Site Characterization Using Geotechnical and Geophysical Techniques - Applicability of 30mAverage Concept in Shallow Bedrock Region of Bangalore, India

Document Type : Seismology and Engineering Seismology


Indian Institute of Science


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 30mshear 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.