Environmental Engineering Reference
In-Depth Information
the ability of Zone 2D to self heal against Zone 2E in the event of a leak). Zone 2E is
compacted with a few passes of a 5 tonne non vibrating roller. The face is protected either
by spraying with an asphaltic emulsion or forming a concrete kerb. Fitzpatrick et al. (1985)
indicate that on earlier dams the HEC applied a 2 coat bitumen and chip seal, but for
Mackintosh Dam the stabilization was applied after rolling and consisted of a single coat
of bitumen emulsion with a cover of sand. Shotcrete has been used instead of bitumen
emulsion; e.g. on Golillas and Salvajina dams 40 mm of shotcrete with size 9 mm to
19 mm in maximum size was adopted. At Altamina Dam (Amaya and Marulanda, 2000)
an extruded concrete kerb was constructed at the edge of each fill layer to limit runoff
over the upstream face. This is economical and allows higher construction rate. A similar
concrete kerb was used successfully during the construction of Ita dam in Brazil (Sobrinho
et al., 2000). Construction methods for the concrete kerb are described in Materon and
Mori (2000).
Zones 3, 3A and 3B rockfill are placed using truck and dozer in layers from 0.8-1.0 m
in Zone 3A and 1.6-2.0 m in zone 3B. Zones 3A and 3B using gravel are built in a simi-
lar way with smaller layer thicknesses of 0.6-1.2 m (Materon and Mori, 2000). 1% to
20% of water by volume may be specified for rockfill. Water is usually not specified for
gravel but should be if it aids compaction. At Xingo Dam in Brazil rockfill in the down-
stream portion of the dam was successfully dumped into water before the diversion of the
river. However this is likely to give a relatively low modulus fill compared to compacted
rockfill.
15.4.4
Face slab construction
The concrete face slab is cast using a slip-form, except for the trapezoidal or triangular
starter slabs adjacent to the face slabs, which are screeded by hand methods ahead of the
main face slab to provide a starting plane for the slip-form. These are usually half the
width of the main slabs.
The slip-form is the full width of the panel. For a 1.3 m screed width, the form
can move at 2 m to 3 m per hour placing 60 mm slump concrete (Cooke, 1984). Higher
slump concrete requires a wider screed.
Figure 15.28
shows the slip form used for Khao
Laem Dam.
Varty et al. (1985) give details of steel placement and other construction factors, as
practiced on HEC dams.
Concrete is delivered to the form by bucket, pumping or in a chute.
The concrete used is typically specified as between 20 MPa and 24 MPa at 28 days,
(ICOLD, 1989a), although higher strengths are used, e.g. Cethana and Lower Pieman had
average 40 MPa concrete (Fitzpatrick et al., 1985). (ICOLD, 1989a) indicate that higher
strengths are not desirable as more shrinkage cracking is likely. They suggest the use of air
entrainment to enhance water tightness and durability. The soundness and reactivity of
aggregates used for the concrete is important. Materon and Mori (2000) give some more
recent experiences which are similar to those described above.
The face slab is constructed on the face as it is presented. The face may have moved sub-
sequent to trimming and compaction, due to the continual raising of the dam. The slab
may therefore not end up being on a single plane, but the small variations do not affect
performance or appearance.
The design thicknesses are minimum values, and average thicknesses are likely to be
50 mm to 75 mm greater. Early practice was to require that the face slab not be constructed
until the rockfill placing was virtually completed so as to minimise post construction
movements. More recently it has been shown that staging of the concrete face slab is
acceptable, e.g. Salvajina and Foz do Aeria dams (Sierra et al., 1985 and Pinto et al., 1985),
but cracking may occur if the zones have differential moduli leading to localisation of
