The shaft capacity of small displacement piles in chalk

Abstract

Multi-pile foundation systems are used to support offshore infrastructure in areas of Northern Europe underlain by Chalk. During installation, the tip of the tubular open-ended ‘small displacement’ pile crushes and destructures the Chalk forming a ‘putty’ interface or ‘annulus’ around it. Pile resistance to uplift (i.e. shaft capacity) results from sliding friction between the pile shaft and this putty chalk annulus. The limited knowledge of the characteristics of this interface is associated with pile design guidelines that may be overconservative, which could result in unnecessary increases in economic costs for the offshore renewable energy industry.

The present Thesis investigates the prospect of using soil mechanics to describe shaft friction in small displacement piles in Chalk as function of the critical state strength of the chalk putty that composes the annulus. To this end, oedometer and triaxial tests were conducted on low and medium density destructured White Chalk samples to determine the suitability of a critical state framework to characterise the mechanical behaviour of the material. Monotonic and cyclic simple shear tests were then used to simulate load transfer through the chalk-pile interface and examine the compatibility of interface performance with the prospective critical state framework. Thereafter, model piles of different geometries were installed in intact chalk cores and micro-focus X-ray computed tomography (XCT) was applied to measure the density of the chalk putty annulus. The piles were then tested in tension.

Oedometer and triaxial results demonstrate that a unique one-dimensional normal compression line and a unique critical state line (CSL) exist for the tested materials, regardless of their origin and the varied preparation methods used. This ‘uniqueness’ is associated with comparable grain shapes and size distributions, and to grain breakage processes that resemble those reported for sands. Simple shear experiments show that interface failure states comply with the CSL, although strain localisation precludes the attainment of critical state strengths when shearing initiates in significantly dense of critical conditions. A CSL-based framework defining interface strength as a function of void ratio was produced and found to compare well with the small number of early-life shaft capacity measurements of piles available in the literature. Yet, this framework could not predict the shaft capacity of the model pile experiments, due to heavily dense of critical states at the end of installation. XCT further revealed that pile penetration processes in chalk and sand are generally comparable, and that design approaches based on the cone penetration test might successfully estimate early-life shaft capacity. Though pile set-up was found to possibly result from annulus consolidation, its prediction may need to rely on empiricism for the foreseeable future.

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Identifiers

ORCID for Fernando Jesús Álvarez Borges: orcid.org/0000-0002-6940-9918

ORCID for Christopher Clayton: orcid.org/0000-0003-0071-8437

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