Environmental Engineering Reference
In-Depth Information
Deformation and Fracturing
Tectonic Forces (See
Appendix A)
Natural stress changes result from tectonic forces, causing the earth's crust to undergo
elastic and plastic deformation and rupture. Plastic deformation is caused by steady long-
term stresses resulting in the folding translation of beds and some forms of cleavage, and
occurs when stresses are in the range of the elastic limit and temperatures are high. Creep,
slow continuous strain under constant stress, deforms rock and can occur even when
loads are substantially below the elastic limit, indicating that time is an element of defor-
mation. Short-term stresses within the elastic limit at normal temperatures leave no per-
manent effects on masses of competent rock.
Rupture and fracture occur at conditions of lower temperatures and more rapid strain,
resulting in faults, joints, and some forms of cleavage.
Other Forces and Causes
Tensile forces occur during cooling and contraction and cause the jointing of igneous
rocks. They are created by uplift (rebound) following erosion and cause jointing in sedi-
mentary rocks. Slope movements result in failure surfaces evidenced by slickensides.
Significance
Faults are associated with earthquake activity and surface displacements, and produce
slickensides; small faults are associated with folding and produce foliation shear and
mylonite shear zones.
Faults, joints, bedding planes, etc., divide the mass into blocks. All discontinuities rep-
resent weakness planes in the mass, controlling deformation and strength as well as pro-
viding openings for the movement of water.
Residual Stresses
Residual stresses are locked into the mass during folding, metamorphism, and slow cool-
ing at great depths. They can vary from a few tons per square foot (tsf) to many times over-
burden stresses, and can result in large deflections in excavations and “rock bursts,” violent
explosive ejections of rock fragments, and blocks occurring in deep mines and tunnels.
Alteration
New minerals are formed underground by chemical reactions, especially when heated,
and by heat and pressure associated with faulting and metamorphism. New minerals are
formed on the surface by chemical weathering processes. The result of chemical weather-
ing is the decomposition of minerals, the final product being residual soil. Weathering
products are primarily functions of the parent rock type and the climate.
Mechanical weathering also results in the deterioration of the rock mass.
Altered masses generally have higher permeability and deformability, and lower
strengths than the mass as formed originally or deformed tectonically.
6.1.2
Terrain Analysis
Significance
Regional and local rock types and structural features are identified through terrain analy-
sis (see also
Section 2.2.3).
