The behaviour of Bolders Bank glacial till under undrained cyclic loading

Abstract

Analysis of foundation behaviour under repeated loading can be important to the design of offshore facilities, towers, bridges, wind turbines and other structures. Although detailed guidance is available on how some geomaterials behave under cycling, few such studies have been reported on the glacial formations that are widespread across parts of northern Europe and under the North and Baltic Seas. This paper presents a cyclic laboratory investigation involving block samples of Bolders Bank till, which is one of the most extensive North Sea glacial formations. Static and cyclic triaxial tests are reported on nominally identical, low-plasticity, high-overconsolidation-ratio specimens from Cowden, near Hull in the UK. Twenty-three cyclic tests are reported on specimens that were re-consolidated to K0 stresses before experiencing up to 3500 undrained cycles involving a range of mean and cyclic shear stress combinations. The impacts of cycling on effective stress paths and strain development, cyclic stiffness and damping ratios are reported and interpreted with respect to parallel index, oedometer and monotonic triaxial compression and extension tests. Stable, metastable and unstable patterns of behaviour are identified, along with different cyclic failure modes. The styles of cyclic response are related to the till's static yielding behaviour. While the till can only manifest quasi-elastic behaviour within a very small (Y1) kinematic yield surface (KYS), a larger second (Y2) KYS is identified within which repeated loading leads to a non-linear yet stable response, with negligible mean effective stress drifts or stiffness degradation with low and stabilising strain accumulation rates. Cyclic paths that engage and relocate the Y2 surface lead to greater permanent strains, mean effective stress drifts and stiffness decays that become progressively more marked as the magnitudes of the cyclic stress perturbation and the number of cycles increase. While cycling under stable or metastable conditions increased the ultimate undrained strength of specimens that did not develop excessive strains, most cyclic paths that exceeded and climbed the till's Hvorslev surface were unstable and eventually failed. The experiments identified the key factors that affect the till's response to repeated loading and provide a basis for developing, testing and calibrating cyclic constitutive models and establishing simplified design procedures for comparable tills.

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

“…Smooth and continuous stiffness degradation trends were observed for the Group C specimens as they developed ductile response towards large strains. Kuwano & Jardine (2007), Liu (2018) and Ushev & Jardine (2020) show that unbonded sands, clays and siltdominated tills generally display small Y 1 kinematic yield loci, which often extend for just a few kPa in q-p 0 space and are dragged by even small movements of the current effective stress point. Their Y 1 yield surfaces tend to expand or shrink in proportion to their current p 0 0 ; stiffness generally increases in proportion to p 0 0 raised to a fractional power between 0.5 and unity.…”

Optimization of Advanced Laboratory Monotonic and Cyclic Triaxial Testing on Fine Sands

“…Two sizes of cell are considered; the more capacious 100mm diameter specimen system enables a comprehensive suite of local instruments to be mounted on specimens including: (a) three 'floating' type LVDT local axial strain sensors (Cuccovillo & Coop, 1997) attached over the central 70mm height; (b) 'fixed' LVDTs to measure local radial deformations from three radially spaced points around the specimen's middle height section through the hinged 'L-shape' system developed by Ackerley et al (2016); and (c) three pairs of platen-mounted or lateral 'T-configuration' piezoceramic bender element shear velocity transducers. Local pore pressure probes may also be mounted when working with clays (Ushev & Jardine, 2022). Table 3 lists the transducers' working limits and the resolutions achieved over the voltage ranges considered with a high-quality auto-regulated, 16bit A/D converter.…”

“…Strains gauged over the central ≈50 mm height of the test specimens by high-resolution axial LVDTs were combined with the measured deviator force to defined the pre-failure stiffness trends shown in Figure 16 as equivalent vertical tangent Young's moduli (E′v,tan = dq/dεa = dσ′v/dεa) against local axial strains (εa). Each E′v,tan-εa trace was determined by a combined approach of linear regression and multiple section polynomial curve fitting (see for examples by Ushev & Jardine (2022) and Liu et al (2020)), with the traces' initial plateaux representing the maximum stiffnesses. The end of the E′v,tan-εa plateau indicates Y1 yielding and the onset of sharp stiffness degradation.…”

The mechanical behaviour of Bolders Bank till

“…Samples were set up for triaxial testing as described by Ushev (2018). Natural specimens were saturated undrained by applying cell pressure at a rate of 20kPa/h.…”

“…Gravel exists at all depths derived from chalk, limestone, sandstone, igneous and metamorphic rocks. Relatively weak chalk, with occasional igneous inclusions, dominates below ≈4.5m (Ushev 2018). Trenter (1999) and Clarke (2018) indicate that matrix dominated behaviour should be expected in such tills.…”