Metro construction combines several methods rather than relying on one universal technique. Long, regular running tunnels are often bored underground with a tunnel boring machine (TBM); shallow sections and many stations are excavated from the surface by cut-and-cover; large or irregular caverns are mined in short, supported stages using the Sequential Excavation Method (SEM), also called NATM. Shafts, cross-passages, ventilation structures and staging areas connect those pieces into an operating railway.
The method selected for each location depends on ground and groundwater, depth, tunnel shape, nearby foundations and utilities, available street space, spoil-removal routes, traffic requirements, cost and programme. “Underground” therefore describes the finished railway, not a project with no surface footprint.
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A metro line is a chain of underground structures
A completed metro route is more than two parallel tubes. It normally includes running tunnels, stations, ventilation and emergency facilities, shafts for access, cross-passages between tunnels and, in some locations, wider caverns for crossovers or junctions. Each structure has a different size and purpose, so the construction method can change several times along one line.
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Running tunnels
A two-track running tunnel has a relatively small, repeatable cross-section. That regular geometry is well suited to a TBM, which can excavate and install a permanent lining as it advances. In places where a machine cannot provide the required shape or access, crews may excavate the tunnel in short stages with SEM.
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Stations and ancillary spaces
Stations must fit platforms, passenger circulation, stairs, lifts, ventilation, services and often crossovers. Their width and internal arrangement usually make them unsuitable for a standard circular TBM drive. A station is therefore commonly built as a large station box from the surface or as a mined cavern underground. Shafts provide routes for workers, equipment and excavated material, while cross-passages connect the running tunnels for operations and emergencies.
The three primary tunnel-building methods
Los Angeles County Metropolitan Transportation Authority states that “There are three primary methods for building tunnels.” In practice, agencies combine these methods to suit each ground condition and structure.
Tunnel boring machine (TBM)
A shielded TBM cuts a circular opening while supporting the excavation face and the surrounding ground. As the machine moves forward, hydraulic jacks push against the lining already installed behind the cutting head. Crews assemble precast-concrete segment rings (and, in some systems, metal segments) to form the permanent tunnel shell. Excavated soil or rock is conveyed away or removed through the tunnel logistics system.
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- Excavate and control the face. The cutterhead removes ground while the shield and operating system maintain support appropriate to the soil, rock and groundwater.
- Install each lining ring. Segment pieces are placed behind the cutterhead and bolted or otherwise connected into a ring that becomes the structural tunnel lining.
- Advance and manage spoil. The machine repeats the excavation-and-lining cycle while trains, conveyors, pumps or trucks move excavated material to the surface.
- Retrieve or dismantle the machine. The route needs a retrieval shaft, station box, portal or another engineered location where the TBM can be removed or taken apart.
TBMs are efficient for long drives with a consistent profile and can reduce the amount of continuously open excavation at street level. They do not eliminate surface work: launch and retrieval shafts, stations, ventilation structures, utility relocation, spoil handling and construction compounds still require access. Machine configuration depends on whether the alignment passes through sand, clay, silt, boulders, mixed ground or bedrock, and on how much water the ground contains. Metrolinx highlights that these materials behave differently and require different support and water-control strategies.
Cut-and-cover
Cut-and-cover creates a shallow tunnel or station box from the surface down. It is particularly common for stations and for sections where a bored tunnel rises toward the surface. The basic sequence is visible at street level:
- Map and protect utilities. Water, sewer, gas, power, communications and other services are surveyed, supported, relocated or kept in service under an agreed protection plan.
- Build the excavation support. Retaining walls, piles, diaphragm walls or other systems hold back soil and limit movement at the edges.
- Maintain surface access where required. On a busy road, crews may install a temporary deck so vehicles and pedestrians can cross while excavation continues below.
- Excavate in stages. Soil is removed inside the supported perimeter, with temporary bracing or slabs added as depth increases. Nearby building foundations may need underpinning or other protection.
- Construct the permanent structure. Crews place the base, walls, roof slab and internal station or tunnel elements, then complete waterproofing and services.
- Backfill and restore the street. Material is placed above the roof, utilities and roadway layers are rebuilt, and the surface is returned to its planned use.
Some projects use a top-down variation. Retaining walls and a roof slab are constructed early, allowing the street to reopen sooner while excavation and internal construction proceed beneath it. The exact order depends on ground, traffic and structural design. Cut-and-cover can be straightforward for shallow work, but the large surface work zone may affect traffic, sidewalks, businesses, noise, dust and utility access for a long period.
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Sequential Excavation Method (SEM/NATM)
SEM, widely called NATM, is used when a tunnel, cavern, crossover or connection is too large or irregular for a conventional TBM and a broad open trench is undesirable. Instead of removing the entire cross-section at once, crews excavate a portion, install immediate support and then continue with the next portion.
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- Apply immediate support. Sprayed concrete (shotcrete), steel ribs, rock bolts, lattice elements or other measures stabilize the newly exposed ground.
- Observe ground response. Survey points and instruments track movement, loads and groundwater behavior so the support pattern can be adjusted.
- Repeat and close the structure. Successive increments are excavated and supported until the full cavern or tunnel profile is formed, followed by the final lining and fit-out.
SEM can create broad station caverns and complex junctions while limiting the surface footprint to shafts or portals. It still requires access for workers, machinery, ventilation and spoil removal. “NATM” does not mean that surrounding ground is simply allowed to move without control; the excavation sequence, support and monitoring thresholds are engineered for the actual site.
| Method | Best fit | Main surface interface | Important limitation |
|---|---|---|---|
| TBM | Long, regular running tunnels in suitable soil, mixed ground or rock | Launch and retrieval sites, shafts, stations, spoil handling and ventilation works | Needs a suitable machine, launch/retrieval arrangement and continuous material logistics |
| Cut-and-cover | Shallow tunnels and many station boxes | Large, usually sustained excavation and traffic or utility management at street level | Can cause substantial disruption to streets, buildings, utilities and nearby activity |
| SEM/NATM | Large, irregular or cavernous spaces, crossovers and connections | Access shafts or portals, spoil routes and surface staging | Requires carefully sequenced excavation, immediate support and continuous monitoring |
Why stations are often built differently from running tunnels
A TBM repeats a compact circular section efficiently. A station needs a much wider and more complicated volume, with platforms, passenger halls, vertical circulation, ventilation, equipment rooms and sometimes track crossovers. That is why a line can be bored between stations while each station is excavated as a box or mined as a cavern.
The World Bank’s Urban Rail Development Handbook notes that even systems using bored tunnels commonly construct many underground stations by cut-and-cover. Station sites also need room for cranes, reinforcement, concrete, workers, excavated material and, in some projects, TBM launch or retrieval. Los Angeles Metro says underground station construction in its project context can take five to seven years; that is an agency estimate for its circumstances, not a universal duration for every station.
When deep access avoids a major street closure
Melbourne’s Metro Tunnel illustrates how access can be adapted to a constrained city centre. Victoria’s Big Build describes shafts as deep as 11 storeys used to move equipment and workers underground to excavate stations while avoiding disruption to Swanston Street. Other stations on the same project were built by cut-and-cover. Temporary acoustic sheds were used to limit community noise, light and dust, and cross-passages were provided so passengers and emergency services can move between the running tunnels.
When a cavern is preferable
A mined cavern can preserve important surface routes where an open station box would occupy too much land. It requires a carefully staged excavation and support design. Los Angeles Metro gives a 300-foot downtown Regional Connector crossover cavern as an example of SEM use; that length describes that project’s cavern, not a general limit for the method.
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How engineers select the method
Method selection is a linked engineering and city-planning decision rather than a ranking in which one technique is always cheapest, safest or least disruptive.
| Question | What it changes |
|---|---|
| What are the ground and groundwater conditions? | Face support, lining, dewatering or pressure control, machine type and the risk of settlement or inflow |
| How deep is the alignment? | Whether a surface trench is practical or whether shafts, a TBM or mined excavation is more suitable |
| What shape and size is required? | Regular running tubes favor TBMs; wide stations, crossovers and irregular links favor boxes or SEM caverns |
| What lies above and below? | Foundation protection, utility relocation, heritage constraints, basements and interaction with other transport infrastructure |
| Where can equipment and spoil move? | Launch and retrieval locations, shaft size, haul routes, conveyor or rail logistics and the possibility of multiple work fronts |
| How much street access must remain? | Decking, staging layout, working hours, traffic diversions, noise controls and the choice between surface and deep access |
| What are the programme, cost and operational requirements? | Construction sequencing, redundancy, ventilation, emergency access and the balance between capital cost and community impact |
Before excavation, teams investigate geology, groundwater, existing structures and utilities. During construction they compare monitoring data with design expectations and act if movement, vibration, groundwater or structural behavior approaches project-specific thresholds. There is no single settlement limit or universal safety checklist that applies to every city; protection levels and contingency actions are set by the project’s design, permits and risk assessment.
How a project moves from survey to a finished railway
1. Investigate and design the alignment
Boreholes, mapping, utility records, building surveys and groundwater studies establish what the route will encounter. Designers then choose depths, station locations, shaft sites, support systems and construction sequences around those constraints.
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Utilities are protected or relocated, properties and foundations are surveyed, traffic and pedestrian routes are arranged, and environmental controls are installed. A TBM launch shaft, station box, portal or access shaft becomes the principal connection between the surface and underground work.
3. Excavate with the selected support system
The TBM, cut-and-cover crew or SEM team removes ground while maintaining face and perimeter stability. Spoil is transported out, water is controlled, and instrumentation records how the ground and nearby structures respond.
4. Form the permanent envelope
TBM segment rings, cast-in-place station structures or final cavern linings create the durable tunnel shell. Waterproofing, drainage and joints are completed to the project design before internal systems are installed.
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5. Fit out and connect the railway
Track, power, signalling, communications, ventilation, fire protection, lighting, lifts, stairs and passenger finishes are installed and connected through the stations, shafts and cross-passages. Testing verifies that the separate structures operate as one safe route.
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Safety comes from matching support and sequencing to the ground, not from simply excavating farther below the surface. Retaining walls and decks support cut-and-cover sites; shields and controlled face pressure support TBM drives; sprayed concrete, ribs and bolts stabilize SEM stages. Adjacent foundations can be underpinned or otherwise supported where excavation changes their ground conditions.
Monitoring is continuous around sensitive work. Survey prisms, settlement points, vibration instruments, groundwater gauges and structural sensors can provide early warning. Engineers define action levels, inspection routines and contingency measures for each site. If readings exceed an agreed level, work may pause while crews add support, modify the sequence, reduce production, control water or protect an affected structure. The actual thresholds are project-specific and should not be replaced by a generic number.
Why an “underground” project still affects the city above
Bored tunnels reduce continuous trenching but do not remove construction interfaces. Stations and shafts need cranes, deliveries, ventilation and worker access. Spoil must reach a handling site; concrete, segment rings and other materials must arrive. Utility relocation, temporary decks, traffic management and emergency access can occupy streets even when the train tunnels themselves are deep underground.
Surface impacts vary by method and location. Cut-and-cover typically has the broadest and longest street footprint. TBM projects concentrate major works at launch, retrieval, station and shaft sites. SEM projects can keep the surface relatively open between access points but still require shafts, spoil handling and monitoring. Acoustic sheds, restricted work hours, dust suppression and carefully staged traffic arrangements are examples of controls used where local conditions demand them.
What the headline numbers do—and do not—mean
- 11 storeys: the depth of access shafts described by Victoria’s Big Build for Melbourne’s Metro Tunnel station construction. It is a project example, not a standard shaft depth.
- Five to seven years: Los Angeles Metro’s estimate for underground station construction in its project context. It is not a universal station schedule.
- 300 feet: the stated length of the downtown Regional Connector crossover cavern cited by Los Angeles Metro as an SEM example. It is not a maximum or minimum SEM cavern size.
Official sources reviewed for this subject do not provide a comparable multi-city dataset for cost per kilometre, construction duration, safety rates or universal tunnel dimensions. Any such figure should be tied to a named project, scope, geography, edition and date.
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Common misconceptions
“A TBM builds the entire metro line.”
A TBM may build the long running tunnels, but stations, shafts, ventilation works, cross-passages and portals generally require other construction methods and surface sites.
“If the tunnel is deep, there is no disruption.”
Depth can reduce continuous trenching, yet station boxes, access shafts, spoil routes, utility work and staging areas remain visible at the surface.
“Cut-and-cover is always cheaper.”
Its suitability depends on depth, ground, traffic, property, utility and restoration costs. A method that is efficient structurally can be unacceptable where the surface cannot be occupied.
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“NATM means letting the ground move freely.”
SEM/NATM uses controlled, incremental excavation, immediate support and monitoring. Ground behavior is observed within engineered limits; uncontrolled movement is not the objective.
Further technical reading
For a broader treatment of urban-rail planning and underground construction choices, see the World Bank’s Urban Rail Development Handbook. It places station, tunnel, access and delivery decisions in the wider context of developing an operating urban railway.
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