Evaluation of design methods for uplift capacity of bored piles

Abstract

Shaft resistance is a major design factor for piles supporting structures such as transmission towers, harbor structures and offshore platforms. Several studies have been conducted to correlate the estimated capacity of piles with the actual capacity determined from load test in compression. However, few studies focused on the determination of the tension capacity of piles. The main objectives of this study are to compare the estimated tension capacity of bored piles with results obtained from uplift load tests and to evaluate the current design methods used to determine the uplift capacity of bored piles. In this study, uplift load tests were conducted on 21 bored piles in several sites. Capacity of the piles was determined from load test data. Capacity of the piles was also estimated from the sub-soil characteristics using five methods–Meyerhof (1968), Murthy (1992), Tomlinson (1977), the German Code of Practice (DIN 4014), and British/American Method (1974). The first three methods use two soil parameters–cohesion, c and angle of internal friction, φ. The British/American method uses δ (based on φ) while the German Code of Practice (DIN 4014) uses only SPT N-value for determining the pile capacity. The results of the pile load test correlate reasonably well with the results estimated from theory. Linear regression of the experimental data showed that the capacity estimated by Meyerhof and Murthy equation and the German Code of Practice (DIN 4014) need to be multiplied by factors of 1.08, 1.22 and 1.28 respectively to get the actual pile capacity. The Tomlinson equation and the British/American method over estimated the pile capacity. The capacity estimated by the Tomlinson equation and the British/American method need to be multiplied by factors of 0.86 and 0.81 respectively to get the actual pile capacity. As far as regression is concerned, the British/American method provided the best regression. The results of these experiments are based on 21 bored pile load test data. The large scatter in the experimental data suggests that a larger sample size is required for better correlation. Nevertheless, the study conducted in this thesis provides an appropriate ground work for further study in this area.