Jawaharlal Nehru Technological University Hyderabad B.Tech III Year TUNNELING ENGINEERING Previous Question Paper (August - 2022 - R18)

Geological structures significantly influence the stability, alignment, and construction method of tunnels. Faults and shear zones act as planes of weakness, often leading to instability, excessive water inflow, and the risk of rock bursts or squeezing ground conditions. Folds, particularly anticlines and synclines, alter the stress distribution within the rock mass, potentially causing uneven loading on the tunnel lining. Joints and bedding planes dictate the blockiness of the rock mass; if these planes are unfavorably oriented relative to the tunnel axis, they increase the likelihood of wedge failures and overbreak. Additionally, the presence of dykes or igneous intrusions can create sharp contrasts in rock hardness, complicating excavation and requiring frequent changes in drilling or blasting parameters.

Geological investigations for tunneling are conducted in stages to minimize risks and optimize design. The primary methods include:

• Surface Geological Mapping: Detailed mapping of rock outcrops, structural features (faults, joints), and surface topography to identify potential hazards and determine the preliminary alignment.
• Geophysical Surveys: Non-destructive methods such as Seismic Refraction and Electrical Resistivity Tomography (ERT) are used to map subsurface layers, identify the depth to bedrock, and detect potential fault zones or cavities.
• Subsurface Exploration (Boreholes): Drilling core holes along the tunnel alignment is essential to obtain rock samples for laboratory testing. This provides direct data on rock quality designation (RQD), weathering grades, and groundwater levels.
• In-situ Testing: Tests like the Lugeon test for permeability, hydraulic fracturing for stress measurement, and plate load tests are conducted to determine the mechanical and hydrological properties of the rock mass in its natural state.
• Laboratory Analysis: Core samples are tested for uniaxial compressive strength, tensile strength, and mineralogical composition to assess the rock's excavatability and support requirements.