July/August 2021 - 80

Installing downstream sheet pile cutoff wall
Completed south crest sheet piling and apron extension
performed using an assumed average
permeability of the encountered soils.
Analyses for below-dam seepage were
performed with a two-dimensional
model for sheet piles driven to depths
ranging from 15 to 30 ft (4.6 to 9.1 m)
below the top of the upstream concrete
cladding at 5 ft (1.5 m) increments.
Analyses for around-dam seepage were
performed using a three-dimensional
model with a similar approach, with
sheet piling being extended laterally to
a distance of up to 30 ft (9.1 m) beyond
the abutments at 10 ft (3 m) increments.
For the case of under-dam seepage,
the relative impact of each sheet pile
depth increment in decreasing flow
was evaluated in terms of the percentage
reduction in flow from that
modeled for the preexisting conditions,
while assuming that the existing dam
structure was impermeable for simplification.
The analyses showed that
sheet piling installed to depths of 15, 20,
25 and 30 ft (4.6, 6.1, 7.6 and 9.1 m)
would result in under-dam seepage
reductions of 40, 54, 62 and 67%,
respectively. As modeling for existing
flow conditions did not address that
there are likely shallow, isolated zones
that have experienced piping - and
therefore are likely to be significantly
more pervious than the surrounding
soils - it was expected that the calculated
80 * DEEP FOUNDATIONS * JULY/AUG 2021
percentage decreases in under-dam
flow for the shallower cutoff depths are
conservative. Specifically, it was
estimated that the 15 ft (4.6 m) long
sheet pile cutoffs would likely result in
a minimum 50% reduction in underdam
seepage.
Another consideration in selecting
the sheet pile length was awareness of
coarse gravel and cobbles existing
within the alluvial soils, and of boulders
that were observed to exist at least at the
surface. The longer the sheet piling, the
greater the chance of encountering
these oversized materials that might
damage the sheet pile interlock and
render the cutoff ineffective. Fifteen foot
(4.6 m) cutoffs were selected to minimize
the possibility of damaging the
interlocks. The estimated 50% seepage
decrease was considered to be in line
with prestorm event magnitudes, and to
reduce the seepage magnitude at the
dam to a level that could be considered
safe against a piping-related breach.
The around-dam seepage case was
evaluated for the cases of no lateral
cutoff, and then for the noted 10, 20 and
30 ft (3, 6.1 and 9.1 m) wide lateral
cutoffs. The bottom elevation of the
sheet piles was maintained from that of
the under-dam seepage analyses. The
analyses showed that a 10 ft (3 m) lateral
extension resulted in an approximately
23% reduction in around-dam seepage,
and that a 20 ft (6.1 m) lateral extension
would result in an approximately 58%
reduction in around-dam seepage. The
Completion of dam seepage control, without causing a visual impact
July/August 2021

Table of Contents for the Digital Edition of July/August 2021
