Part 3 STRAIN INCOMPATABILITY

The most important aspect of applied rock mechanics is appreciating the strain incompatibility between rocks of dissimilar stiffness, strength, and deformability, such as shale and limestone. Strain Incompatibility Shale seam between sandstone beds

Strain Incompatibility A thin seam of shale can exert enormous impact on slope morphology, by engendering extension fractures in stiffer materialsA thin seam of shale can exert enormous impact on slope morphology, by engendering extension fractures in stiffer materials The shale tends to become increasingly plastic where confinement decreases, most often near the ground surfaceThe shale tends to become increasingly plastic where confinement decreases, most often near the ground surface

Slight changes in matrix cementation can have significant impacts on rock strength and stiffness, as shown by these adjacent beds in the Entrada Sandstone.Slight changes in matrix cementation can have significant impacts on rock strength and stiffness, as shown by these adjacent beds in the Entrada Sandstone.

Schematic views of bottomset, foreset, and topset beds in a prograding delta; as well as aeolian crossbeds on the continent.Schematic views of bottomset, foreset, and topset beds in a prograding delta; as well as aeolian crossbeds on the continent.

Materials with low permeability (such as bottomset beds) tend to compress to greater density over a much longer period of time, and thereby develop much greater strength and stiffness.Materials with low permeability (such as bottomset beds) tend to compress to greater density over a much longer period of time, and thereby develop much greater strength and stiffness.

Stiff, brittle units often exhibit profuse exfoliation, known as “secondary jointing” because it these curvaliner joints form in response to local perturbations of the stress field, due to erosion or excavation.Stiff, brittle units often exhibit profuse exfoliation, known as “secondary jointing” because it these curvaliner joints form in response to local perturbations of the stress field, due to erosion or excavation. The base of cliffs and benches or steps in cliffs are often structurally controlled by shale seams or brittle strata a few feet thick, as shown here.

Example of brittle fracture around a stiff tunnel lining. Gravity loads are “drawn” to stiff inclusions, causing over-stressing and brittle fracture. This tunnel opening had to be filled in because of the problem. Zion-Mt Carmel Tunnel, Zion National Park.Example of brittle fracture around a stiff tunnel lining. Gravity loads are “drawn” to stiff inclusions, causing over-stressing and brittle fracture. This tunnel opening had to be filled in because of the problem. Zion-Mt Carmel Tunnel, Zion National Park.