Part 3 STRAIN INCOMPATABILITY. The most important aspect of applied rock mechanics is appreciating the strain incompatibility between rocks of dissimilar.

Part 3Part 3

STRAIN STRAIN INCOMPATABILITYINCOMPATABILITY

• 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 IncompatibilityStrain Incompatibility

Shale seam between Shale seam between sandstone bedssandstone beds

Strain Strain IncompatibilityIncompatibility

• A thin seam of shale can A thin seam of shale can exert enormous impact exert enormous impact on slope morphology, by on slope morphology, by engendering extension engendering extension fractures in stiffer fractures in stiffer materialsmaterials

• The shale tends to The shale tends to become increasingly become increasingly plastic where plastic where confinement decreases, confinement decreases, most often near the most often near the ground surfaceground surface

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

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

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

• Stiff, brittle units often exhibit profuse exfoliation, Stiff, brittle units often exhibit profuse exfoliation, known as “secondary jointing” because it these known as “secondary jointing” because it these curvaliner joints form in response to local perturbations curvaliner joints form in response to local perturbations of the stress field, due to erosion or excavation. 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 Example of brittle fracture around a stiff tunnel lining. Gravity loads are “drawn” to stiff inclusions, causing over-stressing and loads are “drawn” to stiff inclusions, causing over-stressing and brittle fracture. This tunnel opening had to be filled in because of brittle fracture. This tunnel opening had to be filled in because of the problem. Zion-Mt Carmel Tunnel, Zion National Park. the problem. Zion-Mt Carmel Tunnel, Zion National Park.