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

Geotechnical investigations are critical for tunnel design and planning to minimize risks, ensure structural stability, and optimize construction methods. The process involves a systematic approach to characterizing the subsurface environment.

The primary stages and components of these investigations include:

1. Desk Study and Geological Mapping: Reviewing existing geological maps, borehole records, and aerial photographs to identify regional structural features, fault zones, and potential hazards. Field mapping confirms surface outcrops and identifies surface drainage patterns.
2. Geophysical Surveys: Non-destructive methods such as Seismic Refraction, Electrical Resistivity Tomography (ERT), and Ground Penetrating Radar (GPR) are used to delineate rock head profiles, detect cavities, and identify major discontinuities or water-bearing zones along the tunnel alignment.
3. Borehole Drilling and Sampling: Strategic drilling along the tunnel axis provides direct access to subsurface strata. Core recovery is essential for determining Rock Quality Designation (RQD), fracture frequency, and weathering grades. In soft ground, Standard Penetration Tests (SPT) and Cone Penetration Tests (CPT) are conducted.
4. In-situ Testing: Tests such as the Lugeon test (water pressure test) are performed to determine the hydraulic conductivity of the rock mass, which is vital for assessing groundwater inflow risks. Pressuremeter or dilatometer tests are used to estimate the deformation modulus of the ground.
5. Laboratory Testing: Collected samples undergo rigorous testing to determine physical and mechanical properties, including uniaxial compressive strength (UCS), triaxial shear strength, density, porosity, and swelling potential.
6. Groundwater Monitoring: Installation of piezometers to monitor water table fluctuations and pore water pressure is essential for assessing stability and potential drainage requirements during excavation.

The collected data is synthesized into a Geotechnical Baseline Report (GBR), which defines the ground conditions expected during construction. This baseline is fundamental for selecting the appropriate excavation method (e.g., TBM vs. Drill and Blast), determining support requirements (e.g., shotcrete, rock bolts, or steel ribs), and managing contractual risks associated with unforeseen geological conditions.