Tunnels are in many ways unique in the kind of engineering challenges they present. Unlike the vast majority of geotechnics that human beings engage in, tunnels are mostly about removing material to create a structure rather than adding it. Yet, tunnel infrastructure is one of the most ancient kinds, and represents incredible utility in cases where surface infrastructure is undesirable or simply impossible. However, tunnel excavation is highly dangerous, with many horrific tunnel collapse accidents recorded over the years. So what are the basic engineering principles that need to be observed when building a tunnel?
Imagine you have a building in front of you and you are tasked with removing a load-bearing structure in the very middle of the ground floor of it to make way for a corridor. That is essentially what tunnel excavation is. Subterranean infrastructure implies a large, heavy layer of terrain right over it, and removing some soil from underneath it means removing the thing that supported all that earth above. Now the assumption is that the ground is generally hard and can withstand a small cavity no problem. That may be true in situations where you are excavating a monolithic formation of rock, like a cliff, which has a degree of tensile strength if it is uniform. But this is an exceptional scenario, even for rock terrain, which usually has cracks and joints all over. In most other places in the world earth is a loose substance. The density and solidity is due to the pressures that are being put on it from above. Earth isn’t naturally adhesive, but thanks to its coarseness, friction caused by pressure can make it behave like a solid object. By removing soil, pressure in certain sections is removed, potentially creating a chain-reaction of loosening soil.
Most tunnels overcome this problem with reinforcing the ceiling and walls with lining. Historically those were stainless steel beams and plates or reinforced concrete that were installed over the entire ceiling and wall surface of the tunnel. The ceilings of tunnels are usually curved into a half circle, allowing for better distribution of pressure around the tunnel. Unfortunately, this relatively simple solution adds an enormous expense to tunnel construction. Many tunnels never got built simply because of how expensive it would be to install these lining systems. Unfortunately there is no easy way around it. The expense for loose soil tunnel excavation can be reduced through economies of scale, where the lining materials are standardised and produced in quantities that make their use economically feasible. Jointed rock is easier to solve, but still requires work. It works in a similar way to the method by which concrete is reinforced by rebar. Jointed rock tunnels are reinforced by rock bolts – long, thick steel bars that are drilled into the tunnel ceilings and walls. These bolts are usually anchored by wedges at their ends. This creates a pattern of steel reinforcement that spreads pressure around the tunnel evenly regardless of the natural join pattern of the rock formation.