Since 2006, the City of Alexandria, Va., has been aggressively rehabilitating its sanitary sewers to address infiltration and inflow. With the bar set high at bringing wet-weather peaks from 10 times to just two times dry-weather peaks, the job has been challenging.

But with the help of consulting engineers for data analysis and project criteria development, and with outside contractors handling specific tasks like video inspection and trenchless repair, the city is well on the way to meeting its goals.

Serving a population of 130,000, the city’s sewer system covers 15.2 square miles and includes 220 miles of sanitary sewer and 11 miles of combined sewers. With some sections of the system dating back to the 1800s, age has taken a toll on the pipes, mostly VCP and concrete. Root intrusion and just plain growth have had impacts, as well.

“We’re a very tree-friendly city as well as being a very dense urban area,” says Suzanne Salva, civil engineer III in the Transportation and Environmental Services Department, Engineering Division. “Root intrusion, coupled with lots of taps per linear foot on our mains, and the overall age of our system, contributed to the high incidence of I&I issues we were facing and looking to eliminate.”

Knowing the limits

Having limited personnel available to conduct studies and determine the best plan of action, the city enlisted the Greenley & Hansen consulting engineering firm in Richmond to evaluate three critical watersheds known to have the highest I&I: Four Mile Run, Commonwealth and Taylor Run.

The city collected pipeline survey data in those areas from 2000 to 2008. In the beginning, city crews performed the CCTV inspection, but that soon changed. “We had one crew tasked with the I&I project as well as the regular duties of new construction inspection and emergencies or complaints,” says Salva. “After about six months, we realized we weren’t going to get far with our program, and we made the move to outsource the evaluation and cleaning.”

Another consideration was the age of the city’s CCTV equipment, which broke down frequently from the extra workload. “By hiring contractors to do the CCTV work, when their equipment breaks down, as part of the contract, they are required to continue making progress,” Salva says. “They have more robust resources than we would for backup crews and spare equipment. We just didn’t have the funds available for the dedicated crew and an updated rig that were necessary for the project to move forward at the speed we needed.”

With the inspection contractors in place, all sewers and manholes in the three watersheds had been inspected by summer of 2007. The data revealed that many lines in all of the areas needed to be rehabilitated.

The best approach

Greeley & Hansen also helped Alexandria determine the best form of trenchless technology for the rehabilitation. The engineers closely considered cured-in-place pipe (CIPP) lining, fold-and-form lining, sliplining and pipe bursting.

Because of the city’s dense population, a key issue was the number of taps per linear foot on the mains. Based on that and other criteria, the engineers performed a matrix comparison of the technologies. “The rehabilitation method had to have a simple way of reinstating the taps quickly, so CIPP stood out for that reason as the best method for our situation,” says Salva.

Another factor was cost-effectiveness in view of the varied pipe diameters in the system (mostly 8 to 12 inches). Some technologies were cost-prohibitive on smaller lines, others on larger lines. Overall disruption time to the community was also important, and that made factors like installation and curing times a major factor in decision-making.

In the end, the assessment for the three watersheds recommended rehabilitating 204,000 feet of mainlines using CIPP lining, making 518 dig-up point repairs, and repairing 2,122 manholes with a combination of methods.

The first contract, to rehabilitate the lines and manholes in the Four Mile Run area, began in 2004, using Insituform Technologies Inc. The following year’s contract addressed the Commonwealth watershed, and the city chose AM-Liner East Inc. That company also won the contract for the final section, Taylor Run, where work began in fall of 2008 and is to be completed in spring 2010.

All of the contracts specified CIPP using a felt liner and resin system, cured with hot water or steam in the interest of shortening curing time. Most liner installations used steam, which offers the fastest cure. (The city looked at UV-cured CIPP lining for its even shorter curing times but decided to reserve that option for consideration on future projects.)

Manhole rehabilitation in all three watersheds used various technologies depending on the part of the structure affected. To stop infiltration from the manhole covers, contractors installed heavy plastic inserts. Chimney repairs used mechanical manhole seals from various vendors. “We selected this method for the chimney because it accommodates flexing of the structure near the surface while keeping it watertight,” Salva relates.

For the balance of the barrel of the chimney, crews use a quick-setting grout for spot repairs of active leaks. Then, once the spot repairs are set, a calcium aluminate cementitious liner with additives for corrosion protection in mild hydrogen sulfide environments is applied over the rest of the barrel. For the Taylor Run project, an epoxy topcoat was added because the manholes there showed signs of more aggressive corrosion.

Community buy-in pays

Any rehabilitation project as large and lengthy as Alexandria’s needs community support: It was critical to educate public officials and for funding to evaluate the sewer system’s health and address I&I.

“We were very proactive, going to homeowner associations and other public venues for months and months, giving talks about the problems in our system,” Salva says. “As a result of that public outreach, we were able to secure the funding to really move forward and create a very robust I&I program.”

The results appear promising. An analysis of the Four Mile Run area, the first section fully rehabilitated, showed an overall 33 percent decrease in I&I. An average of 17 to 64 percent reduction has been recorded within that area’s sub-basins.

“We are trying to be very cautious about our apparent success,” Salva says. “For us, the question is: Is it enough? We really don’t have enough data to say we’re absolutely meeting our criteria of two times dry-weather peaks, so we are making plans to collect some long-range post-rehabilitation data.”

To do that, the city plans to contract for installation of permanent flow metering devices throughout selected areas of the system. That project will go forward in cooperation with the Alexandria Sanitation Authority (ASA), which shares jurisdiction on some areas of the system. Some flow meters will be in trunk sewers that ASA maintains and operates.

Meter data will also help the city assess the necessity and feasibility of rehabilitating customers’ sewer laterals, and of locating and remediating illicit sewer connections.

Steadily forward

Alexandria will continue its public outreach and get help from consulting engineers and contractors as it works to reduce I&I and improve the health of its underground infrastructure.

Looking back, Salva observes, “Knowing your limits, enlisting help when you need it, educating your community and public officials and finding ways to creatively work within your budgets — it’s all critical to putting together the type of infrastructure rehabilitation programs needed to address aging infrastructure on a large scale.”