 

- Proper equipment and work plan 

- Meeting project deadlines 

- Safety 

- Field monitoring 

- Additional subsurface exploration as required to properly 

prepare dynamic compaction plan 

- Verification of end product 

Iontractor is responsible for: 

To obtain a quality work product, the designer should 

require: 

. Providing adequate equipment to complete the work in a 

timely manner 

. Safety of personnel and equipment 

- Work plan subject to approval of designer 

- Only experienced dynamic compaction contractors to bid 

- Submittal of work plan for review and comment 

- A method for adjusting differences of opinion between 

designer and contractor 

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In addition to preparing the specifications, the responsibility of the owner is to provide: 

l Access to the site for the contractor. 

l Information related to subsurface conditions including the soil borings and geotechnical 

report. 

l Trained personnel at the site during dynamic compaction to make any adjustments in,the 

field operations that are deemed necessary. 

l Soil borings as a check on the degree and depth of improvement; monitoring load tests in 

the field; or measurement of pore water pressure. 

With a method specification, different types of contractors can bid the work. Frequently, this 

work has been completed by earth moving contractors, wrecking contractors, or specialty 

contractors. The local contractors would have the advantage of lower mobilization and general 

knowledge of the area and can be very competitive. This is especially true for the tampers in the 

low to moderate range of generally less than 14 Mg and for moderate drop heights where 

conventional lifting equipment is not over-stressed. For the higher tamper weights or the large 

drop heights, specialty contractors have experience with strengthening of the lifting drums and 

using heavy-duty equipment to minimize field breakdowns. 

The general requirements to be included in a method specification are listed as follows. Typical 

specifications are included in appendices A and B. 

If a method specification is to be used, sufficient information must be provided within the 

specifications so that it is clear to the contractor exactly what must be provided. Some of the 

important considerations are: 

. . 

l Des- - A general description of the new facility to be constructed along 

with a general description of dynamic compaction should be in the specifications to 

acquaint contractors with the project. All pertinent information including topographic 

mapping, surveys, soil boring logs, and geotechnical information should be provided. 

l a - The extent of the area to be improved by dynamic compaction should be 

outlined on a drawing or set of plans. This would include the plan dimensions of the 

embankment or building plus the additional area that is to be improved beyond the 

limits of the embankment or the building. Any utilities or other subsurface features 

should be shown on these drawings because they might affect the dynamic compaction 

operations. If some areas are designated for one type of tamper and drop height, these 

areas should be differentiated. The quantity of area to be dynamically compacted 

should be shown on the drawings. 

l Red - The tamper mass and the drop height should be calculated in 

advance to match the depth of required improvement and the soil type and not left up 

to the contractor. The range in contact pressure of the tamper should be specified. It 

should also be pointed out that the tamper must be raised and dropped with a single 

cable with a free spool drum or by free fall methods. 

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In order to complete the job on a timely schedule, either the number of pieces of 

equipment should be specified or a starting and completion date should be given so 

that the contractor can plan the proper number of pieces of equipment to complete the 

work within the time frame. 

* . 

a nhc&~~ - The amount of energy to apply at ground surface should be 

specified. If different energy levels are to be specified for different areas, these areas 

should be clearly delineated on the drawings. 

The grid spacing, number of drops at each grid point, and number of passes required 

should be specified. A maximum crater depth that can be tolerated for each pass 

should be specified. If this crater depth is reached before the desired number of blows 

is reached, either the crater must be filled before applying additional blows or an 

additional pass or passes are required after ground levelling, so that the specified 

applied energy is imparted. 

After the primary energy has been applied, the amount of energy to apply during the 

ironing pass to compact the surface of the deposit should be specified. If the surface is 

to be compacted with conventional compaction equipment instead of an ironing pass, 

this should also be specified. 

l acm and Ground I,eveb - If a backfill material is required to raise the grade or to 

provide a working mat on weak ground, the thickness and type of backfill should be 

specified. The specifications should state when additional fill is to be brought in to fill 

craters. 

On most projects fill is not required either in advance or during dynamic compaction. 

In this case, the contract should state that ground leveling be undertaken after each 

pass using a dozer to blade the ground from the high areas into the craters and to track 

roll the surface. This is necessary to form a smooth surface from which the equipment 

can work for the next pass as well as to obtain ground surface elevations. 

l red Testing - As the work is underway, certain tests should be performed to 

evaluate the effectiveness of dynamic compaction. These tests could include 

measuring crater depths, measuring heave adjacent to certain craters, determining 

ground losses from settlement readings following each pass, and borings with 

conventional soil testing. The specifications should state who will do this testing and 

how many tests will be performed. If the contractor is responsible for the soil borings, 

then the type of test and type of samples and sampling intervals should also be 

specified. If the owner is to perform some specialized testing within boreholes, such 

as pressuremeter testing, this should also be stated in the specifications, so the 

contractor can provide the proper equipment and money in the budget to compensate 

for time lost during this testing. 

42 

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If dynamic compaction will take place adjacent to built-up areas, a seismograph should 

be used to measure the magnitude of ground vibrations being transmitted off site. The 

specifications should state who will be responsible for taking the readings, the 

frequency of readings and who will interpret the readings. 

* 

l ecordkeepmg - Records should be kept of amounts of fill brought onto the site, the 

number of drops per day, the number of drops at each grid point, the number of passes 

completed to date, plus other general field records. The specification should point out 

who will keep these records and to whom they will be made available. 

l Paymeti - An equitable form of payment for a method specification contract would be 

to have a lump sum for undertaking the dynamic compaction for the energies and area 

specified and then have unit rates for additional work. The additional work could 

consist of additional drops, where needed, or for undercutting and removing soil that 

will not compact properly and for replacement with new fill, where required. A 

separate bid item should be used for placement of granular fill as a working mat or to 

fill craters. These unit price items for work incidental to dynamic compaction will 

take some of the risk out of the total operation and allow the contractor to figure his 

budgets for the dynamic compaction work in the most economical fashion. 

PERFORMANCE SPECIFICATION 

If the design agency and/or their consultant do not have expertise in dynamic compaction, a 

performance type contract should be selected for the work. In this method, the design agency 

specifies the required degree and depth of improvement and the contractor selects the proper 

equipment to achieve this goal. The responsibility of the design agency in this case includes: 

l Providing subsurface information including the geotechnical report to the bidders. 

l Defining the extent of the area to be improved. 

l Specifying the end product to be achieved. For instance, if the oal is to increase the SPT 

value, the minimum value at varying depths below grade shoul 8 be specified. 

l Listing the minimum prequalification requirements since the goal of this method of 

contracting is to obtain an experienced contractor to do the work. 

The contractor assumes a greater risk with this type of a contract. If the equipment selected to do 

the work does not achieve the desired end product, the contractor must alter his field procedures 

and perhaps even use a heavier tamper or a larger drop height to achieve the goals. Normally, 

this work is undertaken on a lump sum basis and the contractor absorbs the additional costs. 

Under this type of contract, the contractor is responsible for: 

l Selecting the tamper and drop height to achieve the depth of improvement required. 

l Selecting the proper energy to apply to reach the required degree of improvement. 

l Determining whether a stabilizing layer is necessary for a working mat. 

l Selecting the grid spacing and number of passes. 

43 

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l Providing sufficient equipment to complete the work within the required time schedule. 

l Maintaining a safe operation to prevent accidents. 

l Keeping records of the rate of progress and submitting monthly reports as well as a final 

report. 

l Performing their own independent tests as a check on the de th and degree of 

improvement and comparing them with the owner’s test resu P ts. 

Only specialty contractors can undertake performance-based work because of the specialized 

nature of this work and the expertise required in dynamic compaction. Four or five specialty 

contractors experienced in dynamic compaction are based in the United States. 

Guidelines for preparing a performance specification are included below. A typical performance 

specification is included in appendix C. 

If a performance specification is used, the required improvement of dynamic compaction should 

be clearly stated so the contractors can plan the field densiflcation program to meet this 

objective. Some of the important considerations to be included in the specifications are listed 

below: 

l Proiect Description - A general description of the project should be provided in the 

specifications. Soil boring logs, the geotecbnical report, property line surveys, 

topographic maps, and enough drawings showing the new facility should accompany 

the specifications to fully acquaint the contractors with the site conditions and the 

proposed new construction. 

l Work Area - The extent of the area to be improved by dynamic compaction should be 

outlined on a drawing or set of plans. This would include the entire area of the 

embankment or building plus the extension beyond the limits of the new facility that is 

also to be improved. Any utilities or subsurface features should be shown on these 

drawings because they could affect the dynamic compaction operations. If a different 

amount of improvement is to be achieved in different areas, these areas should be 

differentiated on the drawings. The total square footage of area to be dynamically 

compacted should be shown on the drawings. 

. 

0 rred IILMprovem& - The amount of improvement to be achieved at the project site 

should be presented in the specifications. This means that a certain amount of 

engineering must be undertaken prior to writing the specifications by the owner or his 

consultant. First, it must be ascertained whether dynamic compaction is appropriate 

for the prevailing subsurface conditions. If appropriate, then determine the minimum 

improvements that are needed so that the new structure or embankment will function 

satisfactorily. For example, if dynamic compaction is done to reduce the potential for 

liquefaction under a design earthquake, the owner or his consultant could undertake an 

analysis which will show the minimum required SPT value at various depths or the 

44 

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minimum relative density at various depths. The specifications could then reflect the 

minimum SPT or relative density values that are needed at the depths of concern. The 

contractor is then free to select the right amount of energy to attain these minimum test 

values. If after application of all the energy the minimum values are not met in certain 

areas, additional energy must be applied to improve the soils to the minimum 

standards. The owner (designer) should not choose a value of soil improvement that 

cannot be achieved. 

In certain cases where the minimum improvement needed at a site is difficult to 

predetermine, such as at a recent landfill site, the specifications could state that the 

amount of deflection of a test embankment after dynamic compaction shall be less 

than a certain predetermined value. The test embankment must be designed to induce 

the pressures imposed by the final embankment. The test embankment may not be a 

true indicator of the final performance of the embankment because some long-term 

decomposition of the landfill will take place and settlements will increase over a 

period of time. However, no better way exists of specifying performance of landfill 

sites when conventional testing will not work. 

On projects where conventional soils are being densified, such as natural or fill 

deposits of sand or silt or mine spoil, conventional soil sampling techniques consisting 

of SPT, CPT, or PMT could be used to evaluate the stability and settlement of these 

deposits with and without dynamic compaction. Minimum values of SPT, CPT, or 

PMT following dynamic compaction could then be specified. 

It is important that a sufficient amount of preengineering is performed to identify the 

problem and determine the minimum parameters that are required following 

densification to ensure that the structure or embankment will perform satisfactorily. 

The minimum parameters should then be made clear to the contractor in the 

specifications. 

l Preautication - Because the desired end product is stated in the specifications 

without the methodology to achieve this end product, only qualified contractors should 

be allowed to bid on these projects. The contractors will have to rely upon their 

previous experience, engineering ability, and judgment to determine the right amount 

of energy to apply, the grid spacing, time delays between passes, drop heights, and 

size of tampers to achieve the final goal. 

One method of prequalification would be to allow only contractors who have 

completed some number of successful dynamic compaction projects to bid for the 

work. The documentation should be presented at the time of the bid. An alternative 

method of prequalification would be to require the contractor to submit a detailed 

work plan and an equipment list at the time of bidding for the owner (designer) to 

evaluate as a condition of accepting the bid. This presumes that the owner (designer) 

is sufficiently knowledgeable to screen out inexperienced contractors. 

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. 

v-c ConZpactl ‘on - Some means of assuring that the project 

will be completed on time should be specified in the specifications. This could be 

accomplished most easily by providing a certain length of time for the work to be 

accomplished, thereby requiring the contractor to bring in the proper pieces of 

equipment to meet the schedule. An alternate method would be to specify a minimum 

number of dynamic compaction rigs, which at an average amount of production per 

day would also complete the work within time. The difficulty with the latter approach 

is that if there are equipment breakdowns or if there is poor weather, the time to 

complete the work might take longer than desired. With the first approach, the 

contractor might choose to work longer hours or on weekends to complete the work 

within the time schedule. 

& - The condition and elevation of the existing site should be discussed 

in the specifications. If site preparation is required prior to dynamic compaction, it 

should be stated whether it is part of the dynamic compaction bid or whether some 

other subcontractor will be handling it. Site preparation could include removal of trees 

or surface debris, flattening a hilly terrain to a more nearly level surface, or placing 

new fill to change the grade. When this work is undertaken with a different 

subcontractor the specifications should be clear as to what the grades and condition of 

the area will be at the time of the dynamic compaction. 

a red Testing - To confirm that the minimum value of improvement has been 

achieved, certain tests must be performed. This would include SPT, CPT, or PMT 

tests in boreholes or monitoring of test embankments with settlement plates to 

determine the amount of ground deformation under load. Whatever method is selected 

for evaluating the improvement should be clearly spelled out in the specifications. 

One test method should be selected as the acceptance criteria to avoid confusion in the 

event that two or three different test methods all show different degrees of 

improvement. It should also be clearly stated who will perform these tests. 

Verification testing is generally done by the contractor. A representative of the owner 

should be present during the testing to provide quality control and interpret the tests. 

The specification should also detail how many tests will be performed and at what 

time intervals during the course of the project. 

If dynamic compaction is to be done adjacent to built-up areas, it will be necessary to 

specify that the contractor is required to obtain seismic readings to determine the 

magnitude of ground vibrations being transmitted off site. 

0 cordkeeois - The contractor should be required to keep records of their operations. 

This would include, but not be limited to, grid patterns, drop heights, drop weights, 

number of blows, depth of crater penetration at each location, number of passes over 

the entire area, vibration readings, and ground settlement. The types of records that are 

kept should be agreed upon in advance and should be provided to the field engineer 

throughout the project. 

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* pavment - Work done on a performance basis is usually done on a lump sum basis. 

Separating the bid into itemized unit quantities is not possible because only the 

contractor knows what types of equipment will be provided at the site, how much 

energy will be applied, and whether any special additional work such as dewatering or 

bringing in more fill will be required. The contractor will include in the bid all the 

items necessary to undertake the work plus some engineering time for planning the 

dynamic compaction and for borings to monitor the work. The contractor must also 

include in the bid some additional funds to cover uncertainties and risks tht will tend 

to raise the bid price. On the other hand, an experienced contractor could use 

ingenuity and past experience to develop an economical field program to accomplish 

the goal, thereby off-setting some of the costs associated with assuming more risk. 

Exceptions to undertaking the work totally on a lump sum basis include landfill sites, 

since the quantity of granular material required to stabilize the surface of the landfill is 

usually unknown. Such contracts should be written for a lump sum for the dynamic 

compaction work with a unit rate for granular material to be brought in and placed 

over the area as required to maintain a stable ground surface. 

COORDINATION BETWEEN DESIGNER AND SPECIFICATION WRITER 

Close coordination between the design engineer and the person writing the specifications is 

essential. The designer has become familiarized with the project site by studying the subsurface 

conditions from the borings or test pits and has used guidelines and judgment in determining the 

proper approach and equipment required to successfully complete the dynamic compaction. This 

information needs to be conveyed via the specifications to the contractors, who are usually 

unfamiliar with the site but must prepare a proper bid. 

Information gaps between the designer and the specification writer will result in information 

either not properly conveyed or understood by the contractor. Many times, the specifications 

contain standard language information that is not meaningful for a particular project site. This 

only tends to contuse the bidders and increase the bid price. 

Consequently, the designer should work closely with the specification writer as a team so that a 

meaningful and clear specification can be produced to avoid confusion by the bidders. A 

commentary prepared by the designer indicating the intent or objective of the densification 

program would provide the bidders a better understanding of the designers’ concerns. This 

commentary would be a non-binding part of the specification but would solicit cooperation 

between the designer and the contractor to work as a team to accomplish the goals of the 

densitication. 

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CHAPTER 5 

CONSTRUCTION MONITORING 

INTRODUCTION 

Monitoring of dynamic compaction should be undertaken to confirm that the work is completed 

in accordance with the specifications. Furthermore, a trained observer can determine if 

adjustments in the energy application or the total applied energy need to be made while the work 

is underway. Normally, dynamic compaction is undertaken on fill deposits that are quite erratic 

in composition. For example, field adjustments may be required if unusually loose pockets or 

soils that won’t densify by dynamic compaction are present. 

While dynamic compaction is underway, observations coupled with measurements should be 

made of: 

l Ground heave and pore water pressure. 

l The average induced settlement following application of the energy. 

l Vibration monitoring. 

l Soil borings with in situ testing or, alternatively, load tests. 

Table 10 lists the type of monitoring to be undertaken and the action item pertinent to each 

observation or measurement. Details are presented in the following sections. 

GROUND HEAVE AND PORE WATER PRESSURE 

Ground heave was discussed in chapter 3, but a few additional comments relative to construction 

monitoring of ground heave and pore water pressure measurements are appropriate. Field 

personnel should observe the change in the surface of the land mass adjacent to the craters 

following the application of energy in any given area. A rise in ground surface between drop 

point locations is an indication of plastic soil deformation associated with high pore water 

pressures. Multiple passes with enough delay time allowed between passes can reduce the 

detrimental effect of heave on the ground improvement. Pore water pressure measuring units can 

be used to ascertain when the pressures dissipate sufficiently to allow for resumption of tamping. 

Rapid response piezometers should be used because the time between drops is less than 1 minute 

and it is essential to know the change in pore water pressure with each drop. 

Heave which cannot be controlled even by delayed time between passes is an indication of a very 

saturated soft material that is not suitable for improvement by dynamic compaction. This deposit 

may need to be excavated and replaced with a more suitable material. Alternatively, if the weak 

deposit is not too thick, stabilization may be possible by the addition of a granular material that is 

driven into this deposit to intermix, resulting in a more favorable densified deposit. Figure 23 

illustrates a ground heave between the drop point locations. 

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Table 10. Construction monitoring. 

Type of Monitoring Action Item 

L. Site Observations 

1. Crater depths 

2. Ground heave 

If greater than height of tamper plus 0.3 m, stop energy 

application, level ground, and resume drops. 

This is a sign of excess pore water pressure in deposit. 

Take site measurements (item B) and adjust drops. 

). Site Measurements 

1. Ground heave Measure ground heave with elevation readings on pins 

embedded in ground at various distances from crater. 

If incremental volume increase in crater is equal to 

volume of ground heave, stop dynamic compaction. 

Allow excess pore water pressure to dissipate before 

resuming work. Take additional heave readings. 

2. Pore water pressures 

3. Induced settlement 

Install piezometers in saturated fine-grained deposits to 

measure pore water rise and dissipation during impact. 

Use data to adjust number of passes or phases required. 

Measure surface settlement on grid basis before and 

after dynamic compaction. Settlement should be 5 to 10 

percent of the improved layer thickness except in 

landfills. 

4. Ground vibrations Use seismograph on ground adjacent to buildings or 

over buried utilities. Compare measured particle 

velocity with permissible values. 

Z. Verification Tests 

1. Standard penetration test (SPT) 

Pressuremeter test (PMT) 

Cone penetrometer test (CPT) 

Other in situ tests 

Compare SPT, PMT, and CPT values after dynamic 

compaction with values before dynamic compaction to 

check depth and degree of improvement. If data is 

obtained while the dynamic compaction equipment is 

on site, additional energy can be applied if needed. 

2. Load tests For landfills and nonhomogeneous deposits, load tests 

before and after dynamic compaction are sometimes 

used. Load is applied with a soil stacked to heights of 

9 to 11 m over a settlement monitoring plate. 

3. Energy at impact If there is a doubt about the efficiency of the equipment ta 

deliver the required energy, measurements of the tamper 

,velocity can be made with a radar gun. The kinetic 

energy can then be calculated. 

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Time - Days 

2 4 6 a 

Allor Dynamic CcmfJWhI 

TFST 4 

FiUDepth=Q.l Metm 

k&k: Loading by conical pile of stone, 10.1 meter height 

and 24.4 meter diameter at 1.78 Mgh?. 

Figure 25. Results of a load test. 

compaction at a project site. Ordinarily, the readings are taken for a period of at least 7 days or 

at least until the settlement readings stabilize. 

At sites such as minespoils or landfills, the SPT values after dynamic compaction are frequently 

the same order of magnitude or only slightly better than the SPT values before dynamic 

compaction. However, the ground subsidence has reached reasonable values such as 5 to 10 

percent of the original thickness of the formation, which indicates that densification has taken 

place. At these same sites, pressuremeter tests have frequently shown significant improvements. 

The reason for the discrepancy between the different methods of field testing is that the 

pressuremeter test measures the stiffness of the soil deposit, which is one of the primary reasons 

for dynamic compaction. The increase in stiffness results in the reduction in compressibility of 

the soil mass. In the standard penetration test, the sampling method is insensitive to the stiffness 

because of the remolding of the soil as the sampler is being driven. 

Deposits that are more granular generally exhibit a marked increase in the SPT value following 

dynamic compaction because of the interlocking of the soil grains as a result of densification. 

Another verification measurement that can be undertaken is to record the actual velocity of the 

tamper just prior to impact. From the velocity measurement, the energy delivered from a single 

drop can be calculated. Measurements of efficiency of a single drop have been made as part of 

the FHWA study. (32) Subsequent to this investigation, additional measurements of tamper 

velocity have been made. Table 11 summarizes these results. For a tamper with a single cable, 

the measured velocity at the point of impact is about 90 percent of the theoretical velocity for a 

tamper falling in a vacuum. The results are fairly consistent for different size tampers and 

different geographic locations. However, measurements can be made at any specific project site 

using a radar gun to measure the velocity of the falling tamper to see if the equipment is 

performing satisfactorily. 

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Table 11. Velocity of Tamper Prior to Impact.@) 

Site 

Tamper Drop Height Theoretical Measured Ratio 

Weight Less 0.6 m Velocity Velocity V Mewured 

(Mg) Cm) (mlsec) (m/set) V Theoretical 

Becancour, 15 3 7.7 7.1 0.92 

Canada 6.1 10.9 9.6 0.88 

17.7 18.6 16.8 0.90 

Tulsa, 16.3 5.5 10.4 9.3 0.9 

Oklahoma 11.6 15.1 13.1 0.87 

22.3 20.9 18.3 0.88 

St. Mary’s, (FreYFall) 29.9 24.2 23.8 0.98 

Georgia 29.9 24.2 23.6 (Radar) 0.97 

Great Lakes, 5.4 5.5 10.4 9.5 0.91 

Illinois 10.1 14.1 13.0 0.92 

Roseville, 31.8 33.5 25.7 23.8 0.93 

California 

Vernal, Utah 27.2 32.9 25.4 23 0.9 

Jote: 1.) The laser device extends 0.6 m above grade so the drop heights were adjusted accordingly. 

i.) The tampers at all the sites except the Georgia site were lifted by the cranes using a single cable and a free 

pool drum. The tamper at the Georgia site was lifted by cables but then allowed to drop free fall. 

;.)The energy of impact can be calculated as 0.5~ IV)@ where g = acceleration of gravity. 

OWNER AND CONTRACTOR RESPONSIBILITIES 

The responsibility for the monitoring will depend to some degree on whether the job is structured 

as a method or a performance specification. 

If a method specification is selected, the owner should provide the monitoring services to 

ascertain that the intent of the owner’s design objectives and the intent of the specifications are 

being fulfilled. Depending upon ground response, adjustments in the field operation may also be 

necessary so the owner’s field representative can communicate this information through the 

proper channels and get field changes authorized. 

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If a performance specification is selected, the contractor generally provides field monitoring and 

the verification borings to confirm that the minimum design value of SPT, CPT or PMT is met. 

The contractor will adjust the field densification program as necessary to obtain the desired 

result. For these projects, it is still useful to have an owner’s field engineer on the project to 

monitor, record, and interpret the dynamic compaction operations in case questions or disputes 

arise later. 

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Whenever sites are very soft at the ground surface, a working mat of granular material should be 

used. The purpose of the granular material is to prevent the sticking of the weight and deep 

penetrations of the tamper into the deposit. The thickness of the stabilizing layer should be 

limited so that the tamper does not lose its effectiveness for deep densification. A thick granular 

layer also increases the costs. The thickness of the granular mat could be adjusted as the work is 

underway. 

GROUND WATER CONTROL 

At sites where the ground water table is closer than about 2 m from ground surface, the depth of 

the craters might approach the water table, and the effectiveness of dynamic compaction will be 

reduced. At some sites, drainage ditches have been dug along the perimeter to lower the ground 

water table. At other sites, the grade has been raised by the placement of fill to provide a greater 

distance between the water table and the working surface. 

If raising the grade or dewatering in advance of dynamic compaction is not possible, then water 

must be pumped from the craters as the work is underway. Stone or other granular material must 

also be added as the craters are being formed so that the dynamic compaction is always being 

undertaken from a higher level. 

GROUND VIBRATIONS 

If dynamic compaction will be undertaken in close proximity to adjacent facilities, vibrations 

will be transmitted to adjacent structures. Methods for predicting ground vibrations were 

discussed in chapter 2, but field measurements of ground vibrations are necessary. Soils are a 

complex medium and vibrations may be transmitted to greater or lesser magnitudes than 

predicted. The dynamic compaction energy should be regulated to keep the ground vibrations 

below damage level. This can be accomplished by adjusting the drop energy, digging isolation 

trenches, or densification in layers as discussed in chapter 2. 

BURIED ENERGY ABSORBING LAYERS 

Frequently fill deposits contain a layer of a softer fine-grain deposit such as clays or organic 

materials that are nearly saturated. These deposits act as energy absorbing layers and do not 

transmit the full energy to the underlying deposits. A thorough subsurface exploration in 

advance will generally reveal the presence of these layers. However, isolated pockets may be 

present at the site and may have to be dealt with only after the work is underway. Additional 

energy could be applied in these locations. If the weak deposits are close to the ground surface, 

they can be undercut and replaced with a more suitable material. Alternatively, coarse granular 

material can be introduced into these deposits and stabilized by intermixing the coarse granular 

materials with the line-grain soils. 

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CHAPTER 7 

CASE HISTORIES AND DESIGN EXAMPLES 

INTRODUCTION 

Two case histories are presented in this chapter to illustrate the dynamic compaction design and 

planning and also to indicate the results obtained. Each case history follows a certain format. 

First, the type of project is described along with the structural loading and design requirements. 

In addition, the typical soil profile is shown. 

The second portion of the example uses the guidelines suggested in this manual to determine the 

anticipated depth of improvement for the tamper and drop height selected, the amount of energy 

to apply to achieve the improvement, and the anticipated degree of improvement. 

The third portion of the example shows the actual energies used at the job site plus the measured 

improvements. 

Finally, there is a commentary at the end illustrating the lessons to be learned from that particular 

case history. 

DENSIFICATION OF A LANDFILL DEPOSIT 

A highway embankment was constructed over a landfill that had been closed approximately 13 

years. The thickness of the landfill typically ranged from 7.3 m to 8.2 m but was found to be as 

deep as 9.1 m at one location. The landfill was operated from 1965 to 1975 and then covered 

with 0.6 m of clay. Methane gases were still exiting from vent pipes installed in the landfill at 

the time the initial soil boring investigation was made. However, the majority of the highly 

organic materials had already decomposed. The landfill was described as a mixture of soil that is 

primarily silts and clays with wood, cinders, glass, and brick fragments. Figure 27 is a 

generalized profile through a portion of the area. 

106.68 405 407 I409 I 411 I 413 I I I 

415 417 

STATION (fi) 

Figure 27. Cross section of highway embankment over landfill, Indiana. 

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Standard penetration tests performed prior to site improvement indicate SPT values ranging from 

about 5 to 25 with an average of 13. A typical SPT profile is shown in figure 28. 

Standard Penetration Resistance - Blows / 30 cm 

# ..s - 

115 - 

114 - 

113- 

112, 

NW All standard penetration test 

values over 60 were not included 

on this plot. KEY 

&---.Q Initial 

~--~ After 2nd Pass 

)b---X After 4th Pass 

Figure 28. Increase in SPT values with energy application. 

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Pressuremeter tests were also performed in the fill deposits prior to dynamic compaction, and the 

variation in pressuremeter parameters in the fill is shown in figure 29. 

2 

s 

E 

I 

C 

0 

‘ij 

9 

u 

w 

112 

111 

110 

109 

Pressuremeter Limit Pressure - kPa 

Figure 29. Increase in PMT values with energy application. 

The new embankment to be built on top of the landfill was to range in height from a low of 2.7 m 

to as much as 7.3 m. In the area where the embankment height was the greatest, the prediction of 

settlement for the typical existing conditions ranged from 43 to 74 mm. Secondary settlement 

was estimated at 200 mm. Because of the possibility that there could be voids and loose pockets 

within the landfill, dynamic compaction was selected to reduce the anticipated differential 

settlement. Because the surface of the landfill was quite soft, a 0.6 m thick layer of crushed stone 

was specified as a working mat. 

I I I I 1 

2500 5000 7500 10000 

Pressuremeter Modulus - kPa 

KEY 

0 * ---O Limit Pressure - Initial 

H Limit Pressure - After 4th Pass 

a--* Modulus - Initial 

a- a Modulus - After 4th Pass 

Landfill 

+ 

Natural 

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DYNAMIC COMPACTION CONSIDERATIONS 

Typical grain size gradation tests of the landfill deposits are shown in figure 30. The significant 

number of fines within the formations resulted in the deposit being ranked as a Zone 2 type of 

soil for dynamic compaction (see figure 5). This means that the soils will dens@, but phasing of 

the energy application and/or multiple passes are required because the generation of pore water 

pressures will take some time to dissipate. The dynamic compaction method specification 

prepared for this project is included in appendix B. 

SIEVE OPENINGS (IN) SIEVE NUMBERS HYDROMETER 

0 

IO 

20 

!z 

30 8 

40 g 

50 ! 

60 8 

70 fj 

80 0. 3 

90 

loo 

GRAIN SIZE IN MILLIMETERS 

GRAVEL 

COBBLES I SAND 

CO IMEDI FINE [ CO 1 MEDIUM 1 I 

SILT OR CLAY 

FINE 

Figure 30. Gradation of landfill deposits, Indiana site. 

For a desired depth of improvement of 8.2 m and an empirical n value of 0.35, the required 

energy per blow (WH) computes to 5.4 MJ. 

height was selected at 29.9 m. 

The contractor had an 18.2 Mg tamper so the drop 

This provides an energy per blow of 5.35 MJ. 

Using table 8 for applied energy requirements as a guide, the suggested applied energy for a 

landfill would be in the range of 600 to 1100 kJ/m3. Because this deposit is of middle age, and 

not in a loose condition except in local areas, the unit applied energy was selected at 735 kJ/m3. 

For an 8.5 m depth of improvement, the suggested total applied energy comes to 6.25 MJ/m2. 

This energy should be applied in increments to allow for pore water pressure dissipation during 

energy application. Two phases with two passes per phase was selected. 

The maximum anticipated degree of improvement following dynamic compaction according to 

table 2 would be an SPT value in the range of 20 to 40 and a limit pressure between 0.5 and 1 .O 

MPa. These are upper bound values, and the degree of improvement could be less depending 

upon the amount of energy applied. Based on this anticipated final value of SPT or PMT value, 

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the amount of immediate settlement under the 7.6 m embankment was calculated to be about 25 

mm to 152 mm, respectively, which was considered acceptable. 

ACTUALPROJECTRECORDS 

The work pattern followed at this site is shown in table 12. Two phases of energy application, 

labelled drop points 1 and 2 were made throughout the area. Two passes were also made at each 

point. The total applied energy was 6.7 MJ/m’. The measured ground compression after 

dynamic compaction calculated to be 10 percent of the thickness of the landfill. The contractor 

placed a 0.9 m thick working mat of crushed stone prior to dynamic compaction. Figure 3 1 is an 

aerial view of the site illustrating the widely spaced drop point locations of phase 1 plus the 

working blanket of crushed stone. 

Table 12. Work pattern at Indiana landfill site. 

Work Pattern - Indiana Landfill 

4.6 m 

Tamper: 18.2 Mg 

Drops: 6 Corn 29.9 m 

Working Mat: 0.9 m 

Phase I,2 passes E=3.12MJ/m2 

Phase 2,2 passes E=3.12h4J/mZ 

Ironing pass = MJ/m2 

E=6.66MJ/m2 

Unit Energy = &&MJ/m2 = 0.73MJ/m3 

9.15 m 

= 1.20 Std. Proctor 

SPT and PMT tests were performed at approximately the same location as the tests prior to 

dynamic compaction, and the results are shown on figures 28 and 29. The pressuremeter 

modulus after dynamic compaction was approximately 3 to 5 times larger, and the limit pressure 

also experienced a similar increase. However, the SPT results after the fourth pass of dynamic 

compaction were only slightly higher than the initial values. The SPT after the second pass was 

higher than after the fourth pass, which is unusual. This is attributed to the variability of landfill 

deposits. 

Three static load tests were performed before and after dynamic compaction using a 10.7 m high 

conical pile with a settlement plate embedded at the original ground surface. Readings were 

taken prior to placement of the fill and for 7 days after placement. One of the load test results 

has previously been presented in figure 25. Before dynamic compaction, the ground 

compression ranged from 0.37 m to 0.58 m. After dynamic compaction, load tests taken at an 

area immediately adjacent to the initial tests indicated ground compressions ranging from 0.07 m 

too. 14 m. This represents a reduction on the order of 4 to 5 times in settlement, which agrees 

with the approximate increase in pressuremeter modulus and limit pressure, 

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DENSIFICATION OF LOOSE POCKETS AND VOIDS 

INTRODUCTION 

A three-story structure was planned over an 8000 m* site in Florida. The structural loads were 

relatively light; but the initial subsurface exploration indicated the presence of sinkholes and 

voids due to dissolution of the limestone formations. In addition, there was a large amount of 

heterogeneity in the subsurface profile throughout the site, which led to large predicted 

differential settlements. 

A typical boring log is shown in figure 32. The predominant soil type is a silty fine sand grading 

to a fine sand with seams of sandy clay. The low SPT values are indicative of either a void or a 

soil that has collapsed into a void. Other soil borings that are not shown indicate a relatively 

dense soil profile especially where the calcareous materials within the silty sand have caused 

some cementation. Thus, the foundation support would range from very good load support on 

the cemented materials to very poor load support in the cavernous areas. 

The initial soil profile led to settlement predictions ranging from 23 mm to 74 mm assuming no 

large collapse of voids. The resulting 51 mm differential settlement was considered too large for 

the structure to tolerate. In addition, the presence of a cavity a short distance below foundation 

level would result in a very risky design. 

The designer indicated that shallow foundations could be used for this project provided the soils 

were made more homogeneous as far as load support and no voids were present within the depth 

range of 7.6 m to 9.1 m below ground surface. 

DYNAMIC COMPACTION CONSIDERATIONS 

The soils at this site are predominantly a silty sand formation that would place them into the 

Zone 2 category according to figure 5. This means that the soils would be suitable for dynamic 

compaction, but that multiple phases and/or passes would need to be made throughout the area 

since the generation of pore water pressures takes time to dissipate. 

For a depth of improvement of 7.6 m, the use of equation 1 and an empirical n value of 0.4, the 

energy per blow (W. computes to 3.56 MJ. The local contractor doing dynamic compaction had 

a 15 Mg tamper, available and for this size tamper the required drop height computes to be 24 m. 

Using table 8 for applied energy requirements as a guide, the average applied energy would 

calculate to be approximately 300 kJ/m3 multiplied by the required depth of improvement of 

7.6 m,resulting in an average applied energy at the surface of 2.28 MJ/m*. This energy should be 

applied with two phases and two passes per phase to allow pore water pressures to dissipate 

between each pass. Because of the possibility of voids or caverns at any location, additional 

energy might need to be applied where large ground depressions would occur. 

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BONING NO. R-6 

01 

t 

nouN0 SUWCI a: ..k&J .- 

e& 

o.o0.5 ’ 

1.8 . 

2.7 s 

4.0 

5.2 

6.9 

8.4 

9.9 

12.7 

fv sli&tly silty fine SAND 

rERY LOOSE to LOOSE tan fine SAND 

L 

- 

t 

1 

1 

I 

T 

/ 

1 

OuCRvTlON 

I 

tan rme SAND with roots 

m gray sandy CLAY with some 

:oots 

: 

clayey fine SAND 

VERY IDOSE tan fine SAND with 

roots 

LOOSE yellow-tan silty fine SAND 

with limestone fragments 

DENSE white calcareous silty 

SAND with limestone fragments 

LOOSE tan calcareow very silty 

SAND with limestone fragments 

limestone fra 

r 

, 

i 

Figure 32. Soil boring log - Florida site. 

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The maximum degree of improvement following dynamic compaction according to table 2 would 

be an SPT value on the order of 35 and a maximum limit pressure of a pressuremeter test of 1.4 

to 1.9 MPa. These are upper-bound values, and the degree of improvement would be less than 

this depending on the amount of energy applied. 

ACTUALPROJECTRECORDS 

The site improvement was undertaken using a performance specification with a specialty 

contractor. The contractor selected a 15 Mg tamper and a drop height of 20 m. The energy was 

applied in 2 phases with 3 passes in the first phase and 2 passes in the second phase. Additional 

drops were made at sinkhole locations. The energy application is summarized in table 13. The 

average energy application was 1.6 MJ/m*. The induced ground compression calculated to be 

9.1 percent of the anticipated depth of improvement of 7.6 m. 

Table 13. Florida project. 

Induced 

Grid Energy Settlement 

Phase Pass (m) Location Blows/print @J/m’) (mm) 

1 1 9.1 x 9.1 Primary 8 281 121 

1 2 9.1 x 9.1 Primary 9 317 98 

1 3 9.1 x 9.1 Primary 9 317 70 

2 1 9.1 x 9.1 Intermediate 9 317 97 

2 2 9.1 x 9.1 Intermediate 10 352 93 

Ironing Over- Continuous 1 35 89 

lapping 

7 Void At observed 10 --- 

sink hole 

locations 

Total Total of 8895 

blows 

1,619 694 

A comparison of average SPT values taken before and after dynamic compaction is shown in 

figure 33. Although some improvement occurred in the standard penetration resistance values, 

the improvement is still less than one would predict. The specialty contractor felt the SPT values 

were somewhat misleading for this project. For this reason, pressuremeter tests were also 

performed before and after dynamic compaction. Figure 34 indicates the average limit pressure 

and pressuremeter modulus values before and at two time intervals after completion of the 

dynamic compaction. The limit pressure and the modulus show a relatively uniform degree of 

improvement with depth, which was one of the desired results, and the limit pressure is also in 

accordance with the predicted value shown in table 2. 

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7.6 

IO 20 30 40 so N (SPTT) 

I b 

, 

DEPTH (m ) 

PL (MPa) E, WW 

S IO IS 2” 25 

I I I 

---+I AFTER S PHASES - I DAY REST 

- AFTER 6 PHASES 21 DAYS REST 

Limit Pressure and Pressuremeter Modulus before and after 

Dynamic Compaction 

rI#lIC J4. rlw I values Derore ancl alter dynamic compaction. 

68 

Archived
The pressuremeter test results performed at various intervals of time after dynamic compaction 

illustrate the improvement that takes place well after the energy has been applied. 

During dynamic compaction, settlements were taken on a grid basis throughout the project site. 

Figure 35 shows the induced settlement contours following the first three phases of dynamic 

compaction. These contours indicate that there are two locations where the settlement is much 

greater than normal. This would correspond to approximate column locations K-6 and C-6. The 

greater settlement in these areas indicates the presence of cavities or very loose deposits. For this 

reason, additional energy was applied in these areas, 

IMPORTANT CONCLUSIONS FROM THIS PROJECT 

l The depth of improvement of 7.6 m was reached even though the energy per blow was 

slightly less than recommended by equation 1. 

l The energy that the contractor used for densification was slightly less than suggested by 

table 8. The pressuremeter test shows good improvement was reached, but the SPT 

values show that there could have been more improvement if additional energy had been 

applied 

l Plotting of the settlement pattern following different phases of energy application was 

very helpful in determining where cavities or sinkholes were present. In these areas, 

additional energy was applied. 

l The increase in pressuremeter properties with time is clearly demonstrated by figure 34. 

This phenomena of strength increase following rest periods has been measured at many 

sites ranging from sandy soils to fine grain soils. Borings with tests made during 

dynamic compaction or immediately thereafter will therefore not measure the total 

improvement. 

69 

Archived
‘4 

‘3 

2 

I 

0 I .” * . . 

P 0 N M L K J I 12-H 

DYNAMIC !WllEMENT CHART 

PHASES I, 2 AND 3 (Settlement contours retained 

in inches from original drawing.) 

LEGEND 

APmoxIWITC BUlLDlNs LWTS 

--.- DYNAMIC COMPACTION AJC?A 

100 FEET 

I5 0 J5 30 h4Jzrms 

Figure 35. Induced settlement contours. Note the two locations of potential cavities at column lines K-6 and C-6. 

70 

Archived
1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

REFERENCES 

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Bayuk, A.A., and Walker, A.D., “Dynamic Compaction: Two Case Histories Utilizing 

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Butler, R.C., “Ground Improvement Using Dynamic Compaction,” Geotechnical News, pp. 

21-27, June 1991. 

Castro, G.V., Keller, T.O., and Rogers, J.H., Ground Modification Test Program for Steel 

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Chow, Y.K., Yong, D.M., Yong, K.Y., and Lee, S.L., “Dynamic Compaction of Loose Sand 

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10. . “Dynamic Compaction of Loose Granular Soils: Effect of Print Spacing,” ASCE 

Journal of Geotechnical Engineering, Vol. 120, No. 7, pp. 1115-l 133, July 1994. 

11. . “Monitoring of Dynamic Compaction by Deceleration Measurements,” 

Computer Geotechnology, Vol. 10, No. 3, pp. 189-209, 1990. 

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Liquefiable Embankment Foundation Soils,” ASCE Geotechnical Special Publication No. 

45, In-Situ Deep Soil Improvement, pp. l-25, October, 1994. 

71 

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13. Drumheller, J.C., Dynamic Compaction and Highway Construction, Case 

Histories/Summaries of Dynamic Compaction Projects for Highway Construction in the 

U.S., 1970-I 991, Transportation Research Board, February 1992. 

14. Dumas, J.C., Morel, J.F., and Beaton, N.F., “Dynamic Compaction Using Select Fill 

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Silty Sand - A Case History,” ASCE Geotechnical Special Publication No. 45, In-Situ Deep 

Soil Improvement, pp. 80-89, October 1994. 

16. Durrani, M., and Van&sin, S., “Application of Dynamic Densification Techniques in Areas 

with Completed Infrastructure,” International Journal of Housing Science Applications, Vol. 

12, No. 3, pp. 189-204, 1988. 

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1992. 

18. Holeyman, A., and Vanneste, G., “A New Approach to the Execution and Control of 

Dynamic Compaction,” Compaction Technology, Thomas Telford, London, 1987. 

19. Holtz, W.G., and Gibbs, H.J., Consolidation and Related Properties of Loessial Soils, 

ASTM, STP 126,195 1, 

20. Hussin, J.D., and Ali, S.S., “Soil Densification at the Trident Submarine Facility,” ASCE 

Geotechnical Special Publication No. 12, Soil Improvement - lo-year Update, pp. 215-23 1, 

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21. Kramer, S.L., and Holtz, R.D., Soil Improvement and Foundation Remediation with 

Emphasis on Seismic Hazards, University of Washington, Seattle, WA, August 1991. 

22. Klinedinst, G.L., and DiMaggio, J.A., “Geotechnical Innovations: Why Seldom Used in 

Highways?,” Civil Engineering - ASCE, Vol. 54, No. 1, pp. 58-61, January 1984. 

23. Lo, K., Ooi, P.L., and Lee, S.L., “Unified Approach to Ground Improvement by Heavy 

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24. Lukas, R.G. and Seiler, N.H., “Settlement of Dynamically Compacted Deposits,” ASCE 

Geotechnical Special Publication No. 40, Vertical and Horizontal Deformations of 

Foundations and Embankments, pp. 1580-1601, June 1994. 

25. Lukas, R.G., “Dynamic Compaction for Transportation Projects”, ASCE Illinois Section, 

March, 1992. 

72 

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26. . Dynamic Compaction - Engineering Considerations, ASCE Grouting, Soil 

Improvement and Geosynthetics, Geotechnical Special Publication, No. 30, pp. 940-953, 

February, 1992. 

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Nuclear Waste,” ASCE Grouting, Soil Improvement and Geosynthetics, Geotechnical 

Special PublicationNo. 30, pp. 1311-1323, February 1992. 

28. . “Dynamic Compaction of Nuclear Waste,” Civil Engineering - ASCE, Vol. 62, 

No. 3, pp. 64-65, March 1992. 

29. . Nuclear Waste Densifjcation by Dynamic Compaction, USEPA Conference on 

Design and Construction Issues at Hazardous Waste Sites, Dallas, pp. 1382-1408, May, 

1991. 

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Guidelines, Federal Highway Administration, Report FHWA/RD-86/133, July 1986. 

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927-939, June 1991. 

36. Lutenegger, A.J., “Dynamic Compaction in Friable Loess,” ASCE Journal of Geotechnical 

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73 

Archived
38. Mayne, P.W., Jones, J.S., and Dumas, J.C., “Ground Response to Dynamic Compaction,” 

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39. Menard, L., “Theoretical and Practical Aspects of Dynamic Consolidation,” Geotechnique, 

Vol. 25, No. 1, pp. 3-17, March 1975. 

40. Mitchell; J.K., and Zoltan, V.S., “Time-Dependent Strength Gain in Freshly Deposited or 

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1559-l 576, November 1984. 

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the 12th International Conference on Soil Mechanics and Foundation Engineering, Rio De 

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42. Moseley, M.P. and Slocombe, B.C., “In-Situ Treatment of Clay Fills,” Conference on Clay 

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1975. 

45. Poran, C.J., Heh, KS., and Rodriguez’ J.A., A New Technique for Quality Control of 

r>mamic Compaction: ASCE, Grouting, Soil Improvement and Geosynthetics, Geotechnical 

Special Publication, No. 30, pp. 915-926,1992. 

46. Poran, C.J., and Rodriguez, J.A., “Design of Dynamic Compaction,” Canadian Geotechnical 

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Transportation and Public Facilities, Report FHWA-AK-RD-87-05, Fairbanks, AK, August 

1986. 

48. Rollins, K.M., and Rogers, G.W., “Stabilization of Collapsible Alluvial Soil Using Dynamic 

Compaction,” Geotechnical Special Publication ASCE, New York, NY, pp. 322-333, June 

1991. 

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74 

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50. Rollins, KM. and Kim, J.H., “U.S. Experience With Dynamic Compaction of Collapsible 

Soils,” ASCE Geotechnical Special Publication No. 45, In-Situ Deep Soil Improvement, pp. 

26-43, October 1994. 

5 1. Schmertmann, J., Baker, W., Gupta, R., and Kessler, K., CPT/DMT QC of Ground 

Modzfzcation at a Power Plant, ASCE Geotechnical Special Publication No. 6, In Situ ‘86, 

Blacksburg VA, 1986. 

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Surface Mine Blasting, Bureau of Mines, Department of Investigation, RI 8507, 1980. 

53. Skaggs, R.L., Dynamic Compaction: A Review With Recent Case Studies in the San Joaquin 

Valley, Submitted for M.S. Thesis, University of California, Davis, February 1985. 

54. Slocombe, B.C., “Dynamic Compaction,” Chapter 2 of Ground Improvement, CRC Press, 

Inc., Boca Raton, FL, 1993. 

55. Smits, J.H., E&ct of Dynamic Compaction on Dry Granular Soils, Proceedings of the 12th 

International Conference on Soil Mechanics and Foundation Engineering, Rio De Janeiro, 

Brazil, August 1989. 

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Report FHWA/OK 86(3), January 1986. 

57. Swarm, L.H., Improvement of Sabkha Soils by Preloading and Dynamic Compaction At A 

Housing Site in Saudi Arabia; Piling and Ground Treatment, Proceedings of the 

International Conference on Advances in Piling and Ground Treatment for Foundations, 

Institution of Civil Engineers, London, England, pp. 101-l 09, March 1983. 

58. Von Thun, J.L., “Preliminary Results of Dynamic Compaction Stage I and Stage II 

Treatment of Jackson Lake Dam Foundation,” US Cold News, Denver, CG, March 1988. 

59. Wahls, H.E., Tolerable Deformations, ASCE, Proceedings of Settlement ‘94, Vol. 2, pp. 

1611-1628, June 1994. 

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No. 12, April 1987. 

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Structures Specialty Conference, GT Div/ASCE, Fort Collins, CO, pp. 663-675, August 

1988. 

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62. . “In Situ Testing For Ground Modification Techniques”, ASCE Geotechnical Special 

Publication No. 6, Use of In Situ Tests in Geotechnical Engineering, Blacksburg, VA, pp. 

322-335,1986. 

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Division, Vol. 107, GT2, pp. 167-181, February 1981. 

76 

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GLOSSARY 

Certain words or terms unique to dynamic compaction are used in the text and are described as 

follows: 

Average energy applied at ground surface, which is calculated on 

the basis of the sum of all the energy applied by dynamic 

compaction divided by the surface area of the densified soil. The 

typical units are Joules per meter squared. 

Depression in the ground at the drop point location that results 

from energy application. 

DH'THOF 

ROVDROPENRRGY Maximum depth to which measurable improvement is attained. 

Energy per blow, which is calculated on the basis of the tamper 

mass multiplied by the drop height. 

Energy applied to cause densification to the depth of improvement. 

Average ground settlement following densiflcation, which is 

determined by elevation readings taken before and after dynamic 

compaction. 

Energy applied to compact the surface deposits to the depth of 

crater penetration following high-level energy application. 

Low-level energy application frequently is called the ironing pass. 

The application of a portion of the planned energy at a single drop 

point location. Multiple drops are required to deliver the energy at 

each drop point. If all the drops cannot be applied at one time 

because of deep craters or excess pore water pressures, another 

pass or passes will be required after excess pore water pressures 

dissipate or the craters are filled with granular fill. There is 

generally a waiting period of at least a few days between passes. 

Describes the pattern in which the energy will be applied. For 

example, every other drop point of the grid pattern could be 

selected to be densifled as Phase 1. After completion of Phase 1, 

the intermediate drop points could be densified as Phase 2. Some 

projects use only one phase but others have been undertaken with 

five phases. 

Archived
Archived
APPENDmA 

TYPICAL METHOD SPECIFICATION PREPARED FOR 

MISCELLANEOUS FILL 

DESCRIPTION OF SITE 

The existing fill deposits that are present below the proposed alignment of the mainline between 

approximate Stations 435 and 445, as well as the existing fill along the southeast ramp between 

approximate Stations 435 and 447 will be densified in place by dynamic compaction. The 

dynamic compaction operations will be undertaken prior to any embankment filling for the 

roadways. 

Along the mainline, the thickness of the fill is typically on the order of 1.5 to 4.5 m in thickness, 

and consists primarily of clayey fill that was presumably placed as part of the railroad 

embankment construction. The thickness of the fill along the southeast ramp is typically on the 

order of 4.5 to 9 m and is a former landfill which contains miscellaneous types of fill deposits. 

Soil borings have been made throughout this area and the logs are included on Drawings -. 

DESCRIPTION OF DYNAMIC COMPACTION OPERATIONS 

Dynamic compaction is a process whereby a heavy tamper is repeatedly raised and dropped from 

a specified height to impact onto the ground surface, thereby transmitting high compaction 

energy into the soil mass. The depth of improvement depends upon the tonnage of the weight 

and the height of the fall. The degree of improvement depends upon the amount of energy 

applied per unit area. The tamper shall be raised and dropped by a single cable with a free spool 

hoisting drum. 

The tamper weight shall be constructed to resist the high impact stresses. The high energy 

compaction shall be applied in two phases which is designated on the drawings as the primary 

(first phase) and secondary (second phase) locations. After energy application at any primary 

location, there shall be a minimum waiting period of three days before energy application at the 

immediately adjacent secondary location. Multiple passes may be required to deliver the full 

energy. One pass is described as either of the following, whichever occurs first: 

A. 

B. 

The application of all the specified drops at a grid point location, or 

Whenever the crater depth reaches 1.5 to 2.0 m regardless of the number of drops 

at a grid point location. 

After the last pass has been completed, the ground surface shall be levelled and a low-level 

energy pass called an ironing pass shall be applied. The ironing pass consists of dropping the 

tamper from heights of 4.5 m to impact the surface at a tight spacing, 

79 

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DYNAMIC COMPACTION EQUIPMENT 

The high energy dynamic compaction shall be undertaken with an 18.2 Mg tamper with a 

minimum 18 m drop height. The contractor shall provide the equipment capable of raising and 

dropping this tamper on a repeated basis without a significant amount of down time. The 

contractor shall also provide equipment to level the ground surface between passes. 

The contractor shall provide a minimum of two cranes and two tampers for the dynamic 

compaction operations. Each tamper shall be 18.2 Mg and the contact pressure which is the 

weight of the tamper divided by the base area shall be in the range of 38 to 72 kPa. 

The contractor shall stake (with wood lathe or wire markers with flags) all of the grid point 

locations as shown on figures. The tamper will be dropped with the staking accurate to the 

nearest f 0.3 m. Following each pass and after the ground is level, the stakes or wire markers 

with flags shall be replaced for the next pass. 

SITE PREPARATION 

The site has been cleared of trees and surface vegetation. However, there is some surface debris, 

including chunks of concrete that may not be able to be broken up by the dynamic compaction 

operations that will have to be removed. The existing grade shall also be smoothed such that 

dynamic compaction takes place from an essentially level surface. This does not mean that 

ravines such as adjacent to the railroad shall be filled prior to dynamic compaction, since the 

intent of the dynamic compaction is to apply the energy from the existing grade. However, sharp 

vertical drops shall be flattened to provide access for the cranes for lifting and dropping of the 

weights. Small hills or stockpiles of miscellaneous rubble should be smoothed out and bladed to 

a more nearly level surface. 

ENERGY APPLICATION 

Drawing __ indicates the limits of the dynamic compaction operation. Dynamic compaction is 

proposed along the mainline from Station 435 to Station 445, and along the southeast ramp from 

Station 434+75 to Station 447. The proposed drop point locations are shown on a grid spacing of 

4.6 m center to center. The open circles or squares represent the primary drop points and the 

solid circles or squares are the secondary drop point locations. 

The total area for dynamic compaction can be divided into smaller work sections with one 

section completed before another section is started. However, the contractor will level the 

ground at the end of each work day for safety reasons and to prevent ponding of water within the 

craters if rain were to occur. Within any one portion of the overall dynamic compaction area, the 

construction sequence shall consist of one pass of dynamic compaction at all primary drop point 

locations, followed by ground levelling and then one pass at the secondary drop point locations 

followed by ground levelling. 

80 

Archived
The proposed dynamic compaction work is divided into two portions, i.e., Area A and Area B. 

Different amounts of compactive energy will be applied in each area as discussed below. 

Area A - Within this area, the fill depths range from 1.5 to 4.5 m and the fill is primarily clayey 

and is presumed to have been placed as part of the original railroad embankment construction. 

Within this area, 1.6 MJ/m2 of energy will be applied exclusive of the ironing pass. This energy 

application can be achieved with a total of five drops at each grid point location using the 18.2 

Mg tamper and 18 m drop. It is anticipated that this amount of energy can be applied in one 

pass. However, energy application will stop if the depth of the crater exceeds 1.5 to 2 m as 

measured from the ground surface. Whenever this occurs the ground wi 1 1 be levelled and an 

additional pass or passes will be applied to administer the remaining drops. / 

After five total drops have been applied at each grid point location, an ironing pass will be used 

to densify the upper portion of the land mass. The ironing pass will consist of three drops from a 

height of 4.6 m, with an 18.2 Mg tamper at a grid spacing of 3 m center to center. This will 

result in an additional 270 kJ/m2 of energy application, raising the total amount of energy for 

Area A to 1.9 MJ/m2. 

Area B - Within Area B, the depth of the fill is on the order of 6 to 9 m and this is the site of a 

former landfill. The amount of energy that will be applied to this area is 5.4 MJ/m2 exclusive of 

the ironing pass. This amount of energy can be achieved by a total number of 17 drops at each 

grid point location using the 18.2 Mg tamper and a drop height of 18 m. It will be necessary to 

apply this energy using multiple passes because of the crater depth limitation of 1.5 to 2.0 m per 

pass. At least two passes are anticipated. Between each pass, the ground shall be levelled and 

trackrolled to allow for passage of the construction equipment to apply the dynamic compactive 

energy. 

After all 17 drops have been applied at all the grid point locations, an ironing pass will be used to 

densify the upper portion of the fill. The ironing pass shall consist of three drops from a height 

of 4.6 m with the 18.2 Mg tamper at a grid point spacing of 3 m center to center. This will result 

in an additional energy application of 270 kJ/m2 for a total energy application of all energy 

within Area B of 5.7 MJ/m2. 

ADDITIONAL WORK 

Because of the varying subsurface conditions of the existing fill deposits, some additional energy 

application may be required at select locations. This decision will be made in the field as the 

work is underway by the field engineer who is monitoring the operation. This additional work 

will be paid at a pro-rated rate based upon the bid item for energy application. 

It may also be necessary to import granular fill such as coarse gravel, rubble, or broken rock to 

stabilize the upper portion of the soil mass in poor ground support areas. The need for additional 

fill will not be known until the project is underway. A pay item should be provided for granular 

backfill in the event granular fill is needed. 

81 

Archived
MONITORING 

A. By State Highway Department Representative 

As the work is underway, the dynamic compaction operations will be monitored and the 

monitoring will be paid for by the State Highway Department. The monitoring will include, but 

not be limited to observations of crater depths, monitoring ground vibrations adjacent to 

building, determining if heave is occurring adjacent to certain craters, deciding on the need for 

additional tamping at select locations or the need for importation of granular fill to stabilize weak 

ground. The monitoring will also include soil borings at select locations to determine the depth 

and degree of improvement. 

B. By the Contractor 

The contractor shall be responsible for obtaining ground elevations on a 30 m grid pattern. 

These ground elevations shall be made immediately prior to dynamic compaction and additional 

elevation readings will be taken following ground levelling after each pass of the dynamic 

compaction. The elevations shall be obtained at the same locations to determine how much 

ground loss was induced by the dynamic compaction. The contractor shall also be responsible 

for counting the number of drops at each grid point location to be sure that the proper energy is 

applied. If any additional tamps are requested by the field engineer, these shall also be recorded 

by the contractor. If granular fill is brought in at select locations, the contractor shall record the 

amount of fill that was hauled onto the site. 

PROTECTION OF PERSONS AND ADJACENT PROPERTIES 

A detailed safety program will be required to provide protection for job site personnel, off-site 

personnel, and adjacent properties. This safety program should be submitted prior to 

commencement of the work for review by the State Highway Department. The safety program 

should include required setback distances from the point of impact from the weight for personnel 

so that they are not within range of flying particles that may occur from impact of the weight into 

the ground. It should also include the manner in which the cable is attached to the tamper and 

the frequency of safety checks on the cable and associated equipment to prevent failure during 

dynamic compaction operations. Photographs should also be made of nearby structures that 

might be affected by the dynamic compaction operations and included with the safety plan 

submittal. All items associated with the safety program shall not be paid for separately, but shall 

be included in the cost of the dynamic compaction operations. 

RECORDS 

The contractor shall keep accurate records of the construction operation, i.e., the locations where 

the drops have been made, the number of drops per location, depth of crater penetration, the pass 

number, ground surface elevations following each pass, and these results shall be available in the 

contractor’s trailer on a daily basis. On a weekly basis, this information shall be submitted to the 

field engineer representing the State Highway Department. 

82 

Archived
Any circumstances affecting the work or intended improvement as a result of dynamic 

compaction should be brought to the attention of the field engineer. 

BASIS OF PAYMENT 

The work to be paid for shall be the measured quantity in square yards of dynamic compaction as 

shown on the drawings. If additional work is necessary based upon field observations or borings 

made as the work is underway, this additional work will be paid for at the prorated energy rate 

per square yard basis as per the bid item. 

83 

Archived
Archived
APPENDIX B 

TYPICAL METHOD SPECIFICATION 

PREPARED FOR A LANDFILL 

DESCRIPTION OF WORK 

The work shall consist of increasing the density of the soil by Dynamic Compaction to the area 

and extent shown on the drawings. No compaction work shall be done on this project unless the 

water table is at least 2 m below the grade of the working areas. A 0.6 m thick layer of rock 

backfill shall be placed over the portion of the site to be densitied. 

BLANKET FILL 

Prior to beginning dynamic compaction operations, the contractor shall level the terrain. Rock 

backfill shall then be placed over the landfill area to a thickness of 0.6 m. This material shall be 

graded and compacted to support the construction equipment during dynamic compaction. This 

work platform shall be graded to drain and shall be suitable for movement of large crawler cranes 

and other equipment. Rock backfill shall consist of quarry limestone or dolomite conforming to 

Class E or better requirements as set out in 903.02(b). The maximum size shall be 300 mm. The 

material shall not have more than 15 percent passing a 41 mm sieve and not more than five 

percent passing a 20 mm sieve. 

After each pass, additional rock backfill shall be placed in the craters to raise the grade to 

prevailing level. 

DEFINITION 

Dynamic compaction is a process whereby a heavy tamper is repeatedly raised and dropped from 

specified heights to impact into the ground surface thereby transmitting high compaction energy 

into the soil mass. The depth of improvement depends upon the mass of the tamper and the 

height of the fall. The degree of improvement depends upon the amount of energy applied per 

unit area. The tamper is raised and dropped by either of two methods: 

1. 

2. 

A single cable with a free spool hoisting drum. 

A number of cables or single cable extending through a series of pulleys to lift the 

tamper, provided the tamper is dropped free fall. 

The tamper is generally constructed of solid steel to resist the high impact stresses, however it 

could be constructed of another approved material. 

SPECIFIC JOB REQUIREMENTS 

The work area is shown on the drawings. It is identified as the BFI Landfill. In this area, 

municipal landfill extends to depths ranging from 6 to 9 m below present grade, except at the 

very edges of the landfill. 

85 

Archived
The contractor shall employ an 18.2 Mg tamper with a minimum drop of 23 m or as directed by 

the Engineer. The energy shall be applied on a grid system in two phases. The primary phase 

shall be completed with a grid spacing of 4.5 m. The secondary phase will also be undertaken at 

a grid spacing of 4.5 m but at locations intermediate to the primary locations. At least one week 

of time must elapse between energy application of the primary and secondary phases. The 

average energy to be applied to this area exclusive of the ironing pass is 6.3 MJ/m2. This will 

require 16 drops at each grid point location. Energy application will cease if the depth of the 

crater exceeds 1.5 m. It is anticipated that eight blows can be applied at each grid point location. 

If a crater depth greater than 1.5 m occurs before the application of this energy, additional passes 

will be required. Following each pass, the craters will be filled with additional rock backfill and 

the ground levelled. Care should be taken to minimize the amount of material falling into craters 

from areas adjacent to the craters. After the 6.3 MJ/m2 energy is applied, the ground shall be 

levelled and the upper soil mass densified with an ironing pass which is a low-level energy 

application applied over the entire surface. The average energy applied during the ironing pass 

shall be 450 kJ/m2. Thus, the total energy applied from the primary through the ironing passes 

will be 6.75 MJ/m2. 

For the area designated as the Lambert fill site which is located south of the levee, the dynamic 

compaction shall be undertaken with a minimum 13.6 Mg tamper and a minimum 23 m drop or 

as directed by the Engineer. The energy shall be applied on a grid basis with a grid spacing of 

4.5 m. The energy will be applied in three phases to result in an average applied energy of 3.6 

MJ/m2. This is exclusive of the ironing pass. No further energy will be applied at a grid point 

when the crater depth exceeds 1.5 m. Care should be taken to minimize the amount of material 

falling into craters from areas adjacent to the craters. This may require multiple passes to apply 

the full required energy. 

The first phase of energy will be applied at 4.5 m centers, and the second phase at the same grid 

spacing at points intermediate to the first phase. The third phase will be applied at the same 

location as the first phase. Following application of this energy, the surface materials will be 

leveled and the upper soils compacted with an ironing pass. A minimum energy of 400 kJ/m2 

shall be applied during the ironing pass. Thus, the total energy applied to this area will be 4 

MJlm’. 

The unit contact pressure of the base of the tamper applying this high energy shall be in the range 

of 42 to 60 kPa. The unit contact pressure at the base of the tamper used for the ironing pass 

shall be in the range of 19 to 38 kPa. 

SOIL BORINGS AND PRESSUREMETER TESTS 

As the work progresses, soil borings will be made as a check on the degree and depth of 

improvement achieved. The depth of the borings will be equal to the thickness of the existing fill 

plus 1.5 m. Samples will be obtained at 0.8 m intervals using split barrel sampling procedures in 

accordance with ASTM D-1557. Approximately twenty-five (25) borings will be 

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made on five separate occasions to check on ti work lc~s the work progresses. The contractor 

shall be responsible for completing the borings iud the final results to the owner. 

While the borings are being made, press- tests will be p&armed within the boreholes at 

selected intervals by a representative of the Highway &pa&meat. A s@ndby time of three hours 

per boring will be required to allow the press~~~~ M&S 40 be performed while the crew is 

standing by. 

Payment for the borings will be made at the contract u&t price per each &W “Boring,” as set out 

in the Itemized Proposal. 

ADDITIONAL WORK 

Based upon the results of the boring tests, or ko& tests aa deseri&d in a subsequent ,section, or 

information obtained from ground subsidence re&ings, it mrcy be necessary to apply additional 

energy to certain areas. This additional work will be p&d far as an extra to the basic contract but 

will be paid at the pro-rated rate on a per w me&r bute BS per the bid item. It is anticipated 

that not more than ten percent of additional v over that origin&y specified will be applied 

as an extra. 

MONITORING 

Monitoring of ground elevations and the Amos& of .a&m brow&t into the dynamically 

compacted areas shall be maintained by the con&&. Ground elevations shall be obtained on a 

30 m grid pattern immediately prior to start of the dynamic compaction in that area. Additional 

elevation readings will be taken after each pass of the dynamic compactin at the same locations 

to determine how much settlement was induced by the hyllBIIIic comp&on. The amount of 

stone placed in the area shall also be estimated 081 tke l&s oft& member of trucks used to haul 

the stone into the area. 

’ LOADING TESTS 

Four static load tests shall be conducted before and fiorr at&c load tests after the dynamic 

compaction at locations directed by the Engineer. The plrvlrose of these Us is to assist in 

demonstrating how much settlement has been removed frrmn the landfill by dynamic compaction. 

The contractor shall construct a pile of material approxim&e!y 21 m in diameter at the base and 

11 m high with one to one side slopes, exerting a pry of 98 kPa on a steel plate 1.2 m by 1.2 

m by 130 mm placed on the surface of the existing grade with a 75 to 100 mm diameter PVC 

pipe extending up through the center. An individual readiag shall be taken when fill commences, 

and readings every 1.5 m interval as material is placed ztp to the maximum height. The 

contractor shall supply and utilize this pipe and plate for testing over a period of seven days. 

Settlement readings shall be made every four hours for the first day and every eight hours for the 

remaining six days of each load test. After completion of the individual load test, the loading 

material must be removed by the contractor. The contractor will be allowed to utilize the rock 

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backfill used in dynamic compaction for the initial test loading and “B” borrow in the final test 

loading. However, no direct payment will be allowed for the construction and removal of the test 

load and payment of these materials will be allowed only if these materials are used in the final 

completed roadway structure. 

PROTECTION OF PERSONS AND ADJACENT PROPERTIES 

A detailed safety program will be required to verify that job site personnel, off-site personnel and 

adjacent properties are protected. The safety program will require the contractor to perform, by a 

specialist approved by the Department who is qualified in seismic testing, measurements before, 

during and after the Dynamic Compaction. Seismic readings shall be obtained at locations 

determined by the Engineer. 

The safety program will address the proposed barriers, fences, etc., to be utilized in protecting 

off-site personnel. The safety program shall include a complete discussion of the special 

programs utilized to assure the crane safety. This will include, but not be limited to, the before 

modification to all parts of the crane affected by Dynamic Compaction, i.e. the crane boom, 

cables, drums, brakes, clutch, outriggers, etc., and the daily, weekly, and monthly maintenance 

program. A detailed safety program shall be submitted prior to the commencement of work 

outlining how the Contractor intends to protect his personnel, other personnel on the site, the 

safety of the adjacent structures, and the maintenance program required to assure the safe 

operation of the crane. The Contractor will be required to photograph all immediately adjacent 

structures before starting this work. All items needed for the safety program shall not be paid for 

separately, but shall be included in the cost of “Dynamic Compaction.” If the seismic specialist 

determines that unacceptable conditions occur, the Engineer shall be notified immediately and all 

compaction work shall cease until such time as the Contractor takes all necessary precautions 

needed to meet safety requirements. Such precautions shall be approved by the Engineer. 

RECORDS 

The contractor shall keep adequate records of the construction operations, i.e. the locations where 

drops have been made, number of drops per location, depth of penetration of the tamper, the pass 

number, etc., and submit these daily. Any circumstances affecting the work or intended 

improvement as a result of dynamic compaction should be brought to the attention of the 

engineer. 

BASIS OF PAYMENT 

The work to be paid for shall be the measured quantity in square yards of “dynamic deep 

,compaction.” If additional work is necessary based upon the borings or load tests, the work will 

be paid for at the pro-rated energy rate per square meter basis as per bid item. 

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APPENDIX C 

TYPICAL PERFORMANCE SPECIFICATION 

DESCRIPTION OF WORK 

Extent of Dynamic Compaction is indicated on drawings. 

The work shall consist of densifying the landfill by Dynamic Compaction (DC) to the area and 

extent shown on the drawings. The work shall be performed by a Specialty Contractor who can 

meet the requirements as outlined below. The Specialty Contractor shall furnish all supervision, 

equipment (including cranes), labor and materials necessary or incidental to the completion of 

the DC for this project. 

DEFINITION 

DC is a process whereby a large tamper is raised above the ground and allowed to fall from 

heights up to 36 m impacting with high compactive energy. The depth of compaction 

improvement depends upon the mass of the tamper and the height of the fall. 

SPECIALTY CONTRACTOR’S QUALIFICATIONS 

The Specialty Contractor shall be regularly engaged in DC work and shall document that they 

have performed a minimum of the following work in the United States: 

Provided all supervision, labor, material and equipment to successfully densify by 

Dynamic Compaction, 25 separate projects utilizing energy inputs of between 13.6 and 

27 Mg dropped from heights of over 24 m to improve soil for both liquefaction, landfill 

densification and bearing capacity using free fall, and single and double lines. Five of the 

documented, successfully completed projects shall be similar to this project in type of 

landfill to be densified, depth to be densified, energy input required and the type of 

modified cranes to be used. 

Contractor shall submit specialty contractor’s qualifications to the Engineer for approval prior to 

the preconstruction conference. 

TESTING 

Construction fills, sanitary landfills, and mine spoils do not lend themselves to in-place testing 

such as standard penetration, cone penetrometer and dilatometer tests, due to their 

non-penetrability, non-homogeneity, etc. These strata can be best tested by before and after load 

tests. The contractor shall install a settlement plate 460 by 460 mm approximately 0.3 m beneath 

the existing ground with a 50 mm diameter steel pipe extending from the settlement plate to 4.5 

m above the existing ground. The ground shall then be filled to a height of 3 m with soil in a 

conical shape approximately 9 m in diameter at a location over the landfill selected by the 

engineer. Settlement shall be monitored for seven days. After Dynamic Compaction a similar 

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settlement test shall be conducted adjacent to the first test. If settlement has not been reduced 75 

percent, additional compactive effort shall be applied until a 75 percent reduction can be 

obtained. This testing shall be the responsibility of the contractor and performed under the 

observation of an independent registered Professional Engineer who shall submit six (6) copies 

of certified test reports to the Owner. 

INSPECTION SERVICES 

The City will furnish full time inspection; however, this shall not relieve the Specialty Contractor 

from keeping adequate records of the operations, including but not limited to, location number, 

number of drops, rate of penetration of weight, pass number, and submit these daily. 

PROTECTION OF PERSONS AND ADJACENT PROPERTIES 

A detailed safety program will be required to verify that job site personnel, off-site personnel and 

adjacent properties are protected. The safety program will require the contractor to perform by a 

qualified specialist a detailed monitoring and documentation before, during and after the DC of 

all structures within 90 m of the densification and structures noted on the plans outside this zone. 

The safety program will address the proposed barriers, fences, etc., to be utilized in protecting 

off-site personnel. The safety program shall include a complete discussion of the special 

programs utilized to assure the crane safety. This will include, but not be limited to, the before 

modification to all parts of the crane affected by Dynamic Compaction, i.e., the crane boom, 

cables, drums, brakes, clutch, outriggers, etc., and the daily, weekly and monthly maintenance 

program. 

PRODUCTS 

Provide equipment, materials and personnel required to achieve the results shown on the 

drawings. 

Crane shall be rigged so that at least 75 percent of potential energy is realized at the point of 

impact. 

EXECUTION 

The Specialty Contractor shall submit a detailed work plan showing impact layout, schedule, etc. 

Prior to beginning the DC, a mat of stone, crushed concrete, earth, sand or other suitable material 

shall be placed over the site where required to assure 1.2 m of cover over the top of the landfill. 

The DC shall then be performed in a minimum of four passes across the site. It may be necessary 

at the completion of each pass to fill the resulting craters with material from the mat. This shall 

be the responsibility of the Specialty Contractor. 

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