JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III TUNNELING ENGINEERING Previous Question Paper (February - 2022 - R18)

The geological requirements for a tunnel site are critical for assessing stability, excavation methods, and long-term safety. Key requirements include:

• Lithology and Stratigraphy: Identification of rock types, sequence of layers, and the presence of weak or swelling materials like clay or shale.
• Structural Features: Mapping of faults, folds, joints, and shear zones, as these discontinuities significantly influence rock mass quality and stability.
• Hydrogeological Conditions: Determination of the water table level, permeability of strata, and potential for water inflow, which dictates drainage and grouting needs.
• Rock Mass Quality: Assessment using classification systems like RMR (Rock Mass Rating) or Q-system to estimate stand-up time and support requirements.
• In-situ Stress State: Evaluation of tectonic stresses and overburden pressure to anticipate potential hazards such as rock bursts or squeezing ground.
• Weathering and Alteration: Assessment of the depth and extent of weathered rock zones, which may require heavier support or different excavation techniques.
• Presence of Gases: Detection of hazardous gases like methane or hydrogen sulfide that may be trapped in geological formations.

Geotechnical considerations in tunnel design focus on the interaction between the tunnel structure and the surrounding ground mass. The primary factors include:

• Ground-Structure Interaction: Designing the tunnel lining to accommodate the redistribution of stresses caused by excavation. This involves selecting between rigid, yielding, or flexible support systems based on ground deformation characteristics.
• Excavation Method Selection: Choosing between TBM (Tunnel Boring Machine), Drill and Blast, or NATM (New Austrian Tunneling Method) based on the strength, abrasivity, and blockiness of the ground.
• Stability Analysis: Evaluating the potential for roof collapse, face instability, or squeezing ground. This requires calculating the factor of safety against failure modes and determining the required support pressure.
• Groundwater Control: Implementing strategies such as pre-drainage, grouting, or using pressurized face TBMs to manage hydrostatic pressure and prevent inflow, which could lead to soil erosion or instability.
• Settlement and Surface Effects: Predicting the magnitude and extent of ground settlement above the tunnel, especially in urban environments, to mitigate damage to existing surface structures and utilities.
• Support Design: Determining the timing and type of primary support (e.g., shotcrete, rock bolts, steel ribs) to ensure the tunnel remains stable during and after construction.