Site Characterization Using Geotechnical and Geophysical Techniques - Applicability of 30m Average 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 30 m based site classification scheme. About 370 bore logs previously drilled up to the bedrock were selected from a geotechnical database. Equivalent N values for 30 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 58 locations. The shear wave velocities were measured and then used to estimate the equivalent 30 m shear wave velocities (Vs30), upon the results of which the study area was then classified as class B, C, and D sites. The average 30 m SPT N and Vs values were included rock N and Vs values when the depth of the rock is less than 25 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 30 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 30 m and engineering bedrock. If the area had clearly defined engineering bedrock within 30 m, then average 30 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 30 m and engineering bedrock. Response spectrum and amplification values are much higher than 30 m based spectrum and amplifications. Separate site classification scheme needs to be developed for the shallow bedrock region with response spectrum and amplification values.