Environmental Engineering Reference
In-Depth Information
1986; McMahon, 1986). One of these zones was locally deepened by up to 8.5 m and
backfilled with concrete. Throughout the limestone sections the base of the trench was
covered by a reinforced concrete slab anchored into the rock. Cavities exposed on the
downstream face of the trench were sealed with concrete. A single row grout curtain
below this grout cap used 32 m deep vertical holes at 0.5 m spacing. Each 8 m stage,
before testing and grouting, was flushed by high pressure air and water until the wash
water was substantially clear. McMahon (1986) believes that this washing was unsuc-
cessful. Relatively high grout pressures were used (3 times the reservoir head), aimed at
achieving hydraulic fracturing of the clayey infill materials, like that reported by Zhang
and Hao (1982).
The general foundation levels for the rockfill shoulders were reached by scrapers. In the
limestone areas the exposed rock was essentially strong mounds and pinnacles separated
by clay-filled depressions. These were cleaned out to the depth at which the clay filling
was less than 1 m wide and backfilled with high quality rockfill.
At sites where cavities are numerous and/or large and partly or wholly filled with clayey
soils, cement grouting alone is not usually relied on to form the cutoff. Other methods
have included
- Cleaning out of some individual caves and backfilling with concrete;
-Walls formed by mining out slots of cavernous rock and backfilling with concrete;
- Diaphragm walls comprising overlapping boreholes backfilled with concrete;
- Closely spaced drilled holes, washed out with compressed air and water and backfilled
with high-slump mortar, poured in and needle vibrated.
The last three methods were used, together with cement grouting, at the 85 m high,
1050 m long concrete face rockfill Khao Laem dam in Western Thailand. At the dam they
were used to construct the upper 20-60 m of a curtain which extended up to 200 m (but
generally less than 100 m) below the plinth. In one section, high pressure flushing with air
and water was used to flush out sandy silt (weathered calcareous sandstone) from cavities
and open joints before grouting.
Another curtain, 3.5 km long and up to 200 m deep, was constructed beneath the right
abutment ridge, which was formed entirely by cavernous limestone, with its water table
below the proposed storage level. All four methods were used in parts of this curtain,
together with cement grouting. Lek et al. (1982) and Somkuan and Coles (1985) describe
details of the methods.
3.7.2.1
Dams which have experienced significant leakage problems
which have never stored water because the leakage rates exceeded the inflow. Two
recently completed dams which have recorded very high leakages are discussed below.
Attaturk Dam.
Riemer et al. (1997) describe cavities met during the planning and con-
struction of this 179 m high, 1800 m long rockfill dam in Turkey, completed in 1990. The
site is formed by a folded sequence of
Category O
limestones including marly, cherty and
bituminous units. The rock substances are moderately strong to weak, dense and imper-
meable. The rock mass is cut through by many steeply dipping faults and joints. Solution
along these has resulted in a network of chimneys and near-horizontal tunnels. Despite
the use of many adits and drill holes, the design stage studies failed to indicate the magni-
tude of this cavity network and of the grouting program required.
The main 3-row grout curtain is 5.5 km long and extends generally 175 m below the
foundation with a local extension down to 300 m. Even at these depths the curtain is
described by Riemer et al. (1997) as “suspended”. An elaborate monitoring program
involving 3.6 km of adits and about 300 piezometers has indicated seepage discharge rates
