Environmental Engineering Reference
In-Depth Information
6
Rock-Mass Characteristics
6.1
Introduction
6.1.1
Characteristics Summarized
Development Stages
Rock-mass characteristics can be discussed under four stages of development:
1.
The original mode of formation with a characteristic structure (
Section 5.2)
2.
Deformation with characteristic discontinuities
3.
Development of residual stresses that may be several times greater than over-
burden stresses
4.
Alteration by weathering processes in varying degrees from slightly modified to
totally decomposed
Rock-mass response to human-induced stress changes are normally controlled by the
degree of alteration and the discontinuities. The latter, considered as mass defects, range
from weakness planes (faults, joints, foliations, cleavage, and slickensides) to cavities and
caverns. In the rock mechanics literature, all fractures are often referred to as joints.
Terrain analysis is an important method for identifying rock-mass characteristics.
Structural features are mapped and presented in diagrams for analysis.
Original Mode of Formation
Igneous Rocks
Intrusive masses form large bodies (batholiths and stocks), smaller irregular-shaped bod-
ies (lapoliths and laccoliths), and sheet-like bodies (dikes and sills). Extrusive bodies form
flow sheets.
Sedimentary Rocks
Deposited generally as horizontal beds, sedimentary rocks can be deformed in gentle
modes by consolidation warping or by local causes such as currents.
Cavities form in the purer forms of soluble rocks, often presenting unstable surface con-
ditions.
Metamorphic Rocks
Their forms relate to the type of metamorphism (see
Section 5.2.5).
Except for contact
metamorphism, the result is a change in the original form of the enveloping rocks.
