Drilling and Blasting Techniques in Tunnel Construction

Posted on Mar 30, 2025 in Geology

Overbreak Definition and Consequences

Overbreak is an undesirable effect produced when the volume of rock excavated due to blasting is larger than the theoretical volume. It can be expressed as the ratio of the difference between the actual cross-section (Sa) and the theoretical one (St): (Sa – St) / St. The over-profile or extra-profile can also be defined as the ratio of the difference between the theoretical and real areas of the cross-sections to the perimeter of the tunnel, excluding the floor: (Sa – St) / p.

Disadvantages include:

  • Extra grouting and more concrete backfilling required.
  • Additional mucking time.
  • Increased risk of caving.
  • Diminished safety.

Drilling and Blasting Excavation Method

Drilling and Blasting is an excavation method carried out using explosives, utilizing their energy to break the rock.

Conditions of Applicability:

  • Suitable for practically any tunnel where the Rock Mass Rating (RMR) is greater than 25.
  • The unique viable method in cases of extremely strong rock mass and/or very abrasive rock.
  • Best conditions are found in tunnels through strong rock mass and of short to medium length.

Alternatives: If the rock mass is weak (RMR < 25), hydraulic excavators or hammers (mechanical methods) are preferred. For very long tunnels, Tunnel Boring Machines (TBM) are typically used.

Steps of the Advancing Cycle:

  1. Drilling blast holes in the tunnel face.
  2. Charging explosives into the blast holes.
  3. Blasting and cleaning/ventilating gases produced.
  4. Removing the muck pile from the tunnel face.
  5. Support installation.

A typical cycle takes around 12 hours, allowing for approximately 2 blasts per day.

Charging Explosives

Typically, 300-400 kg of explosive are used per blast, with a powder factor of 1.5-2 kg/m³. Manual charging is used for dynamite-type explosives, while pneumatic charging is used for ANFO (Ammonium Nitrate/Fuel Oil). This step takes 2-2.5 hours, often using an elevator platform as auxiliary machinery.

Blast and Ventilation

Blast hole zones include cut holes, breaking holes, floor holes, and contour holes. Explosives in the blast holes are detonated in a sequence. The average air speed required in the tunnel for cleaning is 0.5 m/s, equating to 35-40 m³/s. Gases are diluted and removed from the face by fresh air. Fan power required is approximately 10 kW per 100m of tunnel length, with an air flow rate of 0.1 m³/s per kg of explosive. A waiting period of 15 minutes to 1 hour is necessary for ventilation.

Removing the Muck Pile

Each blast typically produces 120-240 m³ of rock volume (representing 2-4 meters of advance). This involves removing the excavated rock from the face and transporting it outside. Typical productivity is 60 m³/h using equipment like a loader (approx. 134 kW, 2-3 m³ capacity) and articulated dump trucks (approx. 183 kW, 20t capacity). This stage takes 2-4 hours.

Support Installation

Support typically involves a combination of bolts, steel ribs, and shotcrete. Common installations include 1 to 2 steel ribs per meter, 5-20 cm thick shotcrete, and 1-2 bolts per m². This takes 2-4 hours, often utilizing jumbos (for drilling/bolting) and robots for spraying concrete.

Cyclic Nature Considerations

Drilling and blasting is a cyclic method. If one of the operations cannot be finished on time, the subsequent blast schedule will likely be delayed. If a cycle is lost, it is not possible to recover the advance corresponding to that cycle. It is also not possible to restart the cycle immediately without losing time, because blasting is often carried out at fixed times during the day.

Advantages of Drilling and Blasting

  • Suitable for any cross-section shape.
  • Allows curves of small radius and larger longitudinal inclinations.
  • Adaptable to other auxiliary works.
  • The face is accessible, making some related work easier.
  • Less dependent on specific geological conditions compared to TBMs.
  • Less influenced by abrasive rocks compared to mechanical methods.
  • Lower initial investment, faster amortization, and less economic risk compared to TBMs.
  • Smaller machinery is easier to transport, with shorter setting up and dismantling times.

Disadvantages of Drilling and Blasting

  • Not suitable for weak rock or soils.
  • Discontinuous method, resulting in slower overall advance rates.
  • In large tunnels, several working faces (headings) might be necessary.
  • Blasting negatively affects the surrounding rock mass, potentially weakening it.
  • Consequently, stronger and heavier support systems are often required.
  • Blasting produces ground vibrations.
  • Generally involves a lower level of safety compared to TBMs.

Environmental Impacts of Tunnel Blasting

Atmospheric Pollution

  • Dust: Generated not only by tunneling but also by related works (e.g., muck transport, crushing).
  • Acoustic Noise: From fans, machinery, generators, etc.
  • Airblast Wave: Pressure wave produced by blasting.
  • Methane Emissions: Can pose safety problems in the tunnel (explosive atmosphere) and contribute to the greenhouse effect if present in the ground.

Earth Impacts

  • Ground Vibrations: Caused by blasting, potentially affecting nearby structures.
  • Settlements/Subsidence: Influence on the ground surface above the tunnel.
  • Chimney/Sinkhole Problems: Potential for localized collapse extending to the surface.
  • Influence on Water/Aquifers: Risk of pollution or alteration of groundwater regimes.
  • Landscape Influence: Impact from inert waste landfills (muck disposal).
  • Influence on Industrial Heritage: Potential damage to existing underground structures or surface heritage sites.
  • Influence on Geological and Paleontological Heritage: Disturbance or destruction of scientifically valuable features.

Groups Affected by Tunneling: Neighbours, local councils, environmental groups (ecologists).

Subsidence in Tunneling: Causes and Control

There are two main types of subsidence:

  1. Sinkhole Formation: Related to face instability, produced by the collapse of the advancing face. Tunnel collapse can lead to chimney formation, resulting in a sinkhole at the surface.
  2. Settlements: Related to ground deformation propagating from the tunnel excavation zone to the surface. This is not directly related to advancing face stability but rather to the overall ground response to excavation.

Influencing Factors

  • Depth: Particularly relevant for shallow tunnels.
  • Strength: Ground exhibiting soil-like behavior is more susceptible.
  • Structures: Buildings or infrastructure on the surface can be damaged. Critical for metro or railway tunnels under cities.

Analysis

Subsidence analysis involves:

  • Identification of magnitudes and variables.
  • Use of prediction methods and models (empirical, analytical, numerical).
  • Establishment of damage criteria for surface structures.

Measures to Control Subsidence

Project Phase:

  • Use empirical, analytical, or numerical methods to analyze potential subsidence.
  • Propose a tunnel layout that minimizes risks.
  • Propose specific mitigation actions in critical locations.

Excavation Phase:

  • Monitor ground displacements.
  • Calibrate prediction models used during the design phase with real data.
  • Carry out specific actions if necessary, such as compensation grouting, ground consolidation, structural reinforcements, or controlling excavation face pressure.