Shear strength (discontinuity)
Encyclopedia
The shear strength of a discontinuity
in a soil or rock mass may have a strong impact on the mechanical behavior of a soil or rock mass. The shear strength of a discontinuity is often considerably lower than the shear strength of the blocks of intact material in between the discontinuities, and therefore influences, for example, tunnel
, foundation, or slope
engineering, but also the stability of natural slopes
. Many slopes, natural and man-made, fail due to a low shear strength of discontinuities in the soil or rock mass in the slope. The deformation characteristics of a soil or rock mass are also influenced by the shear strength of the discontinuities. For example, the modulus of deformation is reduced, and the deformation becomes plastic
(i.e. non-reversible deformation on reduction of stress) rather than elastic
(i.e. reversible deformation). This may cause, for example, larger settlement of foundations, which is also permanent even if the load is only temporary. Furthermore the shear strength of discontinuities influences the stress distribution in a soil or rock mass.
in a soil or rock mass in geotechnical engineering
is governed by the persistence
of the discontinuity, roughness of discontinuity surfaces, infill material in the discontinuity, presence and pressure of gasses and fluids (e.g. water, oil), and possible solution (e.g. karst
) and cementation along the discontinuity. Further the shear strength is dependent on whether the discontinuity has moved before in the geological history (i.e. are the asperities
on opposing walls of the discontinuity fitting or non-fitting, or have the asperities been sheared off).
Discontinuity (geotechnical engineering)
A discontinuity in geotechnical engineering is a plane or surface that marks a change in physical or chemical characteristics in a soil or rock mass. A discontinuity can be, for example, a bedding, schistosity, foliation, joint, cleavage, fracture, fissure, crack, or fault plane...
in a soil or rock mass may have a strong impact on the mechanical behavior of a soil or rock mass. The shear strength of a discontinuity is often considerably lower than the shear strength of the blocks of intact material in between the discontinuities, and therefore influences, for example, tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
, foundation, or slope
Slope stability analysis
The slope stability analyses are performed to assess the safe and economic design of a human-made or natural slopes and the equilibrium conditions. The term slope stability may be defined as the resistance of inclined surface to failure by sliding or collapsing...
engineering, but also the stability of natural slopes
Slope stability
The field of slope stability encompasses the analysis of static and dynamic stability of slopes of earth and rock-fill dams, slopes of other types of embankments, excavated slopes, and natural slopes in soil and soft rock...
. Many slopes, natural and man-made, fail due to a low shear strength of discontinuities in the soil or rock mass in the slope. The deformation characteristics of a soil or rock mass are also influenced by the shear strength of the discontinuities. For example, the modulus of deformation is reduced, and the deformation becomes plastic
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...
(i.e. non-reversible deformation on reduction of stress) rather than elastic
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
(i.e. reversible deformation). This may cause, for example, larger settlement of foundations, which is also permanent even if the load is only temporary. Furthermore the shear strength of discontinuities influences the stress distribution in a soil or rock mass.
Shear strength
The shear strength along a discontinuityDiscontinuity (geotechnical engineering)
A discontinuity in geotechnical engineering is a plane or surface that marks a change in physical or chemical characteristics in a soil or rock mass. A discontinuity can be, for example, a bedding, schistosity, foliation, joint, cleavage, fracture, fissure, crack, or fault plane...
in a soil or rock mass in geotechnical engineering
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...
is governed by the persistence
Persistence (discontinuity)
Persistence determines the possibilities of relative movement along a discontinuity in a soil or rock mass in geotechnical engineering. Discontinuities are usually differentiated in persistent, non-persistent, and abutting discontinuities ....
of the discontinuity, roughness of discontinuity surfaces, infill material in the discontinuity, presence and pressure of gasses and fluids (e.g. water, oil), and possible solution (e.g. karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
) and cementation along the discontinuity. Further the shear strength is dependent on whether the discontinuity has moved before in the geological history (i.e. are the asperities
Asperity (geotechnical engineering)
In Geotechnical engineering the term asperity is mostly used for unevenness of the surface of a discontinuity, grain, or particle with heights in the range from approximately 0.1 mm to many decimetre...
on opposing walls of the discontinuity fitting or non-fitting, or have the asperities been sheared off).
Determination shear strength
Only for simple models of discontinuities the shear strength can be analytically calculated. For real discontinuities no analytical calculation method exists. Testing on various scales in the laboratory or in the field, or empirical calculations based on characterizing the discontinuity are used to establish the shear strength.Discontinuity shear strength tests
- Direct shear testDirect Shear TestA direct shear test also known as shearbox test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuties in soil or rock masses.- Soil :...
or (Golder) shearbox test - Tilt test (Geotechnical engineering)Tilt test (geotechnical engineering)In geomechanics a tilt test is a simple test to estimate the shear strength parameters of a discontinuity. Two pieces of rock containing a discontinuity are held in hand or mounted in test equipment with the discontinuity horizontal. The sample is slowly tilted until the top block moves...
- Triaxial test
Empirical calculations based on characterization
- Barton-Bandis shear criterion
- Sliding criterionSliding criterion (geotechnical engineering)Sliding criterion The sliding criterion is a tool to estimate easily the shear strength properties of a discontinuity in a rock mass based on visual and tactile characterization of the discontinuity...