Pile driveability in low- to medium-density chalk

Abstract

Pile driving in low to medium density chalk is subject to significant uncertainty. Predictions of Chalk Resistance to Driving (CRD) often vary considerably from field driving behaviour, with both pile refusals and free falls under zero load being reported. However, recent field studies have led to better understanding of the processes which control the wide range of behaviour seen in the field. This paper describes the primary outcomes of the analysis of dynamic tests at an onshore and an offshore site and uses the results to propose a new method to predict CRD. The method is based on phenomena identified experimentally: the relationship between cone penetration resistance and CRD, the attenuation of local stresses as driving advances and the operational effective stress interface shear failure characteristics. The proposed method is evaluated through back analyses of driving records from independent pile installation cases that were not included in developing the method, but involved known ground conditions, hammer characteristics and applied energies. The proposed method is shown to lead to more reliable predictions of CRD than the approaches currently applied by industry.

The behaviour of a low- to medium-density chalk under a wide range of pressure conditions

“…As noted earlier and shown by Buckley et al (2020b), large-strain, high-pressure, shearing occurs in pile tips during driving that leads to very low strength putty annuli forming around their shafts under low mean effective stresses and tightly kinematically controlled (near zero volume change) conditions (Jardine, 2020). Final stages were incorporated into some high-pressure tests to try to match aspects of the regime within the putty annuli.…”

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

“…For the modified Fugro-05 method, q b = 8.5q c,avg p a /q c,avg 0.5 A r 0.25 (19) τ f = 0.06q c σ v0 /p a 0.05 (h/R * ) −0.90 , for h/R * ≥ 4 (20)…”

“…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.…”

Cyclic characterisation of low-to-medium density chalk for offshore driven pile design

“…Dynamic percussion applied during impact driving of open steel piles creates de-structured, chalk 'putty' beneath their advancing tips which spreads and further softens around their shafts (Lord et al, 2002). Buckley et al (2018) and Vinck (2021) The key aspects associated with driven pile installation and ageing are illustrated in Figure 1, after Buckley et al (2020), noting the development of significantly higher than in-situ driving stresses as the driven piles' tips approach the relevant depth, followed by rapid stress degradation and relaxation as the tips pass by. Pore-water pressures rise to several MPa near the pile tips during driving but dissipate rapidly through the chalk's fissures.…”

“…Figure 1. Illustration of stress states during installation, equalisation, and ageing of driven piles (Buckley et al, 2020) 3 Laboratory testing procedures and programmes Laboratory dynamic compaction was employed to de-structure intact chalk specimens in an analogous way to pile driving and produce uniform batches of chalk putty for laboratory testing. An automatic 'Proctor' compactor applied blows at ≈2s intervals with a 4.5kg ram and a 300mm drop height to 350-450g chalk lumps contained in a 100mm diameter annular split mould.…”