Study of local site effects using microtremor survey method

• Main Author: Zhu, Cheng.
• Other Authors: Kusnowidjaja Megawati
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Civil engineering::Geotechnical
• Online Access: http://hdl.handle.net/10356/50586
• Institution: Nanyang Technological University

Local site effects are typically characterized by the shear-wave velocity profile of the subsurface layers or the fundamental frequency of the site, which can be obtained by various means. Microtremor survey method is one of the most appropriate methods for urban areas as it is noninvasive and can be carried out easily. Triaxial measurements of microtremors have been carried out at 162 locations in Singapore to investigate the fundamental frequencies of sites underlying different geological conditions. The fundamental frequency of each site is estimated from the Nakamura’s horizontal-to-vertical (H/V) spectral ratio. The site fundamental frequencies estimated are well correlated with the surface geological formations. The frequencies obtained from soft soil sites are typically smaller than 2.5 Hz, whereas those from hard rock formations give higher frequencies or flat H/V spectrum. The site natural frequencies also show good agreement with the N values of the Standard Penetration Test (SPT) at 66 neighbouring boreholes. The relationships between the average shear wave velocity of the upper 30 m soil and topographic slope as well as fundamental frequency are established for site classification, taking into account the uncertainty involved in the estimation. By comparing the fundamental frequency obtained from the H/V spectrum with that of a horizontally layered model based on the SPT data, the maximum effective depth for subsurface soil structure profiling using microtremor measurements is estimated. Additionally, empirical relations are proposed between fundamental frequency and sediment thickness for different shear-wave velocity ranges. Therefore, bedrock mapping is feasible using microtremor survey method for places where site information is not available. Further study on amplification spectrum reveals different impacts by peak ground acceleration (PGA) and underlying soil structure of the site.