Foundation Design for High-Rise Tower in Karstic Ground

Abstract

Ground investigations for a high-rise tower in Jeddah Saudi Arabia revealed the presence of cavities in some of the boreholes. It was decided that a piled raft would be a suitable foundation option as it would provide a measure of redundancy in case some of the piles encountered cavities. Under such circumstances, the raft would permit the transfer of load from the affected piles to those founded in sound limestone. This paper sets out the design process adopted for the piled raft foundation, and describes the post-design analyses undertaken to investigate the mechanism of load transfer from cavityaffected piles. Comparisons are made between the behaviour of the piled raft when no cavities are present and that when cavities are randomly encountered by some of the piles. The increases in pile axial load and raft bending moments in such circumstances are presented. INTRODUCTION Karstic limestone is relatively widespread around the world, including many parts of the Middle East. The identification of cavities in karstic limestone often creates, at best, a sense of anxiety among foundation designers, who may then proceed to take extreme measures to overcome the perceived dangers and high risks associated with the proximity of cavities to a foundation system. For a high-rise project in Jeddah Saudi Arabia, involving a tower over 390-m high, potentially karstic conditions were identified in some parts of the site. A piled raft foundation system was developed for this tower, as it was considered that such a system would allow the raft to redistribute load to other piles in the group if cavities caused a reduction of capacity or stiffness in some piles within the group. This paper provides a brief description of the foundation design aspects of the project, and then describes a post-design investigation to assess the consequences on foundation performance of cavities being present within the underlying limestone. It is demonstrated that such consequences, while not insignificant, may not be as serious as might be feared, because of the inherent redundancy of the piled raft foundation system. GEOLOGICAL AND GEOTECHNICAL CONDITIONS The city of Jeddah is located within the Makkah quadrangle in the southern part of the Hijaz geographic province in Saudi Arabia. Eastward of the flat, low––lying coastal plain are the Sarawat mountains that culminate in a major erosional escarpment that has resulted from uplift associated with Red Sea rifting. The underlying reefoidal limestone is considered to be a