Environmental Engineering Reference
In-Depth Information
Table 10.18.
Factors influencing the enlargement of the pipe, piping through the foundation - flow
limitation (Foster, 1999, Foster and Fell, 1999b).
Influence on likelihood of pipe enlargement
Factor
More likely
Neutral
Less likely
Hydraulic gradient
(1)
High
Average
Low
Filling of “cracks” or
Homogeneous zoning or
Cohesive layer
Zoned type dam with
voids by washing in
upstream zone of cohesive
upstream of the
gravel or rockfill
of embankment or
material
dam (may crack)
upstream shell
foundation materials
Low permeability
High permeability
cohesionless foundation
layer upstream of dam
layer upstream of the dam
Restriction of flow path
Flow path unrestricted
Flow path of small
dimensions, or flow path
restricted width (e.g.
restricted but large
piping through cracks
dimensions (e.g. large
in cutoff walls or
solution channels in
narrow rock joints)
limestone)
(1)
Even dams with very low overall gradients across the foundation, e.g. 0.05, can experience
piping failure.
Note:
embankment/foundation interface. However, there appear to be no examples of this occur-
ring in the foundation piping failures.
From consideration of case studies Foster (1999) concluded that to form a pipe foun-
dation soils needed a high, more plastic fines content than embankment soils. The appar-
ent difference in behaviour in the foundation and the embankment is possibly due to
differences in the degree of saturation of the soil.
Foster (1999), Foster and Fell (1999b) concluded that the formation of an open pipe is
more likely in the foundation if any of the following features are present:
(i) The erodible material is cohesive;
(ii) There is cohesive material overlying the erodible material. Examples are layers of
clay, cemented soil or rock overlying erodible soil or interbedded cemented and non-
cemented layers;
(iii) Solution features in rock, for example solution channels or cavities in limestone filled
with erodible materials;
(iv) The erodible materials are below a rigid structure such as a concrete dam, concrete
spillway structure or below an outlet conduit.
The factors influencing enlargement of a pipe in the foundation are summarised in
Table 10.18 for factors related to mechanisms of flow limitation and
Table 10.19
for fac-
tors influencing the erodibility of the foundation soils.
Zoning is an influencing factor for the “crack filling” mechanism in the foundation, as
piping may progress back into the upstream zone of the dam. If the upstream zone is com-
prised of well-graded material (i.e. gravel or rockfill), washing in of the material may clog
the pipe. Foundation soil layering upstream of the dam is also influential as it may limit
flows into the area of piping.
The factors influencing erodibility are assumed to be essentially the same as those for
piping through the embankment except the compaction characteristics are replaced with
density (for cohesionless soils) and consistency (for cohesive soils).
