Improved Pile Driving Predictions in Carbonate Soils and Rock

Abstract

Driving piles into carbonate strata, particularly weak rock (calcarenite) and cemented sands is traditionally thought to carry high risk in terms of premature refusal, exceedance of fatigue criteria or pile tip buckling. Although soil resistance to driving (SRD) algorithms have been developed for carbonate deposits they are based on limited experience (primarily Arabian Gulf) and there is a need to expand this database to include installation experience elsewhere in carbonate conditions such as the Australian north west shelf where significant oil and gas developments are planned.Much of the uncertainty surrounding the prediction of pile driving in carbonate strata can be attributed to the difficulty in establishing a reliable and continuous soil resistance profile which is primarily due to i) the difficulty obtaining suitable samples of rocks such as calcarenite for compressive strength testing, and ii) the equipment limitations for pushing cone penetrometers into weak rock.It is standard industry practice to base hammer selection, fatigue damage analyses and driveability assessments on upper bound plugged conditions which, for weak rocks such as calcarenite, gives significantly higher resistances than for the unplugged case and this can lead to the conclusion that driving is either completely unfeasible or not possible beyond a certain depth.This study presents data from a pile installation in the Timor Sea to support modifications that may be made to existing SRD algorithms to improve reliability of the predictions, at least for this particular site. Back-analyses of the installation data show that unplugged conditions are far more likely to prevail when driving a large diameter pile which suggests that automatically assuming plugged conditions is over-conservative. Possible correlations between cone tip resistance and unconfined compressive strength (UCS) of calcarenite are made by fitting SRD algorithms that require UCS as an input to those that are based on cone tip data.

Pile Installation Assessment of Offshore Wind Jacket Foundation in Completely Weathered Rock: A Case Study of the South China Sea

“…σ rc = 0.029q c σ v0 /p a 0.13 [max(h/R * , 8)] −0.38 (18) where q c,avg is the average q c ± 1.5D over the pile tip; D is the pile external diameter; D CPT is the diameter of penetrating cone (0.036 m); A r is the area ratio (= 1 − (D i /D) 2 ); D i is pile internal diameter; σ rc and ∆σ rd are radial effective stress after pile installation and change in radial effective stress during pile loading (∆σ rd ≈ 4G∆y/D); G is operational level of shear modulus (≈ 185q c q −0.7 c1N ); q c1N = (q c /p a )/ σ v0 /p a 0.5 ; and ∆y is radial displacement during pile loading.…”

“…Meanwhile, for a more high-quality rock, the end bearing can be computed based on the unconfined compression strength (UCS). Brunning and Ishak [18] calculated the SRD in carbonate rock with Stevens's method and proposed that the unit end bearing is 2-3.5 times the unconfined compressive strength for the carbonate rock. Based on the onshore and offshore dynamic test results for low-medium-density chalk, Buckley et al [19] studied the pile driving in low-to medium-density chalk and proposed a CPT-based ICP-05-like formula to calculate SRD considering the friction fatigue and the ratio of the pile diameter to the pile wall thickness.…”