Question

If you are asked to conduct geologic survey of rocks for potential sites of tunnels in Himalayas, summarize the activities as per geologic investigation.

Answer 1

The following are geologic investigation done for the rock tunning for when a tunnel or shaft is exavated. 1. The physical topography: A key constrains mainly geology watersource, historic structures; and land use. 2.Geotechnical Investigations :Geotechnical investigations are critical for proper planning of a tunnel. Selection

of the alignment, cross section, and construction methods is influenced by the

geological and geotechnical conditions, as well as the site constraints. Good knowledge of

the expected geological conditions is essential. Tunnel alignment is sometimes changed

based on the results of the geotechnical to minimize construction cost or to reduce risks.

3.The type of the ground encountered along the alignment would affect the selection

of the tunnel type and its method of construction.

4. Study of the impact of geological features on the tunnel alignment in the presence of

active or inactive faults. During the planning phase, avoid crossing a fault zone. If it is un

avoidable then proper measures for crossing it should be implemented. Presence of faults

or potentially liquefiable materials would be of concern during the planning process.

5.Geotechnical issues such as the soil or rock properties, the ground water regime, the

ground cover over the tunnel should be analysed. The investigation should address not just

the soil and rock properties, but also their anticipated behaviors during excavation.

6.The investigation should also address groundwater. For example, in soft grounded

tunneling, the stability of the excavated face is greatly dependent on control of the

groundwater. Dewatering, pre-draining, grouting, or freezing are often used to stabilize the

excavation.

7.Analysing the ground behavior during tunneling will affect potential settlements on the

surface. Measures to minimize settlements by using suitable tunneling methods or by

preconditioning the ground to improve its characteristics would be required.

8. Risk assessment is an important factor in selecting a tunnel alignment.

9. Design Process

The basic process used in the design of a road tunnel is:

• Define the functional requirements, including design life and durability requirements;

• Carry out the necessary investigations and analyses of the geologic, geotechnical and

geohydrological data

• Conduct environmental, cultural, and institutional studies to assess how they impact the

design and construction of the tunnel;

• Perform tunnel type studies to determine the most appropriate method of tunneling.

• Establish design criteria and perform the design of the various tunnel elements.

• Appropriate initial and final ground support and lining systems are critical for the tunnel

design, considering both ground conditions and the proposed method of construction.

• Perform the design in Preliminary and Final design phases. Interim reviews should be

made if indicated by ongoing design issues.

• Establish tunnel alignment, profile and cross-section.

• Determine potential modes of failure, including construction events, unsatisfactory long-

term performance, and failure to meet environmental requirements.