September/October 2021 - 18

The stem length was established by striking a balance
between the need to increase the moment arm of the
structural section and the practical limits of slurry trench
construction. The depth of the upstream diaphragm wall was
iteratively established to minimize deformations. The final
height of the most heavily loaded section was 130 ft (40 m),
and included a 35 ft (11 m) rock socket.
Bedrock at the site consisted of dolomitic and argillaceous
shales overlain by alternating beds of limestone and sandstone
with unconfined compressive strengths ranging from
approximately 3,000 to 9,000 psi (21 MPa to 62 MPa). Borehole
pressure meter testing was used to measure the in situ modulus
of the bedrock, which was critical for design of the wall.
The construction sequence for each T-shaped element
required three overlapping excavation passes, or " bites, " that
were nominally 10.5 ft (3.2 m) long by 5 ft (1.5 m) wide to
achieve the design geometry. The average verticality
achieved for the excavation was 0.38%, using a combination
of clamshell and hydromill excavation methods through soil
and rock, respectively.
The diaphragm wall reinforcing cages weighed up to 90
tons (82 m tons) each, with much of the reinforcement at the
end of each element's long stem. A 4,500 psi (31 MPa) tremie
concrete mix with an initial set time of 16 hours and
negligible bleed was developed for the extended duration that
was required to place concrete for each T-shaped element.
The heavily reinforced T-shaped cantilevered elements
eliminated the need for anchors to support the diaphragm
wall. In addition to retaining the embankment dam along the
side of the new intake channel, the wall was also utilized for
temporary excavation support during construction of the
intake structure and upstream penstocks.
To address risks associated with the intrusive excavations
into the existing dam, a second diaphragm wall was
constructed along the center line of the embankment. This 5 ft
(1.5 m) thick wall extends 100 ft (30 m) off the end of the dam's
concrete gravity monoliths and was designed to prevent
potential excavation-induced cracking of the embankment
dam from propagating through the dam (upstreamdownstream).
The second wall also mitigates against the
potential for internal erosion of the soil due to seepage that
can occur along such features.
Deep Excavation Design and Analyses: The construction of
the permanent works would require several excavations into
the existing dam and its foundation in immediate proximity
to the existing dam's spillway.
The design for the excavation support systems considered
differential loading due to the embankment dam slopes and
minimized deformations to maintain the integrity of the
existing dam during construction.
A nonlinear staged three-dimensional finite element
analysis was performed to design the excavation system for
the intake structure excavation, and to confirm that the
18 * DEEP FOUNDATIONS * SEPT/OCT 2021
stability of the existing spillway approach wall would not be
negatively impacted by the excavation. Similar analyses were
conducted for the two downstream penstock excavations,
which extended up to 70 ft (21 m) deep and consisted of
internally braced combi-walls (combination walls composed of
interlocked steel beams and sheet piles).
The excavation support was designed for the 2H:1V
(2 horizontal to 1 vertical) embankment cross-slope acting
across the penstock excavations. The bracing arrangement for
the downstream penstock excavations had to accommodate
the installation of 21 ft (6.4 m) diameter steel liners. This
required an open area up to 30 ft (9.1 m) high and wide at the
base of the excavation, which was accomplished by installing
temporary intermediate struts and then removing those
struts after the lowest level of struts along the bottom of the
excavation were installed. The irregular geometry and staged
construction were incorporated into the finite element
modeling that was used to design the penstock excavations.
An anchored secant pile wall was designed to retain the
downstream slope of the dam for the powerhouse excavation.
This wall was designed to minimize deflections that could
induce potential cracking in the embankment, and thus
effectively had to be designed for at-rest earth pressures
instead of active pressures.
Fall 2017 steel liner installation into penstock
September/October 2021

Table of Contents for the Digital Edition of September/October 2021
