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Overlay and Asphalt Pavement Rehabilitation Manual | Road | Road Surface
Uploaded by Mike Nderitu
DESIGN for ROADS and BRIDGES PART 4 – Overlay Design
DESIGN for ROADS and BRIDGES
MATERIALS & PAVEMENT DESIGN
b) –Overlay Design and Asphalt Pavement Rehabilitation
The Republic of Kenya - Ministry of Roads Draft Document – September 2009
1 Summary ................................................................................................................... 1
2 Definitions and Abbreviations ................................................................................... 2
3 Introduction ............................................................................................................... 5
4 Network level evaluation ........................................................................................... 7
4.1 Visual Inspection ........................................................................................... 7
4.2 Roughness Condition Data ........................................................................... 8
5 Project Level Evaluation ......................................................................................... 13
5.1 Detailed Visual Condition Survey ................................................................ 13
5.2 Falling Weight Deflectometer (FWD) Survey .............................................. 15
5.3 Traffic Estimation ........................................................................................ 15
5.3.1 Classified Traffic Counts and Axle Loading ......................................................... 15
5.3.2 Conversion to design traffic loading .................................................................... 16
5.3.3 Effect of Road Geometry ..................................................................................... 16
5.4 Homogeneous Sections .............................................................................. 17
5.4.1 DCP and Test Pit Investigations .......................................................................... 20
5.5 Use of DCP data for remedial work ............................................................ 20
5.5.1 Test Pits .............................................................................................................. 21
6 Calculation of Structural Number ............................................................................ 23
6.1 Definitions .................................................................................................... 23
6.1.1 variation of bituminous layer coefficient with temperature ................................... 24
6.2 Use of Structural Number for Overlay Design ............................................ 25
6.3 Use of the FWD to estimate SNPExisting ................................................... 26
6.4 Overlay Design Procedure using the FWD ................................................. 27
6.4.1 SNP for Future Traffic (SNPDesign) .................................................................... 27
6.4.2 Structural Deficiency ............................................................................................ 28
6.4.3 Designing thick overlays ...................................................................................... 30
6.5 Overlay Design Procedure using the DCP ................................................. 32
7 Remedial Works Prior to Overlay ........................................................................... 34
8 References .............................................................................................................. 35
9 Appendices ............................................................................................................. 36
9.1 Appendix 1 : DCP Test ............................................................................... 36
9.1.1 Description .......................................................................................................... 36
9.1.2 Operation ........................................................................................................... 36
9.1.3 Interpretation of results ....................................................................................... 37
9.1.4 Calculation of Structural Number ........................................................................ 39
9.2 Test Pit ........................................................................................................ 41
9.2.1 Labour, equipment and materials ........................................................................ 41
9.2.2 Sampling and testing procedure .......................................................................... 41
9.2.2.1 Field Procedure ................................................................................ 42
9.2.2.2 Laboratory procedure ....................................................................... 43
PART 4 – Overlay Design
The purpose of this Manual is to update the document of the same title produced for the
Ministry of Works, Roads Department in May 1988. The Manual recommends a practical
• design asphalt overlays; and
• audit overlay designs submitted by Consultants for major projects.
The procedure is based upon the AASHTO Design Guide (1993) which uses the concept of
Structural Number (SN) to establish the thickness of the overlay. The procedure uses a
correlation between Falling Weight Deflectometer (FWD) deflection measurements and the
Adjusted Structural Number (SNP) of the existing pavement. This correlation must be
calibrated for Kenya conditions.
Overlay design thickness is based on the equation:
( ) ] 4 . 25
SNP mm ckness Overlaythi
SNPDesign = Structural Number for future traffic
SNPExisting = Structural Number of the existing road
= Layer coefficient of asphalt overlay
25.4 = conversion mm to inches
SNPDesign values are determined by the AASHTO (1993) design equation.
SNPExisting values are based on FWD deflection measurements.
The use of the FWD allows designs to be completed quickly and at relatively low cost. In
common with all overlay design procedures the method described in this must be critically
reviewed and adjusted according to local experience.
The Manual also provides guidance on a method of designing overlays using the Dynamic
Cone Penetrometer (DCP), when FWD results are not available.
Republic of Kenya - Ministry of Roads 1
Draft Document – September 2009
Adjusted structural number A numerical indicator of the overall strength of the pavement
layers including the subgrade. It consists of a summation of
the product of the thickness (in inches), layer coefficient and
drainage coefficient (if applicable) of each of the pavement
layers plus a contribution from the subgrade. It is independent
of where the boundary layer of the subgrade is selected.
Asphalt A generic term for any mixture of bitumen, filler and
aggregate. This includes asphalt concrete.
Asphalt concrete A mixture of bitumen, filler and crushed stone aggregate
proportioned to meet specific strength, deformation and
volumetric criteria related to the Marshall test method for
Base course A pavement layer lying between the surfacing and the sub-
base. This can be constructed from asphalt, granular or
stabilised material.
Binder course The lower bituminous course of the pavement, usually asphalt
concrete. It is not always present ie the wearing course may
rest directly on the base course.
California Bearing Ratio This is the standard test for characterising subgrade material
and some granular layers (test method AASHTO T193).
Dynamic Cone Penetrometer This is a portable, hand-operated, percussive penetrometer
for rapidly assessing the strength of subgrade and other
granular layers, on site. The results can be converted to CBR
Empirical A method of engineering design based on observation of the
performance of structures. New designs are extrapolated or
interpolated from the observations without necessarily
reverting to the calculated stresses and strains in the road
ESAL Equivalent Standard Axle Load
Equivalent Standard Axle This is the standard unit of measurement of the
damaging effect of traffic.
Falling Weight Deflectometer A road testing device that generates a pulse load on the road
surface and measures the peak vertical deflection at the
centre of the loading plate and at several radial positions by a
series of sensors.
Layer coefficient A number (a
value) to indicate the strength of asphalt, base
course or unbound sub-base layers when calculating the
structural number of a road pavement.
Maintenance measures undertaken to preserve the pavement, consisting
 routine: eg grass cutting, ditch & culvert cleaning
 recurrent: eg patching, pothole-filling, crack-sealing
 periodic: eg re-sealing road, re-gravelling shoulders
Republic of Kenya - Ministry of Roads 2
 urgent: eg debris removal, erecting warnings
Mechanistic Method of engineering design based on mathematical models
of material behaviour and determination of stresses and
strains within the structure.
Modified Structural Number A numerical indicator of the overall strength of the pavement
layers plus a contribution from the subgrade
Overlay A strengthening layer of either granular or asphalt placed on
top of an existing road to strengthen the road.
Rehabilitation measures undertaken to increase significantly the functional
life of a road pavement
Reliability The reliability of a pavement design is the probability that the
pavement section will perform satisfactorily for the traffic and
environmental conditions over the design period.
Regulating course A layer of material, usually asphalt concrete, placed on an
irregular or unsatisfactory road surface primarily to achieve a
substantially smoother surface or a changed surface profile.
Its thickness will be variable and is typically used when
overlaying existing pavements which have ruts. The
maximum particle size may be fairly small as this material is
sometimes laid to less than 20mm thickness.
Selected subgrade Imported, good quality soil or rock fill material, which is placed
at the top of the subgrade. Its purpose is to increase the
strength and stiffness of low strength in-situ material and thus
Serviceability The serviceability of a pavement is its ability to serve the type
of traffic using the pavement.
SNC Modified Structural Number
SNP Adjusted Structural Number
Sub-base A medium quality granular layer resting on the subgrade and
supporting the base course.
Subgrade All the material below the sub-base. It may consist of in-situ
material, ordinary fill or “selected subgrade “. (Subgrade has
the same meaning as the AASHTO term “roadbed”.)
Surfacing The layer(s) of asphalt or surface dressing forming the surface
of the pavement. If constructed of asphalt it may include a
wearing course and an optional binder course.
Structural Number A numerical indicator of the overall strength of the pavement
layers. It consists of a summation of the product of the
thickness (in inches), layer coefficient and drainage coefficient
(if applicable) of each of the pavement layers above the
Structural deficiency The difference between the required strength of a road to be
overlaid and its existing strength. Recorded in units of
Terminal serviceability index This is the index of the lowest serviceability that will be
tolerated by the road users, before rehabilitation, resurfacing
Republic of Kenya - Ministry of Roads 3
or reconstruction becomes necessary. The value will depend
on the status of the road and generally lies between 3 and 2.
Wearing course The uppermost bituminous course of the pavement, usually
asphalt concrete. The top surface of this layer should provide
a smooth surface but with adequate texture to provide
adequate friction for safe vehicle braking and turning. See
also surfacing
WMAAT Weighted Monthly Average Annual Temperature.
Republic of Kenya - Ministry of Roads 4
Roads in Kenya vary widely in their geometric standard and the traffic they carry. They have
been constructed and maintained over a period of years; indeed, many have ‘evolved’
rather than been designed and constructed by a formalized process. The range in
topography and traffic loading results in roads having a wide range of construction thickness
and strength. However, the common theme is that they have a granular road base and
either a relatively thin asphalt concrete or surface dressing surfacing. The road network,
both in flat and hilly terrain, is also criss-crossed with patches and utility trenches. These
often contribute to road deterioration through poor reinstatement.
The present overlay practice is either to mill the existing surface and overlay with 40-50mm
(periodic maintenance) of asphalt, or to apply a new surface dressing, or to engage
consultants to carry out the overlay design for major projects. The overlay designs
submitted by Consultants are generally based on the methods described in ‘Design of
Pavement Structures’ (AASHTO, 1993).
The proposed empirical overlay design method, described in this , is also based upon the
AASHTO recommendations (1993) and uses the concept of Structural Number (SN) to
establish the thickness of the overlay. The design process is illustrated in Fig 3.1.
The procedure uses a relationship to convert FWD deflection measurements to the
Adjusted Structural Number (SNP) of the existing pavement, allowing designs to be
completed quickly and at relatively low cost.
In common with all overlay design procedures the method described in this recommends a
method to formulate designs which must be reviewed by the Engineer and adjusted based
on his/her own local experience.
The design process envisages the following two levels of survey:
• Network level surveys, consisting of roughness and visual condition, carried out to
demarcate road sections of equivalent condition, followed by:
• Project level surveys, more detailed in scope, consisting of visual condition, FWD,
DCP and Test Pit investigations, carried out to determine the level of maintenance
Republic of Kenya - Ministry of Roads 5
Figure 3.1: Design Process
Republic of Kenya - Ministry of Roads 6
Results from Network VCS and Roughness Survey
identifies sections of road for rehabilitation
Carry out non
Project Level VCS
FWD deflections
Traffic count (where necessary)
Axle load survey (where necessary)
Identify homogeneous sections of road
using FWD deflection (do)
Plan and carry out destructive tests
(DCP and Test Pits)
Establish Adjusted Structural Number
at each FWD point
Establish required Design Structural Number
for future traffic
Design thickness of strengthening overlay
for each homogenous section
Correct Adjusted Structural Number
Establish BoQ of
remedial works from
Calculate Structural Deficiency from Existing
4 NETWORK LEVEL EVALUATION
The network level evaluation categorises road sections into the following :
• those where only minimal routine or periodic maintenance needed
• those where major treatment, such as reconstruction needed, and
• those of intermediate condition where further project-level investigation is needed to
decide what measures to take.
The objectives are to identify the type and severity of the distress in a quantitative manner
in order to estimate maintenance interventions and also to enable the function of
performance modelling tools (eg HDM) if required.
The inspection is implemented by examining the condition of road cross sections at discreet
intervals, or samples. The condition is evaluated at each sample and combined with a
qualitative assessment of the interval between each sample.
The inspection should be undertaken by a trained engineer who also has knowledge of the
software system that he will use to process the data recorded.
Each sample is subdivided into a number of sub-samples and the distress in each sub-
sample is recorded. The severity of the distress is estimated as the proportion of the total
number of sub-samples affected.
The total number of sub-samples influences the survey precision. The number of samples
influences the reliability of the survey. The level of detail required is governed by the
purpose for the data and the resources available to do the work. For network surveys,
sample points could be spaced at up to 1km spacings, each with 2 sub-sample points. For
project surveys, sample points would be more frequent (from 0.01km to 0.1km spacings),
each sample point having 4 sub-sample points.
Table 4.1 gives details of the assessment criteria, Table 4.2 the roughness values to be
expected, Table 4.3 the recommended threshold values for all the assessment criteria and
Table 4.4 lists the risks associated with these partial surveys and recommended follow-up
work. Table 4.5 is a recommended field form.
For 2-lane roads ( ≥ 5.5m), the defects will be assessed in 4 transverse strips
corresponding to each wheel path, covering the full width of the pavement. For road widths
< 5.5 m wide, such that the inner wheel paths overlap leading to 3 rather than 4 wheelpaths,
the assessment shall be carried out over three strips corresponding to the wheelpaths. The
centre wheel path ratings shall be allocated to both strips 2 and 3.
Table 4.1: Assessment criteria for Visual Condition Survey
Wide cracks > 2m 0, 1, 2, 3 or 4 depending on
number of strips with this defect
Depressions with cracks 0, 1, 2, 3 or 4 depending on
Republic of Kenya - Ministry of Roads 7
Rutting (Visible, >10mm) 0, 1, 2, 3 or 4 depending on
Edge failures 0, 1 or 2 depending on number of
edges with this defect
Shallow potholes - No base
(Include shallow local failures on
the basis of 1m
= 1 pothole )
Number per sampling interval
Deep Potholes - Base exposed
“Shiny” Surface 0, 1 or 2 depending on number of
(Surveyor’s estimate of the required
maintenance for the sample length)
NB. Not all the indicated defects
need to be present to qualify for a
1 Routine: No depressions, only a
few cracks and shallow potholes,
very minor rutting.
2 Thin Overlays: Slight depressions,
some shallow potholes, some
cracks, slight rutting.
3 Thick overlays: Major
depressions with cracks, some deep
potholes, wide cracks and
significant rutting.
4 Reconstruction: Broken up
pavement areas, deep potholes,
depressions with cracks and
substantial rutting.
The results of the survey can be evaluated according to Table 4.3 and this will enable the
road sections to be categorised.
4.2 Roughness Condition Data
Roughness is normally measured using a Bump Integrator and expressed through the
International Roughness Index (IRI), in m/km. Typical values of the IRI with reference to the
type and condition of the road are indicated in Fig 4.2.
Table 4.2: Roughness criteria
Republic of Kenya - Ministry of Roads 8
IRI Ranges Road Condition
Lower than 6 very good
6 to 11 good
11 to 15 fair
15 to 19 poor
Larger than 19 very poor
Republic of Kenya - Ministry of Roads 9
Table 4.3: Proposed Analysis of Network Visual Condition Survey Data
(Patching and crack sealing)
Thin Overlays ≤ 40mm Thick overlays Reconstruction
Purpose • Local repairs and sealing • Restore surface
• Prevent moisture entry
• Restore transverse
shape ( + reg. Layer)
• Reduce roughness
• Replace excessively
weakened and distorted
AC and Base
Small number of local failures or
(Strength, Ride and Surface OK)
Some local failures, minor
rutting, cracked or poor surface.
(Strength OK)
More frequent failures and
depressions, some weakness in
AC and/or Base,
Frequent and severe failures and
deformation, general weakness in
AC and/or Base.
Road Class Int. P + S Local Int. P + S Local Int. P + S Local Int. P + S Local
Roughness (IRI
m/km)
≤ 4 ≤ 5 ≤ 6 ≤ 4 ≤ 5 ≤ 6 4 to 6 5 to 7 6 to 8 > 6 > 7 > 8
Wide cracks 0 0 1 1 1 2 2 2 3 ≥ 3 ≥ 3 4
Depressions with
≤ 1 ≤ 1 ≤ 2 ≤ 1 ≤ 1 ≤ 2 ≥ 2 ≥ 2 ≤ 3 > 3 > 3 4
Rutting ≤ 1 ≤ 1 ≤ 2 ≤ 1 ≤ 1 ≤ 2 ≥ 2 ≥ 2 ≤ 3 > 3 > 3 > 3
Edge Failures ≤ 1 ≤ 2 ≤ 2 ≤ 1 ≤ 2 ≤ 2 ≤ 1 ≤ 2 ≤ 2 ≤ 1 ≤ 2 ≤ 2
(No. per km)
≤ 20 ≤ 20 ≤ 30 ≤ 30 ≤ 30 ≤ 40 > 30 > 30 >40 > 30 > 30 >40
≤ 10 ≤ 10 ≤ 20 ≤ 15 ≤ 15 ≤ 30 ≤ 20 ≤ 20 >30 > 20 > 20 >30
“Shiny” Surface ≤ 1 ≤ 1 ≤ 2 > 1 > 1 > 2 > 1 > 1 > 2
> 1 > 1 > 2
General Condition Yes/No Yes/No Yes/No Yes/No
Republic of Kenya - Ministry of Roads 10 Draft Document – September
Table 4.4: Risks involved and action required on completion of Network Surveys
Thin Overlays ≤ 40mm (or
Surface Dressing or
Microsurfacings)
Thick overlays Reconstruction
Risks of Fast-
Track VCS
More serious deterioration
between sample lengths may be
missed. There is a need for
condition checks of some of the
non-sample lengths to confirm
May over or under-estimate
deterioration but this will be
corrected during follow-up
Detailed VCS.
May exaggerate deterioration.
There is a need for condition
checks of some of the non-
sample lengths before
proceeding with FWD
Patching Works Records only Essential:
• Detailed VCS
(100% in 5m sample lengths)
FWD + Cores + DCP + Test pits
Detailed VCS (100%
in 5m sample lengths)
FWD @ 50m, staggered L + R,
outer wheel paths.
Axle weight survey
• Cores + DCP + Test pits at
frequencies and locations to
suit FWD d1 values.
• FWD @ 100m, staggered
L + R, outer wheel
• DCP@ 200m
• Axle weight survey
Republic of Kenya - Ministry of Roads 11 Draft Document – September
Region:…………………………………. Road Name:……………………………. Road ID:…………… Sampling Interval:……..
…m Direction:………………………. Single / Dual Width:…………. m Date:……………….. Form Start Time:
Start of Survey: 1 2 3 4 5 6 7 8 9 10
Wide cracks 0 1 2 3 4
Rutting 0 1 2 3 4
Edge Failures 0 1 2
(No. per sample i’val)
Deep Potholes (No.
per sample interval)
“Shiny” Surface 0 1 2 3 4
General Condition 1 2 3 4
Remaining interval
Similar = 1
Surveyed by – Name:…………………………….. Signed:…………..………………..
Republic of Kenya - Ministry of Roads 12 Draft Document – September
5 PROJECT LEVEL EVALUATION
5.1 Detailed Visual Condition Survey
The project Visual Condition Survey (VCS) is more detailed than the network survey,
covering the whole of the road. It is carried out during on foot, each sample length (5, 10 or
20 metre) of the road being examined to identify defects in the wheelpaths. Notes on the
collection of the defects are presented in Table 5.1.
Table 5.1: Explanation of Defects
Defect Unit Notes
Wide single cracks m Cracks (wider than 3mm) to be sealed, in m
Wide connected cracks m Cracks wider than 3mm, separating pavement into
blocks, to be sealed, in m
Alligator cracks, no
Cracks separating pavement into small pieces, but no
depression or rut
Alligator cracks,
As above, with associated depression
Deep Potholes N Potholes that penetrate through base
Structural rutting m
>10mm depth, originating in base
Edge failure m Loss of pavement surface >50mm
Trench/Patch Failure m
Rutted (>10mm) or broken-up patch
Shallow potholes N Potholes that occur just in surfacing
Asphalt shoving N Pushing-up of asphalt surfacing
Surface rutting m
>10mm depth but just in surfacing
Slippage cracks N Adhesion failure of asphalt surfacing to base
Neither the condition of the road shoulders, nor of the drainage are covered in this type of
survey, which refers to bituminous-surfaced roads only.
Any one defect should only be counted once: for example, ‘rutting with cracks should be
counted as “Depressions with cracks” only and not also recorded as “Rutting”. The data
from the VCS is transferred to a spreadsheet that automatically calculates the Bill of
Quantities for the remedial work prior to overlay. Quantities for crack sealing should be
adjusted to an area not affected by previous deep or surface patching.
Prior to overlay a number of these defects will need remedial work. The survey enables the
quantity of materials required to be estimated.
Republic of Kenya - Ministry of Roads 13
Republic of Kenya - Ministry of Roads 14 Draft Document – September
TABLE: PAVEMENT CONDITION SURVEY (Project Level)
:ةقطننملا
:هاجنتلا
Sta 0+000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Unit
Wide Single cracks l.m 0 0
l.m 0 0
sq.m 0 0
Alligator Cracks with
Base Shoving sq.m 0 0
Deep Potholes sq.m 0 0
Structural Rutting sq.m 0 0
Edge Failure sq.m 0 0
Trench / Patch Failure sq.m 0 0
Shallow Potholes sq.m 0 0
Asphalt Shoving sq.m 0 0
Surface Rutting sq.m 0 0
Slippage Cracks sq.m 0 0
“Shiny” Surface sq.m 0 0
:قنيرطلا مسإ
:ةنينننيعلا ةفاسملا
:قنيرطلا مقر
قنيرطلا ضرع
5.2 Falling Weight Deflectometer (FWD) Survey
On 2 lane single carriageway roads FWD tests should be carried in both lanes (ie both
directions) in the outer wheel-path (closest to the shoulder of the road). The location of the
FWD tests should be ‘staggered’ to allow for maximum coverage. For multi-lane dual
carriageways FWD measurements should be carried out, as a minimum, in the outer wheelpath
of the heaviest loaded lane. In addition, tests should be carried out in other lanes where the
condition of the lane is worse than the heaviest loaded lane.
On Class A, B and C roads, the tests should be carried out at 50 metre intervals. On 2 lane
single carriageway roads the location of the tests should be ‘staggered’ by 25 metres so as to
result in an FWD test every 25 metres along the road.
On Class D and E roads, the tests should be carried out at 100 metre intervals. On 2 lane
single carriageway roads the location of the tests should be ‘staggered’ by 50 metres so as to
result in an FWD test every 50 metres along the road.
The FWD tests should be normalised to a standard load of 50 KN.
5.3 Traffic Estimation
5.3.1 CLASSIFIED TRAFFIC COUNTS AND AXLE LOADING
Reference is made to the Design for New Bituminous, Gravel and Concrete Roads for
procedures to undertake traffic and axle load surveys. Fig 6.1 summarises the steps described.
Figure 5.2: Steps for carrying out Traffic and Axle Load Surveys
Republic of Kenya - Ministry of Roads 15
Determine Initial Traffic
Volume (Initial AADT) per
Determine Cumulative
Traffic Volumes over the
Estimate Mean Equivalent
Axle Load (ESA) per Class
Estimate Cumulative ESAs
over the Design Period (in
The individual weights of each axle of a particular vehicle class are converted to ESA, which
are added to produce a total for the vehicle. It is usual to determine the average ESA for each
vehicle class based on the results of an axle load survey, allowing for the proportions of loaded
and unloaded vehicles in each class. (These may vary in each direction and between routes.)
It is essential that the individual axle weights are converted to ESA before any aggregation or
averaging of data is carried out for either the individual vehicle or for all the weighed vehicles of
After the total ESA for each vehicle have been calculated, the average value of ESA is
calculated for the whole vehicle class. These average values of ESA are sometimes termed
“vehicle wear factors” or “vehicle damage factors”.
The total contribution to pavement loading of a vehicle class is the product of the vehicle
damage factor of the vehicle class and the number of such vehicles recorded on the road either
on a daily or annual basis. The process is repeated for the other classes and the total loading
per unit of time is determined by summation.
5.3.2 CONVERSION TO DESIGN TRAFFIC LOADING
The pavement loading calculated above must be summed for the whole of the design period
(normally 10 to 15 years), adjusted for annual traffic growth. The estimate of annual traffic
growth is usually based on historic trends and affected by predictions of future economic
activity, but normally increases vary between 2 and 7 per cent per annum. In some instances
there is the possibility of a sudden increase (or reduction) of traffic, when for example a new
factory, quarry or port comes into operation.
The Design Traffic Loading in the performance period is calculated from Equation 1:
Equation 1: Calculation of design traffic loading
ESAL ESALs
N = Performance Period in years
ri = Growth rate (%)
ESAL1 = Daily number of ESA in the first year in traffic class ‘i’
5.3.3 EFFECT OF ROAD GEOMETRY
The vehicle damage to the road pavement is influenced by the road geometry. On narrow
single carriageway roads the wheelpaths can overlap in the centre of the road, causing more
damage. On multi lane dual carriageways medium and heavy traffic can use other lanes
besides the outer lane (most heavily trafficked lane). The criteria presented in Table 6.1 can be
used to calculate the design traffic.
Table 5.6 Effect of road geometry on design traffic loading
Carriagway
Calculation of ESALs
Single 2 <6.7 80% of the ESALs in both
directions is used in order to
allow for overlap on the
Republic of Kenya - Ministry of Roads 16
central section of the road.
Single 2 > 6.7 The ESALs in the most
heavily trafficked direction is
Dual 2 - With less than 2000
in one direction, 90% of the
ESALs in one direction is
Dual 2 - With more than 2000
in one direction, a special
study shall be carried out to
establish the distribution of
Dual > 2 - A special study shall be
carried out to establish the
distribution of commercial
5.4 Homogeneous Sections
Results from the Network Visual Condition and Roughness Surveys will identify lengths of road
in need of rehabilitation. Those lengths of road that require Detailed Design (see Fig 6.1) need
further investigation with the FWD or other equipment as described below, as part of the
rehabilitation design process.
Figure 5.3: Maintenance Thresholds
All roads vary in pavement thickness and strength along their length. For instance, the strength
of the underlying subgrade will vary along the road alignment as the road passes from areas of
cut to fill. Rehabilitation measures cannot be tailored to each and every variation in road
characteristic, so to produce cost-effective designs the road should be divided into lengths
Republic of Kenya - Ministry of Roads 17
Surfacing Integrity & Texture
IRI ≤ Min
IRI ≥ Max
Min<IRI <Max
Cracked &
0 to .3
Patch, Seal Cracks
Patch, Seal Cracks, Grinding
Patch, Seal Cracks, Surface Dressing [SD]
Patch, Seal Cracks, Inlay, Double SD
Patch, Seal Cracks, Mill & Replace
Patch, Seal Cracks, Thin Overlay
Patch, Mill & Overlay[Detailed Design]
Patch, Mill & Strengthen[Detailed Design]
Repair, Mill & Strengthen[Detailed Design]
Reclaim Base & Repave[Detailed Design]
Reconstruction & Improvement
where the strength properties are similar, known as homogeneous lengths. Each
homogeneous length is then treated as a separate overlay design exercise. This will result in
reduced costs as the overlay thickness changes, reflecting the existing strength of each
homogeneous section.
This procedure is best carried out by using the Cumulative Sum Method (CUSUM) on FWD
central deflection measurements (do). The method involves plotting the cumulative sum of the
differences of the FWD deflection from the mean FWD value calculated from all the results.
The calculations are based on Equation 2 and a worked example is shown in Table 6.1:
Equation 2: CUSUM calculation
FWDmean = Mean FWD deflection of the road
FWDi = FWD deflection at chainage i
Si = Cumulative sum of the deviations from the mean deflection
Table 5.7: Cusum Calculations on FWD central deflections (d0)
Republic of Kenya - Ministry of Roads 18
Chainage FWD D0 - Mean Cusum
(m) D0
0 0.381 -0.061 -0.061
50 0.407 -0.035 -0.096
100 0.313 -0.129 -0.225
150 0.404 -0.038 -0.263
200 0.261 -0.181 -0.444
250 0.314 -0.128 -0.572
300 0.305 -0.137 -0.709
350 0.301 -0.141 -0.850
400 0.308 -0.134 -0.984
450 0.435 -0.007 -0.990
500 0.261 -0.181 -1.172
550 0.215 -0.227 -1.398
600 0.261 -0.181 -1.580
650 0.166 -0.276 -1.856
700 0.482 0.041 -1.815
750 0.769 0.327 -1.488
800 0.366 -0.076 -1.564
850 0.247 -0.195 -1.759
900 0.366 -0.076 -1.835
950 0.228 -0.214 -2.049
1000 0.313 -0.129 -2.178
1050 0.273 -0.169 -2.346
1100 0.245 -0.197 -2.543
1150 0.318 -0.124 -2.667
1200 0.304 -0.138 -2.805
1250 0.483 0.041 -2.764
1300 0.559 0.117 -2.647
1350 0.665 0.223 -2.424
1400 1.003 0.561 -1.863
1450 0.559 0.117 -1.747
1500 0.769 0.327 -1.420
1550 0.665 0.223 -1.196
1600 0.559 0.117 -1.080
1650 0.769 0.327 -0.753
1700 0.462 0.020 -0.733
1750 0.467 0.025 -0.708
1800 0.467 0.025 -0.684
1850 0.462 0.020 -0.664
1900 0.479 0.037 -0.627
1950 0.665 0.223 -0.403
2000 0.559 0.117 -0.287
2050 0.404 -0.038 -0.325
2100 0.476 0.034 -0.291
2150 0.559 0.117 -0.174
2200 0.462 0.020 -0.155
2250 0.467 0.025 -0.130
2300 0.435 -0.007 -0.136
2350 0.559 0.117 -0.020
2400 0.462 0.020 0.000
The FWD d0 deflection values and CUSUM plot are given in Fig 6.2 and Fig 6.3 respectively.
Error: Reference source not foundA change in slope of the graph indicates a change in
strength along the road. In Fig 6.3 five distinct homogeneous sections can be identified. These
sections should be treated as separate overlay designs.
Figure 5.4: FWD d0 deflection values
Republic of Kenya - Ministry of Roads 19
Figure 5.5: CUSUM plot showing homogeneous sections
5.4.1 DCP AND TEST PIT INVESTIGATIONS
Destructive testing may be needed after the non-destructive testing is completed to establish
the thickness and strength of the existing pavement layers and relate these to the road failure.
Two methods are available, either the Dynamic Cone Penetrometer (DCP) or Test Pits. Details
of these field methods are presented in Appendices respectively
DCP tests are relatively quick and therefore should be used where there is no risk of damaging
any utilities in the road pavement. The results from DCP tests are particularly useful in
identifying areas of weak base course and sub-base layers which will need deep patching
required prior to overlay.
Test pits are best used when the road is to be partially or fully reconstructed. In this case
laboratory tests are carried out on the samples collected from the various granular layers in the
road to establish whether they can be used in the reconstruction process.
5.5 Use of DCP data for remedial work
DCP tests should be carried out at points in the road where the Detailed Visual Condition
Survey and FWD deflection profile show the road to be abnormally weak. In Fig 6.4 the FWD
test are high at chainages 750 and 1400 metres. DCP would be carried out the outside
Republic of Kenya - Ministry of Roads 20
wheelpath at these chainages to establish the cause of the weakness. Prior to testing a
detector should be used to ensure there are no utilities beneath the test location.
Figure 5.6: FWD deflections and points for DCP testing
The DCP is driven through the road pavement under a standard force to a maximum depth of
approximately 800mm. The strength of the layers is related to their resistance to penetration,
measured as mm per blow, and there are correlations to convert the DCP values to in-situ
values of CBR. The thickness of the road layers are identified by the changes in mm/blow as
the apparatus penetrates the pavement layers.
Where the in-situ CBR of the granular base course and sub-base are below 80% and 30%
respectively (as measured from the DCP), the base course and sub-base (if necessary) shall
be deep patched. Sometimes it is difficult to differentiate between the base and subbase and
test pits may be necessary as a last resort to determine the layer interval.
5.5.1 TEST PITS
Where the FWD results indicate that the road should have a thick overlay or be either partially
or fully reconstructed then test pits will be needed. If the road is to be overlaid then the pits
should be dug in areas where the FWD shows the road to be weak.
If the road is to be partially or fully reconstructed the Test Pits should be dug at regular intervals
where the road is weak. Two test pits would normally be dug in every one kilometre of road.
The test pit data are used to determine the reasons for the weaknesses identified from the
FWD investigation, which could include:
 whether the existing granular base course and sub-base meet normally acceptable
material standards for partial or full reconstruction.
standards for thickness for the appropriate road class.
 confirmation of the pavement layers identified during DCP analysis.
 to enable mechanistic analysis of FWD measurements
Test Pits will be dug at points in the road where the Detailed VCS and FWD deflection profile
show the road to abnormally weak. The measurements and tests required are listed in Table
Republic of Kenya - Ministry of Roads 21
The results of these tests should be compared to standard material specifications, listed in the
Design for New Bituminous, Gravel and Concrete Roads. Where the road base and sub-base
material do not meet these specifications the length of road affected should be deep patched.
Pavement material Test Description Test
Field Asphalt surfacing
Sub base/Selected Subgrade
Moisture Content KS 999 Part 2 2001
Layer density KS 999 Part 9 2001
Laboratory Road base
KS 999 Part 2 2001
KS 999 Part 4 2001
Republic of Kenya - Ministry of Roads 22
6 CALCULATION OF STRUCTURAL NUMBER
The Structural Number approach is probably the most reliable method of evaluating the
‘strength’ of pavements of similar type in terms of their likely traffic carrying capacity. It is
Equation 3: Definition of Structural Number
h a SN 0394 . 0
ai = Layer coefficient of layer i
hi= Thickness of layer i (mm)
The calculation of layer coefficients for existing pavement layers is based on the stiffness of
bituminous materials and the CBR of granular materials. They are indicated in Tables 6.1 and
Table 6.9: Layer Coefficients for Existing Asphaltic Concrete and Granular Materials
MATERIAL SURFACE CONDITION COEFFICIENT, ai
AC Surface Little or no alligator cracking and/or only low-severity
<10 percent low-severity alligator cracking and/or
<5 percent medium- and high-severity transverse cracking
>10 percent low-severity alligator cracking and/or
<10 percent medium-severity alligator cracking and/or
>5-10 percent medium- and high-severity transverse cracking
>10 percent medium-severity alligator cracking and/or
>10 percent medium- and high-severity transverse cracking
>10 percent high-severity alligator cracking and/or
>10 percent high-severity transverse cracking
Roadbase or
No pumping, degradation, or contamination by fines.
Some pumping, degradation, or contamination by fines.
Table 6.10: Layer Coefficients for Existing Stabilised Road Bases
Little or no alligator cracking and/or only low-severity
Republic of Kenya - Ministry of Roads 23
<10 percent high-severity alligator cracking and/or
The Structural Number was developed during the AASHO Road Test, which considered the
performance of trial sections constructed over a uniform subgrade having a particular strength.
A further parameter, the Modified Structural Number (SNC) (Hodges et al, 1975), was later
developed to take into account different subgrade strengths. This relationship is defined in
Equation 4: Definition of Modified Structural Number
SN SNSG SNC + ·
SNSG = Structural Number contribution from the subgrade= 3.51 Log10 (CBR) – 0.85 (Log10
(CBR))
SNC = Modified Structural Number
CBR = In situ CBR of the subgrade.
6.1.1 VARIATION OF BITUMINOUS LAYER COEFFICIENT WITH TEMPERATURE
The AASHO Road Test was carried out in Illinois, USA. The layer coefficient taken for a new
asphalt concrete surfacing during the Road Test was 0.44. This was for asphalt concrete
having an elastic modulus of 3100 MPa at a temperature of 20
C. It is therefore necessary to
derive a strength coefficient suitable for Kenya, where the ambient temperatures, and hence
road temperatures, are different to those in Illinois. This is obtained by calculating the effective
elastic modulus of asphalt concrete using the Shell Method of Weighted Monthly Average
Annual Temperature (WMAAT) (Shell, 1978), shown below. The analysis shows that the layer
coefficient of asphalt concrete used in Kenya should be:
Altitude 0 – 600 metres = 0.38 (WMAAT=22.5
Altitude 600 – 1200 metres = 0.40 (WMAAT=19.6C)
Altitude > 1200 metres = 0.44 (WMAAT) = 11.3C
The analysis is shown below:
Calculate equivalent modulus at the AASHO Road Test site (WMAAT = 15
C) of asphalt
concrete having an elastic modulus of 3100MPa at 20
C tested in laboratory, using the
Equation 5: Variation of Elastic Modulus with temperature
where b = 0.024 and T1, T2 are two asphalt temperatures.
• ET=15 = 3100*10
-0.024(15-20)
= 4086 MPa
Republic of Kenya - Ministry of Roads 24
Calculate the elastic modulus of similar material, for instance, in the Coastal Region (WMAAT =
C) to that in Illinois (WMAAT = 15
• ET=22 = 4086*10
-0.024(22.5-15)
= 2700 MPa
Calculate layer coefficient of asphalt concrete in the Coastal Region having an elastic modulus
of 2700MPa:
• aT1/aT2 = (ET1/ET2)
aT1 and ET1 are the layer coefficient and elastic modulus respectively at temperature T1.
• aT=22/aT=15 = (2700/4086)
• aT=22 = 0.44*0.87 = 0.38
6.2 Use of Structural Number for Overlay Design
The overlay thickness is derived from:
Equation 6: Derivation of overlay thickness from Structural Number
( ) [ ] 4 , 25 * / ,
a SNP SNP mm ckness Overlaythi
SNPExisting = Structural Number of existing road
a1 = Layer coefficient of asphalt overlay
Therefore to calculate the thickness of required overlay, the Structural Number of the existing
road (SNExisting) has to be measured. There are a number of ways of doing this, all of which have
various advantages and disadvantages, as enumerated in Table 6.3.
Table 6.11: Advantages and Disadvantages of Investigative methods
Method Procedure to calculate
SNExisting
Test Pits Direct calculation from
(laboratory) of the
different pavement
Direct calculation from
estimated thickness and
in situ strength of the
Test Pits needed to
actual pavement layer
Back calculation DCP or Test Pits
Estimate of SNC from
FWD deflection bowl
Republic of Kenya - Ministry of Roads 25
The Structural Number and Modified Structural Number concept, whilst simple in principle,
gives rise to a number of practical difficulties, especially on roads that have been in existence
for many years. When DCP tests and Test Pits are carried out, the boundaries between the
different materials are sometimes indistinct and differentiating base courses from sub-bases,
and sub-bases from the subgrade can be difficult. Changes of strength are expected to occur
when passing from one layer to another but significant changes of strength also occur within
reasonably well-defined layers. When the same pavement is tested with a DCP a more
complex, many-layered structure is often revealed.
This can cause a problem in defining the layers in Test Pits for calculating the Modified
Structural Number. The same difficulty also applies when trying to define the appropriate layer
thickness for back-analysis of FWD data and often makes this form of analysis somewhat
A procedure is therefore required which takes account of the contribution to Structural Number
of a pavement from all the pavement layers and the contribution of the subgrade, which is
independent of where the subgrade boundary is defined. This value is called the Adjusted
Structural Number (SNP) (Rolt and Parkman, 2000).
6.3 Use of the FWD to estimate SNP
The most suitable tool to measure the Adjusted Structural Number of an existing road
(SNPExisting) is the DCP; its use to design overlays in Kenya is, however, often not ideal. This is
• it may not be practicable to take sufficient DCP measurements along each road to cope
with the possible high variability found in Kenya, and
• the coarse granular road base in the Kenya roads prevent the instrument’s penetration.
An overlay procedure based on DCP results is described in Section 6.6 for Secondary and
Local roads where FWD results are not available.
As FWD deflection data can be measured very quickly and accurately, the proposed overlay
procedure uses the data to estimate the SNPExisting of the existing road, rather than the DCP.
Previous work (Rolt, 2000) showed that the most effective form of the correlation between FWD
measurements and SNP takes the form below:
Equation 7: Correlation between SNP and FWD
* 760 . 1
) 8 . 0 *
( * 548 . 4 394 . 1
d0 = Central deflection (mm)
d900 = Deflection at 900mm from the load (mm)
d1200 = Deflection at 1200mm from the load (mm)
(FWD deflection is measured in mm at a load of 50KN)
Figure 6.7 : Correlation between SNP and Deflection
Republic of Kenya - Ministry of Roads 26
FWD Tests @ 50 KN (mm)
The equation above has been used to convert FWD measurements taken from hypothetical
Trial Sites. The predicted values of SNP are shown plotted against the central deflection d0 in
Fig 6.1. The limited scatter around the ‘line of best fit’ (R
= 0.96) shows the suitability of this
form of general relationship for the analysis of FWD results.
However, to enable the equation above to be used in for Kenya a series of comparative tests
between the FWD and the DCP must be carried out on a selection of Category A and B roads.
6.4 Overlay Design Procedure using the FWD
The required overlay thickness is calculated based on a comparison of the strength of the road
required for the future traffic and the existing strength of the road, as assessed by FWD
6.4.1 SNP FOR FUTURE TRAFFIC (SNP
The first step in the process is to establish the value of Structural Number (SNPDesign) that is
required for each homogeneous section of road for future traffic loading. This is achieved by
using the AASHTO (1993) equation for flexible pavements, shown below:
Equation 8: Computation of SNP Design
( ) 07 . 8 log 32 . 2
20 . 0 1 log 36 . 9 log
10 0 16 . 8 10
+ − + × + × ·
W8.16 = predicted number of 8.16 tonne ESALs,
ZR = Standard normal deviate for required reliability,
S0 = Combined standard error of the traffic and performance predictions - see below,
∆ PSI = drop in serviceability over the performance period,
MR = subgrade resilient modulus in psi,
SN = structural number to carry W8.16 ESALs.
Republic of Kenya - Ministry of Roads 27
The recommended Reliability factors and decrease in Pavement Serviceability Index (PSI)
used in the equation are shown in Table 6.4. The Standard Deviation is set at 0.49 as
recommended by AASHTO (1993). The calculated values of SNPdesign for various values of ESA
are presented in Table 6.5.
Table 6.12: AASHTO Design Criteria: Reliability factors and Servicability Indices
Road Class Reliability Standard
International 90 0.49 2.7 1.5
Primary 90 0.49 2.2 2.0
Secondary 85 0.49 2.0 2.2
Local 50 0.49 1.7 2.5
Table 6.13: Design SNP
Future Traffic (Million ESA)
Road Class <0.5 0.5–1 1-2 2-5 5-10 10-20 20-50
A - - - 5.68 6.25 6.84 7.67
B - - - 5.22 5.76 6.28 -
C 3.54 3.93 4.32 4.90 5.40 -
Local 2.93 3.25 3.57 4.05 4.45
To use the AASHTO design equation when the Adjusted Structural Number (SNP) is used
rather than SN and subgrade strength separately, the subgrade resilient modulus value must
be assumed at 4325 psi in the equation. This was the subgrade resilient modulus used in the
Road Test and therefore at this value the subgrade contribution is zero. Thus SN is then the
same as SNP. In using either method, the difference between SN and SNP needs to be
understood. In the normal AASHTO design method SN is used rather than SNP. The overlay
procedure described in this Manual uses SNP.
This results in the principle that if any two pavements have the same value of Adjusted
Structural Number (SNP) then they should carry the same level of traffic.
In the following paragraphs that describe the overlay procedure it has been assumed that the
road under investigation is a Category A road with a design traffic loading of between 5-10
million ESA. Therefore, from Table 6.5, the SNPDesign is 5.76.
6.4.2 STRUCTURAL DEFICIENCY
It is necessary to plot the ‘Structural Deficiency’, that is the difference between the required
design Structural Number of the road (SNPDesign) and the existing Structural Number at each
FWD test (SNPExisting), for each FWD test. This is simply :
Equation 9: Definition of Structural Deficiency
SNP SNP Deficiency Structural − ·
After calculation the Structural Deficiency is plotted as a bar chart, which allows the engineer to
identify the following actions for the homogeneous sections based on the criteria given in Table
Republic of Kenya - Ministry of Roads 28
Table 6.14: Structural Deficiency Criteria
Mean Structural
Zero or negative Maintain A thin overlay may be
correct other defects
0 to 0.6 Thin overlay Remedial works possible
0.6 to 1.5 Thick overlay
(40/50mm)
Remedial works probable
> 1.5 Reconstruction
Fig 6.2, the results of an actual FWD survey, illustrates these principles:
Figure 6.8: Structural Deficiency
Republic of Kenya - Ministry of Roads 29
Deficiency = -0.92
Deficiency = -0.95
Deficiency = +0.55
Deficiency = +1.45
No strengthening overlay
Thick strengthening overlay
plus patching
Thin strengthening overlay
No strengthening is required if the Structural Number Deficiency is either zero or
predominantly negative. Any occasional positive values should be investigated and deep
patched where necessary. If the road has been identified as having a poor profile (ie high
IRI value) a thin overlay can be constructed as periodic maintenance. The minimum
thickness of these thin overlays is governed by the aggregate grading of the overlay
material. Where the mix has a Maximum Stone Size of 25mm the overlay will need to be
50mm thick. Where the Maximum Stone Size is 19mm the material can be laid with a
minimum thickness of 40mm.
If the mean structural deficiency lies in the range ranges from 0 to 0.6 a thin overlay should
be constructed. Points with high structural deficiency should be investigated and deep
patched where necessary.
If the mean structural deficiency ranges from 0.6 to 1.5 then a thick overlay is necessary.
The need for some deep patching is also very likely to be required. The thickness design
procedure is described in Section 6.4.3.
The need for partial or full reconstruction is less easy to define, but becomes probable if the
structural deficiency is greater than 1.5. Under such circumstances the visual condition
data, DCP and test pit data needs to be re-assessed.
The design of roads that require reconstruction should be done in accordance with design
recommendations set out in the Design of New Bituminous, Gravel and Concrete Roads. In
general, roads with good foundations can be partially reconstructed by making use of much
of the existing material in the form of enhanced sub-base or even lower base course layers.
Roads which have a very weak or non uniform pavement structure and or sub-grade
require more elaborate remedial works and full reconstruction is possibly required.
6.4.3 DESIGNING THICK OVERLAYS
The final step in the process is to calculate the thickness of overlay for those homogeneous
section where a thick strengthening overlay (>50mm) is required.
Republic of Kenya - Ministry of Roads 30
The overlay at each FWD test is calculated using the equation below. Where no overlay is
required at an FWD test, a value of zero is assigned.
Equation 10: Calculation of Overlay Thickness
( ) ( ) 4 . 25 * 1 / a SNP SNP Dtestmm cknessatFW Overlaythi
Where a1 = layer coefficient for the asphalt overlay.
The overlay thickness for each homogeneous section is then calculated as follows:
Equation 11: Calculation of Overlay Thickness for Homogeneous Section
SD CF ythickness Meanoverla ssmm laythickne Designover * + ·
SD = Standard deviation of the overlay thickness in the homogeneous section
CF = Probability of achieving design life.
Values of CF that should be used for different levels of probability are given below:
Table 6.15: Values of 'CF'
Probability of Achieving Design Life CF Factor
85% 1.037
80% 0.841
75% 0.674
A value of 85% is usually recommended. The use of a higher level of probability can result in
overlays being too thick if the road construction is highly variable. In Fig 6.3 a value of 1.037
Figure 6.9: Design of Overlay Thickness from FWD data
Chainage (mm)
Thick Overlay 112mm
Republic of Kenya - Ministry of Roads 31
6.5 Overlay Design Procedure using the DCP
This procedure should only be used on Secondary and Local roads where FWD data is
unavailable and where the road structure allows the DCP to penetrate the road structure to
a depth of 800mm. The required overlay thickness is calculated based on a comparison of
the strength of the road required for the future traffic and the existing strength of the road,
as assessed by DCP measurements. The following steps should be followed:
1. Establish the value of Structural Number (SNPDesign) that is required for each
homogeneous section of road for future traffic loading. This is done using the AASHTO
(1993) equation in the same way as described in Sections 6.4.2 and 6.4.3. The
resultant values of SNPDesign are given in Table 6.5 for different levels of traffic for
Category B roads and Local roads.
2. Calculate the adjusted Structural Number of the existing road (SNPExisting) from DCP
tests. On Secondary and Local roads, the DCP tests should be carried out at 100 metre
intervals. The location of the tests should be ‘staggered’ by 50 metres to result in a DCP
test every 50 metres along the road. The DCP data shall be analysed in purpose
designed software called UKDCP. This software enables the user to analyse each DCP
test and then calculate the SNPExisting for each test.
3. Identify homogeneous sections of road for strengthening. Each length is treated as a
separate overlay design exercise. This procedure is best carried out by using the
Cumulative Sum Method (CUSUM) on the value SNPExisting calculated from each DCP
test. The homogenous sections are identified in the same way as is shown in Section
5.4. The UKDCP software allows the designer to identify the homogeneous sections
automatically and this process is described in the User for the software.
4. Calculate the ‘Structural Deficiency’ for each DCP test. The value of Structural
Deficiency is simply the difference between the required design Structural Number of
the road (SNPDesign) and the existing Adjusted Structural Number at each DCP test
(SNPExisting). After calculation the Structural Deficiency should be plotted as a bar chart
and the required actions are described above.
5. Calculate the thickness of overlay for those homogeneous section where a thick
strengthening overlay (>40/50mm) is required. The overlay at each DCP test is
calculated using the equation below. Where no overlay is required at a DCP test, a
value of zero is assigned.
Equation 12: Calculation of Overlay Thickness from DCP data
( ) ( ) 4 . 25 * 1 / a SNP SNP Ptestmm cknessatDC Overlaythi
Where a1 = layer coefficient for the asphalt overlay (See Section 6.1.1).
The overlay thickness for each homogeneous section is then calculated using the
Equation 13: Overlay Thickness for Homogeneous Section
Republic of Kenya - Ministry of Roads 32
CF = Probability of achieving design life
Values of CF that should be used for different levels of probability are given in Table 6.7.
Under most circumstances a value of 80% is recommended for Secondary roads and 75%
for Local roads. The use of a higher level of probability can result in overlays being too thick
if the road construction is highly variable.
Republic of Kenya - Ministry of Roads 33
7 REMEDIAL WORKS PRIOR TO OVERLAY
A bituminous overlay will only perform as designed if the correct remedial works are carried out
before overlay. Otherwise defects in the existing road will cause the new overlay to deteriorate
and premature failure will occur.
The type of remedial work will depend on the type of road defect and these are recorded during
the Detailed Visual Condition Survey. The remedial works are summarised in Table 7.1.
Table 7.16: Remedial Works prior to Overlay
Defect Remedial Works
Wide single cracks
Wide connected cracks
Alligator cracks with depressions
Base course shoving
Trench/Patch failure
Deep patch affected area
Alligator cracking without depressions
Shallow patch affected area
Asphalt shoving
Mill and patch affected areas prior to overlay
Note 1. Where the existing surface has a poor texture and polished stone, the top
surface should be lightly milled to ensure the new overlay does not ‘slip’ on the
old surface and fail prematurely.
2. Wide cracks should be sealed prior to overlay to prevent water entering the granular base
course if reflection cracking occurs.
When deep patching is needed the required minimum thickness of base course and sub-base
materials are given in Table 11. For low levels of traffic, granular base course materials should
be used. However, for higher levels of traffic on Category A and Category B roads a bituminous
base course material can also be used.
Table 7.17: Minimum layer thickness for patching prior to overlay
Million ESAL
Base course (mm) Sub-base (mm)
Granular Bituminous Granular
> 20 250 200 200
< 20 225 175 200
> 5 200 150 200
< 5 175 - 200
> 1 175 - 200
< 1 150 150
> 2 175 200
< 2 150 - 150
Where the high deflections are related to lengths of road with poor or poorly maintained
drainage then these shortcomings should be rectified prior to overlay construction. Details on
the construction and maintenance of road drainage are described in Design for New
Bituminous, Gravel and Concrete Roads.
Republic of Kenya - Ministry of Roads 34
AASHTO (1993). Guide for the design of pavement structures. AASHTO, Washington DC, USA
HODGES J W, J ROLT and T E JONES (1975). The Kenya road transport cost study: research
on road deterioration. TRL Report 673, TRL, UK.
ROLT J and C PARKMAN (2000). The characterisation of pavement strength in HDM-III and
improvements adopted for HDM-4. 10
REAAA Conference, Tokyo, 2000.
ROLT J (2000). Pavement structural number from FWD measurements for network analysis.
TRL Unpublished Report PR\INT\664\00
SHELL INTERNATIONAL PETROLEUM CO. (1978). Shell Pavement Design , London.
Republic of Kenya - Ministry of Roads 35
9.1 Appendix 1 : DCP Test
The DCP is an instrument which can be used for the rapid measurement of the in situ strength
of existing pavements constructed with unbound materials. Measurements can be made down
to a depth of approximately 800mm and where the pavement layers have different strengths,
the boundaries between them can be identified and the thickness of each layer estimated.
The DCP uses an 8 Kg hammer dropping through a height of 575mm and a 60° cone having a
maximum diameter of 20mm. The instrument is assembled as shown in Figure 3.1. The
instrument is usually split at the joint between the standard shaft and the coupling for carriage
and storage and it is important that when in operation the joints do not become loose.
Operating the DCP with any loose joints will significantly reduce the life of the instrument.
· 60° INC
2 Hammer (8kg)
3 Hammer shaft
6 Clamp ring
8 1 metre rule
9 60° cone
After assembly, the first task is to record the zero reading of the instrument. This is done by
standing the DCP on a hard flat surface, such as concrete, checking that it is vertical and then
entering the zero reading in the appropriate place on DCP Test Data Sheet shown in Figure
The DCP needs three operators, one to hold the instrument, one to raise and drop the weight
and a technician to record the readings. The instrument is held vertical and the weight raised to
the handle. Care should be taken to ensure that the weight is touching the handle, but not lifting
the instrument, before it is allowed to drop. The operator must let it fall freely and not partially
lower it with his hands.
Republic of Kenya - Ministry of Roads 36 Draft Document – September 2009
It is recommended that a reading should be taken at increments of penetration of about 10mm.
However it is usually easier to take a reading after a set number of blows. It is therefore
necessary to change the number of blows between readings, according to the strength of the
layer being penetrated. For good quality granular bases readings every 5 or 10 blows are
usually satisfactory but for weaker sub-base layers and subgrades readings every 1 or 2 blows
may be appropriate. There is no disadvantage in taking too many readings, but if readings are
taken too infrequently, weak spots may be missed and it will be more difficult to identify layer
boundaries accurately, hence important information will be lost.
After completing the test the DCP is removed by tapping the weight upwards against the
handle. Care should be taken when doing this; if it is done too vigorously the life of the
instrument will be reduced.
The DCP can be driven through surface dressings but it is recommended that thick bituminous
surfacings are cored prior to testing the lower layers. Little difficulty is normally experienced
with the penetration of most types of granular or lightly stabilised materials. It is more difficult to
penetrate strongly stabilised layers, granular materials with large particles and very dense, high
quality crushed stone. Penetration rates as low as 0.5mm/blow are acceptable but if there is no
measurable penetration after 20 consecutive blows it can be assumed that the DCP will not
penetrate the material. Under these circumstances a hole can be drilled through the layer using
an electric or pneumatic drill, or by coring. The lower pavement layers can then be tested in the
normal way. If only occasional difficulties are experienced in penetrating granular materials, it is
worthwhile repeating any failed tests a short distance away from the original test point.
If, during the test, the DCP leans away from the vertical no attempt should be made to correct it
because contact between the shaft and the sides of the hole can give rise to erroneous results.
If the lean becomes too severe and the weight slides down the hammer shaft, rather than
dropping freely, the test should be abandoned and the tests repeated approximately one metre
away from the first test. DCP is used extensively for hard materials, wear on the cone itself will
be accelerated. The cone is a replaceable part and it is recommended that it should be
replaced when its diameter is reduced by 10 per cent. However, other causes of wear can also
occur hence the cone should be inspected before every test.
The correlation between DCP readings and CBR value has been determined by a number of
authorities and a selection of these are given in Figure 3.3. Agreement is generally good over
most of the range but differences are apparent at low values of CBR in fine grained materials. It
is expected that for such materials the relationship between DCP and CBR will depend on
material state and therefore, if more precise values are needed it is advisable to calibrate the
DCP for the material being evaluated.
Republic of Kenya - Ministry of Roads 37 Draft Document – September 2009
Until local calibration is carried the following relationship given should be used
Log10 (CBR) = 2.48 - 1.057 Log10 (mm/blow)
The results can be either be plotted by hand, as shown in Figure 3.3, or processed in a
Republic of Kenya - Ministry of Roads 38 Draft Document – September 2009
9.1.4 CALCULATION OF STRUCTURAL NUMBER
If required the Structural Number of the pavement can then be calculated from the DCP results
using the following general equation.
SN = 0.0394 Σ I aI dI
where aI = Layer coefficient of layer I
dI = Thickness of layer I (mm)
Republic of Kenya - Ministry of Roads 39 Draft Document – September 2009
DCP TEST DATA FORM
Date: Wheelpath
Road No: Started test at:
Test No: (Surfacing / Base/ Sub-base / Subgrade)
Chainage: Operator:
Direction: Zero reading of the DCP (mm):
Republic of Kenya - Ministry of Roads 40 Draft Document – September 2009
9.2 Test Pit
Test pits should only be necessary on roads requiring rehabilitation. Roads requiring
maintenance with thin overlays (periodic maintenance) will only have test pits dug where FWD
or DCP measurements indicate short lengths of weak pavement.
The purpose of carrying out a test pit investigation is to confirm the information obtained from
surface condition survey, and FWD and DCP surveys. Pit digging is a time consuming and
expensive operation and for this reason the location of each test pit should be carefully
selected to maximise the benefit of any data collected.
The responsible engineer will select the number and position of the test pits to establish:
• the thickness and material properties of the road pavement in each homogeneous section
• the thickness and material properties of any lengths of road pavement, within any
homogeneous length, which have been shown to be significantly weaker by either FWD or
DCP testing.
The minimum number of test pits dug in any one homogeneous length of road should not be
less than one every 2 kms. In general the test pits will be dug in the near-side wheelpath. ie the
wheelpath adjacent to the shoulder of the road.
9.2.1 LABOUR, EQUIPMENT AND MATERIALS
Test pits can be excavated by hand or by machine, depending on the availability of plant and
the test pit programme required. Machine operations are usually more productive but more
costly than methods.
• traffic controllers - a minimum of one at each end of the site (but see above);
• 2 (if machine excavation) or 3 (if excavation) labourers;
• 1 machine operator if applicable;
• 1 driver for vehicle; and
• 1 supervising technician.
The following equipment and materials are required:
• 1 backhoe (for machine excavation);
• 1 jack hammer with generator (to assist with excavation);
• 1 pick;
• 1 or 2 spades (a fence post hole digger can also be useful);
• 1 tamper or plate compactor for backfilling test pit;
• material to backfill and seal test pit : gravel, cement for stabilising gravel, water and cold
mix for resurfacing;
• 1 broom to tidy area on completion;
• 1 chisel is often useful to assist with inspecting the wall of the test pit;
• equipment necessary to complete any required on-site testing;
• 1 tape measure and thin steel bar to span pit (to assist with depth measurements);
• sample bags and containers, with some means of labelling each;
• test pit log forms and clipboard; and
• sample log book.
9.2.2 SAMPLING AND TESTING PROCEDURE
Republic of Kenya - Ministry of Roads 41 Draft Document – September 2009
9.2.2.1 Field Procedure
Before commencing the survey in the field, the responsible engineer should be clear as to the
information required from each test pit. This will depend on the results of previous surveys, the
materials specifications in use and an understanding of the pavement behaviour. Some field
testing might be necessary as well as subsequent laboratory testing of samples extracted from
the pit. Table 9. summarises the various tests that may be required and references the relevant
standards. Not all these tests may be necessary , depending on the situation found.
A safe working environment should be maintained at all times. Reference should be made to
the appropriate regulations in this regard.
Once it has been decided what testing is to be carried out and the location of the trial pits has
been confirmed, the following procedure should be adopted:
1. Set up traffic control.
2. Accurately locate position of test pit and record this on the Pavement Test Pit Log (see
Figure G1). Usually, the position of a pit will be apparent after completion due to the
patched surface. However, if long term monitoring is required, a permanent location marker
should be placed at the roadside. Record any relevant details such as surrounding
drainage features, road condition and weather.
3. Define the edge of the test pit and remove surfacing. The required size of pit will depend on
the sample sizes necessary for the selected tests, but it can be increased later if found to
be too small. Usually an area of about 0.8m by 0.8m will be sufficient for excavation, and
the minimum working area required for a backhoe operation will be sufficient for machine
excavations. The edge of the pit can be cut with a jack hammer or pick and the surfacing
‘peeled’ off, taking care not to disturb the surface of the aggregate roadbase. The average
thickness of surfacing should be recorded.
4. If density tests are to be performed, a smooth, clean and even surface is required. It is
important for the accuracy of the test that the layer is homogeneous. For the sand
replacement method, no prior knowledge is required of the layer thickness since this
becomes obvious as the hole is excavated. If a nuclear density meter is used, the thickness
of the layer can either be estimated from previous DCP results or construction details to
determine the depth of testing.
5. On completion of any required density testing, the layer can be removed over the extent of
the trial pit, a visual assessment made of the material and samples taken for laboratory
testing. Care should be taken not to disturb the adjacent lower layer. The thickness of the
layer and the depth at which samples are taken should be measured. All information should
be recorded on the Pavement Test Pit Log.
6. Continue to sample, test and excavate each pavement layer following the procedure above.
Once it has been decided that there is no need to excavate further, the total depth of pit
should be recorded along with any other information such as appearance of water in any of
7. All samples should be clearly labelled and proposed tests for the pit materials should be
logged in a sample log book to avoid later confusion in the laboratory.
8. The pit should be backfilled in layers with suitable material which should be properly
compacted. It is often good practice to stabilise the upper layer with cement accepting that
Republic of Kenya - Ministry of Roads 42 Draft Document – September 2009
full compaction will not be achieved. A bituminous cold mix can be used to patch the
backfilled pit.
9. The site should be cleared and left in a tidy and safe condition for traffic.
9.2.2.2 Laboratory procedure
Table 18: Tests to be carried out
Property Possible Tests Field
Sieve analysis Lab KS 999:Part 2:2001 Initial visual
assessment on site.
Plasticity Plastic and Liquid
Limits, Plasticity Index
Lab KS 999:Part 2:2001 Initial visual
Linear Shrinkage Lab KS 999:Part 2:2001 Correlated to PI
Elongation Index Lab
KS 1238 Part 6
Flakiness Index Lab
KS1238 Part 11
given in ASTM
C 131-96 and C 535-
10% Fines Value Lab
KS1238 Part 12
KS1238 Part 13
Aggregate Abrasion Lab
KS 1238
Accelerated Polishing Lab ASTM D 3319-90
Sulphate test Lab
Particle Density Particle density Lab
Particle density Lab
Moisture Content Oven dry
‘Speedy’ Field Suppliers
Nuclear Density Meter Field Suppliers
Tests at various levels
KS 999
Density Sand Replacement Field
Republic of Kenya - Ministry of Roads 43 Draft Document – September 2009
Core Cutter Method Lab
Bearing Capacity DCP Field See Appendix 9.1
California Bearing
Vane test Field
Various load tests Lab
In some cases, the possible tests listed for a given property are alternatives. In other cases all
the tests listed for a given property might be required. The engineer must decide for which
properties information is required and then design a suitable testing programme.
Field tests require testing at the site and possibly further analysis in the laboratory. Laboratory
tests require only sampling in the field. All sampling should be carried out in accordance with
the general guidance of KS 999 or KS 1238, whichever is applicable, as well as any specific
Kenya Standards (KS) are quoted where available. Where no Kenya Standard is available, an
alternative is quoted.
These tests will only be required for surfacing or base materials.
The layer must consist of homogeneous material for these tests.
These tests will only be required where a slope stability or settlement problem is being
evaluated and will only apply to subgrade materials.
For moisture content determination, the oven-drying method is recommended since it provides
a fundamental measure of the moisture content. Both the `Speedy' and the Nuclear Density
Meter methods require accurate calibration and validation, since they derive the moisture
content by indirect analysis, but they have the advantage of providing instant results. Validation
should always be made with reference to the oven-dry method.
Republic of Kenya - Ministry of Roads 44 Draft Document – September 2009
Republic of Kenya - Ministry of Roads 45 Draft Document – September 2009
Test Pit Data
Chai nage :
Pavement Conditi on:
Purpose of Investi gation:
Pit Number:
Notes for Completing Test Pit Data
Material Descri ption:
Sa mple Depth:
S-Surfacing, R-Roadbase , SB-Sub-b ase, SF -Select Fil l, SGR -Subgrade
Subject ive assessment of material type and p roperti es
Dept h (range) at which any samples t aken
Note any laborat ory tests required
Note any part icula r points o f i nterest such as pavement or drain age condit ion, on si te
t est s (moisture, de nsit y), evidence of groundwater e tc.
Material Descri ption Samp le
Dept h (mm)
Method of Pitting:
DESIGN for ROADS and BRIDGES 2009
1 Summary...................................................................................................................1 2 Definitions and Abbreviations...................................................................................2 3 Introduction...............................................................................................................5 4 Network level evaluation...........................................................................................7 4.1 Visual Inspection...........................................................................................7 4.2 Roughness Condition Data...........................................................................8 5 Project Level Evaluation.........................................................................................13 5.1 Detailed Visual Condition Survey................................................................13 5.2 Falling Weight Deflectometer (FWD) Survey..............................................15 5.3 Traffic Estimation........................................................................................15
5.3.1 Classified Traffic Counts and Axle Loading.........................................................15 5.3.2 Conversion to design traffic loading ....................................................................16 5.3.3 Effect of Road Geometry.....................................................................................16
5.4 Homogeneous Sections..............................................................................17
5.4.1 DCP and Test Pit Investigations..........................................................................20
5.5 Use of DCP data for remedial work............................................................20
5.5.1 Test Pits..............................................................................................................21
6 Calculation of Structural Number............................................................................23 6.1 Definitions....................................................................................................23
6.1.1 variation of bituminous layer coefficient with temperature...................................24
6.2 Use of Structural Number for Overlay Design............................................25 6.3 Use of the FWD to estimate SNPExisting...................................................26 6.4 Overlay Design Procedure using the FWD.................................................27
6.4.1 SNP for Future Traffic (SNPDesign)....................................................................27 6.4.2 Structural Deficiency............................................................................................28 6.4.3 Designing thick overlays......................................................................................30
6.5 Overlay Design Procedure using the DCP.................................................32 7 Remedial Works Prior to Overlay...........................................................................34 8 References..............................................................................................................35 9 Appendices.............................................................................................................36 9.1 Appendix 1 : DCP Test...............................................................................36
9.1.1 Description..........................................................................................................36 9.1.2 Operation ...........................................................................................................36 9.1.3 Interpretation of results.......................................................................................37 9.1.4 Calculation of Structural Number........................................................................39
9.2 Test Pit........................................................................................................41
9.2.1 Labour, equipment and materials........................................................................41 9.2.2 Sampling and testing procedure..........................................................................41
9.2.2.1 Field Procedure................................................................................42 9.2.2.2 Laboratory procedure.......................................................................43
The Republic of Kenya - Ministry of Roads
DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009
The purpose of this Manual is to update the document of the same title produced for the Ministry of Works, Roads Department in May 1988. The Manual recommends a practical procedure to: • • design asphalt overlays; and audit overlay designs submitted by Consultants for major projects.
The procedure is based upon the AASHTO Design Guide (1993) which uses the concept of Structural Number (SN) to establish the thickness of the overlay. The procedure uses a correlation between Falling Weight Deflectometer (FWD) deflection measurements and the Adjusted Structural Number (SNP) of the existing pavement. This correlation must be calibrated for Kenya conditions. Overlay design thickness is based on the equation:
Oe l y i v ra th cns k es  (m ) = S P m   N   Ds n e ig − S P    N Eis g x tin   / a1  
] ∗ 5 .4 2
Where: SNPDesign = Structural Number for future traffic SNPExisting = Structural Number of the existing road a1 = Layer coefficient of asphalt overlay 25.4 = conversion mm to inches SNPDesign values are determined by the AASHTO (1993) design equation. SNPExisting values are based on FWD deflection measurements. The use of the FWD allows designs to be completed quickly and at relatively low cost. In common with all overlay design procedures the method described in this must be critically reviewed and adjusted according to local experience. The Manual also provides guidance on a method of designing overlays using the Dynamic Cone Penetrometer (DCP), when FWD results are not available.
Republic of Kenya - Ministry of Roads
Adjusted structural number A numerical indicator of the overall strength of the pavement layers including the subgrade. It consists of a summation of the product of the thickness (in inches), layer coefficient and drainage coefficient (if applicable) of each of the pavement layers plus a contribution from the subgrade. It is independent of where the boundary layer of the subgrade is selected. Asphalt A generic term for any mixture of bitumen, filler and aggregate. This includes asphalt concrete. Asphalt concrete A mixture of bitumen, filler and crushed stone aggregate proportioned to meet specific strength, deformation and volumetric criteria related to the Marshall test method for asphalt mixes. Base course A pavement layer lying between the surfacing and the subbase. This can be constructed from asphalt, granular or stabilised material. Binder course The lower bituminous course of the pavement, usually asphalt concrete. It is not always present ie the wearing course may rest directly on the base course. California Bearing Ratio This is the standard test for characterising subgrade material and some granular layers (test method AASHTO T193). CBR California Bearing Ratio Dynamic Cone Penetrometer This is a portable, hand-operated, percussive penetrometer for rapidly assessing the strength of subgrade and other granular layers, on site. The results can be converted to CBR values. DCP Dynamic Cone Penetrometer Empirical A method of engineering design based on observation of the performance of structures. New designs are extrapolated or interpolated from the observations without necessarily reverting to the calculated stresses and strains in the road structure. ESAL Equivalent Standard Axle Load Equivalent Standard Axle This is the standard unit of measurement of the damaging effect of traffic.
Falling Weight Deflectometer A road testing device that generates a pulse load on the road surface and measures the peak vertical deflection at the centre of the loading plate and at several radial positions by a series of sensors. FWD Falling Weight Deflectometer Layer coefficient A number (a1 value) to indicate the strength of asphalt, base course or unbound sub-base layers when calculating the structural number of a road pavement. Maintenance measures undertaken to preserve the pavement, consisting of:  routine: eg grass cutting, ditch & culvert cleaning  recurrent: eg patching, pothole-filling, crack-sealing  periodic: eg re-sealing road, re-gravelling shoulders
Selected subgrade Imported. Its purpose is to increase the strength and stiffness of low strength in-situ material and thus reduce the pavement thickness. resurfacing Mechanistic Republic of Kenya . good quality soil or rock fill material. It may consist of in-situ material. If constructed of asphalt it may include a wearing course and an optional binder course. which is placed at the top of the subgrade. The maximum particle size may be fairly small as this material is sometimes laid to less than 20mm thickness. Rehabilitation measures undertaken to increase significantly the functional life of a road pavement Reliability The reliability of a pavement design is the probability that the pavement section will perform satisfactorily for the traffic and environmental conditions over the design period. layer coefficient and drainage coefficient (if applicable) of each of the pavement layers above the subgrade. erecting warnings Method of engineering design based on mathematical models of material behaviour and determination of stresses and strains within the structure. It consists of a summation of the product of the thickness (in inches). Serviceability The serviceability of a pavement is its ability to serve the type of traffic using the pavement. Recorded in units of Structural Number Terminal serviceability index This is the index of the lowest serviceability that will be tolerated by the road users. Modified Structural Number A numerical indicator of the overall strength of the pavement layers including the subgrade. ordinary fill or “selected subgrade “.Ministry of Roads 3 Draft Document – September 2009 . layer coefficient and drainage coefficient (if applicable) of each of the pavement layers plus a contribution from the subgrade Overlay A strengthening layer of either granular or asphalt placed on top of an existing road to strengthen the road. Structural Number A numerical indicator of the overall strength of the pavement layers. Regulating course A layer of material. SN Structural Number SNC Modified Structural Number SNP Adjusted Structural Number Sub-base A medium quality granular layer resting on the subgrade and supporting the base course.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009  urgent: eg debris removal. before rehabilitation. Structural deficiency The difference between the required strength of a road to be overlaid and its existing strength. (Subgrade has the same meaning as the AASHTO term “roadbed”. It consists of a summation of the product of the thickness (in inches). usually asphalt concrete.) Surfacing The layer(s) of asphalt or surface dressing forming the surface of the pavement. Subgrade All the material below the sub-base. Its thickness will be variable and is typically used when overlaying existing pavements which have ruts. placed on an irregular or unsatisfactory road surface primarily to achieve a substantially smoother surface or a changed surface profile.
The value will depend on the status of the road and generally lies between 3 and 2. See also surfacing Weighted Monthly Average Annual Temperature. The uppermost bituminous course of the pavement.Ministry of Roads 4 Draft Document – September 2009 .DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Wearing course or reconstruction becomes necessary. WMAAT Republic of Kenya . usually asphalt concrete. The top surface of this layer should provide a smooth surface but with adequate texture to provide adequate friction for safe vehicle braking and turning.
The proposed empirical overlay design method. The design process envisages the following two levels of survey: • • Network level surveys. FWD. is also criss-crossed with patches and utility trenches. DCP and Test Pit investigations. In common with all overlay design procedures the method described in this recommends a method to formulate designs which must be reviewed by the Engineer and adjusted based on his/her own local experience. The present overlay practice is either to mill the existing surface and overlay with 40-50mm (periodic maintenance) of asphalt. more detailed in scope. indeed. The overlay designs submitted by Consultants are generally based on the methods described in ‘Design of Pavement Structures’ (AASHTO. or to apply a new surface dressing. consisting of visual condition. These often contribute to road deterioration through poor reinstatement. carried out to demarcate road sections of equivalent condition. the common theme is that they have a granular road base and either a relatively thin asphalt concrete or surface dressing surfacing. The range in topography and traffic loading results in roads having a wide range of construction thickness and strength.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 3 INTRODUCTION Roads in Kenya vary widely in their geometric standard and the traffic they carry. consisting of roughness and visual condition. They have been constructed and maintained over a period of years. both in flat and hilly terrain. carried out to determine the level of maintenance required. or to engage consultants to carry out the overlay design for major projects. followed by: Project level surveys. Republic of Kenya . many have ‘evolved’ rather than been designed and constructed by a formalized process. The procedure uses a relationship to convert FWD deflection measurements to the Adjusted Structural Number (SNP) of the existing pavement. is also based upon the AASHTO recommendations (1993) and uses the concept of Structural Number (SN) to establish the thickness of the overlay.1. 1993). allowing designs to be completed quickly and at relatively low cost.Ministry of Roads 5 Draft Document – September 2009 . However. described in this . The design process is illustrated in Fig 3. The road network.
Ministry of Roads 6 Draft Document – September 2009 .DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Figure 3.1: Design Process Establish BoQ of remedial works from VCS. Results from Network VCS and Roughness Survey identifies sections of road for rehabilitation Carry out nondestructive tests Project Level VCS FWD deflections Traffic count (where necessary) Axle load survey (where necessary) Identify homogeneous sections of road using FWD deflection (do) Plan and carry out destructive tests (DCP and Test Pits) Establish Adjusted Structural Number at each FWD point Correct Adjusted Structural Number for temperature Establish required Design Structural Number for future traffic Calculate Structural Deficiency from Existing Structural Number Design thickness of strengthening overlay for each homogenous section Calculate Costs Republic of Kenya .
sample points would be more frequent (from 0.Ministry of Roads . The total number of sub-samples influences the survey precision.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 4 NETWORK LEVEL EVALUATION The network level evaluation categorises road sections into the following : • • • those where only minimal routine or periodic maintenance needed those where major treatment. 1. the assessment shall be carried out over three strips corresponding to the wheelpaths.5m). 4. 3 or 4 depending on 7 Draft Document – September 2009 Depressions with cracks Republic of Kenya . Table 4.1km spacings). each with 2 sub-sample points.1 Visual Inspection The objectives are to identify the type and severity of the distress in a quantitative manner in order to estimate maintenance interventions and also to enable the function of performance modelling tools (eg HDM) if required. The number of samples influences the reliability of the survey. The severity of the distress is estimated as the proportion of the total number of sub-samples affected.3 the recommended threshold values for all the assessment criteria and Table 4. Each sample is subdivided into a number of sub-samples and the distress in each subsample is recorded.2 the roughness values to be expected. Table 4.5 is a recommended field form. Table 4. For network surveys. The level of detail required is governed by the purpose for the data and the resources available to do the work. 1.1 gives details of the assessment criteria.4 lists the risks associated with these partial surveys and recommended follow-up work.5 m wide. such that the inner wheel paths overlap leading to 3 rather than 4 wheelpaths. the defects will be assessed in 4 transverse strips corresponding to each wheel path.01km to 0. Table 4. each sample point having 4 sub-sample points. 2. 2. For 2-lane roads ( ≥ 5. For project surveys. The condition is evaluated at each sample and combined with a qualitative assessment of the interval between each sample.1: Assessment criteria for Visual Condition Survey Feature Rating Wide cracks > 2m 0. The inspection should be undertaken by a trained engineer who also has knowledge of the software system that he will use to process the data recorded. or samples. and those of intermediate condition where further project-level investigation is needed to decide what measures to take. For road widths < 5. Table 4. such as reconstruction needed. sample points could be spaced at up to 1km spacings. The centre wheel path ratings shall be allocated to both strips 2 and 3. 3 or 4 depending on number of strips with this defect 0. The inspection is implemented by examining the condition of road cross sections at discreet intervals. covering the full width of the pavement.
Ministry of Roads 8 Draft Document – September 2009 . some shallow potholes.3 and this will enable the road sections to be categorised. some deep potholes.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 number of strips with this defect Rutting (Visible. 4.2. 1 or 2 depending on number of edges with this defect 1 Routine: No depressions. 3 or 4 depending on number of strips with this defect 0. only a few cracks and shallow potholes. 4 Reconstruction: Broken up pavement areas. wide cracks and significant rutting. 3 Thick overlays: Major depressions with cracks. 2. slight rutting. 2 Thin Overlays: Slight depressions.2 Roughness Condition Data Roughness is normally measured using a Bump Integrator and expressed through the International Roughness Index (IRI). 1 or 2 depending on number of edges with this defect Number per sampling interval Edge failures Shallow potholes .2: Roughness criteria Republic of Kenya . Not all the indicated defects need to be present to qualify for a particular treatment.Base exposed (Include shallow local failures on the basis of 1m2 = 1 pothole ) “Shiny” Surface General Condition (Surveyor’s estimate of the required maintenance for the sample length) NB.No base exposed (Include shallow local failures on the basis of 1m2 = 1 pothole ) Deep Potholes . Table 4. some cracks. depressions with cracks and substantial rutting. The results of the survey can be evaluated according to Table 4. Number per sampling interval 0. very minor rutting. Typical values of the IRI with reference to the type and condition of the road are indicated in Fig 4. in m/km. >10mm) 0. 1. deep potholes.
Ministry of Roads 9 Draft Document – September 2009 .DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 IRI Ranges Lower than 6 6 to 11 11 to 15 15 to 19 Larger than 19 Road Condition very good good fair poor very poor Republic of Kenya .
DESIGN for ROADS and BRIDGES 2009 Table 4. Int. Layer) Some local failures. general weakness in AC and/or Base. P+S Local ≤ 4 ≤ 5 ≤ 6 1 ≤ 1 ≤ 1 ≤ 1 ≤ 30 ≤ 15 >1 1 ≤ 1 ≤ 1 ≤ 2 ≤ 30 ≤ 15 >1 Yes/No 2 ≤ 2 ≤ 2 ≤ 2 ≤ 40 ≤ 30 >2 • 3 Thick overlays • • PART 4 – Overlay Design 4 Reconstruction • Replace excessively weakened and distorted AC and Base Strengthening Reduce roughness Typical Pavement Condition Road Class Roughness (IRI m/km) Wide cracks Depressions with cracks Rutting Edge Failures Shallow Potholes (No. per km) “Shiny” Surface General Condition Small number of local failures or cracks (Strength.Ministry of Roads 2009 10 Draft Document – September . P+S Local >6 >7 >8 ≥ 3 >3 >3 ≤ 1 > 30 > 20 >1 ≥ 3 >3 >3 ≤ 2 > 30 > 20 >1 Yes/No 4 4 >3 ≤ 2 >40 >30 >2 Republic of Kenya . Int. cracked or poor surface. P+S Local 4 to 6 5 to 7 6 to 8 2 ≥ 2 ≥ 2 ≤ 1 > 30 ≤ 20 >1 2 ≥ 2 ≥ 2 ≤ 2 > 30 ≤ 20 >1 Yes/No 3 ≤ 3 ≤ 3 ≤ 2 >40 >30 >2 Frequent and severe failures and deformation. P+S Local ≤ 4 ≤ 5 ≤ 6 0 ≤ 1 ≤ 1 ≤ 1 ≤ 20 ≤ 10 ≤ 1 0 ≤ 1 ≤ 1 ≤ 2 ≤ 20 ≤ 10 ≤ 1 Yes/No 1 ≤ 2 ≤ 2 ≤ 2 ≤ 30 ≤ 20 ≤ 2 More frequent failures and depressions.3: Proposed Analysis of Network Visual Condition Survey Data Class Treatment Purpose 1 Routine (Patching and crack sealing) • Local repairs and sealing 2 Thin Overlays ≤ 40mm Restore surface characteristics • Prevent moisture entry • Restore transverse shape ( + reg. per km) Deep Potholes (No. minor rutting. Ride and Surface OK) Int. some weakness in AC and/or Base. (Strength OK) Int.
Traffic survey Axle weight survey Discretionary: • Cores + DCP + Test pits at frequencies and locations to suit FWD d1 values. There is a need for condition checks of some of the non-sample lengths to confirm this classification. DCP@ 200m Traffic survey Axle weight survey • • • Republic of Kenya .DESIGN for ROADS and BRIDGES 2009 Table 4. outer wheel paths. Patching Works Records only 1 2 Thin Overlays ≤ 40mm (or Surface Dressing or Microsurfacings) May over or under-estimate deterioration but this will be corrected during follow-up Detailed VCS. staggered L + R. outer wheel paths. staggered L + R.4: Risks involved and action required on completion of Network Surveys Class Treatment Routine (Patching and crack sealing) Risks of FastTrack VCS assessment More serious deterioration between sample lengths may be missed. May exaggerate deterioration. There is a need for condition checks of some of the nonsample lengths before proceeding with FWD Essential: Further Surveys Essential: • • Detailed VCS (100% in 5m sample lengths) Traffic survey Essential: Detailed VCS in 5m sample lengths) (100% • Discretionary FWD + Cores + DCP + Test pits FWD @ 50m. Thick overlays 3 PART 4 – Overlay Design 4 Reconstruction May over or under-estimate deterioration but this will be corrected during follow-up Detailed VCS. FWD @ 100m.Ministry of Roads 2009 11 Draft Document – September .
. Form Start Time: Date:……………….. Single / Dual Width:…………. …m Direction:………………………. Republic of Kenya .. m Road ID:…………… Sampling Interval:…….DESIGN for ROADS and BRIDGES 2009 Region:…………………………………. ……………… Start of Survey: 1 2 3 4 5 6 PART 4 – Overlay Design Road Name:……………………………. Signed:………….. per sample i’val) Deep Potholes (No.Ministry of Roads 2009 12 Draft Document – September . 7 8 9 10 Roughness m/km) Wide cracks Depressions with cracks Rutting Edge Failures (IRI Ex database 01234 01234 01234 012 Number Number 01234 1 2 3 4 Worse = 0 Similar = 1 Better = 2 Shallow Potholes (No. per sample interval) “Shiny” Surface General Condition Remaining interval condition Surveyed by – Name:……………………………..……………….
nor of the drainage are covered in this type of survey. no m2 Cracks separating pavement into small pieces. The survey enables the quantity of materials required to be estimated.1 Detailed Visual Condition Survey The project Visual Condition Survey (VCS) is more detailed than the network survey.1. separating pavement into blocks. to be sealed. Republic of Kenya . It is carried out during on foot. each sample length (5. Any one defect should only be counted once: for example. Quantities for crack sealing should be adjusted to an area not affected by previous deep or surface patching. in m Alligator cracks. m As above. Prior to overlay a number of these defects will need remedial work. Notes on the collection of the defects are presented in Table 5. 10 or 20 metre) of the road being examined to identify defects in the wheelpaths.Ministry of Roads 13 Draft Document – September 2009 . covering the whole of the road. originating in base Edge failure m Loss of pavement surface >50mm Trench/Patch Failure m2 Rutted (>10mm) or broken-up patch Shallow potholes N Potholes that occur just in surfacing Asphalt shoving N Pushing-up of asphalt surfacing Surface rutting m2 >10mm depth but just in surfacing Slippage cracks N Adhesion failure of asphalt surfacing to base Neither the condition of the road shoulders.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 5 PROJECT LEVEL EVALUATION 5. The data from the VCS is transferred to a spreadsheet that automatically calculates the Bill of Quantities for the remedial work prior to overlay. which refers to bituminous-surfaced roads only. ‘rutting with cracks should be counted as “Depressions with cracks” only and not also recorded as “Rutting”. in m Wide connected cracks m Cracks wider than 3mm. but no depressions depression or rut 2 Alligator cracks.1: Explanation of Defects Defect Unit Notes Wide single cracks m Cracks (wider than 3mm) to be sealed. Table 5. with associated depression depressions Deep Potholes N Potholes that penetrate through base 2 Structural rutting m >10mm depth.
DESIGN for ROADS and BRIDGES 2009 PART 4 – Overlay Design TABLE: PAVEMENT CONDITION SURVEY (Project Level) : ‫رقمالط‬ ‫رينق‬ Road ID : ‫إسمالط‬ ‫رينق‬ Road Name : ‫ل ننط‬ ‫م قة‬ Region : ‫ساف ة العينننينة‬ ‫الم‬ Sample Interval ‫ض رينق‬ ‫عر الط‬ Carriageway Width : ‫التنجاه‬ Direction BILL OF QUANTITIES Start of Survey: Sta 0+000 CRACK SEALING 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Unit Mean Width Sum Quantity Wide Single cracks Wide Connected cracks Alligator Cracks without Depression Alligator Cracks with Depression Base Shoving Deep Potholes Structural Rutting Edge Failure Trench / Patch Failure Shallow Potholes Asphalt Shoving Surface Rutting Slippage Cracks “Shiny” Surface l.m sq.Ministry of Roads 2009 14 Draft Document – September .m sq.m sq.m sq.m sq.m sq.m sq.m sq.m sq.m sq.m 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 MILL AND REPLACE FULL DEPTH PATCHING Republic of Kenya .m sq.m l.m sq.
1 CLASSIFIED TRAFFIC COUNTS AND AXLE LOADING Reference is made to the Design for New Bituminous.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 5. the tests should be carried out at 50 metre intervals. Fig 6. Gravel and Concrete Roads for procedures to undertake traffic and axle load surveys. For multi-lane dual carriageways FWD measurements should be carried out. On 2 lane single carriageway roads the location of the tests should be ‘staggered’ by 50 metres so as to result in an FWD test every 50 metres along the road. 5. On 2 lane single carriageway roads the location of the tests should be ‘staggered’ by 25 metres so as to result in an FWD test every 25 metres along the road.1 summarises the steps described. On Class A. as a minimum. The location of the FWD tests should be ‘staggered’ to allow for maximum coverage.3.2 Falling Weight Deflectometer (FWD) Survey On 2 lane single carriageway roads FWD tests should be carried in both lanes (ie both directions) in the outer wheel-path (closest to the shoulder of the road). On Class D and E roads. in the outer wheelpath of the heaviest loaded lane.2: Steps for carrying out Traffic and Axle Load Surveys Select Design Period Determine Initial Traffic Volume (Initial AADT) per Class of Vehicle Determine Traffic Growth Determine Cumulative Traffic Volumes over the Design Period Estimate Mean Equivalent Axle Load (ESA) per Class of Vehicle Estimate Cumulative ESAs Republic of Kenya . tests should be carried out in other lanes where the condition of the lane is worse than the heaviest loaded lane. the tests should be carried out at 100 metre intervals. The FWD tests should be normalised to a standard load of 50 KN. Figure 5.3 Traffic Estimation 5.Ministry of Roads 15 Draft over the Design Period (in Document – September 2009 one direction) . B and C roads. In addition.
) It is essential that the individual axle weights are converted to ESA before any aggregation or averaging of data is carried out for either the individual vehicle or for all the weighed vehicles of a single class.6 Effect of road geometry on design traffic loading Single/Dual No of Width of Calculation of ESALs Carriageways Lanes Carriagway (m) Single 2 <6.7 80% of the ESALs in both directions is used in order to allow for overlap on the Republic of Kenya . On narrow single carriageway roads the wheelpaths can overlap in the centre of the road. allowing for the proportions of loaded and unloaded vehicles in each class. Table 5.Ministry of Roads 16 Draft Document – September 2009 . The total contribution to pavement loading of a vehicle class is the product of the vehicle damage factor of the vehicle class and the number of such vehicles recorded on the road either on a daily or annual basis. but normally increases vary between 2 and 7 per cent per annum. quarry or port comes into operation. On multi lane dual carriageways medium and heavy traffic can use other lanes besides the outer lane (most heavily trafficked lane). the average value of ESA is calculated for the whole vehicle class. when for example a new factory. which are added to produce a total for the vehicle. 5. adjusted for annual traffic growth.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 The individual weights of each axle of a particular vehicle class are converted to ESA. After the total ESA for each vehicle have been calculated. (These may vary in each direction and between routes.1 can be used to calculate the design traffic. causing more damage. The process is repeated for the other classes and the total loading per unit of time is determined by summation. It is usual to determine the average ESA for each vehicle class based on the results of an axle load survey.3 EFFECT OF ROAD GEOMETRY The vehicle damage to the road pavement is influenced by the road geometry.3. These average values of ESA are sometimes termed “vehicle wear factors” or “vehicle damage factors”. The estimate of annual traffic growth is usually based on historic trends and affected by predictions of future economic activity.2 CONVERSION TO DESIGN TRAFFIC LOADING The pavement loading calculated above must be summed for the whole of the design period (normally 10 to 15 years). In some instances there is the possibility of a sudden increase (or reduction) of traffic. The criteria presented in Table 6. The Design Traffic Loading in the performance period is calculated from Equation 1: Equation 1: Calculation of design traffic loading N   r   1+  n  100  − 1   ESALs = ∑  ESAL1 * 365 *    r 1   100     Where: N = Performance Period in years ri = Growth rate (%) ESAL1 = Daily number of ESA in the first year in traffic class ‘i’ 5.3.
For instance. a special study shall be carried out to establish the distribution of commercial vehicles. Surface Dressing [SD] Patch. the strength of the underlying subgrade will vary along the road alignment as the road passes from areas of cut to fill. Figure 5.5 to 1 < 1 1 to 2 < 3 > 10 < 30 <5 > 30 Min< IRI <Max 5 to 10 < 30 > 30 IRI ≥ Max > 10 to Rehabilitate Patch. Seal Cracks.7 central section of the road.3 < . With more than 2000 commercial vehicles per day in one direction. Seal Cracks. Mill & Strengthen [Detailed Design ] Repair. Rehabilitation measures cannot be tailored to each and every variation in road characteristic.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Single Dual 2 2 > 6. 90% of the ESALs in one direction is used. Double SD Patch. Seal Cracks Patch. A special study shall be carried out to establish the distribution of commercial vehicles Dual 2 - Dual >2 - 5.Ministry of Roads 17 Draft Document – September 2009 .5 . Thin Overlay Maintainable No < 10 IRI ≤ Min < 15 <2 < 20 0 0 to . as part of the rehabilitation design process.3: Maintenance Thresholds Roughness IRI Surfacing Integrity & Texture Cracked & depressed area % Cracked area % <5 Potholed area % Shiny area % < 10 Maintenance Operation Patch.4 Homogeneous Sections Results from the Network Visual Condition and Roughness Surveys will identify lengths of road in need of rehabilitation. With less than 2000 commercial vehicles per day in one direction. Seal Cracks.1) need further investigation with the FWD or other equipment as described below. Inlay. Mill & Replace Patch. Grinding Patch. Those lengths of road that require Detailed Design (see Fig 6. Seal Cracks. Mill & Strengthen [Detailed Design ] Reclaim Base & Repave [Detailed Design ] Reconstruction & Improvement All roads vary in pavement thickness and strength along their length. Seal Cracks. so to produce cost-effective designs the road should be divided into lengths Republic of Kenya . The ESALs in the most heavily trafficked direction is used. Mill & Overlay[Detailed Design ] Patch.
DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 where the strength properties are similar. This procedure is best carried out by using the Cumulative Sum Method (CUSUM) on FWD central deflection measurements (do).Ministry of Roads 18 Draft Document – September 2009 . The method involves plotting the cumulative sum of the differences of the FWD deflection from the mean FWD value calculated from all the results. known as homogeneous lengths. The calculations are based on Equation 2 and a worked example is shown in Table 6. Each homogeneous length is then treated as a separate overlay design exercise. This will result in reduced costs as the overlay thickness changes.1: Equation 2: CUSUM calculation S = FW D − FW D +S i i m ean i −1 Where: FWDmean = Mean FWD deflection of the road FWDi = FWD deflection at chainage i Si = Cumulative sum of the deviations from the mean deflection Table 5.7: Cusum Calculations on FWD central deflections (d0) Republic of Kenya . reflecting the existing strength of each homogeneous section.
DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Chainage (m) 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 2350 2400 M ean FW D D0 0.733 -0.128 -0. Error: Reference source not foundA change in slope of the graph indicates a change in strength along the road.169 -0.467 0.850 -0.276 0.215 0.197 -0.061 -0.665 0.435 0.580 -1.020 Cusum -0.020 0.025 -0.3 respectively.049 -2.318 0.665 0.856 -1.M ean -0.559 0.305 0.403 -0.007 0.130 -0.225 -0.291 -0.214 -0.561 0.543 -2.038 -0.Ministry of Roads 19 Draft Document – September 2009 .138 0.003 0.572 -0.304 0.2 and Fig 6.769 0.155 -0.034 0.061 -0.462 0.559 0. Figure 5.117 0.117 -0.263 -0.404 0.181 -0.559 0.136 -0.483 0.366 0.117 0.247 0.398 -1.261 0.863 -1.835 -2.444 -0.041 0.313 0.178 -2.124 -0.000 The FWD d0 deflection values and CUSUM plot are given in Fig 6.287 -0.117 0.096 -0.420 -1. In Fig 6.753 -0.301 0.327 0.984 -0.080 -0.424 -1.990 -1.195 -0.435 0.273 0.327 -0.559 0.664 -0.325 -0.764 -2.245 0.381 0.129 -0.037 0.404 0.4: FWD d0 deflection values Republic of Kenya .462 0.076 -0.308 0.627 -0.747 -1.684 -0.025 0.479 0.020 0.261 0.172 -1.137 -0.025 0.564 -1.223 0.769 0.134 -0.366 0.488 -1.261 0.467 0.181 -0.129 -0.346 -2.007 -0.313 0.020 0.020 0.196 -1.117 0.708 -0.805 -2.035 -0.462 0.3 five distinct homogeneous sections can be identified.442 D0 .181 -0.709 -0.759 -1.117 0.223 0.141 -0.462 0.667 -2.228 0.467 0.041 0.223 0.815 -1.476 0. These sections should be treated as separate overlay designs.482 0.327 0.665 1.407 0.559 0.166 0.038 0.647 -2.227 -0.174 -0.769 0.076 -0.559 0.314 0.
000 laboratory tests are carried out on the samples collected from the various granular layers in the road to establish whether they can be used in the reconstruction process.000 0. In this case -2. In Fig 6.500 test are high at chainages 750 and 1400 metres.5: CUSUM plot showing homogeneous sections FWD (D0) @ 50KN 0. 0 10 0 20 0 50 0 60 0 70 0 30 0 40 0 80 0 90 0 1 Test pits are best used when the road is to be partially or fully reconstructed.1 DCP AND TEST PIT INVESTIGATIONS Destructive testing may be needed after the non-destructive testing is completed to establish the thickness and strength of the existing pavement layers and relate these to the road failure.000 0.200 1.200 Cumulative Sum 0.Ministry of Roads 20 Draft Document – September 2009 -3. Two methods are available.5 Use of DCP data for remedial work DCP tests should be carried out at points in the road where the Detailed Visual Condition Survey and FWD deflection profile show the road to be abnormally weak.500 DCP tests are relatively quick and therefore should be used where there is no risk of damaging any utilities in the road pavement. either the Dynamic Cone Penetrometer (DCP) or Test Pits.000 . Details of these field methods are presented in Appendices respectively -1. The results from DCP tests are particularly useful in identifying areas of weak base course and sub-base layers which will need deep patching required prior to overlay. DCP would be carried out the outside Republic of Kenya .4 the FWD -2.500 0.000 -1.400 5.000 0. 5.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 1.800 Figure 5.600 0 500 -0.4.
which could include:     whether the existing granular base course and sub-base meet normally acceptable 0. The measurements and tests required are listed in Table 6.000 material standards for partial or full reconstruction. Two test pits would normally be dug in every one kilometre of road.2. measured as mm per blow.200 where the road is weak.Ministry of Roads 21 Draft Document – September 2009 .6: FWD deflections and points for DCP testing 1. whether the existing granular base course and sub-base meet normally acceptable standards for thickness for the appropriate road class. and there are correlations to convert the DCP values to in-situ values of CBR. Figure 5. If the road is to be overlaid then the pits should be dug in areas where the FWD shows the road to be weak.600 Where the in-situ CBR of the granular base course and sub-base are below 80% and 30% respectively (as measured from the DCP). Prior to testing a detector should be used to ensure there are no utilities beneath the test location.5. The thickness of the road layers are identified by the changes in mm/blow as the apparatus penetrates the pavement layers. The strength of the layers is related to their resistance to penetration. confirmation of the pavement layers identified during DCP analysis. The test pit data are used to determine the reasons for the weaknesses identified from the FWD investigation.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 wheelpath at these chainages to establish the cause of the weakness.1 TEST PITS 0.800 The DCP is driven through the road pavement under a standard force to a maximum depth of approximately 800mm. 0. If the road is to be partially or fully reconstructed the Test Pits should be dug at regular intervals 0.400 Where the FWD results indicate that the road should have a thick overlay or be either partially or fully reconstructed then test pits will be needed.200 1. to enable mechanistic analysis of FWD measurements 0 10 0 20 0 50 0 60 0 70 0 30 0 40 0 80 0 90 0 1 Test Pits will be dug at points in the road where the Detailed VCS and FWD deflection profile show the road to abnormally weak.000 DCP Test 0. FWD (D0) @ 50KN 5. Republic of Kenya . Sometimes it is difficult to differentiate between the base and subbase and test pits may be necessary as a last resort to determine the layer interval. the base course and sub-base (if necessary) shall be deep patched.
Where the road base and sub-base material do not meet these specifications the length of road affected should be deep patched.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 The results of these tests should be compared to standard material specifications. Table 5. Gravel and Concrete Roads.8 Field/Lab Test Field Pavement material Asphalt surfacing Road base Sub base/Selected Subgrade Subgrade Road base Sub base/Selected Subgrade Subgrade Road base Sub base/Selected Subgrade Subgrade Sub base/Selected Subgrade Subgrade Test Description Thickness Description Moisture Content Layer density Atterberg Limits Grading Compaction CBR Test KS 999 Part 2 2001 KS 999 Part 9 2001 KS 999 Part 2 2001 KS 999 Part 2 2001 KS 999 Part 4 2001 KS 999 Part 2 2001 Laboratory Republic of Kenya .Ministry of Roads 22 Draft Document – September 2009 . listed in the Design for New Bituminous.
Ministry of Roads 23 Draft Document – September 2009 .2.and high-severity transverse cracking >10 percent high-severity alligator cracking and/or >10 percent high-severity transverse cracking No pumping.35 0.0394 ∑ a i hi i Where: ai = Layer coefficient of layer i hi= Thickness of layer i (mm) The calculation of layer coefficients for existing pavement layers is based on the stiffness of bituminous materials and the CBR of granular materials.20 to 0.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 6 CALCULATION OF STRUCTURAL NUMBER 6.35 to 0.and high-severity transverse cracking >10 percent medium-severity alligator cracking and/or <10 percent medium-severity alligator cracking and/or >10 percent medium.15 to 0. COEFFICIENT.14 0. Table 6.and high-severity transverse cracking COEFFICIENT. or contamination by fines.10 to 0.15 0.and high-severity transverse cracking 0.9: Layer Coefficients for Existing Asphaltic Concrete and Granular Materials MATERIAL AC Surface SURFACE CONDITION Little or no alligator cracking and/or only low-severity transverse cracking <10 percent low-severity alligator cracking and/or <5 percent medium.20 Republic of Kenya .08 to 0.10: Layer Coefficients for Existing Stabilised Road Bases MATERIAL Stabilized Roadbase SURFACE CONDITION Little or no alligator cracking and/or only low-severity transverse cracking <10 percent low-severity alligator cracking and/or <5 percent medium. or contamination by fines.25 >10 percent low-severity alligator cracking and/or <10 percent medium-severity alligator cracking and/or >5-10 percent medium.35 0.40 0. It is calculated from the following: Equation 3: Definition of Structural Number SN = 0. Some pumping.1 and 6.and high-severity transverse cracking >10 percent low-severity alligator cracking and/or <10 percent medium-severity alligator cracking and/or >5-10 percent medium.15 to 0. degradation.30 0. ai 0.25 to 0. degradation.00 to 0.10 Granular Roadbase or Subbase Table 6. They are indicated in Tables 6.1 Definitions The Structural Number approach is probably the most reliable method of evaluating the ‘strength’ of pavements of similar type in terms of their likely traffic carrying capacity.20 0.20 to 0.14 to 0. ai 0.
08 to 0.6C) Altitude > 1200 metres = 0. The analysis shows that the layer coefficient of asphalt concrete used in Kenya should be: Altitude 0 – 600 metres = 0.024(15-20) = 4086 MPa 24 Draft Document – September 2009 Republic of Kenya .20 0. 6. was later developed to take into account different subgrade strengths. where the ambient temperatures.38 (WMAAT=22. • ET=15 = 3100*10-0. using the equation 5: Equation 5: Variation of Elastic Modulus with temperature E1 = E 2 *10 −b (T1 −T2 ) where b = 0.85 (Log10 (CBR))2 – 1. and hence road temperatures.51 Log10 (CBR) – 0. This is obtained by calculating the effective elastic modulus of asphalt concrete using the Shell Method of Weighted Monthly Average Annual Temperature (WMAAT) (Shell. It is therefore necessary to derive a strength coefficient suitable for Kenya.44 (WMAAT) = 11.1. The layer coefficient taken for a new asphalt concrete surfacing during the Road Test was 0.10 to 0. 1975). T2 are two asphalt temperatures.44.15 The Structural Number was developed during the AASHO Road Test.024 and T1. 1978). This relationship is defined in Equation 4: Equation 4: Definition of Modified Structural Number SNC = SNSG + SN Where: SNSG = Structural Number contribution from the subgrade= 3. are different to those in Illinois. This was for asphalt concrete having an elastic modulus of 3100 MPa at a temperature of 20oC. shown below. which considered the performance of trial sections constructed over a uniform subgrade having a particular strength.1 VARIATION OF BITUMINOUS LAYER COEFFICIENT WITH TEMPERATURE The AASHO Road Test was carried out in Illinois.40 (WMAAT=19.and high-severity transverse cracking >10 percent high-severity alligator cracking and/or >10 percent high-severity transverse cracking 0. the Modified Structural Number (SNC) (Hodges et al. A further parameter.43 SNC = Modified Structural Number CBR = In situ CBR of the subgrade.3C The analysis is shown below: Step 1 Calculate equivalent modulus at the AASHO Road Test site (WMAAT = 15oC) of asphalt concrete having an elastic modulus of 3100MPa at 20oC tested in laboratory. USA.Ministry of Roads .5 oC) Altitude 600 – 1200 metres = 0.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 >10 percent medium-severity alligator cracking and/or <10 percent high-severity alligator cracking and/or >10 percent medium.
87 aT=22 = 0. Table 6.3.5-15) = 2700 MPa Step 3 Calculate layer coefficient of asphalt concrete in the Coastal Region having an elastic modulus of 2700MPa: • aT1/aT2 = (ET1/ET2)0. • • aT=22/aT=15 = (2700/4086)0.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Step 2 Calculate the elastic modulus of similar material. in the Coastal Region (WMAAT = 22.Ministry of Roads Good coverage Good coverage 25 Draft Document – September 2009 . the Structural Number of the existing road (SNExisting) has to be measured. all of which have various advantages and disadvantages.333 Where: aT1 and ET1 are the layer coefficient and elastic modulus respectively at temperature T1.11: Advantages and Disadvantages of Investigative methods Method Test Pits Procedure to calculate SNExisting Direct calculation from thickness and strength (laboratory) of the different pavement layers Direct calculation from estimated thickness and in situ strength of the different pavement layers Back calculation Requirements Field and Laboratory testing Operational restrictions Poor coverage DCP tests Test Pits needed to gain information on actual pavement layer thickness and material DCP or Test Pits needed to establish pavement layer thickness - Fair Coverage FWD Estimate of SNC from FWD deflection bowl (SNP) Republic of Kenya . for instance. There are a number of ways of doing this.024(22. as enumerated in Table 6.44*0. mm = ( SNPdesign − SNPexisting ) / a1 * 25.87 = 0.2 Use of Structural Number for Overlay Design The overlay thickness is derived from: Equation 6: Derivation of overlay thickness from Structural Number Overlaythi ckness .5oC) to that in Illinois (WMAAT = 15oC): • ET=22 = 4086*10-0.333 = 0.38 6.4 [ ] Where: SNPDesign = Structural Number for future traffic SNPExisting = Structural Number of existing road a1 = Layer coefficient of asphalt overlay Therefore to calculate the thickness of required overlay.
the boundaries between the different materials are sometimes indistinct and differentiating base courses from sub-bases. whilst simple in principle. This can cause a problem in defining the layers in Test Pits for calculating the Modified Structural Number. Previous work (Rolt. This value is called the Adjusted Structural Number (SNP) (Rolt and Parkman.3 Use of the FWD to estimate SNPExisting The most suitable tool to measure the Adjusted Structural Number of an existing road (SNPExisting) is the DCP. especially on roads that have been in existence for many years.760 *  900 0  d 900  −0. many-layered structure is often revealed. Changes of strength are expected to occur when passing from one layer to another but significant changes of strength also occur within reasonably well-defined layers. gives rise to a number of practical difficulties. The same difficulty also applies when trying to define the appropriate layer thickness for back-analysis of FWD data and often makes this form of analysis somewhat unreliable.6 for Secondary and Local roads where FWD results are not available. and sub-bases from the subgrade can be difficult. 2000) showed that the most effective form of the correlation between FWD measurements and SNP takes the form below: Equation 7: Correlation between SNP and FWD SNP =1.5 −1.5     Where: d0 = Central deflection (mm) d900 = Deflection at 900mm from the load (mm) d1200 = Deflection at 1200mm from the load (mm) (FWD deflection is measured in mm at a load of 50KN) Figure 6.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 The Structural Number and Modified Structural Number concept.548 * (d d −d 1200 * 0. This is because: • • it may not be practicable to take sufficient DCP measurements along each road to cope with the possible high variability found in Kenya.8) −0. its use to design overlays in Kenya is. As FWD deflection data can be measured very quickly and accurately.7 : Correlation between SNP and Deflection Republic of Kenya . A procedure is therefore required which takes account of the contribution to Structural Number of a pavement from all the pavement layers and the contribution of the subgrade. which is independent of where the subgrade boundary is defined. and the coarse granular road base in the Kenya roads prevent the instrument’s penetration.394 + 4. 6. When DCP tests and Test Pits are carried out. When the same pavement is tested with a DCP a more complex. the proposed overlay procedure uses the data to estimate the SNPExisting of the existing road. rather than the DCP. often not ideal. An overlay procedure based on DCP results is described in Section 6.Ministry of Roads 26 Draft Document – September 2009 . 2000). however.
0.200 Republic of Kenya . shown below: Equation 8: Computation of SNP Design log 10 (W8.00 0. to enable the equation above to be used in for Kenya a series of comparative tests between the FWD and the DCP must be carried out on a selection of Category A and B roads.96) shows the suitability of this form of general relationship for the analysis of FWD results. as assessed by FWD measurements.16 ) = Z R ×S 0 4.00 10.00 6. The limited scatter around the ‘line of best fit’ (R2 = 0.4 Overlay Design Procedure using the FWD The required overlay thickness is calculated based on a comparison of the strength of the road required for the future traffic and the existing strength of the road.40 + ( SN +1) 5. = Standard normal deviate for required reliability. = Combined standard error of the traffic and performance predictions . = drop in serviceability over the performance period.2 −1.00 R = 0.16 ZR S0 ∆ PSI MR SN = predicted number of 8.16 tonne ESALs.07 log 1094 0.16 ESALs. 8. 6.19 Where: W8.1 SNP FOR FUTURE TRAFFIC (SNPD The first step in the process is to establish the value of Structural Number (SNPDesign) that is required for each homogeneous section of road for future traffic loading. The predicted values of SNP are shown plotted against the central deflection d0 in Fig 6.32 × 10 ( M R ) −8.00 SNP(Existing) The equation above has been used to convert FWD measurements taken from hypothetical Trial Sites.4. This is achieved by using the AASHTO (1993) equation for flexible pavements.00  ∆ PSI  log 10   4.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 12.000 0.36 × 10 ( SN +1) −0.1.00 ESIGN ) 2. = structural number to carry W8. However.20 + log + 2.5  + 9. = subgrade resilient modulus in psi.96 2 6.Ministry of Roads 27 Draft Document – September 2009 .see below.
5 2. from Table 6. The overlay procedure described in this Manual uses SNP.4.2 STRUCTURAL DEFICIENCY It is necessary to plot the ‘Structural Deficiency’.76 5.6: Republic of Kenya .22 C 3.93 4.67 - To use the AASHTO design equation when the Adjusted Structural Number (SNP) is used rather than SN and subgrade strength separately. that is the difference between the required design Structural Number of the road (SNPDesign) and the existing Structural Number at each FWD test (SNPExisting). In the following paragraphs that describe the overlay procedure it has been assumed that the road under investigation is a Category A road with a design traffic loading of between 5-10 million ESA.13: Design SNP Future Traffic (Million ESA) Road Class <0. which allows the engineer to identify the following actions for the homogeneous sections based on the criteria given in Table 6.32 4.5–1 1-2 2-5 A 5.68 B 5. This results in the principle that if any two pavements have the same value of Adjusted Structural Number (SNP) then they should carry the same level of traffic.Ministry of Roads 28 Draft Document – September 2009 .2 2.25 3.5 0.76.40 4.7 1. Table 6.0 2.05 5-10 6.28 20-50 7.93 3.4.45 10-20 6. Therefore. This is simply : Equation 9: Definition of Structural Deficiency Structural Deficiency = SNP design − SNP existing After calculation the Structural Deficiency is plotted as a bar chart. the difference between SN and SNP needs to be understood.54 3. for each FWD test. The Standard Deviation is set at 0.84 6. In using either method.5.49 as recommended by AASHTO (1993).57 4. the SNPDesign is 5.0 1.25 5.49 0.49 0. the subgrade resilient modulus value must be assumed at 4325 psi in the equation. The calculated values of SNPdesign for various values of ESA are presented in Table 6. In the normal AASHTO design method SN is used rather than SNP.90 Local 2.49 0. 6. Thus SN is then the same as SNP. This was the subgrade resilient modulus used in the Road Test and therefore at this value the subgrade contribution is zero.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 The recommended Reliability factors and decrease in Pavement Serviceability Index (PSI) used in the equation are shown in Table 6.7 2.12: AASHTO Design Criteria: Reliability factors and Servicability Indices Road Class Reliability Standard Terminal Decrease Deviation PSI in PSI International Primary Secondary Local 90 90 85 50 0.49 2.5.5 Table 6.2 2.
5 Thin overlay Thick overlay (40/50mm) Reconstruction probable Fig 6.14: Structural Deficiency Criteria Mean Structural Deficiency Zero or negative Action Maintain Notes A thin overlay may be required to correct other defects Remedial works possible Remedial works probable 0 to 0.Ministry of Roads 29 Draft Document – September 2009 .8: Structural Deficiency Republic of Kenya .6 0. the results of an actual FWD survey.5 > 1.2. illustrates these principles: Figure 6.6 to 1.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Table 6.
6 a thin overlay should be constructed.Ministry of Roads 30 Draft Document – September 2009 80 .00 No strengthening overlay 3. The need for some deep patching is also very likely to be required.6 to 1. Republic of Kenya .5.00 predominantly negative.00 Mean Structural Deficiency = -0.00 6. -2.3. Under such circumstances the visual condition data. but becomes probable if the structural deficiency is greater than 1.0. roads with good foundations can be partially reconstructed by making use of much of the existing material in the form of enhanced sub-base or even lower base course layers. Structural Deficiency 10 0 20 0 30 0 40 0 50 0 60 0 70 0 0 If the mean structural deficiency lies in the range ranges from 0 to 0.92 Patching 2.00 the Maximum Stone Size is 19mm the material can be laid with a Where minimum thickness of 40mm.00 No strengthening is required if the Structural Number Deficiency is either zero or 1. If the road has been identified as having a poor profile (ie high IRI value) a thin overlay can be constructed as periodic maintenance. Roads which have a very weak or non uniform pavement structure and or sub-grade -4. The minimum thickness of these thin overlays is governed by the aggregate grading of the overlay material. The thickness design procedure is described in Section 6.00 If the mean structural deficiency ranges from 0. The design of roads that require reconstruction should be done in accordance with design recommendations set out in the Design of New Bituminous. Any occasional positive values should be investigated and deep patched where necessary. In general.5 then a thick overlay is necessary.3 DESIGNING THICK OVERLAYS The final step in the process is to calculate the thickness of overlay for those homogeneous section where a thick strengthening overlay (>50mm) is required.00 The need for partial or full reconstruction is less easy to define.4. Where the mix has a Maximum Stone Size of 25mm the overlay will need to be 50mm thick. -3.00 require more elaborate remedial works and full reconstruction is possibly required. -1.4. Points with high structural deficiency should be investigated and deep patched where necessary. DCP and test pit data needs to be re-assessed.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 4. Gravel and Concrete Roads.
15: Values of 'CF' Probability of Achieving Design Life 90% 85% 80% 75% 50% CF Factor 1.4 Where a1 = layer coefficient for the asphalt overlay. Where no overlay is required at an FWD test.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 The overlay at each FWD test is calculated using the equation below.037 has been used: Figure 6.674 0. The overlay thickness for each homogeneous section is then calculated as follows: Equation 11: Calculation of Overlay Thickness for Homogeneous Section Designover laythickne ssmm = Meanoverla ythickness + CF * SD Where: SD = Standard deviation of the overlay thickness in the homogeneous section CF = Probability of achieving design life. a value of zero is assigned. Equation 10: Calculation of Overlay Thickness Overlaythi cknessatFW Dtestmm = ( ( SNPdesign − SNPexisting ) / a1 ) * 25 . Values of CF that should be used for different levels of probability are given below: Table 6.282 1. In Fig 6.037 0. The use of a higher level of probability can result in overlays being too thick if the road construction is highly variable.841 0.9: Design of Overlay Thickness from FWD data 180 160 140 120 100 31 Draft Document – September 2009 Deep pa Republic of Kenya .Ministry of Roads thickness (mm) 80 .3 a value of 1.0 A value of 85% is usually recommended.
Calculate the ‘Structural Deficiency’ for each DCP test. Identify homogeneous sections of road for strengthening. The overlay at each DCP test is calculated using the equation below. The required overlay thickness is calculated based on a comparison of the strength of the road required for the future traffic and the existing strength of the road. The value of Structural Deficiency is simply the difference between the required design Structural Number of the road (SNPDesign) and the existing Adjusted Structural Number at each DCP test (SNPExisting). The DCP data shall be analysed in purpose designed software called UKDCP. On Secondary and Local roads. Equation 12: Calculation of Overlay Thickness from DCP data Overlaythi cknessatDC Ptestmm = ( ( SNPdesign − SNPexisting ) / a1) * 25. as assessed by DCP measurements. Where no overlay is required at a DCP test.Ministry of Roads 32 Draft Document – September 2009 . The following steps should be followed: 1.3. 2.4.1. The overlay thickness for each homogeneous section is then calculated using the following.5 for different levels of traffic for Category B roads and Local roads. the DCP tests should be carried out at 100 metre intervals. This procedure is best carried out by using the Cumulative Sum Method (CUSUM) on the value SNPExisting calculated from each DCP test. The resultant values of SNPDesign are given in Table 6.5 Overlay Design Procedure using the DCP This procedure should only be used on Secondary and Local roads where FWD data is unavailable and where the road structure allows the DCP to penetrate the road structure to a depth of 800mm. After calculation the Structural Deficiency should be plotted as a bar chart and the required actions are described above. This software enables the user to analyse each DCP test and then calculate the SNPExisting for each test.1). Calculate the thickness of overlay for those homogeneous section where a thick strengthening overlay (>40/50mm) is required.4. The location of the tests should be ‘staggered’ by 50 metres to result in a DCP test every 50 metres along the road.4 Where a1 = layer coefficient for the asphalt overlay (See Section 6. Each length is treated as a separate overlay design exercise.4. The UKDCP software allows the designer to identify the homogeneous sections automatically and this process is described in the User for the software. This is done using the AASHTO (1993) equation in the same way as described in Sections 6. Calculate the adjusted Structural Number of the existing road (SNPExisting) from DCP tests.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 6. The homogenous sections are identified in the same way as is shown in Section 5.2 and 6. Establish the value of Structural Number (SNPDesign) that is required for each homogeneous section of road for future traffic loading. Equation 13: Overlay Thickness for Homogeneous Section Republic of Kenya . 4. 3. a value of zero is assigned. 5.
7. Under most circumstances a value of 80% is recommended for Secondary roads and 75% for Local roads. Republic of Kenya .Ministry of Roads 33 Draft Document – September 2009 . The use of a higher level of probability can result in overlays being too thick if the road construction is highly variable.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 Designover laythickne ssmm = Meanoverla ythickness + CF * SD Where: SD = Standard deviation of the overlay thickness in the homogeneous section CF = Probability of achieving design life Values of CF that should be used for different levels of probability are given in Table 6.
the top surface should be lightly milled to ensure the new overlay does not ‘slip’ on the old surface and fail prematurely. Wide cracks should be sealed prior to overlay to prevent water entering the granular base course if reflection cracking occurs.16: Remedial Works prior to Overlay Defect Wide single cracks Wide connected cracks Alligator cracks with depressions Deep potholes Base course shoving Trench/Patch failure Alligator cracking without depressions Shallow potholes Asphalt shoving Slippage cracks Polished surface1 Note Remedial Works Crack Sealing2 Deep patch affected area Shallow patch affected area Mill and patch affected areas prior to overlay 1. Gravel and Concrete Roads. granular base course materials should be used. for higher levels of traffic on Category A and Category B roads a bituminous base course material can also be used.1. 2. Table 7. Otherwise defects in the existing road will cause the new overlay to deteriorate and premature failure will occur.17: Minimum layer thickness for patching prior to overlay Road Class International Primary Secondary Local Future Traffic Million ESAL > 20 < 20 >5 <5 >1 <1 >2 <2 Base course (mm) Granular 250 225 200 175 175 150 175 150 Bituminous 200 175 150 Sub-base (mm) Granular 200 200 200 200 200 150 200 150 Where the high deflections are related to lengths of road with poor or poorly maintained drainage then these shortcomings should be rectified prior to overlay construction. For low levels of traffic. When deep patching is needed the required minimum thickness of base course and sub-base materials are given in Table 11.DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 7 REMEDIAL WORKS PRIOR TO OVERLAY A bituminous overlay will only perform as designed if the correct remedial works are carried out before overlay. The remedial works are summarised in Table 7. Details on the construction and maintenance of road drainage are described in Design for New Bituminous. Republic of Kenya . Table 7. However. The type of remedial work will depend on the type of road defect and these are recorded during the Detailed Visual Condition Survey.Ministry of Roads 34 Draft Document – September 2009 . Where the existing surface has a poor texture and polished stone.
The Kenya road transport cost study: research on road deterioration. TRL Unpublished Report PR\INT\664\00 SHELL INTERNATIONAL PETROLEUM CO. AASHTO. Republic of Kenya . Shell Pavement Design .DESIGN for ROADS and BRIDGES PART 4 – Overlay Design 2009 8 REFERENCES AASHTO (1993). 2000. Tokyo. UK. J ROLT and T E JONES (1975). (1978). TRL. The characterisation of pavement strength in HDM-III and improvements adopted for HDM-4. London. Guide for the design of pavement structures. TRL Report 673. Washington DC. ROLT J (2000). Pavement structural number from FWD measurements for network analysis. 10th REAAA Conference. ROLT J and C PARKMAN (2000). USA HODGES J W.Ministry of Roads 35 Draft Document – September 2009 .
the boundaries between them can be identified and the thickness of each layer estimated. This is done by standing the DCP on a hard flat surface.1.1 Appendix 1 : DCP Test The DCP is an instrument which can be used for the rapid measurement of the in situ strength of existing pavements constructed with unbound materials.9 APPENDICES 9. The DCP needs three operators. Measurements can be made down to a depth of approximately 800mm and where the pavement layers have different strengths. The instrument is assembled as shown in Figure 3. such as concrete.2 OPERATION After assembly. one to hold the instrument. one to raise and drop the weight and a technician to record the readings. 9. before it is allowed to drop.1.1. Operating the DCP with any loose joints will significantly reduce the life of the instrument. checking that it is vertical and then entering the zero reading in the appropriate place on DCP Test Data Sheet shown in Figure 3.1 DESCRIPTION The DCP uses an 8 Kg hammer dropping through a height of 575mm and a 60° cone having a maximum diameter of 20mm. but not lifting the instrument. Republic of Kenya . 1 Key:1 Handle 2 Hammer (8kg) 3 Hammer shaft 4 Coupling 5 Handguard 6 Clamp ring 7 Standard shaft 8 1 metre rule 9 60° cone 2 3 4 5 6 Ø 20mm 7 9 8 9 · 60° INC 9. The operator must let it fall freely and not partially lower it with his hands. Care should be taken to ensure that the weight is touching the handle.2. The instrument is usually split at the joint between the standard shaft and the coupling for carriage and storage and it is important that when in operation the joints do not become loose. The instrument is held vertical and the weight raised to the handle. the first task is to record the zero reading of the instrument.Ministry of Roads 36 Draft Document – September 2009 .
the test should be abandoned and the tests repeated approximately one metre away from the first test. Penetration rates as low as 0. If only occasional difficulties are experienced in penetrating granular materials. if more precise values are needed it is advisable to calibrate the DCP for the material being evaluated. There is no disadvantage in taking too many readings. Agreement is generally good over most of the range but differences are apparent at low values of CBR in fine grained materials. but if readings are taken too infrequently. weak spots may be missed and it will be more difficult to identify layer boundaries accurately. if it is done too vigorously the life of the instrument will be reduced. It is therefore necessary to change the number of blows between readings. However it is usually easier to take a reading after a set number of blows. However. or by coring. Under these circumstances a hole can be drilled through the layer using an electric or pneumatic drill. It is more difficult to penetrate strongly stabilised layers. granular materials with large particles and very dense.3 INTERPRETATION OF RESULTS The correlation between DCP readings and CBR value has been determined by a number of authorities and a selection of these are given in Figure 3. during the test. 9. according to the strength of the layer being penetrated. For good quality granular bases readings every 5 or 10 blows are usually satisfactory but for weaker sub-base layers and subgrades readings every 1 or 2 blows may be appropriate. wear on the cone itself will be accelerated. If the lean becomes too severe and the weight slides down the hammer shaft. other causes of wear can also occur hence the cone should be inspected before every test.Ministry of Roads 37 Draft Document – September 2009 .5mm/blow are acceptable but if there is no measurable penetration after 20 consecutive blows it can be assumed that the DCP will not penetrate the material. it is worthwhile repeating any failed tests a short distance away from the original test point. After completing the test the DCP is removed by tapping the weight upwards against the handle. It is expected that for such materials the relationship between DCP and CBR will depend on material state and therefore.1.It is recommended that a reading should be taken at increments of penetration of about 10mm. the DCP leans away from the vertical no attempt should be made to correct it because contact between the shaft and the sides of the hole can give rise to erroneous results. high quality crushed stone. The DCP can be driven through surface dressings but it is recommended that thick bituminous surfacings are cored prior to testing the lower layers. Republic of Kenya . If. rather than dropping freely. The cone is a replaceable part and it is recommended that it should be replaced when its diameter is reduced by 10 per cent. The lower pavement layers can then be tested in the normal way. DCP is used extensively for hard materials. hence important information will be lost. Care should be taken when doing this. Little difficulty is normally experienced with the penetration of most types of granular or lightly stabilised materials.3.
48 .3. as shown in Figure 3.057 Log10 (mm/blow) The results can be either be plotted by hand. Republic of Kenya .1.Until local calibration is carried the following relationship given should be used Log10 (CBR) = 2. or processed in a spreadsheet.Ministry of Roads 38 Draft Document – September 2009 .
Ministry of Roads 39 Draft Document – September 2009 .4 CALCULATION OF STRUCTURAL NUMBER If required the Structural Number of the pavement can then be calculated from the DCP results using the following general equation. SN = 0.0394 Σ I aI dI where aI = Layer coefficient of layer I dI = Thickness of layer I (mm) Republic of Kenya .9.1.
Ministry of Roads 40 Draft Document – September 2009 .Date: Road No: Test No: Chainage: Direction: No of Blows DCP TEST DATA FORM Wheelpath Started test at: (Surfacing / Base/ Sub-base / Subgrade) Operator: Zero reading of the DCP (mm): Blows Σ mm No of Blows Blows Σ mm No of Blows Blows Σ mm Republic of Kenya .
within any homogeneous length. 1 jack hammer with generator (to assist with excavation). 1 machine operator if applicable. 1 or 2 spades (a fence post hole digger can also be useful). and FWD and DCP surveys. material to backfill and seal test pit : gravel. test pit log forms and clipboard.2 Test Pit Test pits should only be necessary on roads requiring rehabilitation.9. cement for stabilising gravel. water and cold mix for resurfacing. EQUIPMENT AND MATERIALS Test pits can be excavated by hand or by machine. depending on the availability of plant and the test pit programme required.2 SAMPLING AND TESTING PROCEDURE Republic of Kenya .2. 1 broom to tidy area on completion. ie the wheelpath adjacent to the shoulder of the road.1 LABOUR.Ministry of Roads 41 Draft Document – September 2009 .2. The following equipment and materials are required: • • • • • • • • • • • • • 1 backhoe (for machine excavation).a minimum of one at each end of the site (but see above). 1 chisel is often useful to assist with inspecting the wall of the test pit. which have been shown to be significantly weaker by either FWD or DCP testing. Machine operations are usually more productive but more costly than methods. In general the test pits will be dug in the near-side wheelpath. 9. 1 tape measure and thin steel bar to span pit (to assist with depth measurements). The purpose of carrying out a test pit investigation is to confirm the information obtained from surface condition survey. Pit digging is a time consuming and expensive operation and for this reason the location of each test pit should be carefully selected to maximise the benefit of any data collected. 1 driver for vehicle. Roads requiring maintenance with thin overlays (periodic maintenance) will only have test pits dug where FWD or DCP measurements indicate short lengths of weak pavement. The responsible engineer will select the number and position of the test pits to establish: • • the thickness and material properties of the road pavement in each homogeneous section the thickness and material properties of any lengths of road pavement. 9. and sample log book. The following personnel are required: • • • • • traffic controllers . sample bags and containers. 2 (if machine excavation) or 3 (if excavation) labourers. with some means of labelling each. 1 tamper or plate compactor for backfilling test pit. 1 pick. The minimum number of test pits dug in any one homogeneous length of road should not be less than one every 2 kms. and 1 supervising technician. equipment necessary to complete any required on-site testing.
All information should be recorded on the Pavement Test Pit Log. and the minimum working area required for a backhoe operation will be sufficient for machine excavations. the total depth of pit should be recorded along with any other information such as appearance of water in any of the layers. The average thickness of surfacing should be recorded. Care should be taken not to disturb the adjacent lower layer. All samples should be clearly labelled and proposed tests for the pit materials should be logged in a sample log book to avoid later confusion in the laboratory. no prior knowledge is required of the layer thickness since this becomes obvious as the hole is excavated. Define the edge of the test pit and remove surfacing. If a nuclear density meter is used. Not all these tests may be necessary . the materials specifications in use and an understanding of the pavement behaviour.8m by 0. 7. Some field testing might be necessary as well as subsequent laboratory testing of samples extracted from the pit. The edge of the pit can be cut with a jack hammer or pick and the surfacing ‘peeled’ off. This will depend on the results of previous surveys. a permanent location marker should be placed at the roadside.Ministry of Roads 42 Draft Document – September 2009 . Once it has been decided that there is no need to excavate further. a visual assessment made of the material and samples taken for laboratory testing. the position of a pit will be apparent after completion due to the patched surface. a smooth.2. On completion of any required density testing. summarises the various tests that may be required and references the relevant standards. It is often good practice to stabilise the upper layer with cement accepting that Republic of Kenya . The pit should be backfilled in layers with suitable material which should be properly compacted. If density tests are to be performed. 3. Usually. depending on the situation found. Reference should be made to the appropriate regulations in this regard. if long term monitoring is required.8m will be sufficient for excavation. the following procedure should be adopted: 1. the thickness of the layer can either be estimated from previous DCP results or construction details to determine the depth of testing. However.1 Field Procedure Before commencing the survey in the field. The required size of pit will depend on the sample sizes necessary for the selected tests. 8. For the sand replacement method. Set up traffic control. 2.9. A safe working environment should be maintained at all times. Continue to sample. The thickness of the layer and the depth at which samples are taken should be measured. Accurately locate position of test pit and record this on the Pavement Test Pit Log (see Figure G1). road condition and weather. taking care not to disturb the surface of the aggregate roadbase. but it can be increased later if found to be too small. 5. the responsible engineer should be clear as to the information required from each test pit. Table 9. test and excavate each pavement layer following the procedure above. 6. the layer can be removed over the extent of the trial pit. Record any relevant details such as surrounding drainage features. 4. Once it has been decided what testing is to be carried out and the location of the trial pits has been confirmed. It is important for the accuracy of the test that the layer is homogeneous.2. clean and even surface is required. Usually an area of about 0.
Plasticity Index Linear Shrinkage Particle Shape4 Elongation Index Flakiness Index Particle Strength4 Aggregate Crushing Value 10% Fines Value Aggregate Impact Value Particle Durability4 Aggregate Abrasion Accelerated Polishing Field or Lab Lab Lab Lab Lab Lab Lab Lab Lab KS 1238 Lab Lab ASTM D 3319-90 KS 1238 Particle Soundness Particle Density Sulphate test Particle density Particle density Moisture Content Oven dry7 ‘Speedy’ Nuclear Density Meter Moisture Density Relationship Layer5 Density Tests at various levels of compaction Sand Replacement Lab KS 1238 Lab KS 1238 Lab KS 1238 Lab Field Field Lab KS 999 Field Suppliers instructions Suppliers instructions KS 999 Hazardous radioactive material Recommended method For aggregates For soils Los Angeles Abrasion Value given in ASTM C 131-96 and C 53596 Procedure KS 999:Part 2:2001 KS 999:Part 2:2001 KS 999:Part 2:2001 KS 1238 Part 6 2003 KS 1238 Part 6 2003 KS1238 Part 11 2003 KS1238 Part 12 2003 KS1238 Part 13 Remarks Initial visual assessment on site.Ministry of Roads 43 Draft Document – September 2009 . 9. Correlated to PI Los Angeles Abrasion Value given in ASTM C 131-96 and C 53596 Republic of Kenya .full compaction will not be achieved.2.2 Laboratory procedure Table 18: Tests to be carried out Property Particle size distribution Plasticity Possible Tests Sieve analysis Plastic and Liquid Limits. 9. Initial visual assessment on site.2. A bituminous cold mix can be used to patch the backfilled pit. The site should be cleared and left in a tidy and safe condition for traffic.
In other cases all the tests listed for a given property might be required. All sampling should be carried out in accordance with the general guidance of KS 999 or KS 1238. as well as any specific requirements for each test. Kenya Standards (KS) are quoted where available. but they have the advantage of providing instant results. since they derive the moisture content by indirect analysis. whichever is applicable. These tests will only be required where a slope stability or settlement problem is being evaluated and will only apply to subgrade materials.1 KS 999 Hazardous radioactive material In some cases.Method KS 999 Core Cutter Method Nuclear Density Meter Bearing Capacity DCP California Bearing Ratio Shear Strength6 Vane test Various load tests Lab Field Field Lab or Field KS 999 Field KS 999 Lab Suppliers instructions See Appendix 9. For moisture content determination. an alternative is quoted. Both the `Speedy' and the Nuclear Density Meter methods require accurate calibration and validation. Validation should always be made with reference to the oven-dry method. The engineer must decide for which properties information is required and then design a suitable testing programme. The layer must consist of homogeneous material for these tests. Field tests require testing at the site and possibly further analysis in the laboratory.Ministry of Roads 44 Draft Document – September 2009 . the oven-drying method is recommended since it provides a fundamental measure of the moisture content. These tests will only be required for surfacing or base materials. Laboratory tests require only sampling in the field. the possible tests listed for a given property are alternatives. Republic of Kenya . Where no Kenya Standard is available.
on si te t est s (m oisture.Ministry of Roads 45 Draft Document – September 2009 . SGR -Subgrade Subjective assessment of material type and p roperti es Depth (range) at which any samples t aken Note any laborat ory tests required Note any particu r points o f i nterest such as pavement or drain la age condition. R -Roadbase . SF -Select Fil l.TEST PIT LOG Location Data Road Num ber: Chainage : P avement Conditi on: P urpose of Investigation: Date: From : Position: Done by: To: Weather: Section: Pit Num ber: Test Pit Data D epth (m m) Layer Fun on cti Method of Pitting: M aterial Descri ption Samp le Depth (mm) Tests Required Remarks 500 1000 1500 Notes for Co mp leting Test Pit Data Layer Functio n: Material Description: Sa mp le Depth: Tests Requ ired: Remar ks: S -Surfacing. de nsit y). Republic of Kenya . SB -Sub -b ase. evidence of groundwater e tc.
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