136 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



The general treatment is excellent, and Sharpe shows how the classi- 

 fication of mass movements of earth rest on variable factors such as 

 (1) type, size, cause, and rate of movement; (2) water content; (3) 

 type of material involved; (4) characteristics of internal friction and 

 organization of material within the moving mass; and (5) relation- 

 ship of the moving mass to surface material and substrata. The two 

 principal types of mass movement, flows and slides, depend on the 

 presence or absence of a slip plane. Within the limitations of a geolo- 

 gist who makes no claim of also being an expert in soil mechanics 

 techniques and the mechanics of soil movement, Sharpens work is 

 excellent. Subsequent writers, including soil mechanics specialists, 

 have not improved on his classification, although they have materially 

 aided in presenting some idea of the mechanics of movement. 



Terzaghi and Peck devote considerable space not only to the theory 

 relative to stability of slopes (1948, pp. 181-191) but also to stability 

 of hillsides and slopes in open cuts (1948, pp. 354-371). They also 

 suggest a classification for slides based on the types of soils in which 

 they occur: (a) detritus, (6) sand, (c) loess, {d) fairly homogeneous 

 soft clay, (e) clay flows, and (/) stiff clays. These are analyzed on 

 the basis of the assumption of more or less homogeneity. In two ad- 

 ditional groups more complex types are considered: (g) clay with 

 layers or pockets of water-bearing sand; and (h) sudden spreading 

 on clay slopes. 



They state that slides may occur slowly or suddenly, in any con- 

 ceivable manner, but claim that they are usually due to excavation 

 or to undercutting of the foot of an existing slope. Initiation of slid- 

 ing may be caused by gradual disintegration of the soil, starting at 

 hair cracks which subdivide the soil into angular fragments, or by 

 increase of pore-water pressure in permeable layers, or by shock which 

 liquefies the soil beneath the slope. They admit that, because of the 

 variability of the factors and processes leading to slides, the condi- 

 tions for slope stability generally defy theoretical analysis. In addi- 

 tion they agree that secondary structures and conditions may invali- 

 date the results of computations. 



For cohesionless dry sand, they express the slope factor of safety 

 relative to sliding by the equation 



tan <b 

 G 8 = 



tan p 



where the angle /?, made by the slope with the horizontal, is equal 

 to or less than the angle of internal friction cj> for loose sand. 



