Abstract:
The ground motion parameters such as amplitude, frequency and duration are altered greatly due to the presence of soil deposit in the earth’s shallow crust which is known as site response. Proper site response analysis is required for proper hazard quantification due to an earthquake. In BNBC 2020, site response is quantified by multiplying a factor with bedrock response curve based on soil type. BNBC 2020 divides Bangladesh into four seismic zones with different levels of ground motion. Each zone has a seismic zone coefficient (Z) which represents the peak ground acceleration (PGA) on very stiff soil/rock (site class SA) in units of g (acceleration due to gravity) at maximum considered earthquake (MCE) level. The design basis earthquake (DBE) ground motion is selected at a ground shaking level that is 2/3 of the maximum considered earthquake (MCE) ground motion. PGA on rock (site class SA) for Chattogram city is 0.28g at MCE level and 0.19g (2/3 x 0.28) at DBE level. If the soil type of our study site is “SD”, PGA at surface of that place will be 0.2565g (0.19g X 1.35). Similarly, if the soil type of our study site is “SC”, PGA at surface of that place will be 0.2185g (0.19g X 1.15).
In this research Chattogram city in Bangladesh has been taken into consideration. Site response of different locations of this city have been developed employing 1-D site response analysis using nonlinear (NL) and equivalent linear (EL) methods and compared with the site response developed with BNBC 2020 formula. For this analysis, 10 time histories from a recognized international source PEER has been used as input motions at the bedrock level.
In this study, 1-D site response analysis has been performed using DEEPSOIL software. The analysis has been conducted by utilizing both the equivalent linear and nonlinear methods within the DEEPSOIL software.
The nonlinear Peak Ground Acceleration (PGA) for the soil type “SD” was determined to be 0.2680g through averaging of PGA measurements taken at the surface, using DEEPSOIL software, across 18 boreholes. Interestingly, it has been observed that the PGA value calculated using the equivalent linear (0.4405g) method exceeds that of the nonlinear method (0.2680g).
Based on the microtremor survey reports acquired during a prior testing initiative carried out in 2009 by the Asian Disaster Preparedness Center and OYO International Corporation as part of the Comprehensive Disaster Management Programme (CDMP) at a depth of 125 meters, it is evident that the shear wave velocity registers at around 800 meters per second in our study site. Hence, it is important to note that the bedrock is assumed to be located approximately 125 meters below the ground's surface at the study site.