Influence of trapped soft/stiff soil layer in seismic site response analysis
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Ó Indian Academy of Sciences (0123456789().,-volV)(0123456789( ).,-volV)
InCuence of trapped soft/stiA soil layer in seismic site response analysis NARAYAN ROY*, SIBAPRIYA MUKHERJEE and RAMENDU BIKAS SAHU Jadavpur University, Kolkata 700 032, India. *Corresponding author. e-mail: [email protected] MS received 19 November 2019; revised 14 May 2020; accepted 21 May 2020
As the ground response analysis serves an integral part of site speciBc seismic hazard study to obtain design ground motion, a proper and accurate estimation should be of prime importance. The paper presents a study on the eAect of trapped soft and stiA soil layer on equivalent linear ground response analysis implemented in computer program STRATA. For this purpose, the concept of normally stiA and inversely stiA soil proBles have been introduced. The study clearly indicates the higher impact of a trapped soft soil layer proBle, i.e., inversely stiA soil proBle with soft layer, in comparison with trapped stiA layer proBle, i.e., inversely stiA proBle with stiA layer. For low to moderate ground motions, as the depth of the trapped soft layer increases, peak ampliBcation and peak frequency reduces, and for high intensity input ground motion, significant reduction only in the peak frequencies is observed. On the other hand, as the depth of trapped stiA soil layer increases the outcome of ground response analysis remain quite similar. Peak transfer function, peak frequency, peak spectral ratio and peak spectral acceleration are found to exhibit a COV B 60 to 100% for all combinations of IS-Soft proBles, whereas, IS-StiA proBles exhibit a maximum COV B 15 to 25% for all the considered input ground motions. Computed normalized-root mean square error (Norm-RMSE) values also clearly indicate the higher deviations in the ground response analysis for different combinations of trapped soft soil proBles, from that of normally stiA proBle, whereas, trapped stiA layer proBles show lower deviations in the Norm-RMSE values. The proBles with trapped soft layer exhibit a Norm-RMSE value B0.8, whereas, trapped stiA layer proBles exhibit a Norm-RMSE value B0.2, which clearly demonstrate the large deviations in the outcome of ground response analysis in case of trapped soft layer proBles. Keywords. Ground response; soil ampliBcation; normally stiA proBle; inversely stiA proBle; trapped soft layer; trapped stiA layer.
1. Introduction Widespread destruction from past earthquakes exhibited the need to estimate the site speciBc seismic hazard. It is a well known fact that a shallow soil deposit may significantly modify the hazard level at the ground surface (Boore et al. 1993; Anderson et al. 1996), which is known as local site eAects. Seismic ground response analysis
is generally performed to study the local site eAects. Thus, ground response analysis (GRA) has become an integral part in a site speciBc seismic hazard study (Roy and Sahu 2012; Shukla and Choudhury 2012; Desai and Choudhury 2014; Shiuly et al. 2015; Roy et al. 2018). GRA requires small strain shear m
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