Most large urban sewer tunnels are built from below ground with a tunnel boring machine (TBM), launched and recovered through a limited number of deep shafts. The TBM excavates the alignment while crews install a structural lining behind the cutting head; shafts also provide the route for workers, equipment and excavated material, and for connecting the new tunnel to existing sewers. This avoids a continuous open trench along the street, so traffic can usually continue over most of the route. It does not make construction invisible: shafts, connection works, haul roads, deliveries, noise controls and occasional closures still occupy selected surface sites.
The basic idea: excavate below the road instead of along it
An open-cut sewer is built by repeatedly removing pavement, digging a trench, installing a pipe and backfilling. On a busy city corridor, that can close lanes, relocate utilities and disrupt access for the full length of the sewer. A deep tunnel changes the geometry of the work. Engineers select a route and depth, then concentrate major excavation at launch, retrieval and connection shafts. Between those points, the TBM advances underground.
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The machine’s rotating cutter head breaks the soil or rock. As the machine moves forward, crews install a lining that becomes the tunnel’s structural wall. On segment-lined projects, precast concrete pieces are assembled into rings immediately behind the cutter. The exact cutter, pressure-control system, lining and excavation sequence depend on geology, groundwater, tunnel size and the project’s hydraulic purpose. There is no single TBM design used for every sewer.
Toronto’s Fairbank Silverthorn project describes a 2.4-kilometre trunk tunnel with a 4.5-metre diameter at depths of 15 to 40 metres. Its municipal description explains that the TBM installs pipe segments to form the tunnel walls and that shafts provide the surface access needed for construction and connections. City of Toronto project information
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In Washington, DC Water’s Anacostia River Tunnel used an earth pressure balance machine. The project page describes seven-piece bolted and gasketed concrete rings installed behind the machine. DC Water’s Anacostia River Tunnel description
How a city builds the tunnel, step by step
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Choose the alignment, depth and construction method
Designers map the existing sewer network, streets, utilities, buildings, foundations, waterways and ground conditions. The alignment must provide enough gradient or hydraulic control for the intended flow while leaving room to pass beneath obstacles. The route also determines where shafts can be built without blocking critical intersections or damaging sensitive sites. Toronto says its Fairbank work includes coordination with other infrastructure projects plus settlement and vibration monitoring.
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Construct launch, retrieval and connection shafts
A shaft is a vertical work site, not merely a hole for lowering the TBM. It can accommodate the machine, lifting equipment, workers, ventilation, temporary services and systems for removing excavated material. Other shafts house diversion structures, drop shafts or connections to existing sewers. Surface excavation is therefore concentrated at selected locations rather than repeated continuously along the tunnel.
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Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy.Toronto reported that 20 planned shafts had been completed by 2024 for the Fairbank project. DC Water describes diversion facilities and deep shafts ranging from 20 to 60 feet wide for the Anacostia system. Shaft dimensions and spacing are project-specific.
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Lower and assemble the TBM underground
Large machines are commonly delivered in sections and assembled at the launch shaft or in an underground chamber. The cutting head, shield, propulsion equipment, guidance system and support trains are connected before excavation begins. An earth pressure balance machine, such as the one identified by DC Water for Anacostia, controls pressure at the face by conditioning and supporting the excavated ground. Other projects may require different machine types; the Anacostia specification should not be treated as a universal sewer-tunnelling standard.
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Excavate while advancing through the ground
Hydraulic jacks push the TBM against the installed lining or another reaction surface. Guidance equipment keeps the machine on the designed line and grade. The excavated material is conveyed or transported back through the tunnel to the shaft, where it is handled at the surface. The sources establish that shafts provide access for excavated material and equipment, but the exact spoil-handling system varies by machine and project.
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Install the permanent lining
On a segmental concrete design, a ring is assembled in the tail skin of the TBM from precast sections. Bolts, gaskets and the surrounding ground or backfill system create a watertight, load-bearing tunnel wall. DC Water describes seven-piece rings that were bolted and gasketed on the Anacostia drive. Toronto likewise describes the TBM installing segments to form the tunnel walls. Some smaller or different ground-support systems use other linings, so the ring details cannot be generalized to every sewer.
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Connect existing sewers and control wet-weather flow
A trunk tunnel is useful only when flows can reach it safely. Diversion chambers, drop shafts and connection structures intercept portions of the existing network and route them into the deep tunnel. During storms, a tunnel may temporarily store combined sewer overflows or stormwater and then release the water toward a treatment plant when capacity is available. Connections often require separate surface work, bypass pumping or temporary flow controls even when the main drive remains underground.
DC Water says captured Anacostia overflows are conveyed to the Blue Plains Advanced Wastewater Treatment Plant. A historical U.S. Environmental Protection Agency report on Chicago’s Lawrence Avenue Underflow Sewer describes a pumping station dewatering the tunnel after a storm. Those are examples of operating arrangements, not a universal sequence for every city.
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Recover the machine, restore the sites and verify movement
At the retrieval shaft, the TBM is dismantled or removed. Crews complete permanent structures, surface the roads or paths, remove temporary compounds and restore parks or other public land. Monitoring continues during and after excavation. Toronto describes property surveys plus settlement and vibration monitoring and coordination intended to reduce utility disruption. Such controls manage risk; they do not mean that construction has no surface effect.
Why traffic can keep moving over most of the route
The traffic benefit comes from the location of the excavation. If a tunnel runs hundreds of metres below a street, vehicles can use the roadway above while the TBM works below. The U.S. EPA’s 1979 Lawrence Avenue Underflow Sewer report said modern TBMs made the project economically competitive with conventional sewers while “reaping additional benefits of ease in construction, no disturbance to traffic and least inconvenience to public.” That sentence describes the advantage reported for that historical Chicago project, not a guarantee for every contemporary tunnel. U.S. EPA report
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Surface traffic still encounters the shafts and the logistics around them. Trucks may enter and leave a site, cranes may occupy lanes, and connection work can require a road or sidewalk closure. The route may also cross other construction zones. DC Water’s current Potomac River Tunnel project page lists traffic advisories, illustrating why an underground alignment still needs a surface traffic-management plan. DC Water Potomac River Tunnel project
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A precise claim is therefore that tunnelling avoids a continuous open trench and limits disruption along most of the alignment. Saying that traffic is never affected overstates what the method can deliver.
Published project figures show how much designs vary
The following figures are reported by the named agencies for particular projects. They are examples, not universal dimensions, production rates or performance guarantees.
| Project | Tunnel size and depth | Machine or excavation rate | What the figure represents |
|---|---|---|---|
| Fairbank Silverthorn trunk sewer, Toronto | 2.4 km long; 4.5 m diameter; 15–40 m deep | Approximately 8–10 m per day | Toronto’s approximate TBM excavation rate in the project’s dense soil, sediments and materials; the page does not state a publication year for this rate. |
| Anacostia River Tunnel, Washington, DC | 12,500 ft long; 23 ft diameter; approximately 80–120 ft deep | 39 ft per day average for the Nannie machine | DC Water’s reported average performance during that drive; the page does not state a publication year for the figure. |
| Lawrence Avenue Underflow Sewer, Chicago | Approximately 200–250 ft below the surface | Not stated | A depth reported in the 1979 EPA interim report, describing a historical deep-tunnel project. |
Different units and ground conditions make the rates unsuitable for a direct productivity contest. A machine’s advance can slow for mixed geology, groundwater, interventions, maintenance, shaft changes or connection work. The only defensible schedule estimate for a new tunnel comes from its own geotechnical and construction plan.
What the tunnel does after construction
Storage and treatment of combined sewer overflows
In older combined systems, the same pipes carry sanitary sewage and rainfall runoff. During a heavy storm, capacity can be exceeded, causing an overflow. A deep tunnel provides temporary storage and a controlled route to treatment, reducing the frequency or volume discharged to a river. DC Water reports a 90 percent reduction in combined sewer overflows to the Anacostia River since completion of its tunnel in March 2018. That is an agency-reported outcome for that completed system, not a standard reduction rate for all sewer tunnels.
De-watering after a storm
Storage is temporary. Pumps or gravity conveyance empty the tunnel once the treatment system can accept the flow. The EPA’s Chicago report specifically describes a pumping station dewatering the Lawrence Avenue tunnel after a storm. Other systems use different hydraulic layouts, pump capacities and treatment connections.
Drop shafts and energy control
Water entering a tunnel from a shallow sewer can fall a considerable distance. Drop shafts and diversion chambers are designed to control that energy, protect the lining and prevent unsafe turbulence. Their location is one reason a project needs several carefully selected shafts rather than only a single launch site.
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Surface impacts that residents should expect
Underground construction reduces the length of roadway excavation, but it does not remove local construction impacts. A shaft compound can include fencing, cranes, lighting, ventilation equipment, generators, spoil trucks and worker access. Typical effects include:
- temporary occupation of a road, sidewalk, park or parking area;
- truck movements and lane restrictions near shafts;
- noise, dust and vibration, with working-hour and mitigation requirements;
- utility surveys, relocations or protection measures;
- tree removal or protection and later landscape restoration;
- temporary closures where new drop shafts or sewer connections are built.
Toronto says part of Fairbank Memorial Park would be closed during shaft and tunnel work while access to some facilities remained open. The city also describes surveys and monitoring for settlement and vibration. A closure at one shaft does not mean the entire tunnel route is closed to traffic, but it is a real neighborhood impact.
Vancouver’s December 18, 2025 release provides a smaller trenchless example: a remote-controlled micro-tunnel boring machine installed a concrete storm sewer main beneath a park footpath, preserving 24 mature trees. The city still expected a temporary closure of the playground and pedestrian path for later connections and sliplining. City of Vancouver project release
Large deep tunnels are not the only trenchless option
Microtunnelling
A remote-controlled micro-tunnel boring machine can install a comparatively small pipe beneath a road, rail line or park path. It is suited to shorter alignments and smaller diameters than a city-scale storage tunnel. Vancouver’s storm-sewer work illustrates this use: the machine installed a concrete main under a footpath while keeping most of the surface open.
Sliplining
Sliplining rehabilitates an existing sewer by inserting a new, usually smaller pipe inside the old one. It does not excavate a new deep tunnel and does not have the same storage capacity as a large combined-sewer-overflow tunnel. Vancouver used sliplining as a separate method for an existing combined sewer main. Treating sliplining and deep TBM excavation as interchangeable obscures the different engineering problems they solve.
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Open cut remains practical where a pipe is shallow, the route is short, utilities are sparse or surface access is already available. Its trade-off is a longer and wider surface work zone. Cities choose among methods by balancing diameter and depth, geology and groundwater, surface footprint, traffic management, lining and ground support, connection complexity, duration and restoration requirements.
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How engineers limit risk at the surface
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Survey before excavation
Property-condition surveys, utility records and baseline measurements establish what is present before the TBM or a shaft begins. This gives the project a reference for detecting movement and resolving claims.
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Monitor settlement and vibration
Instruments and inspections track ground movement, building response and vibration around shafts and along sensitive sections. Toronto explicitly identifies settlement and vibration monitoring as part of its project controls.
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Stage traffic and deliveries
Authorities can schedule heavy deliveries, narrow the work zone, maintain pedestrian routes and issue advisories. The controls are local and may change as a shaft moves from excavation to connection work.
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Coordinate with other utilities and projects
Mapping and coordination reduce the chance that a new shaft or connection conflicts with water mains, transit works, electrical ducts or road reconstruction. Toronto notes coordination with other infrastructure work for this reason.
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Restore each work site
After the machine leaves and connections are tested, temporary compounds are removed and roads, sidewalks or parks are rebuilt. Restoration is part of the project scope, not evidence that no disruption occurred during construction.
What determines whether a tunnel is the right answer
Owners generally consider a deep tunnel when the required diameter, route length, hydraulic storage or surface constraints make a continuous trench impractical. The decisive questions are:
- How much flow must be conveyed or stored, and how will it reach treatment?
- What soils, rock layers and groundwater pressures will the machine encounter?
- How deep must the tunnel run to pass utilities, foundations, waterways and existing sewers?
- Where can shafts be placed without unacceptable traffic, park or property impacts?
- What lining, sealing and ground-support system will remain safe over the design life?
- How will excavated material, workers, ventilation and emergency access move through the shaft system?
- Can a smaller microtunnel, rehabilitation method or open-cut pipe meet the need at lower overall risk?
Because these variables are site-specific, published dimensions and rates should inform public understanding rather than serve as a template. Toronto’s 8–10-metre daily estimate, DC Water’s 39-foot daily average and the historical Chicago depth each describe a particular set of conditions.
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The practical answer to “How do they build it without closing the streets?”
They do not keep every street, park or sidewalk untouched. They keep the main excavation underground, using a TBM launched from a limited number of shafts, and concentrate unavoidable surface work at those shafts and at the points where the new tunnel joins the sewer network. That arrangement can preserve through traffic along most of the alignment while still requiring closures, deliveries, monitoring, utility coordination and restoration at local work sites.
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