Over the past 30 years a number of factors in combination have contributed to a marked increase in the use of underground space. One obvious factor is an increase in transportation needs around the globe. We have also seen an increase in the number of people living in dense urban environments and desert climates that make the use of underground space particularly practical. Along with an increase in the use of using underground space have come advances that have made it possible to build underground in ways that would not have been feasible even 10 or 15 years ago. Engineering advances in areas such as rock mechanics, concrete technology, geotechnical investigation, and numerical analysis have been notable, but the most significant change in underground construction may be from advances in construction technology, such as the development of tunnel boring machines, machinery for drilling large-diameter shafts, new methods of excavation support, new waterproofing materials, and advanced guidance systems.
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