Hazel tunnels across the finish line

Hazel tunnels  across the finish line
Hazel'’s cutterhead was lifted from the tunnel on March 21. (Photo/AlexRenew)

By Mark Eaton

Her job is to bore, but Hazel is anything but boring.

Hazel, AlexRenew’s 380-ton tunnel boring machine, recently completed a 2.2-mile precast concrete tunnel that runs 100 to 130 feet under Southeast Alexandria and the Potomac River. The tunnel is 12 feet wide.

On March 21, Hazel was disassembled and lifted from the tunnel to the surface by a large crane. The tunnel, part of the RiverRenew project, extends from AlexRenew’s water treatment facility to an outfall at the foot of Pendleton Street. The tunnel will divert combined sewer overflows during rain storms to the treatment plant to prevent discharges in the river.

Hazel, more than 150-yards-long when fully assembled, had tunneled six days a week in 24-hour shifts since November 2022 up until her recent dismantlement.

A typical day involved two 10-hour shifts with the remaining time dedicated to maintenance. Hazel’s massive cutterhead allowed her to tunnel around 50 to 60 feet every day and up to 130 feet in a single day.

The tunnel is constructed from rings of precast concrete formed in jigsaw piece-like sections that Hazel helped push into place.

Why Hazel?

Alexandria’s combined sewer system is antiquated and over capacity, such that whenever there is a significant accumulation of rain, the outfalls spill millions of gallons of raw sewage into the Potomac River. The city in 2016 approved plans to address the issue over the next decade, under which it would have begun engineering assessments on the Oronoco Bay outfall, the last of the four, by 2026.

However, under pressure from Republican state lawmakers, whose districts border the Potomac River downstream from Alexandria, the Virginia General Assembly passed bills during its 2017 session, which were signed into law by then- Gov. Terry McAuliffe, a Democrat, that required Alexandria to have its aged sewer system outfalls updated by 2025.

In April 2018, the City of Alexandria approved the remediation plan under which the RiverRenew project, and Hazel was born. The plan involved bringing all outfalls together in a unified tunnel at AlexRenew in addition to the construction of a dual use facility for overflow during wet weather conditions. After passage of the remediation plan, City Council then approved a transfer of outfall ownership to AlexRenew with the approval of Virginia’s Department of Environmental Quality.

In a tradition that goes back several hundred years, tunneling and other subsurface equipment was often given female names in honor of Saint Barbara, the patron saint of miners and other underground workers.

Hazel is named after Hazel Johnson, who was known as the “mother of the environmental justice movement.” Johnson worked to improve environment-related health outcomes, particularly cancer and respiratory illnesses, in public housing on Chicago’s South Side from the 1970s until her death in 2011.

What comes next

RiverRenew encountered pandemic-related challenges that contributed to project delays.

“You have issues associated with transportation, supply chain issues,” Matt Robertson, AlexRenew’s director of communications, said in an interview. “The costs of supplies and construction materials for certain things, like rebar, dramatically increased.”

Robertson said Hazel was built in Germany and the reduced availability of cargo ships because of the war in Ukraine added to the delay.

The General Assembly, without significant resistance or controversy, recently voted to extend RiverRenew’s statutorily-mandated completion date from mid-2025 to mid-2026.

RiverRenew’s remaining work involves constructing and installing mechanical components including the underground diversion chamber at the foot of Pendleton Street and the pumping facility at Holland Lane.

With the tunnel completed, Robertson addressed Hazel’s future.

“The hope is that another group, potentially operating on the East Coast, [will need] a tunneling machine of similar size and will reuse her,” Robertson said.

AlexRenew will host an open house at 501 North Union St. on April 13 from 10 a.m. to 4 p.m. to celebrate Hazel’s achievements.

An earlier version of this article stated that the Hazel boring machine is more than 150-feet-long when fully assembled. Hazel is 150 yards long when fully assembled. The Times regrets the error.