Axial cyclic loading of piles in low-to-medium-density chalk

Abstract

Comprehensive field investigations into the axial cyclic loading behaviour of open-steel pipe piles driven and aged in low-to-medium density chalk identify the conditions under which behaviour is stable, unstable or metastable. Post-cycling monotonic tests confirmed that stable cycling enhanced pile capacity marginally, while unstable cases suffered potentially large losses of shaft capacity. Metastable conditions led to intermediate outcomes. The patterns by which axial deflections grew under cyclic loading varied systematically with the normalised loading parameters and could be captured by simple fitting expressions. Cyclic stiffnesses also varied with loading conditions, with the highest operational shear stiffnesses falling far below the in-situ seismic test values. The monotonic and cyclic axial responses of the test piles were controlled by the behaviour of, and conditions within, the reconsolidated, de-structured, chalk putty annuli formed around pile shafts during driving. Fibre-optic strain gauges identified progressive failure from the pile tip upwards. Large factors of safety were required for piles to survive repetitive loading under high-level, two-way, conditions involving low mean loads, while low amplitude one-way cycling had little impact. A simple ‘global’ prediction procedure employing interface shear and cyclic triaxial tests is shown to provide broadly representative predictions for field behaviour.

An Axial Load Transfer Model for Piles Driven in Chalk

“…Phase Transformation was developed at mobilised angles of shearing resistance (𝜑 𝑚𝑜𝑏 ′ ≈ 26.5 𝑜 ) far below those required to induce interface failure (𝛿 𝑓 ′ ≈ 32 𝑜 ), as suggested by Buckley et al (2023). Buckley et al (2020) show that the dilative post-PT behaviour contributes to the ultimate shaft resistance of piles driven in chalk, as captured in the Chalk ICP-18 and ALPACA methods.…”

“…𝜆 𝑎 , 𝜆 𝑏 , 𝜆 𝑐 , 𝜆 𝑑 are model parameters calibrated from triaxial tests mentioned above. Assuming that the puttified chalk zone can be modelled on the basis of isotropically consolidated laboratory tests, despite the currently unquantifiable effects of pile installation, the initial local effective radial stress 𝜎 𝑟,0 ′ prior to loading is approximated as the radial effective stress 𝜎 𝑟,𝑓 ′ at failure, neglecting any normal stress changes provoked by shaft loading (Buckley et al, 2023). The 𝜆 𝑐 and 𝜏 𝑠,𝑐 terms can then be expressed as follows:…”

The axial behaviour of piles driven in chalk

“…SUMMARY AND CONCLUSIONS The ALPACA and ALPACA Plus JIP programmes addressed the uncertain behaviour of piles driven in low-to-medium density chalks through comprehensive dynamic, cyclic, monotonic, axial and lateral testing of piles covering a wide range of scales, materials, groundwater conditions, ages and loading modes, generating a unique high-quality experimental database against which design approaches may be assessed and developed. Buckley et al (2023) report the cyclic axial pile loading experiments and show how their responses may be predicted from laboratory element tests and ALPACA AWG (2022) summarised the lateral loading study. The main outcomes regarding monotonic axial loading are:…”

Driven pile behaviour in low-to-medium density chalk: the ALPACA JIP outcomes

“…Liu et al (2023b) summarise the laboratory cyclic testing on both intact and de-structured SNW chalk. Jardine et al (2023) and Buckley et al (2023) employed the tests to interpret the monotonic and cyclic axial pile tests. Extensive interface shear testing was also conducted, coupled with metallurgical investigations into the rates of steel-pile corrosion under a range of in-situ conditions (Vinck, 2021).…”

“…Large factors of safety were required for piles to survive repetitive loading under high-level, two-way, conditions involving low mean loads, while low amplitude oneway cycling had little impact. Buckley et al (2023) set out a simple 'global' prediction procedure employing interface shear tests and undrained cyclic triaxial tests on 'putty' chalk which provided broadly representative predictions for the field behaviour.…”