An experimental investigation into the behaviour of destructured chalk under cyclic loading

Abstract

Low-to-medium density chalk can be de-structured to soft putty by high-pressure compression, dynamic impact or large-strain repetitive shearing. These process all occur during pile driving and affect subsequent static and cyclic load-carrying capacities. This paper reports undrained triaxial experiments on de-structured chalk, which shows distinctly time-dependent behaviour as well as highly non-linear stiffness, well-defined phase transformation (PT) and stable ultimate critical states under monotonic loading. Its response to high-level undrained cyclic loading invokes both contractive and dilative phases that lead to pore pressure build-up, leftward effective stress path drift, permanent strain accumulation, cyclic stiffness losses and increasing damping ratios that resemble those of silts. These outcomes are relatively insensitive to consolidation pressures and are distinctly different to those of the parent intact chalk. The maximum number of cycles that can be sustained under given combinations of mean and cyclic stresses are expressed in an interactive stress diagram which also identifies conditions under which cycling has no deleterious effect. Empirical correlations are proposed to predict the number of cycles to failure and mean effective stress drift trends under the most critical cyclic conditions. Specimens that survive long-term cycling present higher post-cyclic stiffnesses and shear strengths than equivalent ‘virgin’ specimens.

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

“…Y 1 yield points are shown to mark the ends of any initial linear portions, as are Y 3 points at the midpoints of the large-scale yielding stages where marked changes in stress-strain response occur. As shown later, the experiments did not provide any systematic indication of Y 2 yielding which was located by Jardine (1992), Smith et al (1992), Kuwano & Jardine (2007), Gasparre et al (2007) and Liu (2018) in clays and sands as the points where abrupt changes in strain increment directions and the development of strain rate dependency occur in drained shearing tests. These changes were interpreted as reflecting the onset of particle contact phenomena that are not expected to apply to more strongly cemented geomaterials.…”

“…1, taking / 0 = 31°and K 0 o =1 À sin/ 0 = 0.48 when OCR = 1. It is interesting to note the intact chalk's critical state line runs at a steeper slope (higher k) than that of the dynamically compacted fully de-structured chalk reported by Liu et al (2022), reflecting intact chalk's progressive destructuration under isotropic compression and shearing, as observed and modelled with crushable granular media (Altuhafi et al, 2017;Ciantia et al, 2019). It may be spec-ulated that the critical state lines of intact and destructured chalk converge over even higher consolidation and shearing stress ranges as the de-structuration process terminates fully.…”

“…2. Group C tests were ductile with (q/p 0 ) ult % 1.25, equivalent to critical state / 0 values of % 31°, as seen also with fully de-structured chalk (Doughty et al, 2018;Liu et al, 2022) or sub-rounded fine silica sands (Aghakouchak et al, 2015). 3.…”

“…Local radial strains are inherently more difficult to measure than axial (Ackerley et al, 2016). Liu (2018) describe how stick-slip and other local radial strain measurement problems were overcome in the low-pressure tests, but equivalent measurements proved difficult to achieve consistently in the high-pressure tests; the radial sensors failed to respond correctly in several cases. Reliance therefore had to be placed on corrected external volume measurements, which are affected by porous stone compliance, end imperfections, compression of drainage filter papers and compliance of the cell and connecting tubes.…”

“…3(b) the intact chalk's post-Y 3 isotropic compression line with N = 3.14 and k = 0.16. Also plotted for comparison are the postyield isotropic compression and critical state lines for the intact chalk, the normal compression line for reconstituted chalk and the (common) critical state line established from tests on reconstituted and fully de-structured chalk by Liu et al (2022). The log-linear isotropic-NCL* was constructed from the K o -NCL* shown in Fig.…”