Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 287

likelihood of which depends on the nature of the formations on the site and
their spatial arrangement.
It is in the immediate surroundings of the dam that the risk of
percolation is highest, since it corresponds to the shortest path, but it can
also affect deeper or more distant areas, when the arrangement of natural
discontinuities enables it (stratifi cation planes, foliation, diaclases, and
faults).
2.1 Headward erosion and internal erosion effects
This is a risk particularly associated with mobile formations, covering the
bottom and sides of a valley, and through which water circulation can occur
and gradually create favored channels, as well as washing out fi ne particles
(alluvium, colluvium, moraines, scree, bedrock weathering mantle).
It can also affect massive bedrock formations, when they are
heterogeneous and susceptible to degradation in the presence of water,
and then to allow headward erosion and the removal of material (sandy
sandstone, puddingstone, breccia, marly sandstone fl ysch).
These processes are sometimes accompanied by upward pressure
under certain particular conditions (favorable structures and very marked
permeability contrasts).
The 15 m high Bouzey gravity dam in the Vosges (France) was built in
1880, spanning 472 m along its crest.
Anchored entirely in Lower Triassic sandstone, its showed partial
deformation of its facing as early as 1884, at the same time as the increase
in discharge of the springs downstream (from 75 to 232 L·s -1 ).
An anchor block was built in the affected area in 1889, but the dam
failed on April 27th, 1895 (Lugeon, 1933; Gignoux & Barbier, 1955;
Letourneur & Michel, 1971). Apparently, the disaggregation into sand of
poorly consolidated sandstone layers favored the migration of fi ne-grained
particles by internal erosion, and the creation of pressures that the dam
could not support.
The Cedar River dam near Seattle (USA) was built on moraine deposits.
When the reservoir was fi lled in 1918, water seeped into gravelly beds,
prompting internal erosion. A landslide at the base of the dam then caused
its partial failure (Letourneur & Michel, 1971).
The Cheurfas dam near Oran (Algeria) was built in 1880 on the Oued
Mebtouh. It rests on highly tilted Tortonian (Miocene) limestone on the left
bank, but its right bank consists of ancient alluvium and reworked slope
deposits masking the underlying limestone bedrock (Figure 133).