Monitoring and surveillance of embankment dams - Geotechnical Engineering of Dams - page 827

Table 20.4. Influence of factors on the progression of erosion - likelihood of pipe enlargement (and rate of

erosion) (adapted from Foster and Fell, 1999b, 2000; Fell et al., 2001, 2003).

Influence on likelihood of pipe enlargement

Factor More likely Neutral Less Likely

Embankment and Foundation
Hydraulic gradient

High Average Low

across core (2)
Soil type

Very uniform, fine Well graded l Plastic clay (PI 15)

cohesionless material with

sand. (PI 6) clay binder

or Well graded (6 PI 15)

cohesionless soil.
(PI 6)

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

Dispersion test (4) pinhole D1, D2 pinhole PD1, PD2 pinhole ND1, ND2

Embankment
Compaction density Poorly compacted, 95-98% standard Well compacted, 98%

ratio 95% standard compaction density standard compaction

compaction ratio density ratio

density ratio (1)

Compaction water Dry of standard Approx standard Standard optimum or wet

content optimum water OWC-1% of standard optimum

content (approx. to OWC-2% water content

OWC-3%
or less)

Saturation (5) As-compacted, Saturated after

partially compaction

saturated

Foundation
Relative density or Loose Medium dense Dense

Consistency Soft Stiff Very stiff

Erosion Rate Erosion Rapid or Very Rapid if most factors are “more likely” slow if most factors

are “less likely”, and intermediate, if most factors are “neutral” or there is a mix of
“more” and “less likely”.

Notes: (1) 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. (4) Using Sherard Pinhole Test. (5) Based on tests at

UNSW.

Table 20.5. Influence of factors on the progression of erosion - limitation of flows by upstream zones (Foster

and Fell, 1999b, 2000; Fell et al., 2001, 2003).

Influence on likelihood of upstream flow limitation

Factor Unlikely Neutral Likely

Filling of cracks by Homogeneous zoning. Zone upstream of core capable

washing in of material Upstream zone of of crack filling (cohesionless

from upstream cohesive material soil)

Restriction of flow by Homogeneous zoning. Medium to high In zoned dam, medium to low

upstream zones or Very high permeability permeability zone permeability granular zone

concrete element zone upstream of core upstream of core upstream of core. Central

in dam concrete corewall and concrete

face rockfill dams