Engineering geophysics for site characterization for local seismic hazard assessment in Barishal city, Bangladesh

Abstract

Theme 7: Urban Geology, Urban Planning and Management – Engineering geological perspective

Sub-Theme 7.1:Hazard Assessment and Geo-Information Management for Climate-Change-Resilient Urbanization

Abstract: Various engineering geophysical methods are available for seismic site characterization, including invasive and non-invasive techniques. Multichannel analysis of surface waves (MASW), downhole seismic testing (DST) and Standard Penetration Tests (SPT) were carried out to for local seismic site effect (amplification) analysis in the mapped area. A regional seismic hazard map based on a probabilistic seismic hazard assessment (PSHA) for Bangladesh was prepared by Azari Sisi et al. (2021), reflecting regional Peak Ground Acceleration (PGA) at bedrock with an exceedance probability of 10 % in 475 years. The mapped area covers an area of approximately 92 km² and is characterized by very thick deposits of unconsolidated or loosely consolidated late Holocene to recent age fluvial and deltaic sediments. The research presented in this contribution gives an estimation of the near surface one-dimensional (1D) shear (S) wave velocity (Vs) models at every 1 m depth interval, limited to the upper 30 m of soil deposits at various locations in Barishal city using DST which was done in the framework of the GPAC project carried out by BGR, LIAG and GSB.  MASW results were cross-checked with DST data. 13 boreholes with DST and SPT information were used to establish a regression model to calculate shear wave velocity using the SPT data at each SPT location. A SAGA (system for Automated Geoscientific Analyses)-ISEG (the Information System Engineering Geology)-based algorithm is used to correlate each SPT N-value (uncorrected/1.5 m interval) with the corresponding measured shear wave velocity (mean value). The two parameters, shear wave velocity and SPT are thus correlated and the resulting exponential regression formula of Barishal is 107.9x^0.192 with R² = 0.6308. The local site conditions are classified using the mean shear wave velocity value representative for the top 30 m soil column (VS₃₀ value in m/s). The VS₃₀ values range between 145 m/s and 216 m/s. Based on VS₃₀, the studied areas are classified into two seismic site classes, SC and SD according to the national building code (BNBC, 2020).