Geological investigation for any tunnel in any river valley project is a very important activity to begin with. The basic objective is to decipher in the field the following subsurface and surface geological information
1.Main morphological features of the proposed project area
2.Geological features of engineering importance.
3. What type of rocks lie down below, their location in three dimension, their geological sequence, folding, faulting and any abrupt change in the geological setting.
4.Probable engineering properties of soils and rocks.
5. Hydrological parameters, including ground water flow pattern and ambient water temperatures.
6. Subsurface gas presence, tectonic activities etc.
7. Data for applying rock mass classification systems.
8. Environmental information to generate base line data
The above in formations are needed to prepare geological cross section along the proposed tunnel alignment, delineating different rock formations, faults, folds, joints and shear, their disposition vis- a- vis the tunnel alignment.
The data base needed is quite varied and usually difficult to gather in the field due to various reasons, being inaccessible, exposures buried under thick soil and vegetation covers. This calls for special techniques, hence the investigations are multi disciplinary activity to be under taken with determination.
2. Geological Investigations are carried out in three phases as follows, a, Pre construction Explorations, b, Construction stage investigations, and c. Post construction monitoring
Pre construction explorations are a major activity done over a vast area of potential project site. The objective is to locate the tunnel with an insight in to the associated problems and probable solutions for the guidance for the construction engineers. Locating infrastructure like roads, buildings societal needs, and source of required construction material are also explored.
Construction stage investigations are primarily aimed at comparing the postulates with the realities. They are confined to the area of main project activity. The data revealed are used to effect rationalized corrections in construction designs and the construction process.
The post construction monitoring is to assess how the project components are behaving and is there any departure from the forecast made.

3. Pre- Construction Geological Exploration
Elaborate planning is needed so that the investigations can be conducted smoothly resulting in obtaining reliable data. It is essential to mobilize essential man power and the equipments well in advance so that there are no delays on that account.
3.1 Geographical map of the area be obtained on a large scale. The following should be available
a. Political, physiographic and location details like latitude, longitude and elevation be obtained.
b, Survey of India topo- sheets of the area.
c. Key plan of the area be prepared.
d, Any other prepared map of the area with higher resolution.
3.2 Necessary survey instruments be procures and placed on site ( as per the existing code
,if any). The appropriate BIS codes be used for planning and executing the field investigations
4.0 Identifying a Tunnel Alignment
Siting of a tunnel is a long drawn process. It is usually done routinely by GSI and the concerned state and central agencies. The investigations are done to further develop the basic idea by providing the desired data base for planning, designing and executing the project successfully.
5.0. Elements of Geological Exploration
5. 1. After prima- facie fixing up a tunnel alignment, efforts are made to decipher subsurface geological features of engineering importance.
5.2. The important features are as follows,
a, Bedding planes.
b, Fold/faults
c, Joints/shears
d, Surface exposure tracing
5.3 Tracing the exposures, their trends on and below the ground, dips, strikes by observation /estimation.
5.4 Preparing a rough map of the area and trying to generate a geological cross section along the tunnel alignment. The geological investigations as summarized above give a general picture of the subsurface conditions. This may form a basis for detailed geological exploration.
6.0 Detailed Field Survey
6.1 Minute details are gathered in detailed study with the help of various techniques. The following are the important techniques,
a, Direct Methods
i, Drifting

ii, Drilling
iii, Core logging
b, Indirect Methods
i, Geophysical field tests by various techniques like seismic, electromagnetic, gravimetric etc.
ii, Ariel surveys employing air born geophysical instruments
iii, Geo penetrating radar
6.2
Further Details
6.2.1.Drifting
Standard size drifts ( appropriate BIS code) are driven to prove the rock formation under investigation. Three-D logs are prepared ( as per appropriate BIS code) This provides direct access to the rock formations and one gets the perfect visual impression of the rock condition. The three D logs be prepared on map 1.100 to 1.200 scale. The logs should help in assessing Rock Mass Rating (RMR) and Q (Rock Mass Quality)
6.2.2 Drilling
Drilling is to be as per the appropriate BIS code. The location, depth to drill, bore hole spacing, machine type to be used, speed of drilling, need for double or triple tube barrel are to be decided as per the existing practices.
6.2.3 Core Logging
The cores are to be kept in boxes ( as per the BIS Codes). The cores be numbered, depth be marked, fines, sludge etc be properly collected and kept in bags. Lengths of individual core pieces be recorded. There are special tabulation process for this purpose.
6.2.4
The data generated out of exploration needs to be analysed so as to arrive at a better understanding of rock mass behaviour. Modern tools like 3 D modeling be used to predict the tunnel behaviour at critical places. The results will help in optimizing the tunnel support requirements. It may also lead to better engineering judgment.
6.2.5
Laboratory Tests
The rock cores are to be used for determining physical and mechanical properties like specific gravity, modulus of elasticity, poisons ratio, unconfined compressive strength, tensile and shear strength. Swelling, slake durability and hardness tests should also to be done.
6.2.6
Insitu tests
Some insitu tests are suggested such as primitive stress conditions, deformation properties and rock load and plastic field by repeated loading and unloading tests.

6.2.7
Engineering Geological Modeling
Field and laboratory investigations usually result in large data base. This data can be syatamitised so as to help in preparing geological model of the tunnel site. Computer based techniques are available for such model preparation.
6.2.8
Hydrological Studies
Ground water table position , its seasonal fluctuation should be studied. Permeability and porosity are to be determined in places where ground water may pose problems.
7.0 Construction Stage Investigations
The investigations are aimed at determining the actual rock conditions. These conditions are helpful in reassessing the rock mass behaviour and effecting necessary design changes. The tunnel support requirements can be optimized effecting economy and ensuring safety. The tunneling technique can be decided from a large number of techniques like conventional drill blast method, tunnel boring machine, heading benching, multiple drifting, fore poling and shield tunneling.
7.1Three D logging is usually done to record the actual geological features on scale 1.100 or 1.200. The data can also help in re assessing RMR and Q and using them for further designing purposes in balance of tunnel driving.
7.2 The data obtained during construction can again be subjected to numerical modeling on 3 DEC computer programme for better understanding of rock mass behaviour.
7.3
Instrumentation
Critical areas in tunnel can be instrumented to have a better understanding of rock mass behaviour affecting the tunnel stability and support competence.
The following instruments are generally installed to monitor the tunnel stability while construction,
1.
Closure pags on the tunnel wall to record periodically the closing in of tunnel walls internally.
2.
Multi point bore hole extensometers to assess the extent of broken zone around tunnel opening and its behaviour.
3.
Load cells on supports to measure rock loads being transferred by rocks to the installed supports.
4.
Pressure cells between the supports and tunnel wall rocks.
Instrumentation is a very specialized activity and is to be done under the guidance of specialists. A knowledgeable expert, and reliable instrument manufacturing company and an agency to install and monitor instruments are to be carefully chosen for better results.
The results are to be analysed by a seasoned expert who may also recommend design changes. The data obtained may be published for use by others . Instrumentation is a

growing subject and dissemination of knowledge will help in developing the subject for better use in future.
7.0
Post Construction Investigation
At this stage it is essential to know the behaviour of tunnel during operation. Some instruments can be installed during construction to monitor rock pressures and deformations. In the Himalayan region tectonic movements are believed to be going on .
They are very slow . Such movements are to be measured. How it is to be done can be decided by going in to the details of the scheme and its continuation during the project operation.
8.0
Some Suggestions
There is a great debate going on adequate geological explorations and every day new techniques and instruments are being developed. Some times there is a tendency to suggest tests right from the text books and project engineers are faced with enormous task.. There is thus a need to evolve a optimum exploration programme which can be implemented successfully.
The consensus between the engineers and geologist is a must.
Many insitu tests are in the process of evolution. One important test is insitu stress determination. The hydro fracture technique is the latest and preferred for use. The experience shows that better results are obtained at depth i.e beyond 500 metres. At shallow depths horizontal stresses are usually higher as compared to vertical ones. It is not supported by theory. Insitu stress determination is costly and be determined in situations where it is essential In tunnels it may be avoided however in case of large caverns it is essential.
Insitu tests for compression, tension and shear are also expensive and they result in large data scatter. More tests one does larger scatter results. Such tests are good for dam foundations but in tunnels it may be done under exceptional circumstances.
Elaborate investigations and rock tests be done where tunnel boring machine( TBM) is contemplated. Machines usually designed to tackle a predetermined geomining conditions. TBM may be unsuitable in fast changing rock conditions with water inrush.
Such conditions are common in the Himalayan region. The decision to induct TBM may therefore be taken with great caution in the Himalayan Region. We had no good experience so far in the immediate past.
Systematic instrumentation be done in all the tunneling projects to generate data for support optimization and data base generation to develop the subject of rock mechanics.