4.2 Unsupported Excavations

An unsupported cut with free slopes in soft cohesive soils are limited to very shallow depths (max. 3 m). Independent from the depth, however, unsupported excavations require protection using different techniques, such as:

• slope protection against erosion using geotextile membrane (e.g. Fig. 4.1),

• providing a filter layer made of gravel or single-grained concrete (e.g. Fig. 4.2), whereby the filter stability requires additional attention. Geotextile filter mat, for example, may help to insure the filter stability, and

• stabilising the slope by means of vacuum drainage (e.g. Fig. 4.3)

Figure 4.1: Sloped excavation with surface protection

Figure 4.2: Slope protection using single-grained concrete

Figure 4.3: Slope stabilization using vacuum drainage, a) without additional measure, b) supported with timber piles and geotextile mat (intermediate excavation stage)

If berms and sloped excavations remain unprotected or unstabilised, delayed slides and mass flow can occur as shown in Fig. 4.4.

Figure 4.4: Slope slide and collapse of unprotected slope in soft soil a) in lacustrine soils; b) in alternate peat and clay layer

The depth of excavation of unprotected excavation in soft soils should not normally exceed 3 m. A slope up to 45 (1:1) can be used for temporary slopes without a need for further verification of its safety, provided that slope is located above the groundwater. The height of a safe and long-term stable slope is also dependant on the plasticity and the undrained strength of the soft soil. Soils with low plasticity are usually prone to flow, specially when they contain...