Control of seepage, internal erosion and piping for embankment dams - Geotechnical Engineering of Dams - page 432

Table 10.12. Influence of factors on the progression of erosion - likelihood of pipe enlargement (erodi-

bility) (Foster and Fell, 2000).

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 Dispersive soils Potentially dispersive Non-dispersive soils

soils

Dispersion test (3)

Pinhole D1, D2 Pinhole PD1, PD2 Pinhole ND1, ND2

4 dyne/cm 2

Critical shear stress Soils with c Soils with Soils with c 9

9 dyne/cm 2 dyne/cm 2

(Arulanandan and 4 c

Perry, 1983)
Compaction density Poorly compacted, 95% 95-98% standard Well compacted, 98%

ratio standard compaction compaction density ratio standard compaction

density ratio (1)

density ratio

Compaction water Dry of standard optimum Approx standard Standard optimum

content water content (approx. OWC-1% to OWC-2% or wet of standard

OWC-3% or less) optimum water content

Hydraulic gradient High Average Low

across core (2)

(1)

Notes: 93% Standard, dry of OWC, much more likely.

(2)

Even dams with very low gradients, e.g. 0.05, can experience piping failure. (3) PI

Plasticity index.
(3) Using Sherard Pinhole test.

Table 10.13. Mechanism of breach formation, failures by

piping through the embankment.

Mechanism of breach formation No. of failure cases

Gross enlargement of pipe 22

Crest settlement/sinkhole 3

Unravelling or sloughing 1

Instability 1

Not known 24

Total 51

Table 10.14 summarizes the influence of factors on these mechanisms. These are largely
based on a review of case studies, (Foster, 1999; Foster and Fell, 1999b) with some data
on unravelling and sloughing failures from Leps (1973), Olivier (1967) and Solvick
(1991).
As listed in Table 10.14, the nature of the downstream zone has a major influence on
possible breach mechanisms. If the downstream zone is free draining rockfill, large leak-
age flows can be sustained without breaching of the dam.
Table 10.15 lists some dams experiencing large flows without or before unravelling.
Free draining downstream zones are also very unlikely to experience slope instability
which may occur in low permeability materials even under modest seepage flows. The
ability of rockfill to experience large through flows can be assessed using the methods
described in Olivier (1967), Leps (1973) and Solvick (1991).