Infinite Slope Analysis (Slope Stability)

ERDC TN-DOER-N5

July 2000

Infinite Slope Analysis (Slope Stability). For long slopes where the end effects on the sliding

material can be neglected, an infinite slope analysis of the stability of the slope can be expressed as

FOS =

(5)

γz 1 2 sin 2α

where

FOS = factor of safety against sliding

= undrained shear strength, lbf/ft2

γ = unit weight (submerged unit weight for submerged slope), lb/ft3

z = depth at which sliding occurs, ft

α = slope angle

An infinite slope analysis is appropriate for the long, continuous slopes such as those formed in

dredged material deposits. In this type analysis, the thickness of the unstable material is small

compared to the height of the slope, and each vertical block of soil above the potential slip plane

will have the same forces acting on it (Taylor 1954; Lambe and Whitman 1979). The infinite slope

analysis will indicate the various combinations of slope angle and maximum deposit height that will

result in a stable deposit. It is a simple analysis to conduct, and it is one that should be evaluated

in any comprehensive slope stability analysis.

Spencer's Analysis Method (Slope Stability). A detailed analysis of potential sliding

surfaces should be made using Spencer's method, which is generally considered to be the most

reliable of the method of slices analysis techniques and which provides the best solutions for meeting

both force and moment equilibrium requirements. Calculations can be made using the UTEXAS3

computer program developed at the University of Texas (Edris and Wright 1992).

When the program is used, piezometric levels should be assumed to coincide with mean water

surface elevation. The water should be modeled with normal loads applied to the slope surface as

is recommended by Edris and Wright (1992) for submerged slopes. Analysis conditions should

include the dredged material prior to capping and soon after capping, if a cap will be used on the

project. If stability problems are indicated in the analysis, it would be advisable to simulate

consolidation for some reasonable period of time, then correlate output from a consolidation analysis

(such as output from Primary Consolidation, Secondary Compression, and Desiccation of Dredged

Fill (PSDDF)) to density and strength at that time, additional stability calculations can be made to

assess the effect of consolidation on the stability of the material. In simulations with a cap, the cap

should be modeled as a load normal to the slope surface.

The condition of the foundation soil (firm versus weak) beneath the soft dredged material mound

has a major impact on the formation of the slope failure surface. Several failure modes should be

postulated and examined individually for each project, as illustrated in Figure 2, which is based on

data for a capping project in the Mud Dump Site (Rollings and Rollings 1998b). The computerized

analysis using Spencer's method of slices locates the specific surface of these various potential