Environmental Engineering Reference
In-Depth Information
The cutoff trench was excavated through the limestone to the water table and a reinforced
concrete grout slab formed. A high-pressure grout curtain, with close hole spacing, was
adopted. The core was kept narrow to minimize the cost of excavation and foundation treat-
ment. McMahon (1986) gives more details of the project. See also Chapter 3, Section 3.7.2.
The Lungga Dam (
Figure 8.9
), in the Solomon Islands, was to have been constructed on
60 m of alluvial sand and gravel. A partially penetrating diaphragm wall was to assist in
reducing seepage flow in the foundation, but substantial seepage was still anticipated. The
available rockfill was relatively low permeability, so a substantial horizontal drain and
protective filters was incorporated into the design to cope with the very large anticipated
under seepage.
Blue Rock Dam (
Figure 8.10
)
is a fairly conventional central core earth and rockfill dam,
constructed on a foundation of mainly Silurian mudstones with thin interbedded sand-
stones. The rockfill was zoned to accommodate poorer quality rock in the random rockfill
zone. An unusual feature of the embankment is the use of reinforced shotcrete on the
upstream face for wave protection, as the available rock was of inadequate size and quality.
Hinze Dam (
Figure 8.11
) is a central core earth and rockfill dam which is founded on
greywacke, greenstone and chert. It is unusual (for a central core dam) in that it has been
designed to be constructed in three stages. This leads to a modified form of geometry, and
necessitates placing some rockfill under water.
Blue Rock and Hinze Dams, would be improved if the filters were taken higher to control
internal erosion and piping under flood conditions higher than anticipated by the designers.
Thomson Dam is 166 m high with a saddle dam on the right abutment.
Figures 8.12
and
8.13
show sections through the dam and saddle dam.
Each of the two dams is a rockfill dam with a central earthfill core. The cores are
flanked by processed chimney filters, a fine and a coarse filter on the downstream side, a
fine filter on the upstream side below 7 m below the crest and a coarse filter on the
upstream side in the top 11 m of the dam. The main dam has a partial processed two-
filter blanket under its downstream shoulder; the saddle dam has a full filter blanket
under its downstream shoulder. Both dams have riprap zones for wave protection on their
upstream faces. Both dams have multi-row grout curtains up to 80 m deep in places. The
grout curtain extends beyond the left end of the main dam and right end of the saddle dam
and between the dams to form a continuous seepage barrier.
A stabilizing fill up to 50 m high was placed upstream of the dam and saddle dam. This
is discussed in Section 2.10.3.5.
The sources of the various zones were:
Zone 1
“Impervious” cores.
Decomposed granite slope wash and residual
soil from borrow pits.
Zones 3A, 3C and 4
All rockfill except a small
Sandstones and siltstones from quarry
amount of initial main
on left bank. Initial placement
dam stabilizing fill.
in stabilizing fill came from
sedimentary rock right bank
quarry.
Zone 3B
Upstream face wave
Fresh granitic rock from right bank
protection (riprap).
quarry.
Zones 2A and 2B
Filters - process materials.
Mainly indurated sandstones from
quarry on right bank.
8.3.2.4
Sloping upstream core earth and rockfill dam
An example of a sloping upstream core dam is given in Figure 19.25. The section has been
designed to facilitate staged construction.
