IN-SITU MEASUREMENT OF THE STRENGTH OF DEFORMING SUBGLACIAL TILL

The shear strength of deforming till beneath Storglaciaren, Sweden, was continuously measured with a new instrument, a ''dragometer''. A cylinder with conical ends, roughened with till from the glacier bed, was dragged through the till, and the force on the cylinder was recorded. Consistent with studies of cone penetration in soils, it was assumed that the till behaves as a Coulomb plastic material. This allows the residual strength of the till to be calculated from the measured force. Results from laboratory experiments confirm the validity of the analytical procedure. The average residual strength of the till was similar to 55 kPa. A source of error is the potential generation of pore pressure in excess of hydrostatic ahead of the cylinder, which could significantly weaken the till. Calculations indicate that excess pore pressure did not develop during the experiment, but could develop during similar experiments beneath rapidly sliding glaciers with less permeable basal till layers.