Tunnel shafts are vertical passages that connect the ground surface to the tunnel roof. 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. Shafts have many advantages and are used in constructing tunnels.
The rectangular shape may be adopted for temporary shafts, which are filled up after the tunnel’s construction . The tunnel shafts are generally circular or rectangular in shape. The permanent shafts are lined with pressed steel linear plates or concrete of circular shape.
The excavation of such shafts proceeds in the upward direction. These shafts are used when the shaft depth is less. The removal of muck by gravity is achieved by using a suitable gradient of 45 degrees or less. The excavated material falls downward and is removed from the tunnel.
In some cases, a pilot shaft is first driven upward before the shaft is excavated to the full section in the downward direction. This type of tunnel shaft is usually easier and more economical to excavate when compared to the inclined shaft. Then, the pilot shaft is used for mucking out—the widening of the shaft to full-size proceeding downwards.
These shafts are usually circular and are lined with pre-stressed steel liner plates or concrete.
Shafts in rock/hard soil 2. 1. Shafts in soft ground
The construction of the tunnel shaft involves the following steps:
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 help mucking and drilling simultaneously.
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.
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.
This is divided into two compartments, a ladderway, and a hoistway. Timber ‘sets’ are frames of two side plates and a pair of endplates. 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.
Shafts are usually wet due to the groundwater table. The wet seams are sealed off by cement grouting. The pump must be disconnected and hoisted up before commencing blasting. A “sinking pump” is used when pumping is necessary to drain the excess water logged in the shaft.
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.
Soft soil tunnels are broadly divided into two types:
For shafts in soft shallow soils, an open excavation is adopted to a suitable depth at the given location.
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 wood sets, and the sheets are kept in the position by double wedges.
It is based on the horizontal pressure at several depths. Therefore, the formula for the design of shaft support is:
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:
Tunnel shafts are vertical passages that connect the ground surface to the tunnel roof. 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. Shafts have many advantages and are used in constructing tunnels.
Inclined Shaft 2. The types of tunnel shafts based on their shape are 1. Circular Shaft Vertical Shaft 2.
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.