Environmental Engineering Reference
In-Depth Information
20.4.3
Surface displacements
Regular accurate survey of displacements of the surface of the dam embankment can be a
most useful guide to performance.
Such measurements can be useful as a check on design assumptions, e.g. deformations of
a CFRD, and as an indication of developing problems, including marginal slope stability
and internal deformations due to softening or internal erosion and piping in the embank-
ment or the foundation.
Figure 20.10
shows design of surface settlement points suitable for dams.
It is useful to extend the survey to surrounding areas, particularly if they are affected by
slope instability.
In almost all cases, horizontal as well as vertical displacements should be measured, so
that movement vectors can be determined. Such vectors can often give a good indication
of the mechanism causing the displacement.
It is important to consider carefully where the survey markers are positioned, because
quite different movements occur in the upstream and downstream slopes, and between the
dam core and adjacent filter and rockfill zones.
To properly monitor movements, survey markers should be positioned centrally over
the earthfill core, on the upstream and downstream edges of the crest and at several levels
on the upstream and downstream slopes. Survey markers (at least those on the crest of the
dam) should be positioned over the whole length of the dam, not just one or two cross sec-
tions, because one of their most valuable outcomes is to detect local larger settlements.
Some survey markers should always be positioned where embankments abut spillway
concrete walls, because this is where local seepage, softening and abnormal deformation
is often a guide to developing problems.
For monitoring of soil landslides or earthfill dams, it will be necessary to embed the sur-
vey points below the depth of shrink and swell due to seasonal moisture change. This may
involve a system as shown in
Figure 20.11
, with the base at 2 m to 4 m below ground sur-
face and the pillar isolated from the upper soil by a casing.
ICOLD (1988b) indicate that survey of displacements is falling out of favour. The
authors' opinion is that this is unfortunate, and seemingly unnecessary, given the availabil-
ity of quick accurate electronic survey methods. Our experience is that survey data has
been very valuable in early identification of problems and has the great advantage of
being readily checked is some results appear inconsistent.
20.4.4
Pore pressures
The measurement of pore pressures in the embankment and foundations of a dam can
give vital quantitative information for use in assessing stability, potential “heave” condi-
tions in foundations and for identifying unusual seepage pressure, which may be a pre-
cursor to internal erosion and piping provided there is adequate coverage along the dam.
The following discussion gives an overview on why and where pore pressures are measured,
the types of instruments available, their characteristics and some practical factors. For more
detailed information on instruments, the reader is referred to Hanna (1985) and to manufac-
turers of the instruments.
20.4.4.1
Why and where are pore pressures measured?
There is a wide range of situations where it is necessary to measure pore pressures.
Examples are given to highlight some of the principles involved in locating the piezome-
ters to obtain meaningful answers.
Dam embankment.
Figure 20.12
shows piezometric conditions in an earthfill dam, con-
structed on an alluvial foundation, consisting of clay over sand. The ratio of horizontal to
