Advanced in-situ and laboratory characterisation of the ALPACA chalk research site

Vinck, K., Liu, T., Jardine, R. J., Kontoe, S., Ahmadi-Naghadeh, R., Buckley, R. M. , Byrne, B. W., Lawrence, J. A., McAdam, R. A. and Schranz, F. (2023) Advanced in-situ and laboratory characterisation of the ALPACA chalk research site. Geotechnique, (doi: 10.1680/jgeot.21.00197) (Early Online Publication)

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.

• Item Type: Articles
• Additional Information: The ALPACA Project was funded by Engineering and Physical Science Research Council’s (EPSRC) grant EP/P033091/1 to Imperial College London and Oxford University.
• Status: Early Online Publication
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Buckley, Dr Roisin
• Authors: Vinck, K., Liu, T., Jardine, R. J., Kontoe, S., Ahmadi-Naghadeh, R., Buckley, R. M., Byrne, B. W., Lawrence, J. A., McAdam, R. A., and Schranz, F.
• College/School: College of Science and Engineering > School of Engineering > Infrastructure and Environment
• Journal Name: Geotechnique
• Publisher: ICE Publishing
• ISSN: 0016-8505
• ISSN (Online): 1751-7656
• Published Online: 19 January 2022
• Copyright Holders: Copyright © 2022 ICE Publishing
• First Published: First published in Geotechnique 2022
• Publisher Policy: Reproduced under a Creative Commons license