Environmental Engineering Reference
In-Depth Information
TABLE 5.14
Characteristics of Metamorphic Rocks with Massive Fabric and other Forms
Rock
Characteristics
Plate
Metaconglomerate
Heat and pressure cause the pebbles in a conglomerate to stretch, deform,
and fuse
Quartzite
Results from sandstone so firmly fused that fracture occurs across the grains,
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which are often imperceptible
Marble
Results from metamorphism of limestone or dolomite and is found with large
and small crystals, and in many colors including while, black, green, and red.
Metamorphosed limestone does not normally develop cavities. Very hard
Serpentinite
Derived from serpentine. Generally compact, dull to waxy luster, smooth to
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splintery fracture, generally preen in color and often soft unless it contains
significant amounts of quartz. Can have foliate fabric
Soapstone
Derived from talc; generally gray to green color, very soft and easily trimmed
into shapes with a knife, without cleavage or grain, and resists well the action
of heat or acids
Hornfels
Rocks baked by contact metamorphism into hard aphanitic material, with
conchoidal fracture, dark gray to black color, often resembling a basalt
Other forms
Migmatite
Signifies a rock that is a complex intermixture of metamorphic and granular
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igneous rocks such as formed by the injection of granite magma into foliated
rocks
Mylonites
Produced by intense mechanical metamorphism: can show strong lamination
but the original mineral constituents and fabric have been crushed and
pulverized by the physical processes rather than altered chemically. Common
along the base of overthrust sheets and can range from very thin, to a meter
or so, to several hundreds of meters thick. Shale mylonites form very unstable
conditions when encountered in cut slopes or tunneling. They are formed by
differential movement between beds
Rock-Mass Description
General
The degree of complexity of description depends upon the nature of the problem under
study and the relative importance of the rock-mass response. For routine problems, such
as the average building foundation on good-quality rock, simple descriptions suffice,
whereas for nonroutine problems, the rock mass is described in terms of intact rock char-
acteristics, discontinuities, and groundwater conditions. Descriptions are made from the
examination of outcrops, exploration pits and adits, and boring cores.
Intact Rock Characteristics
Descriptions should include the hardness, weathering grade, rock type, coloring, texture,
and fabric.
Discontinuities
Joint spacing and joint characteristics are described, and details of joint orientations and
spacing should be illustrated with photographs and sketches to allow for the preparation
of two- or three-dimensional joint diagrams (see Section 6.1.3).
Other mass characteristics, including faults, slickensides, foliation shear zones, bedding,
and cavities, are provided in an overall mass description.
 
 
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