Environmental Engineering Reference
In-Depth Information
fractures, and the degree of decomposition. Some geologic materials defined as rock, such
as halite, many shales, and closely jointed masses, may be removable by machine, or may
be more deformable under applied stress than are some materials defined as soils, such as
glacial tills and cemented soils. Compounding the problem of definition is rock decompo-
sition that, at a given location, can vary over short lateral and vertical distances, resulting
in materials ranging from “hard” to ”soft.”
In the literature, rock can be found defined for engineering purposes as ”intact speci-
mens with a uniaxial compressive strength of the order of 100 psi (6.8 tsf, kg/cm
2
) or
greater.” For the above reasons, this definition is not applicable to rock masses in many
instances.
General Classification
A broad classification which generally includes all geologic materials, together with a brief
description, is given in Table 5.1.
5.1.2
Rock Groups and Classes
Geologic Bases
Based on their geologic aspects, rocks are grouped by origin as igneous, sedimentary, or
metamorphic, and classified according to petrographic characteristics, which include their
mineral content, texture, and fabric.
Engineering Bases
On an engineering basis, rock is often referred to as either intact or
in situ
.
Intact rock
refers to a block or fragment of rock free of defects, in which its hydraulic and
mechanical properties are controlled by the petrographic characteristics of the material,
whether in the fresh or decomposed state. Classification is based on its uniaxial compres-
sive strength and hardness.
TABLE 5.1
A Broad Classification of Geologic Materials
Category
Material
Description
Rock
Fresh rock
Intact
Unweathered rock free of fractures and other defects
Nonintact
Unweathered, but divided into blocks by fractures
Transition zone
Decomposed rock
Intact
Weathered, the rock structure and fabric remain but mineral
constituents are altered and the mass softened
Nonintact
Rock softened and altered by weathering and containing
discontinuities (fractures)
Soil
Residual soils
Most minerals changed by advanced decomposition: fabric
remains or is not apparent: material friable
Transported soils
The residual soils are removed and graded by the agents of
transportation: wind, water, gravity, and glaciers. Degree of
grading varies with transport mode
Sedentary soils
Organic deposits develop
in situ
from decomposition of
vegetation
Either
Duricrusts
Deposits from evaporation of groundwater: caliche, laterite,
ferrocrete, silcrete
