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Tunneling is one of the most intricate and expensive projects in engineering and construction. With increasing demands and the emphasis on reducing the travel time between any two places becoming a priority, the risks and challenges involved in the construction of tunnels have increased manifold.
The concept of tunnels was first executed in the year 1843 when the first underwater tunnel was constructed to connect the north and south bank of the River Thames in London. After a lot of hardship and a series of failures, though the tunnel was built successfully, it is still considered a financial failure.
In order to deal with these failures, extensive planning and surveying goes into tunneling projects. It is, therefore, important to understand the types of tunnel failure, their causes, and the remedial measures to prevent tunnel failures.
Types of Tunnel Failure, Causes, and their Corrective Measures
1. Ground Collapse near the Tunnel Portal
Cause for Ground Collapse
The reason for the collapse of the ground near the tunnel portal is due to the excessive build-up of pore water pressure, which could be due to heavy rainfall.
The umbrella technique which holds the material surrounding the periphery of the tunnel and reduces the risk of sudden collapse should be followed.
2. Wrapping and Twisting of Tunnel Support System
Reasons for Wrapping and Twisting
Due to the continuous ingress of water to the bottom of the tunnel portal, the bearing capacity of ground soil near the portal decreases, which causes the concrete lining to settle into the ground and therefore leads to the wrapping and twisting of the tunnel support system.
The portion which is subjected to the maximum stress level is the invert level of the tunnel; therefore, to protect the invert, a systematic approach in designing the drainage system should be followed.
3. Improper Blasting Techniques
Reasons for Improper Blasting
During the construction of a tunnel in mountainous regions, it is difficult to hold the tunneling process using Tunnel Boring Machines (TBM) due to the cost factor involved in the transportation of TBM and strengthening of the existing transportation system. Therefore, the alternative approach is adopted, i.e., the use of blasting for the cut and cover method.
If the blasting is not designed correctly, then it can lead to the squeezing and bursting of the rock mass, which may eventually lead to the complete collapse of the tunnel.
The blasting system should be designed in such a way that it incorporates all the functional parameters such as burden, stemming length, bench spacing, control of fly rock, etc.
4. Cavity Formation
Reasons for Cavity Formation
After the conclusion of the blasting process, cavities may generate in the surrounding rock mass. If the assessment of cavities in the alignment of the tunnel is not worked out before the advancement of each and every trail, then it can lead to the failure of the tunnel near the advancement portal, which consequently may increase the accident rate.
Before each and every advancement in the tunnel section, trail holes should be made, and if any cavity is observed, it should be filled with bentonite slurry.
5. Presence of Fault and Fracture Zones
Reasons for Fault and Fracture Zones
Due to the blasting technique, the faults and fractures get loosened, and therefore the loosened rock mass creates an excessive shear force on the periphery of the existing tunnel, which can cause shear failure and cracks into the concrete lining.
During the construction of a tunnel, if any significant fault or fractures are present, then the alignment of the tunnel should be changed instead of proceeding with the same alignment. If alignment can’t be changed due to unavoidable reasons, then the spacing of fault should be monitored with proper monitoring equipment.
If it exceeds the permissible limit, then immediately, chemical stabilization (grouting) should follow. Near the fault or fracture regions, the strength of tunnel support should be higher than the normal capacity.
Reasons for Heaving
After the commencement of the tunnel service, if swelling clay soil is observed above the tunnel crown, it will be subjected to hydrothermal pressure, which in turn, can cause an increase in the volume of soil particles, and therefore causing heaving at the base of the pavement surface.
Rock bolts or anchor bolts are very useful in case of swelling clay soils because they reduce the passive flow of material on the soil particles, and hence they should be placed around the periphery of the tunnel and also below the base of the pavement surface.
1. Tunnel Construction Techniques and Their Details
2. Tunnel Surveying -Methods and Procedures of Tunnel Surveying
3. Tunnel Engineering -Features, Advantages and Methods of Tunneling in Construction