Novel Approach in the Design of Friction Piles Resting on the Rock – Reverse Engineering

Abstract

Abstract Offshore structures are supported by pile foundations to transfer vertical and lateral loads to the sub-soil. Based on the type of sub-soil, pile foundations are designed as driven or drilled & grouted piles. Design of the offshore pile foundation generally follows the guidelines of API RP 2A and API RP 2GEO. However, these guidelines being general it's applicability for complex soil-rock stratifications need to carefully reviewed. For the soils with sand/clay layers in upper stratum and rock or hard carbonate rock layers below these soft stratums, combined pile foundations are generally followed. These include driven pile in top soil with drilled & grouted insert pile in the rock below. However, this combined pile foundation has great impact on offshore installation time which directly influences cost of the project. From the design point of view, drilling through soil plug of driven pile has major impact on pile frictional and end bearing capacity. New design approach was developed for one of the platforms in the Arabian Gulf, where top soil is sand and clay up to 40 m followed by 10 m of Gypsum and 10 m of sand layers. Based on soil strata, composite pile option was recommended in the soil report with driven piles up to 40 m below mudline and insert piles of lesser diameter up to 55 m. However, this recommendation had an impact on project cost and also there was uncertainty in achieving required design pile capacity. For safe and economical design, a novel design was proposed by making use of rock end bearing to achieve the required capacity. However, in the geotechnical report rock design parameters for axial capacity were not reported and design guidelines are silent on capacity computation of driven piles in the rock. Therefore, end bearing capacity in the rock was back-calculated from the pile driveability analysis using the GRLWEAP software. With this back calculations and required design changes, composite pile option was eliminated and pile was designed with minimum penetration into the rock. This paper includes discussion on methodology, results, pre-cautionary measures, other additional requirements and cost comparison with conventional method. This new approach of pile design in complex soil-rock system is cost-effective and safe. This can be implemented for future platforms with the similar type of soil strata.