large. Similarly, high discount rates for calculating the present value of future benefits will make those benefits less valuable if provided long into the future. For example, the federal fiscal year 2011 real test discount rate for a 30-year planning horizon was established at 2.7 percent (OMB, 2010). With this discount rate, $1.00 of real benefits received 30 years later would have a 2011 value of 1/1.02730 = $0.45, or less than half. One dollar of benefits received 100 years in the future would either be disregarded as beyond the planning horizon or would have a 2011 value of only $0.07.

A long lifetime in itself also may affect planning for future alternatives. Particular underground development can preclude other uses or make them more expensive to implement. For example, underground transportation tunnels such as the Boston Central Artery project required rebuilding and relocating existing underground utilities in the tunnel right-of-way. Building foundations may make re-use of their underground locations prohibitively expensive, precluding new underground parking, tunnels, or other uses in that location. In effect, underground construction may increase cost and reduce flexibility of options for alternative future uses. Because most underground facilities are left in the ground even after their useful life ends, the extra cost or difficulty of re-using the space continues nearly indefinitely. A comprehensive planning effort would recognize that underground space is a resource that should be used in the best manner possible, rather than letting initial uses preclude later uses. Similar conclusions have been drawn with respect to limiting space debris in orbits around Earth that may prevent use of those orbits for other purposes (e.g., UN, 1999).

In addition to assessing the life cycle of underground infrastructure itself, sustainability suggests that impacts of the infrastructure also be considered for the entire life cycle of a project. Lifecycle assessment “studies the environmental aspects and potential impacts throughout a product’s life (i.e., cradle-to-grave) from raw material acquisition through production use and disposal” (ISO, 1997). Figure 5.1 illustrates a generic supply chain life cycle. For underground infrastructure, the supply chain would include the various materials and processes involved in construction as well as inputs such as energy for lighting and ventilation during facility operation. Closure and decommissioning costs would be included in the disposal phase in Figure 5.1. The landfill phase would be expected to include the costs of providing an engineered landfill for disposal or any costs associated with legacy structures underground.

Metrics to use in assessing sustainable development overall, as well as to assess specific economic, environmental, and social impacts, are still a subject of widespread debate even without consideration of the special circumstances of underground development (Jeon and Amekudzi, 2005). Economic impacts are typically expressed in monetary units, but a variety of impacts may be considered for environmental and social impacts. For example, Reijnders (1996) suggests that broad environmental impacts be considered in preparing a lifecycle assessment including:



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