SITE CLASSIFICATION AND SEISMIC RESPONSE OF DHAKA CITY SOILS

ABSTRACT Dhaka, one of the fastest growing megacities of the world, is located in a seismic zone with an equivalent peak ground acceleration value of 0.15g. One of the major concerns during an earthquake in Dhaka is the presence of soft soils which may result in amplification of ground motion. Many low lying lands in the city area have been filled up for urban construction to meet the population growth, which have not been properly compacted. Soil boring data including Standard Penetration Test (SPT) values from 34 sites around Dhaka have been collected to estimate equivalent dynamic properties of the top 30 m soil. These sites can be classified into three groups S2, S3 and S4 as per the 1993 Bangladesh national building code. This code, based on UBC 1990, recognizes site amplification effects and specifies design response spectrum with maximum spectral acceleration ratio of 2.5 for soil types S1, S2 and S3. Soil type S4 needs site-specific analysis. Seismic site response of the top 30 m soil at eighteen selected sites of the city belonging to S2, S3 and S4 class is computed using the computer program 'SHAKE'. Seven different earthquake records scaled and suited to soil type S2 have been used. Results obtained suggest that the maximum spectral acceleration ratio of the response spectrum should be around 2.75 for soil type S2 and around 3.75 for soil type S3. The current building code is thus on the unsafe side. The results presented thus indicate the necessity of revising the existing national code