JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III TUNNELING ENGINEERING Previous Question Paper (September - 2021 - R18)

Tunneling techniques are selected based on geological conditions, tunnel length, diameter, and project constraints. The primary methods are categorized as follows:

1. Full-Face Method: The entire cross-section of the tunnel is excavated simultaneously. It is suitable for small to medium-sized tunnels in stable rock conditions. This method allows for rapid progress but requires robust support systems to be installed immediately after blasting.

2. Heading and Bench Method: The tunnel is excavated in two stages: a top heading followed by a lower bench. This is advantageous in medium-strength rock where the top heading acts as a pilot tunnel, allowing for easier control of the roof and safer installation of initial support before removing the lower section.

3. Drift Method: A small pilot tunnel, or drift, is excavated first, followed by enlargement to the full size. It is used in poor ground conditions or for very large tunnels to manage stability and facilitate ventilation and drainage. Common types include top, bottom, or side drifts.

4. Tunnel Boring Machine (TBM) Method: A mechanical method using a rotating cutter head to excavate the tunnel. It is highly efficient for long tunnels in consistent rock or soil. TBMs provide continuous excavation, integrated support installation, and minimal disturbance to the surrounding ground, making them ideal for urban areas.

5. Cut and Cover Method: Primarily used for shallow tunnels, a trench is excavated from the surface, the tunnel structure is built, and the trench is backfilled. This is common for metro stations and underpasses in urban environments where depth is limited.

6. New Austrian Tunneling Method (NATM): This approach treats the surrounding rock mass as a load-bearing component. It involves controlled deformation of the rock, monitored by instrumentation, and the application of flexible support (shotcrete, rock bolts, and wire mesh) to stabilize the ground before it reaches a state of failure.

7. Shield Tunneling: Used in soft, unstable ground or water-bearing strata, a protective shield supports the excavation face while the tunnel lining is installed behind it. This prevents collapse and controls groundwater inflow.

Tunneling in water-bearing strata requires specialized techniques to control groundwater inflow, stabilize the face, and prevent ground subsidence. The choice of method depends on the geological conditions, hydrostatic pressure, and the scale of the project.

The primary methods adopted include:

1. Compressed Air Tunneling: This method involves maintaining air pressure within the tunnel face higher than the external hydrostatic pressure. This balances the water pressure, preventing inflow and stabilizing the soil. It requires air locks for personnel and equipment, and strict safety monitoring to prevent decompression sickness.
2. Ground Freezing: This technique involves circulating a refrigerant (like brine or liquid nitrogen) through pipes installed in the ground around the tunnel alignment. This freezes the pore water, creating a temporary, impermeable, and structurally stable ice wall that allows excavation to proceed in dry conditions.
3. Dewatering: Using deep wells, well points, or vacuum systems, the groundwater table is lowered below the tunnel invert level before excavation begins. This is effective in permeable soils but may cause settlement in surrounding structures due to consolidation.
4. Grouting: Chemical or cementitious grouts are injected into the ground ahead of the tunnel face to fill voids, reduce permeability, and strengthen the soil mass. This creates a consolidated zone that limits water seepage.
5. Shield Tunneling with Slurry or Earth Pressure Balance (EPB): Modern TBMs are designed for water-bearing ground. A Slurry Shield TBM uses pressurized bentonite slurry to support the face and balance hydrostatic pressure, while an EPB TBM uses the excavated soil itself, pressurized within the working chamber, to counteract water pressure.

Selection of the appropriate method is based on a detailed geotechnical investigation to determine the permeability of the strata, the magnitude of hydrostatic head, and the potential impact on surface structures.