ETL 1110-2-544
31 Jul 95
Figure 35. Section view through centerline of powerplant
c. Finite element model, code, and material
little under the structure itself. Contours of total head
properties. The program CSEEP (Tracy 1983) was to
for this case are shown in Figure 38. Most head drop
assess the uplift pressure on the structure from a plan
occurs along the walls outside of the powerplant
view model. Two simplifying assumptions were made
structure and downstream channel lining. The uplift
so that the problem could be solved as a 2-D problem.
pressure distribution along the centerline of the
First, it was assumed to be a confined flow problem.
structure is plotted in Figure 39 for this case. Uplift
The ground surface adjacent to the structure and
pressures resulting from the various cutoff wall
channels is covered by a natural clay and silt blanket or
permeabilities used in the parametric study are shown
in Figure 40. At the highest permeability of 10-2
by a man-made clay blanket. These make the surface
of the surrounding area highly impermeable, therefore,
ft/min, the distribution of head is nearly linear under
water will flow mainly from the upstream channel and
the structure. Another analysis performed in which the
the surrounding subsurface stratum under the structure
cutoff wall was modeled as impervious (zero
to the downstream channel. The second assumption is
permeability) gave results which were almost identical
to the results for 10-6 ft/min. The results of the analysis
that the seepage
occurs in one uniformly thick layer having a constant
clearly display the influence of the cutoff wall on the
permeability. Figure 36 shows the finite element mesh
uplift under the structure. The design permeability
used in the analysis. The boundaries were chosen so
makes the cutoff wall act as a relatively impervious
they would not unduly influence the seepage pattern
barrier causing water to flow around the structure
around the power plant. The boundary conditions
resulting in a longer flow path and reduced uplift
(shown in Figure 36) were selected to represent a
pressures under the structure. Conversely, with the
highest permeability, 10-2 ft/min, the wall hardly
piezometric level equal to the water level of the
Mississippi River. The extreme differential hydraulic
impedes flow at all because this value is near the same
order of permeability as the surrounding soil, 10-1
head conditions, 41 ft, were applied in these analyses.
The soil is considered to be homogeneous and isotropic
ft/min.
with a permeability of 0.14 ft/min. The design
permeability for the cutoff wall was 2 10-6 ft/min.
The effect of the cutoff walls on the seepage was of
4-5. Case History: Cerrillos Dam
particular interest with respect to the resulting uplift
pressures under the powerplant and downstream lining.
a. Project description. Palmerton (1993)
Several analyses were performed in which the
reported on a 3-D steady-state seepage analysis of
permeability of the cutoff walls was varied from 10-6 to
Cerrillos Dam near Ponce, Puerto Rico, for the U.S.
10-2 ft/min to determine the range of effectiveness of
Army Engineer District, Jacksonville. Cerrillos Dam
the cutoff walls in controlling uplift pressures.
is 323 ft high and has a crest length of 1,555 ft. The
dam consists of a central clay core, a grout curtain
d. Results. Figure 37 shows a vector plot of
extending to a depth of 200 ft, and upstream and
flow for the case of the cutoff walls having a
downstream rockfill shells with the appropriate
permeability of 2 10-6 ft/min. Most flow occurs
transition zones. The geologic structure near the dam
around the cutoff walls from upstream to downstream
is characterized by steeply dipping planar and parallel
with
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