Numerical Modelling of the Positive Water Head Technique for Hole Stabilisation

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Drilled and grouted piles are a relatively common foundation solution for offshore structures where the design objective is to take advantage of competent strata to provide the required axial capacity through shaft resistance. The installation of such piles involves drilling a hole first to sufficient depth so that the pile can be lowered, then grouted in the drilled hole. However, if the soil in the hole is insufficiently cemented, or exhibits a drained to partially drained response, there could be a risk of hole collapse prior to grouting operations. To mitigate this risk, a stabilisation technique typically adopted offshore Australia involves the application of a positive water head (i.e. above hydrostatic) through an elevated and sealed drill riser. This paper presents a numerical model routinely used to determine positive head requirements for the construction of drilled and grouted piles. A hypothetical case study is presented on a soil profile under transitional drainage conditions to illustrate the effectiveness of the positive head technique to maintain a drilled hole stable throughout the course of the drilling and grouting operations. It is shown that the numerical model is capable of predicting the onset and progression of hole instability with time, thus allowing the installation contractor to determine the appropriate positive head requirement for the construction schedule.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

6 - Pile Installation Challenges and Decommissioning in sands and clays: monopiles, anchor piles, pin piles