Ch. 13] METHODS OF COMBATING SLIDES 241 



Pore-water pressure has been studied principally in regard to the 

 stability of earth dams. Several failures of earth dams, notably those 

 of the Alexander Dam in Hawaii (Anonymous, 1930) and the Belle 

 Fourche Dam (Anonymous, 1933) have been ascribed to the develop- 

 ment of excess pore-water pressures through lack of drainage. Pore 

 pressures developed in the walls of cuts or natural slopes sometimes 

 produce very rapid and disastrous slides. Stratified deposits consisting 

 of layers of sand and clay and masses of cohesive soil that contain 

 irregular lenses or pockets of sand and silt are particularly susceptible 

 (Terzaghi and Peck, 1948, p. 365). Glacial and glacial-outwash de- 

 posits are commonly of this type. Beds or pockets of sand or silt be- 

 come saturated during wet weather and may develop high pore-water 

 pressure. These beds have little or no cohesion, and their shear 

 strength is determined by (Terzaghi and Peck, 1948, p. 367) 



s = (p — U w ) tan <f> 



As the pore-water pressure U w increases, the shear strength de- 

 creases until the porous bed can no longer support the overlying mass. 

 Failure takes place by outward spreading and breaking up of the over- 

 lying material on the underlying viscous bed. Such a failure by 

 spreading may occur within a few minutes, in contrast to the slower 

 action of slump in homogeneous clay. 



It is hardly necessary to comment on the natural agencies of erosion 

 and the work of man in the production of slopes that may become 

 unstable. It should perhaps be emphasized that the stability of a 

 given slope is not a constant for all time. As erosion proceeds, as the 

 ground-water conditions fluctuate, and as changes in the state of 

 hydration and structure of clays occur with the passage of time, the 

 state of stress throughout the slope may change greatly. The original 

 slope design must take these possibilities into consideration. 



One must also consider the possibility of transitory stresses that 

 arise from vibrations, blasting, earthquakes, or earth tides, triggering 

 the failure of an otherwise stable slope. 



METHODS OF COMBATING SLIDES 



The methods employed in combating slides are generally curative 

 rather than preventive. They depend on the kind of material, the 

 area involved, and the type and rate of movement. These factors 

 vary so greatly from slide to slide that the details of remedial measures 

 are tailored to fit each slide. 



The design of high embankments and cuts, or the combating of 



