Correlations between cone penetration test (CPT), tip resistance, qc', and pile shaft friction, tf, have been shown to be reliable for evaluating axial pile capacity. The correlation between tf and qc is not direct, and pile shaft friction is influenced by many more factors than those which affect cone tip resistance, namely
differences between open- and closed-ended piles/penetrometers;
the reduction in local friction with continued pile penetration (friction fatigue);
changes in radial stress during loading; and
interface friction angle.
This paper presents results from analytical studies, model pile test results and field pile load tests in siliceous, micaceous and calcareous sands to assess the influence of sand grain mineralogy on input parameters for CPT qc based shaft friction calculations. While the data for calcareous and micaceous sands are limited, observations based on laboratory and field studies are consistent. The paper concludes that while input parameters may differ, the same framework is valid for evaluating shaft friction in siliceous, calcareous and micaceous sands from CPT data. Calcareous and micaceous sands appear to have higher rates of degradation of local friction than siliceous sands, but this degradation tends to be bounded by a minimum shaft friction value. Input parameters to a general expression for shaft resistance based on the UWA-05 design method are proposed for each sand type.
A sound design method leads to a safe and cost effective engineering solution with consistent levels of reliability for the anticipated range of situations encountered over the design lifetime. For piled foundations, previous successful experience and static or dynamic load testing plays a significant role in developing such design methods. However, experience alone cannot guarantee reliability, and there is a clear need to develop design frameworks that reflect the current best understanding of the underlying factors controlling pile capacity. Such understanding is critical for offshore piles currently being considered in new regions and in soil conditions that have not been previously encountered1. Of the 600 failures of civil engineering systems reviewed by Bea2, a majority of failures during operation and maintenance were attributed to flawed engineering design. While these structures and foundations may have been designed to accepted standards, failures occurred due to limitations and imperfections embedded in the standards2.
This paper is concerned with the design of axially loaded driven piles in sand. The comments of Bea2 are particularly relevant to this topic, as the underlying behaviour governing the installation and subsequent axial capacity of piles driven in sand is poorly understood. This uncertainty is compounded by a lack of relevant data to support design formulations. Virtually no measurements of the axial capacity of a driven pile with the dimensions relevant to new offshore developments exist. The databases used to calibrate current design methods predominantly comprise short, small diameter piles. Every design of a full-scale offshore pile is therefore reliant on the design method providing a correct extrapolation from the database pile geometry to the field conditions. It is therefore essential that the design method formulation captures the underlying mechanisms as closely as possible.