Sewer renovation by open cut excavation

In 1989, officials of the City of High Point, NC, 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...

HIGH POINT, NC, Feb. 26, 2008 -- Sometimes the solution to a problem is so ideal that it can be applied repeatedly. This was the case in 1989 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. (TC) of Hillsborough, NC, was the contractor.

The many visitors to the city recognize it as a center for diversified industry and commerce, especially furniture, but give little thought to the miles of utility infrastructure. However, to the city, infrastructure is of utmost importance. With a population of about 100 thousand, the city has more than 70 furniture stores and claims the title of "the Home Furnishings Capital of the World™." It draws participants from 50 states and more than 100 countries for the biannual International Home Furnishings Market, the largest event of its kind in the world.

Ben Palmer of DMP said, "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."

Ed Powell, president of DMP, agreed with Palmer, "We wanted a pipe that would be inert to the hydrogen sulfide and HOBAS fit the bill."

Palmer added, "The city had installed a short segment of 66-inch HOBAS pipe into a new wastewater pumping station that they had built in the early 90's, which actually was the starting point for this major outfall replacement. They had also used some 36-inch HOBAS for a section of the force main coming out of the station. That was really their first experience with HOBAS, they liked what they saw and that was what led to the selection of HOBAS for the remainder of the outfall line.

"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.

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

HOBAS was the only pipe named in the specs. 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. Powell explained, "HOBAS CCFRPM pipe can be fabricated into a variety of fittings such as elbows, wyes and tee-base manhole risers, so the city could have the required qualities throughout. The FWC couplings on the main line could also connect to the fittings maintaining the leak-free quality."

High Point started specifying HOBAS pipe for a variety of applications in the early 1990's, and has used more than 10 miles of this pipe in a variety of applications for corrosive environments including sanitary sewers and foul air. Of the many jobs, Deep River Outfall Segment 2 and Segment 3 are the most recent. Powell said, "When the entire Deep River outfall replacement is complete, including all of the jobs since we started using HOBAS, more than 40 miles of HOBAS pipe will have been used,"

Ken Glasgow, mid-Atlantic sales manager for HOBAS, who was a regular visitor to the project, said, "It was a satisfying job: the owner was happy; the engineer was happy; the contractor was happy; so I would say it was a successful job for everyone involved."

TC installed 13,700 feet of HOBAS 54- to 66-inch sewer pipe at depths of 35 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- 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 TC, was involved in every aspect. He said, "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.

"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.

"The job turned out really well. We were very pleased with the pipe. It was the first time that I personally had worked with that pipe. It's easy to install. It's lighter so your rigging and equipment can be lighter."

This project was one of many phases that High Point is undertaking to replace the interceptor system on the east side of town. As it neared completion, the city continued its reliance on HOBAS, starting a new job using 54-inch pipe.

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 to 20 feet for this first part of the excavation.

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

Bedding was six 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 traverses. The cover was completed with "select native material," which is material obtained from project excavations cleaned of objectionable materials and used as backfill.

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. Powell said, "The HOBAS was ideal. It was easy to handle for direct bury and worked just as well in the tunnels."

The extreme depth extending to about 60-feet in much 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.

The depth creates 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.

Commented on the city's acceptance of the new line, Greg Hall said, "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 have not used any RCP in a long time but we do know from surrounding communities that they have had considerable problems with it.

"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.

"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."

Hall also addressed future plans, "HOBAS will continue to be allowed for use on any project we have. The company is great to work with. In fact that is something we were talking about last week in the public services department. Their technical support and their technical people are Johnny-on-the-spot. If we've got any questions or when we're doing something a little bit different and new for us, that we're not quite comfortable with, Ken's out there and then all the guys from Houston -- they're right on top of it."

HOBAS pipe is manufactured in sizes from 18 inches to 110 inches in pressure and non-pressure classes.


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