SPECIAL SECTION: CIVIL ENGINEERING RESEARCH CURRENT SCIENCE, VOL. 97, NO. 8, 25 OCTOBER 2009 1162 *For correspondence. (e-mail: hari@colorado.edu) Natural analogs for improved understanding of coupled processes in engineered earth systems: examples from karst system evolution Harihar Rajaram*, Wendy Cheung and Abhijit Chaudhuri Department of Civil Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309-0428, USA There is an increasing need to understand the behav- iour of engineered earth systems, from the viewpoint of safe waste disposal and exploration of renewable energy sources. Often, human activities lead to signifi- cant perturbations of the earth systems, involving hy- drologic, mechanical, thermal and chemical processes. Prediction of the long-term response of earth systems to large perturbations is critical for evaluating their design, performance and operation. Because many of the processes involved in system response will mani- fest over decades or centuries, field-testing during the design stage is infeasible. In this connection, we pro- pose that development of coupled process simulators and testing them on natural analogs provided by geo- logic systems may be fruitful. As example, we illus- trate our attempts to simulate the development of two types of cave systems – branchwork in meteoric environments and mazework in hypogene or hydro- thermal environments. Our computational models combine hydraulic, thermal and chemical processes in limestone fractures and consider the influence of sub- surface heterogeneity as well. Our computational results vividly demonstrate the mechanisms by which branchwork patterns develop in meteoric environ- ments and demonstrate how sustained dissolution along upward flow channels can be established in hypogene environments, thus creating favourable conditions for development of maze patterns. Investi- gations of system sensitivities in both types of envi- ronments indicate that a surprisingly robust pattern of behaviour results, thus serving as a target for de- veloping simplified conceptual models of these sys- tems. We also discuss the implications of our results for design, operation and risk analysis of engineered earth systems. Keywords: Engineering earth systems, karst system evolution, natural analogs. Introduction THE increased energy demands of the industrial societies of the 20th century prompted the exploration of alterna- tive energy sources, including nuclear energy, geother- mal, wind, tidal and wave energy. In the case of nuclear energy, safe disposal of radioactive waste generated in the fuel cycle emerged as an important need, prompting the concept of subsurface or geological waste reposito- ries 1 . At the same time, consumption of fossil fuels has increased significantly, leading to increased emission of carbon dioxide that is implicated in the recently docu- mented warming climate 2 . Recent efforts have focused on reducing carbon dioxide input to the atmosphere by sub- surface storage of fossil power plant emissions 3 . Since the 1960s, geothermal heat has been viewed as a promis- ing source of renewable energy 4 , and the design and operation of geothermal power plants remains an active area of research 5 . All the above activities involve an in- teraction between engineered and natural systems. They require a refined understanding of the long-term response of subsurface environments to significant perturbations, involving several coupled processes, including evolution of flow/thermal regimes, deformation (and perhaps even fracturing), and geochemical alteration of medium prop- erties by precipitation/dissolution reactions. Coupled process (thermo–hydro–mechano–chemical) simulation has been in development for the last 20 years 6,7 , and advanced in step with the dramatic advances in computational technology during the same period. To- day, the term ‘multi-physics’ is more commonly used to describe such problems. Both the complications resulting from nonlinear coupled processes and the inherent het- erogeneity of natural subsurface environments pose sig- nificant computational challenges. The impacts of nuclear waste disposal and subsurface carbon storage will only be evident a few hundred years after disposal/storage opera- tions commence. Thus, it is practically impossible to validate coupled process simulators for evaluating the long-term impacts of these operations. Although short- term tests have been conducted, the wide separation in time scales between deformation (fast), thermal and geo- chemical (very slow) processes preclude full evaluation of slower processes in these tests. Natural analogs in geo- logical systems, where interacting coupled processes are manifest in nature over very long time scales are promis- ing in this context. Examples of such natural analogs include: karst and cave systems 8,9 , organized cementa-