September/October 2022 - 19

For the production and delivery of
the base grout mix, Trevi deployed
three main mixing plants outside the
gallery to feed new grout pipelines.
Each automated mixing plant included
a Soilmec SGM 45, which is capable of
producing 30 m3/hr (1,059 ft3/hr) of
grout; 12 silos each with a 45 m3 (1,589
ft3) capacity for cement, water and dry
and hydrated bentonite; and four
horizontal piston pumps. These
Soilmec SGP-20.12 pumps delivered
the fresh grout mixture from the three
main mixing plants to the grouting
stations, while keeping it in circulation
through a 20 km (12.4 mi) long closed
circuit split in six pipelines.
Twenty grouting stations were in
the gallery and on the dam crest. Each
station included a Soilmec SGP-9.12M
automated batching grouting unit
equipped with a vertical piston pump
and a dosing-dilution system to prepare
the design mixes as needed.
Initially, six Soilmec SM-5E electric
Grouting work in the gallery
The holes bored from the crest were
crawler-mounted rigs were used. A few
months later, four additional rigs were
added to increase productivity. These
new rigs were custom designed to
better fit within the limited space of the
gallery. This also facilitated drilling
works under the spillway, where the
gallery is even smaller at 2 m (6.6 ft) wide by 3 m (9.8 ft) high.
Seven diesel rigs (SM5 and SM-16) were supplied for the
outdoor drilling works. The three SM-16 (long-mast rigs)
were equipped with a powerful double rotary, to allow the
simultaneous driving of auger rods and steel casing for safely
drilling through the embankment. All rigs were equipped
with a drilling parameter recording device.
Outlet work tunnels and grout hole orientations
drilled through 10-40 m (3-131 ft) of
embankment using auger and temporary
casing, without any drilling fluid,
to prevent any possible hydraulic/
pneumatic fracturing. A permanent
poly-vinyl chloride casing was
cemented inside the borehole and
embedded 1 m (3.3 ft) into the rock
beneath the embankment. Foundation
rock was then drilled by water flushing
rotary drilling.
Most of the drilled boreholes were vertical or subvertical.
Drilling and Grouting Methods
In accordance with the outcomes of the risk assessment, the
dam's foundation was divided into critical areas that defined
the grouting priorities. The grout curtain initially included two
lines, one on the upstream and one on the downstream. Each
line comprised primary, secondary, tertiary and quaternary
holes, split spaced, from 12 m-1.5 m (39-5 ft). Quinary holes
were drilled in between, mainly for coring and verification.
Additional holes were added in critical areas based on review
of grout takes, subsurface investigation, verification results
and monitoring instrumentation trends. Almost all the
boreholes were grouted to 100-150 m (328-492 ft) depth.
However, arrays of inclined boreholes were drilled to allow
the continuity of the curtain under the seven tunnels in the
foundation beneath the grouting gallery, and to connect the
east bank and underneath the concrete spillway structure.
Grouting from either within the tunnels or within the
grouting gallery was based on the reservoir level and the need
to operate the tunnels.
The upstage method (≤ 5 m [16.4 ft] long stages), was the
most applied grouting method. Downstage and downstagezone
methods were used where needed because of borehole
instability. Each borehole was pressure-grouted using a single
packer, lowered in the hole at top of the stage to be grouted.
A series of stable grout mixes - characterized by different
cement content, viscosity and setting time - were designed
and tested to treat both the large voids and the extensive net
of fine fissures that characterize the foundation rock.
As a rule, the grouting of each stage began with thin grout,
both in density and viscosity, and continued with thicker
grouts. The grouting volume for each type of mix was directed
DEEP FOUNDATIONS * SEPT/OCT 2022 * 19
September/October 2022

Table of Contents for the Digital Edition of September/October 2022
