Development of seismic microzonation map and mapping of soft sediment thickness in Kuala Lumpur by using microtremor survey
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Date
2017-06
Authors
Chiew, Chang Chyau
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Abstract
Seismic site effect is one of the major concerns in earthquake design of structures. Soft ground tends to amplify the seismic wave in surficial geological layers. This dissertation presents a simple and convenient approach to estimate the soft sediment cover thickness at site using microtremor observation technique. A total number of 103 observation points were conducted at selected sites around Kuala Lumpur area using microtremor measuring instrument. The bedrock of Kuala Lumpur area is formed by Kenny Hill Formation, limestone, granite and the Hawthornden Schist but the thickness of soft sediment formed by alluvial deposits, mine tailings and residual soils still remains unknown. Hence, the predominant frequency of ground in each site was determined based on Nakamura method. The results show that the predominant frequency observed in Kuala Lumpur ranges from 1.48 Hz to 5.85 Hz. Kuala Lumpur has an average predominant frequency of 3.11 Hz. Seismic microzonation map based on predominant frequency for Kuala Lumpur was produced for seismic design purpose. In order to develop an empirical correlation to relate the ground predominant frequency and sediment cover thickness, a total number of 14 sites with known depth to engineering bedrock from geotechnical reports in the study area were referred. The relationship of thickness of soft sediment (h) and predominant frequency of soil (f) was established as h = 121.16f -1.538. Based on the established relationship, the thickness of soft sediment in Kuala Lumpur was determined from the result of microtremor survey and plotted in a map. The depth of the soft soil in Kuala Lumpur region ranges from 8 m to 66 m. The findings from this project provide useful information for engineers to estimate the soft sediment cover thickness in Kuala Lumpur area based on the dynamic characteristics of the ground measured from microtremor observation.