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.
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.
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.
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.
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