Cyclic and Post-Cyclic Behavior of Bootlegger Cove Formation Clay

Colorado Urrea, Gabriel Jaime

doi:https://doi.org/10.21985/n2-hqjf-gv09

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Cyclic and Post-Cyclic Behavior of Bootlegger Cove Formation Clay

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Cyclic loading, from earthquake, has caused the instability and failure of slopes composed of sensitive clays. During the 1964 9.2 moment magnitude (M) earthquake in Anchorage, Alaska, several slopes comprised of Bootlegger Cove Formation (BCF) clays failed during the strong seismic motion. Some researchers have proposed the use of residual strength to account for strength degradation to overcome the seismic failure of clay slopes. The residual strength is determined in monotonic tests that involves large deformations that may never occur during an earthquake on a clayey slope. This conservative approach fails to provide a comprehensive analysis to consider dynamic parameters (e.g., magnitude and duration) on clay specimens that can control the possible strength degradation during and after a seismic event. The goal of this research is to understand the cyclic behavior and post-cyclic degradation of facies III and IV of Bootlegger Cover Formation (BCF) clay based on laboratory tests to provide key information needed to develop a more comprehensive analysis. This work evaluates the field and the laboratory investigation performed by Northwestern University and others at Lynn Ary Park adjacent to the Turnagain Heights landslide escarpment in Anchorage The field exploration consisted of drilling two borings to obtain thin-walled samples for subsequent laboratory testing and one boring to perform field vane and downhole seismic tests. Field data used herein also included the results from CPT, field vane and torvane tests performed by other researchers. These data can provide the necessary elements to establish the soil characterization of the BCF clay at Turnagain Heights. Laboratory tests were used to establish soil characteristics and in situ conditions of the BCF clay and to evaluate the cyclic and post-cyclic response of BCF clay. For the evaluation of in situ conditions, the laboratory tests include index property, consolidation, monotonic undrained triaxial shearing results. These results complemented the field work and provided a full evaluation of the conditions of the BCF at the site and the possible properties that can impact its cyclic and post-cyclic behavior. The laboratory testing program also includes cyclic and post-cyclic loading triaxial tests on BCF specimens collected in thin-walled tubes. The analyses focus on the behavior of the BCF clay under equivalent earthquake loadings. Both undrained cyclic and post-cyclic strength are evaluated with respect to stress history and liquidity index. The BCF specimens were tested under stress-controlled cyclic loading with diﬀerent cyclic stress ratios (CSR). The degradation of undrained shear strength directly is evaluated based on the results of post-cyclic undrained shearing. This degradation is presented as a function of the accumulated strain during the consolidation and cyclic loading or during cyclic loading only. The degradation of post-cyclic strength was also analyzed in terms of effective stresses. The role of sensitivity, quantified by the liquidity index, and conventional axial strain failure criteria are assessed. A limited study of the cyclic loading rate was made to illustrate its effects on the cyclic responses of clay. Field and laboratory tests were conducted to measured OCR, strength, sensitivity, cyclic and post-cyclic strength. Results from analysis of field and laboratory data showed that the OCR at the BCF at the Lynn Ary Park site decreases with depth, presumably as a result of desiccation. The undrained strengths measured by CPT and field vane were consistent with the OCR and decreased with depth. Material sensitivity measured by field vane varied from 2 to 6 with the highest sensitivity near a sand layer in predominately clay profile. The post-cyclic normalized undrained strength of BCF specimens depends on OCR, liquidity index, and the total strain prior the post-cyclic shearing (i.e., axial strain from consolidation and cyclic shearing). Reduction of post-cyclic strength of NC specimens was observed after 15% axial strain accumulated during cyclic loading. There is an influence of the liquidity index in the degradation of the undrained strength when it is higher than 1.0. Based on the current design guidelines, a reduction of 10 to 15% of the undrained strength does not apply to the BCF clays evaluated in this thesis, wherein results indicate more degradation can occur in specimens when accumulating a large amount of axial strain. The cyclic and post-cyclic strength from specimens prepared by SHANSEP are conservative and may be more representative of the intrinsic behavior of BCF clay.

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