## Construction Activities

Construction activities near the toe of an existing slope can cause failure because lateral resistance is removed (Figure). We can conveniently divide slope failures due to construction activities into two cases. The fi rst case is excavated slope and the second case is fill slope.

**Excavated Slopes :
**

When excavation occurs, the total stresses are reduced and negative porewater pressures are generated in the soil. With time the negative porewater pressures dissipate, causing a decrease in effective stresses and consequently lowering the shear strength of the soil. If slope failures were to occur, they would most likely take place after construction is completed. We can use our knowledge of stress paths to provide insight on the possible effects of excavation on slope stability. Let us consider a construction activity involving excavation of a normally consolidated fi ne-grained soil to construct a reservoir (Figure ). Let us consider an element of soil, X, at a depth z below the surface. Groundwater is assumed to be at the surface. The soil element, X, is under plane strain condition, but we will use axisymmetric condition for illustrative purposes.

The initial vertical effective stress is srzo 5 grz, and the lateral effective stresses are. The initial porewater pressure is uo = gw z. The stress invariants are

In stress space {p9( p), q}, the initial total stresses are represented by point A and the initial effective stresses are represented by point A9. The excavation will cause a reduction in sx (i.e., Dsx , 0) but very little change in sz (i.e., Dsz > 0) and sy (i.e., Dsy > 0). The change in mean total stress is then D_{p} = 2Dsx/3, and the change in deviatoric stress is Dq = Dsx. The total stress path (TSP) is depicted as AB in Figure 16.3h. Although B is near the failure line, the soil is not about to fail because failure is dictated by effective, not total, stresses.

If the soil were a linear, elastic material, the ESP would be A9B0 (recall that for elastic material, Dp9 5 0 under undrained condition). Assuming our soil is elastoplastic, then A9B9 would represent our ESP. The ESP moves away from the failure line. This is because the excess porewater pressure is negative due to the decrease in lateral stress, and consequently the effective stress increases. Therefore, failure is unlikely to occur during the excavation stage.

**Fill Slopes:
**

Fill slopes are common in embankment construction. Fill (soil) is placed at the site and compacted to specifi cations, usually greater than 95% Proctor maximum dry unit weight. The soil is invariably unsaturated, and negative porewater pressures develop. The soil on which the fi ll is placed, which we will call the foundation soil, may or may not be saturated.

If the foundation soil is saturated, then positive porewater pressures will be generated from the weight of the fi ll and the compaction process. The effective stresses decrease, and consequently the shear strength decreases. With time the positive porewater pressures dissipate, the effective stresses increase, and so does the shear strength of the soil. Thus, slope failures in fi ll slopes are most likely to occur during or immediately after construction.

**Rapid Drawdown:**

Reservoirs can be subjected to rapid drawdown. In this case the lateral force provided by the water is removed and the excess porewater pressure does not have enough time to dissipate (Figure ). The net effect is that the slope can fail under undrained condition. If the water level in the reservoir remains at low levels and failure did not occur under undrained condition, seepage of groundwater would occur and the additional seepage forces could provoke failure .