Site Characterization Using Geotechnical and Geophysical Techniques - Applicability of ۳۰mAverage Concept in Shallow Bedrock Region of Bangalore, India

In this study, a shallow region of engineering bedrock in Bangalore, India, was chosen to investigate the application of ۳۰ m based site classification scheme. About ۳۷۰ bore logs previously drilled up to the bedrock were selected from a geotechnical database. Equivalent N values for ۳۰ m were estimated using measured standard penetration test (SPT) N values, which mean that the study area may be classified as site class C and D. In the study area, a geophysical test and a multi-channel analysis of the surface wave (MASW) were conducted at ۵۸ locations. The shear wave velocities were measured and then used to estimate the equivalent ۳۰mshear wave velocities (Vs۳۰), upon the results of which the study area was then classified as class B, C, and D sites. The average ۳۰ m SPT N and Vs values were included rock N and Vs values when the depth of the rock is less than ۲۵ m. Therefore, an attempt was made to estimate the equivalent SPT N values and shear wave velocity of the thickness of the overburden soil up to the engineering bedrock and compared with ۳۰ m values. This study shows that, by excluding the N and Vs values of the bedrock, this area can be classified as site class D. There were large spatial site class variations between the site classifications based on the ۳۰ m and engineering bedrock. If the area had clearly defined engineering bedrock within ۳۰ m, then average ۳۰ m SPT N values and the shear wave velocity would mean a higher classification. Site response analysis carried out considering selected SPT N and Vs profiles by giving regional synthetic ground motion at ۳۰ m and engineering bedrock. Response spectrum and amplification values are much higher than ۳۰ m based spectrum and amplifications. Separate site classification scheme needs to be developed for the shallow bedrock region with response spectrum and amplification values.

Keywords:

Site Classes , SPT-N Values , MASW , Shear wave velocity , Engineering Bedrock