The Inclination of Talus, Arctic Talus Terraces, and Other Slopes Composed of Granular Materials

Abstract

Recent engineering research into the strength of rockfill suggests that the minimum angle of shearing resistance ($$ø'{cv}$$) of most materials composing talus slopes is likely to be about 39°-40°. The assumption that the typical 35° inclination of well-drained talus slopes reflects the shearing resistance of the talus material is therefore incorrect. The inclination of well-drained talus slopes will reflect the angle of shearing resistance only if (1) the slopes are being rapidly eroded, or (2) deposition rates are high compared with the degradation rates resulting from processes on the talus slope. In both instances, the slope angles will generally be 39° or steeper. This suggestion is supported by laboratory shear-test data from some Spitsbergen talus terraces which have eroded, and consequently relatively steep (up to 39°), outer slopes. Further support is provided by a number of observations reported in the literature. Shallow landslips sometimes occur on these and other "granular" slopes in Spitsbergen at angles considerably less steep than 39°-40°. This appears to be the result of impeded drainage due to the presence of permafrost at a shallow depth. A further observation suggests that average degradation rates over the past 10,000 years for the faces of two Spitsbergen talus terraces originally steepened by marine erosion are about 0.3° per 1,000 years.