Robertson's Remarks #4: Evaluation of Cyclic Softening in Clays |
Boulanger and Idriss (2007, ASCE) recently presented an excellent summary for the evaluation of the cyclic softening in silts and clays. They correctly suggest that the first step is the evaluation of the behavioral characteristics of the soil. They suggest that the transition from being more fundamentally like sands to more like clays occurs at around a plasticity index (PI) of 7. Others have suggested slightly higher values of PI to define this transition (e.g. Seed et al, 2003 and Bray and Sancio, 2006). This issue will likely become resolved in the fullness of time as more field data becomes available. I prefer a slightly higher PI criteria to define clay-like behavior (PI > 10) since sand-like soils tend to have a lower resistance to cyclic loading than clay-like soils and the simplified methodology requires a certain level of conservatism in its application for low to moderate risk projects or in the screening stages of high risk projects. The resistance to cyclic loading for silts and clays is essentially controlled by the undrained strength ratio of the clay-like soil. It is clear that if an earthquake applies a cyclic stress ratio that is close to the undrained strength ratio of the soil, large deformations are likely. The amount of deformations will likely depend on the size and duration of cyclic loading and the plasticity of the soil. Boulanger and Idriss suggest a value of CRR(M=7.5) = (0.8 x undrained strength ratio), to define the limit when deformations are likely to become large. In clay-like soils the CPT is also commonly used to obtain estimates of both undrained shear strength (su) and consolidation stress history (OCR) profiles. The CPT has the advantage of providing continuous, reliable profiles of tip resistance in a highly cost effective manner. The undrained shear strength of clay-like soils can be estimated from the CPT using the following equation: su = (qt – sv) / Nkt Normalized CPT parameters are used to identify Soil Behavior Type (SBT), where the normalized CPT parameters in clay-like fine-grained soils are: Hence, it is possible to identify contours of undrained shear strength ratio in clay-like fine-grained soils on the Normalized Soil Behavior Type (SBTn) chart suggested by Robertson (1990). These undrained shear strength ratio contours can be reduced by the recommended factor of 0.8 to produce contours of Cyclic Resistance Ratio (CRRM=7.5) for clay-like fine-grained soils on the Robertson (1990) Soil Behavior Type chart, as shown in Figure 1. Robertson and Wride (1998) suggested that the approximate boundary between sand-like and clay-like behavior could be estimated using the Soil Behavior Type Index, Ic, and that the boundary was approximately Ic = 2.6, as shown on Figure 1. Zhang et al, 2001, recently updated the normalization of cone resistance for application to liquefaction assessment to allow for a variable stress exponent in different soils. The updated approach for normalized cone resistance (Q) is recommended for Figure 1. Also included in Figure 1 are the contours for CRRM=7.5 for sand-like soils derived using the method described by Robertson and Wride (1998). Robertson and Wride (1998) and Youd et al (2001) suggested that when Ic > 2.6, samples should be obtained and Atterberg Limits determined to evaluate whether the soils were more clay-like or sand-like. Figure 1 illustrates that when the Robertson and Wride (1998) approach for sand-like soils is extended into the clay-like region (i.e. Ic > 2.6) the approach produces generally conservative estimates of cyclic resistance ratio compared to the clay-like response suggested by Boulanger and Idriss (2007). |
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