Progressive deformation of glacial till due to
viscoplastic straining and pore pressure variation
Ruth Harley, Dr. V. Sivakumar, Dr. D. Hughes, Kaine Lynch
School of Planning, Architecture and Civil Engineering, Queen’s University
Belfast, UK
Prof. S. L. Barbour
Department of Civil and Geological Engineering, University of Saskatchewan, Canada
ABSTRACT
Cuttings are typically designed to a factor of safety (FOS) of 1.25 in accordance with Eurocode 7. The stress regime of a
cutting however is complex, with different principle stress directions along the potential failure plane. For example,
loading may be primarily in extension near the toe of the slope, while compressive loading represents conditions at the
crest of a slope. Cuttings in heavily overconsolidated clay are known to be susceptible to progressive failure, developing
from the toe of the slope. Softening and the development of a rupture surface has been observed in the field, and is well
documented for London Clays; however, this failure mechanism is yet to be established for glacial tills. This process can
be triggered by softening, however initial observations from laboratory testing of reconstituted samples of glacial till
indicate that insitu stress states (between 80-90% of peak strength) may also undergo viscoplastic straining as a result of
the combination of pore-pressure cycling and elevated stress conditions.
This paper outlines the stress path tests carried out on reconstituted samples of the fine fraction of glacial till under
positive and negative loading conditions. It also presents an evaluation of the samples response to rest, and pore water
pressure variations, when subjected to deviator loading close to failure. Observations indicate that once the material
reaches a stress level causing irrecoverable deformation, the material will continue to undergo viscoplastic straining
under a constant deviator load, clearly demonstrating that progressive deformation is of concern in glacial till cut slopes.
Keywords: progressive deformation, viscoplastic straining, strain softening, glacial till