Friction to Unconfined Compression Strength Correlation for Evaluating Capacity of Weak Rock Foundation Sockets

Abstract

Historically, deep foundations in weak rock have been designed as friction elements using frictional resistance (fs), calculated from the unconfined compressive strength (UCS) of rock. Most of the published correlations of fs to UCS were developed based on load tests on low-capacity piles in specific geological conditions, using UCS values not necessarily representative over the test depth. There is a large variation in foundation design depths calculated using these correlations. This paper presents a correlation between fs and UCS of weak rock, developed using data from 44 bidirectional load tests from high-capacity deep foundations in weak rocks. The dataset used in this study, is one of the largest used for weak rocks, with high test loads in the range of 100–320MN and the depth of foundations mostly in the range of 20–87 m below ground level. Bi-directional load test data from La Maison tower site is then simulated in Plaxis, and ultimate skin friction developed is compared against the skin friction calculated using the new corelation. The actual ground profile and foundation layout of La Maison tower is then modelled in Plaxis with the required foundation depth derived using the recommended corelation, to check serviceability limits. The resulting maximum settlements are found to be well within the acceptable limits. The correlation factor of 0.5 between fs and UCS is thus recommended for estimating rock socket friction for design of deep foundations in weak carbonate rock formations.