In Situ Stresses

Geological Controls Stress and Strain Outline

How do we estimate the stress acting on a material?

The vertical stress acting on a rock or soil at a depth z beneath the surface is simply the depth x the unit weight of the soil or rock. Unit weight is the force equivalent of density or the density x g (the acceleration due to gravity). Typical soil unit weights are around 18 kN/m3. Rocks have higher values, typically 25 kN/m3.

At a depth of 10 metres below the surface in a typical soil the vertical stress will be 10 x 18 = 180 kN/m2 or 180 kPa. For comparative purposes, atmospheric pressure is normally 101.3 kPa.

For rock at a depth of 1000 metres (the depth of Saskatchewan potash mines), the stress will be about 1000 x 25 = 25,000 kN/m2 or 25 MN/m2 or 25 MPa.

The increasing rock pressure with depth in the crust is called the lithostatic gradient, typically 25 MPa/km. If the rocks are saturated with water, the water pressure increases with depth at a rate known as the hydrostatic gradient, typically 10 MPa/km.

The ratio of horizontal to vertical stresses is usually between 0.3 and about 3.0. Horizontal stresses greater than vertical stresses occur in compressive tectonic zones. Low H:V stress ratios occur in zones of crustal extension. If the horizontal and vertical stresses are equal, the stress field is said to be lithostatic and no shear stresses are present. Shear is generated by differences between vertical and horizontal stresses called deviatoric stresses.

Geological Controls Stress and Strain Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9