Environmental Engineering Reference
In-Depth Information
Table 20.3.
Influence of factors on the likelihood of progression of erosion-ability to support a roof
(Foster and Fell, 1999b, 2000; Fell et al., 2001, 2003).
Influence on likelihood of fill or foundation materials supporting
the roof of a pipe
Factor
More likely
Neutral
Less likely
(a) Embankment materials
Fines content (% finer
Fines content
15%
Fines content
15%
No fines or fines
than 0.075 mm)
and
5%
content
5%
Degree of saturation
Partially saturated (first
Saturated
filling)
(b) Foundation materials
Piping through soils with
Well graded sand
Homogeneous,
cohesive fines
and gravel
cohesionless sands
Cohesive layer overlying
piped material
Piping through open
features in rock
Piping below rigid
structure (e.g. spillway)
breach enlargement which is used in estimates of dam-break floods. It also does not equal
the available warning time to evacuate persons at risk, since this is dependent on whether the
dam is under surveillance at the time of piping, and the measures set in place to advise per-
sons downstream of an impending dam failure.
The times for initiation and continuation of erosion are dependent on the initiation
mechanism, and the ability of filters, if present, to control erosion. Fell et al. (2002, 2003)
have made a judgemental assessment of the usual rates of development and ease of detec-
tion. These are summarized in
Tables 20.6
to
20.9
.
The classifications for ease of detection assume the dam is well instrumented to measure
seepage and pore pressure, readings are frequent and the dam is subject to regular inspec-
tion by experienced personnel.
Fell et al. (2001, 2003) conclude that:
(a) Monitoring of seepage, either by visual surveillance, or measurement, is the most
common means of identifying whether internal erosion and piping have occurred.
(b) It is not common to have sufficient change in the seepage, or in other factors such
as pore pressure changes or settlement, to identify conclusively that internal erosion
has initiated and is continuing. It is more common to recognize when the erosion has
progressed to the stage that a pipe has developed, or that there are changes in pore
pressures, seepage or settlement which may be related to internal erosion, but this is
not conclusive and it may reflect other factors. These changes may be a pre-cursor to
a higher likelihood of internal erosion and piping, so it is important they are observed
and, when they occur, investigated.
(c) The inability to detect that internal erosion has initiated relates to the common mech-
anisms of initiation. For piping in embankment failures, initiation is most common in
cracks, high permeability zones or hydraulic fractures in the embankment or around a
conduit. These mechanisms could be expected to initiate very rapidly or rapidly once
the reservoir level reaches the critical level at which erosion begins in cracks or high
permeability zones or the critical level at which hydraulic fracture initiates. However
this depends on the rate of erosion of the soil.
