Methods of Tunnelling

Twin tunnel

        Twin tunnel is useful in railways.

        Sometimes it proves more advantageous to have two small tunnels instead of single large tunnel.

        Each of twin tunnel is excavated separately & tunnels are so spaced that disturbance caused in ground from construction of one tunnel does not affect the other .


Advantages of twin tunnel

        The facility for transportation is increased.

        If one tunnel is blocked up due to any reason the other tunnel can be conveniently used.

        If there is trouble of hot water it can be easily & effectively   managed.

        There is considerable improvement in the ventilation & drainage of tunnels.




        The actual doorways or main entrance of the tunnel are known as portals.

        Portals indicates the intersection point between the underground opening & the ground surface.

        The portals should be made massive in appearance. They  should look beautiful & attractive.

        Such a construction would encourage a layman to enter the tunnel with confidence of safety.



Pilot tunnel

        Sometimes it may be found that a horizontal approach to the tunnel line may be to closer & shorter compared to the deep vertical shafts .

        In such circumstances, a tunnel of small size called a pilot tunnel is driven parallel &close to the proposed main tunnel & short cross connecting tunnels are driven from it reach the proposed main tunnel to create operational faces.



Advantages of pilot tunnel

        The cross headings are convenient places to store tools & material during the construction work.

        It is found to be less costly than the shaft.

        The chances of any material failing accidentally down the shaft in to the tunnel during the construction work are considerable reduced

        It avoids the dislocation of the strata at the sides of the tunnel after the work is completed, the cross headings may be used as passage for the workers

        The shaft can be used as means of artificial ventilation by the use of fans.   



Following are methods of tunneling in soft

        Forepoling method

        Needle beam method

        Army method or case method

        American method

        British method

        Belgian method



Factors affecting methods of tunneling in soft soils

        Factors affecting in Forepoling method

        The process is slow & tedious requiring skilled labour

        Tunnels of small dimensions, for laying sewers , gas pipes,etc.,at ordinary depths, could advantageously be constructed by this method .

        it is very important that the sequence of operations has to be strictly adhered to , in the correct order.



Needle beam method

        Factors affecting in Needle beam method:

        It makes use of beams & bracings .Hence,there is difficulty in mechanical methods of concrete lining.

        The pushing of heavy beam by hand is laborious & tedious.

        It requires large number of trench jacks & they interfere with the efficient working of the gang.




Factors affecting Army method or Case method

        The only drawback of this method is that its use or application is limited for the construction of short tunnels of small cross-section.

        Factors affecting American method:

        The method is unsuitable for tunnels having flat bottoms.  



English method

        This method involves the use of  a lot of timber & frequent shifting of heavy timber logs back.

        The masons & excavators have to work alternatively & thus , the process requires more time as compared to other methods of tunneling.

        If the soil is unstable , the excavation of full section of tunnel will increase the danger of caving.



Factors affecting belgian method

        The disadvantage is due to the system of underpinning of the built arch .

         particularly when the avoidable subsidence of the soil may take place , causing settlements can be reduced by adopting R.C.C.arches.



Liner plates methods

        The liner plates method is somewhat  expensive.

        It is also relatively weak as plates are attached with nuts & bolts.



Factors affecting tunneling in rock compared with tunneling in soft soil

        It is more costly than in soft ground.

        It requires extreme care in carrying out the work as slight mistake may result in heavy loss of money.

        Timbering required will be less as rocks are self-supporting.

        Over-cutting of section has to be avoided.

        It allows the tunneling operations to be carried out at many sections along the tunnel length.



The following are different methods  of tunneling in rock

        Drift method

        Heading &bench method

        Full face method

        Cantilever car dump method



Factors affecting in drift method

        The enlargement work cannot be started until the central holr is constructed for the full length.

        That as the enlarging & benching work commences , mucking tracks have to be shifted frequently from bench to bench.

        This method is extremely costly & it is generally recommended for ground conditions which are difficult to solve.



Factors affecting in heading & bench method

        In this method enlargement work cannot be started until the central hole is constructed for full length.

        This method is extremely costly .

        It is generally recommended for ground conditions which are difficult to solve.