Predictive Relationship for Estimation of V_(s30) Using Shallow Bore Logs for Karachi

摘要

Standard Penetration Test (SPT) is one of the most widely adopted methods for determination of the average shear wave velocity for the top 30 m of soil at a given site, V_(s30) However, in many cases, the bore logs do not extend to the required depth of 30 m. In such scenarios, V_(s30) needs to be predicted using the average shear velocity of shallow bore logs (depth <30 m), V_(sd) This paper investigates the relationship between the average shear wave velocity for a shallow bore log and the average shear wave velocity for the top 30 m of soil at a given site for Karachi. To this end, a large dataset of bore logs is congregated which includes 140 deep bore logs with the depth extending up to or exceeding 30 m and 1,350 shallow bore logs with the depth below 30 m. The most suitable relationship is selected from the literature for conversion of the SPT N-values to corresponding shear wave velocity. Utilizing the deep bore logs, a predictive relationship is developed herein to correlate the average shear wave velocity for the shallow bore log with V_(s30). A comparative analysis of the relationship developed in this paper is carried out with two existing methods, namely the extrapolation assuming constant velocity and extrapolation using the regression coefficients proposed for California. Based on the statistical study, it is noted that the extrapolation assuming constant velocity and extrapolation using regression coefficients proposed in this work produce better estimates of V_(s30). Finally, V_(s30) maps are generated for Karachi.