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.

 

 

Portals

•        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.

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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.