Understanding Tunnel Construction Methods in Rock

Tunnel construction involves excavating underground passages. The choice of method depends heavily on the ground conditions, such as soil type, rock strength, presence of groundwater, and the required tunnel size and length. Different methods are better suited for specific geological environments.

Analysis of Tunnel Construction Methods and Their Suitability in Rock

Let's examine the suitability of the given methods for tunneling in rock:

• Drift Method: This method involves excavating a smaller pilot tunnel, often along the intended path of the main tunnel or as a means of exploration and ventilation. It is a common technique used in various ground conditions, including rock, especially when dealing with variable rock quality or as a preliminary step for larger excavations. It is considered suitable for rock.
• Compressed Air Method: This method is primarily used for tunneling in soft ground, particularly below the water table or where significant groundwater is present. Compressed air is pumped into the tunnel face to equalize the external water pressure, preventing water inflow and ground collapse. Rock formations are generally stable and impermeable compared to soft soil, making the use of compressed air unnecessary and ineffective. Applying high air pressure in a stable rock tunnel offers no benefit and can create logistical challenges. Therefore, this method is not suitable for tunneling in hard rock.
• Heading and Benching Method: This is a traditional and widely used method for excavating tunnels in rock, especially for larger cross-sections. The excavation is done in stages: the upper part (heading) is excavated first, followed by the lower part (benching). This allows for better control of the excavation process and ground support installation. It is highly suitable for a wide range of rock conditions.
• Full Face Method: In this method, the entire cross-section of the tunnel is excavated simultaneously. This is often done using powerful Tunnel Boring Machines (TBMs) in stable rock formations, or sometimes with conventional drilling and blasting in competent rock where the ground is stable enough to stand unsupported across the full face for a short period. It is suitable for stable rock conditions.

Suitability Comparison Table

Based on the analysis, the Compressed air method is the one that is not suitable for tunneling in rock formations. It is a method specifically designed to counter groundwater pressure in soft or permeable ground, a condition typically not found in stable hard rock.

Revision Table: Tunneling Methods

Additional Information: Challenges in Rock Tunneling

While rock is generally considered more stable than soil for tunneling, it presents its own challenges:

• Rock Hardness and Strength: Hard, strong rock requires powerful excavation methods like drilling and blasting or heavy-duty TBMs.
• Geological Features: Faults, joints, fissures, and variable rock types can affect stability and require specific support measures.
• Groundwater Inflow: Although generally less problematic than in soft ground, significant water inflows can occur through fissures and joints, requiring drainage or grouting.
• Stress Conditions: High in-situ stresses in deep rock tunnels can lead to squeezing ground or rock bursts, requiring substantial support.

Methods like Heading and Benching, Full Face (especially with TBMs), and variations of the Drift method are adapted to handle these specific challenges encountered when constructing tunnels in rock.