Researcher: Dr Ruth Fearon

Supervisors: Professor R.J. Chandler, Dr J. Bommer

Sponsor: EPSRC

Duration of Project: January: 1998 - June 1999

The sudden decrease in soil strength on the application of earthquake loading has been considered to be the cause of several catastrophic landslides (e.g. Keefer 1984). This implies that after the shear failure of a mass of a soil mass the shear strength does not simply drop to a constant residual value, but rather depends on the magnitude and rate of displacement. A thorough understanding of this phenomenon therefore requires the testing of soil samples at rates of displacement appropriate to earthquake loading, that is in excess of 1 mm/second, and also shearing to large displacements, in excess of 500 mm.

Changes in the residual strength with the rate of displacement have been studied in detail by Lemos (1986) and Tika (1989) who carried out tests in the IC/NGI ring shear apparatus (Bishop et al., 1971). The shear strength of a soil sample, which had been sheared at a slow rate of displacement until a residual value is reached, increased to a peak strength higher than that of the slow residual when the rate of displacement is increased suddenly. This was called the fast peak. The strength then either remained constant at an elevated strength falls to the slow residual or falls below that of the slow residual. The constant strength that was achieved after a large displacement was termed the 'fast residual'.

One of the problems with the tests that were carried out by Lemos (1986) and Tika (1989) was that they were unable to maintain the high shear velocities for sufficient magnitudes to ensure that stabilisation had occurred. Hence Parathiras (1994) designed a new set of rings that used ball bearings to maintain a constant gap between the upper and lower confining rings and hence reduce the amount of soil loss through the gap.
 

The purpose of this research is to try and determine the mechanisms that are associated with the changes in the residual strength at higher displacement rates. The figures below show a positive and negative effect of the displacement rate for samples of London Clay sheared at rates of 100 mm/min and 1000mm/min respectively. These tests were carried out in the IC/NGI ring shear apparatus using the confining rings developed by Parathiras (1994).

Displacement Rate = 100 mm/min

Displacement Rate = 1000 mm/min

One possible reason for the increase or decrease in the residual strength measured surface at faster displacement rates is that the changes are due to variations in the pore pressures in the slip. Pore pressure measurement was carried out by Lemos (1986) and Tika (1989). However their results were thought to be inconclusive due to their inability to maintain the displacement rate for sufficiently large displacements to achieve sustainability. Tests have therefore been carried out with miniature pore pressure transducers to determine if the drop in the residual strength can be accounted for by a corresponding rise in pore pressure during shearing.

The drop in the residual strength at higher displacement rates can also been associated with an increase in the thickness of the shear surface as has been shown by viewing thin sections of the shear surface under cross polarised light. Further tests have been carried out to determine whether the increase in the thickness of the shear band is a result of local disturbances in the sample.
 

Bishop, A.W., Green, G.E., Garga, V.K., Andersen, A. and Brown J.D. 1971. A new ring shear apparatus and its application to the measurement of residual strength. Geotechnique Vol. 21, pp. 273-328.

Keefer, D.K. 1984. Landslides caused by earthquakes. Geol. Soc. America Bull., Vol. 95, pp. 406-421.

Lemos, L.J.L. 1986. The effect of the rate of shear on residual strength. Ph.D. Thesis, Imperial College, University of London.

Parathiras, A.N. 1994. Displacement rate effects on the residual strength of soils. Ph.D. Thesis, Imperial College, University of London.

Tika, T.M. 1989. The effect of the rate of shear on residual strength. Ph.D. Thesis, Imperial College, University of London.