- the consequencies of uncontrolled internal seepage;
- parameters controlling the stability of slopes;
- deformation and settlement parameters;
- design flood estimation, overflow capacity and freeboard;
- internal interface effects.
Internal erosion, with migration of core material, plus an undesirably high
phreatic surface in the downstream shoulder, are obvious outcomes of 1.
Problems arising from the absence of internal drainage and protective
filters are difficult and costly to remedy. The only practicable shorter-term
response may be a limit on retention level and increased surveillance.
Remedial options based on sophisticated geotechnical procedures
to install drains or reconstruct a damaged core may then be evaluated.
Major rehabilitation work on a defective rolled-clay core seriously
damaged by erosion following hydraulic fracturing is described in
Vaughanet al., 1970.
Inadequate stability of the downstream slope is a relatively straight-
forward problem. The primary causes are either an excessively steep slope
or a high phreatic surface. The problem can be resolved by construction of
a downstream berm of free-draining fill, e.g. a rockfill.
Excessive core settlement and loss of freeboard can be indicative of
ongoing consolidation processes. Raising the core to restore freeboard is a
common procedure. In a more extreme case at Ladybower, a UK dam
where core settlement in excess of 1.2 m was recorded over the 50 year life
of the 45 m high dam and was continuing, a major re-construction and
upgrading of the dam which included re-profiling the downstream shoul-
der was undertaken (Vaughan, Chalmers and Mackay (2002)).
The flood discharge capacity at many older dams has been shown to
be inadequate. Considerable effort has therefore been expended on recon-
structing and upgrading overflow and outlet works on older dams, an exer-
cise which is costly and, on a congested site, is very difficult. An alternative
option is installation of an auxiliary overflow which can take one of several
forms, e.g. a separately located erodible ‘fuse-plug’ (Dan, 1996) or con-
crete flap-gates operated by hydrostatic pressure.
Interface effects are principally associated with discontinuities and
strain incompatibilities, e.g. at the interface between clay core and coarser
shoulder material or an outlet culvert, or at a step in the base of a deep
cut-off trench. They remain a complex aspect of dam behaviour, and while
amenable to resolution in the design of a new structure they are seldom so
in the case of old dams; total reconstruction is then the only option.
Recent examples of major upgrading or reconstruction works are
fully described in Vaughan, Chalmers and Mackay (2000), and in Banyard,
Coxon and Johnston (1992). Further descriptions of such work are pre-
sented in Macdonald, Dawson and Coleshill (1993) and in Chalmers,
Vaughan and Coats (1993).
110 EMBANKMENT DAM ENGINEERING
