underwater work. A system of buoys, subsea
levels and a gyroscope with built-in accelerometers
positioned the structure within one
inch of its design position. The intricately
choreographed concrete termie pour two
and a half miles offshore took place over 12
days with crews working 24 hours a day. The
12,000 cubic yards of concrete were batched
onsite and transported by 1,086 concrete
trucks on four barges assembled from 143
The three-mile long intake tunnel is being
bored through rock cover ranging from 587
feet (at launch) to about 50 feet where it
enters the intake structure tremie block base.
The prototype hybrid TBM was designed
and supplied by Herrenknecht of Germany
and is sufficiently flexible to operate in
open mode at atmospheric pressure and
closed mode where ground conditions are
less solid or stable, high inflow of water or
water inflows at high pressure, all of which
have been encountered onsite. The tunnel
will have a one-pass lining of 20-foot ID
steel-reinforced, 14-inch thick bolted-andgasketed
precast concrete segments. Waterproofing
is ensured by the gasket design,
which was tested at a pressure of 36 bars
(one bar being approximately equal to the
atmospheric pressure of the earth).
Source spoke at length with SNWA
Project Manager Erika Moonin, P.E. about
the construction challenges and logistics of
mounting such a complex project.
Source: What kind of decision-making went
into the development of this third intake?
Moonin: Given the rapid decline of the lake
during the current prolonged drought on the
Colorado River, we were concerned that in
the near future we could lose Intake No. 1,
which is at elevation 1,050. Additionally, we
were beginning to see some adverse water
quality impacts on our treatment processes
during the summer. The lake stratifies in
the summer months and some of the lesser
quality water from urban runoff and treated
wastewater discharges stays near the surface.
A deeper intake substantially mitigates the
potential for those adverse impacts.
Source: In addition to a deepwater intake
what other possibilities did you consider?
Moonin: We looked at extending one of the
other existing two intakes and at a floating
barge pumping station as well as expanding
our existing pumping stations and at modifications
to our water treatment plants. The
floating barge didn’t have sufficient capacity
and would have been difficult to supply power
to. Extension of one of the existing intakes
with a pipe wasn’t an option because of the
lake bottom topography and inadequate
pump depth in the higher existing intake.
Source: Other than size is there anything else
unique about the intake?
Moonin: The intake structure and the tunnel
have to withstand the higher external forces
in the dewatered condition because they have
to handle the full lake elevation. And the soft
eye of the intake where the TBM is going to
mine into the structure is reinforced with
fiberglass instead of steel because fiberglass
won’t damage the cutter head.
Another fascinating aspect of the project
is the system VTC used to survey the intake
riser and get it in alignment and positioned.
They placed a guiding frame at the bottom of
the lake with cigar buoys attached to the four
corners. There was approximately 1,000 pounds
of tension on these lines and they surveyed the
tops of these buoys with GPS for several weeks.
Based on the pattern of variation, they knew
they were within an inch of tolerance.
There was also the logistics of doing a major
tremie concrete placement at the bottom of
a deep lake. VTC employed a special tremie
feature called a dobber to facilitate the
Tremie concrete: A special mix of
concrete that can be placed under
water; critical to the Intake No. 3
project because of the amount of
concrete that had to be poured over
an extended period, in over 350 feet
of water. Tremie concrete placement
uses a pipe through which the
concrete is placed below water
level. The lower end of the pipe is
kept immersed in fresh concrete
so the rising concrete from the
bottom displaces the water without
washing out the cement content.
Soft Eye: A circular section of the
intake structure that utilized fiberglass
rebar in place of traditional
steel rebar. Used where the TBM
will bore through to connect the
tunnel to the intake to avoid
damaging the cutter head.
Moon pool: A section of floating
barge that was hollowed out,
effectively a pond in the middle
of the barge constructed by an
arrangement of the individual floats.
concrete placement. Other projects have used
the dobber system but not at this depth. The
first pour of concrete comes in contact with
the water and the rest fills from below. We
didn’t want any cold joints and so were very
concerned about any interruption that would
cause the concrete to set up. This meant the
mix had to be very flowable and have a bit
longer set time. VTC effectively had a full
redundant system on the placement barge
in case a pipe clogged, which happened with
some of the early test placements.
Probably the biggest challenge was the
depth of the placement, which required
that everything had to be monitored and
performed from the surface with an ROV.
The blasting stirred up the bottom of the lake
and prior to that, placing the grid of shape
charges stirred up the fines. Underwater
visibility was 12-18 inches, which meant
that a lot depended on the ROV operator,
who was extremely skilled.
Source: The tunnel alignment hardly looks
like the closest distance between two points.
Moonin: When we did the initial geotechnical
investigation and selected the intake
location, we did a straight line shot from
Saddle Island north and discovered the
geology was not very good. So we took a little
more sweeping alignment, found there was
still quite a bit of highly fractured volcanics
and moved the alignment even further, going
north and then turning east out into the lake.
Even with that we’ve had to run the TBM a
long distance in closed mode because we’ve
had high water pressure most of the way and
more sections of unstable ground than we
anticipated, where the material won’t stand
on its own in front of the machine.
Running in closed mode involves pumping
slurry in front of the machine. The chippedup
material mixes with the slurry mixture, a
rock crusher attachment crushes the large
pieces so they can be pumped out in the
slurry system pipes and it all goes up to the
surface to a slurry separation plant. Production
in closed mode can be much slower
and more complicated and more expensive
because of increased energy costs.
The connection tunnel from the pumping
station to Intake No. 2 was also challenging
because the contractor,Barnard of Nevada,
Inc., had to blast a shaft between our two
operating pumping stations, which meant
they had to blast when we weren’t pumping.
We also had to dewater the whole system
from Intake No. 2, the tunnel through the
mountains and the River Mountains Water
Continued on page 16