September/October 2021 - 19

Secant pile wall for powerhouse construction (at front), with diaphragm wall cage being lifted for upstream placement (background)
Due to the intrusive nature of the large excavations, the
designs needed to consider construction staging to reduce
potential impacts to the dam's integrity and maintain dam
safety throughout construction. As a result, construction
sequence drawings were developed to prescribe the
upstream and downstream work. The construction sequence
included coordination with the design of the braced
excavations, cofferdams and other temporary structures. The
sequence defined by those drawings became part of the
design criteria for the excavation support systems and the
basis of load cases for design of the permanent structures.
Construction
Excavation for the 7 ft (2.1 m) deep braced intake structure
was supported by the upstream diaphragm wall along the
embankment dam side of the excavation. The other three
sides were supported by 5 ft (1.5 m) diameter secant pile
walls. The primary and secondary piles were both reinforced.
The piles were keyed into rock along the upstream and
downstream sides of the excavation and notched into the
backside of the existing spillway approach wall along the
spillway side of the excavation. The intake excavation was
braced by steel walers and pipe struts. The struts were
arranged to avoid interfering with concrete forms for the
water passages through the intake structure.
A differing site condition consisting of cobbles within the
embankment fill was encountered during installation of the
penstock combi-wall. The project team modified the combiwall
system to incorporate elements of a traditional lagging
wall, and deep soil mixing was locally used to provide a cutoff
so that progress could continue.
The upstream side of the powerhouse excavation was
formed by a 63 ft (19 m) high anchored secant pile wall, which
retained the downstream slope of the dam above a 40 ft (12 m)
high rock cut. The rock anchors for this wall were installed at
a 45-degree angle to avoid encroaching on the concrete
gravity dam monoliths and the impervious fills that wrap
around the end of the monoliths.
Treatment of Solutioned Gypsum Deposits: Prior to the
rock excavation for the powerhouse, grouting was performed
around the perimeter of the excavation site, primarily to
strengthen the rock mass but also to reduce seepage inflows
during construction. For much of the perimeter, the rock was
fairly tight, and grout takes were relatively low. However, for
a 160 ft (49 m) extent along the land-side of the excavation,
extensive grouting was required to treat a 2 to 5 ft (0.6 to 1.5 m)
thick zone characterized by artesian flows from grout holes.
The flows were on the order of 20 gallons per minute (gpm, or
75 L/min) or higher from a voided zone in the rock that was
partially filled with sand, clay and rock fragments. The
voided zone was assessed to have been created by a
previously undetected water-soluble evaporite (gypsum
anhydrite) deposit.
Communication was observed between widely spaced
grout holes during drilling in this area, indicating that there
were indirect hydraulic connections in rock across distances
of 50 ft (15 m) or more. Despite the large grout takes, return
was seldom seen through neighboring grout holes, even
when closely spaced.
The treatment of this solutioned zone ultimately entailed a
combination of thick cement grout and polyurethane grout,
with holes spaced as close as 18 in (45 cm) on-center; two additional
rows of grout holes were required
for closure in this area
prior to commencing rock excavation for the powerhouse.
The observed sequences of pressure increase and release
during grouting indicated that the grout was moving and/or
compacting soil in the voided zone. Upon excavation, a 6 to
9 in (15 to 23 cm) seam of grout was observed in the excavation
DEEP FOUNDATIONS * SEPT/OCT 2021 * 19

September/October 2021

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