Geostatic Stress

Synonyms

Lithostatic stress; Overburden pressure

Definition

The weight of Earth materials in an imaginary vertical column acting on an imaginary horizontal surface at the base of the column.

A simple three-layer model of the Earth (Fig. 1) consists of 12 m of alluvium overlying 18 m of siltstone, overlying granite. An underground structure, such as a tunnel, may be planned at this location at a depth of 50 m, so the geostatic stress at 50 m needs to be computed. Assume that the relative densities (Dr) of alluvium, siltstone, and granite are as listed in Fig. 1; the Dr. of water is 1.0. Mass density is Dr times the mass density of water, which is 1000 kg/m³. Force is mass times acceleration; weight is mass times the acceleration of gravity; thus, the unit weight of water is 9802.26 N/m³. Hydrostatic pressure at a depth of 1 m would be 9.802 kPa (the slope of the hydrostatic pressure curve; H in Fig. 1). Therefore, at the base of a 1-m column of alluvium in the example, the unit...