For thousands of years, the underground has provided refuge, resources, foundations for surface structures, and a place for spiritual or artistic expression. More recently, important infrastructure has been placed underground because of proximity to services, to preserve surface space, provide climate or security isolation and containment, reduce construction and energy costs, improve traffic flow, and for various aesthetic benefits. Underground space can provide three-dimensional freedom often unavailable in densely developed areas. Infrastructure systems can be placed beneath cities, under rivers, and even through mountains. Millions of people rely on these systems with little thought to the comfort and conveniences provided. Placing new infrastructure underground also may encourage or support the redirection of urban development into sustainable patterns. Resilient, well-maintained, and well-performing underground infrastructure, therefore, becomes an essential part of sustainability.
At the request of the National Science Foundation (NSF), the National Research Council (NRC) conducted a study to summarize current underground engineering knowledge, identify needed research and direction for a new research track to support sustainable development through underground engineering, and examine drivers that promote or inhibit underground development (see Box S.1 for statement of task). The NRC convened a panel including researchers and practitioners with expertise in geotechnical engineering, underground design and construction, trenchless technologies, risk assessment, visualization techniques for geotechnical applications, sustainable infrastructure development, lifecycle assessment, infrastructure policy and planning, and fire prevention, safety, and ventilation in the underground. The committee’s report is intended to inform
Statement of Task
An ad hoc committee of the National Academies will conduct a study to explore the potential advantages of underground development in the urban environment, to identify the research needed to take advantage of these opportunities, and to develop an enhanced public and technical community understanding of the role of engineering of underground space in the sustainability of the urban built environment, specifically the minimization of consumption of nonrenewable energy resources, construction materials, and negative impact on the natural, built, and social environments. In particular the study will:
• Summarize current geological and geotechnical engineering knowledge about underground development in the urban environment and how utilization of underground could increase sustainability, including knowledge of geologic site characterization, construction and geotechnical monitoring techniques, energy requirements, use of excavated materials, and lifecycle costs and benefits of underground infrastructure development.
• Identify the research needed to capitalize on opportunities for enhancing sustainable urban development through underground engineering, in the following areas:
• Underground characterization, prediction of the geologic environment, and ground response critical for successful design and construction of underground projects and critical facilities to maximize sustainability and resiliency;
• Construction and monitoring methodologies and enhanced excavation
public- and private-sector audiences engaged in research, urban and facility planning and design, underground construction, and safety and security.
Based on discussions with study sponsors, this report focuses on contributions of engineered underground space to sustainable development and outlines needs in the research, educational, regulatory, and social environments that would maximize those contributions. The report provides a set of overarching observations, conclusions, potential actions, and research topics related to integrated and interdisciplinary infrastructure systems design and management; underground engineering education, training, research, and practice; approaches to management and technological development; infrastructure lifecycle assessment; underground space use acceptance and safety; and underground space as a resource. These conclusions address all aspects of the charge generally rather than specifically. Important research topics are highlighted with the conclusions, but more are found throughout the main body of the report.
• Smart underground structures and conduits that report their status;
• Health and safety considerations, such as cost-effective ventilation, light, and concerns related to radon exposure or fire control;
• Lifecycle cost and benefit issues, including reduced energy needs for heating and cooling, reduced construction material use, use of excavated materials, increased longevity of underground structures and reduced maintenance associated with stable temperatures and isolation from surface weathering effects;
• The potential sustainability benefits of increased use of underground space for human transportation systems, including roadways and mass transit, and freight;
• The potential for integrating of energy, water, and waste systems for certain urban regions to improve sustainability; and
• How underground development might address concerns related to the impacts of climate change on the urban environment.
The committee will recommend directions for a new underground engineering research track focused on earth systems engineering and management to ensure future human resources for sustainable underground development, will analyze the advantages and disadvantages of establishing a new research center in this area, and consider other potential options for enhancing the human resource capacity for sustainable underground development (including the status quo). The committee also will consider from a social science point of view, the policy, economic, and human behavioral drivers that promote or inhibit the development of the subsurface in a sustainable manner, but will not make policy or funding recommendations.
THE UNDERGROUND FOR SUSTAINABILITY
Sustainability is defined in this report as the ability to meet present societal needs without compromising the ability of future generations to do the same. Maintaining or improving quality of life and maintaining long-term ecological balance are among societal needs. An unhealthful natural environment can negatively impact food, water, and air supplies and degrade quality of life and health to unacceptable levels. Resilience, an important aspect of sustainability, is defined as the ability to respond to environmental changes—especially natural or human-caused adverse events—with minimum impact on functioning.
Master plans of some cities (e.g., Singapore) include extensive underground use. Well-planned underground infrastructure can positively influence land use and development decisions and can reduce vehicle use and associated impacts. High-density urban centers may depend on centralized services but can capitalize on centralization to increase sustainability. Underground transportation infrastructure (e.g., urban roads and highways, public transit subways, grade-separated and underground freight railroads, high speed rail, and pedestrian rights of way) can address multiple growth-related challenges in urban areas
(e.g., congestion, urban sprawl) if infrastructure elements are optimally designed and located. Well-planned and operated underground infrastructure can, in many cases, improve quality of life and sustainability more so than can similar-purpose surface infrastructure.
UNDERGROUND INFRASTRUCTURE AS PART OF A SYSTEM
Observation: There is little strategic coordination of underground infrastructure development in the United States.
Conclusion 1. Coordinated formal administrative support and management of underground infrastructure as part of an integrated, multi-dimensional, above- and belowground system of urban systems is vital to urban sustainability.
a. Recognize responsibilities related to formal support for underground infrastructure as part of the total urban system through coordinated planning and operations, fostered technological development, and local and regional rule making.
b. Develop and encourage use of a system for consistent data collection, archiving, and access to be used by all facility owners and operators to aid decision making.
a. Explore within the federal government the most appropriate technical and administrative approaches to facilitate coordinated management of the underground as part of a total urban system. Recognize and coordinate with ongoing research in this area, for example, that conducted by the NRC Transportation Research Board related to road projects.
b. Conduct a technology scan of how countries and cities around the world collect, manage, make available, and use three-dimensional geological and buried structure information.
Infrastructure development, operation, and maintenance require management of the complex physical, social, and environmental systems influencing proper functioning. Development of underground infrastructure suffers in the United States from the lack of a mission agency or organization within the federal establishment dedicated to coordinating development activities across sectors. Project and research funding mechanisms tend to focus on solving particular problems with little consideration of long-term impacts on the total urban system.
Coordination could lead to better management of research investments, optimized decision making, reduced risk for federal development projects, and better leveraging with state and local entities. Although some planning and zoning by local governments of outward and upward city growth does occur, there is little analogous control of underground space, and even less control that coordinates above- and belowground development. It is possible and desirable to identify, protect, and effectively zone prime subsurface resources for optimal use as is done, for example, in Helsinki, Finland, Montreal, Canada, and Singapore. Some policy changes can result in lower costs through, for example, streamlining time-consuming permitting processes as is done in Japan.
Observation: Market forces in the United States encourage workforce capacity growth and urban and infrastructure development, but often in an ad hoc manner that may not be consistent with urban sustainability.
Conclusion 2. Development of underground space as part of sustainable urbanization requires expanded and coordinated communication with stakeholders to better incorporate site-specific conditions, greater flexibility, and long-term community needs into infrastructure system design and optimal lifecycle management.
a. Establish a federally led interdisciplinary network or organization of organizations and institutions to guide sustainable patterns in underground infrastructure development and encourage interdisciplinary research and communication of findings among all disciplines and stakeholders. Stakeholders include, for example, designers, long-term planners, architects, safety specialists, and an array of engineering, geologic, geophysical, environmental, and contracting specialists from industry, government, and academia.
b. Develop mechanisms for integrated and holistic three-dimensional research and planning that include information management and communication technologies to facilitate complex research, design, construction, operation, and management of underground infrastructure.
a. Explore models for designing sustainability into engineered systems of urban systems that recognize interdependencies, vulnerabilities, complexity, and adaptability. Coordinate ongoing research in the United States and elsewhere on, for example, complex adaptive systems and human factors engineering (e.g., incorporating behavioral science, human performance and capacity, personnel and training, and human biology and physiology into engineered systems).
b. Develop conceptual models of the complex interactions among multiple systems (e.g., mechanical, human, and environmental) to improve understanding, reduce risk, and effectively manage infrastructure amid changing technologies, societal conditions, and expectations.
c. Research the behavior of those operating, maintaining, and using underground infrastructure during normal and worst-case operation scenarios to optimize the human-technical interfaces in a manner consistent with long-term values.
Underground infrastructure development is a multidisciplinary endeavor. A sustainable urban system is possible if decisions are informed by the links between the social, technical, and governing elements of society (as occurs to some extent today). Underground infrastructure projects, however, are often undertaken on a project-by-project basis with minimal consideration of long-term maintenance or societal needs. This approach is inconsistent with sustainability. Decisions are often made among decision makers with competing political, social, and economic interests and security concerns that influence if, how quickly, where, and by what methods underground development occurs. To maximize sustainability, multidisciplinary efforts are needed during the entire infrastructure life cycle.
Better informed decision making is possible when engineers understand the complex and interactive social and economic factors that contribute to sustainability and when urban planners have realistic expectations about the underground environment. Some interdependencies are obvious, but other interdependencies—some critically important to national security—may remain unknown without appropriate communication and planning among experts and stakeholders.
The capacity for flexibility is needed to address emerging issues, technologies, and societal expectations during and beyond the operational life of underground infrastructure. New hazards associated with vulnerable and deteriorating infrastructure systems, climate change, and security concerns, for example, may affect provision of service, environmental quality, or personal safety. Extreme events (e.g., terrorist acts or natural disasters) present still other hazards and risks. Sustainability depends on planners and engineers building and pooling capacities to anticipate and accommodate human behavior and the constantly evolving urban environment. Accounting for human behavior in underground space can lead to creation of environments that allow more intuitive understanding of safety in the underground under varied circumstances.
A new institutional framework for professional planning, architecture, engineering, public administration, and social and economic policy committed to sustainable development will be difficult to create but could recharge U.S. educational and research capacities to address sustainable urban underground space use. Federal, state, and local agencies, the engineering and construction industries,
Observation: Complex ownership models for underground infrastructure confuse responsibility for routine inspections, maintenance, repairs, guidelines, budgets, and liability.
Conclusion 3. There is a need to understand the ownership and control models of underground space and to develop guidelines for funding and performing essential periodic inspections, maintenance, and repair of individual infrastructure elements.
a. Analyze multidisciplinary and holistic approaches to view the complex web of ownership, control, and responsibilities associated with maintenance and safety of underground infrastructure.
b. Examine multidisciplinary approaches to aid transition to more modern systems management.
Underground infrastructure in the United States is typically owned and controlled by numerous individuals, partnerships, corporations, and local, state, and federal government. Responsibility for routine inspections, maintenance, and repairs is confused, and ambiguity regarding applicable guidelines, budgets, and terms can arise. Separate agencies deal independently with transportation, housing and urban development, homeland security, and energy issues. Sustainability goals will be hindered without more coordinated control and management.
STATUS OF U.S. RESEARCH, TECHNOLOGY DEVELOPMENT, AND EDUCATION
Observation: The United States was a world leader in many areas of underground science and technology when there was federal and industry investment in underground engineering research and development.
Conclusion 4. Maintaining global competitiveness in underground engineering education, technology development, and practice supports urban sustainability, resilience, and the standard of living of the United States.
Allocate resources for broader interdisciplinary education and technology development in underground design and construction.
Expand U.S. research that advances and revolutionizes, for example, materials technologies, robotic construction technologies, laser guidance systems, geographic information systems, and enhanced computer analysis and visualization systems that improve the ability to model, design, plan, and reduce risk associated with complex underground systems (see Chapter 6 for more detail).
Geotechnical expertise will always be critical to delivery of underground facilities with lower costs and risk and to better lifecycle performance. Geotechnology education, research, and practice need to better integrate all disciplines related to site investigation, design, construction, operation, and risk management of underground facilities. The complexity and unpredictability in underground construction indicate that many challenges remain. Technological advances improve our ability to understand, model, construct, and reduce risk associated with underground infrastructure. It is not in the country’s best interest, however, to rely heavily on imported technological advances and expertise to create and maintain underground facilities, as has become a trend in the United States. Much new knowledge, technology, and project-specific memory may leave the country at the completion of construction, to the possible long-term detriment of underground infrastructure operation, maintenance, and security.
Observation: Lack of funding continuity that allows meaningful investment in equipment and faculty has resulted in a substantial reduction in the number of U.S. university programs dedicated to integrated underground engineering research and education.
Conclusion 5. There is a critical shortage in educational, training, and research opportunities for engineers who wish to learn and practice underground engineering in the United States.
a. Develop national multidisciplinary, multi-institutional, cross-sector research centers that focus on different areas in underground engineering and sustainable urban infrastructure to produce the next generation of leaders in underground engineering.
b. Integrate graduate underground engineering studies with research programs or a critical mass of coordinated faculty activity to anchor research to existing programs. Create opportunities to specialize in particular aspects of underground engineering, but with a multidisciplinary approach.
c. Develop university consortia to aggregate faculty expertise; strengthen industry-university faculty relationships.
d. Teach better facility planning and management with a multidisciplinary approach through traditional, distance, or hybrid-style education formats. Traineeships (e.g., NSF’s Integrative Graduate Education and Research Traineeships) could help to fund programs.
e. Expose undergraduates to multiple disciplines, issues, challenges, and opportunities associated with sustainable underground space use and engineering.
f. Develop continuing education opportunities for professionals.
g. Develop appropriate credentialing for inspectors, technicians, and operators of complex underground facilities.
Underground engineering knowledge, expertise, and training in the United States today are obtained mostly through mentoring and on the job experience, rather than through higher education. Such training provides hands-on experience and benefits the workforce, but competitiveness and liability concerns can limit information sharing more generally within the industry and can limit exposure for young engineers to a range of technologies and methodologies. Because few commercial incentives exist for industry to embrace the challenges associated with long-term infrastructure or urban system sustainability, young engineers may not be exposed to potential solutions for these issues. In contrast, U.S. students educated within multidisciplinary U.S. research institutes are more likely to benefit from the advances and broad knowledge and technology dissemination that takes place and become a better prepared domestic workforce.
Optimized design and more judicious use of resources can result from detailed knowledge of the underlying and nearby geology and human-development histories (e.g., existing infrastructure and legacy construction materials) and the ability to minimize unanticipated ground conditions. Traditional undergraduate programs do not teach an integrated approach to practice, and few graduate programs offer interdisciplinary programs in underground engineering, certification in specific areas (e.g., tunneling), or specialization within more general graduate degree programs that allow for optimization. Knowledge of technologies for tunneling (including trenchless), excavation, ground support, ground improvement, and natural and built systems monitoring, and other functions is essential. But good education programs also will include mechanical, electrical, civil, structural, geotechnical and geological engineering; planning; architecture; public policy; fire safety; and information technology in their curricula.
Few U.S. university faculty research tunnel design and construction performance. The lack of a continuous government focus on infrastructure issues, and the fragmentation of U.S. government-sponsored underground development research across several disciplines at the core of underground engineering (e.g., structural, geotechnical, and mining engineering), result in little expectation of program funding continuity. Opportunities in specialized areas such as tunneling are disappearing as a result of mandated reductions in credit hour requirements
Observation: The complexity of urban infrastructure systems and uncertainties associated with system design and performance increase with greater and more varied demands on both above- and belowground infrastructure.
Conclusion 6. Engineers and urban planners could better improve whole lifecycle facility performance and overall urban sustainability with documented and validated risk-informed approaches to project planning and design that balance lifecycle project needs in terms of service delivery, initial costs, resilience against extreme events, and effective maintenance and operations.
a. Advance existing and develop new technologies for modeling uncertainty during all phases of infrastructure life cycle. These include invasive and noninvasive technologies for geologic site characterization (including existing and legacy infrastructure and materials); analytical and computational design methods; excavation, ground support, and monitoring technologies; and technologies for asset management including related to the management of data and security (see Chapters 6 and 7 for more details).
b. Develop strategies to investigate potential hazards, impending problems, and cascading evolution of problems, especially given current underinvestment in infrastructure system rehabilitation.
c. Engineers and planners could use extreme events to understand complex systems behaviors and interdependencies and to validate computational models of system performance.
Full assessment of lifecycle costs and benefits may convince owners and planners that greater initial investment in underground infrastructure can be economical in the long term. Security and resilience of urban areas can be enhanced if decisions are informed by complete evaluation of the merits, deficiencies, and interactions of infrastructure elements with respect to all potential hazards and risks. Long-term performance of infrastructure can be improved with access to good models and data for analyses. However, the validity of models developed for individual system functionality and performance is often questionable, and uncertainty increases when modeling systems of greater complexity. Models of integrated systems of systems such as urban infrastructure have yet to be developed and validated.
Observation: Aging underground infrastructure may be susceptible to deterioration and issues associated with changing technologies, changing climate, and societal needs.
Conclusion 7. Underground space development requires a long-term commitment to technological advancements in an environment that is friendly to improved planning, innovation, and implementation.
a. Design infrastructure that allows ease of access for inspections, maintenance, repairs, upgrades, and reconfigurations in response to new needs or technologies that allow such work to be completed at lower costs.
b. Consider resource needs, availabilities, and access when making administrative and technical decisions concerning development. These include energy resources (e.g., oil, gas, and other energy resources), industrial minerals, high-value or critical strategic minerals (e.g., gold, uranium, rare earth elements), and construction materials (e.g., gravel, sand, building stone).
c. Use appropriate models that demonstrate multiple potential scenarios and allow better infrastructural system planning based on local conditions.
a. Academia and system stakeholders could collaboratively develop long-term performance simulation models for complex systems and validate the results over time to understand dynamic responses and emerging system behaviors.
b. Explore how technologies and innovations from other industries (e.g., exploration tools, in situ analytical techniques, measurement-while-drilling systems, laser scanning, fusion of multi-sensor data) and civilian application of military research could be applied to underground engineering.
c. Conduct long-term research on the effects of the underground infrastructure on the natural and built environments to increase the capacity of decision making for society’s best long-term interests.
d. Research comprehensively and on a common risk-cost-reward basis the long-term effects on sustainability of underground storage or disposal of urban wastes (e.g., municipal, sewage, or energy-related products).
Lifecycle planning aids long-term infrastructure health. Age, deterioration, and changes in technologies and use mean that underground infrastructure systems constantly require attention. Selecting the most sustainable approaches to underground space use may be more likely if the best available science, technology,
and ideas can evolve, keep up with societal needs, and become less expensive to use. For example, combining utility services into common utility tunnels (called utilidors) can isolate utilities from the surface in a continuously accessible location. Tangles of utility infrastructure in many urban areas can be reduced or avoided (such infrastructure typically remains in place long after its operational life), and more of the subsurface can remain available for other uses. This is particularly beneficial in areas with narrow rights-of-way, and can be economically advantageous when cost considerations include the value of the underground.
Strategic construction and long-term maintenance of underground infrastructure may result in fewer adverse environmental impacts than for surface infrastructure. Technological advances can help minimize noise and vibrations, protect air quality, and allow for recycling and reuse of waste construction materials, including soil and rock from a site. Technological advances that allow better prediction of impacts to water quality, groundwater flow, soil geochemistry, and underground temperatures and heat flow that may impact the natural and built environments are needed.
Observation: Few data exist regarding the environmental and social impacts and lifecycle sustainability of urban development that can inform technology and administrative decisions related to long-term (decades to centuries) infrastructure operation, maintenance, and reduced costs.
Conclusion 8. Comprehensive and scientific retrospective studies of the direct and indirect costs and impacts of various types of underground projects are needed to evaluate usefulness and economic, environmental, and social impacts so that future planning can maximize sustainability.
a. Conduct comprehensive and scientific investigations to retrospectively identify the lifecycle performance of various types of underground infrastructure and to identify the aspects of project planning, design, construction, and operation that contribute most to project costs and performance. For example, track financial (both direct and indirect), environmental (e.g., air and water quality), and social impacts over an extended period (e.g., decades) following a project such as Boston’s Central Artery alignment.
b. Develop common metrics for assessing sustainable development more generally, and for assessing specific economic, environmental, and social impacts.
c. Develop quantitative methods to compare the value of underground space on a par with other urban resources (e.g., linked to market value of surface
d. Compile data about sustainability aspects of various construction methods and materials (e.g., the availability of materials and energy embodied in production of materials).
Planning horizons for decision makers are often far shorter than the useful life of underground infrastructure. Underground infrastructure development may require seemingly cost-prohibitive initial investment for construction when compared to similar-use surface infrastructure. Few data exist to validate investment support when long-term benefits are not valued. Lifecycle assessment can provide data through consideration of costs, impacts, and benefits—from raw materials acquisition, to construction and operations, through closure, decommissioning, and post-operational use. Additional inputs such as energy (e.g., for lighting and ventilation) also are factors. Similarly, understanding how some underground development has precluded or made other uses of underground space more expensive may inform decisions that affect future options. The costs and challenges of re-using occupied underground space remain long-term issues.
USER ACCEPTANCE, SAFETY, AND COMFORT
Observation: Underground infrastructure can safely enhance the lives of millions, but few federal-level safety regulations exist to guide operational safety at a time when underground system complexity is increasing.
Conclusion 9. Greater user acceptance and occupancy of underground infrastructure and facilities are likely if underground spaces are planned with more consideration of utility, ease of access, wayfinding, safety, and aesthetics.
a. Develop and adopt performance-based safety mechanisms and codes that not only account for today’s underground occupancies (e.g., mixed use, multilevel) and risks, but also allow for expansion and change of use. The International Code Council technical requirements, applicable National Fire Protection Association standards, and other related standards and guidelines could be expanded and made applicable to underground facilities.
b. Incorporate human factor and complex systems engineering concepts to guide threat recognition and technical and operational decision making for normal operations and for operations during times of stress (e.g., in response to extreme events).
a. Research the state of practice and best practices related to safety systems (e.g., hazard detection, notification, ventilation, fire suppression, emergency egress, and system integration). Develop appropriate minimum safety system requirements to incorporate into national-level guidelines and standards.
b. Compare international underground safety codes and guidelines with those applicable in the United States to identify inadequacies and guide future practice, recognizing existing efforts in this area (e.g., by FHWA).
Underground space can be as safe, attractive, stimulating, functional, productive, and healthy as similar-use surface space. Negative perceptions about underground space, however, can be as difficult to overcome as complex safety and technical challenges. Acceptance and use of underground space may increase with greater convenience and comfort of use (e.g., by incorporating better connectivity among underground systems that limit pedestrian travel time and lengthy vertical movements by stairs, escalators, or elevators). More intuitive understanding of safety in the underground by its occupants will also increase acceptance.
Safety in the underground is achieved by avoiding, transferring, or reducing risks associated with naturally occurring phenomena (e.g., gases, radiation, extreme temperatures, water, and lack of oxygen) and human activity (e.g., fire, smoke, hazardous materials, intentional or accidental explosions, structural failure, or simple human failure). Safety is more challenging with increasing infrastructure complexity. Human factors engineering becomes essential to increasing the ability of people to operate and occupy the underground safely.
Safety codes are often written in response to incidents or litigation and are not flexible enough to accommodate evolving technologies. Safety is created operationally or through technical solutions, but it is dependent on designing and operating beyond mere compliance with often inadequate codes. The few federal-level safety regulations for underground infrastructure mostly apply to construction rather than to operational usage of most facilities. State-level fire safety codes do not fully address underground structures and will likely be inadequate when different occupancy types are combined in one underground space (e.g., public transportation and commercial).
Capital construction and operational risk mitigation costs for underground space can be substantial and could preclude an underground project from being started, or could result in improper system maintenance. Human factors engineering can help to minimize costs associated with avoiding or transferring risk, for example, by identifying ways to reduce risk through safety regulations and education when technological solutions are not feasible. Innovation in design and
THE UNDERGROUND AS A RESOURCE
Observation: Underground space is a valuable but decidedly nonrenewable resource.
Conclusion 10. Underground space can enhance urban sustainability only if the underground is thoroughly understood and if underground use and reuse and the protection of the natural and built environments are incorporated into long-term total urban infrastructure system planning.
a. Institute planning of all underground space as part of an evolving urban system to be carefully engineered or preserved for optimal long-term use and regional sustainability.
b. Establish reasonably intensive groundwater, soil, and infrastructure monitoring practices to track the health of the underground urban environment according to the general geologic conditions and use. Use data generated from a range of environments and situations to inform urban planning in other areas.
The underground is not a universal alternative to the surface, but many uses of underground space contribute to urban sustainability. It is critical that policies and administrative structures provide appropriate and comprehensive guidance, that the public develops a long-term community vision, and that community expectations regarding underground services are informed and met. An adequate institutional commitment to enhancing interdisciplinary and cross-sector research, education, and training capacity is needed to ensure the nation develops the types of underground infrastructure that support sustainable urban development economically, securely, and in a manner consistent with national priorities.