Environmental Engineering Reference
In-Depth Information
15.3
CONCRETE FACE
15.3.1
Plinth
The principal purpose of the plinth (or “toe-slab”) is to provide a “watertight” connec-
tion between the face slab and the dam foundation. The plinth is usually founded on
strong, non erodible rock which is groutable, and which has been carefully excavated and
cleaned up with a water jet to facilitate a low permeability cutoff. For these conditions the
plinth width is of the order of 1/20 to 1/25 of the water depth (ICOLD, 1989b; Cooke and
Sherard, 1987). Marulanda and Pinto (2000) suggest 1/10 to 1/20 depending on rock con-
ditions. Up the dam abutment, the width is changed according to the water head. This is
done in several steps (not gradually) for construction convenience. The minimum width
has generally been 3 m, although Cooke & Sherard (1987) suggest that for dams less than
40 m high on very good rock, 2 m could be used. For poorer rock conditions a wider
plinth and/or other erosion control measures may be used. This is discussed in Section
15.5.2. Cooke (2000) and Marulanda and Pinto (2000) indicate that a recent evolution in
design is to reduce the length of the plinth to say 4 m to 5 m, and maintain an overall seep-
age gradient beneath the plinth by using an undowelled reinforced slab under the rockfill.
This can give economies in rock excavation.
The minimum plinth thickness is usually between 0.3 m and 0.4 m, but may be up to
0.6 m for the lower plinths of high dams. The actual thickness is usually more because of
the need to fill over-excavation, and to make up for irregularities in the topography.
Where this extra concrete is significant it is common to construct the plinth in two stages;
the first stage is to fill the irregularities.
Figure 15.13
and
Figure 15.14
show plinth designs for Mangrove Creek, Boondooma,
Cethana and Reece dams. These designs are typical for CFRDs.
It is necessary to ensure that the plinth is stable under the imposed forces. For a plinth
of normal thickness there is adequate friction resistance on the base, unless there are
unfavourably oriented low strength bedding planes, joint or shears in the foundations.
The plinth is usually anchored to the rock with grouted dowels, which are generally 25 mm
to 35 mm diameter, reinforcing steel bars, 3 m to 5 m long and are installed at 1.0 m to
1.5 m spacing. Cooke (2000) indicates 25 mm bars at 2 m spacing are adequate, that the
anchor design is usually empirical, and that the bars are grouted full length into the rock
and hooked on to the layer of reinforcing steel in the plinth. The anchors are provided
nominally to prevent uplift during grouting, although Cooke and Sherard (1987) claim
uplift will not develop in most cases.
For plinths which are thicker than normal, due to overbreak or irregularities in the
foundation, the stability of the plinths should be analysed assuming the uplift pressure
under the slab is zero at the downstream toe and varies linearly to full reservoir head at
the upstream toe. No support should be assumed from the face slab on the understanding
that the perimetric joint may have opened, and no support from the rockfill should be
allowed for, since significant displacement into the rockfill would be necessary to mobi-
lize the resistance (Marulanda and Pinto, 2000).
The plinth must be stable against sliding and overturning. A proper assessment of the
sliding friction angle should be made depending on the rock type, orientation of weak
planes etc, not just a check against some arbitrary “sliding factor” or assumed friction
coefficient.
It may be necessary to install additional buttress concrete downstream to maintain stability.
If the plinth is high, a possible further problem is that excessive settlement of the face
slab may occur and disrupt the perimetral joint. Problems were encountered in Golillas
Dam due to such movements (in this case the valley sides were near vertical), (Amaya and
Marulanda, 1985). Particular attention should always be paid to compacting the rockfill
