Environmental Engineering Reference
In-Depth Information
Piezometers
(see
Section 4.4.2)
are installed to monitor pore pressures and changes in
groundwater conditions.
Conclusions should be reached regarding the worst conditions likely to be encountered
during the life of the project, and to foresee possible changes in groundwater conditions
brought about naturally or by construction.
During Construction
Changes in groundwater conditions occurring during construction are monitored with
piezometers.
During tunnel construction, pilot holes drilled in advance of the heading provide data
to forewarn of hazardous conditions.
After Construction
Where the possible development of seepage forces may endanger the performance of the
structure, conditions are monitored with piezometers.
Where the problem of excessive seepage through a dam embankment, abutment, or
foundation exists, attempts to locate the seepage paths are advisable to aid in designing
treatment. Various dyes and tracers have been injected into boreholes and observations
made of their exit points, but the positive location of seepage channels is usually extremely
difficult (see
Section 2.3.7).
The acoustical emissions device (see Section 4.3.5), which
measures microseisms, may provide useful information. Testing indicates that minimum
flow rates required for detection with acoustical emissions devices are of the order of
45 mL/sec for clear water seepage and 10 mL/sec for turbid water, such as that in a dam
undergoing piping erosion (Koerner et al., 1981).
Control Methods Summarized
Control methods may be placed in three main categories:
1.
Cutoffs and barriers
, which, when constructed and installed properly, have the
potential to seal off flow
2.
Dewatering systems
, which serve to lower the water table and reduce pore-water
pressures, or in some cases only to reduce pore pressures
3.
Drains
, which serve to control flow, in some cases lowering the water table, in
others reducing pore-water pressures and seepage forces
Other factors:
Filters
are provided between zones with significant differences in permeability to
control flow velocities and prevent migration of fines and piping. They are most
important adjacent to drains where it is desirable to prevent clogging or piping.
●
Surface treatments
are provided to deter or prevent infiltration of water on slopes.
●
Groundwater and seepage control methods and their common applications are
summarized in
Table 8.6.
A general comparison of many of the various methods
in terms of soil gradation characteristics is given in
Figure 8.35.
Not included are
liners, walls, and drains, which are normally an integral part of a structure.
●
8.4.2
Cutoffs and Barriers
Liners, Blankets, and Membranes
Clay blankets
are placed on materials with moderate permeability, extending upstream from
a dam embankment to increase the horizontal length of flow paths and decrease seepage
quantities and pressures at the downstream toe, as discussed also in
Section 8.4.7.
