Control of seepage, internal erosion and piping for embankment dams - Geotechnical Engineering of Dams - page 438

Table 10.19. Influence of factors on likelihood of pipe enlargement, piping through the foundation -

erodibility. (Foster 1999 and Foster and Fell, 1999b).

Influence on likelihood of pipe enlargement

Factor More likely Neutral Less likely

Soil type Very uniform, fine Well graded material Plastic clay (PI 15)

cohesionless sand (PI 6) with clay binder

Well graded cohesionless (6 PI 15)

soil (PI 6)

Pinhole dispersion Dispersive soils, Pinhole Potentially dispersive Non-dispersive soils,

test (AS1289.3.8.3) D1, D2 soils, Pinhole PD1, PD2 Pinhole ND1, ND2

Relative density Loose Medium dense Dense

Consistency Soft Stiff Very stiff

10.7.2.4 Formation of a breach
Most cases of failure through the foundation where the breach mechanism was known,
failed by gross enlargement of the pipe, leading to emptying of the reservoir without
breaching the crest. Two cases led to settlement of the crest and overtopping. The factors
influencing the formation of a breach mechanism are generally similar to those as for pip-
ing through the embankment, described in Section 10.7.1.
Gross enlargement. The factors are similar to those for piping through the embankment
shown in Table 10.14(a) . However, dam zoning would only be important if the pipe exits
through the downstream zone of the dam. Gross enlargement is likely if there is continu-
ing enlargement of the pipe and the roof of the pipe can be supported along its full length
and these factors are considered in the preceding branches of the event tree.
Sinkhole or crest settlement. The assessment of breach by sinkhole or crest settlement
leading to overtopping is assumed to be similar to that for piping through the embank-
ment (Table 10.14b). However, piping can also occur through the abutments of the dam
and therefore the freeboard of the abutments would be important in this situation.
Unravelling. The likelihood of unravelling or sloughing initiating is related to the dis-
charge capacity and characteristics of the soil at the exit point of seepage. It is assumed
the assessment is similar to piping through the embankment (Table 10.14c), except that
the characteristics of the soils at the exit point of seepage need to be considered. For pip-
ing through the foundation, seepage may exit through the dam, downstream of the dam
or on the abutments.
Slope instability. The assessment is assumed to be similar to that for piping through the
embankment, except the characteristics of the foundation soils (in terms of permeability
and loss of strength on shearing) need to be considered as well as the embankment mate-
rials. Like unravelling, slope instability may also be an issue on the abutments of the dam.

10.7.3 Piping from the embankment to the foundation

10.7.3.1 Initiation of erosion
Factors which would influence the likelihood of initiation of erosion are:

- The presence of open joints or coarse soils in the foundation against which the core is
placed;
- The particle size distribution of the core material compared to these joints or coarse
soils;