Going under

Cities strapped for buildable space are starting to look at a new frontier – the area beneath their feet. it certainly has far more potential than just for parking cars and storing excess stuff. Underground temperatures remain conveniently cool and constant year-round, a definite plus in a world threatened by global warming. These conditions are also ideal for artwork, as the louvre museum’s underground expansion has successfully demonstrated. Underground spaces are also more resistant than above-ground structures to unpleasant events such as explosions and earthquakes.

"The whole world is looking at getting more underground space in urban areas – it’s a global problem," explains enAC professor jian Zhao, an expert in rock tunneling engineering.

Zhao points out that the issues involved in building a tunnel under a mountain for a train are quite different from designing and building large underground cavern spaces in which people willingly agree to work or live. It requires a wide range of expertise, from tunneling engineering to architecture and logistics. A multidisciplinary team in enAC led by Professor Aurèle Parriaux studied these issues in detail in projects entitled "Deep City" and "Deep City China". You must first pick a suitable location. Granite is best. Sedimentary rocks are possible, but require a lot more steel and concrete support.

Second, you must choose a suitable construction technology, keeping in mind that the cavern cannot be too large or it will cave in. Concrete supports can be incorporated to guarantee structural stability. The world’s largest unsupported underground rock cavern is a 60 m wide hockey stadium built in norway for the 1994 winter olympics, capable of seating 5,000 people. Third, you must find a way to make the space attractive and safe enough so that people agree to spend large quantities of their time underground. An underground stadium seating 10,000 people sounds like a good idea until you think about how they are going to get out of a limited number of aboveground access holes in the event of a fire. And lighting is crucial; most people won’t willingly cut themselves off from natural sources of light and the diurnal cycle.

For his part, Zhao explores the effects of "hazards loading" – a term which encompasses earthquakes, construction vibrations, bombs, and explosives – on these kinds of underground structures. In fact we know very little about how seismic waves propagate through the ground and how they would affect an underground cavern far away from an earthquake epicenter. Given their rarity and unpredictability, it’s difficult to field test using real earthquakes; explosions are easier to arrange, but still prohibitively expensive. So Zhao is studying seismic wave and energy propagation using a combination of experiments and numerical modeling. He is developing a tool to simulate and predict the effects of earthquake or explosion loading on underground structures, and will then use it to find a means of minimizing the effects of these events and thus protect the structures from destruction. "If you understand the kind of loading your structure will encounter, then you can always support it better," he explains. "In most cases, tunnels are safer in earthquakes, because they are moving with the ground. You might have cracks or shifts, but they don’t generally collapse."