Explain geological consideration for successful tunneling.

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 unsaf....

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.