1 Introduction

In 1962 Karl Terzaghi describes in a seminal paper systematically the progressive damage of brittle rock slopes through geological time, driven by fluvial and subglacial glacial incision into steep slopes. According to Terzaghi the removal of lateral support of steep and fractured rock slopes leads to increased shear stresses in the rock mass and the breakage of "many cohesive bonds connecting the blocks between the joints", driving the formation of a new fracture system and a basal rupture plane. He stresses that in the layer of frost penetration and of significant daily and seasonal temperature variations, the effective cohesion of the rock may be completely eliminated and failure is characterized by periodic rock fall activity. On the other hand, deep and delayed sliding is supposed to origin from a combined effect of the increase of shear stresses and a slow creep-deformation of the rock acting upon by these stresses. These processes are overlain by "cleft-water pressures reducing the frictional resistance along the joint walls" from rainfall and snow melt water infiltrating into the strongly damaged near surface layer, where the groundwater table (or pore pressures) show seasonal variations measured in "tens of feet". These pressure variations are strongest during rapid snowmelt in spring, when "the exit of the joints are still plugged with ice".

Terzaghi (1962) also discusses the successive stages in the evolution of slopes. He states that if the rate of fluvial or glacial down-cutting decreases, the slope angle of the valley walls is gradually reduced by weathering and removal of weathering products until the slope angle reaches a stable value defined by the orientations and friction angles of "continuous joint sets" and persistent faults. Special consideration is given to the exceptional conditions prevailing after the retreat of large valley glaciers, including the observation that strong variations in delayed response times after exposure to "all the agencies which weaken the slope" are observed.

While many of these ground-breaking concepts have been verified and refined in the following decades, several open key questions could not be clearly resolved until today. This lecture discusses the current understanding of Terzaghi's concepts based on new field observations and modeling results carried out at the Chair of Engineering Geology in the last 10 years.

This content is only available via PDF.
You can access this article if you purchase or spend a download.