232 vaknes. LANDSLIDES [Ch. 13 



State of Stress 



The state of stress determines directly whether or not an earth mass 

 is stable. Gravity is, of course, the prime mover and is occasionally 

 aided by other forces arising within the material. The state of stress 

 is determined largely by topographic form, but is also influenced by 

 changes in composition and structure as they affect the distribution 

 of stress among the solid particles, the interstitial water, and en- 

 trapped air. 



OPERATION OF MAJOR FACTORS 



Texture 



The physical ana physicochemical characteristics of sedimentary 

 material and the relative abundance of water are of prime importance 

 in the determination of stability. 



The size of individual particles in a sedimentary deposit in itself 

 does not greatly affect its stability, unless the size is of fine sand or 

 smaller, where capillary forces, surface effects, or the peculiar hydra- 

 tion properties of clay minerals come into play. The effect of size in 

 uniform conglomerate, gravel, coarse sandstone, and sand is primarily 

 to produce high transmissibility, allowing water to migrate readily — 

 possibly to adjacent shales where it may do real damage. The Tel- 

 luride (San Miguel) conglomerate is believed by Cross (1899) to have 

 so contributed to large landslides in the San Juan Mountains of Colo- 

 rado. Coarse gravels capping shale mesas have been instrumental in 

 guiding irrigation waters to slides along Cedar Creek, near Montrose, 

 Colorado (Varnes, 1949). In even-grained deposits the permeability 

 increases as the size increases (Fraser, 1935). 



In graded deposits, the filling of voids with fine material affects 

 the permeability and may affect other physical properties of the sedi- 

 ment. In dry cohesionless sands, size distribution apparently, has 

 little effect on compressibility (Chen, 1948), but in a cohesive clay 

 soil, addition of gravel has been found to decrease compressibility 

 (Murdock, 1948). 



The shape of the constituent particles also influences the physical 

 properties of soils and sedimentary deposits. In dry material, both 

 the compressibility and the angle of internal friction (and hence 

 shearing resistance) increase with increasing angularity of the con- 

 stituent grains (Chen, 1948). The shape of the particles also influ- 

 ences the fine structure developed during the process of sedimenta- 



