CPT-based Axial Static Capacity Approaches to Evaluate Pile Driveability in Sand

Abstract

The demand for offshore wind farm installation is increasing in recent years as the concern in using sustainable energy source is rising. One of the primary steps in constructing offshore wind farm is the pile installation which is a high-risk activity due to the expensive offshore installation vessels requirement. Any factor which can result in delaying the pile installation process will lead to financial losses. Therefore, a comprehensive driveability analysis using an efficient pile model is favourable. From the driveability model, the suitable pile equipment can be selected consider the underlying soil condition. The selected equipment must be capable of installing pile into the target depth without overstressing the pile within the design time.

An essential component in driveability model is to estimate the static resistance to driving (SRD). The SRD in analogues with axial static pile capacity approaches. These approaches use the Cone Penetration Test (CPT) data to determine the axial pile capacity. Factors such as the friction fatigue effect, stress equalisation and soil plugging are related and affect axial pile capacity. These factors are integrated into driveability analysis in this study to provide a more reliable result when using CPT-based approaches to calculate static axial capacity.

Pile installation data records from Blessington, Ireland are used as a part of the axial static load test programme. Pile load tests have been performed on open-ended steel piles with a diameter of 0.34m. The site condition consists of glacial deposit dense sand. From this database, the performance of driveability models by using the available and modified CPT-based approaches (e.g., the UWA-05, ICP-05, and Fugro-05) are assessed in this study. The modified model considers the friction fatigue effect, the pile ageing effect, the soil plugging condition, the pile tip mobilisation and the base residual stresses while the SRD is calculated by using the available CPT-based approaches.

The recent CPT-based axial static capacity methods are investigated to see if they can be used as a reliable method to determine static resistance to driving (SRD) profiles. The SRD profiles are comparable to the axial static capacity approach which account for the pile ageing effect, the soil tip displacement, and residual stresses during driving. The pile ageing effect that is incorporated in the model as installation resistance is set for the time equals to zero, unlike with the static capacity load test which is derived after a certain time after end installation. The total resistance that is recorded for open-ended small diameter piles is calculated to model the pile tip mobilisation which is associated with the base resistance. The residual base stresses are modelled for each hammer blow during driving. The wave equation analysis uses a combination of the SRD profiles and dynamic soil components, pile properties and installation equipment resulting in total resistance as the blow count prediction.

This study provides information on how to model driveability analysis from the recent CPT-based axial static capacity approaches. The models one modified to include related factors that affected pile installation process. The performance of predicted blow counts result from unmodified and modified CPT-based methods are appraised and compared to the recorded blow count as a model verification. The UWA-05 modified model can be considered as an appropriate model to estimate pile driveability from CPT-based axial static capacity approach.