The proper investigation and the study of the geological characteristics of the soil in the site through which the work of constructions of tunnels, dams, roads, buildings etc are to be carried out is known as geological investigation.Site investigation is overall evaluation of specific site condition which is selected for construction of any civil engineering Infrastructures. The principal objective of site investigation are as follows:

1. To determine the sequence, thickness and Lateral extent of the soil Strata and bedrock. 

2. To identify the groundwater condition.

3. To obtain the representatives samples of soils or rocks for identification and classification and if necessary for use in laboratory test to determine the relevant soil properties.

4. To conduct in-situ test to assess appropriate soil characteristics.

5. To explore the source of the construction materials.

The proper investigation and the study of the geological characteristics of the soil in the site through which the work of constructions of tunnels, dams, roads, buildings etc are to be carried out is known as geological investigation.Site investigation is overall evaluation of specific site condition which is selected for construction of any civil engineering Infrastructures. The principal objective of site investigation are as follows:

1. To determine the sequence, thickness and Lateral extent of the soil Strata and bedrock. 

2. To identify the groundwater condition.

3. To obtain the representatives samples of soils or rocks for identification and classification and if necessary for use in laboratory test to determine the relevant soil properties.

4. To conduct in-situ test to assess appropriate soil characteristics.

5. To explore the source of the construction materials.

The types of geological investigation are as follows:

1. Surface Investigation and

2. Subsurface investigation

Surface investigation:

 In this type of site investigation, data are collected from the surface which can be done by two methods:

1. Direct method and 

2. Indirect method

Direct methods:

 In this method, direct field visits are made and data are collected by observations, measurements and photography. The data includes information about:

1.  Lithology

2.  Soil and rock types, strength etc.

3.  Weathering grade of rocks.

4.  Folds, faults, joints etc.

5.  Landslides and geohazards .

Indirect methods:

Investigation in which preliminary data are obtained about any specific site without visiting directly in the site is termed as indirect method. There may be different means by which such investigation can be carried out:

 Literature review:

The study of existing books, articles, maps and previous reports of project area is known as literature review.

 Map analysis:

The study and analysis of available topographic map, aerial photographs and satellite images to evaluate terrain, slope orientation, drainage and other geomorphic feature is known as map analysis.

 Subsurface investigation:

 In this type of Investigation, subsurface details are done. This is generally done after site investigation. It is done by two methods:

1.  Direct subsurface investigation and

2.  Indirect subsurface investigation

 Direct subsurface investigation:

Direct contact with the subsurface details are made in this method. This method helps in:

1. Determination of depth, thickness, types of rocks and soil.

2. Identification of weathering grade.

3. Prediction of GWT conditions.

4. Prediction of presence of aquifers.

 This method includes the following parameters:

1. Excavation works.

2. Drilling of boreholes.

Indirect Subsurface Investigation:
It is the type of Investigation in which  sub-surface data are obtained without direct contact with subsurface materials and structures. This method helps in:

1. Predictions of presence of aquifers.

2. Determination of GWT.

3. Finding discontinuities.

 Various indirect methods of site investigations are:

  Seismic method:

 The velocity of Elastic waves produced by shocks are analysed and compared for different subsurface structure to obtain the information or data.

 Gravity survey:

 The normal `g` calculated using altitude and the observed `g` by experiment are compared. Considerable difference shows the presence of unusual rock mass below the place.

 Electrical method:

 Resistivity, electro-chemical activity and dielectric constant are compared to obtain the data.

 Magnetic method:
The observed and theoretical values of magnetic intensity are compared to obtain and interpret the data.

The types of geological investigation are as follows:

1. Surface Investigation and

2. Subsurface investigation

Surface investigation:

 In this type of site investigation, data are collected from the surface which can be done by two methods:

1. Direct method and 

2. Indirect method

Direct methods:

 In this method, direct field visits are made and data are collected by observations, measurements and photography. The data includes information about:

1.  Lithology

2.  Soil and rock types, strength etc.

3.  Weathering grade of rocks.

4.  Folds, faults, joints etc.

5.  Landslides and geohazards .

Indirect methods:

Investigation in which preliminary data are obtained about any specific site without visiting directly in the site is termed as indirect method. There may be different means by which such investigation can be carried out:

 Literature review:

The study of existing books, articles, maps and previous reports of project area is known as literature review.

 Map analysis:

The study and analysis of available topographic map, aerial photographs and satellite images to evaluate terrain, slope orientation, drainage and other geomorphic feature is known as map analysis.

 Subsurface investigation:

 In this type of Investigation, subsurface details are done. This is generally done after site investigation. It is done by two methods:

1.  Direct subsurface investigation and

2.  Indirect subsurface investigation

 Direct subsurface investigation:

Direct contact with the subsurface details are made in this method. This method helps in:

1. Determination of depth, thickness, types of rocks and soil.

2. Identification of weathering grade.

3. Prediction of GWT conditions.

4. Prediction of presence of aquifers.

 This method includes the following parameters:

1. Excavation works.

2. Drilling of boreholes.

Indirect Subsurface Investigation:
It is the type of Investigation in which  sub-surface data are obtained without direct contact with subsurface materials and structures. This method helps in:

1. Predictions of presence of aquifers.

2. Determination of GWT.

3. Finding discontinuities.

 Various indirect methods of site investigations are:

  Seismic method:

 The velocity of Elastic waves produced by shocks are analysed and compared for different subsurface structure to obtain the information or data.

 Gravity survey:

 The normal `g` calculated using altitude and the observed `g` by experiment are compared. Considerable difference shows the presence of unusual rock mass below the place.

 Electrical method:

 Resistivity, electro-chemical activity and dielectric constant are compared to obtain the data.

 Magnetic method:
The observed and theoretical values of magnetic intensity are compared to obtain and interpret the data.

Road is an important infrastructure, the planning, designing, construction and maintenance of which are the major duties of Civil Engineers.  Geological consideration for the arrangement and stability of road can be explained under the following points:

1. Topography

2. Lithology

3. Geological structures

4. Weathering

5. Groundwater condition

Topography:

1.  A topographic map can be used to reveal the existence of various types of land features like valley, hills, in-flowing or out-flowing streams and their rugged topography, which are entirely responsible for the alignment of the road.

Lithology:

 The following points are to be considered:

1. Consolidated, competent massive rock is favorable and more stable since it requires less maintenance but it is costly and difficult to blast.

2. Unconsolidated incompetent rock is unfavorable and many complicated problem can arise in construction, but it is easy in blasting.

Geological Structures:

 If some plane of weakness like bedding plane, foliation plane, joint, shear zone are present in a rock in such a way that these planes are inclined towards the free side of the valley, then the rock has very high chance to fall along these planes. 

Joints:

It influences the stability of the cuts in the same way as the bedding planes. 

1. when joints are present in the large number, joints reduce even the hardest rock to a mass of loosely packed up blocks on the side of a cuts which could fall down. 

2. Even if the joints are few but are continuous and inclined toward the free side of cut, then there is a risk of slop during the presence of moisture. Jointed rocks are supported by the different retaining structures in the major road construction programme.

Faults:

1. Faults are the worst type of planes of potential failure and quite unfavourable.

2. Faults should not be left untreated in any case.

 Weathering :

1. Weathering can result on  undermining, rocks or debris fall etc  and hence is unfavourable.

2.  Better design is required when cuts pass through weathered zone of the rock.

Ground Water Condition:

 It is very necessary to find out the position of the ground water table of the site. Following points should be considered:

1. Ground water makes soil/rocks weak and unstable

2. Presence of aquifers,GWT at shallow depth makes the site hazardous.

3. Special consideration (drainage,lowering of GWT, etc.) are to be taken before construction of roads in such regions.

Road is an important infrastructure, the planning, designing, construction and maintenance of which are the major duties of Civil Engineers.  Geological consideration for the arrangement and stability of road can be explained under the following points:

1. Topography

2. Lithology

3. Geological structures

4. Weathering

5. Groundwater condition

Topography:

1.  A topographic map can be used to reveal the existence of various types of land features like valley, hills, in-flowing or out-flowing streams and their rugged topography, which are entirely responsible for the alignment of the road.

Lithology:

 The following points are to be considered:

1. Consolidated, competent massive rock is favorable and more stable since it requires less maintenance but it is costly and difficult to blast.

2. Unconsolidated incompetent rock is unfavorable and many complicated problem can arise in construction, but it is easy in blasting.

Geological Structures:

 If some plane of weakness like bedding plane, foliation plane, joint, shear zone are present in a rock in such a way that these planes are inclined towards the free side of the valley, then the rock has very high chance to fall along these planes. 

Joints:

It influences the stability of the cuts in the same way as the bedding planes. 

1. when joints are present in the large number, joints reduce even the hardest rock to a mass of loosely packed up blocks on the side of a cuts which could fall down. 

2. Even if the joints are few but are continuous and inclined toward the free side of cut, then there is a risk of slop during the presence of moisture. Jointed rocks are supported by the different retaining structures in the major road construction programme.

Faults:

1. Faults are the worst type of planes of potential failure and quite unfavourable.

2. Faults should not be left untreated in any case.

 Weathering :

1. Weathering can result on  undermining, rocks or debris fall etc  and hence is unfavourable.

2.  Better design is required when cuts pass through weathered zone of the rock.

Ground Water Condition:

 It is very necessary to find out the position of the ground water table of the site. Following points should be considered:

1. Ground water makes soil/rocks weak and unstable

2. Presence of aquifers,GWT at shallow depth makes the site hazardous.

3. Special consideration (drainage,lowering of GWT, etc.) are to be taken before construction of roads in such regions.

Geological Considerations in the site investigation of buildings:

1. The site selected should be hard enough to construct a building. The side should not be in marshy area. If the area is marshy, special care can be taken according to the need.

2.  The foundation should rest on the stable soil layer. If it is a bedrock, that will be the best. 

3. The stability of the building depends upon the compaction and consolidation.

4.  The groundwater conditions should be studied before construction of the building because due to the presence of water, the soil layer cannot bear load up to its original strength.

5. Consolidated, hard rock is favorable site for building construction.

6. Small or no dipping of the strata is quite favorable as such rocks are more competent and stable.

7. Unconsolidated soft rock or soil, presence of Steep bedding or shear planes are unfavourable and special design consideration of foundation should be taken in this case.

Geological Considerations in the site investigation of buildings:

1. The site selected should be hard enough to construct a building. The side should not be in marshy area. If the area is marshy, special care can be taken according to the need.

2.  The foundation should rest on the stable soil layer. If it is a bedrock, that will be the best. 

3. The stability of the building depends upon the compaction and consolidation.

4.  The groundwater conditions should be studied before construction of the building because due to the presence of water, the soil layer cannot bear load up to its original strength.

5. Consolidated, hard rock is favorable site for building construction.

6. Small or no dipping of the strata is quite favorable as such rocks are more competent and stable.

7. Unconsolidated soft rock or soil, presence of Steep bedding or shear planes are unfavourable and special design consideration of foundation should be taken in this case.

Geological Consideration in the Site investigation of Bridges:

Bridge is defined as a structure built over a river to provide the line between opposite sides.some considerations are:

Lithology:

•  Nature of Bed Rocks:

1. The foundation Bedrock should be massive, hard, consolidated, stable and durable enough to bear different loads throughout the proposed life of the bridge.

2. Weaker rocks are unfavourable and special consideration should be taken.

• Depth of bedrock below River water:

1. Competent and hard bedrock should be available at shallow depth as far as possible.

2. Special measures should be taken if competent bedrock is not available even upto large depths.

Geological Structures:

1. Folding and faulting may cause some uncertainty in establishing a perfect geological profile but are not, otherwise a negative factor.Acute fracturing and abundant jointing is, however, undesirable at the foundation level as these might cause settlement beyond the allowable limits.

2. When the bridge site are located in the zone of seismic activity, the foundations are required to be designed for additional seismic loads as specified in the codes of the respective areas. Factor of a scour cannot be ignored. 

Some additional factor that should be considered as follows:

1. The site should be at the minimum bank scouring of river due to water velocity.

2. Narrow width of river is favourable.

3. Less jointed and less fractured rocks are favourable.

4. Beds should be competent.

5. Fracture in filling should not be clay type.

6. Bedding should be across the river flow.

7. Site should be avoided from geological structures like faults, folds.

8. Water drainage should be studied. Special attention should be given to origin discharge of water catchment area of river. It is also studied for high level flood,debris potentiality etc.

Geological Consideration in the Site investigation of Bridges:

Bridge is defined as a structure built over a river to provide the line between opposite sides.some considerations are:

Lithology:

•  Nature of Bed Rocks:

1. The foundation Bedrock should be massive, hard, consolidated, stable and durable enough to bear different loads throughout the proposed life of the bridge.

2. Weaker rocks are unfavourable and special consideration should be taken.

• Depth of bedrock below River water:

1. Competent and hard bedrock should be available at shallow depth as far as possible.

2. Special measures should be taken if competent bedrock is not available even upto large depths.

Geological Structures:

1. Folding and faulting may cause some uncertainty in establishing a perfect geological profile but are not, otherwise a negative factor.Acute fracturing and abundant jointing is, however, undesirable at the foundation level as these might cause settlement beyond the allowable limits.

2. When the bridge site are located in the zone of seismic activity, the foundations are required to be designed for additional seismic loads as specified in the codes of the respective areas. Factor of a scour cannot be ignored. 

Some additional factor that should be considered as follows:

1. The site should be at the minimum bank scouring of river due to water velocity.

2. Narrow width of river is favourable.

3. Less jointed and less fractured rocks are favourable.

4. Beds should be competent.

5. Fracture in filling should not be clay type.

6. Bedding should be across the river flow.

7. Site should be avoided from geological structures like faults, folds.

8. Water drainage should be studied. Special attention should be given to origin discharge of water catchment area of river. It is also studied for high level flood,debris potentiality etc.

Dams are the solid barrier constructed across a river valley for impounding water. Geological investigation for dam site is performed in the surface or subsurface and carried out either by direct or indirect method.

 The important geological requirement which should be considered in the selection of dam site are as follows:

1. Narrow river valley

2. Bedrocks with shallow depths

3. Horizontal strata

4. Tilted beds

 Narrow river valley:

 At the proposed site, if the river valley is narrow, only a small dam is required, which means that the cost of dam construction will be least. On the other hand, if the valley is wide, a bigger dam is necessary which means the construction cost will be very high. But such sites should not be blindly selected without further investigations because sometimes it may have severe defect which may lead to serious leakage from the dam.

 Bedrocks at shallow depths:

 To ensure the safety and stability of the dam, it has to be necessarily on very strong and very stable rock. If such bedrock is at shallow depth, cost of dam will be ultimately lower. On the other hand, if such bedrock occur at great depth, the cost of foundation will be very high because it enables extensive work of excavation of loose overburden and concrete refilling. if the thickness of the loose overburden is negligible, the country rocks may be found to occur exposed frequently along the river valley.

 Horizontal strata:

 This geological situation is good at the dam site because the load of dam acts perpendicular to the bedding plane, which means that the beds are in an advantageous position to bear the load with full competence. Further, the seepage of reservoir water that may take place beneath the dam is effectively prevented by the weight of dams which acts vertically downward. Thus, the possible uplift pressure which is dangerous to the stability of the dam is effectively reduced. This also means that the leakage of the reservoir water is checked. However, if the strata are composed of alternating hard and soft rocks like quartzites and shales, it will be undesirable.

Tilted beds:

 There are many condition of tilting of beds affecting the dam load differently as follows:

 Beds with gentle upstream slope($10^0-30^0$)

 Such situation is ideal and much better and this is a typical example where the occurrence of a geological structure brings the advantage to the associated rock of a given site. This is because the resultant force acts more or less perpendicular to the bedding plane which are dipping in the upstream side. Hence, rocks are the best positioned to take the load effectively.

 Vertical beds:

 In the case of vertical beads, it will not pose problem of uplift. However, it will not have any advantage in terms of competence of rocks.

Dams are the solid barrier constructed across a river valley for impounding water. Geological investigation for dam site is performed in the surface or subsurface and carried out either by direct or indirect method.

 The important geological requirement which should be considered in the selection of dam site are as follows:

1. Narrow river valley

2. Bedrocks with shallow depths

3. Horizontal strata

4. Tilted beds

 Narrow river valley:

 At the proposed site, if the river valley is narrow, only a small dam is required, which means that the cost of dam construction will be least. On the other hand, if the valley is wide, a bigger dam is necessary which means the construction cost will be very high. But such sites should not be blindly selected without further investigations because sometimes it may have severe defect which may lead to serious leakage from the dam.

 Bedrocks at shallow depths:

 To ensure the safety and stability of the dam, it has to be necessarily on very strong and very stable rock. If such bedrock is at shallow depth, cost of dam will be ultimately lower. On the other hand, if such bedrock occur at great depth, the cost of foundation will be very high because it enables extensive work of excavation of loose overburden and concrete refilling. if the thickness of the loose overburden is negligible, the country rocks may be found to occur exposed frequently along the river valley.

 Horizontal strata:

 This geological situation is good at the dam site because the load of dam acts perpendicular to the bedding plane, which means that the beds are in an advantageous position to bear the load with full competence. Further, the seepage of reservoir water that may take place beneath the dam is effectively prevented by the weight of dams which acts vertically downward. Thus, the possible uplift pressure which is dangerous to the stability of the dam is effectively reduced. This also means that the leakage of the reservoir water is checked. However, if the strata are composed of alternating hard and soft rocks like quartzites and shales, it will be undesirable.

Tilted beds:

 There are many condition of tilting of beds affecting the dam load differently as follows:

 Beds with gentle upstream slope($10^0-30^0$)

 Such situation is ideal and much better and this is a typical example where the occurrence of a geological structure brings the advantage to the associated rock of a given site. This is because the resultant force acts more or less perpendicular to the bedding plane which are dipping in the upstream side. Hence, rocks are the best positioned to take the load effectively.

 Vertical beds:

 In the case of vertical beads, it will not pose problem of uplift. However, it will not have any advantage in terms of competence of rocks.

Reservoirs are very large artificial lake formed along the course of a water when dams are constructed across it due to impounding of the natural flow of river water.

From the geological point of view, a reservoir can be claimed to be successful if it is water tight. That is if it does not suffer from any serious leakage of water and it has a long life due to a very low rate of silting in reservoir basin.

Geological consideration in Reservoirs:

Geological considerations in site investigation of reservoir are:

Influence of Rock types:

Water tightness of a reservoir is very much influenced by the kind of rock that occurs in the site. If the rocks are porous and permeable i.e, aquifers, it will cause the leakage of water and hence such rocks are undesirable at the reservoir site. Hence, it is said that the occurrence of aquifer rock is undesirable and aquifuge or aquiclude are desirable at the reservoir sites.

Influence of geological structure:

As a consequence of the role played by tectonic force, the rock which occurs in nature often has different geological structures such as faults, folds, joints etc.The presence of such structure has a significant influence in decreasing or increasing the leakage through the rocks at the reservoir.

 For example: granite is known for its impermeable character.

Water tightness and influencing factor:

As a consequence of weathering which is a natural process in general, the surface is covered by loose soil and below it lies the fracture rocks i.e, subsoil. The bedrock which is massive occurs further below. When a river flows over such loose or fractured ground, it is natural that some water of river percolates underground. Before the construction of the dam, this leakage shall be less and Limited only to the extent over which the river flow occurs. But when the dam is constructed, the impounded water accumulates in large quantities in a reservoir which covers a very large area. 

Buried river channels:

Buried river channels which are more frequent in glaciated region are also a serious source of leakage when it occurs at the reserve site. This is because it is generally filled with loose and coarse sediments and allows heavy leakage of water along the course of the old channel.

Reservoir silting:

Silting of Reservoir is as harmful as the leakage of reservoir and both can cause the failure of the reservoir. The silting and leakage of the reservoir have a similar effect or result on the reservoir water quantity. In both cases, the quantity of water stored get reduced. Silting is totally different from leakage. In the case of silting, the deposition or settlement of sediments on the floor reduces the capacity of the reservoir; there by indirectly reducing the volume of reservoir water. In the case of leakage, water percolation directly causes the reduction in the volume of reservoir water.

Seismic activity in reserve areas and occurrence of landslides:

When a reservoir is put into active seismic zone, landslides are likely to occur. Hence, the possible consequences of the creation of a reservoir should be considered carefully. Due to a reservoir, large masses are likely to be waterlogged by the raising of the water table around the reservoir. This makes it necessary to determine which rock will be affected and how they will behave on the new conditions. Since, water is the single most important causes for landslide, the scope for their occurrence should be examined carefully. Further the raising and lowering of water levels in a reservoir represents the application and withdrawal of hydrostatic pressure and this could induce movement of the valley due to slipping.

Reservoirs are very large artificial lake formed along the course of a water when dams are constructed across it due to impounding of the natural flow of river water.

From the geological point of view, a reservoir can be claimed to be successful if it is water tight. That is if it does not suffer from any serious leakage of water and it has a long life due to a very low rate of silting in reservoir basin.

Geological consideration in Reservoirs:

Geological considerations in site investigation of reservoir are:

Influence of Rock types:

Water tightness of a reservoir is very much influenced by the kind of rock that occurs in the site. If the rocks are porous and permeable i.e, aquifers, it will cause the leakage of water and hence such rocks are undesirable at the reservoir site. Hence, it is said that the occurrence of aquifer rock is undesirable and aquifuge or aquiclude are desirable at the reservoir sites.

Influence of geological structure:

As a consequence of the role played by tectonic force, the rock which occurs in nature often has different geological structures such as faults, folds, joints etc.The presence of such structure has a significant influence in decreasing or increasing the leakage through the rocks at the reservoir.

 For example: granite is known for its impermeable character.

Water tightness and influencing factor:

As a consequence of weathering which is a natural process in general, the surface is covered by loose soil and below it lies the fracture rocks i.e, subsoil. The bedrock which is massive occurs further below. When a river flows over such loose or fractured ground, it is natural that some water of river percolates underground. Before the construction of the dam, this leakage shall be less and Limited only to the extent over which the river flow occurs. But when the dam is constructed, the impounded water accumulates in large quantities in a reservoir which covers a very large area. 

Buried river channels:

Buried river channels which are more frequent in glaciated region are also a serious source of leakage when it occurs at the reserve site. This is because it is generally filled with loose and coarse sediments and allows heavy leakage of water along the course of the old channel.

Reservoir silting:

Silting of Reservoir is as harmful as the leakage of reservoir and both can cause the failure of the reservoir. The silting and leakage of the reservoir have a similar effect or result on the reservoir water quantity. In both cases, the quantity of water stored get reduced. Silting is totally different from leakage. In the case of silting, the deposition or settlement of sediments on the floor reduces the capacity of the reservoir; there by indirectly reducing the volume of reservoir water. In the case of leakage, water percolation directly causes the reduction in the volume of reservoir water.

Seismic activity in reserve areas and occurrence of landslides:

When a reservoir is put into active seismic zone, landslides are likely to occur. Hence, the possible consequences of the creation of a reservoir should be considered carefully. Due to a reservoir, large masses are likely to be waterlogged by the raising of the water table around the reservoir. This makes it necessary to determine which rock will be affected and how they will behave on the new conditions. Since, water is the single most important causes for landslide, the scope for their occurrence should be examined carefully. Further the raising and lowering of water levels in a reservoir represents the application and withdrawal of hydrostatic pressure and this could induce movement of the valley due to slipping.

Tunnels are the underground passage through Hills and mountains used for transportation as well as in hydropower sector.

Geological consideration for successful tunneling:

The safety, success and economy of tunneling depends heavily on the various geological conditions prevailing at the site. The relevance of this aspect in tunneling is described below:

Lithology:

1. Tunneling through hard rock is safe and no temporary support is required.

2. Tunneling through soft rock is unsafe and temporary and permanent supports are required.

Importance of geological structures:

The knowledge of geological structures is very important for the following two reasons:

1. They modify the competency and suitability of a rock for tunneling.

2. They can create or prevent groundwater problems which are important in tunneling.

   Joints, fault, fold and tilted characters are most common structural features associated with rocks.

Effect of joints at the tunnel site:

Closely spaced joints in any type of rocks are harmful. Constant roof fall, heavy over break and abundant leakage occurs in closely spaced joints.This is the reason, tunneling is very expensive and difficult. Closely spaced joints may further lead to overbreak which is undesirable. Another problem that is likely to arise due to joints is the groundwater problem.

Effect of faults at tunnel site:

Faults are harmful and undesirable because they create a variety of problems. The active fault zones are the place where there is scope for the recurrence of faulting which will be  accompanied by the physical displacement of litho units. Such faults lead to dislocation and discontinuity in the tunnel alignment. 

Effect of folds at tunnel site:

1. Folds represent the deformation of rocks under the influence of tectonic forces. Hence, folded rocks will be under considerable strain. When excavation for tunnels is made in folded rocks, it gets the opportunity to release the strain. Such release may occur in the form of rock burst. This complication may appear when tunneling is done in such folded structures.

2. Arrangement parallel to the axis of the fold is desirable because formation with similar stress, strain condition are encountered along the course of the tunnel.

3. Tunnel alignment along crest may cause frequent fall of the rocks from the roof and is undesirable.

4. Tunnel alignment along the trough is unfavorable because rock masses there will be harder and more resistant. This means exhibition is a difficult process.

Effect of undisturbed or tilted Strata at tunnel site:

In case of horizontal strata, tunneling will be safe if the strata are thick because the overlying rock layers acts as a natural beam. But if the strata are thin or fractured, it is unsafe as the strata are not self-supporting and need support after/during excavation.

The following two cases may arise in inclined strata:

1. Tunnel axis parallel to dip direction

2. Tunnel axis parallel to strike.

Tunnel axis parallel to dip direction:

It is Safe, uniformly distributed load is offered and roof rock acts like an arch that transfer load on the side wall.

Tunnel axis parallel to the strike:

This case is unsafe, load distribution is not uniform and there are chances of sliding of Rock 

Importance of groundwater condition:

Among different problems that may occur in tunnels, the groundwater problems is the most dangerous and serious one. Groundwater not only creates difficulties but also aggravates other difficulties.

• Groundwater makes the moment of rock masses easier upon each other and it will therefore promotes slips along divisional plane like joint and bedding planes. 
• If the tunnel lies below the position of water table then groundwater problem is expected .

Over break:

Over break indicates the quantity of rock broken and removed in excess of what is required by the perimeter of the proposed tunnel. An excavation through hard rock necessarily involves the removal of some of the rock outside the proposed perimeter of the tunnel. The quantity of rock broken and removed in excess as required by the perimeter of proposed tunnel is known as over break.

 The geological factor which governs the amount of over break are;

1. Nature of rocks 

2. Orientation and spacing of joint or weak zones in them.

3. Orientation of the bedding plane and thickness of bed with respect to the alignment of tunnels in case of sedimentary rocks,

Massive and soft rocks of a homogeneous nature cause less over break than harder rocks with well developed joints or weak zones.  In sedimentary rocks, thin formations and those with alternating hard and soft strata produce more over break. This is so because, during exhibition, softer rock yield more readily than the harder ones. In metamorphic rocks, foliated and soft formations like Slate and schist produce more over break if the tunnel lies parallel to them and less over break if they are mutually across. 

The factor of over break is important because it adds to the cost of tunneling, particularly if lining is required. Hence, it is desirable that over break should be as minimum as possible.

Tunnels are the underground passage through Hills and mountains used for transportation as well as in hydropower sector.

Geological consideration for successful tunneling:

The safety, success and economy of tunneling depends heavily on the various geological conditions prevailing at the site. The relevance of this aspect in tunneling is described below:

Lithology:

1. Tunneling through hard rock is safe and no temporary support is required.

2. Tunneling through soft rock is unsafe and temporary and permanent supports are required.

Importance of geological structures:

The knowledge of geological structures is very important for the following two reasons:

1. They modify the competency and suitability of a rock for tunneling.

2. They can create or prevent groundwater problems which are important in tunneling.

   Joints, fault, fold and tilted characters are most common structural features associated with rocks.

Effect of joints at the tunnel site:

Closely spaced joints in any type of rocks are harmful. Constant roof fall, heavy over break and abundant leakage occurs in closely spaced joints.This is the reason, tunneling is very expensive and difficult. Closely spaced joints may further lead to overbreak which is undesirable. Another problem that is likely to arise due to joints is the groundwater problem.

Effect of faults at tunnel site:

Faults are harmful and undesirable because they create a variety of problems. The active fault zones are the place where there is scope for the recurrence of faulting which will be  accompanied by the physical displacement of litho units. Such faults lead to dislocation and discontinuity in the tunnel alignment. 

Effect of folds at tunnel site:

1. Folds represent the deformation of rocks under the influence of tectonic forces. Hence, folded rocks will be under considerable strain. When excavation for tunnels is made in folded rocks, it gets the opportunity to release the strain. Such release may occur in the form of rock burst. This complication may appear when tunneling is done in such folded structures.

2. Arrangement parallel to the axis of the fold is desirable because formation with similar stress, strain condition are encountered along the course of the tunnel.

3. Tunnel alignment along crest may cause frequent fall of the rocks from the roof and is undesirable.

4. Tunnel alignment along the trough is unfavorable because rock masses there will be harder and more resistant. This means exhibition is a difficult process.

Effect of undisturbed or tilted Strata at tunnel site:

In case of horizontal strata, tunneling will be safe if the strata are thick because the overlying rock layers acts as a natural beam. But if the strata are thin or fractured, it is unsafe as the strata are not self-supporting and need support after/during excavation.

The following two cases may arise in inclined strata:

1. Tunnel axis parallel to dip direction

2. Tunnel axis parallel to strike.

Tunnel axis parallel to dip direction:

It is Safe, uniformly distributed load is offered and roof rock acts like an arch that transfer load on the side wall.

Tunnel axis parallel to the strike:

This case is unsafe, load distribution is not uniform and there are chances of sliding of Rock 

Importance of groundwater condition:

Among different problems that may occur in tunnels, the groundwater problems is the most dangerous and serious one. Groundwater not only creates difficulties but also aggravates other difficulties.

• Groundwater makes the moment of rock masses easier upon each other and it will therefore promotes slips along divisional plane like joint and bedding planes. 
• If the tunnel lies below the position of water table then groundwater problem is expected .

Over break:

Over break indicates the quantity of rock broken and removed in excess of what is required by the perimeter of the proposed tunnel. An excavation through hard rock necessarily involves the removal of some of the rock outside the proposed perimeter of the tunnel. The quantity of rock broken and removed in excess as required by the perimeter of proposed tunnel is known as over break.

 The geological factor which governs the amount of over break are;

1. Nature of rocks 

2. Orientation and spacing of joint or weak zones in them.

3. Orientation of the bedding plane and thickness of bed with respect to the alignment of tunnels in case of sedimentary rocks,

Massive and soft rocks of a homogeneous nature cause less over break than harder rocks with well developed joints or weak zones.  In sedimentary rocks, thin formations and those with alternating hard and soft strata produce more over break. This is so because, during exhibition, softer rock yield more readily than the harder ones. In metamorphic rocks, foliated and soft formations like Slate and schist produce more over break if the tunnel lies parallel to them and less over break if they are mutually across. 

The factor of over break is important because it adds to the cost of tunneling, particularly if lining is required. Hence, it is desirable that over break should be as minimum as possible.

It involves the following points:

1. Feasibility study,

2. RMR or Q system rock mass quality index,

3. Tunnel portal mapping

4. Face mapping

5. Support system design

6. Daily report about progress, geological structures, support system and materials used

7. Weekly report

8. Monthly report

9. Monitoring and evaluation.

It involves the following points:

1. Feasibility study,

2. RMR or Q system rock mass quality index,

3. Tunnel portal mapping

4. Face mapping

5. Support system design

6. Daily report about progress, geological structures, support system and materials used

7. Weekly report

8. Monthly report

9. Monitoring and evaluation.

Borehole logs are the graphical and vertical representation of core materials which are found from core drilling.It is the systematic presentation and tabulation of data obtained from such way. It includes

• Depth of bore holes,
• Lithological vibration in respective depth,
• Characteristic of discontinuity of rock,
• Core recovery,
• RQD,
• In-situ test conducted within hole,
• Water table etc.

For engineering geological propose, the basic objective of bore hole logging or core logging is to provide factual data/accurate and concise record of important geological and physical properties of materials that is significant for engineering purpose i.e, design and cost estimation. Borehole log provides in-situ facts of any site.

Borehole logs are the graphical and vertical representation of core materials which are found from core drilling.It is the systematic presentation and tabulation of data obtained from such way. It includes

• Depth of bore holes,
• Lithological vibration in respective depth,
• Characteristic of discontinuity of rock,
• Core recovery,
• RQD,
• In-situ test conducted within hole,
• Water table etc.

For engineering geological propose, the basic objective of bore hole logging or core logging is to provide factual data/accurate and concise record of important geological and physical properties of materials that is significant for engineering purpose i.e, design and cost estimation. Borehole log provides in-situ facts of any site.