b. Seepage and Groundwater Control. Surface control of drainage
decreases infiltration to potential slide area. Lowering of groundwater
increases effective stresses and eliminates softening of fine-grained soils
at fissures. Details on seepage and groundwater control are found in
(1) Application. Walls or large diameter piling can be used to
stabilize slides of relatively small dimension in the direction of movement
or to retain steep toe slopes so that failure will not extend back into a
(2) Analysis. Retaining structures are frequently misused where
active forces on wall are computed from a failure wedge comprising only a
small percentage of the total weight of the sliding mass. Such failures may
pass entirely beneath the wall, or the driving forces may be large enough to
shear through the retaining structure. Stability analysis should evaluate a
possible increase of pressures applied to wall by an active wedge extending
far back into failing mass (see Figure 4, DM-7.2, Chapter 3), and possible
failure on sliding surface at any level beneath the base of the retaining
(3) Piles or Caissons. To be effective, the piles should extend
sufficiently below the failure surface to develop the necessary lateral
resistance. Figure 11 shows how the effect of the piles is considered in
calculating the factor of safety. The distribution of pressure along the
pile can be computed from charts shown in Figure 12. This assumes full
mobilization of soil shear strength along the failure surface and should be
used only when the safety factor without the piles is less than 1.4. This
criteria is based on results of analysis presented in Reference 10, Forces
Induced in Piles by Unsymmetrical Surcharges on the Soil Around the Pile, by
DeBeer and Wallays.
See Figure 13 for example computations. Note the computations
shown are for only one of the many possible slip surfaces.
(1) Other potential procedures for stabilizing slopes include
grouting, freezing, electro osmosis, vacuum pumping, and diaphragm walls.
See Table 7 of DM-7.2, Chapter 1 for further guidance on these methods.
1. SLOPE EROSION. Slopes which are susceptible to erosion by wind and
rain-fall should be protected. Protection is also required for slopes
subjected to wave action as in the upstream slope of a dam, or the river and
canal banks along navigational channels. In some cases, provision must be
made against burrowing animals.