THE CBR TEST – A CASE FOR CHANGE? · PDF fileThe reproducibility and repeatability of...

Australian Geomechanics Vol 49 No 1 March 2014 41

Figure 1: Laboratory CBR Test, Porter 1938

THE CBR TEST – A CASE FOR CHANGE?

Ralph Rallings Geotechnical Consultant - Pitt and Sherry Consulting Engineers

ABSTRACT The origin of this paper lies in a perceived inconsistency between the laboratory CBR’s poor reproducibility and the CBR’s special place within pavement technology. The paper summarises the reproducibility and repeatability of the laboratory CBR and discusses the implications of the poor reproducibility on design and product quality decisions. It shows that the load-deformation properties assumed for the standard crushed rock (CBR=100) differ significantly from the measured properties of crushed rocks and that this inconsistency results in an undesirable bias in the reported CBR. It explores the laboratory test’s ability to replicate the in situ CBR and investigates the practicality of replacing the CBR of cohesive subgrades with the undrained shear strength. It contends that continuing reliance on the CBR hinders the development of pavement technology.

1 INTRODUCTION The following explores four matters relating to the use and interpretation of the CBR.

1. The reproducibility and repeatability of the laboratory CBR test. 2. The load - penetration properties of the standard crushed rock. 3. The reliability of the laboratory CBR test as a measure of the in situ CBR. 4. The future of CBR based pavement design.

Reproducibility is essentially the closeness of agreement of test results from independent laboratories. The National Association of Testing Authority’s (NATA) proficiency test programs have shown that AS 1289.6.1.1-1998 “Determination of the Californian Bearing Ratio of a soil-Standard laboratory method for a remoulded specimen” has poor reproducibility. The repeatability, from limited data, appears to be significantly better. It is questioned if the test can provide a sound basis for design and material quality decisions, apart from those made in-house, and if its status as an Australian Standard is justified.

The CBR is the piston load, Figure 1, at 2.5 mm or 5 mm penetration, expressed as a percentage of a load nominated in the test method for the particular penetration, Table 1. It was observed that the 2.5 mm estimate is most often reported in in situ tests and the 5 mm estimate in the laboratory test. It is shown that there is a marked bias in the laboratory test towards the 5 mm estimate particularly at CBRs greater than 15. A major contributor to this bias is the ratio of the loads (at 2.5 mm and 5 mm penetration) for the standard crushed rock. A case is put for abandoning the 5 mm penetration estimate.

THE CBR TEST – A CASE FOR CHANGE? RALPH RALLINGS


The Austroads thickness design chart for granular pavements, Figure 2, is an adaptation of charts, developed by the US Corp of Engineers, for airfield runway pavements. The airfield charts were based on in situ CBRs and solely on shear failure of the subgrade. Two lines of enquiry have been followed. The first concerns the relationship between laboratory and in situ CBR and the second between CBR and undrained shear strength for cohesive subgrades. These enquiries have led to a view that pavements, designed in accord with Figure 2 for low traffic levels, may be vulnerable to subgrade shear failures when subjected to short-term heavy loads.

The following includes arguments for change in the definition and method of calculation of the CBR, in the preparation of laboratory samples and in the conduct of both the laboratory and field test procedures, though as interim measures until the CBR is replaced. A case is outlined for replacement of the CBR by the undrained shear strength for cohesive subgrades.

It is argued that the CBR’s dominant position within pavement technology is not justified, not only because of its poor reproducibility, but also that it is not amenable and is never likely to be with the information requirements of numerical modelling of pavements.

2 THE CBR TEST In his 1938 paper “The Preparation of Subgrades” O J Porter provided the following description “Tests are made by wetting the material to optimum moisture content and then consolidating the sample in a 6 inch diameter mould under a load of 2000 pounds per square inch. The sample contained in the mould is then tested for bearing value (Figure 1) by penetrating the centre of the compacted specimen at a rate of 0.05 inches per minute with a piston 3 square inches in area. The loads for each 0.1 inch penetration, to a total of 0.5 inch are recorded.”

Figure 2: Granular Pavement Design Chart (Figure 8.4 – Austroads 2004).

Porter then describes how the sample is loosened, re-compacted and soaked for 4 days and the bearing value again determined, and continues “The bearing value for both conditions (soaked and un-soaked) of the specimen, are reported in pounds per square inch and also in percentages of the loads required to obtain like penetrations of a standard sample of crushed rock.” (Table 1)

Table 1: Standard Curve for Nominal Crushed Rock CBR=100 (ASTM1883-73).

Penetration (Inches) Bearing Pressure (psi) Penetration (mm) Bearing Pressure (MPa)

0.1 1000 2.5 6.9 0.2 1500 5.0 10.3 0.3 1900 7.5 13.1 0.4 2300 10.0 15.9 0.5 2600 12.5 17.9



Earlier references (eg ASTM1883-73) opined that the 0.1inch (2.5mm) estimate was normally larger than the 0.2 inch (5 mm) estimate. The earlier practice was to repeat the test when the 0.2 inch penetration estimate was greater than the 0.1 inch penetration estimate. If the retest gave the same result, the 0.2 inch value was adopted.

AS1289.6.1.1-1998 broadly follows Porter’s test procedure though there have been significant changes. Static compaction has been replaced by impact compaction. A surcharge is now applied during the loading phase. The CBR is calculated only at 2.5 mm and 5.0 mm penetration and the higher estimate is adopted without re-test. Bearing stresses are not reported.

In addition to AS1289.6.1.1, there are two other Australian Standards for CBR testing:

• AS1289.6.1.2-1998 “Standard laboratory method for an undisturbed specimen”. • AS1289.6.1.3-1998 “Standard field-in-place method”.

3 DEFINITIONS Reproducibility is defined here as: A measure of the closeness of agreement achieved in the testing of a homogeneous material using identical test procedures by different laboratories.

Repeatability: A measure of the closeness of agreement achieved in the testing of a homogeneous material using identical test procedures within the same laboratory.

Load Ratio (Lr): Load on the CBR piston at 5 mm penetration divided by the load at 2.5 mm penetration.

CBR Ratio (Cr): The CBR calculated from the load at 5.0 mm penetration (CBR5mm) divided by the CBR calculated from the load at 2.5 mm penetration (CBR2.5mm).

Because of the assumption that Lr =1.50 for the standard crushed rock, the following applies when:

o Lr ≤1.50, CBR2.5 mm is reported o Lr > 1.5, CBR5mm is reported and Cr =1+ (Lr-1.5)/1.5. Equation (1)

4 REPRODUCIBILITY OF THE LABORATORY TEST LABORATORY

4.1 DATA The following is based on reports covering NATA’s proficiency test programs between 1992 and 2011. Proficiency testing involves NATA registered laboratories testing a sample, from the same nominally homogeneous source, in accordance with prescribed instructions. NATA at the same time undertakes homogeneity tests of that source. Random samples, usually six (6) or ten (10) from the source are tested by a nominated laboratory. The data considered here was contained in six (6) detailed reports, (NATA and Proficiency Testing Australia, pt:a) dating from August 2002 and summary statistics of three (3) earlier programs for which the detailed reports were not located. The reports include the results of individual laboratories, without identifying the laboratory and statistical summaries.

The “homogeneity” test results of the distributed material are included. These are somewhat akin to repeatability tests in that they are made within the one laboratory on representative samples of the distributed material.

The inverse of the Coefficient of Variation (CV) has been used here as a measure of the reproducibility and repeatability of the proficiency and homogeneity tests, respectively. While it is recognised that non-uniformity within the distributed material would have contributed to the proficiency CVs, this matter is not discussed further.

Except for the two most recent proficiency programs (Reports 695 and 710), participating laboratories were required to report results in accordance with the test method, AS1289.6.1.1, that is:

For CBR (%) <5 5-20 20-50 >50

Report CBR to nearest % 0.5 1.0 5 10

Table 2 summaries the results achieved in each program. It includes:

• The specified compaction requirements and if soaking is required.

• The mean CBR and CV for the proficiency tests (P) and homogeneity tests (H) at both 2.5 mm and 5 mm penetration.



Table 2: Summary of Proficiency (P) and Homogeneity (H) Tests.

Report Compaction Soak Mean 2.5/5.0 mm P H

CV % 2.5/5.0 mm P H

710 Note 1 Yes 8.0/7.5 6.9/6.7 44/46 9/7.7 695 100% Mod No 113/142 158/174 34/27 8/7 607 100% Mod Yes 50/70 49/67 43/38 5/7.2 457 100% Mod Yes 140/185 39/31 453 100% Std Yes 23/34 42.5 44/40 11.4 396 100% Std (N2) Yes 18/20 23 35/31 17 387 (N3) No 15 14.7 189 92% Std (N2) No 12.2 23 189 98% Std (N2 No 16.5 19 120/A Std Comp (N4) No 2.9 32 120/B Std 100% (N4) No 112 22

Note 1; The compaction dry density (1.55t/m3) and moisture content (5%) were specified. The higher CV may be due to instability of loose sand subjected to inundation.

Note 2: The moisture contents and densities were specified by NATA. Note 3: This report was not sighted. The information was extracted from summaries given in subsequent reports Note 4:The moisture content was nominated by NATA. For 120A this was 5% above the nominated OMC of 15%.

Table 3 includes estimates of the percentage of proficiency test results within ±10, 20 and 30% of the median values. The estimates are approximate because rounding of the test results has led to the repetition of the same numerical value. Appendix A includes plots of the distribution of the reported CBR2.5mm values. Program 120A is limited to the results of state road authority (SRA) laboratories.

Table 3: Distribution of Results of the Proficiency Tests.

Program Number of Results

Median Value CBR 2.5mm

% age within ± 10%*



710 49 8.0 20 35 45 695 43 113 15 40 55 607 58 50 20 40 55 457 38 140 30 40 55 453 50 22.5 15 35 40 396 32 18 30 45 50

* Of the median value.

4.2 OBSERVATIONS In all the programs covered in Table 3, less than 60% of results were within ± 30% of the median value. The largest relative spread of reported proficiency test results was from 11 to 100 (Report 607) and the smallest, 80 to 290 (Report 457). The spread of results was generally less for the CBR 5.0mm..

The CVs of the homogeneity tests were commonly less than 25% of the proficiency test CVs, Table 2. The largest CV recorded in the homogeneity tests was 17% in Program 396. It is nearly double that of the other programs. The rounding of the test results, whereby a value of 22 would be rounded down to 20 and 23 up to 25, may have contributed to the higher CV. In three (3) of the five (5) programs where comparisons could be made, the mean CBR of the homogeneity tests exceeded the mean of the proficiency tests, and quite significantly in two programs, 695 and 453.

The reproducibility of the un-soaked tests was consistently better than for the soaked tests, though marginally in two cases. In one of these, Program 120A, the sample was prepared at 5% moisture content above optimum. In the other case, Program 695, the loading immediately followed compaction. The average CV for the other three un-soaked samples was 21%, about half that of the soaked tests.

The CVs achieved in compaction densities ranged from 0.5 to 1.4 % (median 0.9%) for the proficiency tests and from 0.2 to 1.1% (median 0.5 %) for the homogeneity tests.

4.3 DISCUSSION The poor reproducibility of the laboratory test has been reported previously in a number of sources, including, Jervis and Eustis (1950) and Black (1979).



The purpose of testing is usually to improve confidence in decision making. How then will confidence be improved when two generally competent laboratories provide markedly different estimates of the CBR? Based on the reproducibility data supplied above, it would be unexceptional if the two laboratories differ in estimates of:

• The subgrade CBR, one laboratory indicating a value of 4 and the other 6. The difference in subgrade cover is in the order of 150mm. (Figure 2). Similar uncertainties arise with the mechanistic design methods from its dependence on E=10 CBR as the resilient modulus of the subgrade.

• The soaked CBR of a potential base course gravel, one laboratory indicating non-compliance based on a value of 70 and the other 120 and compliance.

With respect to the above scenarios, all estimates are probably valid. Confidence in estimates can usually be improved by increasing the frequency of testing. However it is not known if the laboratories that produced a relatively low or high result would continue to produce low or high results in repeat tests.

The cause/s of the poor reproducibility have not been identified. It is evident that reproducibility is reduced with soaking and when the compaction moisture content is well above the OMC. The better repeatability points to variations in the sample preparation procedures. The cause does not appear to lie in variations in average sample density, which was consistently small and similar to the homogeneity test results.

The test method, while placing tight limits on layer thickness, has no requirements about the placement of the compaction hammer or the sequence of compaction. It is conceivable that some sequences in compaction may locally reduce densities and others may increase densities. The required diameters of the mould and the 19 mm high surcharge are 152±1 mm and 150±0.5 mm respectively. The gap between these two items could be as low as 0.25 mm. This seems too small to eliminate the possibility of the surcharge jamming against the mould if it is tilted during the loading phase.

It is evident that different laboratories testing the same material can produce markedly different estimates of the CBR. If this is an inherent feature of the test method, then is it appropriate that it should have the status of an Australian Standard? If it remains as an Australian Standard, repeatability and reproducibility data should be provided.

5 CALCULATION OF THE CBR 5.1 DATA

The test method requires that the CBR is the higher of the two estimates of:

1. The load (kN) at 2.5 mm penetration divided by 13.2. 2. The load (kN) at 5.0 mm penetration divided by 19.8, i.e. 1.5 times 13.2.

Figure 3 plots the load ratio (Lr) against the CBR2.5mm of 160 laboratory soaked CBR tests done in accordance with AS1289.6.1.1-1998. The materials ranged from high plasticity clays to high quality crushed rocks. Compaction levels, expressed in terms of Maximum Dry Density (MDD) varied from 95% Standard to 98% Modified.

Figure 3: Laboratory CBR 2.5mm vs Load Ratio.



Table 4 provides a summary of the means and CVs of the CBR ratio (Cr) of NATA’s proficiency test program. Appendix B includes plots of CBR2.5mm versus CBR 5mm. for these programs.

Table 4: Proficiency Tests, CBR Ratios (Cr)

Program Mean CBR 2.5mm Mean Cr CV of Cr %

710 8 0.99 33 695 115 1.26 21 607 50 1.31 15 457 140 1.30 8 453 23 1.39 16 396 18 1.05 10

Figure 4, which concerns in-place CBR tests on clay subgrades, was prepared on the same basis as Figure 3. The data source was Scala 1954. Scala’s investigation involved over 110 in situ CBR tests. The CBR at each of the five penetration levels in Table 1 was reported at each site. The subgrades were almost exclusively CL and CH clays with Liquid Limits spread relatively uniformly from 13 to 90.

Figure 4: In situ CBR 2.5mm vs Load Ratio

5.2 OBSERVATIONS With respect to Figure 3:

o The minimum Lr increases with increasing CBR2.5mm, when CBR2.5mm >15. o There are no points in the immediate vicinity of CBR 2.5mm= 100 and Lr=1.5. o Near CBR2.5mm=100, Lr generally exceeds 1.75, and often exceeds 2.0. , o In the low CBR range (<15), the majority of results where Lr< 1.5 are clays. When Lr>1.8 the

majority are sands or gravely materials.

Note: When Lr=1.8, 2.0 or 2.2, CBR5mm will exceed CBR2.5mm by 20, 33 and 46% respectively (Equation 1). An Lr of 2.0 indicates that the load-deformation plot is effectively linear to 5mm penetration. An Lr above 2.0 indicates that the plot is concave upwards and that the material has stiffened with increasing penetration.

Almost all points in Figure 4 lie below the 1.50 load ratio line. This is in stark contrast to Figure 3 where the majority of points at and above CBR 15 lie above 1.5.

5.3 DISCUSSION No evidence was found supporting the test method’s apparent assumption that the load ratio for a crushed rock at CBR of 100 is at or near 1.5. The recorded load ratios for crushed rocks in the vicinity of CBR=100 vary from 1.6 to 2.15



which suggests the test method’s assumption is too low. This underestimation results in a bias towards the adoption of CBR5mm as the CBR.

The underestimation of the load ratio for the crushed rocks may have resulted from the change from static to dynamic compaction and from the inclusion of a surcharge during loading by the US Corp of Engineers. The Corp replaced the 13.8 MPa static compaction pressure with Modified AASHO compaction. It was reported that the densities achieved by impact compaction on sandy soils of low plasticity were significantly higher than those achieved by static compaction, Jervis and Eustis (1950). They reported that “ Laboratory soaked specimens of soils with low plasticity compacted at optimum moisture content … by the dynamic method have greater CBR values than corresponding specimens compacted by the static method”. In spite of the significant changes to the test method, no change was made to the load-penetration characteristics of the standard crushed rock.

In Figure 3, at CBRs >15, the minimum load ratio increases with CBR 2.5mm. No such dependence is evident in Figure 4. It is possible that the differences between Figures 3 and 4 are, to a significant degree, due to differences in material types and moisture/ soaking effects. Mould influences, as discussed below are thought to be particularly significant.

The increase in recorded Lr with increasing CBR in the laboratory test may from confinement of the sample within the mould and expansion of dense granular soils under shear. In combination they are likely to elevate confining stresses and increase stiffness. Clays of low CBR are less likely to dilate and produce smaller plastic zones which are less likely to impinge on the mould.

Table 5 compares the applied stresses in AS1141.22-2008 “Wet/dry strength variation” to those applied in a CBR test at the same numerical value. The applied stress in AS1141.22 for a wet strength of 100 kN is 55% of the stress at 5.0 mm penetration for CBR=100. It is conceivable that crushing could occur in weaker materials at 5.0 mm penetration.

Table 5: Comparison of Loading Stresses for Wet Strength and CBR

Value Loading Stress MPa Wet Strength CBR 2.5mm CBR 5.0mm

50 2.8 3.4 5.1 100 5.7 6.9 10.3 150 8.5 10.3 15.4

The points below the trend line in Figure 3 in the vicinity of CBR 80 –100 were mostly gravels and crushed rocks derived from rocks with wet strengths sometimes below 100kN. The lower load ratios may be the result of crushing and may not be representative of conventional crushed rocks.

The in-built bias towards the adoption of the 5 mm penetration estimate could be removed by increasing the load ratio of the standard crushed rock to around 2.0. However removal of the 5.0 mm estimate from the test method is preferable. The stress and strains at 2.5 mm penetration far exceed those that occur within pavements. Tyre pressures rarely exceed 1 Mpa, (Chowdhury &Rallings, 1994) compared to 6.9 MPa imposed at 2.5 mm penetration at CBR2.5mm =100. The average vertical strain (penetration÷sample depth) at 2.5 mm penetration within the laboratory test mould is near 2x104 µε, about 10 times the design strain for a weak pavement.

6 REPLICATING THE IN SITU CBR

6.1 DATA Figure 5, which was sourced from Smith and Pratt, 1983, compares laboratory and in situ CBRs over a range of moisture contents and dry densities. The authors, at each of 30 locations in an existing pavement, carried out three in-situ CBRs, moisture content and sand replacement density measurements on a sandy clay subgrade of medium plasticity. It contained ironstone nodules. Samples extracted from each location were cured and compacted in a CBR mould, as closely as possible to the field moisture contents and densities, and the CBR determined.

6.2 OBSERVATIONS With respect to Figure 5, and relative to the in situ CBR, there are observable, but not universally consistent patterns:

o The laboratory CBR in the higher density range, dry of OMC, has increased and sometimes substantially, e.g. from 54 to 145.

o The laboratory CBR on the wet side of OMC is sometimes significantly lower, eg 16 reducing to 2.

The range in recorded values has increased from 4.5 to 58 for the in situ tests to 1.1 to 145 for the laboratory tests.



Figure 5: Laboratory vs In situ CBR (Source: Smith and Pratt).

6.3 DISCUSSION The US Corp of Engineers airfield charts were based on the in situ CBR (Turnbull 1950). In consequence, the ability of the laboratory test to replicate the in situ test is of significance.

The patterns described above are consistent with the confinement hypothesis discussed previously. An equally valid explanation is that in situ densities were overestimated, evidenced by points occurring on the wet side of assumed zero air voids lines. An overestimation of in situ density could lead to higher CBRs in the laboratory test on the dry side of OMC. On the wet side compacted soils may lie very close to the zero air voids line and show instability.

While nuclear moisture density meters may have greater consistency in density measurement, they provide no assurance that actual field densities are precisely known. In consequence there is likely to remain uncertainty with respect to the comparison of laboratory and field CBRs. Furthermore, the fabric and structure of the in situ soil is unlikely to be reproduced in laboratory prepared samples.

7 THE FUTURE OF THE CBR IN PAVEMENT THICKNESS DESIGN

7.1 INTRODUCTION AND SCOPE The following explores:

• The undrained shear strength (su) as a possible replacement of the CBR for the estimation of subgrade cover. • The limitations arising from reliance on subgrade CBR in modelling pavement behaviour.

The undrained shear strength is a particular test state within an established body of knowledge covering the load-deformation properties of granular materials. The CBR by it’s a -hoc origins has no specific meaning within the load-deformation behaviour of granular materials. While the use of undrained shear strength would be limited to cohesive subgrades and total stress analyses, familiarity and exploration with it could provide a gateway to a wider conceptual understanding of road material and pavement behaviour.

An advantage of the undrained shear strength is the variety of methods of measurement. These methods include triaxial, unconfined compression and direct shear tests on undisturbed or remoulded samples in the laboratory and by in situ vane tests in the field. Hand penetrometer and hand vane measurements can be done in both laboratory and field. This variety provides avenues for a reasonableness check on a test result. Reasonableness checks on a CBR measurement are likely to be limited to the calculated CBR (at particular moisture and density conditions for CBR< 15) and published tables.



7.2 RELATIONSHIP BETWEEN CBR 2.5MM AND UNDRAINED SHEAR STRENGTH (SU) Figures, 6 and 7 have been derived from Scala’s in situ tests. Figure 6 plots the mean bearing stress (q) at the five penetration levels defined in Table 1 as a %age of the bearing stress at 12.5 mm penetration, q12.5mm. It can be observed that the mean piston bearing stress at 2.5 mm penetration (q 2.5mm) is approximately half of q12.5mm and that the bearing stress is approaching a peak value at 12.5 mm penetration. If it is assumed that q12.5mm is the ultimate bearing stress (qu) and that qu = 6 su (for a circular loading), then q 2.5mm =3 su. As q 2.5mm= 69 CBR2.5mm (Table 1):

CBR2.5mm = su/23. Equation (2)

Figure 6: Percentage (%) of 12.5 mm Bearing Stress Penetration (In situ CBR).

The above approach follows Black 1979. Black developed an identical equation and contended that it applied to remoulded/compacted soils. For undisturbed over-consolidated clays, by assuming that q2.5mm = qu, he arrived at CBR=Cu/11.5. Figure 7 plots CBR2.5mm against the calculated shear strength (su). It has been assumed that that q12.5mm = qu and that qu = 6 su. Almost all of the points lie within the range of q2.5mm/ qu of 0.4 to 0.6. The divisor in Equation 2 when q2.5mm is 0.4 is 28 and at 0.6 is18, for this particular data set.

The ratio q2.5mm /qu is likely to depend on the soil’s loading and reworking history. Undisturbed clays, reflecting a wide variety of past environments, are likely to display a wide variety of load-deformation characteristics and resulting divisors in Equation 2. In consequence, it is unlikely that there is a universal relationship between CBR and su. There is a possibility of an approximation to universality for remoulded clays, because remoulding potentially obliterates the effect of previous environments, but there was insufficient information to reach such a conclusion. Increased understanding of the spread of the ratio q2.5mm /qu might be improved if penetration in the in situ CBR was extended to the ultimate bearing stress.

While the above is based on the statistical correlation between CBR and su, the following shows the pivotal role of shear strength in the development of the airfield design charts and on subsequent transformations, such as Figure 2.



Figure 7: Estimated Undrained Shear Strength (su) versus In situ CBR 2.5mm

7.3 CONCEPTUAL AND MODELLING LIMITATIONS OF THE CBR In 1942 the US Corp of Engineers began investigations into the pavement thickness requirements for airfields under a range of wheel loads. They developed preliminary CBR- pavement thickness relationships for aircraft wheel loads based on the following assumptions, Middlebrooks and Bertram 1950:

• Pavements satisfying Curve A, Porter 1950, could carry “ 9000 lb truck wheel loads…. without distress throughout the life of the pavement”.

• As wheel-tracking was not pronounced on airfields, a 12000 lb (53 kN) aircraft wheel load was considered to be equivalent to the 9000 lb (40 kN) highway wheel load.

• Subgrade shear failures would not occur if subgrade shear stresses did not exceed those of Curve A when loaded under a 12000 lb (53 kN) wheel load.

Subgrade shear stresses under a 53 kN, 410 kPa wheel load were calculated at pavement thicknesses corresponding to subgrade CBRs of 3, 5, 7 and 10% of Porter’s Curve A, (Table 6). These stresses were then used to determine the Minimum Permissible (Pavement) Thickness for a range of aircraft wheel loads of up to 340 kN. The subsequent loading trials confirmed the preliminary designs for aircraft wheel loads to 133 kN, though the minimum wheel load used in the trials was 57 kN. The failure criterion was subgrade shear. Other failure modes were excluded.

Table 6: Subgrade Cover Requirements for Airfield Pavements.

Subgrade CBR 3 5 7 10 Required cover, mm, for 31 kN wheel load, 450 335 280 230 Required cover, mm, for 53 kN wheel load, 570 430 340 285 Required cover, mm, for 67 kN wheel load 635 485 395 325 Required cover, mm, for 89 kN wheel load 710 545 435 350 Estimated Subgrade Shear Stress kPa 35 56 72 98

Notes: 1. The cover and shear stresses have been derived from small scale figures and may not be exact. 2. The cover for the 31 kN and 53 kN wheel loads are the same as Porter’s (1950) Curves B and A

respectively. 3. The cover thicknesses given were derived from Figure 29a, Foster, 1950 4. The shear stresses were extracted from Figure 6 of Middlebrooks and Bertram, 1950.

q2.5mm/qu = 0.4

q2.5mm/qu = 0.6



Figure 8 plots the subgrade cover estimates of Table 6 for aircraft wheel loads of 31, 53, 67 and 89 kN onto the cumulative ESA framework of Figure 2. It can be seen that the CBR- cover relationships for the aircraft wheel loads have been maintained.

Figure 8: Minimum Cover over the Subgrade for a range of Aircraft Wheel Loads

There appears in Figure 8 to be a contradiction between the Corp’s Minimum Permissible Thickness and the cumulative ESA plot with respect to the safe load capacity. The cumulative ESA plot places no upper limit on the safe load. It can be seen that where it coincides with the 31 kN aircraft plot, Figure 8 indicates that a pavement, so constructed, is capable of withstanding 3x105 passages of a 40 kN wheel load and/or 1.2x105 passages of a 50 kN wheel load. The airfield plot suggests that the wheel load should not exceed 31 kN.

Conventional structural design would normally aim to ensure that:

• The structure would maintain its integrity under all conceivable loading conditions. To this writer’s knowledge, this type of check has not been undertaken. While it is not appropriate to claim that the above provides strong evidence that light duty pavements are vulnerable to shear failure, there remains a case for the investigation of the potential for subgrade shear failures using up-to date models of granular materials behaviour and stress-strain distribution.

• The functional/serviceability requirements for the structure are met. It is believed that the basis of Figure 2 is simply a serviceability provision.

Pavements that are most likely to be vulnerable to subgrade shear failures include:

• Low capacity pavements exposed to significantly heavier traffic (than envisaged in design) for relatively short periods. Foster 1950, made the significant observation that subgrade shear failures occur “after a relatively few repetitions. In approximately fifty cases for which reliable data are available, 96% of failures occurred before 2000 coverages had been applied.”

• Pavements weakened by increased subgrade moisture contents but remaining open to heavy commercial vehicles. • Narrow pavements and shoulders.

The cumulative ESA chart appears to have been developed without consideration of the potential subgrade shear failure mechanism. There are likely to be a number of reasons for this omission. Subgrade shear failures might have been uncommon due to lower wheel loads when Figure 2 was developed. Another is that there is no useful benefit in a



quantitative model, based on the CBR. Unlike shear stress, the CBR is not a property that can be directly compared to a calculated stress or strain. A CBR based model would not be responsive to changes in wheel load, tyre pressure, axle configuration or position of the wheel path and configuration of the pavement and shoulders.

Continuing reliance on the CBR restrains the development of coherent and rational models of pavement behaviour. Rallings (2002). The CBR is incompatible with current concepts of materials and pavement behaviour and with the information demands of available analytical tools. These deficiencies and its poor reproducibility should lead to its demise.

7.4 NON-COHESIVE SUBGRADES Undrained shear strength is not appropriate for materials of low or zero cohesion. These material types were not well represented within the data considered here. They can be expected to display distinctly different load-deformation characteristics to clays, particularly within a confined space such as the CBR mould. It was found in the airfield trials Turnbull 1950, that the data for non-cohesive materials was scattered and that “the design curves are generally considered conservative”. As the plastic/disturbed zone in non-cohesive materials is likely to be considerably wider than for cohesive soils, there is possibly a case for wider overburden plates for in situ tests on non-cohesive subgrades.

8 THE CASE FOR CHANGE

8.1 INTRODUCTION The laboratory test is usually undertaken for two quite different purposes:

1. To determine subgrade cover, normally for CBR<15. 2. To determine a material’s suitability within a pavement course, typically above CBR15.

It is recognised that for the immediate future, until the CBR is replaced by other measures, that there will be a need for the laboratory CBR test in the determination of subgrade cover. With this in mind recommendations are made concerning changes to the test method, including the accompanying notes.

AS1289.6.1.2-1998 “Standard laboratory method for an undisturbed specimen,” as a measure of a material’s suitability within a pavement course, has insufficient reproducibility for use within contracts open to public tender.

8.2 THE DEFINITION OF CBR The “Glossary of Austroads Terms” defines CBR as “ The ratio, expressed as a percentage, between a test load and an arbitrarily defined standard load.” This provides no insight into the CBR as a physical quantity. CBR can be precisely and meaningfully defined as:

CBR2.5mm = q /69 Equation (3)

where q is the stress (kPa) at 2.5 mm penetration on a 50 mm diameter footing/piston when driven at a rate of 1 mm/minute.

CBR so defined is not a ratio. A new term, the Pavement Bearing Value (PBV) equal to the CBR, would remove the inconsistency. The 5mm penetration estimate of the CBR would not be required with this definition

8.3 LABORATORY SURCHARGE GAP AND COMPACTION PROCESS The allowable gap between the mould and the surcharge weight seems unnecessarily small. Consideration should be given to specifying limits for the gap. The purpose is to eliminate the possibility of the surcharge plate jamming against the mould during loading.

The test method has no requirement concerning the placement or sequence of the hammer blows during compaction. The face area of the hammer is only about 10% of the surface area of the sample. It is feasible that particular sequences may reduce densities and that other routines may increase densities. A defined compaction routine should be considered for inclusion, though it would need to be shown that the compaction routine affects reproducibility.

8.4 TERMINATION OF THE TEST Clause 8 (c) of the laboratory test requires that loading is continued until 12.5 mm penetration, with the proviso (Note 13) that it “may be terminated after 7.5 mm if the penetration is proceeding without any increase or decrease in the load.” It is recommended that the loading be terminated at 7.5 mm penetration in the laboratory test, with the option to continue to 12.5 mm.



The option should be provided in the in situ test to continue loading until the ultimate stress (qu) is reached. This would enable the estimation of the undrained shear strength (su). The reporting requirements would include the ultimate bearing stress qu, the penetration at peak stress, as well as a plot of penetration versus bearing stress.

8.5 ADVISORY AND CAUTIONARY NOTES The matters dealt with in the Scope and Advisory notes should cover:

• The evidence on the reproducibility and repeatability of the laboratory test methods. • The detrimental effects of soaking on reproducibility.

9 AKNOWLEDGEMENTS Thanks are due to Philip Briggs of pt:a for his assistance in locating proficiency test reports, to Bob Absolom for advice with respect to the test method, to Judy Jensen for locating old papers, to Lily Panton and Rebecca Cooper for their help in figure and report preparation and particularly to my partner Sue for her support, wise words and patience.

10 REFERENCES Austroads 2004. “Pavement Design – A Guide to the Structural Design of Road Pavements” Black, W.P.M, (1979). “The strength of clay subgrades-its measurement by a penetrometer.” TRRL Laboratory

Research Report, 901. Chowdhury F, and Rallings R A (1994). “A Survey of Truck Tyre Pressures in Tasmania,” Road and Transport

Research, Vol3, No3, pp 80-89. Foster, C R. (1950). “Design Curves for Single Wheel Loads”. Trans, A.S.C.E, 115, pp 526-533. Jervis W H and Eustis J B (1950), “Accepted Procedure for the CBR Test”. Trans, A.S.C.E, 115, pp 472 –484. Middlebrooks, T A and Bertram, G E. “Adaptation to the Design of Airfield Pavements” Trans, A.S.C.E, 115, pp 468-

471. NATA (2002) “Soils Proficiency Testing, Round 2 (Vic& Tas).” PTAC Report 396, Aug 2002. NATA (2004) “Soils Proficiency Testing, Round 3 (Qld).” Report No 453, June 2004. NATA (2004) “Soils Proficiency Testing, Round 4 (WA,SA,&NT).” Report No 457, July 2004. Porter, O J (1938). “The Preparation of Subgrades” . Proc.Highw.Res.Bd., Wash 18:2 pp 324-331. Porter, O J (1950). “Development of the Original Method of Highway Design”. Trans, A.S.C.E, 115. pp 461-467. pt:a (2009) “Soils Proficiency Testing, Round 8.” Report No 607, April 2009 pt:a (2011) “Soils Proficiency Testing, Round 11.” Report No 695, Jan 20011. pt:a (2011) “Soils Proficiency Testing, Round 12.” Report No 710, June 20011. Rallings R A (2002). “ Granular Pavements- Geomechanics Lost Child”. Australian Geomechanics, Vol 37, No 4, Sept

2002, pp34-44. Scala, A J (1954). “ Simple methods of flexible pavement design using cone penetrometers”. Proc of 2nd Aust-NZ Conf.

Soil Mech. And Found. Eng, pp73-84. Smith, R.B and Pratt D.N (1983). “A Field Study of In Situ California Rearing Ratio and Dynamic Cone Penetration

Testing for Road Subgrade Investigations”.Australian Road Research, 13(4). Turnbull, WJ (1950). “Appraisal of the CBR Method”. Trans, A.S.C.E, 115, pp 547 –554.



Appendix A: Proficiency Testing – Distribution of Test Results CBR 2.5mm vs. Number

AS1289-6.1.1 Other

Legend

Method not identified



Appendix B: Proficiency Testing – CBR 2.5mm vs CBR5.0mm

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