005 brown soil mech in pavements

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Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) 12 16 September 2004

ISBN Number: 1-920-01718-6 Sun City, South Africa

Proceedings produced by: Document Transformation Technologies cc

APPLICATION OF SOIL MECHANICS PRINCIPLES TODESIGN AND TESTING OF PAVEMENT FOUNDATIONS

S.F. Brown

Nottingham Centre for Pavement EngineeringUniversity of Nottingham, Nottingham, NG7 2RD, United Kingdom.

ABSTRACT

Asphalt pavement engineering embraces knowledge not only of the imported bound materialsbut also of the unbound aggregates and soils, which form the foundation layers of a pavement.These latter obey the principles of soil mechanics, which are gradually being more widelyapplied in practice to replace old empirical index tests such as the CBR. Following an outline ofthe key principles, their application to pavement design is discussed and a description given ofthe steps being taken in the UK to implement a performance-based approach to pavement

foundation testing.

1. INTRODUCTION

Although man has been designing and building roads for many centuries, the identifiablediscipline of pavement engineering has only emerged in recent times. Traditionally, the design ofpavements has been dominated by empirical rules based on experience. Developments overthe past 50 years or so have seen the establishment of theoretical frameworks for design and agradual understanding of the mechanical properties of paving materials. Today's practice tendsto combine theory with empiricism, the relative influence of each depending on thesophistication of the job and the resources available.

Pavement structures consist of layers of various materials, which are processed and imported tothe site and placed over a prepared foundation of the underlying soil. The imported materialsembrace asphalt, cement or other hydraulically bound materials and unbound aggregate,though not all types are always used on a particular site. Traditionally, the lower layers of theroad are constructed by a main contractor and the bound materials, notably the asphalt, aresupplied and paved by specialists. While the design engineer is expected to understand allmaterials, the fact remains that there are few with sufficient detailed knowledge across the fullrange, including the soil. Hence, the complete pavement engineer either in design orconstruction is a relatively rare animal.

In addition to requiring knowledge of soil mechanics for the subgrade and unbound granular

layers, the complete pavement engineer also needs to be competent in asphalt technology,concrete technology and stabilisation techniques and materials. This knowledge has to beblended with that of pavement design and the whole field of pavement maintenance, sincemany organisations today are responsible for the design, construction and long-termmaintenance of highway networks.

Asphalt pavement conferences, such as CAPSA, have tended to be dominated by papersdealing with bituminous materials and related issues so they often do not provide balancedcoverage of important related topics of interest to the pavement engineer, even one who isconcerned just with flexible pavements.


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CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA

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In reality, the performance of an asphalt layer in a pavement is intimately linked to that of thesupporting structure. Consequently, it can be dangerous to consider the properties of an asphaltlayer in isolation. If the asphalt construction is thick, exceeding 160mm, then the detailedproperties become less influential. Hence, for heavy-duty pavements where total asphaltthicknesses up to 400mm or more are involved, simplified assumptions can be made about theproperties of the subgrade and the sub-base at least for design purposes. However, forpavements with thin asphalt layers, the structural significance of the asphalt is small compared

to that of the underlying structure and it is vital that design and maintenance of such pavements,which are common in Southern Africa, should take this into account. Given the dominant role ofthe subgrade and of unbound aggregates in many such structures, it is clear that the principlesof soil mechanics, which govern the mechanical properties of these materials, should beapplied.

Over the past 30 years, the Author and his colleagues at Nottingham have carried out extensiveresearch into the properties of soils and granular materials relevant to pavement engineering(Brown, 1996). In addition, they have proposed ways in which their findings could beimplemented in design practice both for new pavements and for the evaluation of existing ones.The key issues from this research, which are relevant to conditions in Southern Africa, arepresented here together with some new developments in UK practice that involve theimplementation of research.

2. MECHANICAL PROPERTIES OF SOILS AND GRANULAR MATERIALS

2.1 Introduction

Soils and compacted granular materials consist of mineral particles embracing void spaceswhich may contain water. The maximum size of the particles can vary from several mm toseveral microns but the mechanical properties of the mass observe the same principles.Consequently, the laws of soil mechanics apply both for compacted crushed rock bases as wellas for clays and all combinations in between. In the text which follows the term 'soil' is used to

describe all unbound materials used in and below pavements.

The essential aspects of soil mechanics for pavements embrace the following main points:

1. The materials observe the principle of effective stress (') which states that:' = u (Eq.1)

in which is the total externally applied stress and u is the pore water pressure in the voids.

2. Soils in pavements are subjected to large numbers of repeated stress applications at levelsgenerally well below their shear strength. Higher stress levels are involved for granularbases under thin surfacings or when subject to direct loading during construction.

3. The resilient modulus (Er) of the soil and the accumulation of permanent strain underrepeated loading are key parameters for design and are defined in Figure 1.

4. In well-maintained pavements, the water table will be well below formation level and most ofthe soil will be in a partially saturated state.

The determination of effective stress (Equation 1), which is the parameter that controls themechanical properties of soil, is not straightforward. It can only be calculated from the totalstress, which can easily be determined from self-weight and wheel loading, and the porepressure, which, for partially saturated conditions, is not easy to specify. The Author hasdiscussed this in more detail elsewhere (Brown, 1996) but it should be noted that the soilsuction characteristics are crucial. This refers to the negative pore pressure which arises in soilabove the water table under zero external stress and which can achieve very high values in

fine-grained soils that are partially saturated. In such circumstances the effective stress will beapproximately equal to the soil suction, which is the reason why this parameter has been widelyused to assist in defining the mechanical properties of soils for pavement design purposes.


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Figure 1. Definitions of resilient modulus (Er) and plastic strain (p).(q = deviator stress, = a axial strain in triaxial test).

From Equation (1) it is clear that if the pore pressure is positive and high, then the effectivestress will be low causing poor strength and stiffness. It is for this reason that good drainage isessential to avoid high degrees of saturation. This point is illustrated in Figure 2, which showsboth laboratory and field data, the latter covering materials of various qualities from G1 (best) toG4 (worst).

a) Repeated load triaxial tests at identical stressconditions.

b) HVS tests on various granular materials(Freeme and Servas, 1985).

Figure 2. Effects of drainage conditions on the performance of granular materials.

2.2 Resilient Modulus

An important feature of the resilient behaviour of soils is that of non-linearity. The value ofresilient modulus is a function of the applied stresses and this is illustrated in Figure 3 with dataon London clay. This clearly shows how Er increases with soil suction and decreases with thelevel of applied deviator (shear) stress. The suction value dominates when it has a high value,which is the case for dry soils. Brown et al (1990) showed that the value of suction for a range ofsoil types can be estimated from the Liquidity Index which is a function of the Atterberg Limitsand the water content.

Figures 4 and 5 show the importance of stress ratio in determining the resilient strain and,hence the resilient modulus, for both a clay and a crushed rock. This reflects the fact that soil is

a frictional material. Thom and Brown (1989) showed that there is a rough correlation betweenthe surface friction of particles from a variety of sources and the resilient modulus of thecompacted mass (Figure 6). They also showed, perhaps surprisingly, that the actual state of


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compaction did not have a major effect on resilient modulus for crushed limestone over a widerange of gradings. This may seem less surprising when the extreme case of rail ballast isconsidered. This is an essentially single sized material with large voids but is used to distributehigh stresses below sleepers to protect the sub-ballast and subgrade, so it must mobilise areasonably high value of resilient modulus. Thom and Brown's tests involved 18 differentmaterials including crushed rock, sand, gravel, slag and furnish bottom ash all tested in a drycondition. The values of resilient modulus at a stress level representative of the centre of a

typical granular layer in a pavement varied between 100 and 550 MPa.

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60

Deviator Stress (kPa)

Resilient

Modulus(MPa)

Suction = 76kPa Suction = 43kPa Suction = 33kPa Suction = 19kPa

Figure 3. Resilient modulus of a silty clay as a function of stress conditions.

Figure 4. Influence of stress ratio on resilient strains in compacted london clay.


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Figure 5. Effect of stress ratio on resilient strains in a crushed limestone.

Figure 6. Relationship between surface friction and resilient modulus for various aggregate types.

Thom and Brown (1988) also investigated the effect of grading on the permeability and suctioncharacteristics of a crushed dolomitic limestone. Figure 7 shows the low permeability obtainedat the dense end of the spectrum for materials with high fines content. Correspondingly highvalues of suction were measured. Thom and Brown (1987) demonstrated that the very densematerial could easily achieve saturation when exposed to a moderate rainfall because of its highsuction and low drainage performance. This points to the need for such materials to beprotected during construction in wet weather and to be well sealed throughout the pavement life.The problem arises particularly when the fines content is high. Conversely, if a high suction level

can be sustained, then correspondingly high values of resilient modulus are mobilized.


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Figure 7. Influence of grading on permeability of a crushed limestone.

2.3 Permanent Deformation

Rutting in pavement wheel tracks arises because of accumulated vertical permanent strainswithin the structure and, in principle, all materials, except those that are cement bound and verybadly cracked, can contribute. For pavements with thin asphalt surfacings, it will be the granularbase and subgrade that are most likely to be the sources of rutting. The permanent (plastic)

strains arising in soils under repeated loading have been extensively studied and some basiccharacteristics have emerged.

For clays of many types, a threshold stress level has been identified, below which themagnitude of accumulated plastic strain is negligible. Figure 8 shows the results reported byCheung (1994) for a silty clay subjected to 1,000 cycles of load, which indicate a thresholddeviator stress of about 20 kPa at a plastic strain of about 0.2%. His work supported by that ofLoach (1987) showed that the threshold stress (q t) was directly proportional to the soil suction(S), ie:

qt = KS (Eq.2)

The constant of proportionality (K) varied somewhat with the soil type (0.5 to 0.8) and theprecise criterion used to define the threshold stress. However, this characteristic of soils is veryhelpful for pavement design where the objective is to ensure that the maximum deviator stressat formation level does not exceed the threshold value. Brown and Dawson (1992) used thisconcept in their proposals for pavement foundation design.

Similar testing of a compacted crushed limestone (Brown, 1996) yielded a threshold conditionexpressed in terms of the shear to normal stress ratio, = q/p':

= Cf (Eq.3)

where the suffices t and f refer to threshold and failure conditions respectively. The value of C

for a typical hard crushed limestone was 0.7, which implies that negligible plastic strain willdevelop if the peak stress ratio remains below 70% of that at failure.


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Figure 8. Threshold stress from repeated load triaxial tests on a compacted silty clay.

Noting that clays and crushed rock obey the same soil mechanics principles as frictionalmaterials, the results for clays could be examined in the same context as those for granularmaterial. If stress ratio is the key parameter, it would explain why the value of K in Equation (2),varies somewhat between soil types. The comparison may be done in an approximate way byassuming that the effective stress, p', in the granular material is the suction value, S. Hence,from Equations (2) and (3), K = Cf. Table I shows the results of the computations, indicatingvalues of 0.6 and 0.7 for two of the clays but a much lower value of 0.3 for the other. Theseresults are sufficiently encouraging to merit further study but, meanwhile, give some basis forconsidering the pavement design implications.

Table 1. Threshold stress computations.

Soil Type Cf '

(degrees)

f C PlasticityIndex (%)

London clay 0.5 23 0.9 0.6 48

Bothkennar clay 0.4 34 1.4 0.3 29

Keuper marl 0.8 29 1.2 0.7 19

Crushedlimestone

1.54 53 2.2 0.7 0

The threshold stress concept is similar to that of the 'Shakedown Limit' idea, which originated instructural dynamics and was first applied to pavements by Sharp and Booker (1984). This isunder study at Nottingham presently and has been reviewed by Werkmeister et al (2001). Theessential principle is illustrated in Figure 9, which shows that below the shakedown limit amaterial or a complete pavement structure will reach an equilibrium state under which no furtheraccumulation of permanent deformation will develop.

Thom and Brown (1989) also investigated the parameters that influence the accumulation ofpermanent strain when the applied stress ratio exceeds the threshold value. They demonstratedthat good compaction was important for all grading types and that the shear strength wasinfluenced by similar factors to those which affected permanent strain. This is not surprising,since strain will only accumulate when the peak stress conditions under a wheel load arerelatively close to failure.


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Figure 9. Repeated loading leading to shakedown.

With regard to permanent deformation resistance, Brown and Chan (1996) showed that, byadopting the grading design ideas used for asphalt, it is possible to enhance the properties of agranular layer. Standard vibrating hammer compaction tests were used to determine theoptimum grading for a given maximum aggregate size in order to maximise the dry density. Thisenhanced material showed a significant increase in resistance to the development of permanentdeformation both in repeated load triaxial and wheel track testing, the results of which areshown in Figure 10.

0

5

10

15

20

25

30

1 10 100 1000 10000

No. of Load Applications

PermanentDeformation(m

m)

Standard grading Optimum garding

Figure 10. Wheel tracking tests on different gradings of crushed limestone.

3. IMPLICATIONS FOR PAVEMENT DESIGN AND EVALUATION

The marked non-linearity of stress-strain relationships for granular materials has been quantifiedthrough extensive repeated load triaxial and other laboratory-based research. The much-used'k-' relationship, which expresses resilient modulus (Er) as a simple exponential function of thetotal mean normal stress was shown to be a simplification to be used with care and not incircumstances when the detailed stress conditions in the granular layer need to be computed

(Brown and Pappin, 1981). It is normally used in terms of total rather than effective stress whichwill not yield accurate results except for material in a dry state. As noted above, Erdepends bothon the mean normal stress and the deviator stress and, in particular, on the ratio between them


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as illustrated by Figures 4 and 5. For detailed pavement analysis, the recommended approachis to characterise the material in terms of stress-dependent values for the bulk and shearmodulus.

Brown and Pappin (1985) showed that this approach could be used to simplify design sincethey demonstrated, at least for pavements with over 100mm of asphalt, that a single effectivevalue of resilient modulus could be used for linear elastic analysis. The value depended on the

quality of the granular material but could vary between 50 and 100 MPa. Field tests reported byBrown (1996) using the Failing Weight Deflectometer (FWD) showed that the effective resilientmodulus of a granular capping layer was 90MPa when the test was conducted directly on thesurface of this layer but increased to 200MPa when testing was conducted on the completedasphalt pavement. A value of 240MPa was reported for the sub-base.

This background was used by Brown and Dawson (1992) to deal with the design of pavementfoundations as the first part of a two-stage procedure, which involved wheel loading fromconstruction traffic running directly on the sub-base. Stage two involved the completedpavement with its asphalt layers in place. For pavements with thin surfacings, their stage Iprocedure could be used. They recognised that the mobilised resilient modulus for the granularmaterial will be higher when the stresses are higher, which is the situation when there is little or

no asphalt. They recommended 200 to 400 MPa in this situation for good quality material. InFrench practice (LCLC and SETRA, 1994), suggested values range from 200 to 600 MPadepending on material quality.

The above discussion indicates the desirability for testing materials to determine their designparameters whenever possible, rather than to estimate them. This is the approach now taken inthe UK and the next section describes the latest techniques used to implement an end-product,performance based method for design of pavement foundations.

4. TOWARDS PERFORMANCE-BASED DESIGN

In their two-stage design method, Brown and Dawson (1992) proposed that the pavementfoundation should be characterised in terms of its 'Equivalent Stiffness' (E f), defined as the valueof Young's Modulus determined from a dynamic plate loading test assuming the foundation tobe an elastic half space. It is sometimes referred to as the 'Surface Modulus'. Variouscombinations of subgrade and granular layers can achieve the same E f and the approach hasthe particular merit that Ef can be measured on site to check compliance. It also recognises thatthe flexure to which the overlying asphalt is subjected is a function of the effective stiffness ofthe supporting structure. This philosophy has been adopted in the UK and the Highways Agencyis in the process of putting appropriate specifications in place. The concept of an equivalentfoundation based on its measured stiffness modulus provides contractors with the freedom tochoose materials from convenient sources including the use of secondary aggregates and toapply them in a combination which achieves the specified effective stiffness. The approach is

somewhat similar to that used for many years in Germany with a static plate loading test. Fourclasses of foundation are proposed, the upper two of which are intended to apply forfoundations with one or more stabilised layers. The lower two are defined in terms of stiffnessesof 60 and 100MPa respectively and can be achieved with unbound materials.

This approach to design involving field testing, including trial sections to check compliance,recognises that it is difficult to reproduce site conditions in a laboratory test specimen of soil orgranular material, in the way which can be done with asphalt through the use or cores fromcompacted layers. Nonetheless, at the preliminary design stage, it is useful to carry outlaboratory tests to assess candidate materials. In the field of asphalt mixture design, theNottingham team developed a simple apparatus known as the Nottingham Asphalt Tester (NAT)(Cooper and Brown, 1989), which is now in widespread practical use and covered by

appropriate standards. It has revolutionised the approach to asphalt technology in the UK andfacilitated the performance-based approach, allowing asphalt companies to innovate.


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A similar procedure is now in the final stages of development for soils, granular and lightlystabilised materials. The apparatus is known as the 'Springbox' and is described by Edwards etal (2004) and shown in Figure 11. It is designed to be used in the same test frame as a NAT andworks on similar principles to the K-Mould developed by Semmelink and de Beer (1995) but issomewhat simpler and uses a 170mm cubical rather than a 152mm cylindrical test specimen.Lateral spring loading is applied in just one of the horizontal directions. In the developmentphase, a range of materials has been tested to determine both resistance to permanent

deformation and resilient modulus. Realistic results have been obtained and typical data areshown in Figures 12 and 13 for a crushed limestone sub-base material. The programme of initialwork with the apparatus is continuing using a wide range of stabilized and unstabilisedmaterials.

Figure 11. The Springbox.

Figure 12. Resilient modulus results for crushed limestone from the Springbox.


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Figure 13. Permanent strain measurements on a crushed limestone in the Springbox.(Stress conditions as shown in Figure 12).

Specifications for field-testing are also in preparation and include dynamic plate loading todetermine equivalent stiffness and trafficking with a loaded truck to assess rutting resistance.Three dynamic plate-loading devices have been used. The most accurate and sophisticated isthe FWD and this has been regarded as the standard against which to judge the other two; theGerman Dynamic Plate (GDP) and the Prima, which is shown in Figure 14. Details of these andother devices have been presented by Fleming et al (2000). The proposed procedure involves a300 mm diameter plate and contact stresses of 150 to 250 kPa. The GDP cannot quite reachthis stress range and the computed stiffness values are lower than those from the FWD so thatan adjustment factor is required. The Prima is more satisfactory.

After considering various possibilities for rutting tests, loading with a 32t four-axle truck for 75passes has been recommended. The rut depth should be measured under a straight edge insuch a way that the humps on each side of the rut are ignored as this was shown to give morereliable and consistent results. The allowable rut depth has yet to be finalised but valuesbetween 30 and 50 mm have been proposed for tests on capping, the larger values applying forthicker construction. For tests at sub-base level it would be logical to adopt 30 mm.

The developments outlined above indicate that use of the CBR as an index test for pavementfoundations is likely to disappear as more rational and relevant procedures are introduced. Thelimitations of CBR have been well documented by Brown (1996).

The semi-empirical subgrade strain criterion for design to limit rutting (Brown and Brunton,1984) is also in the process of being replaced as knowledge of soil mechanics and the newapproaches to pavement foundation design and testing become more widely used.


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Figure 14. The prima dynamic plate loading test.

5. CONCLUSIONS

This review of basic principles for pavement foundations and their application in practice hasshown that:

1. Application of the soil mechanics principle of effective stress is required to properlyunderstand subgrade and granular material behaviour.

2. Resilient modulus is strongly influenced by soil suction but is principally a function of theratio of shear to normal stress.

3. Granular materials with high density have good resistance to rutting and can sustain highsuctions but are susceptible to water absorption and must be protected from rainfall.

4. The threshold or shakedown concept is useful for design to minimise rutting.5. Dynamic plate loading can be used to determine effective insitu stiffness values for

foundations as part of a performance-based approach to design.6. A new, simple low-cost apparatus known as the 'Springbox' has been developed to test

foundation materials in the laboratory.

7. The introduction of new techniques for characterising soils and foundation materials shouldreplace empirical index tests such as the CBR.

6. ACKNOWLEDGEMENTS

The Author is grateful to a number of colleagues past and present from the pavement researchteam at Nottingham who have contributed to many years' research on pavement foundations.Amongst current staff in the Nottingham Centre for Pavement Engineering and at Scott WilsonPavement Engineering, the assistance of Nick Thom, Robert Armitage and Paul Edwards hasbeen particularly appreciated.


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7. REFERENCES

Brown, S F.,1996. 36th Rankine Lecture: Soil Mechanics in Pavement Engineering.Geotechnique, Vol. 46, No. 3. 383-426.

Brown, S F, O'Reilly, M P and Loach, S C., 1990. The relationship between CaliforniaBearing Ratio and elastic stiffness for compacted clays. Ground Engineering. Vol. 23, No.

8. 27-31.Thom, N H and Brown, S., 1989. The mechanical properties of unbound aggregates fromvarious sources. Unbound Aggregates in Roads, (Ed. Jones and Dawson). Butterworths.130-142.

Freeme, C R and Servas, V., 1985. Advances in pavement design and rehabilitation.Accelerated Pavement Testing, CSIR, Pretoria.

Thom, N H and Brown, S F., 1988. The effect of grading and density on the mechanicalproperties of a crushed dolomitic limestone. Proc. Aust. Road Res. Board. Vol. 14, Pt. 7.94-100.

Thom, N H and Brown, S F., 1987. The effect of moisture on the structural performance ofa crushed-limestone road base. Transp. Res. Record 1121, Transportation Research Board.50-56.

Cheung, L W., 1994. Laboratory assessment of pavement foundation materials. PhDthesis, University of Nottingham.

Loach, S L., 1987. Repeated loading of fine-grained soils for pavement design. PhD thesis,University of Nottingham.

Brown, S F and Dawson, A R., 1992. Two-stage approach to asphalt pavement design.Proc. 7th lnt. Conf. on Asphalt Pavements. Vol.1. Nottingham, 1992. 16-34.

Sharp R and Booker J., 1984. Shakedown of pavements under moving surface loads.ASCE Journal of Transport Engineering. No 1. 1-14.

Werkmeister S, Dawson A R and Weliner F., 2001. Permanent deformation behaviour ofgranular materials and the shakedown theory. Transp. Research Record 1757.Transportation Research Board. 75-81.

Brown, S F and Chan, F W K., 1996. Reduced rutting in unbound granular pavement layersthrough improved grading design. Proc. lnst. of Civil Engineers Transport. Vol. 117. 40-49.

Brown, S F and Pappin, J W., 1981. Analysis of pavements with granular bases.Transportation Research Record 810. Transportation Research Board. 17-22.

Brown, S F and Pappin, J W., 1985. Modelling of granular materials in pavements. Transp.Res. Rec. 1022. Transportation Research Board. 45-51.

LCPC and SETRA, 1994. Conception et dimensionnement des structures de chaussee.

Cooper, K E and Brown, S F., 1989. Development of a simple apparatus for themeasurement of the mechanical properties of asphalt mixes. Proc. EurobitumeSymposium. Madrid. 494-498.

Edwards, J P, Thom, N H and Fleming, P R., 2004. Development of a simplified test forunbound aggregates and weak hydraulically bound materials utilising the NAT. Proc. 6thInt Symposium on Pavements Unbound. (in press).

Semmelink, C J and de Beer, M., 1995. Rapid determination of elastic and shear propertiesof road-building materials with the K-mould. Unbound Aggregates in Roads , (ed. Dawsonand Jones). 151-161.

Fleming, P R, Frost, M W and Rogers, C D F., 2000. A comparison of devices for measuringstiffness insitu. Unbound Aggregates in Road Construction (ed. Dawson). Balkema.193-200.

Brown, S.F and Brunton, J M., 1984. Improvements to pavement subgrade strain criterion.Journ. Transp. Eng. ASCE. Vol.110, No. 6. 551-567.

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