By Leslie Nemo
While residents and visitors of Alexandria, Virginia, are likely aware of street flooding threatening to touch their ankles, water pouring into the Potomac River during storms might be less obvious. An average of 140 million gal. of combined sewer overflow empties into the local waterways every year via a combined sewer system implemented in the late 1800s.
As is the case in many cities in the United States, a patch of the historic section of Alexandria known as Old Town relies on a single pipe to carry stormwater runoff, sewage, and wastewater. On dry days, the system carries all waste to a water treatment plant. Too much rain can exceed the capacity of the pipes, however, and the mixed runoff and sewage flows into local waterways instead in an event called a combined sewer overflow.
To reduce the water pollution stemming from Old Town, the public wastewater authority Alexandria Renew Enterprises is charting a 2.2 mi tunnel system over 100 ft below the city. The giant pipe will channel wastewater that would otherwise empty into the Potomac into a treatment plant instead.
Dubbed RiverRenew, the $454.4 million project should solve an issue that other water innovations created for the city about 130 years ago. In Alexandria, says Jason Tercha, an environmental historian and Ph.D. candidate at Binghamton University, the problem of where to send wastewater didn’t begin until there was a built water supply system.
The origin story
In the late 18th and early 19th centuries, Alexandria was in a predicament many increasingly urban communities in the new country were dealing with: Privies and wells were too close together. Brick-lined pits that functioned as outdoor toilets were designed to slowly leak moisture into the surrounding soil while containing any solids, Tercha says. But if masonry cracked or the structure got too full, waste could seep into aquifers and spread illnesses through people’s use of their nearby contaminated water.
In 1810, the city banned the drilling of private water sources and opted instead to create fewer but deeper wells for entire communities to draw from.
The hunt for more clean water continued and eventually gained an advocate in a local resident named Benjamin Hallowell. The schoolteacher joined the push for a water company and suggested that Alexandria could imitate Moorestown, New Jersey, by building a pumping station and hilltop reservoir. With plans to draw from a stream running south of the city, the Alexandria Water Co. received its charter in March 1850.
The construction of the freshwater system went smoothly. A brick-lined reservoir large enough to hold 2.5 million gal. was built on a local knoll, and 7 mi of cast-iron pipes carried water to the city for the first time in June 1852. But all this new, clean water meant residents suddenly had a lot more of the resource to waste, Tercha says. Excrement and dirty liquid still wound up in the nearby outhouses, albeit now in higher volumes that continued overwhelming the structures.
Drainage to carry away leakage from outhouses or dirty puddles on the ground was sparse. Up until this point, the sewer system in the city consisted simply of the streets themselves. Graded to facilitate runoff into the Potomac and its tributaries, roads were largely dirt with drainage gutters carved along their edges. Without a well-executed, comprehensive street grading plan, rain and waste runoff were often too much for the roads to handle and created a muddy mess that piecemeal modifications failed to fix. In 1795, the city used dirt to try to fill holes that had been carved out by water.
Though it’s not clear when the first public or private sewer line appeared in Alexandria to remedy the muddy streets, Tercha has found two possible instances of the sewer pipes mentioned in records from the 1860s in his research into the city sewer system: One was a permission request from a resident wanting to build a sewer from a hotel directly into the Potomac, while another was a complaint filed by a local who was upset that an embankment protecting the resident’s property had been damaged during waste pipe construction. Both examples indicate a piecemeal approach to drainage and not a comprehensive sewer strategy, Tercha argues.
A chance to advance
By the late 19th century, the thinking was that if individual pipes were replaced with a larger, connected drainage system, the change could help Alexandria compete for business — at least, that was the philosophy of then-Mayor E.E. Downham. In 1889, the public official gave a speech outlining three tactics he thought would attract investors, two of which dealt with improving streets and addressing the city’s standing water. His sewer goals were typical for the reform movements of this time — dubbed the Progressive Era by historians — Tercha says, as cities around the U.S. were focusing on sanitation as a way of combating disease. Downham wanted all streets graded and paved and a citywide system of pipes to carry sewage to the river.
The mayor brought on Alexandria’s first engineer, E.C. Dunn, in 1895 to complete these tasks. Over the next couple decades, Dunn turned the city streets, about three quarters of which were dirt, into vitrified brick or bituminous macadam, that is gravel with a binder coating similar to asphalt. The sewer system was also improved from four disconnected pipes to over 10 mi of terra cotta, concrete, and brick lines by 1914. Dunn and his colleagues led the network to four outfall points at some of the lowest-lying locations in the city, Tercha says, for the combined storm and wastewater to empty into the Potomac or one of its tributaries called Hunting Creek.
The outfall system operated under the understanding that whatever emptied into the river would wash away. However, the Potomac as it passes Alexandria is tidal — a fact which kept that theory from playing out. A first attempt to separate out wastewater for diversion to a treatment plant came in 1956 with the Alexandria Sanitation Authority, which evolved into AlexRenew.
AlexRenew is responsible for revamping the flow of the waste currently going into local waterways through existing combined sewer overflows. In 2017, the Virginia state legislature required four outfalls to reduce their output by July 1, 2025. Specifically, one location, known as CSO-001, needed to drop from an average of 37 combined sewer overflows into the river each year to an average of 4-6, while the other three — CSOs-002, 003, and 004 — had to reduce their total maximum daily load of bacteria discharged into local waterways.
Of the four outfall points that will receive new service, two will funnel into a 6 ft diameter open cut sewer while two will empty into a more than 2 mi long, 12 ft diameter tunnel running south under the Potomac and west into the city. Both pipelines will take the waste to the AlexRenew treatment plant.
Planning for a long stretch of the larger tunnel to extend beneath the riverbed meant AlexRenew avoided a route through Old Town and the additional complications this would have created. “You have to monitor the structures for settlement, so it gets more onerous,” says Kelvin Coles, P.E., PMP, the RiverRenew program manager. The Virginia Clean Water Revolving Loan Fund, a $90 million grant from the American Rescue Plan, and Water Infrastructure Finance and Innovation Act loans are helping fund the project along with eventual fees on residential bills.
To complete the construction, AlexRenew turned to a design-build contract with a joint venture between Traylor Bros Inc . and J.F. Shea Co. Inc. In turn, the two construction firms and their design consultant, Jacobs Engineering, opted for a custom tunnel boring machine to create the 2.5 mi passage.
Built by Germany-based Herrenknecht, the TBM design focuses on durability. While the route avoided Alexandria’s designated historic district, the city contains many other historic sites. “For the most part, we’re either in an area of historic structures or we’re out underneath the river,” says Mike Krulc, P.E., an area manager for the eastern underground division of Traylor Bros.
To ensure the earth pressure balance design chews through the clay, fine sediment, and occasional boulder that it will come across, the project adopted modifications like wear-resistant plating on the rim bar and tungsten carbide inserts on the disc cutters. Ground conditioners will also help the TBM move smoothly: One additive will turn gummy clay into a toothpastelike substance while a polymer will suspend any rocks and ease them through the cutterhead. Airlocks near the front of the TBM will help workers complete what are hopefully infrequent service trips.
Progress at the front of the TBM will allow the back end to assemble the tunnel walls. A crane will use suction to lift curved segments off a service truck and onto a conveyance system for another suction-powered device to pick up. Called a segment erector, the mechanism will lift each piece into place against the tunnel — six in total to make a ring — before workers bolt the sections into place.
On a mission
The machine, named Hazel, started digging in November 2022. The ground surface entry point, which will be repurposed as a pump station, is only 60 ft across — a fraction of Hazel’s 500 ft body length. The cutterhead functioned via an umbilical system of cables for power, hydraulic, and grout services until it had carved enough tunnel for the rest of Hazel to be lowered and attached.
Hazel should finish digging in 2024 when it bores into the shaft of the first outfall site. From there, the construction teams will disassemble the TBM in pieces. The tunnel will run from the pump station through diaphragm wall shafts at two outfalls, which each hold a pile-supported diversion chamber near surface level. When functional, the tunnel and the open cut sewer will feed into AlexRenew’s treatment plant, which is undergoing renovations to handle another 8 mgd.
The tunnels will cut the amount of untreated sewage that overflows into the Potomac from about 140 million gal./year to less than 17 million gal./year during storms when the combined sewer overflows are triggered.
Alexandria has always been competitive, even in the late 1800s, in pursuing large goals, Tercha says. “From the city's beginning, they've created this big plan.” If the RiverRenew system were to ever need replacing, an ambitious mentality could serve Alexandria well.
This article is published by Civil Engineering Online.
Learn more about the RiverRenew project in the ASCE Interchange video, “Renewing infrastructure for a healthier tomorrow.”