Estimation of hydraulic properties of subglacial till from ploughmeter measurements

Force variations on a "ploughmeter" and fluctuations in subglacial water pr essure have been measured in the same borehole at Storglaciaren, Sweden, to investigate hydraulic properties of the basal till layer. A strong inverse correlation of the pressure and force records, in conjunction with a signi ficant time lag between the two signals, suggests that pore-water pressures directly affect the strength of the till. Variations in subglacial water p ressure result in potential gradients across the water-till intel face at t he bottom of the borehole that drive pressure waves downwards through the t ill layer when the borehole water level is high and back upwards when the w ater level is low. Analysis of the propagation velocity of this pressure wa ve indicates that the hydraulic diffusivity of Storglaciaren till is in the range 1.9-3.6 x 10(-6) m(2) s(-1), in good agreement with estimates obtain ed in the laboratory. Hydraulic conductivity values associated with these d iffusivities are between 10(-9) and 10(-8) ms(-1) and thus are well within the range of values for other glacial tills.