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- مبلغ: ۹۱,۰۰۰ تومان
The cyclic strain approach was proposed in the 1980s as a potential alternative to the stress-based simplified liquefaction evaluation procedure. However, despite its fundamental basis and many positive attributes, it has not been embraced by practice. One reason for this may be the need to perform cyclic laboratory tests on undisturbed/reconstituted samples to develop a relationship among excess pore water pressure, cyclic strain amplitude, and number of applied strain cycles. Herein an alternative implementation of the strain-based procedure is proposed that circumvents this requirement, using a strain-based pore pressure generation model in lieu of laboratory test data. To assess the efficacy of the alternative implementation, several hundred small strain shear wave velocity (Vs) and Standard Penetration Test (SPT) field liquefaction case histories are evaluated. The results are compared with both field observations and with predictions from the stress-based procedures. It was found that the stress-based approach yielded considerably more accurate predictions compared to the cyclic strain approach. One likely reason for this is the strain-based procedure's inherent and potentially fatal limitation of ignoring the decrease in soil stiffness due to excess pore pressure when representing the earthquake loading in terms of shear strain amplitude and number of equivalent cycles.
6. Summary and conclusions
The existence of a volumetric threshold shear strain, below which there is no development of excess pore pressures, and the unique relationship between excess pore pressures and cyclic shear strain, make compelling arguments for adopting a strain-based approach for evaluating liquefaction potential. Herein an alternative implementation of the Dobry et al.  cyclic strain approach is assessed by evaluating liquefaction triggering using both Vs and SPT case histories. Toward this end, γc was computed using the Dobry et al.  procedure in conjunction with shear modulus degradation curves by Ishibashi and Zhang . As a corollary to γtv, (amax)t was shown to provide a fast and simple screening for liquefaction triggering, where liquefaction triggering is unlikely when amax is less than ~0.08–0.12 g, which corresponds to γc in the critical stratum less than ~0.03–0.05%.
If either amax > 0.08–0.12 g or γc > 0.03–0.05%, excess pore pressures are predicted to develop and it becomes necessary to quantify these pore pressures to evaluate liquefaction potential. This was accomplished by implementing the pore pressure generation model by Vucetic and Dobry , using correlations by Mei et al.  and Carlton  to obtain the required model calibration parameters and the correlation by Lasley et al.  to estimate neqγ. For the case histories analyzed, the efficacy of the strain-based procedure was identical when Vs versus N1,60cs are known and other parameters are estimated. However, there was a higher percentage of True Negative predictions when N1,60cs was known than when Vs was known; conversely there was a higher percentage of True Positive predictions when Vs was known than when N1,60cs was known. This could be related to the subjective positioning of the deterministic CRRM7.5 curves for the respective procedures or the robustness of the scenarios represented in the respective case history databases.