Performance of a Polyurethane Core Seal

Tested in a Hydrostatic Chamber

RI 9674

REPORT OF INVESTIGATIONS/2008

Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Report of Investigations 9674 

Performance of a Polyurethane Core Seal 

Tested in a Hydrostatic Chamber 

By Dennis R. Dolinar, Michael J. Sapko, and Samuel P. Harteis

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, PA 

May 2008
This document is in the public domain and may be freely copied 

or reprinted. 

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do not constitute NIOSH endorsement of the sponsoring organizations or their programs or 

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DHHS (NIOSH) Publication No. 2008–129 

May 2008 

SAFER • HEALTHIER • PEOPLETM
CONTENTS 

Page

Abstract............................................................................................................................................1 

Introduction......................................................................................................................................2 

Seal construction and testing ...........................................................................................................3 

 Seal construction......................................................................................................................3 

 Test setup .................................................................................................................................6 

 Hydrostatic chamber ........................................................................................................6 

 Instrumentation ................................................................................................................7 

 Test protocol ....................................................................................................................9 

 Test and test results..................................................................................................................9 

 Seal damage ...........................................................................................................................12 

Seal material properties..................................................................................................................16 

 Sampling and testing..............................................................................................................16 

 Lift composition.............................................................................................................16 

 Test procedures and measurements ...............................................................................16 

 Sampling ........................................................................................................................17 

 Seal material properties..........................................................................................................17 

 Change in foam characteristics and properties in the polyurethane and aggregate mixture.....21 

Analysis of seal performance.........................................................................................................23 

 Analysis of seal material properties.......................................................................................23 

 Seal failure .............................................................................................................................24 

 Shear failure and strength of the seal.....................................................................................29 

 Contribution of the concrete block walls to the strength and stiffness of the seal.................30 

 Hydrostatic versus explosion testing of polyurethane seals ..................................................33 

Conclusions....................................................................................................................................36 

References......................................................................................................................................37 

ILLUSTRATIONS 

 1. The dry-stacked concrete block walls comprising the forms for the composite poly- 

 urethane seal.........................................................................................................................4 

 2. Vertical cross-section across thickness of the polyurethane-aggregate seal.............................4 

 3. High-density polyurethane sprayed on roof, ribs, and back form wall.....................................5 

 4. Polyurethane foam applied on the aggregate at the start of a lift..............................................6 

 5. Cross-section of a typical lift used to form the seal..................................................................6 

 6. Schematic of the hydrostatic chamber ......................................................................................7 

 7. Locations of the LVDTs and pressure transducer used to monitor displacement of the 

 seal and pressure behind the seal during the test .................................................................8

 8. Pressure transducer and LVDTs used to monitor pressure and deflection during seal test......8 

 9. Water pressure and flow rate versus time during seal test......................................................10 

10. Displacement for LVDTs along the vertical center line of the seal and pressure recorded 

 versus time .........................................................................................................................10 

11. Displacement for LVDTs along the horizontal center line of the seal and pressure recorded 

 versus time .........................................................................................................................11 
CONTENTS—Continued 

Page 

12. Displacement for LVDTs just off the horizontal center line near the ends of the seal and 

 pressure recorded versus time............................................................................................11 

13. Damage observed in polyurethane core after removing block form.......................................13 

14. Closer view of damage observed along lower left side of polyurethane seal along top of 

 bottom row of blocks .........................................................................................................13 

15. Fracture and damage observed along lower left side of polyurethane seal along top of 

 bottom row of blocks .........................................................................................................14 

16. Internal damage along lower left side of polyurethane seal just above of the bottom row 

 of blocks.............................................................................................................................14 

17. High concentration of aggregate that is poorly bonded with polyurethane observed along 

 upper right side of seal.......................................................................................................15 

18. Separation along the interface between lifts observed in 6-in-diam core drilled 

 horizontally ........................................................................................................................15 

19. Typical stress-strain curves developed from uniaxial compressive tests on polyurethane- 

 aggregate mixture tested both perpendicular and parallel to the direction of foam rise.......19 

20. Typical stress-strain curves developed from uniaxial compressive tests on polyurethane- 

 only cap of lift....................................................................................................................20 

21. Pressure-displacement curve for LVDT 6 at the center of seal shows the behavior of 

 the seal ...............................................................................................................................25 

22. Displacements along horizontal center line of the seal show the change in shape of the 

 seal during the test for a given transducer pressure ...........................................................26 

23. Displacements measured on the left side, center, and right side of seal showing the 

 40-sec delay in the reaction of the left side when the seal failed.......................................27 

24. Water leakage observed from failure zone along upper right side of the seal........................27 

25. Displacements along vertical center line of the seal show the change in shape of the seal 

 during the test for a given transducer pressure ..................................................................31 

26. Stress-strain curves from uniaxial compressive tests on samples from explosion-tested 

 seal from the 1990s and the seal tested in the hydrostatic chamber ..................................35 

TABLES 

 1. Results of uniaxial compressive tests on polyurethane-aggregate mixture samples ..............18 

 2. Results of uniaxial compressive tests on the polyurethane lift caps from the poly- 

 urethane seal.......................................................................................................................19 

 3. Summary of results of uniaxial compressive tests on samples from the polyurethane seal ......20 

 4. Densities of soft, rigid, and combined foam samples taken from the polyurethane-aggregate 

 mixture ...............................................................................................................................22 

 5. Calculated deflections of the seal based on assumed end support and the calculated 

 elastic modulus used to match the actual seal deflections at the center of the seal ...........32 

 6. Summary of explosion tests on polyurethane-aggregate seals conducted in the 1990s .......33 

 7. Average strengths of polyurethane-aggregate mixtures tested perpendicular to foam rise 

 in uniaxial compression from an explosion-tested seal from the 1990s and the seal 

 tested in the hydrostatic chamber.......................................................................................35 
ACRONYMS AND ABBREVIATIONS USED IN THIS REPORT 

 AASHTO American Association of State Highway and Transportation Officials 

 CFR Code of Federal Regulations 

 LLEM Lake Lynn Experimental Mine 

 LVDT linear variable displacement transducer 

 MSHA Mine Safety and Health Administration 

 NIOSH National Institute for Occupational Safety and Health 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

 ft foot 

 ft2

 square foot 

 ft3/min cubic foot per minute 

 g gram 

 gal gallon 

 gpm gallon per minute 

 hp horsepower 

 Hz hertz 

 in inch 

 in3 cubic inch 

 lb pound 

 lb/ft3

 pound per cubic foot 

 min minute 

 psi pound-force per square inch 

 sec second 

 V/in volt per inch 

PERFORMANCE OF A POLYURETHANE CORE SEAL 

TESTED IN A HYDROSTATIC CHAMBER 

By Dennis R. Dolinar,1 Michael J. Sapko,2 and Samuel P. Harteis3

ABSTRACT 

 The National Institute for Occupational Safety and Health’s Pittsburgh Research Laboratory conducted a full-scale test of a composite polyurethane-aggregate seal in a hydrostatic 

chamber at the Lake Lynn Experimental Mine. This composite seal consisted of a polyurethane 

and limestone aggregate core that was formed between two concrete block walls. In the hydrostatic chamber test, water is used to pressurize the seal. 

 During the test, the maximum average water pressure the seal was able to withstand was 

18.9 psi. Further, the seal began to yield at an average water pressure of only 17 psi. At the time 

of the test, the minimum seal design requirement was 20 psi under 30 CFR4

 75.335 (2005). 

Therefore, based on this test, the seal did not meet the regulatory minimum design requirement. 

 Uniaxial compressive tests conducted on samples from the polyurethane-aggregate core 

revealed that both the compressive strength and the elastic modulus of the material were much 

less than that for polyurethane only with a density of 10.0–12.0 lb/ft3

. The significant reduction 

in the strength and stiffness of the polyurethane-aggregate mixture results from the formation of 

a soft polyurethane foam around each piece of aggregate. The formation of the soft foam seems 

to be caused by residual moisture from the aggregate. The moisture also caused the density of the 

polyurethane in the polyurethane-aggregate mixture to be less than the design requirements. The 

lower density of the polyurethane in the mixture, along with the formation of zones of pure polyurethane only, resulted in the overall seal density being less than the design requirements. During 

seal construction, steps were taken to control the moisture, but were not successful. 

 An analysis of the seal failure showed that the calculated shear stresses at failure and the 

estimated shear strength of the polyurethane-aggregate mixture are fairly close. This suggests 

that the strength properties of the polyurethane-aggregate mixture along with a weak zone of 

poorly bonded aggregate controlled the maximum pressure the seal could withstand. 

 At the time of the seal testing, the minimum design requirement was 20 psi, as specified 

at 30 CFR 75.335 (2005). New emergency temporary standards for seal designs were promulgated on May 22, 2007, with a minimum requirement of 50 psi under specified conditions. 

However, it will be shown that the evaluation of the polyurethane seal performance as designed 

to meet the older 20-psi criterion can be relevant to the design requirements under the new 

standards. 

 

1

Lead research mining engineer. 

2

Principal research physical scientist (retired). 

3

Research mining engineer. 

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA. 

4

Code of Federal Regulations. See CFR in references. 
2

INTRODUCTION 

 U.S. mine ventilation plans require seals to protect against explosions. They are also used 

extensively in mining to isolate abandoned areas, fire zones, or areas susceptible to spontaneous 

combustion. To effectively isolate areas within a mine, a seal should control the methane and air 

exchange between the sealed and open areas so as to prevent toxic and/or flammable gases from 

entering the active workings and oxygen from entering the sealed area. A seal must also be capable of preventing an explosion from propagating into or out of the sealed area. Over the years, 

more than 20,000 seals have been erected in U.S. underground coal mines. 

 The composite polyurethane-aggregate seal is considered to be an alternative seal design. 

At the time of the test, alternative seal designs were required to withstand a static 20-psi minimum pressure [30 CFR 75 (2005)]. New emergency temporary standards for seal designs were 

promulgated on May 22, 2007. Under these standards, the seal strength requirement is 50-psi 

overpressure when the atmosphere in the sealed area is monitored and kept inert and 120-psi 

overpressure when the atmosphere is not monitored and not kept inert [72 Fed. Reg.5 28795 

(2007)]. However, it will be shown that the evaluation of the polyurethane-aggregate seal 

performance as designed to meet the older 20-psi criterion can be relevant to the design requirements under the new standards. Based on the test results presented in this report, the physical 

properties of the seal are much less than what is expected from the density of polyurethane used 

to build the seals. It also proved very difficult to maintain quality control of the strength and 

elastic properties of the seal material even under the controlled conditions at the Lake Lynn 

Experimental Mine (LLEM). Further, the results of this investigation are also relevant to the 

seals designed and installed under the 20-psi criterion and to other seals using the same materials 

and construction techniques. 

 Under the old standard, before any new seal design type could be deemed suitable by the 

Mine Safety and Health Administration (MSHA) for use in underground coal mines, the seal 

design was generally required to undergo full-scale performance testing at the LLEM [Triebsch 

and Sapko 1990]. Full-scale explosion testing is very elaborate, time-consuming, costly, and 

often conflicts with other high-priority research conducted in the LLEM. Further, the explosion 

method only determined if the seal met the 20-psi criterion; it did not determine the ultimate 

strength of the seal. With the knowledge of the ultimate strength of the seal, a safety factor for 

the seal design can be established. Therefore, the National Institute for Occupational Safety and 

Health (NIOSH) constructed two test chambers within the LLEM where a hydrostatic pressure 

could be applied to the seals using water. The dimensions of the chambers were 30 ft wide by 

15 ft high, and 20 ft wide by 8 ft high. To evaluate the appropriateness of using the hydrostatic 

chamber to develop the static pressure, a series of tests of various alternate seal designs was 

subsequently conducted. Data from these chamber studies were used to compare the strength 

characteristics with the same seal designs previously tested against full-scale 20-psi explosions 

within the LLEM. 

 As part of this evaluation program, a composite seal consisting of a polyurethane and 

limestone aggregate core formed between two concrete block walls was constructed and then 

tested in the smaller hydrostatic chamber. The design of the seal was based on the 20-psi static 

pressure requirement. For this seal, the main design requirements are (1) a minimum density of 

the cured polyurethane of 10 lb/ft3 and an overall core density of 35 lb/ft3

, and (2) a minimum 

 

5

Federal Register. See Fed. Reg. in references. 
3

core thickness of 16 in for seals under 8 ft high with 1 in of thickness added for each foot of 

height over 8 ft. There is no strength requirement for this seal. 

 Further, physical property testing of samples from the seal was also conducted. Because 

a limestone aggregate is used in combination with the polyurethane in the seal construction, the 

seal consists of sections with a polyurethane-aggregate mixture and sections with only polyurethane. A sampling program was designed to obtain properties of both types of material, as 

well as to evaluate differences in properties as a result of material anisotropy based on the direction of foam rise.6

 Physical properties included the density and uniaxial compressive strength of 

the polyurethane and polyurethane-aggregate mixture. This report details the test results of the 

composite polyurethane-aggregate seal and provides an analysis of those results in terms of the 

seal’s structural behavior. 

SEAL CONSTRUCTION AND TESTING 

Seal Construction 

 The constructed seal was 20.4 ft wide by 9.0 ft high with a total thickness, including the 

forms, of 30 in. Because the entry was higher than 8 ft, the thickness of the polyurethane seal 

portion was designed by the manufacturer to be 18 in, i.e., 2 in thicker than the standard seal. 

However, because of some slight bulging of the forms, the actual core seal thickness was 

19–20 inches in the middle portion of the seal. 

 The standard construction practices were followed, and the recommended materials were 

used for installing this type of polyurethane-aggregate seal. The composite seal tested consisted 

of two dry-stacked concrete block walls spaced 18 in apart with a polyurethane and limestone 

aggregate core (Figures 1–2). The concrete blocks used to construct the walls were nominally 

16 in by 8 in by 6 in. The back wall was constructed first. After the last block in each row was 

placed, a wedge was driven between the block and rib to tighten the blocks in place. Wedges 

were also used between the top row of blocks and the roof. A manhole was left open in the upper 

right7

 corner of the back wall to allow the back side to be plastered with an MSHA-approved 

sealant. Once plastering was complete, concrete blocks with MSHA-approved sealant applied to 

the back side were placed in the manhole until the opening was filled. To complete the back wall 

at the manhole, boards were used between the top of the blocks and the roof. An MSHAapproved sealant was then used to fill the gap at the roof and ribs. The front wall was constructed 

similar to the back wall for the first six courses of block. From this point, the blocks were 

pyramided to the roof so that two blocks were in contact with the roof and wedged in place. 

An MSHA-approved sealant was then applied to the front side of the outside wall. 

 

 6Upon mixing, rigid polyurethane foam will expand as a result of a blowing agent. The direction of expansion is 

termed the “rise direction.” In this application, the rise direction is generally vertical, except at the top of a lift. 

 7

The right or left side of the seal is referenced from the viewpoint of standing outby and facing the seal. 
4

 Figure 1.—The dry-stacked concrete block walls comprising the forms 

for the composite polyurethane seal. 

Figure 2.—Vertical cross-section across thickness of the polyurethane-aggregate seal. 

 Before constructing the inner core, high-density polyurethane was sprayed on the ribs, 

floor, and around the perimeter of the block walls inside the core area. This was, in part, to provide better adhesion of the seal to the surrounding rock mass and block walls (Figure 3). The 

improved adhesion, however, only occurs when the polyurethane is still tacky and is limited to 

the first three lifts. The high-density polyurethane also provided a barrier to protect the lowdensity foam from moisture or water. For this application, higher-strength polyurethane foam 

with a density of 70 lb/ft3 was used. This material was also sprayed along the upper edge of the 

back wall to improve the seal along the roof line. The high-density polyurethane also fixed the 

bottom layer of blocks to the floor and was used to saturate the first layer of aggregate to create 

a good bond with the floor. 

9‘

18“

34”

PolyurethaneAggregate Core

Concrete Block 

Forms

9‘

18“

34”

PolyurethaneAggregate Core

Concrete Block 

Forms
5

Figure 3.—High-density (70 lb/ft3) polyurethane sprayed on roof, ribs, and back form wall. 

 Once the two walls were prepared, the seal core was constructed by placing a limestone 

aggregate in between the forms and then pumping polyurethane over the aggregate. This process 

was conducted in a series of lifts. The limestone aggregate is used to add mass to the seal and to 

reduce the cost. The limestone was sized and graded as AASHTO size number 57 (aggregate size 

gradation ranges from 0.19 to 1.0 in), which was dried and delivered in plastic lined bags to 

minimize the possible effects of moisture. The two-component foam was poured into the seal 

and formulated to have an in-place density of at least 10 lb/ft3

. With the addition of the aggregate, the average seal core density was designed to be a minimum of 35 lb/ft3

. Heating blankets 

were used to keep the polyurethane at the proper temperature for installation. 

 The process of developing a lift began by placing a 4-in layer of aggregate at the bottom 

of each lift, then a polyurethane mixture was poured on top of the aggregate (Figure 4). After the 

aggregate was placed in a layer, the layer was saturated with polyurethane. The foam would then 

rise or expand as a result of the blowing agent in the vertical direction. As the polyurethane 

expanded, the aggregate was carried with and dispersed within the foam. However, depending on 

the degree of saturation, the polyurethane would expand beyond the aggregate, and a zone of 

only polyurethane would form a cap at the top of the lift. Once the polyurethane set, a typical lift 

would be 12–14 in high and consist of an 8- to 12-in thickness of the polyurethane-aggregate 

mixture and a cap of polyurethane with a thickness of 1–4 in. The cap would generally be 1–2 in 

thick in the middle of the lift and 2–4 in at the edges. Steps were taken to minimize the thickness 

of the pure polyurethane cap. Figure 5 shows a typical lift. Eight lifts were used in the seal 

construction. 
6

Figure 4.—Polyurethane foam applied on the aggregate at the start of a lift. 

Figure 5.—Cross-section of a typical lift used to form the seal. 

Test Setup 

Hydrostatic Chamber 

 Figure 6 shows a diagram of the hydrostatic chamber. The chamber used in this test averages 20 ft wide by 8.0–9.5 ft high by 10 ft deep [Sapko et al. 1999a,b; Sapko and Weiss 2001; 

Sapko et al. 2003]. The seal was constructed at the front portion of the chamber. A steel frame 

structure built inside the chamber allows for simulated hitching of the seals. However, for this 

test the polyurethane seal was not in contact with the steel frame and therefore was not considered hitched. The typical design and installation of these polyurethane-aggregate seals does 

not require hitching into the surrounding rock. 

PolyurethaneAggregate

mixture

Polyurethane cap

8-12”

2-4”

2”

19”

1-2”

PolyurethaneAggregate

mixture

Polyurethane cap

8-12”

2-4”

2”

19”

1-2”

PolyurethaneAggregate

mixture

Polyurethane cap

8-12”

2-4”

2”

19”

1-2”
7

Figure 6.—Schematic of the hydrostatic chamber. 

 The chamber was connected via remote-controlled air valves to two diesel-driven air 

compressors that provide 1,000 ft3/min of air. The air compressors are used to conduct the preand postexplosion leakage measurements. The chamber is also connected to a 102-hp diesel 

water pump capable of 1,000 gpm at 90 psi at the chamber inlet with water fed from an underground 130,000-gal reservoir. The pump provided the static horizontal pressure to the seal. 

Instrumentation 

 Eleven linear variable displacement transducers (LVDTs) were used to monitor the 

displacement of the seal. Figure 7 shows the locations the LVDTs. These spring-loaded LVDTs 

capable of measuring up to 6 in of displacement were installed on the outside of the seal by 

attaching them to 2 by 6’s that were set between the roof and floor (Figure 8). The LVDTs measured this displacement by generating an output signal of approximately 1.68 V/in. A strain gauge 

pressure transducer (1,000 Hz) with an internal pressure range of 0–100 psi measured the water 

pressure behind the seal. This transducer was connected to a pipe placed through the seal and 

located 6.44 ft above the floor (Figures 7–8). Data from the pressure transducer and LVDTs 

were recorded at 2,000 samples per second per channel with a WinDaq PC-based data acquisition system. 

Pump

Test Seal

Water Pressure

Water Line

To Reservoir

Water

Hitching Frame

Pump

Test Seal

Water Pressure

Water Line

To Reservoir

Water

Pump

Test Seal

Water Pressure

Water Line

To Reservoir

Water

Pump

Test Seal

Water Pressure

Water Line

To Reservoir

Water

Hitching Frame
8

 

 Figure 7.—Locations of the LVDTs and pressure transducer used to 

monitor displacement of the seal and pressure behind the seal during the 

test. Nominal dimensions are given for the seal and LVDT locations. 

 Figure 8.—Pressure transducer and LVDTs used to monitor pressure and deflection during seal test. 

Schematic of LVDT is shown on the left. 

+

2’

1’

~20’

~8’

1’

2’ 2.5’

2.5’

4’

1’ 1’

2 X 6’s

2 X 6

2 X 6 6.44’

3

1 2

4

5

6

7

8

9 11

10

LVDT Locations

+ Pressure Transducer

+

2’

1’

~20’

~8’

1’

2’ 2.5’

2.5’

4’

1’ 1’

2 X 6’s

2 X 6

2 X 6 6.44’

+

2’

1’

~20’

~8’

1’

2’ 2.5’

2.5’

4’

1’ 1’

2 X 6’s

2 X 6

2 X 6 6.44’

2’

1’

~20’

~8’

1’

2’ 2.5’

2.5’

4’

1’ 1’

2 X 6’s

2 X 6

2 X 6 6.44’

3

1 2

4

5

6

7

8

9 11

10

LVDT Locations

+ Pressure Transducer

3

1 2

4

5

6

7

8

9 11

10

3

1 2

4

5

6

7

8

9 11

10

3

1 2

4

5

6

7

8

9 11

10

LVDT Locations

+ Pressure Transducer

2 x 6 wedged at roof Attached to 

2 x 6 with

conduit clamps/

screws

LVDT

LVDT piston &

Condom cover

Seal face

Top view of LVDT and 2X6 mount

Pressure Transducer

LVDT

2 x 6 wedged at roof Attached to 

2 x 6 with

conduit clamps/

screws

LVDT

LVDT piston &

Condom cover

Seal face

Top view of LVDT and 2X6 mount

2 x 6 wedged at roof Attached to 

2 x 6 with

conduit clamps/

screws

LVDT

LVDT piston &

Condom cover

Seal face

Top view of LVDT and 2X6 mount

2 x 6 wedged at roof Attached to 

2 x 6 with

conduit clamps/

screws

LVDT

LVDT piston &

Condom cover

Seal face

Top view of LVDT and 2X6 mount

2 x 6 wedged at roof Attached to 

2 x 6 with

conduit clamps/

screws

LVDT

LVDT piston &

Condom cover

Seal face

Top view of LVDT and 2X6 mount

Pressure Transducer

LVDT
9

Test Protocol

 Before hydrostatically loading the seal, an air leakage test is conducted to confirm that 

the seal structure met the MSHA-established guidelines. For pressure differentials up to 0.036 

psi, air leakage through the seal should not exceed 100 ft3

/min; for pressure differentials over 

0.11 psi, air leakage should not exceed 250 ft3

/min. The air leakage test is conducted by pressuring the hydrostatic chamber with compressed air to the guideline pressures. The rate of pressure 

decay with time is then monitored. The air leakage is calculated from the volume of the chamber 

and the rate of pressure decay [Sapko et al. 2003]. The results from the air leakage test on this 

seal prior to the hydrostatic test were within the guidelines. 

 Following the leakage test, the seal would be hydrostatically loaded to 20 psi at the pressure transducer (6.44 ft above the floor). When this pressure was achieved, pumping would be 

stopped and the water drained from behind the seal. The seal would then be subjected to a second 

air leakage test to determine if the seal could still function for ventilation purposes. If the seal 

passed the second air leakage test, the seal would be hydrostatically loaded to a pressure where 

the seal failed structurally. 

 Hydrostatically loading the seal creates a differential pressure between the top and 

bottom of the seal. For a 20-psi pressure at the transducer, the pressure is 18.9 psi at the top of 

the seal and 22.9 psi at the bottom. The average pressure across the seal is 20.9 psi. 

Test and Test Results 

 Figure 9 shows the flow rates and water pressures generated during the test. In the initial 

phase, the pumping rate was 250–350 gpm. Once the chamber behind the seal was filled, the 

remaining portion of the test then took 13–14 min to complete. 

 Figure 10 shows the pressure and the displacements with respect to time for the five 

LVDTs placed along the vertical center line of the seal. The responses of the LVDTs along the 

horizontal center line are shown in Figure 11, and the responses of the two LVDTs just off the 

horizontal center line are shown in Figure 12. During the initial pressurization to about 11 psi, 

the pressure-time curve showed a stairstep behavior. This behavior resulted from adjustments to 

the rate of pressurization using a pressure relief value at the pump. At just over 16 psi at about 

3,100 sec into the test, significant leakage began to occur through the seal. This resulted in several sudden pressure drops just before reaching the maximum pressure. A maximum pressure of 

18 psi at about 3,250 sec into the test was achieved at the transducer. Just past the peak pressure, 

a large amount of displacement occurred at all LVDT locations over a short period of time 

(Figures 10–12). After the maximum pressure was reached and the pressure dropped to about 

11 psi because of leakage, the flow rate was increased to 800 gpm to determine if a higher pressure could be achieved. Although the pressure did increase to 14 psi with the higher pumping 

rate, there was an accompanying rapid increase in the seal deflection. This resulted in severe 

leakage from the seal and a rapid pressure drop to about 8 psi. The maximum seal deflection of 

at least 6 in at the center of the seal was recorded at this time. However, several of the LVDTs 

had reached their maximum stroke limit of 6 in. The pumping was then halted and the water was 

allowed to drain from behind the seal at 3,650 sec into the test. As the pressure dropped, there 

was some rebound of the seal with up to 2 in of displacement recovered near the center. 

However, there was still nearly 4 in of displacement that was not recovered by the end of 

monitoring. 
10

0

100

200

300

400

500

600

700

800

900

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Time, sec

Flow, gpm

0

2

4

6

8

10

12

14

16

18

20

Pressure, psig

Pressure

Flow Rate

Figure 9.—Water pressure and flow rate versus time during seal test. 

-1

0

1

2

3

4

5

6

7

2500 2700 2900 3100 3300 3500 3700 3900 4100 4300

Time, sec

Displacement, in

0

2

4

6

8

10

12

14

16

18

20

Pressure, psi

Pressure

LVDT 8 

LVDT 6

LVDT 7

LVDT 5

LVDT 4

 Figure 10.—Displacement for LVDTs along the vertical center line of the seal and pressure 

recorded versus time. 
11

0

1

2

3

4

5

6

7

2500 2700 2900 3100 3300 3500 3700 3900 4100 4300

Time, sec

Displacement, in

0

2

4

6

8

10

12

14

16

18

20

Pressure, psi

LVDT 6

LVDT 2

LVDT 9

LVDT 1

LVDT 11

Pressure

 Figure 11.—Displacement for LVDTs along the horizontal center line of the seal and 

pressure recorded versus time. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

2500 2700 2900 3100 3300 3500 3700 3900 4100 4300

Time, sec

Displacment, in 

0

2

4

6

8

10

12

14

16

18

20

Pressure,psi

Pressure

LVDT 3

LVDT 10

 Figure 12.—Displacement for LVDTs just off the horizontal center line near the ends of 

the seal and pressure recorded versus time. 
12

Seal Damage 

 To evaluate the damage done to the polyurethane seal core, the front stack wall was 

removed. Figure 13 shows the seal after removing the block wall, with the fractures and open 

joints indicated. Fractures developed at or near the lift joints at the lower left and upper right side 

of the seal. There is also a fracture cutting across the lifts along the lower left side of the seal. 

The joints along the lifts near the center portion of the seal also opened. Figure 14 shows the 

major fracture or open joint that formed parallel to the lifts just above the bottom row of blocks 

along the interface of lifts 1 and 2. Further, the area at and below the fracture parallel to the lift 

line was displaced a few inches horizontally over the top of the lowest row of block. Figure 15 

shows the major fracture that developed across lifts 1 and 2 on the lower left side of the seal. 

This fracture was open along the front side of the seal core. A portion of the polyurethane was 

excavated so that internal damage to the seal and the back block wall could be examined. Internal damage to the seal near the floor is shown in Figure 16. The damage includes major nearvertical fractures in the lowest lift. This area is just to the right of the external fracture shown in 

Figure 15. Essentially, the external damage is directly related to the development and movement 

along these internal fractures. Even though there were several inches of horizontal displacement 

of the polyurethane core over the top of the first row of blocks in this area, no horizontal shear 

surface or fracture was observed at this location. A small diagonal fracture was also noted in the 

upper left quadrant of the seal (Figure 13). 

 Another zone of damage and deformation occurred near the roof and extends across most 

of the right side of the seal (Figure 13). In exposed portions of this damaged zone, there is high 

concentration of aggregate with very little polyurethane to lift the aggregate and to act as a 

binder (Figure 17). The seal below the fractured zone was displaced outward. 

 Other fractures opened along the joined surfaces between the top and bottom of the lifts 

(Figure 13). This occurred in the lifts from the upper middle portion of the seal and below and 

involved the interfaces between lifts 1 to 5. Cores taken along some of these joint surfaces reveal 

that the joint is open only to the middle of the seal, thus being restricted to the tension side of the 

seal (Figure 18). 

 Also, two long narrow fissures parallel to the face of the seal were found in the polyurethane caps. These fissures were about 24–30 in long, 2–3 in high, and about 0.5 in wide. They 

seem to have been caused by thermal effects during foam rise or as the polyurethane was setting. 

Possibly, as a result of less heat being generated due to the reduced volume of the polyurethane 

in the polyurethane-aggregate mixture, no fissures were noted in the mixture. These voids, 

however, did not seem to be related to or contribute to the seal failure. 
13

 Figure 13.—Damage observed in polyurethane core after removing block form. Fractures in seal are 

indicated with solid lines; open joints along lifts are indicated with dashed lines. Circles denote where core 

was obtained for compressive tests. 

 

 Figure 14.—Closer view of damage observed along lower left side of polyurethane seal along top of bottom 

row of blocks. Seal has displaced about 2 in over the top of the blocks. The fracture or open joint is along the 

interface of lifts 1 and 2. 
14

 Figure 15.—Fracture and damage observed along lower left side of 

polyurethane seal along top of bottom row of blocks. 

 

 

 Figure 16.—Internal damage along lower left side of polyurethane seal 

just above of the bottom row of blocks. 
15

 Figure 17.—High concentration of aggregate that is poorly bonded with polyurethane 

observed along upper right side of seal. 

 

 Figure 18.—Separation along the interface between lifts observed in 6-in-diam 

core drilled horizontally. The right side of the cores is the front side of the seal. 
16

SEAL MATERIAL PROPERTIES 

 To evaluate the material properties of the seal, uniaxial compressive tests were conducted 

on cylindrical samples taken from the seal. From these tests, compressive strengths could be 

determined and an elastic modulus calculated. The sample densities were also measured. These 

properties were evaluated to determine if the materials in the seal, installed under field conditions at the LLEM, met the seal construction requirements. Further, the results of the tests could 

be used to analyze the seal performance. However, the targeted design requirements for this type 

of seal were based in large part on the density of the seal and not on the strength or stiffness 

properties. 

Sampling and Testing 

Lift Composition 

 Because of the addition of the aggregate in the seal, there are two zones that develop in 

a lift: a zone consisting of a mixture of polyurethane and aggregate, and a polyurethane cap 

(Figure 5). By volume, about 80% of the lift is composed of the polyurethane and aggregate mixture where the polyurethane cap is 1–4 in thick. However, the reported results from previous 

tests on the polyurethane and aggregate mixture prepared under laboratory conditions indicated 

there was no apparent reduction in the elastic modulus when the aggregate was added [Sawyer 

1999]. In the present testing program, samples from a seal constructed under field conditions 

would be tested. Therefore, the testing program was designed to evaluate the materials to determine physical properties developed under field conditions. 

Test Procedures and Measurements 

 Cylindrical specimens were tested in uniaxial compression to determine the strength of 

the seal materials. These specimens were obtained by either drilling the seal in situ or by drilling 

of larger sections of the seal that had been brought back to the NIOSH Pittsburgh Research 

Laboratory after the seal was dismantled. For the polyurethane and aggregate mixture, because 

the nominal aggregate size was 1 in, 6-in- diam samples were used. For the polyurethane cap, 

both 2- and 6-in-diam specimens were prepared. 

 The ends of the cylindrical specimens were cut parallel and perpendicular to the axis with 

a diamond saw to a length that resulted in a length-to-diameter ratio of one for the specimens 

[ASTM 2004b].8

 No other surface preparation was done to the ends of the specimens. A length 

ratio of one was used to accommodate the aggregate size of the polyurethane-aggregate mixture. 

 The uniaxial compressive tests were conducted on a servocontrolled load frame with the 

tests being conducted in displacement control. During the tests, both the load and displacement 

were recorded. To measure the displacement, the main activator LVDT for the load frame was 

used. The specimens were strained 30%–50% over a 10–15 min period. This timeframe is similar 

to that of the seal test. 

 

 8According to ASTM standards for testing rigid cellular plastics, the length-to-diameter ratio of the samples 

should not exceed one. The only specifications for the ends of the samples are that they are parallel and perpendicular to the sides. 
17

 From the uniaxial compressive tests, the compressive strength, elastic modulus, and 

proportional limit were determined. The compressive strength is the stress at the yield point or, 

where there is no distinct yield point, the stress at 10% strain [ASTM 2004b]. The elastic modulus is determined from the near linear portion of the stress-strain curve below the proportional 

limit. 

 The density of the specimens was determined by two methods. For regularly shaped 

specimens, the weight and dimensions were measured directly according to ASTM standards 

[ASTM 2004a]. For irregularly shaped specimens, a water displacement test was used [ASTM 

2004c]. 

Sampling 

 Specimens for the compressive tests were obtained by two methods: in situ drilling of the 

seal and laboratory drilling of large sections removed from the seal. In situ 6-in-diam cores were 

obtained by drilling horizontally into the seal. Figure 13 shows the location of the coreholes with 

respect to the lifts. Because the polyurethane cap and polyurethane-aggregate mixture interface 

was intersected in a significant number of the cores, only cores from four of the holes (L2 (lift 4), 

L3 (lift 5), M1 (lift 6), R1 (lift 5)) were prepared and tested (Figure 18). However, the length of 

the cores allowed for two 6-in-long samples to be tested from these four holes. The resulting 

samples were tested in the direction perpendicular to foam rise and provided properties of the 

polyurethane-aggregate mixture. 

 Several large sections of the seal were brought back and cored in the laboratory. The 

sections were either one or two full lifts in height. Six-inch-diameter cores orientated both 

perpendicular and parallel to foam rise were drilled. For the polyurethane and aggregate mixture, 

samples were obtained from lifts 2 and 5. For the polyurethane cap, 6-in-diam samples were also 

obtained from lifts 4 and 5. However, these samples had a length of only about 2 in because of 

the cap thickness. Further, 2-in-diam cores from the polyurethane cap orientated both perpendicular and parallel to foam rise were drilled from lift 4. 

Seal Material Properties 

 Table 1 shows the results of the uniaxial compressive tests on the cores of the 

polyurethane-aggregate mixture. Test results include samples from lifts 2, 4, 5, and 6. Figure 19 

shows typical stress-strain curves for the mixture for samples tested both perpendicular and 

parallel to foam rise. Table 2 shows the strength and deformation properties for the polyurethane 

cap with samples from lifts 4 and 5. Typical stress-strain curves for samples from the polyurethane cap tested perpendicular and parallel to foam rise are shown in Figure 20. A summary 

of the results is given in Table 3. 

 For the polyurethane and aggregate mixture, the average density is 33.7 lb/ft3

. Parallel 

to foam rise, the average compressive strength is 41 psi with an elastic modulus of 1,187 psi. 

Perpendicular to foam rise, the average compressive strength is 32 psi with an elastic modulus of 

635 psi. 

 For the polyurethane cap, the average density is 11.0 lb/ft3

. The average compressive 

strength is 210 psi parallel to foam rise and 240 psi perpendicular to foam rise. The average 

elastic modulus is 5,810 psi parallel to foam rise and 7,753 psi perpendicular to foam rise. 
18

Table 1.—Results of uniaxial compressive tests on polyurethane-aggregate mixture samples 

Sample Lift Density, 

lb/ft3 Diameter, 

in 

Length, 

in 

Compressive 

strength, 

psi 

Strain,1 

% 

Proportional 

limit, 

psi 

Elastic 

modulus, 

psi 

PARALLEL TO FOAM RISE 

L2VC 2 28.2 5.675 6.13 26 10 13 550 

L2VA 2 27.5 5.675 6.13 35 10 18 1,012 

L2VB 2 32.5 5.675 6.13 39 10 22 1,215 

Average — 29.4 5.675 6.13 33 10 18 926 

PERPENDICULAR TO FOAM RISE 

L2HA 2 28.5 5.675 6.13 24 10 9 452 

L2HB 2 28.1 5.675 5.92 23 10 9 410 

Average — 28.3 5.675 6.03 24 10 9 431 

PERPENDICULAR TO FOAM RISE 

L2 A 4 42.8 5.875 6.125 31 10 7 715 

L2 B 4 35.2 5.875 6 41 10 16 906 

Average — 39.0 5.875 6.06 36 10 12 811 

PARALLEL TO FOAM RISE 

L5VA 5 32.2 5.675 6.25 42 10 20 1,042 

L5VB 5 31.7 5.675 6.13 51 10 29 1,954 

L5VC 5 50.3 5.675 6.13 42 10 21 1,181 

L5VD 5 34.7 5.675 6.38 55 10 24 1,355 

Average — 37.2 5.675 6.22 48 10 24 1,383 

PERPENDICULAR TO FOAM RISE 

L5HA1 5 31.2 5.675 6.19 32 10 12 743 

L5HA2 5 34.4 5.675 5.92 37 10 16 861 

L5HB1 5 28.2 5.675 6.06 36 10 12 559 

L5HB2 5 30.3 5.675 6.06 28 10 15 583 

L3 A 5 37.9 5.875 6.063 40 10 17 789 

L3 B 5 40.1 5.875 6 36 10 12 655 

Average — 33.7 5.742 6.05 35 10 14 698 

PERPENDICULAR TO FOAM RISE 

M1 A 6 37.7 5.875 6.063 20 10 8 246 

R1 A 6 33.4 5.5 6.125 26 10 12 432 

R1 B 6 30.5 5.5 6 38 10 15 900 

Average — 33.8 5.625 6.063 28 10 12 526 

1

Strain at the compressive strength. 
19

-20

0

20

40

60

80

100

0 5 10 15 20 25 30 35

Strain,pct

Stress, psi

Parallel

Perpendicular

 Figure 19.—Typical stress-strain curves developed from uniaxial compressive tests on 

polyurethane-aggregate mixture tested both perpendicular and parallel to the direction of 

foam rise. 

Table 2.—Results of uniaxial compressive tests on the polyurethane lift caps from the polyurethane seal 

Sample Lift Density, 

lb/ft3 Length, 

in 

Diameter, 

in 

Compressive 

strength, 

psi 

Strain,1 

% 

Proportional 

limit, 

psi 

Elastic 

modulus, 

psi 

PARALLEL TO FOAM RISE 

L4VC1 4 11.4 1.98 1.995 215 7 140 7,372 

L4VC2 4 11.2 1.99 1.995 210 6 160 7,495 

L4VC3 4 11.2 1.98 1.995 220 6 170 7,577 

L4VC4 4 11.2 1.96 1.995 210 10 150 5,081 

L4VC5 4 11.2 2.03 1.995 200 6 155 6,607 

Average — 11.2 1.99 1.995 211 7 155 6,826 

L4VCA 4 10.6 2.393 5.675 205 10 160 5,509 

L4VCB 4 10.77 1.793 5.675 215 10 175 4,792 

L4VCC 4 11.02 2.005 5.675 200 10 160 5,018 

L4VCD 4 11.06 1.522 5.675 210 10 170 4,614 

L5VCA 5 11.04 2.069 5.675 210 10 160 4,731 

L5VCD 5 11.08 1.955 5.675 215 10 155 5,112 

Average — 10.93 1.956 5.675 209 10 163 4,963 

PERPENDICULAR TO FOAM RISE 

L4HC1 4 11.2 2.156 1.995 215 10 90 5,390 

L4HC2 4 10.8 1.99 1.995 260 5 220 8,584 

L4HC3 4 11.3 2.2 1.995 250 10 210 8,020 

L4HC4 4 10.5 2.07 1.995 215 5 180 7,746 

L4HC5 4 10.9 1.99 1.995 260 5 215 9,026 

Average — 10.9 2.08 1.995 240 7 183 7,753 

1

Strain at the compressive strength. 
20

-50

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60

Strain, pct

Stress, psi

Perpendicular

Parallel

 Figure 20.—Typical stress-strain curves developed from uniaxial compressive tests on polyurethane-only 

cap of lift. 

Table 3.—Summary of results of uniaxial compressive tests on samples from the polyurethane seal 

Direction 

of foam 

rise 

No. of 

samples 

Density, 

lb/ft3

SD, 

lb/ft3

Compressive 

strength, 

psi 

SD, 

psi 

Strain,1 

% 

Proportional 

limit, 

psi 

SD, 

psi 

Elastic 

modulus, 

psi 

SD, 

psi 

POLYURETHANE-AGGREGATE MIXTURE 

Perpendicular 13 33.7 4.8 31.7 6.9 10 12.3 3.3 635 208 

Parallel 7 33.8 7.7 41.4 7.7 10 21 5 1,187 423 

Average 20 33.7 5.8 35.1 9.1 10 15.4 5.7 828 396 

POLYURETHANE LIFT CAP 

Parallel 11 11.07 0.22 210 6.3 8.6 160 10 5,810 1,200 

Perpendicular 5 10.94 0.32 240 23.2 7 183 54 7,753 1,411 

Average 16 11.03 0.25 219.4 19.4 8.1 167 32 6,417 1,535 

SD Standard deviation. 

1

Strain at the compressive strength. 
21

Change in Foam Characteristics and Properties 

in the Polyurethane and Aggregate Mixture 

 The physical characteristics of some of the polyurethane foam were different from the 

rigid foam in the polyurethane cap, as were the properties of the remaining foam in the mixture. 

The differences in physical characteristics occurred in the foam surrounding each piece of aggregate where a soft foam layer formed. This formation is clearly associated with the aggregate and 

may be caused by residual moisture from the aggregate. Residual moisture can affect the polyurethane in two ways [Zhang 2006]. It can cause the formation of additional carbon dioxide gas 

beyond the specified formulation and result in lower-density polyurethane near the moisture 

source. Further, the stoichiometric balance between the two components (the isocyanate and 

polyalcohol used to form the polyurethane) will be upset with the extra moisture. This could 

cause some of the polyalcohol component not to react, and thus the foam could become softer in 

the vicinity of the residual moisture. 

 The soft foam can easily be deformed with finger pressure. The thickness of the soft 

layers measured about 0.06–0.15 in. Essentially, the material has different characteristics from 

the rigid foam and now behaves as soft foam. Moving away from the aggregate, the foam 

becomes rigid, similar to the polyurethane cap. Therefore, the polyurethane-aggregate mixture 

consists of aggregate surrounded by soft foam that is dispersed in a matrix of rigid foam. 

 The density and the variation of the density of the polyurethane in the mixture caused by 

the residual moisture can be used as a measure of the changes in the polyurethane foam and to 

determine if the design requirements were achieved. The estimated amount of soft foam in the 

mixture can also be determined from the density. For rigid polyurethane, there is a strong, direct 

correlation between density and the strength and stiffness [DuPont 1987; Piping Technology & 

Products 2008]. However, it is assumed that the soft foam has little or no strength regardless of 

the density, and the amount of soft foam would then have a significant effect in reducing the 

overall strength and stiffness of the mixture. 

 To determine the density, the aggregate was removed from a 6-in-diam cylindrical sample from the polyurethane-aggregate mixture from lift 2. The combined sample had a volume of 

88.2 in3

 with a weight of 1.77 lb, resulting in a density of the mixture of 34.7 lb/ft3. The weight 

of the aggregate was 1.56 lb with a density for the aggregate of 160 lb/ft3

. The density of the 

aggregate was measured by using a water displacement method [ASTM 2004c]. Based on weight 

and density, the volume of the aggregate in the sample was calculated to be 16.9 in3

. The weight 

of the polyurethane foam was 0.21 lb with a volume of 71.3 in3

. The calculated foam density in 

the mixture is 5.1 lb/ft3. This is much less than the design requirement of 10.0 lb/ft3. Based on 

this sample, the percent by volume of the aggregate in the mixture is 19.2, and the percent 

by volume of the polyurethane foam is 80.8. 

 The amount of softer foam that has formed in the mixture can be estimated from the 

potential difference in the density between the two foams. Table 4 shows the densities of various 

samples of the soft, rigid, and combined foams. These foam densities were determined using a 

water displacement method [ASTM 2004c]. The samples were obtained by taking pieces of the 

foam from a lift and removing the aggregate. Some of the foam samples were further dissected to 

separate the soft and rigid foam. Because of this process, measurements were made on small, 

highly irregular samples. 
22

Table 4.—Densities of soft, rigid, and combined foam samples 

taken from the polyurethane-aggregate mixture1

Sample Weight, 

g 

Specific 

gravity 

Density, 

lb/ft3 Location 

RIGID FOAM 

L4-R 1.05 0.15 9.31 Middle lift 4 

L3-R 1.52 0.14 8.81 Lift 3 

L3/L4-R 3.01 0.15 9.26 Lift 3/Lift 4 

L3/L4-R 3.71 0.16 9.83 Lift 3/Lift 4 

Average — 0.15 9.30 — 

SOFT FOAM 

L3-Sa 2.18 0.06 4.02 Middle lift 3 

L5-S 1.71 0.08 4.89 Lift 5 

L4-S 1.66 0.09 5.38 Lift 4 

L3-Sb 2.33 0.07 4.55 Lift 3 

Average — 0.08 4.71 — 

COMBINED RIGID AND SOFT FOAM 

L3-Ca 4.98 0.09 5.54 Lift 3 

L3-Cb 5.31 0.12 7.18 Middle lift 3 

L4-Ca 2.37 0.11 7.01 Bottom lift 4 

L4-Cb 6.6 0.14 8.54 Lift 4 

L5-C 5.78 0.10 6.31 Lift 5 

Average — 0.11 6.92 — 

1

Densities were measured by water displacement method. 

 The density of the soft foam was measured at 4.7 lb/ft3 and the density of the rigid foam 

at 9.3 lb/ft3. The combined density was 6.9 lb/ft3. Based on these densities, the polyurethane 

portion of the polyurethane-aggregate mixture is composed of about 50% soft foam. It must be 

recognized that these densities are based on small grab samples. Further, the amount of soft foam 

is dependent on the amount, size, and gradation of the aggregate. These parameters vary through 

a lift. Also, the density of the combined foam samples as measured by this technique is higher 

than the apparent density of 5.1 lb/ft3 based on a much larger sample. The higher density results 

from the penetration of water into the foam during the water displacement tests caused by voids 

along the surface. With the small, highly irregular samples used, the surface area is quite large 

compared to the volume; therefore, the density determined by water displacement can be higher 

than the apparent density. However, the assumption is made that the porosity along the sample 

surfaces is approximately the same for the two types of foam, and the densities measured by this 

method can be used to estimate the percent of each type of foam. Regardless of these assumptions and sampling issues, it is clear that the amount of soft foam in the mixture is substantial. 

 The formation of soft foam in a polyurethane seal has also been noted from a seal that 

was destroyed in a mine explosion [MSHA 1996]. In the 1996 explosion at the Oasis Mine in 

West Virginia, three polyurethane seals were destroyed and one partially destroyed. The seals 

contained a 16-in-wide core composed of polyurethane and aggregate. From the explosion, the 

seals were blown apart into primarily small pieces. Based on the examination of a small piece of 

the seal that was primarily polyurethane, MSHA [1996] reported that “[t]his polymer could be 

compressed or broken by hand.” However, as a result of the hazardous conditions at the site and 

in the mine, there were no further site visits to examine the failed seals. 
23

ANALYSIS OF SEAL PERFORMANCE 

Analysis of Seal Material Properties 

 Based on the results of the seal material testing, the properties of the two sections of a lift 

are significantly different. The two sections include the polyurethane cap and the polyurethaneaggregate mixture, with the mixture comprising about 80% of a lift and of the polyurethane seal 

(Figure 5). 

 The cap consists of polyurethane with a density of about 11.0 lb/ft3

. This does fall above 

the designed minimum density of 10.0 lb/ft3

 for the polyurethane. In general, the stress-strain 

curves show the type of performance expected from rigid polyurethane foam with this density 

(Figure 20) [DuPont 1987; Huber and Gibson 1988; Piping Technology & Products 2008]. 

Initially, there is an elastic portion to the curve that is nearly linear. This is followed by a sharp 

flattening of the curve or even a slight drop in the stress as the foam structure collapses. Beyond 

the compressive strength, the stress-strain curve continues to rise, although at a much reduced 

rate with a nearly plastic material behavior. Further, there does not seem to be a large difference 

in the compressive strength between the vertical and horizontal direction. Typically, the collapse 

or compressive strength may be 30%–50% lower in the direction perpendicular to foam rise 

[Huber and Gibson 1988]. This anisotropic behavior results from a polyurethane cell structure 

that is more elongated in the direction of foam rise. In this case, the compressive strength is 

about 14% higher in the direction considered to be perpendicular to foam rise. The strength 

properties indicate either that the aspect ratio of the cells is close to one, where there is no cell 

elongation, or the orientation of the cell structure is not vertical and horizontal [Hawkins et al. 

2005]. At the top of a lift, the foam that forms the cap will be free to expand not only in the 

vertical direction, but also in the horizontal direction, which could result in a variation of the 

orientation of the cell structure. 

 The density of the polyurethane-aggregate mixture that was sampled is about 34 lb/ft3

. 

Based on the estimated volumes and densities of the polyurethane cap and polyurethaneaggregate mixture, the overall density of the core seal is 29 lb/ft3

. The minimum design density 

for the seal is 35 lb/ft3. To meet the minimum density requirement, the polyurethane-aggregate 

mixture would need to have a density of 40 lb/ft3

. In part, the lower measured density of the 

mixture could be due to sampling. However, the lower density of the polyurethane foam in the 

mixture could account for a significant portion of this difference. 

 The polyurethane-aggregate mixture has significantly lower compressive strengths and 

deformation properties than the cap with only polyurethane. The compressive strengths and 

elastic modulus are only about 15% of those of the polyurethane cap. This significant reduction 

in properties is the result of the formation of the soft foam around the aggregate. Further, there is 

not a sharp transition from the pre- to the postcompressive strength regions on the stress-strain 

curves (Figure 19). 

 The reduction in strength results from the rigid foam forming a stiff structural matrix 

around the soft foam and aggregate, where the soft foam may comprise up to 50% by volume of 

the polyurethane in the mixture. However, the soft foam contributes little to the compressive 

strength, and the aggregate surrounded by the soft foam could be approximated as a hole or void 

until the matrix collapses. Essentially, the rigid foam matrix structure must support the load. This 

rigid matrix does provide some initial stiffness to the specimen until the rigid foam in the matrix 

structure collapses. Because the aggregate and soft foam are distributed through the samples and 
24

the lift, the strength is based to a large extent on the matrix structure and not the percent 

by volume of the soft and rigid foam. 

 Based on the measured compressive strengths and elastic moduli from the two orthogonal 

directions, the polyurethane-aggregate mixture seems to be anisotropic. In the direction of foam 

rise, the compressive strength is about 31% higher and the elastic modulus about 87% higher 

than perpendicular to foam rise. It is possible that the rigid foam within the polyurethaneaggregate mixture may be anisotropic, thus resulting in a higher strength and stiffness in the 

direction of foam rise. However, this effect is small compared to the reduction in properties from 

the addition of the aggregate. 

 The density of the polyurethane foam in the mixture is a further indication of the changes 

that occurred with the addition of the aggregate. The foam density in the mixed zone is 5.1 lb/ft3. 

It is much lower than the cap density of 11.0 lb/ft3

. This in part reflects the formation of a large 

amount of the soft foam. However, as measured by the water displacement method, the density 

of the rigid foam within the mixed zone may have a lower density (9.3 lb/ft3

) than the polyurethane in the cap. Because of the porosity, this measured density (9.3 lb/ft3

) may be overestimated in making a comparison with the density measured for the cap. Essentially, the rigid 

foam within the mixed zone could have a correspondingly lower compressive strength than the 

foam cap based on the density. 

 Clearly, the formation of the soft foam is closely associated with the aggregate, as is 

the overall lower density of the foam in the mixture. It is assumed these changes are occurring 

because of residual moisture on the aggregate. The aggregate was added to provide additional 

mass; however, there was also a significant reduction in the compressive strength and stiffness of 

the polyurethane-aggregate mixture due to the addition of the aggregate. The importance of this 

compressive strength is that once it is exceeded, the material will exhibit nearly plastic behavior. 

Seal Failure 

 Figure 21 shows the pressure-displacement curve for LVDT 6 located at the center of the 

seal. The pressure used is that at the pressure transducer. From 0 to 4 psi, the curve is linear, 

indicating a linear elastic behavior for the seal. From 4 to 16 psi, the curve shows some nonlinear 

characteristics, possibly associated with inelastic behavior that is time-dependent. Fitting a line 

from 4 to 16 psi, the decrease in slope indicates some softening of the seal. However, from 16 to 

18 psi a significant change in behavior occurs. This was accompanied by a larger increase in 

visible leakage from the seal. Again, fitting a line between the pressure peak at 16 psi and the 

maximum pressure at 18 psi shows a further and significant softening of the seal. This suggests 

that structural damage to the seal was taking place after the pressure reached 16 psi at the pressure transducer. Therefore, 16 psi can be considered the yield strength for the seal. This calculates to an average pressure across the seal of about 17 psi based on the stress distribution of the 

hydraulic load. 
25

0

2

4

6

8

10

12

14

16

18

20

0.0 0.2 0.4 0.6 0.8 1.0

Displacement, in

Pressure, psi

 Figure 21.—Pressure-displacement curve for LVDT 6 at the center of seal shows the behavior of the seal. 

General slopes of curves are indicated by dashed lines. 

 Based on the average pressure on the seal of 17 psi at yield and the seal surface area 

of 184 ft2, the total force on the seal was about 450,000 lb. With a seal thickness of 18 in and 

assuming no contribution from the forms, the calculated shear stress along the boundary at yield 

was 35.3 psi. Assuming the seal acts as a plate with simply supported edges, a center deflection 

at yield of 0.46 in, an elastic modulus of 828 psi for the polyurethane-aggregate mixture, and a 

seal thickness of 18 in, the maximum stress due to bending is 3.2 psi.9

 The shear stresses are an 

order of magnitude higher than the bending stresses. Therefore, the yielding is probably the 

result of the shearing stresses developed near the boundaries. 

 The maximum pressure that the seal withstood at the pressure transducer was 18 psi. As 

shown in Figures 10–12, at that point there was a large increase in displacement with a sudden 

drop in pressure. This occurred as the seal failed and the pumping could not keep up with the 

leakage. Further loading at an increased pumping rate did not achieve a higher maximum pressure, but did significantly increase the deflection and water leakage. Essentially, the 18-psi static 

pressure can be considered the pressure where the seal structurally failed. When this occurred, 

the stress was 16.9 psi at the top of the seal and 20.8 psi at the bottom. This calculates to an average stress across the seal of 18.9 psi. This is below the 20-psi criterion mandated by the Code of 

Federal Regulations at the time of the test [30 CFR 75 (2005)]. 

 

 9For a seal simply supported at the edges, the maximum stresses from bending will be in the middle of the seal, 

with the back of the seal core in compression and the front in tension. If the seal edges are assumed to be fixed, the 

maximum stress from bending would increase to 10.5 psi. This stress is still much lower than the shear stress. 

With the edges fixed, the maximum stresses from bending are in the center of the long side of the seal. The back of 

the seal would be in compression and the front in tension at the maximum stress location. 
26

 From the observed damage and the pressure and displacement measurements, an understanding of how the seal may have failed can be developed. Figure 22 shows the displacement 

versus the location of the LVDTs along the horizontal center line of the seal for various pressures. The lines on the graph indicate the change in shape of the seal for a given pressure. The 

heavy line (third from left) is the deflection at the maximum transducer pressure of 18 psi. All 

curves past this point are for a failed seal. At the initiation of failure, there is a change in the 

geometry of the seal. Failure is also indicated by the continued displacement even with a pressure drop. However, the change in shape and movement initially occurs on the right side of the 

seal. After the failure on the upper right side, at a pressure of 13.5 psi, the displacement at the 

location 6 ft to the right of the center was initially larger than the displacement at the center. 

Little or no additional movement is seen on the left side of the seal until the pressure has dropped 

to 10.9 psi. Figure 23 shows the displacement versus time for LVDTs 2 (left side), 6 (center), 

and 9 (right side). There is about a 40-sec delay before the increase in displacement is seen on 

the left side as opposed to that on the right side and center. The main damage observed along the 

right side of the seal is near the top where the aggregate was not well bonded with the polyurethane (Figures 13 and 17). A major leak also developed at this location (Figure 24). This high 

concentration of poorly bonded aggregate can be considered a construction flaw. 

0

2

4

6

8

10

12

14

16

18

20

01234567

Displacement, in

Location, ft

10.5 psi

16.1psi

18 psi

13.5 psi

10.9 psi

14.4 psi

10.1 psi

Right Side of Seal

Left Side of Seal

 Figure 22.—Displacements along horizontal center line of the seal show the change in shape of the seal 

during the test for a given transducer pressure. Curves to the right of the 18-psi line represent the behavior 

after the peak pressure was achieved. 
27

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 3600

Time, sec

Displacement, in

0

2

4

6

8

10

12

14

16

18

20

Pressure, psi

Time delay 

Pressure

LVDT 6

LVDT 9

LVDT 2

LVDT 2

LVDT 6

 Figure 23.—Displacements measured on the left side (LVDT 2), center (LVDT 6), and right side (LVDT 9) 

of seal showing the 40-sec delay in the reaction of the left side when the seal failed. 

 

Figure 24.—Water leakage observed from failure zone along upper right side of the seal. 
28

 About 40 sec after the failure on the right side, there was a significant change in the 

geometry on the left side of the seal. This change in geometry probably developed as a result of 

increased shear stresses and displacements along the top of the bottom row of blocks due to the 

failure on the upper right side of the seal. This increased shearing and displacement resulted in 

the development of the near vertical fracture near the lower left corner of the seal (Figures 13 

and 15). This was the external manifestation of the internal fractures that extended across the 

thickness of the seal at this location (Figure 16). This fracture system developed because the 

lower left corner was more constrained by the rib and blocks than the seal to the right of the 

fractures. When sufficient shearing and displacement had taken place, the right portion of the 

seal was torn or sheared away from the lower left corner of the seal, thus creating the fractures. 

Mostly likely these fractures developed at or just past the peak pressure. 

 After yield, the seal behaves nearly plastically with a low stiffness, indicating that the 

seal is approaching maximum strength. With sufficient displacement, the failure was initiated 

near the roof on the right side of the seal probably as a result of shear failure through the seal 

along a poorly bonded aggregate layer. Shortly after, failure occurred along the lower center and 

left side of the seal. Essentially, the strength of the seal material was not sufficient to prevent 

deformation and damage in this area near the floor even though the pressure was dropping. 

Although there may have been a weak area near the roof on the right side, the area near the floor 

did not seem to be much stronger. Further, even though the initial failure occurred at a weakened 

zone, the seal was in yield, indicating that the seal was close to the maximum strength even without a weak point. 

 After the seal had failed structurally, the water flow rate was increased to 750 gpm and 

there was a subsequent increase in pressure to 14.5 psi. However, the seal could not sustain this 

added pressure, resulting in a large increase in deformation and a sharp drop in pressure to less 

than 8 psi. At this point, the fracture through the seal on the lower left side would probably have 

been fully developed (Figure 15). 

 Following yield and failure, the seal was subjected to a significant increase in displacement and bending. Approximately 95% of the displacement occurred after yield and 90% after 

failure. As a result of the bending of the seal, the horizontal interfaces opened between lifts 

toward the center of the seal. These interfaces were only open to the middle of the seal, indicating that the joint opened because of tension caused by bending. The interfaces do not appear 

to have affected the failure. The open joints, however, indicate that the interfaces between the 

lifts were weaker than the polyurethane-aggregate mixture in tension. This suggests that the 

polyurethane is not bonding well to itself after curing. Much of the damage to the seal along the 

top of the lower row of blocks probably occurred in the postfailure period (Figure 15). The large 

seal displacements and the opening of the joint along the interface of lifts 1 and 2 at this location 

were probably caused by shearing along the top of the bottom row of blocks. This shearing 

created a sharp bend in the seal core just above this location that resulted in the opening up of 

the interface. 

 During the test there was only a little over 1 in of deflection recorded by the two LVDTs 

just 1 ft from each rib. This indicates that there was little or no slippage of the seal along the rib 

boundaries. To a large extent, the seal was restrained at the ribs. 
29

Shear Failure and Strength of the Seal 

 The assumption is made that the seal was in yield because of the shear stresses developed 

from the water pressure. Further, the failure seems to have occurred in shear near the upper right 

corner and lower left boundary. Therefore, an estimate can be made of the shear stresses along 

the boundary at failure. These average shear stresses can then be compared to the strengths measured for the seal material. 

 Based on the average pressure on the seal of 18.9 psi at failure and the seal surface area 

of 184 ft2

, the total force on the seal was about 500,000 lb. With a seal thickness of 18 in and 

assuming no contribution from the forms, the calculated shear stress along the boundary at failure was 39.4 psi.10 Because of how the failure initiated, this may represent the shear strength 

along the upper right side of the seal, a zone of poorly bonded aggregate that can be considered a 

construction flaw. However, the seal was in yield before the failure occurred at this weakened 

zone; therefore, the shear strength of the seal without this construction flaw may not be that 

much higher. 

 Since the failure occurred through the polyurethane-aggregate mixture, the strength properties of this material are used to evaluate the strength of the seal. From the tests, only a uniaxial 

compressive strength was measured and not the shear strength. However, a similar reduction in 

the shear strength that occurred with the compressive strength from that of a pure polyurethane 

sample is assumed. 

 Strength equations have been developed that can be used to estimate the expected 

strength of a rigid polyurethane from the density [DuPont 1987; Piping Technology & Products 

2008]. For the compressive strength, the equation is 

Co = 12.77D1.416 (1) 

where Co = compressive strength, psi, 

and D = density of polyurethane, lb/ft3. 

For the shear strength, the equation is 

So = 14.9D1.077 (2) 

where So = shear strength, psi. 

These equations are for the strength in the direction of foam rise or in the strong direction. 

Further, although these equations were developed for specific polyurethane formulations, they 

can be used as general equations showing the effect of the density on strength. 

 The average compressive strength for the polyurethane-aggregate mixture is 35 psi 

(Table 3). In this analysis, no consideration is being given to the affects of foam anisotropy since 

there is such a large reduction in strength with the addition of the aggregate. Based on the average compressive strength for the polyurethane-aggregate mixture, the density of polyurethane 

foam that would result in the same compressive strength can be calculated. From the above 

 

 10The maximum bending stresses for the seal core are for a fixed-end beam 2–3 times less and for a simply supported beam 6–10 times less than this calculated shear stress. 
30

equations, a foam density of 2 lb/ft3 would result in a compressive strength of 34 psi and a shear 

strength of 31 psi.11

 The estimated shear strength of 31 psi for the polyurethane and aggregate mixture is 

certainly within the same magnitude range as the calculated shear stresses developed at failure. 

Therefore, shear failure through the polyurethane-aggregate mixture could have occurred. 

Essentially, when the stresses in the seal exceeded the strength of the polyurethane-aggregate 

mixture, which had changed because of the residual moisture from the aggregate and the construction flaw, the seal failed. Thus, the properties of the polyurethane-aggregate mixture and 

the construction flaw determined the maximum pressure that the seal could withstand. 

Contribution of the Concrete Block Walls 

to the Strength and Stiffness of the Seal 

 One can ask whether the dry-stacked concrete block wall forms increased the strength or 

stiffness of the seal and thus could be an important element in maintaining the structural integrity 

of the seal. The possible effects of the forms on the stiffness can be evaluated from the pressure 

and deflection data prior to failure. Figure 25 shows the deflection of the seal along the vertical 

center line for different water pressures before and after failure. 

 In the following analysis, the seal performance with both a fixed and simply supported 

edge will be evaluated. For the fixed-edge condition, the assumption is made that the middle 

portion of the seal will act as a beam in bending with the edge constraints provided by the ribs 

sufficiently removed so their effects can be ignored [Obert and Duvall 1967]. However, for a 

plate with simply supported edges, the error in using the beam equations is larger and may not be 

sufficiently accurate. Therefore, both the plate and beam equations will be used for this case. The 

actual deflections can then be compared to those calculated from a bending beam or plate. 

Considering only the seal core, the calculations are as follows: beam length is 9 ft, plate length is 

20.4 ft and plate width is 9 ft, and thickness for both is 18 in. The elastic modulus was then 

adjusted so that the calculated deflection in the center of the beam or plate matched the measured 

deflection. Table 5 shows the calculated deflection at locations where the measurements were 

made and the resulting elastic modulus. Values were calculated for both fixed-end and simply 

supported beams and for a plate with simply supported edges. To develop the same amount of 

deflection at the center of the seal for the range in pressure from 4.6 to 16.1 psi, the elastic modulus for the fixed-end beam ranges from 22,500 to 32,500 psi and for a plate with simply supported edges from 84,000 to 125,000 psi. 

 

 11The 90% confidence interval for the shear strength equation at a density of 2 lb/ft3 is from 20 to 50 psi. 

However, these confidence intervals are based on testing different formulations of the polyurethane where the 

confidence levels are the variation between the formulations. If the strength of the formulation is less than the 

average strength for a given density, it will be less than the average for all densities.
31

0

1

2

3

4

5

6

7

8

9

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Displacement, in

Location, ft

4.6 psi

8.5 psi

14.4 psi

16.1 psi

18 psi

13.5 psi

Bottom of Seal

Top of Seal

 Figure 25.—Displacements along vertical center line of the seal show the change in shape of the seal 

during the test for a given transducer pressure. Curves to the right of the 18-psi line represent the behavior 

after the peak pressure was achieved. 
32

Table 5.—Calculated deflections (in inches) of the seal based on assumed end support 

and the calculated elastic modulus used to match the actual seal deflections 

at the center of the seal 

Location,1

 ft Support 1 2.5 4.3 6 7.5 

Elastic 

modulus, psi 

PRESSURE = 4.6 psi 

Beam 

Fixed-end 0.02 0.08 0.11 0.08 0.02 32,500 

Simply supported 0.04 0.08 0.11 0.1 0.06 135,000 

Plate 

Simply supported — — 0.11 — — 125,000 

Actual 0.03 0.08 0.11 0.08 0.03 - 

PRESSURE = 8.5 psi 

Beam 

Fixed-end 0.03 0.13 0.19 0.14 0.03 31,000 

Simply supported 0.07 0.16 0.19 0.17 0.08 136,000 

Plate 

Simply supported — — 0.19 — — 117,000 

Actual 0.05 0.13 0.19 0.15 0.08 — 

PRESSURE = 14.4 psi 

Beam 

Fixed-end 0.06 0.24 0.35 0.25 0.06 26,500 

Simply supported 0.12 0.27 0.35 0.29 0.14 124,000 

Plate 

Simply supported — — 0.35 — — 100,000 

Actual 0.1 0.24 0.35 0.28 0.14 — 

PRESSURE = 16.1 psi 

Beam 

Fixed-end 0.08 0.31 0.46 0.32 0.08 22,500 

Simply supported 0.16 0.36 0.46 0.38 0.18 105,000 

Plate 

Simply supported — — 0.46 — — 84,000 

Actual 0.13 0.3 0.46 0.38 0.18 — 

1

Locations are from the roof. 

 For the seal, the elastic modulus varies from an average of 828 psi for the polyurethaneaggregate mixture to 6,417 psi for the polyurethane cap. The actual large-scale elastic modulus 

for the seal is not known, but would not exceed that of the polyurethane cap and would probably 

be much less. However, using the elastic modulus of the polyurethane cap for the seal, the 

concrete block walls apparently increase the stiffness of the seal by a factor of 3.5–5.0 for a 

fixed-end condition and by a factor of 13–19 for a plate with simply supported edges. If the 

polyurethane-aggregate mixture determines the elastic modulus of the seal, the contribution of 

the forms to the seal stiffness would be an order of magnitude higher. Therefore, the form walls 

are substantially increasing the seal stiffness prior to failure. The increased stiffness developed 

from the block forms prevents the seal core from failing in tension or compression through bending. Therefore, this results in a greater likelihood of a shear failure of the seal to occur. After the 

seal yields, the form walls offer little resistance to displacement, as indicated by the flattening of 
33

the pressure displacement curve (Figure 21). This could imply that the on set of yield may be 

occurring because of the loss of constraint or support provided by the form walls. 

 At the lowest pressure, the fixed-end solution is closer to the actual deflections at the 

points away from the center (see Table 5). At the higher pressures, the simply supported solution 

generally calculates deflections that are somewhat closer to the measured values. The smooth arc 

shape of the displacement curves on Figure 25 also indicates that the seal is acting more like a 

simply supported beam at the higher pressures. This could occur as a result of shearing deformations that may be developing at the ends beyond that due to bending because of the low 

deformation properties of the seal (Figure 25). 

Hydrostatic Versus Explosion Testing of Polyurethane Seals 

 Explosion testing of polyurethane seals was conducted at the LLEM in the 1990s [Weiss 

et al. 1996]. These tests were made in openings that were 18 ft wide by 6.5 ft high with an opening area of 120 ft2

. During these tests, the seal thickness, core densities, and types of stacked wall 

blocks were varied. There were six tests on seals that contained a polyurethane-aggregate mixture; only the results of these tests will be used in the comparison to the seal tested in the hydrostatic chamber. Because of the size difference (both area and thickness) between the seal in the 

hydrostatic chamber and those in the explosion tests, the shear stresses along the boundaries at 

failure can be used in comparing the seal performance. 

 Table 6 summarizes the seal design and shear stresses at failure for the explosion tests. 

Because the explosion tests are pass/fail, only a range of the possible shear stresses at failure 

can be determined. All six of the seals are considered to have passed the 20-psi criterion. Four 

of the seals failed at pressures higher than the criterion, and two seals did not fail during subsequent tests. 

Table 6.—Summary of explosion tests on polyurethane-aggregate seals conducted in the 1990s 

[Weiss et al. 1996] 

Pressure,1

 psi Seal 

No. 

Type 

of 

block 

Designed 

core 

thickness, 

in 

Actual 

core 

thickness, 

in 

Core 

density, 

lb/ft3 Passed Passed Failed 

Range of possible 

shear stresses 

at failure, psi 

 1 HC 30 30 NA 21 NAp 32 20–31 

 2 HC 18 20 NA 21 39 NAp >50 

 3 HC 12 16 NA 21 NAp 40 38–72 

 8 S 16 16 46 20 28 NAp >56 

 9 HC 16 16 40 19 NAp 29 34–52 2

10 S 18 18 35 19 NAp 24 30–37 

HC Hollow core block. S Solid core block. NA Not available. NAp Not applicable. 1

The seal passed or failed the ventilation test requirements after being subjected to the indicated pressure. 2

The front form wall contained a pilaster. 

 Based on these tests, the range of possible shear stresses at failure is 20–72 psi for those 

seals that were tested to failure. The range of the maximum shear stress possible at failure is 

31–72 psi. The maximum possible shear strength is the highest shear strength that could be determined from each test and is calculated from the pressure when the seal failed. The true maximum 

shear strengths would therefore likely be lower than these values. As previously calculated, the 
34

shear stress on the hydrostatically tested seal was 39.4 psi at the maximum pressure. The hydrostatically loaded seal, however, began to yield at a shear stress of 35.3 psi. This places the shear 

strength of the seal tested in the hydrostatic chamber within the lower range of the maximum 

shear stresses determined from the explosion tests. The lower strength of the seal may in part be 

due to the construction flaw on the upper right side of the seal though the seal was in yield. 

 Further, a direct comparison can be made to the performance of seal 10, the seal most 

similar in design. The main difference is that seal 10 was constructed with a pilaster in the form 

wall that could have increased the rigidity of the seal. For seal 10, the possible range of shear 

stresses at failure is 30–37 psi. Essentially, the shear stresses at failure are nearly the same. This 

suggests that the two tests can provide similar results and that the shear strengths are relevant to 

seal design. 

 However, the conclusions reached from the two types of tests are different. The seals 

subjected to the explosion tests would be considered to have met the Code of Federal Regulations criterion at the time of the test whereas the hydrostatically loaded seal did not. In part, these 

seals were designed to meet or exceed the criterion with probably little consideration to a factor 

of safety. Further, there are too many variables and unknowns involved with this type of seal 

construction and testing to allow for such precise design and consistent results. 

 In addition, the seal in the static test may have been subjected to more deformation due to 

the sustained load than the seal in the explosion test. The increased deformation could lead to the 

seal fracturing at a lower stress. In part, the time-dependent properties of the polyurethane could 

cause an increase in the displacement of the seal during the static test, whereas this would not 

occur during the short period of loading from the explosion tests. Also, a part of the energy during the explosion test is used to accelerate the seal mass and is not available to deform the seal. 

Therefore, the results of the static test may be more conservative than those of the explosion 

tests. 

 For the explosion tests, a minimum seal mass is required to prevent seal failure with 

the strength of the seals actually increasing with density. However, the effects of the seal mass 

cannot be evaluated in the static tests. This also suggests that there are other failure criteria 

related to the dynamic loading that may need to be considered in seal design, and this forms the 

basis of the minimum density requirements for the seal. 

 Another difference between the seals tested in the 1990s and the seal tested in the hydrostatic chamber is the uniaxial compressive strength of the polyurethane-aggregate mixture. For 

the polyurethane aggregate-mixture from the explosion tests, a total of 12 samples were tested in 

uniaxial compression in the direction perpendicular to foam rise. These samples were obtained 

from a block of seal material saved from prior testing. These samples had a diameter and length 

of 6 in and were prepared and tested in the same manner as those from the seal in the hydrostatic 

chamber. Table 7 shows the average stress at 10% and 30% strain for the 12 samples. These 

strain levels are used because there is no clear yield or collapse stress for the samples. Also 

shown in the table is the average stress at 10% and 30% strain for all of the samples tested 

perpendicular to foam rise for the seal tested in the hydrostatic chamber. 
35

Table 7.—Average strengths of polyurethane-aggregate mixtures tested 

perpendicular to foam rise in uniaxial compression 

from an explosion-tested seal from the 1990s 

and the seal tested in the hydrostatic chamber 

No. of 

samples

Density, 

lb/ft3

SD, 

lb/ft3

Stress at 

10% strain, 

psi 

SD, 

psi 

Stress at 

30% strain, 

psi 

SD, 

psi 

SEAL TESTED IN HYDROSTATIC CHAMBER 

13 33.7 4.8 31.7 6.9 85 24 

EXPLOSION-TESTED SEAL, 1990s 

12 40.6 3.1 59 14 188 26 

SD Standard deviation. 

 Figure 26 shows some stress-strain curves for samples tested in compression for the polyurethane cap and polyurethane-aggregate mixture from the explosion-tested seal and the seal 

tested in the hydrostatic chamber. All three samples were tested in the direction perpendicular to 

foam rise. The densities of the samples were 11 lb/ft3 for the polyurethane cap, 46 lb/ft3 for the 

polyurethane-aggregate mixture from the explosion-tested seal, and 31 lb/ft3 for the 

polyurethane-aggregate mixture from the hydrostatically tested seal. 

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60

Strain, pct

Stress, psi

Polyurethane-Aggregate 

Hydrostatic Chamber Seal

Polyurethane-Aggregate

Explosion Test-1990's

Polyurethane 

Hydrostatic Chamber Seal

 Figure 26.—Stress-strain curves from uniaxial compressive tests on samples from explosiontested seal from the 1990s and the seal tested in the hydrostatic chamber. Direction of testing was 

perpendicular to foam rise. The samples were either pure polyurethane or a polyurethane-aggregate 

mixture. 
36

 The compressive test results indicate that the strength and stiffness of the polyurethaneaggregate mixture from both sets of seals were degraded significantly from those of the polyurethane only. However, the polyurethane-aggregate mixture from the explosion-tested seal from 

the 1990s showed much less decrease in strength and stiffness. The polyurethane aggregate from 

the 1990s seal has about twice the strength at 10% and 30% strain compared to the polyurethaneaggregate mixture from the more recent hydrostatically tested seal. The degradation of the 

strength of the polyurethane-aggregate mixture of the explosion-tested seal resulted from the 

formation of a small amount of soft foam around each aggregate similar to that occurring in the 

polyurethane-aggregate mixture from the seal tested in the hydrostatic chamber. 

 Essentially, moisture seems to reduce the strength of the polyurethane-aggregate mixture 

for both the seal tested in the hydrostatic chamber and the explosion-tested seal from the 1990s 

by causing the formation of soft foam around the aggregate. Apparently, however, the 

polyurethane-aggregate mixture from the explosion-tested seal was affected to a lesser degree. 

Because the compressive strength is much less for the polyurethane-aggregate mixture from the 

more recent seal tested in the hydrostatic chamber, the results of the explosion tests conducted in 

the 1990s may not be applicable for establishing design criteria for seals constructed today. Since 

the seal tested in the hydrostatic chamber did not meet the 20-psi criterion either because of the 

reduced strength or the construction flaw, there seems to be at present no solid technical basis to 

establish the minimum design requirement for such seals from the results of full-scale physical 

testing. Unless the amount of the soft foam and construction flaws can be controlled or 

eliminated, significant safety factors will need to be used in the design of seals built with a 

polyurethane-aggregate mixture. 

CONCLUSIONS 

 From the test of the composite polyurethane-aggregate seal in the hydrostatic chamber 

and the results of the testing of the seal material, several conclusions about the seal performance 

can be reached. 

1. The average pressure applied to the seal at failure was 18.9 psi. This is less than the 

20 psi of horizontal static pressure established in the Code of Federal Regulations 

(at the time of the test) for such a seal [30 CFR 75 (2005)]. Therefore, this seal 

did not meet the regulatory requirements when the horizontal static pressure was 

applied with water. 

2. The seal yielded structurally at an average hydraulic pressure of 17 psi. This was 

accompanied by increased damage and water leakage. 

3. Based on the observed damage to the seal and the measured displacements, the 

failure initiated from shear near the top and along the right side of the seal. This 

occurred along a construction flaw, a zone with a high aggregate content but poorly 

bonded with the polyurethane. The seal then failed along the lower center and left 

side of the seal. 

4. The addition of the aggregate altered the characteristics and properties of the foam 

in the polyurethane-aggregate mixture. A high percentage of the foam in the 

polyurethane-aggregate mixture developed into a soft foam that can carry little load 

prior to the collapse of the foam structure. Further, the density of the foam in the 

mixture is less than half of that for the polyurethane cap and less than the minimum 

design density. These changes have resulted in large reductions in the strength and 

stiffness of the polyurethane-aggregate mixture compared to the polyurethane foam 
37

with no aggregate. Further, the reduction in foam density in the mixture resulted in 

the overall seal density being less than the minimum required to resist the energy of 

an explosion. 

5. The change in foam characteristics in the mixture seems to be caused by moisture 

associated with the aggregate. Steps were taken to minimize the effects of the moisture, but were not successful. 

6. The change in characteristics and properties of the foam in the mixture is important 

because the polyurethane-aggregate mixture comprises at least 80% by volume of 

the seal. Further, it seems that the failures have occurred through the polyurethaneaggregate mixture in the seal. Therefore, the change in foam characteristics as a 

result of the addition of the aggregate has reduced the overall stiffness and strength 

of the seal. 

7. The calculated shear stresses at failure and the estimated shear strength of the 

polyurethane-aggregate mixture are fairly close. This suggests that the strength and 

properties of the polyurethane-aggregate mixture controlled the maximum static 

pressure that the seal could withstand. 

8. The concrete block form walls added considerable stiffness to the seal, which minimizes the amount of prefailure deflection. This composite behavior may reduce the 

potential for a tension or compression failure in bending resulting in the seal failing 

in shear. 

9. Polyurethane-aggregate seals constructed and tested in the 1990s passed the 20-psi 

criterion, but the recently tested seal in the hydrostatic chamber did not. The difference in the outcomes seems to be related to a difference in the strength and stiffness 

properties between the seals. Therefore, it is questionable whether the results of the 

seals tested in the 1990s should be used in establishing or confirming the design criteria for these types of seals. Since, the polyurethane-aggregate seal tested in the 

hydrostatic chamber did not meet the criteria, there are then no results from full-scale 

physical testing that can be used to establish or confirm minimum design requirements for such seals. 

10. Because the formation of the soft foam seems to be highly variable and may be difficult to control, significant safety factors need to be built into the design of the seals 

using a polyurethane-aggregate mixture. To reduce the formation of soft foam, techniques could be developed to control the moisture. 

11. The joints between the lifts are weaker in tension than the surrounding polyurethane 

or polyurethane-aggregate mixture. Further, other defects in the form of long narrow 

fissures in the polyurethane cap developed in the seal. Although the joints and fissures have the potential to decrease the seal strength, these features do not seem to 

have contributed to the seal failure in this case. 

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 ASTM [2004a]. Standard test method for apparent density of rigid cellular plastics. West 

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 ASTM [2004b]. Standard test method for compressive properties of rigid cellular plastics. West Conshohocken, PA: ASTM International. ASTM D1621–04a. 
38

 ASTM [2004c]. Standard test methods for density and specific gravity (relative density) 

of plastics by displacement. West Conshohocken, PA: ASTM International. ASTM D792–00. 

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 Hawkins MC, O’Toole B, Jackovich D [2005]. Cell morphology and mechanical properties of rigid polyurethane foam. J Cell Plastics 41(3):267–285. 

 Huber AT, Gibson LJ [1988]. Anisotropy of foams. J Mater Sci 23(8):3031–3040. 

 MSHA [1996]. Report of investigation (underground coal mine). Noninjury coal mine 

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West Virginia, May 15, 1996, and June 22, 1996. U.S. Department of Labor, Mine Safety and 

Health Administration. 

 Obert L, Duvall WI [1967]. Rock mechanics and the design of structures in rock. 

New York: John Wiley and Sons. 

 Piping Technology & Products [2008]. Polyurethane technical information. 

[http://www.pipingtech.com/products/preis/poly.pdf]. Date accessed: February 2008. 

 Sapko MJ, Weiss ES [2001]. Evaluation of new methods and facilities to test explosionresistant seals. In: Proceedings of the 29th International Conference of Safety in Mines Research 

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mine seal research. In: Proceedings of the 28th International Conference of Safety in Mines 

Research Institutes (Sinaia, Romania). Vol. I, pp. 71–85. 

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NIOSH. In: Sealbarr ’99 - Proceedings of the Polish-American Seminar on Seals and Barriers as 

a Means of Protection Against Fires and Explosions in Mines (Katowice, Poland, May 27–28, 

1999), pp. 39–53. 

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of Safety in Mines Research Institutes. Johannesburg, Republic of South Africa: South African 

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required for polyurethane/limestone mine seals in large openings [Thesis]. Pittsburgh, PA: 

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polymer construction material and water trap designs for underground coal mine seals. Pittsburgh, PA: U.S. Department of Energy, RI 9634. NTIS No. PB96–123392. 

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message to Michael J. Sapko (NIOSH Pittsburgh Research Laboratory), March 17. 
DHHS (NIOSH) Publication No. 2008–129

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