Tunnel Shafts: Types, Construction, and Advantages

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Tunnel shafts are vertical passages that connect the ground surface to the tunnel roof. Shafts have many advantages and are used in constructing tunnels. When the position and course of the tunnel are correctly aligned and accordingly set out on the surface, shafts are sunk at defined points on the tunnel line to aid tunneling operations.

Shapes of Tunnel Shafts

The tunnel shafts are generally circular or rectangular in shape. The rectangular shape may be adopted for temporary shafts, which are filled up after the tunnel's construction. The permanent shafts are lined with pressed steel linear plates or concrete of circular shape.

Types of Shafts

1. Inclined Shafts

These shafts are used when the shaft depth is less. The excavation of such shafts proceeds in the upward direction. The excavated material falls downward and is removed from the tunnel. The removal of muck by gravity is achieved by using a suitable gradient of 45 degrees or less.

2. Vertical Shafts

This type of tunnel shaft is usually easier and more economical to excavate when compared to the inclined shaft. In some cases, a pilot shaft is first driven upward before the shaft is excavated to the full section in the downward direction. Then, the pilot shaft is used for mucking out—the widening of the shaft to full-size proceeding downwards.

3. Circular Shafts

These shafts are usually circular and are lined with pre-stressed steel liner plates or concrete. 

Types of Tunnel Shafts based on Nature of Soil

1. Shafts in rock/hard soil
2. Shafts in soft ground

General Construction Procedure of Tunnel Shaft

The construction of the tunnel shaft involves the following steps:

1. Drilling of Shaft

The center cut or pyramidal cut pattern of drill holes is adopted in soil consisting of rocks or in hard soil. For shafts in soft shallow soils, an open excavation is adopted to a suitable depth at the given location. In the case of large shafts, the stepping method is employed to facilitate mucking and drilling simultaneously. 

2. Mucking

The mucking process is carried out by hand, and the soil is loaded into buckets, and lifted. However, two buckets could be used so that one descends as the other rises.

The explosive charge should be controlled so that the blasted pieces in the tunnel weigh between 9 kg and 90 kg for easy handling. Mucking loader is also used in case the size of the shaft is big.

3. Timbering

Though there is generally no horizontal pressure exerted, timbering is necessary to carry guides for the cages and support lagging in rock shafts. Lagging is used to avoid small pieces of rock from breaking loose and falling on and injuring workers.

Timber 'sets' are frames of two side plates and a pair of endplates. This is divided into two compartments, a ladderway, and a hoistway. The sets are spaced at 5 ft. centers, and each set is hung from the one above using 'hanging bolts.' The sets are fixed to the shaft sides by blocks and wedges.

4. Pumping

Shafts are usually wet due to the groundwater table. The wet seams are sealed off by cement grouting. A "sinking pump" is used when pumping is necessary to drain the excess water logged in the shaft. The pump must be disconnected and hoisted up before commencing blasting.

5. Raising

If the rock is hard and strong, shafts are also "raised" from the tunnel heading instead of sunk from above. This has the advantage that the blasted muck drops into the tunnel, and pumping will not be necessary.

The initial size of the shaft that is raised is not more than about 5 ft. in diameter.

Shaft Sinking in Soft Ground

Soft soil tunnels are broadly divided into two types:

1. Shallow Shaft

For shafts in soft shallow soils, an open excavation is adopted to a suitable depth at the given location.

1. Two timber sets are placed at the proper interval and braced with a diagonal piece of timber.
2. The sheeting consisting of 1.5 inches to 3-inch thick board is placed around the sets and kept in position by the backfilling.
3. The sheets are driven into the ground, and simultaneous digging below the sheets is carried out, ensuring that the sheets are kept vertical.
4. The frames are correctly strutted and wedged after this first sheeting set is entirely driven.
5. A slight benching or margin is given, and the second stage of excavation is commenced and completed similarly.
6. The timber sets should be located and fixed at pre-designed depths to withstand the side pressures as sheets are driven.

2. Deep Shafts

 A modified system of vertical fore-polling is implemented where the shaft depth is more. Short poling sheets of 5 ft to 6 ft are used and driven flaring out from timber sets, and the sheets are kept in the position by double wedges.

Precautions during Shaft Sinking Work

1. Avoid the general movement of the earth behind the sheeting by keeping sheets tightly against the soil's surface.
2. Deflection of sheets should be prevented as this will tend to form a cavity behind the sheets.
3. The top set near the ground surface should be strongly strutted with a heavy section to withstand a considerable amount of surcharge due to living loads, like cranes, material heaps, and machinery usually placed at the mouth of the shaft.
4. A 3 ft high wall is constructed around the edge of the shaft opening to form protective fencing. This also prevents the surface water from entering the shaft.

Design of Shaft Supports

It is based on the horizontal pressure at several depths. Therefore, the formula for the design of shaft support is:

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.
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:

Advantages of Tunnel Shafts

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 a large influx of water.
4. They help correct alignment and help carry the center line into the tunnel properly.
5. In long tunnels, they afford ventilation and are helpful for exhausting smoke and foul air out of the tunnel.
6. The shaft also acts as an escape route in accidents.

FAQs

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