Some notes on the design of pile foundations in sedimentary rock

This paper presents some technical notes on the design of pile foundations in sedimentary rock based on the author’s thirty-year research and consulting experiences. Firstly, the fundamental difference between the superseded working stress method and the current ultimate limit state (ULS) approach within the practicing codes will be discussed from a geotechnical engineer’s perspective. Then some delusion and confusion encountered by practicing geotechnical engineers such as rock classification and the characteristics of each rock class will be highlighted. The importance of soil and structure interaction and establishment of design criteria for structures and substructures will be emphasized. An overview of the published methods for assessing the pile end bearing capacity and lateral resistance will be carried out to appreciate some issues that practicing engineers are often required to deal with. For typical bridge pile foundations and piled deep excavation retention or retaining structures, both serviceability limit state and ultimate limit state assessments are required to satisfy the requirements set out in current codes of practice. For vertically loaded piles in sedimentary rock it is found that the serviceability limit state is governing the design rather than the ultimate limit state condition for most road and railway projects, based on the commonly accepted design parameters. For laterally loaded piles in rock it is noted that the method based on the lateral force and bending moment equilibrium such as described in Hong Kong Geoguide 1 is frequently used to determine the pile socket length. The critical input parameter required by this method is the ultimate lateral resistance of the rock mass, which is often arbitrary with little guidance provided, and a degree of confusion is often noted by the author. It is proposed to undertake a lateral equilibrium assessment under ULS conditions for a piled wall along with analysis of the deformation characteristic of the rock mass to come up with the “mobilised” rather than the ULS lateral pressure for pile socket design. Worked examples will be given to demonstrate how the pile socket in sedimentary rock can be determined with reasonable confidence for a cantilever piled wall for tunnel projects, and for a bridge structure.