Last reviewed 30 Sept 2026 · 7 min read
The method
Cut-and-cover is the construction of an underground structure by excavating from the surface, building the box inside the excavation and then backfilling and restoring the ground above. It is the cheapest and fastest way to build shallow stations (up to about 20–25 m deep) in open or reclaimable streets. The excavation needs temporary or permanent earth-retaining walls and often dewatering.
Bottom-up and top-down construction
| Bottom-up | Top-down | |
|---|---|---|
| Sequence | Retaining walls → excavate to full depth with struts/anchors → cast base slab → walls → intermediate slabs → roof; then remove supports and backfill | Retaining walls → cast roof slab (or first slab) at the top → excavate beneath it → cast the lower slabs downward |
| Supports | Temporary steel struts or ground anchors | The permanent floor slabs act as struts |
| Surface disruption | Long — the road is closed until the roof is built | Short — the road can be reinstated as soon as the roof slab is cast |
| Movements | Larger (before struts are placed); depends on stiffness of supports | Smaller (stiff slabs as props) |
| Speed / cost | Simple, cheaper | Complex, needs access holes and lighting below, but less traffic diversion; suits deep boxes and tight sites |
Earth-retaining systems
| System | Description | Use |
|---|---|---|
| Sheet piles | Interlocking steel sheets driven into the ground | Shallow excavations (up to 8–10 m) in soft soils with a low water table; noise and vibration limit use in cities |
| Soldier piles and lagging | Steel H-piles at spacing with timber/concrete lagging between them | Dry, stable soil; temporary works |
| Contiguous bored piles | Adjacent RCC piles almost touching | Retain soil; not water-tight |
| Secant piles | Overlapping piles (primary soft and secondary hard) | Water-tight, moderate depth |
| Diaphragm walls (D-walls) | Panels of RCC cast in a trench supported by bentonite slurry | Deep excavations, high water tables, permanent walls; the standard for underground stations |
| Soil nailing / anchored walls | Reinforcement bars and shotcrete | Cuttings on slopes |
Diaphragm walls
A D-wall is built panel by panel (each panel 2.5–6 m long, 0.6–1.2 m thick, up to 30–50 m deep):
- Build guide walls at the surface.
- Excavate the panel with a grab or hydro-mill (cutter) under bentonite slurry, which supports the trench by pressure and by forming a filter cake.
- Lower a reinforcement cage (with spacers, lifting points, and openings for later slabs).
- Pour concrete through tremie pipes from the bottom, displacing the slurry upward (the tremie stays embedded in the concrete).
- Form joints between panels with stop-ends (steel tubes withdrawn as concrete sets) with water-stops where water-tightness is needed.
Quality control: slurry density, viscosity, sand content and pH; verticality (within 1:200 or better) and sonic (cross-hole) logging or coring for integrity. Because the wall is permanent, it is designed for the final condition as well as the construction stages.
Supporting the excavation
- Struts — steel pipe or H-section spanning the box, preloaded with jacks to limit movement; spacing about 3–5 m vertically.
- Ground anchors — prestressed tendons drilled into the soil behind the wall (temporary or permanent), keeping the box free of struts but needing space beyond the site boundary.
- Floor slabs (top-down) — very stiff props.
- Sequence: excavate to a level just below the support, install the support, and repeat; the wall is designed for each stage and the final condition.
Design of the retaining wall
- Ground model — soil layers, strength (c, φ), unit weight, ground-water level (including seasonal and long-term changes).
- Earth pressure — active pressure on the retained side and passive on the excavated side. For braced excavations the apparent pressure diagrams (Peck; trapezoidal in clay, rectangular in sand) are used for the strut loads because real pressure is redistributed by arching. At-rest pressure is used for permanent walls against movement-sensitive structures.
- Surcharge — roads, traffic, buildings, cranes.
- Water pressure — hydrostatic difference between the inside and outside, plus seepage.
- Analysis — limit equilibrium (free or fixed earth support) or, in practice, beam-on-springs or plane-strain FEM with staged excavation, giving the wall bending, shear, strut loads and deflection.
- Structural design — flexure and shear of the wall, connection to slabs, the permanent condition with the box in place.
- Stability checks — embedment, basal heave, piping (hydraulic failure), uplift and global slope stability.
Stability against basal heave and piping
Basal heave (in soft clay): the soil at the bottom of the excavation is pushed up because the weight of the soil outside exceeds the bearing capacity below the base. It is checked by a factor of safety using undrained shear strength and depth of excavation (Terzaghi and Bjerrum–Eide methods).
Piping (hydraulic failure) in sand or silt: upward seepage at the base of the excavation reduces the effective stress to zero when the exit gradient equals the critical gradient
The factor of safety is , usually required ≥ 1.5–2.0.
The head difference between outside and inside the excavation is = 4 m, and the effective seepage path along the wall is = 6 m. The soil has = 2.65 and = 0.65.
; (average; the local exit gradient is higher).
on the average gradient — too low for a local exit; the wall is deepened to lengthen the flow path, or the water level is lowered by dewatering or the base is sealed by a jet-grouted plug.