242 varnes. LANDSLIDES [Ch. 13 



large and serious slides, generally requires geologic mapping of both 

 the surface and underground, the studies of ground-water levels and 

 pressures, taking and testing of enough samples to be representative, 

 and the analysis of the stability of the deposit. 



The purpose of a stability analysis is to aid in the design of safe 

 slopes, to aid in the redesign of slopes that have failed, or to approxi- 

 mate the original physical properties of material that has failed along 

 a known surface. Assumptions are necessarily made regarding the 

 gross and fine structure of the deposit and the shape of the sliding 

 surfaces. The dangers of these assumptions are well recognized, but, 

 in many instances, at least an approximate idea of the safety factor is 

 obtained. It is generally assumed that sliding occurs along the arc of 

 a cylinder whose axis is parallel to the slope surface, and that the 

 sliding block rotates about the center of the arc. The factor of safety 

 is then defined as the ratio of the moments about the center of the 

 arc tending to produce movement and tending to resist movement 

 (Terzaghi and Peck, 1948, p. 184) : 



moment of resisting forces 



Safetv factor = — 



moment of driving forces 



Various refinements may be made, such as the assumption that 

 the surface of sliding is composed of a series of arcs of different 

 radii. Many variables enter into the computation of the moments of 

 the resisting and driving forces, and into the selection of centers and 

 radii of the arcs. The factor of safety is computed for several sur- 

 faces, and that having the lowest factor is the one along which sliding 

 may most probably occur. Such analysis is most successful if the 

 material is homogeneous and if data on the strength of the material 

 and subsurface stresses are ample. 



Remedial measures for the cure of slides are generally aimed at 

 changing one or more of the factors of composition, structure, or state 

 of stress to restore equilibrium. Where slump or earth flow is involved, 

 remedial measures include, if possible, changing the composition of 

 the material by removal or diversion of water. Methods of removal 

 include open or rubble-filled trenches, drainage tunnels, wells, reverse 

 filters, drying by heated air, and electro-osmosis. Methods used to 

 prevent access include surface diversions, grouting openings, sodding, 

 and oiling. 



Methods involving change of structure as well as composition, such 

 as the introduction of gelling substances and stabilization by electric 

 current, have been used in the stabilization of foundations, but the 

 author is not familiar with their use in stabilizing slopes. 



