Abstract

Windfarms have become a major player in green energy transformation for the world. With ever increasing demand for clean energy, offshore windfarm sites are slowly moving to rocky seabed conditions that are not suitable for conventional pile designs which are covered by existing codes and standards. Understanding shaft friction in drill and grouted piles in rock is critical for design of offshore foundations in rocky seabed. This paper presents insight into shaft friction using a new database of shaft friction in rock by compiling from literature a comprehensive collection of 365 pile load tests data from various rock types, strengths and socket diameters varying from 0.06m to 3m.

INTRODUCTION

Offshore windfarms sites which have rock seabed conditions require pile foundations in rock. Rock-socketed piles rely on the shaft friction between the grout and the rock for the axial capacity. There are several empirical correlations that provide unit shaft friction for grouted piles in rock based on unconfined compressive strength (UCS) of rock. However, these are often limited to particular rock type and do not account for some of the parameters that govern shaft friction; in particular these correlations do not account for socket diameter which affects dilation induced increase in radial stresses. This paper presents a new master database of unit shaft friction for drilled and grouted piles in rock compiling a comprehensive collection of 365 pile load tests from various rock types, strengths and socket diameters varying from 0.06m to 3m. The new database of shaft friction from pile load tests is used to provide insight into the factors that affect the shaft friction, and to provide lower and upper shaft friction estimates for varying socket diameters, rock types and UCS. Published empirical methods that predict the unit shaft friction in rock based on UCS are also presented and compared to the database of measured shaft frictions.

A detailed literature review was undertaken investigating all available pile load test databases for drilled and grouted piles in rock. The following reference sources were used to collect the available pile load test data: Collingwood (2000); CIRIA R181 (Gannon et al. 1999) ; FHWA (Brown et al. 2010) ; Fragio et al. (1985) ; Gunnink and Kiehne (1998); Horvath et al. (1983); Ng et al. (2001); Shoib et al. (2017); Wainshtein et al. (2008); Williams and Pells (1981); Wood (2015); Manceau et al. (2020)

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