A New Laboratory Apparatus for Investigating Clast Ploughing

  1. Michael J. Hambrey2,
  2. Poul Christoffersen2,3,
  3. Neil F. Glasser2 and
  4. Bryn Hubbard2
  1. Marie Rousselot,
  2. Urs H. Fischer and
  3. Michael Pfister

Published Online: 24 MAR 2009

DOI: 10.1002/9781444304435.ch3

Glacial Sedimentary Processes and Products

Glacial Sedimentary Processes and Products

How to Cite

Rousselot, M., Fischer, U. H. and Pfister, M. (2007) A New Laboratory Apparatus for Investigating Clast Ploughing, in Glacial Sedimentary Processes and Products (eds M. J. Hambrey, P. Christoffersen, N. F. Glasser and B. Hubbard), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304435.ch3

Editor Information

  1. 2

    Centre for Glaciology, Institute of Geography & Earth Sciences, Aberystwyth University, Wales, Ceredigion SY23 3DB, UK

  2. 3

    Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge, CB2 1ER, UK

Author Information

  1. Laboratory of Hydraulics, Hydrology and Glaciology, ETH Zentrum, CH-8092 Zürich, Switzerland

Publication History

  1. Published Online: 24 MAR 2009
  2. Published Print: 14 DEC 2007

Book Series:

  1. Special Publication Number 39 of the International Association of Sedimentologists

Book Series Editors:

  1. Isabel Montanez

Series Editor Information

  1. University of California, Davis, USA

ISBN Information

Print ISBN: 9781405183000

Online ISBN: 9781444304435

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Keywords:

  • new laboratory apparatus for investigating clast ploughing;
  • basal motion of soft-bedded glaciers - attributed to ‘ploughing’;
  • improving understanding of mechanisms of basal motion beneath soft bedded glaciers;
  • sediment compressibility depending on stress magnitude and stress history;
  • sediment chamber showing wake devoid of sediment after ploughing experiment;
  • influence of effective stress and ploughing velocity on excess pore-pressure generation and sediment strength

Summary

A significant portion of the basal motion of soft-bedded glaciers can be attributed to ‘ploughing’. This term designates the transitional state between sliding and bed deformation which occurs when clasts that protrude into the glacier sole are dragged through the upper layer of the sediment. This process may cause pore pressures in excess of the hydrostatic value that could weaken the sediment downglacier from ploughing clasts and thus affect the strength of the ice–bed coupling. A large laboratory apparatus was developed and constructed to study, systematically and under glacially relevant conditions, the influence of sediment properties, ploughing velocity and effective pressure on excess pore-pressure generation and sediment strength. In this device, an instrumented tip is dragged at different velocities through a water-saturated sediment bed subject to different effective normal stresses. The drag force on the tip and the pore-water pressure in the adjacent sediment are measured simultaneously. In preliminary experiments performed with subglacial sediment from Unteraargletscher, Switzerland, the sediment diffusivity was estimated from consolidation records. During ploughing, pore-pressure gradients developed rapidly around the tip. Excess pore pressures were due to sediment compression in front of the tip whereas pore pressures below the hydrostatic value resulted from dilatant shearing and a wake devoid of sediment that was left behind the tip. A zone of compressed sediment formed in front of the tip. The absolute magnitude of the pore pressure changes was small relative to the effective normal stress, so that the pore pressures did not significantly influence the resistance to ploughing. Rather, the drag force on the ploughing tip was influenced by the properties of the sediment in front of the compression zone, with a greater magnitude in a virgin sediment than in one that has been ploughed before.