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Tunneling in Rocks

 Tunneling in the rocks is very difficult and tedious work. Nowadays tunneling in rocks is done with the mechanical methods only, because tunneling by manual labour is not done these days, as it was done during ancient times. 


Methods of tunnel construction in hard rocks differ from tunneling in soft grounds. Tunneling in rocks is much costlier than tunneling in soft ground. As rocks are self supporting, they require less supporting. As slight deviation will involve huge wastage of money, greater care is required in tunneling through rocks. In rocks the tunneling work can be started in various sections for expecting the work. 

SEQUENCE OF OPERATIONS 

All the operations in tunnel construction must be done in proper sequence and in well-planned manner. The sequence of various operations mainly depends on the type and size of tunnel, method of blasting, type of formation encountered etc. 

After locating the center-line, generally the work of tunneling is done in the following sequence: 

  1. Construction of shafts. 
  2. Transferring center-line to the inside of tunnel. 
  3. Deciding method of excavation. 
  4. Setting up and drilling holes for blasting. 
  5. Loading holes with explosives and blasting. 
  6. Ventilation and removing dust after blasting. 
  7. Carting the muck outside the tunnel. 
  8. Pumping and removing ground water from the inside of the tunnel. 
  9. Providing supports inside the tunnel. 
  10. Lining of the tunnel. 

METHODS OF TUNNELING 

All the methods of tunneling have been grouped as follows : 
(A) Tunneling in soft rock or soft ground : 
        1. American Method                          2. English Method 
(B) Tunneling in hard rock : 
        1. Heading                                          2. Heading and Benching 
        3. Full face                                         4. Drift 
        5. Perimeter method                           6. Pilot tunnel 

American Method  
American Method of Tunneling 

In this method first a drift is driven at the top and then it is supported by leggings, cap and vertical posts. After properly supporting the drift, it is widened on both sides gradually and also supported by timber planks and struts. In this way we gradually go on widening and supporting till we reach at springing level of the arch. At springing point, wall plates are fixed, after driving posts shown by the dotted lines. In this way total roof load is transferred to vertical posts. Now we continue the excavation of the remaining portion of the section. After completing one section length we start excavation in forward section in the same way till the complete tunneling is done. 

English Method 
English Method 

The starting work is done in the same way as in American method viz. one drift is driven at the top and is supported by laggings crown bars and posts. After it, the drift is widened and supported on crown bars and laggings. In this way the work is continued up to springing point of the arch. Then sides are widened and sill is extended and supported over underpinning. In case of American method the load is transferred to vertical posts, but this method is taken by underpinning.  

Heading 

This is the most common used for tunneling in hard rocks. First of all one heading at top is excavated. The door level of this heading is kept at the springing level of the arch. After it sections 2 and 3 are removed. In this way the arched opening had been formed. Now the section is complete after excavating portions 4, 5, 6 & 7 is order to sequence. 

Heading and Benching 

In this method, first of all top portion of the tunnel is excavated by drilling holes and then blasting or excavating with other tools. After it, some of the lower portion is removed. The top portion is known as heading and it always remains ahead about 4 to 5 meters than the lower portion known as benching. 

Full Face 

In this method first of all No. 1 portion of top is removed, then No. 2 and 3 are removed and the tunnel is excavated up to its full face. Then this process is again repeated. It differs from the heading and benching, in this respect that in the former, excavation in top portion always remains ahead, whereas in the method, after excavating one full section the work is started in the next section. 

Drift 
Drift Method

In drift method of tunneling, one drift is excavated in the center of the tunnel and then it is widened in all sides by drilling holes and excavating. The position of drift may be in the side or in the center, but the central drift is the best, because blasting can be done easily and the depth of holes will not be more. It also provides good ventilation. 

Perimeter method 

In this method, the excavation is done along the perimeter in order of stages No. 1, 2, 3, 4 and 5 as shown in the figure. This method is also known as German method. 

Pilot Tunnel Method 

This method is used when the tunnel is to be constructed in very short time. One small tunnel known as 'Pilot Tunnel' is first constructed parallel to the main tunnel. Short tunnels are constructed to reach the center of the main tunnel at different sections. In this way workers can reach the main tunnel in different sections and start the excavation work in all the sections simultaneously. The excavated material is taken out through the pilot tunnel. 

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Location: Duliajan, Assam, India

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