Investigation of the Carrying Capacity of the Socketed Region of Bored and Cast-in-Situ Piles in Sri Lanka

Abstract

Approximately 90 % of Sri Lanka is underlain by Proterozoic gneisses consisting of Precambrian basement, with Phanerozoic sediments being restricted to the coastal areas. Due to the availability of bedrock on an average at approximately 20 m below the ground surface, end-bearing bored and cast-in-situ piles socketed into bedrock are very often used in Sri Lanka to support large structures such as high-rise buildings. However, significantly high variations of both the bedrock profile and properties are observed even within small sites. Therefore, termination of the piles within the bedrock is a very important consideration during the installation of piles to achieve the required capacity. Pile termination is generally done using the drilling rate and observing the rock cutting as the criteria for terminating piles. The relationship between the drilling rate, the quality of the rocks, and mobilized skin friction within the rock socket are investigated in this paper based on the results of instrumented pile load tests. The results of the analysis indicate that the mobilized skin friction in the rock socket does not have a direct correlation with the drilling rate but on the overall quality of the bedrock. Due to the low cost and quick execution, engineers use the skin friction distribution and end-bearing capacity obtained from signal matching of high-strain dynamic load test results for the design of piles. In this paper, mobilized pile capacities within the rock socket are investigated using measured field-instrumented maintain load test results and the results from signal matching. The results of the analysis indicate that the static load-settlement curve can be obtained from CAPWAPVR analysis, but the mobilized skin friction distribution and the end bearing cannot be accurately obtained from an acceptable CAPWAP signal matching.