Geological Controls

Laboratory Tests In Situ Stresses Outline

The engineering properties of rocks are influenced by a large number of geological factors. Mineralogy and particle-contacts control strength on a small scale; tectonic deformation, igneous activity aand metamorphism all result in substantial changes in the mechanical behaviour of rocks through recrystallization and fracturing.

Burial and erosion of sediments results a series of consistent and predictable changes. The increase in sediment load during burial combined with cementation and filling pores results in:

Stripping away sediment by erosion and the consequent unloading and weathering results in the development of joints leading to: In general rocks become stronger and less porous and permeable as they get older. Recent sediments are normally weaker than ancient rocks with similar lithology and mineralogy.

Rocks and soils with a level of compaction corresponding to their present burial depth re said to be normally consolidated. Where erosion has occured, rocks may be compacted much more than expected for their current depth of burial. These rocks and soils are said to be overconsolidated. Rocks that have not compacted to the expected extent for their depth of burial, perhaps because fluids could not escape, are said to be underconsolidated. Underconsolidated rocks are often associated with high fluid pressures (overpressure). An overpressure is a pressure in excess of the pressure predicted from the normal hydrostatic gradient.

Laboratory Tests In Situ Stresses Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9