Environmental Engineering Reference
In-Depth Information
Diabase dike
Talus
Ta l u s
Residual soil
Granite
RS
RS
Gneiss and other metamorphic rocks
FIGURE 6.74
Typical section of rock decomposition in the warm, moist climate of the coastal mountains of Brazil.
pumping, in which case solution increases rapidly. Laterite and caliche (see
Section 7.7.2)
are
usually found above the permanent water table in well-drained, nonsaturated zones.
Urbanization and Construction Excavations
Foliated metamorphic rocks and mafic igneous rocks are the least resistant to chemical
decomposition, but in an urban environment where polluted rainwater or sewage from
broken sewer lines may seep downward into the rock mass, deterioration may be sub-
stantially accelerated. Feld (1966) reported a number of instances in New York City where
the foundation rock (Manhattan mica-schist) was a “hard, ringing” material when foun-
dations were installed, and in a matter of 10 to 20 years was found during adjacent exca-
vation to be altered and disintegrated and easily removed by pick and shovel. Depths of
alteration were as high as 25 ft. Bearing pressures as high as 25 tsf were reported, but no
indication of distress in the buildings was given. In one case, a broken steam line 75 ft dis-
tant from a tunnel excavation was attributed to the rock softening.
Marine and clay shales may undergo very rapid disintegration and softening when
exposed to humidity during excavation, as discussed in
Section 6.7.3.
6.7.3
Weathering Profile in Various Rock Types
(See also
Section 7.2,
“Residual Soils.”)
Igneous Rocks
Quartz-Rich Sialic Rocks
Quartz-rich sialic rocks go through four principal stages of profile development as illus-
trated in
Figure 6.75.
Stage 1
: Weathering proceeds first along the joints of the fresh rock surface, and decom-
position is most rapid where the joints are closely spaced. In temperate regions, the initial
change is largely mechanical disintegration and the resulting material does not differ sig-
nificantly from the original rock. The granite begins to alter in appearance; the biotite
tends to bleach and lighten in color, and the ferrous compounds of biotite tend to become
ferric hydrate and to migrate, staining the rock yellowish-red to reddish-brown. Fresh and
slightly weathered granite specimens are illustrated in
Figure 6.76.
During Stage 1 the
joints fill with sand or clay.
Stage 2
: During intermediate decomposition, the granite loses its coherence and becomes
crumbly. In humid climates, a sandy matrix forms around spherical boulders (corestones)
as shown in
Figure 6.77,
especially in partially saturated but continuously moist zones.
The corestone size reflects the fracture spacing. In well-drained zones, Stage 2 soil cover is
