Deep Sewer Installation Upgrades Infrastructure

Edited by Christina Fisher | September 28, 2010

Sometimes the solution to a problem is so ideal that it can be applied repeatedly. This was the case when officials of the city of High Point, North Carolina, decided to replace an entire interceptor system instead of just one sanitary sewer. The old sewer was concrete, but they wanted pipe with a much longer service life. Inherent corrosion resistance and a leak-free system are two of the factors that affect longevity and both were included in the specifications.

High Point works hard to maintain its infrastructure and this project, one of the most recent, was named Deep River Outfall Segment 2. Greg Hall was project engineer for the city. Davis-Martin-Powell & Associates, Inc. (DMP) of High Point was the consulting engineer. Thalle Construction Co., Inc. of Hillsborough, N.C., was the contractor.

Ben Palmer of DMP says, "Basically, the line that was replaced was 40 years old, constructed of concrete pipe. It had reached its life, undersized, deteriorating. It had several point failures in the line just from the degradation of the concrete. That was really the driving force behind replacing the whole line. And they were looking for a replacement material that was going to be resistant to the hydrogen sulfide gasses that were present in the line. And, that led them to the HOBAS pipe."

The centrifugally cast, fiberglass reinforced polymer mortar (CCFRPM) pipe was the unique product that fit the city's requirements without any add-on coatings or linings.

Thalle installed 13,700 feet of 54-inch to 66-inch sewer pipe at depths of 35 feet to 60 feet. The deep sewer construction included more than 200,000 cubic yards of pre-cut benching and restoration, 74,000 cubic yards of rock excavation, 1,427 feet of 96-inch and 90-inch diameter tunnels, and the removal of 4,500 feet of the existing aerial sewer crossings.

The 60-foot burial depth for the outfall is very deep for a sewer. Much of it was installed through granite in a rock quarry and required blasting. Ed Kuehnel, project manager for Thalle, was involved in every aspect. He says, "It was definitely a tough job, up to 60 feet in the ground with an average depth of about 40 feet. We had one 800-foot run that was 60 feet deep from end to end."

The depth of Deep River Outfall Segment 2 called for benching in the excavation. This began with a scraper used to remove layers of dirt, in this case up to 20 feet wide for the first excavation. The crew then dug down 10 feet to 20 feet for this first part of the excavation.

As is typical with the benched excavation technique, they then dug the excavation deeper but narrower, the width equal to the diameter of the pipe plus 2 feet of clearance on each side for the crew to work. The trench box was located in this lower sub-trench.

"There was a tremendous amount of earthwork involved just to get to the point where you could install the pipe. We had to excavate and build ourselves a bench at 20 feet. We had to get the earth down to a 20-foot cut. The biggest challenges were the depth and the rock. We had to drill and blast to get the mass rock and trench rock out," says Kuehnel.

Bedding was 6 inches to 12 inches deep, depending on the depth of the excavation, using crushed granite 3/4-inches in diameter. After the installation crew put the bedding down, they laid the pipe on the bedding and adjusted the final line and grade. The same material was used for backfill. On the shallower depths, it covered up to 70 percent of the O.D. For the deeper covers of more than 15 feet, they placed the same material 12 inches over the top of the pipe.

The bedding material was available from the rock quarry that the pipeline traversed. The cover was completed with select native material.

When pipeline construction goes this deep, the engineers usually consider tunneling, typically the first choice for deep installations. In High Point, the mixed face conditions and hilly terrain made tunneling difficult. The wide-open area of the project allowed for the deep direct bury with the benched construction and only a minimum of tunneling.

The depth, however, created an enormous overburden on the pipe. The engineers specified pipe stiffness of 72 psi for all of the installation and had full confidence in the structural characteristics of the HOBAS pipe, so there was no question about its strength.

"There were a number of discussions between the city and HOBAS regarding the depth it was going to have to be installed at, and HOBAS assured us that with proper installation, it could withstand those burial depths. During and after the installation, there was no noticeable deflection of the pipe. So, we felt pretty comfortable based on the information that HOBAS had provided," says Palmer.

The depth of the line eliminated the need for several lift stations that would have been needed in a shallower installation. Lift stations must function in harsh and corrosive environments, and their pumps and valves can become a maintenance problem. Choosing a deeper installation and a pipe system with an extended, maintenance-free life, the city of High Point demonstrated that it recognizes the long-term benefits of investing in infrastructure.

Commenting on the city's acceptance of the new line, Greg Hall says, "We air test each joint independently. On pipes of 30 inches and larger, a test pressure of 3.5 psi would apply. We've had less than six joints fail and every one of those would be classified as an installation problem. Minimal effort was required to correct all six prior to job completion.

"Overall, counting this job and two previous jobs, we've put in 10 miles of HOBAS pipe. The failure rate is much better than any other pipe. We've been more than pleased with the performance of the HOBAS pipe and the installation has gone smoothly. We're digging through rock, which is always a pain, (but) the precautions in handling the pipe are no greater than with any other material. I've seen little damage if any from handling the pipe. It goes in fine in comparison with other pipe."

The sections that were replaced had some significant infiltration problems. Hall said, "It's hard for us to put a figure on infiltration because that segment is only phase four out of six, so it's hard to get a number till the whole project is built. I can't give any numbers but I know that infiltration has definitely decreased. The same can be said for exfiltration."

"The segment 2 project is completed and holding up as promised. The main trunk line for segment 3 is nearing completion," says Ben Palmer.