Ground Truth: Engineering the Underground
A tunnel boring machine at Yucca Mountain, Nevada, in the process of excavating a circular tunnel, 8m in diameter. The well-lighted trailing gear of the TBM provides work space for the miners as well as the scientists performing experiments as tunneling progresses. Lighting emphasizes the TBM to the right of the trailing gear.
Photograph by David Wehner, provided by the U.S. Department of Energy
- What are the mechanical properties of rock?
- What lies between the boreholes?
- How does rock respond to human activity?
- How does water flow deep underground?
- How can technology lead to a safer underground?
The 21st century exerts increasing demands to go underground. Expanding and developing populations put growing pressure on surface space, driving mass transit systems, hydroelectric plants, energy storage, waste disposal facilities and a host of other systems underground. The depletion of shallow mineral and energy resources sends prospectors ever deeper in search of essential raw materials and new sources of energy. Yet despite the demand for subsurface space and resources, major engineering obstacles to underground use remain.
SERVING SOCIETY
The stability of deep underground structures depends critically on the strength and mechanical properties of rock, the environment in which rock exists, and the consequences of engineered changes. Unlike the properties of engineered materials such as concrete and steel, properties of rock vary markedly in space and time and often defy accurate prediction. All too often, the nature of rock as revealed by excavation and underground construction differs dramatically from the expected.
Photograph by David Wehner, provided by the U.S. Department of Energy.
Source: SKB, Sweden
The resulting uncertainties in the underground engineering process drive up construction costs and increase both the human and economic risks of underground activities. Underground engineering has necessarily followed a conservative path based on empirical rules derived from experience in previous projects–rules that cannot reliably be extrapolated to new underground environments. It will take a systematic and sustained program of underground research to yield new technologies for the accurate prediction of rock conditions and behavior if we are to take full and wise advantage of the underground dimension. Engineering research at a deep underground laboratory would support advances in underground construction practices, making them more cost-effective and less risk-prone. Key engineering elements of an engineering research program would include rock characterization, design and construction, rock engineering, underground technology and safety.
WHAT LIES BETWEEN THE BOREHOLES?
The ability to recognize and characterize rock's complexity is important for design and construction both on the surface and underground. The consequences of inaccurate or incomplete characterization can be catastrophic.
Borehole-based investigations provide little direct evidence of rock's inherent complexity. Developing better remote sensing techniques to image the rock mass at depth would be a core component of an underground engineering research program, a superb opportunity for geoscientists and engineers to work together to develop technologies to characterize rock in all its natural complexity. The characterization of rock variables, such as intact strength, fracture, fluid flow and forces, poorly captured by current technologies, could be revolutionized by the development of emerging remote-imaging technologies.
HOW DOES ROCK RESPOND TO HUMAN ACTIVITY?
The stability of both surface and subsurface structures excavated in rock relies on rock's strength and mechanical properties. Yet systematic knowledge of how rock reacts to such human impositions is inadequate and often indirect, inferred from surface-based studies and borehole data alone. In the absence of direct basic knowledge of the rock, engineers continue to rely on rock-mass classifications developed using the small-scale data sets afforded by surface observation, coring and observation of the behavior of full-scale engineered structures. Building a deep underground laboratory would involve creating a substantial number of tunnels and large caverns at depth. The combination of depth and span for some of the caverns, and the requirement for stability over decades goes beyond current experience. Thus, the construction period itself represents a special opportunity for innovative excavation designs and experiments. Excavating large caverns for physics experiments, for example, would redirect and intensify the gravitational and tectonic forces in the rock around the cavity.
Advances in computing permit geoscientists to incorporate many of the complex and coupled influences on rock mass behavior into predictive models. As yet, however, no laboratory exists where such models can be tested on a useful scale. An underground laboratory would overcome this barrier by allowing engineers to "ground truth" their theoretical models and advance toward more rational and reliable underground design. Design-predicted values could be calibrated against excavation realities, yielding real-time model improvements. The data from an underground laboratory would support an accurate three-dimensional representation of the rock system in situ, which would, in turn, provide accurate input to the sophisticated software packages now available to model and analyze rock and fluid behavior. Instrumentation and long-term monitoring of the rock would further enhance model reliability.
Source: Dr. Peter Blümling, NAGRA (Swiss Radioactive Waste Agency)
Building a deep underground laboratory would involve creating a substantial number of tunnels and large caverns at depth. Excavating large caverns for physics experiments, for example, would redirect and intensify the gravitational and tectonic forces in the rock around the cavity. From small advance tunnels, engineers could monitor these changes and their effects on the rock as cavity excavation proceeds. The use of explosives to excavate the caverns provides opportunities for research on wave transmission in rock. The stability of supports in auxiliary tunnels close to the large caverns can be evaluated under conditions of dynamic stress, a topic with potential implications for national security. The development of precise electronic detonators also opens exciting opportunities for the use of explosives-driven waves in "conditioning" rock to prepare it for excavation by low-cost bulk methods.
SAFETY UNDERGROUND
Safety and health would have the highest priority in the engineering of a deep underground laboratory and would be fully integrated into the design of laboratory activities at every stage of planning, design and construction. An underground laboratory would also provide an ideal laboratory for research and development leading to advances in underground safety systems and technology. Scientists and engineers could carry out safety and health research within a deep, controlled underground setting. Particular attention would go to advances in key areas such as underground communication, ventilation, access, emergency egress and refuge design. Because the temperature of rock increases with depth, typically 10°C-30°C per km, engineers would also undertake mechanical-systems research in the areas of air conditioning and filtration to support human activity and research at depth.
Reproduced from Going Underground: Courtesy of Royal Swedish Academy of Engineering Sciences.
ROCK ENGINEERING–A DEEPER UNDERSTANDING
A major opportunity of a deep underground laboratory would be the availability of a large block of rock in situ exclusively for long-term scientific and engineering research. A broad suite of rock engineering studies would address a wide range of fundamental questions in rock behavior and fluid flow in rocks. Under moderate stress, excavation stability is largely determined by rock fracture. Under conditions of high stress, hazardous ground behaviors increase. Not only does the fall-out of fracture-bound blocks create hazards for underground construction, but underground operations may also encounter "rockburst" conditions. Rockbursts, earthquakes triggered in intact rock by underground operations, are a deadly and costly hazard for mining and underground construction operations. Research on the stability of fracture-bound blocks and the safe and nonviolent release of rockburst energy would have significant benefits for mine safety and efficiency.
A deep underground laboratory would provide an ideal site for the study of groundwater system behaviors at depth. An improved understanding of fracture flow would aid reservoir engineers in protecting drinking water supplies, facilitate the bioremediation of contaminated aquifers and enhance energy recovery from geothermal systems. An improved understanding of fluid flow in rock could enable bioengineering advances for the in-place extraction of mineral resources.
TOWARD A BETTER-ENGINEERED UNDERGROUND
An underground laboratory would provide both the academic and industrial communities with low-cost, long-term access to underground research sites. In a large, dedicated facility, new equipment and material trials could take place under a wide range of in situ conditions, and field tests could go forward without the production constraints imposed by mining operations.
A deep underground laboratory represents a unique opportunity for engineers to conduct both fundamental and applied research that would directly address societal needs. It would catalyze major advances in underground engineering and contribute strongly to a fundamental understanding of the rock mass. It would also provide an extraordinary opportunity for innovation in underground technology with immediate and long-term benefits for the economic well-being of the nation, for the protection and remediation of the environment, and for human life and safety.
Drawing reproduced from Going Underground: Courtesy of Royal Swedish Academy of Engineering Sciences.
