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Investing in healthier waterways for Alexandria

How Is a Tunnel Built?

Look inside a similar state-of-the-art machine that RiverRenew will use to construct its tunnel system.

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Photo: Alexandria ducks at play

RiverRenew is a major initiative in response to a 2017 Virginia law to achieve cleaner, healthier waterways in Alexandria.

Photo of pollution in the Potomac river

The issue

Alexandria’s combined sewer system dates back to the 1800s. In a combined sewer system, only one kind of pipe is designated for transporting both sewage and rainwater. When it’s not raining, these pipes transport sewage for treatment at a water resource recovery facility run by Alexandria Renew Enterprises (AlexRenew).

On rainy days, the capacity of the combined sewer pipes is often exceeded. Instead of being transported and treated at the water resource recovery facility, sewage mixed with rainwater overflows into our waterways via four discharge points, or “outfalls.” In April 2017, the Virginia General Assembly passed a new law requiring the remediation of these outfalls by July 1, 2025.

Next:
Alexandria's water system at a glance

Alexandria waterfront

Alexandria's water system at a glance

As shown in the illustration below, four types of underground pipes make up Alexandria's water system.
  1. Water pipes (operated by Virginia American Water) transport clean drinking water.
  2. Storm sewers (managed by the City of Alexandria) collect rainwater runoff from roofs, sidewalks, and streets.
  3. Sanitary sewers (operated by the City of Alexandria, and treated by AlexRenew) carry sewage from buildings.
  4. Combined sewers (mainly in Old Town) carry a mixture of sewage and rainwater.
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Here’s a closer look at how Alexandria’s water system works today.

ARB_Sewer_Systems_web

In a separate sewer system, water pipes deliver clean drinking water, sanitary sewers carry sewage from buildings to AlexRenew, and storm sewers carry rainwater from roads, roofs, driveways, and sidewalks.

At AlexRenew, trash, debris, and solids are extracted; nutrients are removed and bacteria is virtually eliminated. Cleaned water is returned to the Potomac River.

In a combined sewer system, water pipes deliver clean drinking water, and combined sewers carry a mixture of sewage and rainwater. On dry days, sewage is transported to AlexRenew. On rainy days, these pipes can overflow into our waterways.
The solution
RiverRenew is a major initiative in response to a 2017 Virginia law to achieve cleaner, healthier waterways in Alexandria.

RiverRenew will feature a new network of deep tunnels and sewers that will connect to four combined sewer outfalls, which currently pollute our waterways on rainy days. The tunnels will capture millions of gallons of sewage mixed with rainwater for treatment at AlexRenew and the cleaned water will be returned to the Potomac River.
Map of Alexandria showing proposed tunnel alignment
Next:
Diversion facilities
What are diversion facilities?
Diversion facilities will direct millions of gallons of combined sewage into the new tunnel system for conveyance to AlexRenew's water resource recovery facility. The construction of these structures, which are mainly below ground when complete, requires surface and ground disturbance.
Diversion Facility
Next:
Shaping the tunnels
Shaping the tunnels
To build the new tunnel system, RiverRenew will use state-of-the-art tunnel boring machines, or TBMs, which will:
  1. allow us to build long tunnels without digging major portions of the city,
  2. dig out the soil deep below ground and install a new tunnel in the void that's created, and
  3. remove soil through the TBM and along the newly created tunnel, then lift it to the surface via an elevator.
Boring Machine
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Program timeline

  • April 2017

    Law passed by Virginia General Assembly mandating remediation of existing combined sewer outfalls by July 1, 2025.

  • July 2018

    Virginia Department of Environmental Quality approves plan for remediating existing combined sewer outfalls.

    View plan

  • 2021

    Construction of new tunnel system begins.

  • July 2025

    Construction complete and tunnel system operational per the April 2017 law.

Program timeline

  • April 2017

    Law passed by Virginia General Assembly mandating remediation of existing combined sewer outfalls by July 1, 2025.

  • July 2018

    Virginia Department of Environmental Quality approves plan for remediating existing combined sewer outfalls.

    View plan

  • 2021

    Construction of new tunnel system begins.

  • July 2025

    Construction complete and tunnel system operational per the April 2017 law.

The result

RiverRenew will prevent millions of gallons of sewage mixed with rainwater from contaminating our rivers and streams. This reduction in the frequency and volume of discharges of sewage mixed with rainwater will limit the amount of bacteria, trash, and other pollutants flowing into Hooffs Run, Hunting Creek, and the Potomac River. Through RiverRenew, we can achieve cleaner, healthier waterways for Alexandria.

New controls
in place by:

2025

  • Capture of
    combined
    sewer flows

    0%

Reduced number of
overflows per year:

60

TO
FEWER
THAN

4

X