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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.

Corrective Measures

The umbrella technique which holds the material surrounding the periphery of the tunnel and reduces the risk of sudden collapse should be followed.

Use of umbrella technique to reduce the risk of sudden collapse.
Fig 1: Use of umbrella technique to reduce the risk of sudden collapse.

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.

Corrective Measures

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.

Twisting and wrapping of reinforcement.
Fig 2: Twisting and wrapping of reinforcement.

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.

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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.

Corrective Measures

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.

Proper sequence and formation of blasting hole for excavation of the tunnel.
Fig 3: Proper sequence and formation of blasting hole for excavation of the tunnel.

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.

Corrective Measures

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.

Advancement of drainage hole for accessing the cavity formation.
Fig 4: Advancement of drainage hole for accessing the cavity formation.

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.

Corrective Measures

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.

Grouting of the potential cracks or fractures.
Fig 5: Grouting of the potential cracks or fractures.

6. Heaving

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.

Corrective Measures

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.

Use of rock bolts, anchor bolts, spilling bolts, and radial bolts around the periphery of the tunnel.
Fig 6: Use of rock bolts, anchor bolts, spilling bolts, and radial bolts around the periphery of the tunnel.

Read More:
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

Ravi Panwar

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