MONOTONIC SHEAR FAILURE TRIAXIAL TESTS

Monotonic shear failure tests were conducted on all materials for comparison with the RLT permanent strain results. As many pavements do not fail by shear, the RLT permanent strain tests are considered more representative of actual performance in the road. Nevertheless, the static Mohr-Coulomb yield strength parameters c and φ (Section 2.9.2) are well known. Further, it is commonly thought that safe stress states are those less than 70% of the static shear strength (Section 2.6.1).

Drucker-Prager (Section 2.9.3) and Mohr-Coulomb (Section 2.9.2) yield strength parameters were calculated for all materials and compared to the RLT permanent strain results and the shakedown behaviour Ranges A, B and C (Section 2.11.2) determined. Results of the monotonic shear failure tests were used as a guide for setting stress limits for the RLT tests, although bearing in mindTheyse’s (2002) observation that for fast repetitive loading, stress states in excess of the yield strength are possible.

Monotonic shear failure tests were conducted on 3 to 4 specimens for each material at different constant confining pressures of 25, 50, 75 and/or 100kPa. The shear failure test available on the University of Nottingham triaxial apparatus was a ramp test. The ramp test is a stress controlled test and was set to a constantly increasing stress of 5kPa per second until the sample failed. Strain controlled failure tests are considered best (Niekerk, et al. 2000) but were discounted as they cannot be performed on the Nottingham triaxial apparatus due to the large electrical noise from the LVDT in relation to the tiny displacement incrementssthat would be required each second.

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Each triaxial sample for the ramp test was prepared as detailed in Section 3.5 except that on-sample instrumentation was not used. Instead, the deformation of the sample was measured using an external LVDT (Linear Variable Displacement Transformer). During the ramp tests vertical displacement versus load is electronically recorded. The maximum load achieved along with the cell pressure is noted as one stress state defining failure. This stress state was characterised in terms of stress invariants, p and q (Section 2.4.3). The results of at least 3 tests are required to define the Drucker-Prager yield line or surface in terms of slope and intercept (β and d, Figure 2.17) in p-q stress space. Mohr-Coulomb criteria (φ and c, Section 2.9.2) were then calculated from β and d as per Equations 2.57 and 2.58. Repeat tests were not conducted as the prime reason for the shear tests was to obtain an indication of stress limits for the Repeated Load Triaxial permanent strain tests. However, a fourth ramp test was sometimes needed as some samples did not fail within the loading limits (1200 kPa) of the triaxial apparatus when a 100kPa confining pressure was applied.

Figure 3.10 is the result of the monotonic shear failure tests at a range of cell pressures for the NI Good material. Similar tests were completed for all the materials and their failure surfaces determined as plotted in Figure 3.11. Table 3.3 shows the calculated Drucker-Prager yield failure parameters, intercept d and angle β in p-q stress space (Figure 2.19, Section 2.9.3). Also included are the values of Mohr- Coulomb cohesion (c) and friction angle (φ) (Section 2.9.2).

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Table 3.3. Failure surfaces.

Drucker-Prager (p-q stress space) Mohr-coulomb

Material d (kPa) β c (kPa) φ

NI Good 135 62 74 46 NI Poor 49 62 27 46 CAPTIF 1 61 64 35 50 CAPTIF 2 0 68 0 61 CAPTIF 3 103 61 55 44 CAPTIF 4 7 68 5 61 CAPTIF Subgrade 33 60 17 42

It should be noted that for CAPTIF 2 yield shear failure test results, the best fit straight line resulted in a negative intercept being calculated. A negative intercept or cohesion is not physically possible and therefore the straight line was forced through the origin.

Results of the monotonic shear failure tests show relatively high angles of internal friction for all materials. This means that as the level of confinement increases the yield strength will increase significantly. For a Drucker-Prager friction angle of 64 degrees (CAPTIF 1) then for every 1 kPa increase in confining stress will result in nearly a 2kPa (tan64° ) increase in vertical deviatoric stress, q that can be sustained

before failure. CAPTIF 3 material has the lowest friction angle of the materials and this maybe due to the dense grading that does not allow the build up of pore water pressure to dissipate.

The level of cohesive strength seen in most materials excluding CAPTIF 2 and CAPTIF 4 is due to the effect of matrix suction. Matrix suction is effectively negative pore water pressure that occurs in partially saturated materials. The effect of

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this suction is to pull particles together and significantly increase the apparent cohesion of the aggregate or soil. The importance of the matrix suction in determining the effective strength of partially saturated subgrades and granular materials is well understood and described by many authors (e.g. Theyse 2002, Oloo et al. 1997 and Brown 1996). Another reason for cohesive strength is the particle interlock having the effect of effectively joining stones to a limited degree. Individual stones have tensile strength some of which is transferred to an interlocked arrangement of stones. As pore water pressure was not measured during the shear failure tests, the cohesion is calculated based on total stress values. In reality, in partially saturated materials, the pore water pressure is a negative value and subtracting this value from the total mean princial stress (p) will mean that the principal effective stress, p', is to the right of the value of p. Effective cohesion, expressed ijn this way would be substantially reduced compared to the level of total cohesion. Hence, nearly all of the cohesion reported in Table 3.3 is expected to be due to suction or negative pore water pressure.

The yield strength parameters for the CAPTIF 2 and CAPTIF Subgrade materials are a surprise result. CAPTIF 2 is the same as CAPTIF 1 but with 10% by mass of silty clay soil (actually the CAPTIF Subgrade soil) added. It is expected the effect of this silty clay soil would be to reduce the friction angle due to the introduction of plasiticity to the aggregate and increase cohesion due to soil particles bonding together. However, the opposite occurred for the CAPTIF 2 material and is probably due to the CAPTIF Subgrade being a low plasticity clay (Section 3.3.1). The CAPTIF Subgrade which is a silty-clay soil showed an unexpected high friction angle, nearly as high as several granular materials, however, the friction angle determined from RLT tests defining when shakedown Range C (failure) occurs is significantly less (Table 3.4) and thus suggesting that the friction angle in Table 3.3 is in error.

Ideally the monotonic shear failure tests should have been repeated to reduce the errors in the results by identifying outliers, however, these errors willonly become

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apparent when comparing the stress states at which failure occurred in the RLT permanent strain tests.