would come at greater cost and challenge when engineers were forced to work around existing infrastructure.

As we progress into the 21st century, the underground is used for all the purposes listed above, and many of the problems predicted a century ago have been realized. Table 2.1 describes the estimated lengths of major underground utility services in the United States, totaling approximately 10.8 million miles (17.4 million kilometers). Underground infrastructure has expanded to accommodate growing populations and new infrastructure services (and their multiple providers) but is still installed beneath the same public rights-of-way. As traffic becomes more congested with population growth, underground utility work that must be accessed from the surface results in increased traffic problems and expense. It has been reported that approximately 4 million holes are dug in the United Kingdom’s roads and sidewalks by utilities at a cost of approximately $2.25 billion1 per year and consequent indirect costs of approximately $4.5 billion per year (Farrimond, 2004). Analogous costs in the United States could well be many times larger.

Wastewater systems have also been expanded and the underground now accommodates large wastewater transport systems (e.g., sanitary and stormwater sewer systems; combined sewer systems) and combined sewer overflow (CSO) interceptor and storage tunnel systems with large diameter openings. Most segments of wastewater and drainage systems are designed to flow by gravity through pipes and tunnels and are therefore dependent on closely controlled vertical alignments. These systems are generally placed beneath the hodgepodge of existing shallow utility infrastructure, and they may block usage of that underground space for future services including rapid transit subways and high speed rail (HSR). Protecting access opportunities for such services argues for planning and permitting with a goal of preserving underground corridors for major high-value urban infrastructure. Foresight is vital to sustainability because such complex infrastructure is often not needed until much later in a city’s evolution.

ENGINEERING THE UNDERGROUND FOR SUSTAINABILITY

Tunneling, a component of many underground construction projects, shares many properties with other types of construction done in urban societies. Certain challenges, however, may become amplified in an underground setting (Wood, 2000). For example

• there is greater dependence on the ground and understanding ground properties in terms of risk (see Box 1.3) to the construction project itself, other infrastructure, worker health and safety, the environment, and economic interests;

• there is higher interdependence between planning and project design

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1 Based on 2008 exchange rates.



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