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NUCA contractor
BRH-Garver, Inc. recently tackled a difficult problem–How
to replace a drain line under the floor of a 45-ft tall, 100-ft
wide earthen reservoir with limited access. The innovative
solution they came up with combined two acclaimed trenchless
methods, directional drilling and pipe bursting.
The large-scale
reservoir rehabilitation project was being done for the city
of San Diego. The general contractor encountered grout filled
and broken perimeter drain lines underneath the floor of the
reservoir. Unable to perform conventional excavation because
of the extremely tight access, the contractor approached BRH-Garver
Vice President Mike Arme at their San Diego, California office
for a solution.
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The
BRH Garver crew used a pit-launched mini-directional
Grundopit to drill a 2 1/4-inch diameter, 110-ft
pilot bore, through the difficult soil conditions.
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The Shot
Arme said, "The
drain lines had to be tied back in, there was no way around
it. Any kind of overflow or displacement from the reservoir
goes into these lines, drains off into vaults and is pumped
out. I had an idea. I didn’t know if it was going to
work, but it was the best shot we could come up with."
The idea Arme had
was to utilize a mini-directional drill and a pneumatic pipe
bursting tool to install a new HDPE drain line. The BRH-Garver
crew would first drill a 2 1/4-inch, 110-ft pilot bore, then
run the pipe bursting tool through the pilot bore, pulling
the new 8-inch HDPE.
Mini-Directional
In order to successfully
complete the grade critical pilot bore, BRH-Garver needed
to use a directional drilling system. However, tight access,
a difficult working area and a relatively short bore distance
precluded the use of a conventional drilling rig. Arme decided
to use the pit launched mini-directional Grundopit 40/60
from TT Technologies, Aurora Illinois.
Arme said, "We
have been using the Grundopit on
lateral sewer line replacement jobs in La Jolla [California].
I thought it would be powerful enough to complete the job,
as well as small enough to overcome tight access problems.
In addition, the Grundopit is
trackable and offers the directional capabilities we needed." Arme
was still concerned, though, about the difficult soil conditions
in that area of California.
Soil Conditions
One of the most
challenging facets of trenchless work in Southern California
is the unpredictable nature of the ground conditions. Arme
said, "We have what is referred to as a caliche type
material. It’s almost like a cemented stone with PSI
ratings in the 10,000 to 12,000 range. It comes in waves.
You’ll be going through soft material, then you’ll
hit a section of caliche sediment. It may only be about 4
or 5 feet thick, then you’re back in good conditions
again."
The mini-directional
encountered 2 sections of caliche during the 110-ft bore.
Arme said, "We started on the floor of the reservoir
and bored down to the overflow vault. The project called for
1
1/4% grade but we figured 1% to give ourselves some breathing
room. The bore was progressing
quickly until we hit the first section of sediment. That slowed
us down considerably."
According to Arme,
the first section of sediment was approximately 6-ft long.
The Grundopit was
able to grind through that section and another 10-ft section
during the last 20 feet of the bore. The crew pumped approximately
750 gallons of a water/polymer mixture with a 500 gallon
Grundomudd mixing and delivery system.
Despite concerns
about tracking, they were able to keep in contact with the
Grundopit at
depths up to 45 feet. The bore took 6 hours from start to
finish.
Pipe Bursting
After completing
the pilot bore, the crew attached a swivel and a cable to
the drill stem and pulled it back through the newly drilled
hole. The crew then positioned a 10-ton constant tension Grundowinch at the vault. The winch line was pulled through by hand and
attached to an 8.5-inch diameter Grundocrack Hercules pneumatic
pipe bursting tool.
Arme said, "Initially
we wanted to burst from the vault up. However, the vault was
only 4 ft x 8 ft and wouldn’t function as a usable launch
pit. We needed to maintain grade and didn’t have a host
pipe to help guide us. We needed to start at a reasonable
angle. So we decided to burst down to the vault."
Bursting from the
floor of the reservoir down to the vault also presented a
challenge in the form of a 30-inch ductile iron main located
just under the surface. Arme said, "We actually had to
move back to the bank of the reservoir to get enough clearance
to drop the Herc in at the angle we wanted. Prep work was
considerable, but bursting was completed in about an hour
and a half."
The general contractor
was extremely pleased to have the 8-inch drain line in place
and is looking at using the same technique to replace other
lines. Arme was impressed with the performance of both piece
of trenchless equipment and anticipates more work for the
mini-directional/pipe bursting trenchless team.
UTILITY Contractor,
July 2000
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