The support system for a attack or receiving pit The choice depends on the depth, diameter, geotechnical conditions, water table, permeability, available space, nearby loads, schedule, excavation method, and the well’s function within the project. There is no one-size-fits-all solution: the support system must ensure stability, watertightness, safe access, and the ability to withstand thrusts, accommodate equipment, and support driving operations or the reception of the tunnel boring machine.
In projects of vertical pits for driving and microtunneling, pipe ramming y microtunneling in terrestrial and subway applications, the launch shaft typically requires more space and greater structural strength due to the thrust frame, while the reception shaft is designed primarily to receive, dismantle, or remove the tunnel boring machine and complete the final connection.
Criteria for Selecting a Well Support System
The selection process should begin with a technical review of:
- Soil type: rock, clay, sand, gravel, fill, or mixed soil.
- Depth and diameter of the well.
- Water table, water pressure, and permeability.
- Bottom stability and risk of siphoning.
- Nearby loads: buildings, roads, railways, utilities, or structures.
- Space available for machinery and storage.
- The need for a watertight seal at the tunnel boring machine's entry and exit points.
- Excavation method and type of tunnel boring machine.
- Frame thrusts in the attack pit.
- Timeframe, accessibility, and urban or environmental constraints.
Common Systems Based on Terrain
| Ground Conditions | Common Support Systems | Technical Note |
|---|---|---|
| Competent rock | Excavation using bolts, shotcrete, trusses, or localized support | It may require less confinement, but you need to check for fractures, wedges, and water ingress |
| Stable cohesive clays | Precast rings, shoring, piles, diaphragm walls, or walls constructed in phases | Temporal stability depends on depth, humidity, and exposure time |
| Sand, gravel, or granular soils | Drainage piles, continuous walls, sheet piles, closed shoring, or pretreatment | They usually require water management and containment to prevent soil loss |
| Heterogeneous fillers | Sinking piles, diaphragm walls, rigid shoring, jet grouting, or localized replacement | High risk of service failures, blocks, gaps, low resistance, and unpredictable behavior |
| Land with a high water table | Screens, dewatering piles, sheet piles, bottom plugs, jet grouting, grouting, or controlled pumping | Tightness and basal stability are critical |
| Urban areas with nearby services | Sinking piles, diaphragm walls, rigid systems, and monitoring | The priority is to minimize structural damage and protect existing infrastructure |
| Temporary, shallower wells | Metal shoring, caissons, rings, or modular systems | Solution determined by depth, loads, and access safety |
Most Commonly Used Support Systems
Continuous screens or screen walls:
Suitable for deep wells, urban environments, water-saturated soil, or when deformation must be minimized. They can provide good rigidity and watertightness, depending on the seals, depth, and embedment.
Secant or tangent lines:
Commonly used in heterogeneous soils, backfilled areas, urban areas, or wells with space constraints. Dry piles provide greater water control than tangent piles and can be combined with tie beams, rings, or slabs.
Sheet piles:
They can be useful in certain soils and at certain depths, especially when speed is required and there is space for driving or vibration. Their use should be evaluated if there are sensitive utilities, vibration concerns, boulders, rock, or urban constraints.
Precast rings or well segments:
Ring systems that can be used in circular wells, depending on diameter, depth, soil conditions, and excavation method. They facilitate controlled geometry and can be integrated with access points and connections.
Metal shoring or modular systems:
Suitable for shallower wells or temporary installations, provided that the soil conditions, loads, and water table are compatible.
Support using shotcrete, bolts, and trusses:
More common in rock or sufficiently sound terrain. Requires checking the stability of boulders, fractures, water, and safety during excavation.
Jet grouting, grouting, or bottom plug:
They are not always the primary support, but they are used to improve the soil to increase strength, reduce permeability, control water inflow, or stabilize the bottom of the well.
Differences Between the Attack Pit and the Receiving Pit
The attack shaft It must withstand the forces exerted by the thrust frame, allow for the installation of piping, and accommodate hydraulic equipment, guidance systems, material removal, and assembly operations. Therefore, its structural integrity must be verified with regard to thrust forces, crane loads, pipeline operations, seabed stability, and workplace safety.
The reception shaft It may involve fewer logistical requirements, but it must allow for the precise arrival of the tunnel boring machine, as well as its retrieval, disassembly, or final connection. In both cases, ensuring watertightness at the tunnel boring machine’s entry or exit point is critical, especially where the water table is high or the soil is permeable.
What risks does proper support help prevent?
A well-designed support system helps prevent:
- Collapses or instability of walls.
- Uncontrolled water ingress.
- Siphoning or lifting from the bottom.
- Deformations that affect nearby services.
- Settlement in roadways, buildings, or infrastructure.
- Failures at the entry point or reception area of the tunnel boring machine.
- Lack of reaction for the thrust frame.
- Safety Risks During Lifting, Assembly, and Operation.
- Delays and cost overruns due to corrective measures.
At infrastructure crossings In urban environments, support systems must be coordinated with the monitoring plan, permits, traffic, access points, and the protection of existing utilities.
Minimum checklist for selecting well support: geotechnical engineering, depth, diameter, water table, permeability, water pressure, nearby loads, affected utilities, type of well, anticipated earth pressures, available space, machinery, excavation method, need for watertightness, base stability, monitoring, and emergency plan.
Request a Technical review of the support structures for the entry and reception shafts before determining the auxiliary civil works, the excavation method, or the bidding process.
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