236 varnes. LANDSLIDES [Ch. 13 



or less loose structures in which the larger particles are cemented to- 

 gether or supported by the fine clay-water particles, or by the shearing 

 off of the water films which bind the clay particles together. Upon 

 resting, even at the previous water content, disturbed clays may re- 

 gain a considerable portion of their strength, probably through the 

 reconstruction of the water films (Moretto, 1948). 



This sensitivity of fine-grained deposits to disturbance makes dif- 

 ficult the control of those slides in which slump blocks turn rapidly 

 into semi-liquid earth flows as the material becomes disturbed (see 

 Fig. 2a). 



Vibrations 



Earth flows may be initiated by vibration. In describing earth 

 flows that accompanied the San Francisco earthquake, Anderson 

 (1908) says, "In certain cases the water seems to have risen with 

 a gush, as if actually squeezed from the hills." The author suspects 

 that the vibrations caused the natural structure of the soil to break 

 down and the soil to consolidate. Some of the water held tightly in 

 small voids and bound around the mineral particles was freed, and it 

 locally oversaturated the soil to cause flowage. 



A landslide along the banks of the Gerzenzee in Switzerland (von 

 Moos and Rutsch, 1944) was due to the breaking up of the structure 

 of a marl bed by the felling of trees and blasting of stumps. Here 

 the marl bed beneath a cover of peat had remained stable, even 

 though it was highly saturated, because of a gel structure involving 

 organic material. When disturbed, the mass liquefied and flowed 

 out into the lake, and the shore subsided. 



Fine-grained deposits of clay or silt size, but consisting of rela- 

 tively fresh rock flour rather than clay minerals, are particularly 

 susceptible to disturbance. Such deposits are common in lakes that 

 have received glacial detritus and include varved "clays." The water 

 content of this material in its natural state commonly equals or ex- 

 ceeds its liquid limit, and although it may be trimmed back to stable 

 slopes, it has little cohesion and cannot be handled without loss of 

 strength (Legget and Peckover, 1948; Tschebotarioff and Bayliss, 

 1948). 



Fine sand, if it is saturated and below its maximum density, may 

 flow readily if support is removed. Peck and Kaun (1948) describe 

 the liquefaction of sand deposited by a dredge behind sheet piling 

 when two of the sheet piles were removed. Progressive flow slides 

 in the Dutch province of Zeeland in sands below the critical density 



