Field tests on pavement foundations

A. R. Dawson s & N. H. Thom2

1 Department of Civil Engineering, University of Nottingham. Nottingham, NG7 2RD, UK 2SWK Pavement Engineering, Highfields Science Park, Nottingham, NG7 2QP, UK

Abstract. Generally the California Bearing Ratio (CBR) is used to quantify the properties of subgrades for use in pavement design. In the last few years, a wide variety of less familiar, but potentially more useful, tests has been developed. This paper describes some trials in which the results of Dynamic Cone Penetrometer, Clegg Hammer and Falling Weight Deflectometer tests are compared. Comparisons with the output of an instrumented vibrating roller and Plate Bearing Test results are also made.

The practical problems of relating the collected information to the engineering properties (particularly stiffness) of the soil are discussed and some tentative proposals for dealing with these problems are put forward.

The foundation of a road pavement performs several the former may be due to one application of a large functions of which the following are probably the most load or several applications of moderate loads. important:

(i) carry construction traffic; (ii) provide a stiff base for the compaction of higher

layers; (iii) carry traffic loadings on the completed pavement.

albeit dispersed over a wide area and attenuated by higher layers of the structure.

It addition it should provide for the drainage of pore pressures beneath notionally impervious bound layers and, realistically, drain water percolating through these upper layers. If any of the foundation is frost-susceptible then the upper layers of the foundation. that is the sub-base, must provide frost insulation.

The stress levels imposed by construction traffic. layer compaction and final traffic upon the foundation vary widely. When the pavement is in use the stress levels are relatively small and failure conditions are not approached. In this case the transient deformation levels are important for the pavement as a whole. If they are too high then large transient horizontal tensile strains will be developed at the bottom of higher. stiffer layers in the pavement because inadequate support is being provided by the foundation. In time repeated application of these tensile strains may lead to fatigue failure of the bound lavers. More indirectly, but nevertheless of importance. the vertical transient de-formation can be related to the ease with which a bituminous pavement will develop ruts.

Under construction traffic. and due to the compac-tion of layers onto the foundation, the imposed stress levels are much higher. Accordingly failure conditions may be approached. Fortunately traffic is rarely canal-ized during construction and the number of loadings applied is small. Hence at this stage it is normal to experience both permanent and resilient deformations;

Foundation characteristics

In all these situations the design requirement consists of relating the load which will be applied at the various stages of construction and service to the resulting deformation. For resilient movements the stiffness is required but for permanent deformation indirect rela-tionships with stiffness may be applied. The basic problem for the designer is to determine the stiffness of the consituent layers of the foundation.

Conventional road foundation design has utilized the results of California Bearing Radio Tests (CBR) (Anon, 1975). Although some of the test plunger movement is due to resilient deformation much of it is due to permanent shear deformation. For this reason the CBR test gives results relatable more closely to shear strength than to stiffness. The small diameter of the CBR plunger causes a further problem when attempting to measure values for granular materials. Readings with a large scatter and usually indicating strength in excess of the true value are obtained.

For this reason a number of other tests have been suggested which measure stiffness more directly. Ideal-ly a load as large as that applied during construction needs to be applied over a similar area and period of time. It would also be desirable that this is applied such that the principal stress directions rotate as the load is applied and removed as they do under real traffic loading. No piece of test apparatus can meet all of these requirements with the exception of extremely expen-sive vehicle simulations as used in South Africa (Freeme et al. 1986).

The present paper describes a test which comes close to simulating traffic loading and the results obtained

From BELL, F. G., CULSHAW, M. G., CRIPPS, J. C. & COFFEY, J. R. (eds) 1990, Field Testing in Engineering Geology. Geological Society Engineering Geology Special Publication No 6, pp 137-143

138 A. R. DAWSON & N. H. THOM

from its use are compared with results obtained with simple non-simulative test methods. In addition the use of an instrumented vibrating roller is discussed.

Test methods

Test methods can broadly be broken down into those which are large and those which are small. The former category has been well covered by Cobbe (1988) who describes the Plate Bearing Test (PBT), instrumented rollers, and also falling weight and vibrating weight deflectometers of various kinds. The PBT has been in use for many years and is well known. The right load intensity over the correct area can be applied but the short time period of loading cannot be modelled and hence rheological and pore pressure dissipation errors occur.

Falling weight deflectometers (FWD) overcome this probleni of correctly modelling the test duration and area of loading but. at considerable finanical and interpretive expense. However, the Dynaplaque is a simplified version of this test developed for testing foundation layers and costs can be somewhat reduced (Cobbe, 1988; Benoiste & Shaeffner 1988). Vibrating weight deflectometers (VWD) are also available but, in common with other types of test, these have been developed for measurements on completed pavements. The constant repetition of load by a VWD at founda-tion level may be expected to create artificial pore pressure changes.

Instrumented rollers operate by detecting the re-sponse of the rolling drum to the applied vibration (Cobbe, 1988; Thom, 1987). Apart from the fact that the loading time is very fast and repetitive and therefore not representative of traffic loading (although clearly representative of construction plant) three problems arise. Firstly, the testing can only be carried out during compaction and hence is not truly indepen-dent of the construction process. Secondly, it is largely restricted to sub-base/capping layers of placed aggre-gate. Thirdly, the results are very machine and aggregate/sub-base dependent. These restrictions will probably limit the technique to construction control uses rather than layer property quantification.

More manageable and economical tests can be achieved by the use of a variety of hand-held test equipment. The field CBR apparatus is well known; penetrometers of various types are also available. In 'static' penetrometers a cone or flat end to a rod is pushed into the ground to a pre-set distance and the load required to do so is recorded in terms of CBR. Its maximum reading is 6%. The 'dynamic' cone penetro-meter (DCP) uses a repeatedly dropped weight to drive a point into the ground, the penetration per drop being recorded.

Of a more truly dynamic character is the Clegg Hammer (Clegg, 1976) in which an accelerometer is

used to determine the energy absorbance (or wise) of a layer onto which a hammer rod is drol pre-determined distance. This test is thus of th, kind in that a short pulse loading is applied. In respects it is less than ideal, having a small loadii (50 mm diameter) at a high stress (several th, kPa) for a very brief period (about 1 ms).

The investigation

Using a variety of the test methods described previous section comparative results were obtai testing subgrade and sub-base materials in situ same locations. By this means the relative mt each method were examined and the importance limitations already discussed were highlights testing described in this paper was on the sub-g capping layers of the pavement, further tests sub-base layers are reported by Thom (1987).

A total of five sites were used for the tests. was north-east of Wakefield, Yorkshire where a haul road was constructed for the West Yorkshir Waste Management Board. As part of the roar high embankment consisting of locally obtained ered Coal Measures sandstones, mudstones, a: stones was constructed. Instrumented roller test carried out at this site together with Clegg Fa static cone penetrometer and PBT measurement

The other four sites were situated on part of t Ashby and Measham by-pass, Leicestershire Triassic sandstones and marls were excavated a used as fill. Two of these sites were in cuts resp, in marl (Site B) and sandstone (Site E). Sites C were respectively on marl fill and a limestone e which was obtained from Bredon on the Hill, Le shire. At all four sites FWD testing was compar DCP, static penetrometer and Clegg H measurements.

The results of the testing are presented in Fi in Tables 1-4. The FWD results given show t applied and the resulting deflection at the eentr, platen, at a radius of 300 mm (50 mm beyond the edge) and at a radius of 500 mm. Deflections at radius were measured but are not tabulated were generally small and of doubtful accurac normal practice to ignore readings taken on FWD drop at a particular location as bedding can be significant. Whilst this may be appropriat dealing with paved surfaces, on subgrades the ri application of the platen load may be expected t significant compaction. For this reason both fil and fourth (last) drop data are presented.

The Dynamic Cone (DCP) data are summa Tables 1-4 in terms of the interpolated nun blows to give 150 mm penetration and also to rate of advancement once the cone has bedded soil. The Clegg Impact Values given are the av,

237:

23 19 22 21 >6

21 >6

CIV 23 CBR(Stct,c Cone) >6

22 28 17 >5

1. 1

CIV 19 CBRIStztic Cone) >6 CBRIPIcte Dec,=irg) 6

23 27 /.8

19 ' 11, 16 13 18 CIV 1/. CBR (Stchc Ccne) 1.5

2350 2360 Chaincee Im)

L. STRIP A

3

2 Ui

1

0

STRIP B

Ui

STRIP C

Ui

0

2

0

FIELD TESTS ON PAVEMENT FOUNDATIONS 139

1. Summary of test results from site A.

five measurements taken around the peripher\ of the FWD test position. At site E, where the sand \\us weak enough for a static cone test, an average CBR equivalent (in which units the device is read) after three readings is given.

Test results

At Wakefield, site A. the formation was rolled and a continuous recording of w (the omega value) was made. (This is a relative measure of the effecti\ e power transmitted to the ground (Heinrich, 1985)). Readings were taken over three compaction strips and the results, together with other test results are shown in Fig. 1. This clearly shows that approxinlatel\ half of strip was on relatively soft subgrade and that similar conditions also occurred on strip 'B' in two locations. Since Clegg Hammer tests were carried ou: at 5 m intervals. the readings at chainages 2365 m and 2370 m on strip •C'' are expected to have been affected by the lower subgrade strength in this area. In general, the Clegg Hammer results for strip 'C' are lower than for the other strips, thereby partially correlating \k ith the w values. Between 2350 m and 2360 m this relationship is not evident and there are insufficient PBT or static

penetrometer data for meaningful comparisons to be made although the two CBR values obtained from the factored PBT results do show respectively unexpected-ly low and high values. However, these latter data mar reflect the difficulty of testing mixed sand, shale and sandstone fill.

At Site B (Ashby-Measham bv-pass) on the strong intact marl all the test methods gave fairly uniform results (Table 1). The Clegg, Hammer results are sensibly constant and the two DCP values are yer\ similar to each other. The FWD data is more variable with a central deflection varying by a factor of almost and even greater scatter for the other deflections despite fairly constant loading. A reduction in deflec- tion from first to fourth loading, presumably due to compaction of the marl by repeated loading, was observed.

At Site C on marl fill the opposite effect to tha: observed for site B was observed with slightly increased deflections occurring after the fourth loading when compared with the first one (Table 2). This is no: entirely attributable to the increase in applied load and may be due to load induced pore pressure increases reducing the stiffness of the material. The softer soil as indicated by the FWD results appears also to have given rise to higher DCP rates of penetration. There is no obvious relationship between either the FWD or the DCP results and the CIV data with high (stiffer) and low (softer) values being obtained by the Clegg Hammer for soils identified by the FWD as being respectively softer and stiffer.

At Site D (Table 3) only the central and 300 mm radius deflection gradings were reliably measured by the FWD. The central deflection is fairly consistent as is, with one exception, the deflection at 300 mm radius. The CIV values are also consistent. the small fluctua- tions in value being apparently unrelated to fluctu, lions in FWD deflection. The two DCP readings taken are reasonably close to each other but the difference does not seem to correspond with any significant difference in either the FWD or CIV reading.

At Site E (Table 4) there is a marked variation in the results. Where intact sandstone was tested small FWD settlements and high CIV readings were obtained. Furthermore the rock was too hard to allow any CPT penetration. Where uncemented sand, or sand dis-turbed by the earthworks operation occurred, the opposite behaviour was observed. The type of reading was further complicated in that loose sand commonly lay on top of intact sandstone. The deeper stress bowl of the FWD would be expected to be effected by the stiffer material beneath the surface, whereas the Cleoo Hammer might be less effected. Where disturbed blocks of sandstone were set into the uncemented sand, readings of the smaller devices could vary widely over a few centimetres. For these reasons poor correlation between the results of the different tests was expected and observed. This is clearly demonstrated in the last of the tests where a stiff FWD response was observed for

140 A. R. DAWSON & N. H. THOM

Table 1. Site B: Marl Cutting, summary of results

FWD Load FWD Deflection DCP

(kPa) Centre 300 mm radius 500 mm radius mm/drop drops for 150 mr

1st drops 175 1525 125 0

190 985 89 22 17S 936 312 7 17S 832 187 40

1S7 1124 150 25

mean 182 1117 173 19

4th drops 17S 1256 141 18

196 697 92 28 187 778 309 10 176 647 174 42

190 945 162 34

2.25 63.0

2.09 64.5

7.17 63.8

2.25 63.0

2.09 64.5

mean 185 865 176 26 7 .17 63.8

Back calculated stiffnesses: 57 MPa 0-300 mm

407 MPa 300 mm & deeper.

Table 2. Site C: Marl fill, summary of results

FWD Load FWD Deflection DCP

(kPa) Centre 300 rum radius 500 mm radius mm/drop drops for 150 mm

1st drops 150

2573

986 298

3.91 43.2 152 2581 965 379 141 2802 1251 520

5.41 34.2 138 2813 1055 353 140 2735 1380 396

mean 144 2701 1127 389 4.66 38.7

4th drops 161 2821 932 328

3.91 43.2 158 2824 1046 423 147 3153 1537 608

5.41 32.4 146 2782 1226 440 136 2673 1250 410

mean 150 2851 1198 442

4.66 38.7

Back calculated stiffnesses: 23 MPa 0-300 mm

10 MPa 300 mm-600 mm v large 600 mm & deeper.

FIELD TESTS ON PAVEMENT FOUNDATIONS 141

Table 3. Sift, D: Limestone capping, summary of -es Os

FWD Load FWD Deflection DCP CIV

( k Pa) Centre 0/ 300 mm radius (o 500 mm radius min drop drops for 13, ,imm (mean)

3:ops 176 798 195 42 35.4

3.50 40.4 175 753 105 2 36.4 177 781 175 21- 47.0 176 835 250 96- 43.0

2.79 46.9 171 794 219 40- 38.8

rile,7, 175 792 189 40 3.15 43.7 39.1

4th ,::Tops 176 657 199 66 35.4

3.50 40.4 178 571 III 16' 36.4 175 568 182 53 42.0 176 686 251 108 43.0

2.79 46.9 172 590 209 70 38.8

mean 175 614 190 74+ 3.15 43,7 39.1

Bac: calculated stiffnesses:- 92 MPa Capping

254 MPa 0 mm-300 mm

203 Mpa 300 mm & deeper.

' Irregular deflection bowl, reading suspect. + Ignoring abnormal reading.

Table 4. Site E: Sandstone/sand cutting, summary of results

FWD Load FWD Deflections DCP drops for Static Cone CIV (kPa) Centre 0i,300 mm radius P9 mm radius mm/drop 150 mm (mean of 3)(CBR )̀/0) (mean)

1st drops 161 2430 807 12 3.4 23.0

2.41 55.6 152 2510 748 231 >5 29.4 170 1532 464 119 2.0 24,6 173 1200 172 114 1.6 22.4 175 1342 506 273 11.7/1.5 36.7 0.8 7.4

mean 166 1803 539 172+ 46.2 2.0+ 7 1,4

4th drops 172 1779 667 23 3.4 73.0

2.41 55.6 163 2343 722 146 >5 29.4 176 1226 439 118 2.0 24.6 171 1002 184 111 1.6 77.4 174 981 375 73 11.7/1.5 36.7 0.8 7.4

mean 171 1466 477 94 46.2 2.0+ 21.4

Back calculated stiffnesses: 40 MPa 0-300 mm

132 Mpa 300 mm & deeper.

Irregular deflection bowl, reading suspect. + Ignoring abnormal reading.

Stif

fnes

s -

Mr (

MPa

) de

flect

ion)

(p

mik

p do

/p (

no

rma

lised

mea

n c

0

✓ Cet . .1 after 1st F`,..'D 1c,ad • Centra d .ctien after 4th F1'.'D :cad

10

20 Civ

Fig. 2. Comparison of CICL'Al Hammer and FWD testing, sites 13—E.

142 A. R. DAWSON & N. H. THOM

the soil which (at precisely the same location) exhibited the softest CIV reading. The DCP testing gave a bimodal response with an initially weak (easily penet-rating) part followed by a slow rate of penetration. For this reason two rates of penetration are tabulated. On the looser sand, static cone penetrometer testing was possible and equivalent CBR readings are given. These, too, reflect the variability of the subgrade.

Discussion

FWD testing

It had been hoped that the FWD would provide reliable stiffness information after appropriate back-analysis of the deflection bowls had been performed (Brown er 01, 1987). However, the unreliable readings obtained for the more remote sensing points meant that only the deflections at the three points given in the Tables could be used and this severely limits the amount of stiffness data that can be obtained by back-analysis. However. the most reliable bowls for each site have been back analysed and the results are given at the foot of each table.

The pressure applied by the FWD used on the Ashby-Measham by-pass was between 136 and 196 k.Pa. This pressure is rather high if it is intended to represent off-road vehicles and rather low in terms of delivery lorry traffic operating off-road. The large deflections obtained on the softer soils would prohibit a further increase in stress and the mid-range pressure used is believed to be a reasonable compromise.

Comparison between test methods

The test results show that there is little correlation between different test method results at the same site. At best it may be concluded that the inherent inaccur-acies of each test method mask smaller variations in soil properties. In this case it would appear that the wide variation in behaviour observed at Site E must be explained by reference to the difficulty of testing precisely the same piece of subgrade with different equipment. At worst it may be concluded that the different methods give incomparable results.

A comparison of results from different sites can be used to reveal some trends in the data. Clegg Impact Value appears to relate fairly closely with the logarithm of the central deflection in the FWD tests (Fig. 2). If the mean central deflection from each site is compared with the mean CIV a tentative linear relationship can be produced:

do — 43 x 1.07 -civ (1)

where d o /p is the normalized mean central deflection and CIV is the Clegg Impact Value.

The back-analysed stiffnesses (Mr) have been F. similarly against Clegg Impact Value in Fig. 2 straight line has the form:

12:5 x 1.055cl `'.

This suggests that the CIV reading is relate broad sense, to the value of stiffness of the soil tested. This finding is not surprising as, by the 0: hammer has been dropped four times, an esse elastic ground response might be expected. In s( the central deflection is an indicator of this stif corresponding relationship of CIV and central tion is therefore possible.

The DCP results do not compare well with eitl Clegg or the FWD results. This observation attributed to the dissimilarity between shear beh tested by the DCP and the stiffness behaviour as by the FWD and Clegg Hammer. However 0 ways of recording the DCP data (as used in Tabi are clearly related (Fig. 3) and bedding errors 150 mm penetration appear to be quite small.

The w values obtained at Site A clearly continuous and sensitive measure of the su condition with soft spots being easily identif troughs in the response curve (Fig. 1). Thi readings are not so sensitive although a series readings are obtained on the softer strip c (whe CPT results also occur) and, with some excep CIV of 20 is found at places where (0 is at least 2.

FIELD TESTS ON PAVEMENT FOUNDATIONS 145

softer spots found in the roller test. It could also be related fairly closely to FWD back-analysed stiffness.

None of the methods could be used for reliable determination of a single materials normal variability, The Clegg Hammer and FWD were the best means of identifying material changes.

The number of blows to produce 150 mm penetration of the DCP may be a useful first guide to the soil resistance to probing by this means.

Acknowledgements. The authors \‘. ould like to thank the many organisations who have made this work .Tossibre. by provision of plant. labour, data, access and financ:al sue They include the West Yorkshire Waste Manage:-.et Luc re,a the Department of Transport, the Department of the En\ iron-ment. the Science and Engineering. Research Coo:cil. Bomag Ltd. Scott Wilson Kirkpatrick and Partners. SWK Pavement Engineering, A F Budge and Mowlem Ltd as well s Professor Pell for providing the facilities of the Departnat of Civil Engineering. University of Nottingham.

References drops/mm for straight line portion

Fig. 3. Summary of DCP results, sites B—E.

Conclusions

Stiffness of subgrade and capping layers is critical to the performance of pavements. At present the CBR is the primary guide to soil/capping quality but the test does not give information about stiffness. Large equipment (including PBT, FWD, Dynaplaque instrumented rol-lers) is available to measure stiffness but tests are expensive and/or cumbersome to carry out. Simple alternatives have been tried but these have been shown to have other limitations.

The static cone penetrometer is limited to use in weak fine granular or soft cohesive soils and in the present investigation insufficient material of this type was tested for significant correlations to be drawn. The results obtained from the Dynamic Cone Penetrometer (a cumbersome hand-operated device) do not appear to relate closely to those from the other tests used. The Clegg Hammer related fairly closely with the central deflection of the FWD (a relationship has been tentatively proposed) and also identifed the markedly

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