| 1. |
- Ahmadi Naghadeh, Reza, 1982, et al.
(författare)
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A laboratory characterisation of the response of intact chalk to cyclic loading
- 2022
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Ingår i: Geotechnique. - : Thomas Telford Ltd.. - 0016-8505 .- 1751-7656. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports the cyclic behaviour of chalk, which has yet to be studied comprehensively. Multiple undrained high-resolution cyclic triaxial experiments on low- to medium-density intact chalk, along with index and monotonic reference tests, define the conditions under which either thousands of cycles could be applied without any deleterious effect, or failure could be provoked under specified numbers of cycles. Intact chalk's response is shown to differ from that of most saturated soils tested under comparable conditions. While chalk can be reduced to putty by severe two-way displacement-controlled cycling, its behaviour proved stable and nearly linear visco-elastic over much of the one-way, stress-controlled loading space examined, with stiffness improving over thousands of cycles, without loss of undrained shear strength. However, in cases where cyclic failure occurred, the specimens showed little sign of cyclic damage before cracking and movements on discontinuities led to sharp pore pressure reductions, non-uniform displacements and the onset of brittle collapse. Chalk's behaviour resembles the fatigue response of metals, concretes and rocks, where micro-shearing or cracking initiates on imperfections that generate stress concentrations; the experiments identify the key features that must be captured in any representative cyclic loading model.
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| 2. |
- Liu, Tingfa, et al.
(författare)
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An experimental investigation into the behaviour of de-structured chalk under cyclic loading
- 2022
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Ingår i: Geotechnique. - : Thomas Telford Ltd.. - 0016-8505 .- 1751-7656. ; In Press
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Tidskriftsartikel (refereegranskat)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.
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| 3. |
- Liu, Tingfa, et al.
(författare)
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Laboratory investigation of the cyclic loading behaviour of intact and de-structured chalk
- 2023
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Ingår i: Proceedings of the 8th International Symposium on Deformation Characteristics of Geomaterials (IS-PORTO 2023).
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Konferensbidrag (refereegranskat)abstract
- Chalk is a soft biomicrite composed of silt-sized crushable CaCO3 aggregates. Chalk’s response to cyclic loading depends critically on its sensitive micro fabric and state, which may be altered significantly by high-pressure compression, dynamic impact or prior large-strain repetitive shearing. This paper reports high-resolution undrained cyclic triaxial experiments on low- to medium-density intact chalk and chalk de-structured by dynamic compaction to model the effects of percussive pile driving. The intact chalk manifested stable and nearly linear visco-elastic response under a wide range of the one-way, stress-controlled cyclic loading conditions imposed. However, high level cycling led to sudden failures that resembled the fatigue response of metals, concretes and rocks, with little sign of cyclic damage before sharp pore pressure reductions, non-uniform displacements and finally brittle collapses. However, the de-structured chalk’s response to high-level undrained cycling resembles that of silts, developing both contractive and dilative phases that led to pore pressure build-up, leftward effective stress-path drift, permanent strain accumulation, cyclic stiffness losses and increasing damping ratios. Results from exemplar tests are presented to illustrate these key features and demonstrate how chalk’s undrained cyclic shearing characteristics depend also on effective stress level. The experimental outcomes provide significant scope for developing constitutive and empirical relationships or predictive tools to enable the interpretation and design of driven pile foundations in chalk and other chalk-structure interaction related problems under cyclic loading.
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| 4. |
- Vinck, Ken, et al.
(författare)
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Advanced in-situ and laboratory characterisation of the ALPACA chalk research site
- 2022
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Ingår i: Geotechnique. - : ICE Publishing. - 0016-8505 .- 1751-7656.
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Tidskriftsartikel (refereegranskat)abstract
- Low-to-medium density chalk at St Nicholas at Wade, UK, is characterised by intensive testing to inform the interpretation of axial and lateral tests on driven piles. The chalk de-structures when taken to large strains, especially under dynamic loading, leading to remarkably high pore pressures beneath penetrating CPT and driven pile tips, weak putty annuli around their shafts and degraded responses in full-displacement pressuremeter tests. Laboratory tests on carefully formed specimens explore the chalk’s unstable structure and markedly time and rate-dependent mechanical behaviour. A clear hierarchy is found between profiles of peak strength with depth of Brazilian tension (BT), drained and undrained triaxial and direct simple shear (DSS) tests conducted from in-situ stress conditions. Highly instrumented triaxial tests reveal the chalk’s unusual effective stress paths, markedly brittle failure behaviour from small strains and the effects of consolidating to higher than in-situ stresses. The chalk’s mainly sub-vertical jointing and micro-fissuring leads to properties depending on specimen scale, with in-situ mass stiffnesses falling significantly below high-quality laboratory measurements and vertical Young’s moduli exceeding horizontal stiffnesses. While compressive strength and stiffness appear relatively insensitive to effective stress levels, consolidation to higher pressures closes micro-fissures, increases stiffness and reduces anisotropy.
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