Shaft Friction of Bored Piles in Hard Clay

Abstract

The precise prediction of maximum load carrying capacity of bored piles is a complex problem because it is a function of a number of factors. These factors include method of boring, method of concreting, quality of concrete, expertise of the construction staff, the ground conditions etc. besides the pile geometry. The performance of pile load tests is, therefore, of paramount importance to establish the most economical design of piles especially where bored cast-in-situ piles are to be provided to support a structure. This paper describes the experience gained from four pile load tests at a site in the North West Frontier Province of Pakistan where a new cement plant is going to be installed. Geotechnical investigations at the site were carried out to a maximum depth of 60 m. The subsoils at the site are predominantly hard clays within the investigated depth with thin layers of gravels / boulders below 40 m depth. Perched water was encountered at various horizons. Four piles of diameter varying from 660 mm to 760 mm and length ranging between 20 m and 47.5 m were subjected to axial loads. The load test data were analyzed using various state of the art techniques including intercept of two tangents, point of change of slope, 6 mm net settlement [1], 90 percent and 80 percent Hansen [7], limit value Davisson [2], and Chin [3]. Based on a comparison of pile capacities from these methods with the theoretical values, recommendations are made on the approach to estimate the pile capacity in hard clays. Using the pile load test results, back calculations were also carried out to estimate the appropriate values of pile design parameters such as undrained cohesion and adhesion factor.