Civil Engineering Reference
In-Depth Information
rocks, such as poorly indurated sandstone,
the “primary” hydraulic conductivity of the
rock material may be significant such that
a proportion of the total seepage occurs
through the pores. The rate of seepage is
proportional to the local hydraulic gradient
and to the relevant directional conductiv-
ity, proportionality being dependent on lam-
inar flow. High velocity flow through open
discontinuities may result in increased head
losses due to turbulence.
•
In the case of rock slopes, the preliminary
design estimates will be based on assumed
values of effective normal stress. If, as a result
of field observations, one has to conclude that
pessimistic assumptions of water pressure are
justified, such as a tension crack full of water
and a rock mass that does not drain readily,
then this will clearly influence the slope design.
So also will the field observation of rock slopes
where high water pressures can develop due
to seasonal freezing of the face that blocks
drainage paths.
•
The prediction of ground water levels, likely
seepage paths, and approximate water pres-
sures may often give advance warning of
stability or construction difficulties. The
field description of rock masses must inev-
itably precede any recommendation for field
conductivity tests, so these factors should
be carefully assessed at early stages of the
investigation.
Seepage from individual unfilled and filled dis-
continuities or from specific sets exposed in a
tunnel or in a surface exposure, can be assessed
according to the descriptive terms in Tables II.10
and II.11.
In the case of an excavation that acts as a drain
for the rock mass, such as a tunnel, it is helpful if
the flow into individual sections of the structure
are described. This should ideally be performed
immediately after excavation since ground water
levels, or the rock mass storage, may be depleted
•
Irregular ground water levels and perched
water tables may be encountered in rock
masses that are partitioned by persistent
impermeable features such as dykes, clay-filled
discontinuities or low conductivity beds. The
prediction of these potential flow barriers and
associated irregular water tables is of con-
siderable importance, especially for projects
where such barriers might be penetrated at
depth by tunneling, resulting in high pressure
inflows.
Table II.10
Seepage quantities in unfilled
discontinuities
Seepage
rating
Description
•
Water seepage caused by drainage into
an excavation may have far-reaching con-
sequences in cases where a sinking ground
water level would cause settlement of nearby
structures founded on overlying clay deposits.
I
The discontinuity is very tight and
dry, water flow along it does not
appear possible.
II
The discontinuity is dry with no
evidence of water flow.
•
The approximate description of the local
hydrogeology should be supplemented with
detailed observations of seepage from indi-
vidual discontinuities or particular sets,
according to their relative importance to sta-
bility. A short comment concerning recent pre-
cipitation in the area, if known, will be helpful
in the interpretation of these observations.
Additional data concerning ground water
trends, and rainfall and temperature records
will be useful supplementary information.
III
The discontinuity flow is dry but
shows evidence of water flow, that
is, rust staining.
IV
The discontinuity is damp but no
free water is present.
V
The discontinuity shows seepage,
occasional drops of water, but no
continuous flow.
VI
The discontinuity shows a
continuous flow of water—estimate
l
/
min and describe pressure, that is,
low, medium, high.
