Data are presented concerning mudflows of very silty material occurring on slopes in the range 8½° to 12°. In the stress range within which these flows (or more properly slides) occurred the angle of shearing resistance was at least 36°, so that for movement to have been initiated on slopes less steep than about 16°–18° slope stability analyses require that substantial artesian pore pressures should exist. Both electrical and standpipe piezometer observations confirmed the existence of artesian pressures in this slope, though the pressures at the time of observation were not quite high enough to cause further mudflow movement. This observation is consistent with stability analyses carried out using the measured soil strengths.
The significance of these observations is discussed in the context of the shear surfaces that have recently been reported from low angle clay slopes in midland and southern England.