An introduction to underground tunnels
By definition, a tunnel is a horizontal civil or mining engineering structure whose length is longer than twice the diameter of the structure (International Tunnelling Association, 2009).
Tunnels can be built underground, on the surface or submerged. Underground tunnels are common where construction on the surface is not feasible (Beaver, 1972). For instance, where cities are populated, other land uses exist or legal restrictions hinder surface construction. Underground construction provides a means to optimize land and water resources sustainably especially in critical ecosystems comprising wildlife UNESCO heritage sites (Ghimire & Reddy, 2013). Thus, possible adverse environmental impacts are mitigated whilst generating electricity concurrently (Tshering, 2012; Poff & Hart, 2002). Further benefits of underground structures include a better earthquake tolerance which makes them favourable in earthquake prone regions such as Tokyo (Sousa, 2010).
Underground tunnel construction is increasing at a fast rate globally. In the Ugandan context, tunnels have been used at the Kilembe mines; hydro tunnels are under construction at the Karuma project for electricity generation; and other potential uses for tunnels include utilities, access and transportation of wildlife and traffic. Though an old art on a global scale, tunnel engineering is not very well-known compared to other engineering fields (Road Tunnel Manual, 2009).
Tunnel structures are generally high hazard risk structures which cause tremendous losses including human life. Up to US$100 million in financial losses are registered in catastrophic tunnel failures and average project delays of 6 months (Sousa, 2010). Moreover, tunnel structures are very expensive to construct and are very capital-intensive ranging over millions of dollars (Spackova, 2012). Therefore, understanding the field of tunnelling is very important to avoid disastrous catastrophes and losses. Specifically, underground construction involves high risks and each undertaking is unique necessitating sharing of experiences which can only be built on a good appreciation of the subject. According to Greer (2012), identified potential failures can be mitigated to minimize the escalating record of tunnel failures. Figure 1 shows examples of failed tunnel cases. Figure 2 shows how tunnel failures resulting from works were stopped following an investigation or remedial incident affect economics of resources (Konstantis, Konstantis, & Spyridis, 2016).
According to Prior (2016), the actual origin of tunnels is uncertain but Beaver (1972) writes that in Europe and North America active tunnelling started in the 18th century. Before then, ancient Siberians, Babylonians, Egyptians, ancient Greeks and Romans built tunnels for mining, water transport, military and access (Sousa, 2010; Beaver, 1972). The purposes for which tunnels are built today are similar to former uses. Additional uses of tunnels include utility lines, storage, irrigation and passages of wind and wildlife (Sousa, 2010; Yi, 2006). Traditionally, miner gangs dug out the ground to create tunnel caverns until the advent of mechanical methods. The most remarkable event in the history of tunnelling was the invention of a tunnelling shield and drill.
A main characteristic of the industrialisation era was the need for less congested transportation routes in the 18th century during which attempts were made to tunnel under the navigable River Thames. Initially, manual hand excavation was futile without much progress until the first tunnelling shield was invented by Brunel in 1825 (Prior, 2016; Beaver, 1972). Brunel’s shield was rectangular-shaped and robust although tedious to use since parts had to be reassembled as the excavation advanced.
Later around 1864 Barlow invented the price excavator which was a more superior circular shield (Beaver, 1972). Barlow’s shield was used to construct the Gotthard Tunnel where a fast rate of tunnelling progress was achieved. Both inventions by Brunel and Barlow were unique and patented. Barlow’s shield was not an improvement to the Brunel shield.
Barlow’s shield was eventually modified significantly into the Greathead shield (Sousa, 2010; Beaver 1972). Greathead’s shield was first tested in the construction of the London railway and it earmarked modern tunnelling shields (Transport and Road Research Laboratory, 1973). Figure 3 illustrates the evolution of the tunnelling shields before the drilling age from (a) Brunel to (b) Barlow and eventually to (c) Greathead shield.
In 1953, further advancement included the rotary machinery developed by Robbins Company from which the model of most popular modern tunnel boring machines (TBMs) originated (Sousa, 2010). Figure 4 shows examples of TBM machine varieties. Machines reduce construction risks, are suitable for soft and water-logged conditions and they support and balance the weight of the surrounding ground and pressures (Hoek, Kaiser & Bawden, 1995), they give faster tunnelling progress rates and lower costs in hard ground (Blake, 1989).
Drilling started in 1876 when Brandt invented a pressurized rotary water-driven hydraulic rock drill during construction of the Simplon tunnel. However, its unsatisfactory application led to further research and eventually in 1970, manufacturers developed hydraulic oil percussive drills which earmarked modern drilling (West, 2005). Usually, drilling precedes blasting and both processes together are called the drilling and blasting (D&B) method of excavation. Small diameter holes up to 6 m deep are drilled into the ground following an expert-designed blasting pattern. Explosives are placed in the holes then the area is evacuated to a safe distance away from the excavation for safety purposes before the explosives are detonated to break the ground thereby creating the excavation.
After blasting, the area is scarified and cleared of muck. Figure 4 illustrates the D&B method which is used at the Karuma (600MW) hydropower construction project.
Types of tunnels
Tunnel types vary depending on their purpose, ground material properties, service function and design life. A tunnel name concisely describes it because it usually encompasses the construction method, material, geometry, purpose and method of support. Based on the method of construction, types include mined or bored tunnels, cut-and-cover tunnels, immersed or submerged tunnels, jacked-box tunnels and inverted arch tunnels (RTM, 2009). Cut-and-cover tunnels include top-down and bottom-up tunnels though the latter causes more surface disruptions. Mined tunnels are excavated at depth without unearthing the ground (soil, rock or mixed face) above.
Mixed face is a ground condition which involves combinations of different material types thereby making construction difficult. Based on the earth material out of which they are constructed, tunnel types include soft-ground and rock tunnels.
Tunnels are also differentiated based on their cross-sectional shapes (Figure 6) and method of support generally as either lined or unlined. Tunnel support improves overall stability which ensures functionality over its average design life of 100-150 years.
Where surface disruptions should be limited, top-down construction is preferred. It involves construction of permanent structural slurry support walls from the ground surface. The tunnel roof is tied into the support walls then it is either cast in-situ or built using precast beams and the surface is restored while tunnel construction progresses underneath the roof. After excavation is completed, the floor is constructed in the same way as the roof. Figure 7 illustrates the construction process for cut-and-cover top-bottom tunnels (RTM, 2009).
Bottom-up cut-and-cover tunnels are a suitable option when it is not critical to limit surface disruptions. The vertical sides of the ground aligning the tunnel sidewalls are supported with lateral earth supports and the ground in-between is excavated while piling the muck at the surface. The tunnel is then constructed in-situ, the trench is backfilled and the surface restored. Figure 8 illustrates the steps taken for construction (RTM, 2009).
Immersed or submerged tunnels
These tunnels are immersed or submerged in water and they are used as sea water crossings. Submerged floating tunnels (SFTs) are a variation of immersed tunnels used for deeper sea crossings where ship traffic exists (Ingerslev, 2003). Immersed tunnel elements are constructed from prefabricated structural steel, reinforced concrete or concrete-filled steel elements on shipways, dry docks or improvised floodable basins; then they are floated and carefully installed in a dredged trench (Sousa, 2010; Ingerslev, 2003). They are connected to cut-and-cover tunnels, linked to the surface and covered with backfill to protect them from water traffic. Figure 9 shows how an immersed tunnel is installed in position.
Jacked-box tunnels are constructed when surface disruptions are not acceptable and tunnels should be constructed at shallow depths near the surface. Construction is achieved by jacking a large precast reinforced concrete box horizontally through the ground thereby excavating the enclosed earth prism (Viggiani, 2012) and finally the excavation is supported. Figure 10 shows the construction process illustrating the launch slab along which the jacked box is driven into the ground to construct the tunnel shown in (b).
An invert is the bottom floor of a tunnel. Inverted-arch tunnels are constructed with a curved floor in order to limit deformation at the bottom of the tunnel. According to Zhongming (2015), inverting the arch increases a tunnel’s structural load-bearing capacity. Kawata et al (2014) explain that the increased structural load-bearing capacity is due to formation of a ring structure which causes the bending moment to be altered into an axial compressive force.
Soft ground tunnels
The term ‘soft ground’ refers to earth material of low-bearing strength generally less than 25 MPa (Ongodia et al, 2016). Challenges associated with tunnelling in soft ground include face collapse, flowing ground, squeezing and swelling ground, loose material and limited stand-up times. Therefore, great precaution is required including balancing the excavation by using tunnelling shields or the TBM which support the ground during the excavation process. Tunnelling in soft ground is usually done in stages depending on the stand-up time of the unsupported excavation according to a chosen sequential excavation method (SEM). Figure 11 illustrates examples of SEM construction sequences.
Rock varies in degree of strength and hardness. For softer rock, mechanized excavators are used whereas the TBM, SEM and the D&B methods are used to excavate hard rock. The TBM is most popular because it is fast and has a fairly controlled tunnelling ability despite it being an expensive high technology machine comprising a cutter head, gripper, shield, thrust cylinder, conveyor and rock reinforcement equipment (RTM, 2009). Figure 12 shows cutters with (a) grinding ability for hard intact rock, (b) less teeth for fairly hard rocks and (c) smoother cutters for jointed rocks (Blake, 1989).