238 varnes. LANDSLIDES [Ch. 13 



Inclined Strata 



If the strata have a component of dip toward the free face of a 

 valley wall, the tendency for slippage along incompetent strata is, 

 of course, greatly increased. A large rockslide of this type occurred 

 in the Gros Ventre River valley in Wyoming in 1925 (Alden, 1928). 

 Slippage took place obliquely down dip along clay beds in a series of 

 sandstones and limestones. The slide mass dammed the river and 

 formed a large lake. Two years later the dam washed out, causing 

 a disastrous flood downstream. 



Fractures 



Joints and fissures of various kinds not only weaken any mass of 

 earth material, be it solid rock or clay, but also allow water to enter. 

 The properties of stiff, fissured clays are quite different from homo- 

 geneous intact clays. Stiff, fissured clays are usually very compact, 

 but they contain innumerable fractures that run through the mass. 

 The common characteristic of stiff, fissured clays is that if a lump of 

 such clay, with a moisture content below the plastic limit, is dropped, 

 it breaks into polyhedric fragments with dull or shiny surfaces that 

 may differ in color from the mass (Cassel, 1948; Terzaghi, 1936). 

 Clays of this type have been consolidated under considerably higher 

 pressure than their present surcharge and have been lifted and exposed 

 by erosion. Whereas the vertical pressure has been reduced, the hori- 

 zontal pressure, corresponding to the previous high vertical stress, re- 

 mains in considerable part. The Assuring is believed to result from 

 the differential variation of the vertical and horizontal pressures dur- 

 ing the geologic history of the clay and from the ensuing shear stresses. 

 As soon as the horizontal resistance is decreased through excavation, 

 the clay expands and the fissures open. If water enters the fissures, 

 a progressive softening occurs, gradually reducing the strength of the 

 clay mass. The rate of softening depends on many factors. Skemp- 

 ton (1948) describes slips in London clay that occurred from 5 to 40 

 years after the cuts were made. 



Landslides in hard rock commonly occur along, -or are at least in 

 part controlled by, joint surfaces. Many such slides, generally of the 

 rockslide or rockf all type, have been recorded. A notable example is 

 the Turtle Mountain slide at Frank, Alberta, in which approximately 

 41,000,000 cubic yards of limestone, weakened by joints across the 

 beds, descended into the valley, just touched the outskirts of the town 

 of Frank, and killed 70 people (Daly, Miller, and Rice, 1912). 



Faults may exercise control similar to joints, with the added factor 



