Comparative analysis of Geotechnical and Geophysical Investigations for average shear wave velocity in Metro Manila

Abstract

Philippines is one of the most seismically active countries in Southeast Asia and records at least 20 earthquakes per day according to the PHIVOLCS. Vs30 is defined as the shear wave velocity of the upper 30-meter subsurface profile which is an important seismic parameter to determine the dynamic behavior of soils or rocks. This research aims to determine the Vs30 of three sites in Metro Manila using two methods Geotechnical Investigation(GI) and Geophysical Investigation(GPI) by Multichannel Analysis of Surface Waves (MASW) method and compare their results. The data used for GI are SPT N-values for soils and Rock Quality Designation. Conversely, a seismic survey using the MASW method was conducted along the three sites and Vs30 was determined based on the processed seismic data from Geogiga software. The results of the methods were compared in terms of the ratio between the Vs30 difference from geotechnical investigation and geophysical investigation with the calculated range based on the NEHRP classification table. The calculated ratios showed 43.68% for MNHS, 6.44% for BES, while 48.16% for VFMHS. Comparison of methods showed that ratio between Vs30-difference from GI and GPI with the calculated range based on the NEHRP was found to be acceptable (≤1.0) for all sites. Moreover, the seismic soil classification based on the NEHRP was found to be consistent for both methods.