Essential Techniques in Underground Construction and Tunneling

Posted on Oct 16, 2025 in Civil Engineering

Underground Construction Fundamentals

Underground construction refers to the building of structures beneath the Earth’s surface. This can include various types of facilities like tunnels, subways, basements, mines, parking lots, utility networks (e.g., water, sewer, and electrical lines), and even underground buildings or storage facilities.

Key Aspects of Underground Construction

Purpose and Applications

  • Transportation: Subways, tunnels for highways, and pedestrian passageways are essential for efficient urban mobility.
  • Utility Systems: Sewers, water mains, gas lines, electrical cables, and telecommunications networks are often buried to ensure protection from the elements and minimize interference with surface activities.
  • Storage and Defense: Some underground facilities serve as storage for hazardous materials (e.g., fuel, radioactive waste) or are used for military purposes, such as bunkers and shelters.

Construction Techniques

  • Tunneling: One of the most common methods of underground construction. This can be done using methods like boring (with Tunnel Boring Machines, *TBMs*), drilling, or using the cut-and-cover approach, where a trench is dug, and after construction, the surface is restored.
  • Cut and Cover: A trench is dug from the surface, and after the construction, the site is covered over again. This method is often used for shallow tunnels.
  • Mining Methods: For deep underground projects like subway systems or large caverns, mining techniques such as drill and blast or roadheader use may be employed.
  • Support Systems: Underground construction often requires specialized supports, such as *shotcrete*, steel reinforcements, or tunnel linings to prevent collapse during and after construction.

Major Challenges in Subsurface Projects

  • Geotechnical Considerations: The type of soil and rock, groundwater levels, and other geological factors play a significant role in planning underground construction. Different types of soil or rock require different construction techniques and machinery.
  • Water Ingress: Managing groundwater or nearby bodies of water is crucial, as it can seep into the construction site, causing flooding or instability.
  • Ventilation: For larger spaces like subway systems or underground factories, managing air quality and ensuring proper ventilation is critical to maintain worker safety and operational viability.
  • Safety: Underground construction can involve risks like cave-ins, gas leaks, or even underground explosions, which need to be managed carefully through engineering and safety protocols.

Environmental and Social Impact

  • Surface Impact: While underground structures minimize above-ground disruption, their construction can still affect surface traffic, buildings, and utilities.
  • Sustainability: Many cities are increasingly looking at underground construction as a sustainable solution to urban expansion by reducing the footprint above ground, protecting green spaces, and promoting energy-efficient designs.

Modern Innovations and Technology

  • Tunnel Boring Machines (TBMs): These machines have revolutionized tunneling by making it faster, more efficient, and safer. They can excavate and install tunnel linings simultaneously.
  • New Materials: Advances in materials like high-strength concrete, geosynthetics, and fiber-reinforced polymers (*FRPs*) are improving the durability and efficiency of underground structures.

Underground construction, particularly tunneling, involves several critical techniques and processes. Let’s dive into some of the key components:

Tunnel-Shaft Sinking and Construction

A shaft is a vertical or inclined opening that provides access to underground tunnel construction. It serves as an entry point for workers, materials, and equipment.

Shaft Sinking Process

  1. Surveying and Planning: Before digging, the site is carefully surveyed, and geological conditions are analyzed to choose the right location for the shaft.
  2. Excavation: Shaft sinking involves digging vertically through soil and rock layers to reach the depth of the intended tunnel. The excavation process is carried out in cycles, and the method depends on the type of ground:
    • *Drill and Blast:* Used in hard rock, where explosives are used to break the rock, and the material is removed.
    • *Mechanized Shaft Boring:* TBMs or shaft boring machines are used to bore through softer soils or rock, making the process faster and safer.
  3. Ground Support: As the shaft is deepened, support systems such as steel liners, shotcrete, and rock bolts are installed to stabilize the surrounding ground.
  4. Water Control: If groundwater is encountered, dewatering pumps or grouting techniques are used to manage water ingress and prevent flooding.

Microtunneling: Trenchless Technology

Microtunneling is a trenchless construction method used to install small-diameter tunnels and pipes underground with minimal surface disruption.

Microtunneling Process

  • Remote-Controlled Machine: A microtunneling boring machine (*MTBM*) is used, which is operated remotely from the surface. The machine excavates the soil and simultaneously installs the pipe or tunnel lining behind it.
  • Slurry System: In soft soils, a slurry system is often used to carry the excavated material out of the tunnel. The slurry is pumped back to the surface and separated from the debris.
  • Laser Guidance: Microtunneling systems are highly precise and use laser guidance to ensure the alignment and grade of the pipe.
  • Applications: Microtunneling is commonly used for installing underground utilities (e.g., sewage, water, gas lines) in urban areas where surface disruption must be minimized.

Tunnel Driving in Hard and Soft Strata

Tunneling in hard strata (rock) and soft strata (soil) requires different methods and equipment:

Tunneling in Hard Strata (Rock)

  • Drill and Blast Method: Explosives are used to break hard rock, after which the debris (*muck*) is removed. This is typically followed by reinforcing the tunnel walls with shotcrete, steel arches, or rock bolts.
  • Tunnel Boring Machine (TBM): In modern projects, TBMs are used for driving tunnels through hard rock. These machines have rotating cutter heads that grind the rock while simultaneously installing tunnel linings.
  • Ground Stability: Hard strata generally provide good natural stability but may require localized reinforcement if fractures or faults are encountered.

Tunneling in Soft Strata (Soil)

  • Shield Tunneling: In soft soils, shield tunneling or cut-and-cover methods are used. A shield (protective frame) is driven forward while excavation takes place inside the shield to prevent collapse.
  • Slurry or EPB TBM: Earth Pressure Balance (*EPB*) or slurry TBMs are used in soft ground. These machines maintain balance between external soil pressure and internal tunnel pressure, reducing the risk of cave-ins.
  • Ground Support: Immediate support, such as tunnel linings or segmental rings, is necessary in soft soils to prevent collapse.

Bedding of Conduits and Utility Support

Bedding refers to the preparation of the trench or tunnel floor to support the installation of underground pipes or conduits.

Conduit Bedding Process

  1. Excavation of Trench: The trench or tunnel base is excavated to a specified depth and width.
  2. Placement of Bedding Material: A layer of bedding material (such as gravel, crushed stone, or sand) is spread and compacted to provide a stable, even surface for the pipe or conduit.
  3. Conduit Placement: The conduit is carefully placed on the bedding to ensure proper alignment and support.
  4. Backfilling: After the conduit is installed, backfill material is placed and compacted around it to provide support and protection from external forces.

Importance of Bedding

  • Proper bedding ensures the pipe or conduit is uniformly supported, preventing settlement or misalignment over time.
  • It also protects the pipe from damage due to external loads and soil movement.

Soil Excavation Methods

Soil excavation involves removing earth to create space for underground construction. The method used depends on the type of soil, depth, and scale of the project.

Excavation Methods

  • Manual Excavation: Used for small or shallow excavations where machinery is not feasible.
  • Mechanical Excavation: Excavators, backhoes, and bulldozers are used to remove large volumes of soil quickly and efficiently.
  • Trenchless Excavation: Methods like microtunneling or horizontal directional drilling (*HDD*) are used when surface disruption must be minimized.

Compaction Technology and Soil Stabilization

Compaction is the process of densifying soil to increase its load-bearing capacity and reduce the risk of settlement.

Soil Compaction Methods

  • Vibratory Rollers: Heavy rollers equipped with vibration mechanisms compress soil to increase density, typically used for granular soils.
  • Tampers: Handheld or machine-powered tampers compact soil in small areas or around utilities.
  • Static Rollers: These heavy machines apply downward pressure to compact soil, useful in clay-rich soils.
  • Dynamic Compaction: Involves dropping heavy weights onto the soil surface to increase compaction at deeper levels, often used in large-scale projects.
  • Soil Grouting: For areas where traditional compaction is difficult, grouting involves injecting cement or chemical solutions into the soil to stabilize and densify it.

Importance of Compaction

  • Prevents Settlement: Compaction ensures the ground remains stable under the weight of structures, reducing future settlement.
  • Improves Soil Strength: Well-compacted soil has a higher load-bearing capacity, which is crucial for supporting tunnels, shafts, or underground utilities.
  • Reduces Water Penetration: Compacted soil resists water infiltration, reducing the risk of erosion or water-related issues.