Environmental Engineering Reference
In-Depth Information
TABLE 5.15
Rock-Mass Properties Summarized
Property
Fresh, Intact Rock
Decomposed Rock
Nonintact Rock
Permeability (see
Essentially impermeable
Increases with degree
Water moves with relative
Section 3.3)
except for porous
of decomposition
freedom along fractures,
sandstones vesicular, and
and flow quantity increases
porous rocks. Significant as
as joint openings,
aquifers for water supply,
continuity, and pattern
or seepage beneath dams
intensities increase
(Table 3.12)
Significant in cut slopes
and other excavations,
seepage pressures beneath
dam foundations, or
seepage loss, and for water
supply
Rupture strength
Most rocks essentially
Decreases with degree
Seldom exceeded in the
(see
Section 3.4)
nonrupturable in the
of decomposition
confined state, but in an
confined state, although
unconfined condition, such
under conditions of high
as slopes or tunnels,
tensile stresses and high
strength can be very low
pore pressures under a
along weakness planes.
foundation not confined
Normally controls mass
totally, rupture can
strength
occur, especially in
foliated rocks
Deformability (from
Compression under
Increases with degree
Occurs from the closure of
stress increase)
foundation loads essentially
of decomposition
fractures and displacement
(see
Section 3.5)
elastic, although some rocks
along the weakness plane.
such as halite deform
When confined,
plasticly and undergo
displacements are usually
creep. Plastic deformation
negligible. In open faces,
can also occur along
such as tunnels and slopes,
foliations in a partially
movements can be
confined situation such as
substantial and normally
an excavation, or from high
control mass deformation
loads applied normal to the
foliations
Expansion (stress
Occurs in shales with
Increase with degree of
Residual stresses are locked
decrease)
montmorillonite clays or
decomposition
into the rock mass during
(see Section 3.5)
pyrite, causing excavation
formation or tectonic
and foundation heave,
activity and can far exceed
slope collapse, and tunnel
overburden stresses,
closing, when in contact
causing deflections of
with free water or moisture
walls and floors in
in the air
excavations and under-
ground openings, and
even violent rock bursts
in deep mines
(intact or nonintact)
Groundwater Conditions
Observations of groundwater conditions made in cuts and other exposures must be
related to recent weather conditions, season, and regional climate to permit judgments as
to whether seepage is normal, high, or low, since such conditions are transient.
