Population increase means mega-cities will be growing very fast as "compact cities" for which surface space becomes a priority. This creates a particular urgency to make the underground space of the future cheaper to construct, and reliable in construction and operational performance. The cost and performance of underground projects is intimately linked to the understanding and management of geologic risk for both construction and life-cycle performance of subsurface facilities. This includes "normal" uncertainties, but also the expectation that urban growth will extend into increasingly fragile and poor geotechnical environments, and that the projects will involve larger and deeper openings. This paper assesses the state-of-practice and future possibilities for improved management of geologic risk, including risk avoidance, new materials and methods, ground improvement, life cycle engineering for sustainability, and better subsurface characterization. Some geologic risks have plagued for centuries, e.g., ground water, shallow cover and weathered rock, subsidence and impact on structures, stresses and stress relief, progressive deterioration. New risks have arisen associated with new technologies including unexpected stress-driven ground behavior, and design for higher water inflows and pressures, increased depth, and variety of excavated shapes. In addition, a better understanding of the spatial variability of soil and rock structure is needed a priori, including application of geophysical and remote sensing techniques. Our site investigations of the future need to be increasingly confirmatory rather than exploratory.
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