Predicting the end bearing capacity of rock-socketed bored and cast in-situ piles in Sri Lankan metamorphic rock using rock mass properties

Abstract

Rock socketed end bearing cast in situ bored (CIB) piles are currently among the most widely used types of deep foundations in Sri Lanka to transfer the superstructure load into the competent bedrock. However, deliberations are going on whether the pile designers maximize the advantage on locally available higher-strength-less deformable metamorphic rock characteristics during the design of CIB piles. In this study, appropriateness of commonly used end bearing capacity estimation methods in the design of rock socketed CIB piles are briefly reviewed and then the results obtained through these methods are compared against the actual end bearing capacity levels obtained from pile static load test data. Locally available static pile load test data do not facilitate direct estimation of actual ultimate end bearing capacity values due to non-continuation of pile load tests beyond the required pile capacity limits caused by practical and economic reasons. Therefore, load-settlement curve extrapolation methods proposed by Chin- Kondner (1978), Brinch Hansen (1963) and Decourt (1999), were used in this study to determine the ultimate carrying capacities of piles. It is found that the ultimate carrying capacity values obtained from Chin- Kondner (1978) and Decourt (1999) methods are in a similar range while values obtained from Brinch Hansen (1963) are far below from other two methods Moreover, the Hong Kong Guidelines (HKG) method which has been developed based on Rock Mass Rating (RMR), seems reasonable to estimate of the allowable end bearing capacity of the bedrock corresponding to a settlement of less than 0.5% of the pile diameter. However, HKG method can be used only in instances where RMR value of rock strata is exceeds 40. In order to facilitate locally available high-strength rock terrain conditions, a more appropriate new empirical relationship between RMR and allowable end bearing capacity of rock socketed pile was introduced and the accuracy of the relationship was verified using limited available instrumented pile load test data. It was further noted that this relationship performs well in rock masses with RMR greater than 20.