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    Evaluation of Cyclic Softening in Clays PDF

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    Bárbara Rodríguez
    This document discusses the evaluation of cyclic softening in clays. It suggests that the first step is to evaluate the soil's behavioral characteristics, and that the transition from sand-like to clay-like behavior occurs around a plasticity index of 7-12. The resistance to cyclic loading in silts and clays is controlled by the undrained strength ratio, and large deformations are likely if the cyclic stress ratio approaches the undrained strength ratio. The document also discusses using CPT results to estimate undrained shear strength ratios and contours of cyclic resistance ratio in clay-like soils.

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    Evaluation of Cyclic Softening in Clays PDF

    Uploaded by

    Bárbara Rodríguez
    81 views2 pages
    This document discusses the evaluation of cyclic softening in clays. It suggests that the first step is to evaluate the soil's behavioral characteristics, and that the transition from sand-like to clay-like behavior occurs around a plasticity index of 7-12. The resistance to cyclic loading in silts and clays is controlled by the undrained strength ratio, and large deformations are likely if the cyclic stress ratio approaches the undrained strength ratio. The document also discusses using CPT results to estimate undrained shear strength ratios and contours of cyclic resistance ratio in clay-like soils.

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    This document discusses the evaluation of cyclic softening in clays. It suggests that the first step is to evaluate the soil's behavioral characteristics, and that the transition from sand-like to clay-like behavior occurs around a plasticity index of 7-12. The resistance to cyclic loading in silts and clays is controlled by the undrained strength ratio, and large deformations are likely if the cyclic stress ratio approaches the undrained strength ratio. The document also discusses using CPT results to estimate undrained shear strength ratios and contours of cyclic resistance ratio in clay-like soils.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    81 views2 pages

    Evaluation of Cyclic Softening in Clays PDF

    Uploaded by

    Bárbara Rodríguez
    This document discusses the evaluation of cyclic softening in clays. It suggests that the first step is to evaluate the soil's behavioral characteristics, and that the transition from sand-like to clay-like behavior occurs around a plasticity index of 7-12. The resistance to cyclic loading in silts and clays is controlled by the undrained strength ratio, and large deformations are likely if the cyclic stress ratio approaches the undrained strength ratio. The document also discusses using CPT results to estimate undrained shear strength ratios and contours of cyclic resistance ratio in clay-like soils.

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    of 2

    Robertson’s Remarks

    - 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 to evaluate 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 > 12) 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 (CSR = cy/ 'vc) that is close to the undrained strength ratio of the soil
    (su/ 'vc), 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 CRRM=7.5 = 0.8 su/ 'vc 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 – vc) / Nkt
    Where: su is the peak undrained shear strength; qt is the corrected total cone resistance; vc is the total overburden stress, and;
    Nkt is the empirical cone factor, which typically varies from 10 to 20 depending on the appropriate undrained shear strength
    required.

    Normalized CPT parameters are used to identify Soil Behavior Type (SBT), where the normalized CPT parameters in clay-like
    fine-grained soils are:
    Normalized cone resistance, Q = (qt – vc)/ 'vc
    Normalized Friction Ratio, F = 100 fs / (qt – vc) in percent (%)
    It can be shown that for insensitive clay-like fine-grained soils the undrained shear strength ratio can be estimated using the
    following simplified equation and CPT results: su / 'vc = fs / 'vc = (F . Q) / 100

    For sensitive clays the sleeve friction, fs is essentially the remolded shear strength of the clay. Hence, the above equation
    produces the remolded undrained strength ratio. Hence, it is possible to identify contours of undrained shear strength ratio in
    insensitive, 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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