Changes of Soil Properties During Excavation.

The soil at level 1, below ground level – see Fig. 1.9 – is subject to pressure, and thus consolidation, due to the weight of the soil above, and is in equilibrium. If the overlying soil is removed to form a basement then the pressure, and consolidation effects, at level 1 are also removed. The unloaded soil, in this condition, is known as over-consolidated, and is likely to recover from the consolidation and rise in level (heave). This can be likened to the elastic recovery of contraction on a column when its load is removed.

Fig. 1.9 Heave following removal of overburden.