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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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