Embankment Dam Studies
A paper by
Robert B. Jansen, consulting civil engineer (former chairman of USCOLD (now US
Society on Dams) and director of design and construction for the USBR and the California
Department of Water Resources) on “Dangers at Embankment Dam Boundaries and Embedments,” discusses the variety of issues that might be
experienced by an embankment dam built circa Exchequer Dam in the
mid-1960s. Mr. Jansen is a member of the
National Academy of Engineering, and he has authored and edited books on dams,
including Advanced Dam Engineering and Dams and Public Safety.
“Significant numbers of embankment dams have failed from either external
or internal attack. The threat of inevitable floods and earthquakes, which give
few useful signs of their approach, is different from that of the
already-present but unseen – and often more immediately dangerous – flaws
within a dam and its foundation.
“Embankments differ from other dams in their composition of deformable
natural materials that are largely inaccessible once embodied in the fill. Behaviour of earth and rockfill zones will be more
predictable if they are free of penetrations or other encumbrances. Facilities
located upon, against, or inside the dam body add to uncertainties and thereby
to the possible modes of failure. Dam engineering requires reducing unknowns to
a minimum.
“The long history of earthfills and rockfills shows that many of their problems were spawned by
designers through inadequate safeguarding of embankment boundaries and
introduction of unreliable components into the hearts of the dams.
Vulnerability may be found at internal zone limits, foundation contacts,
structural faces, or works buried in the fill. An embankment should be zoned
for internal stability and capable of conforming naturally to its site without
appurtenances or foundation characteristics that might hamper its behaviour.”
His paper
points to early issues at Exchequer Dam:
“Damaging settlement and cracking has occurred at concrete facings on
rockfill dams built before the 1970s by dumping and sluicing in high lifts.
More recent rockfills compacted by vibratory rollers
and given improved zoning and face slab and plinth details have performed much
better. Methods for repair of faces on the early dams have evolved from
concrete patching to filling of cracks with expansive mortar, mastic, or clay-bentonite slurry to rubber, shotcrete,
or synthetic membrane overlays….
“Some embankments built in the 20th Century as replacements for old dams
incorporate parts of the earlier works. The design of
California
’s 490ft high
(149m) New Exchequer Dam, a concrete-faced rockfill constructed in 1964-66,
used its predecessor, a 326-ft-high (99m) gravity dam, as the upstream toe
block. An ineffective flexible asphalt-impregnated joint seal was provided
where the slab met the downstream face of the old dam 185ft (56m) above its
base. Settlement of the rockfill caused severe separation and spalling of slab joints and leakage reaching a maximum of
about 490cfs (14m3/sec) in 1967. Repairs in that year were concentrated on
sealing the opened joint between the two dams by underwater placement of bentonite-enriched earthfill. This was effective with later
replenishment of lost material and addition of a geotextile
blanket reducing total leakage by 99%. However, as the dam continued to settle,
other face joints needed remedial work, which has included concrete filling,
new flexible waterstops, and membrane covering.”
Mr. Jansen
addresses the issue of remediating inadequate spillways, such as would be
created by raising the crest level of Lake McClure, which, therefore,
necessitates MID’s Spillway modification project.
“When spillways for embankment dams are inadequate, safe and economical remedial
alternatives may be limited. If the present facilities cannot be enlarged, the
additional capacity preferably should be developed by works located away from
the dam. At some projects, however, embankments have been armoured
for overtopping. This previously objectionable practice has won some
endorsement for infrequent discharges over lesser dams, partly because of the
increased quality of roller compacted concrete. The safety of overlays for
higher embankments remains unproven. Caution is required in evaluating the
possible effects of high velocities, overflowing debris, settlement, cracks, underseepage and uplift, and sliding on slopes. Armour must have underdrains that
are kept clean by effective filters. Protection must also be provided against erosion
of the dam at the overlay edges and in the approach area of the embankment
slope immediately upstream. Safe dissipation of the discharge energy at the toe
of the dam would be imperative if this kind of design is pursued for large
structures.”
It is unclear
in our research as to whether the toe of Exchequer Dam has been engineered for
the additional 183 billion lbs of water weight and pressure that the increase
in crest elevation would permit.
Mr. Janson’s paper discusses new science with regard to use of geosynthetics to halt seepage, and the potential for lack
of durability of such retrofits:
“Flexible synthetic sheets have been used increasingly in embankments
for seepage control. In dams typically intended to last hundreds of years, the
durability and service life of these embedded manufactured products are of
principal concern. Survival depends on the materials, the manner of placement,
the function of the installation (e.g., zone divider, filter, drain,
reinforcement), and the effects of embankment deformation and cracking. Geosynthetics implanted in an earthfill become integral
parts of the dam, usually without access for inspection or ready repair. They
must be resistant to the various forces and processes that cause their
deterioration. Clogging of filters and drains may reduce their effectiveness to
intolerable levels.
“Internal geosynthetics constitute
discontinuities that may have lower shearing resistance than the neighboring
zones of natural materials. Some multilayered systems may undergo differential
slipping of the sheets. Synthetics can fail if their junctions with the dam
foundation are deficient. Hydraulic gradients at that contact are high, so
leakage could escape at damaging velocities.
“Use of synthetics as permanent members in an embankment’s zoning has
been under way for little more than thirty years and, while much has been
learned, the relatively short experience has not resolved all questions as to
dependability and durability. Embedment of geotextiles
as internal filters in big dams is not generally accepted as a satisfactory
alternative to well-designed composites of hard sands and gravels, which have
been thoroughly proven in long-time service.”
“Synthetics have also been placed on the upstream slopes of embankments,
and as liners for reservoirs (e.g., San Joaquin Reservoir,
California
, 2004). Such
elements can be accessible for inspection and repair, during drawdown or by
divers, which is an advantage not shared by deeply buried synthetics. Access is
also a merit of lightly covered geotextiles in
drainage works.”
Introduction Page – HR869 & HR2578
MERG Current Thoughts on HR 869
& HR 2578
New Exchequer Dam and
Lake McClure
FERC Project Boundary Question
The FERC Relicensing
Argument
Spillways
at Lake McClure
Dam Safety
Embankment Dam Studies
Is Raising
the Dam Crest Elevation Unlawful?
Known Geologic Issues
The
Effect on the Limestone Salamander
Other Options for Additional
Water Storage
MERG Early Thoughts on HR 869
