Keywords: glaciation, over-consolidation, principal stress rotation, deformation, anisotropy, plasticity, rheology.

There is general consensus, based on field observations, hydrological studies, macro-scale and micro-scale structural evaluation, e.g. [1,2], and rheological models for sub-glacial deformation and deposition [3], that glacier movement is influenced by deformation of sediments underneath the ice. The current over-consolidated state has been considered to be due to one-dimensional consolidation under the effective weight of the ice sheet or glacier e.g. [6]. However, the writers have noticed the following two inconsistencies in this reasoning, namely a) the pre-consolidation pressures of tills from the Pleistocene continental glaciations [4] are significantly higher than the effective stresses measured under modern glaciers [7] and b) the in-situ effective stress state in a glaciated clay deposit in the Netherlands have been found to be anisotropic [5,9]. These observations seem consistent with the hypothesis [8] that the consolidation of tills and glaciated sediments may be the result of both shear and gravity induced stresses under an advancing glacier. Subsequent measurements of anisotropic stress and stiffness in glaciated clays [8,9] support the model of shear and gravity induced consolidation.

The conditions at the base of the continental ice sheets far from the ice margin are unfrozen due to geothermal heating, pressure melting and friction. Investigations of sub-glacial water pressures present evidence of free water and drainage at the base of temperate glaciers [7,10]. To demonstrate the shearing enforced by an overrunning glacier as the potential cause of the anisotropic over-consolidated state, the deformation process of the ground is simulated numerically and the corresponding stress distribution in the ground is calculated. The present state of glaciated soils is considered as essentially the soil's "memory" of the stress conditions that it encountered in the past.

The sub-glacial deformation was modelled using a simple Mohr-Coulomb elastic, perfectly plastic rheology in which the Young's modulus increased linearly with depth. The gravity load was applied and the horizontal incremental deformation was imposed at the top of the finite element shear beam. During large continuing horizontal shearing imposed at the ground surface, the displacement was found to be concentrated in the uppermost element of the mesh. The distribution of the calculated principal stress rotations converged with increasing deformation to the maximum angle of rotation (45 o) in the first element, and slightly smaller values with depth. Measurements of principal stiffness rotations in tills suggest much higher rotation angles than those predicted using a simple elastic-perfectly plastic sub-glacial rheology. This demonstrated that a Mohr-Coulomb rheology with steady state deformation was unlikely the dominant process under historic ice sheets. Whereas this conclusion has been reached in the past, e.g. [3], this is the first attempt to use measurements of principal axis rotation on natural samples of glaciated soils for the validation of sub-glacial rheological models. Several other rheological models are currently being developed and applied to assess whether they yield principal stress rotations closer to those measured.

1

Boulton, G.S., "A paradigm shift in glaciology". Nature, 322, p.18, 1986. doi:10.1038/322018a0

2

Meer, J.J.M. v.d., Menzies, J., Rose, J., "Subglacial till: the deforming glacier bed". Quaternary Science Reviews, 22, 1659-1685, 2003. doi:10.1016/S0277-3791(03)00141-0

3

Iverson, N.R., Iverson, R.M., "Distributed shear of subglacial till due to Coulomb slip". Journal of Glaciology, 47(158), 481-488, 2001. doi:10.3189/172756501781832115

4

Sauer, E.K., Egeland, A.K., Christiansen, E.A., "Compression characteristics and index properties of tills and intertill clays in southern Saskatchewan, Canada", Canadian Geotechnical Journal, 30(2), 257-275, 1993. doi:10.1139/t93-022

5

Schokking, F., "Anisotropic geotechnical properties of a glacially overconsolidated and fissured clay", PhD Thesis, Delft University of Technology, 1998.

6

Tulaczyk, S., Kamb, W.B., Engelhardt, H.F., "Estimates of subglacial effective stresses from till preconsolidation and till void ratio", Boreas, 30, 101-114, 2001. doi:10.1080/030094801750203134

7

Engelhardt, H., Humphrey, N., Kamb, B., Fahnestock, M., "Physical conditions at the base of a fast moving Antarctic ice stream". Science, 248, 57-59, 1990. doi:10.1126/science.248.4951.57

8

Gareau, L.F., Molenkamp, F., Sharma, J., Hegtermans, B.M.H., "Engineering Geology of glaciated soils". Proceedings of the Skempton Memorial Conference: Advances in geotechnical engineering, 2, 1280-1291, 2004.

9

Remijn, M., "Measurement of anisotropic stiffnesses and stresses in Pot Clay". MSc. Thesis, TUDelft, Engineering Geology, 2004.

10

Tulaczyk, S., Kamb, W.B., Engelhardt, H.F., "Basal mechanics of Ice Stream B, West Antarctica. 1. Till Mechanics". Journal of Geophysical Research, 105(B1), 463-481, 2000. doi:10.1029/1999JB900329

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)