SHAFTS AND PORTAL

GET RID OF METHODOLOGY AND MOVE TOWARDS THE APPLICATION OF SHAFTS AND PORTALS IN TUNNEL

Introduction about Shaft and Portal in Tunneling

Shafts are vertical tunnels reaching from ground surface down to tunnel roof . They afford many advantages and used in constructing tunnel. When the position and course of tunnel are correctly aligned and accordingly set out on the surface shafts are sunk at suitable points on this tunnel line to aid tunneling advantages 

Location of Shaft and Portal

1. Shaft are verticals tunnels reaching from ground surface down to tunnel roof .They afford manifold advantages and uses in constructing tunnels
2. Shaft are sunk at suitable points on tunnel line to aid tunneling operations
3. Rectangular shape is preferred for temporary shaft and circular shape is preferred for vertical shaft 
4. Ideal site selection for shaft is valley

Advantages of Shaft and portal

1. They greatly expedite the work by adding two faces per shaft for driving
2. They afford outlets for excavated material and means of access into the tunnel for building materials 
3. They could be used as pumping shafts in case of large influx of water
4. They help correct alignment and help carry the centre line into tunnel proper
5. In long tunnels they afford ventilation and useful for exhausting smoke and foul air out of the tunnel

Operation in Shaft and Portal

The method consists of

• Drilling and Blasting
• Mucking 

Drilling

The centre cut or pyramidal cut pattern of drills holes is adopted.In case of large shafts stepping is resorted to facilitate mucking and drilling to go on simultaneously 

Mucking

Generally done by hand and loaded into buckets and lifted up.Twin buckets could be used so that as one rises the other descends.The explosive charge should be so controlled that the blasted pieces weigh between 20 lb and 200 lbs for easy handing 

Timbering:

As there is generally no horizontal pressure exerted inn rock shafts timbering is found necessary to carry guides for the cages and support lagging. Lagging is used to prevent small pieces of rock breaking loose and falling on and injuring workmen.

Timber sets are frames made of two side plates and pair of end plates.This is divided into two compartments for ladder way and hoist way .The sets are spaced at 5ft centres and each set is hung from one above by means of hanging bolts.The sets are made about 4" less around and fixed to shaft sides by blocks and wedges.

Shaft is being located over two portion:

1)Majority of shaft directly located centre line of tunnel.This shaft have advantage of hoisting out material easily from tunnel

Side shaft not over centre line .A transverse gallery or pilot tunnel required to connect with main tunnel.Turning of machinery at sharp curve is great disadvantage   

Pumping:

Shaft are usually wet.The wet seams are sealed off by cement grouting.When pumping has to be resorted to sinking pump is used ,slung from lowest timber set.The pump should be disconnected and hoisted up before commencing blasting

Pumping in Tunnel

Raising :

If rock is of a good variety ,shafts are also raised from tunnel heading instead of being sunk from above .This has advantage that the blasted muck drops into the tunnel and pumping will not be  necessary.Both these make this method are cheaper .The initial size of shaft is not more than about 5ft diameter.

CLASSIFICATION OF SHAFT

1. Temporary shaft: used for supply of material and removal of muck and generally used in valley
2. Permanent shaft : for providing ventilation
3. Vertical shaft : pilot shaft for mucking out
4. Inclined shaft : for removing muck by gravity

Types of Cut :

1. Central wedge cut – for rectangular shaft
2. Step/alternate cut – when shaft are large in c/s
3. Pyramid cut – for circular shaft

Design of Shaft Support

It is based on the horizontal pressure at several depths and  given by the formula,

                                                  H = K.W.D. 

Where,

H is the horizontal pressure in lbs/sq. ft.

W is the weight of soil in pounds per cubic ft. and

D is the depth in feet below the surface.

K is a constant depending on the type of soil. The value of

K for different soils is given in the table below

 

SOIL	VALUE OF K
Damp clay                     Moist sand and gravel   Wet sand Coarse gravel Water	0.16 0.22 0.34 0.6 1

Note:It is usual to connect 900mm high wall round edge of shaft opening to form protective fencing and prevent surface water flowing down shaft

Points to be kept in mind regarding SHAFT AND PORTAL

Drilling and Blasting: Pyramid or centre cut pattern is adopted 

Mucking:

1. Time for mucking and hauling is 1/2 to 1/3 *total time of construction of tunnel
2. Explosive charge weigh 90N to 900N having 9 m౩ capacity
3. Lashing is done for loading of muck into conveyance for disposal
4. Where gradient of tunnel is 1 in 50 attempt are made to make self mucking at inclination of 40°
5. If tunnel blocks due to non removal of muck called muck bound.

Timbering: Used to prevent lagging and used in self supporting rocks .Timber set are spaced @ 1.5m and timbering is done from 20m

Twin tunnel : In case of railway tunnel with double line it is economical to provide two small tunnel,in case if one block other can be used. 

1. It avoid head on collision
2. Separate entrance and exit of two traffic stream
3. Facility of carry out repairs

TWIN TUNNEL

This is all about Shafts and Portals