September/October 2022 - 20

by a flowchart defining the relationship between the effective
pressure and the volume of mix being injected. Grouting
pressure and methodology varied along the length of the dam
with depths based on encountered geology and observed
conditions while drilling. Maximum effective pressure used
in each borehole was determined to get the best penetration
while limiting rock hydro-fracturing risk. The completion
criteria for each stage were based on the designed refusal
pressure and flowrate, which had to be reached and
maintained constant for a prefixed time. Once the refusal
condition was reached, an additional waiting period allowed
the complete dissipation of the final backpressure.
All grouting operations were controlled and monitored by
T-Grout, Trevi's proprietary automated control system of the
grouting process. The software governed the whole grouting
process and triggered mix changes based on preset criteria.
Data were transmitted to the grouting software, which in turn
regulated the pump pressure and flowrates. The software
automatically computed the effective pressure, correcting the
" raw value " measured at the borehole collar, taking into
consideration dynamic and static head losses and or gains.
The boreholes in the lower part of the gallery were drilled
and grouted in challenging conditions, due to very high
reservoir levels and resultant artesian conditions. In general,
the downstream line (the first line to be grouted) recorded the
highest grout takes. Within each line, the grout takes
generally decreased from the primary holes towards the
secondary, tertiary and quaternary holes. The decreasing
trend of grout take indicates the void filling and gradual
reducing of the residual permeability. In addition to the holes
inspected by an optical televiewer, check holes were coredrilled
and investigated by water pressure tests to evaluate
the grouting efficacy. Nearly 81% of the tested stages
measured values of Lugeons (LU) ≤ 1 and 98% of tested stage at
LU ≤ 3. In general, the results of the Lugeon tests confirmed
great improvement of the foundation.
Monitoring
Onsite instrumentation, including piezometers, weirs,
pendulums, tiltmeters, crackmeters and inclinometers, were
updated and installed. A state-of-the-art automated data
acquisition system was installed to provide near real time
monitoring during and post emergency grouting efforts.
Interferometric Synthetic Aperture Radar was also used to
monitor for surface deformation and aid in select locations for
subsurface investigation.
The record low and a 15-year record high lake levels were
experienced during the emergency grouting and monitoring
efforts. The spillway was operated for the first time in 15 years
and twice in three spring seasons. Surveillance and
monitoring indicated that the dam performed satisfactorily to
lake levels up 4.8 m (15.7 ft) below full storage in the spring of
2019, near the completion of Trevi and USACE efforts on site.
Geospatial Information System
Software governed the grouting process
With the help of the T-Grout system, more than
900,000,000 data points were recorded and stored. This huge
amount of digitized data made it possible to do in-depth
analysis of the grouting behavior and foundation rock
responses in a relatively short amount of time.
Quantities and Results
Drilling and grouting works were performed 24 hours/day, six
days/week. From November 2016 through July 2019, Trevi
drilled and grouted 403,100 m (1,322,507 ft) of holes, injecting
more than 40,800 m3 (1,440,838 ft3) of grout. The drilling
involved 667,800 m (2,190,945 ft) of rotary drilling/redrilling,
85,000 m (278,871 ft) of coring and 35,450 m (116,306 ft) of
predrilling in the embankment.
20 * DEEP FOUNDATIONS * SEPT/OCT 2022
USACE also developed and implemented a geospatial
information system to aid in the management and
visualization of historic and construction data acquired on
site. Data included drilling and grouting hole location, depth,
T-Grout information for each stage of grouting, instrumentation,
subsurface exploration and verification hole data,
water pressure test results, and instrumentation data. Data
were visualized in both 2D and 3D in concert with dam
stratigraphy and design features including grouting gallery,
filters and drains, spillways, and tunnels. The management
and visualization of data as they were produced enabled rapid
and informed evaluation and decision making, facilitating
communication across the globe.
Training Local Staff
Under the contract, Trevi was asked to train the local MoWR
staff and technicians for the operation, maintenance and
repair of the drilling and grouting equipment to ensure good
operations into the future.
September/October 2022

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