Environmental Engineering Reference
In-Depth Information
10.7.1.3
Progression to formation and enlargement of a pipe
There are two issues affecting the progression of piping through the embankment:
- The ability of the soil to support a roof of the pipe, i.e. will the pipe remain open or collapse;
- Enlargement of the hole i.e. will the pipe enlarge and how quickly?
Tables 10.10, 10.11 and
10.12
summarize the factors which influence this. These are
based on the literature, including Sherard (1953), Terzaghi and Peck (1967), Sherard et al.
(1972a and b), Arulanandan et al. (1975), Vaughan and Soares (1982), Arulanandan and
Perry (1983), Sherard and Dunnigan (1985), Chapius and Gatien (1986), Peck (1990),
Bickel and Kuesel (1992), Fell et al. (1992), Hanson (1992) Hanson and Robinson
(1993), Charles et al. (1995), Von Thun (1996), and a review of case studies (Foster, 1999,
Foster and Fell, 1999b). The review of case studies showed that fines content was the crit-
ical factor in whether a roof could be supported. Soils with fines content
15% were
found to be able to hold a roof, even if the fines were cohesionless.
10.7.1.4
Formation of a breach mechanism
Table 10.13
summarizes the mechanics of the breach for failures involving piping through
the embankment (Foster, 1999, Foster and Fell, 1999b).
Table 10.10.
Influence of factors on the likelihood of progression of erosion - ability to support a roof
(Foster and Fell, 2000).
Influence on likelihood of fill materials supporting
a roof of a pipe
Factor
More likely
Neutral
Less likely
Fines content (% finer
Fines content
15%
Fines content
15%
No fines or fines content
than 0.075 mm)
and
5%
5%
Degree of saturation
Partially saturated
Saturated
(first filling)
Table 10.11.
Influence of factors on the progression of erosion - enlargement of the pipe (limitation of
flows) (Foster and Fell, 2000).
Influence on likelihood of pipe enlargement
Factor
More likely
Neutral
Less likely
Action of filter
Considered in assessment of filter performance.
downstream of core
Filling of cracks by
Homogeneous zoning.
Zone upstream of core
washing in of
Upstream zone of
capable of crack filling
material from
cohesive material
(cohesionless soil)
upstream
Restriction of flow
Homogeneous zoning.
Medium to high
In zoned dam, medium to
by upstream zones or
Very high permeability
permeability zone
low permeability granular
concrete element in
zone upstream of core
upstream of core
zone upstream of core.
dam
Central concrete corewall
and concrete face rockfill
dam
