(4) End-of-construction condition for fills built of cohesive soils.

Use the undrained strength of samples compacted to field density and at

water content representative of the embankment.

b. Effective Stress Analysis. The effective shear strength parameters

(c', [phi]') should be used for the following cases:

(1) Long-term stability of clay fills.

Use steady state seepage

pressures where applicable.

(2) Short-term or end-of-construction condition for fills built of

free draining sand and gravel. Friction angle is usually approximated by

correlation for this case. See Chapter 1.

(3) Rapid drawdown condition of slopes in pervious, relatively

incompressible, coarse-grained soils. Use pore pressures corresponding to

new lower water level with steady state flow.

(4) Long-term stability of cuts in saturated clays.

Use steady

state seepage pressures where applicable.

(5) Cases of partial dissipation of pore pressure in the field.

Here, pore water pressures must be measured by piezometers or estimated from

consolidation data.

3. EFFECT OF GROUNDWATER AND EXCESS PORE PRESSURE. Subsurface water

movement and associated seepage pressures are the most frequent cause of

slope instability. See Table 1 for illustrations of the effects of water on

slope stability.

a. Seepage Pressures. Subsurface water seeping toward the face or toe

of a slope produces destabilizing forces which can be evaluated by flow net

construction. The piezometric heads which occur along the assumed failure

surface produce outward forces which must be considered in the stability

analysis. See Table 3 and the example of Figure 1.

b. Construction Pore Pressures. When compressible fill materials are

used in embankment construction, excess pore pressure may develop and must

be considered in the stability analysis. Normally, field piezometric

measurements are required to evaluate this condition.

c. Excess Pore Pressures in Embankment Foundations. Where embankments

are constructed over compressible soils, the foundation pore pressures must

be considered in the stability analysis. See top panel of Table 3.

d. Artesian Pressures. Artesian pressures beneath slopes can have

serious effects on the stability. Should such pressures be found to exist,

they must be used to determine effective stresses and unit weights, and the

slope and foundation stability should be evaluated by effective stress

methods.

7.1-333

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