Spatial Variability of the Depth of Weathered and Engineering Bedrock using Multichannel Analysis of Surface Wave Method P. ANBAZHAGAN 1 and T. G. SITHARAM 1 Abstract—In this paper an attempt has been made to evaluate the spatial variability of the depth of weathered and engineering bedrock in Bangalore, south India using Multichannel Analysis of Surface Wave (MASW) survey. One-dimensional MASW survey has been carried out at 58 locations and shear-wave velocities are measured. Using velocity profiles, the depth of weathered rock and engineering rock surface levels has been determined. Based on the literature, shear-wave velocity of 330 ± 30 m/s for weathered rock or soft rock and 760 ± 60 m/s for engineering rock or hard rock has been considered. Depths corresponding to these velocity ranges are evaluated with respect to ground contour levels and top surface levels have been mapped with an interpolation technique using natural neighborhood. The depth of weathered rock varies from 1 m to about 21 m. In 58 testing locations, only 42 locations reached the depths which have a shear-wave velocity of more than 760 ± 60 m/s. The depth of engineering rock is evaluated from these data and it varies from 1 m to about 50 m. Further, these rock depths have been compared with a subsurface profile obtained from a two- dimensional (2-D) MASW survey at 20 locations and a few selected available bore logs from the deep geotechnical boreholes. Key words: MASW, shear-wave velocity, weathered rock, engineering rock and rock depth. 1. Introduction Spatial variability of the bed rock with reference to ground surface is vital for many applications in geotechnical engineering and geosciences. Rock depth in a site is a very useful parameter for the geotechnical and geotechnical earthquake engineers to find their basic requirement of hard strata (shear-wave velocity of more than 700 m/s) and to arrive at the depth of the foundation or to assign input ground motion for ground response analysis. In most of the geotechnical investigations, knowledge of the hard strata or rock is essential to decide the type of foundations and to design a suitable foundation for a structure. Among the ground response analysis, Peak Ground Acceleration (PGA) and response spectrum for the particular site is evaluated at the bed rock level and further on at the ground level considering local site effects. 1 Department of Civil Engineering, Indian Institute of Science, Bangalore 560 012, India. E-mail: [email protected]; [email protected]; URL: http://www.civil.iisc.ernet.in/*anbazhagan/ Pure appl. geophys. 166 (2009) 409–428 Ó Birkha ¨user Verlag, Basel, 2009 0033–4553/09/030409–20 DOI 10.1007/s00024-009-0450-0 Pure and Applied Geophysics
Spatial Variability of the Depth of Weathered and Engineering Bedrock using Multichannel Analysis of Surface Wave Method
Jun 24, 2023
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