BIO-SOILS WORKSHOP

Overview | Motivation | Logistics | Agenda | Organizers | Letter to participants

MOTIVATION

A number of technical and professional factors motivated the organizers to propose hosting this workshop. To provide a framework for the proposed event, these are summarized below.

Research advances that have used biological processes have provided solutions to engineering problems that have improved society. New solutions have provided more efficient, environmentally friendly, and/or economic solutions relative to traditional treatments that utilize man-made materials. An example of this is Microbial Enhanced Oil Recovery (MEOR), where microbial activity has been used to either mobilize adsorbed oils or to plug large pore spaces via calcite cementation, which in turn enable additional production of oil from a well before abandoning it. Other examples of how biological process can alter environmental materials to a degree significant for engineering applications are the stabilization of metals including radioactive wastes, development of biological shields for zonal remediation of hazardous wastes or reactive landfill liners, environmental stabilization of contaminated soils, encapsulation of hazardous, reactive materials, and other contaminants in natural soils and acid mine tailings. However, the changes that occur in soils from the geotechnical aspect are often not investigated due to the focus on environmental remediation.

Natural biological processes have been shown to alter the properties and behavior of both natural soil deposits and of man-made soil deposits created during construction. One of the most common examples of this is "bio-engineering" techniques that are implemented for near surface stabilization of slopes (Gray and Sotir 1996). Live vegetative materials, or fascines, are integrated into the slope surface and their natural anchoring systems – roots – provides additional stability, enabling construction at higher slope grades than that possible with conventional construction techniques. Less widely recognized is the role that biological processes can have in the failure of geotechnical systems. For example, Mitchell and Santamarina (2005) cited biologic activity as a critical component in the 1984 embankment failure of Carsington Dam in Derbyshire, England, and in the long term stability of a high rock pile near an open pit mine in northern New Mexico.

Perhaps equally unrecognized by geotechnical engineers is the influence geotechnical construction processes can have on biological processes. For example, soil compression during agricultural treatment (compaction by a smooth drum roller in geotechnical terminology) initiates a fundamental shift in biological processes – from aerobic to anaerobic metabolism -, which creates conditions harmful to plant roots and an environment that is conducive to the release of greenhouse gasses (Flynn et al., 2005). Soil compression (due to mechanical compaction) also alters the balance between water stress and mechanical impedance that roots experience, affecting plant growth.

The biological sciences have advanced well beyond on the age of observation. Today many biological processes have been characterized and understood to a degree that the process can be controlled as desired. Some examples where the controlled use of microbial processes is firmly established are the microbial degradation of organic matter in waste waters, the use of microorganisms in the bioremediation of hazardous substances in groundwater, soils and sediments, in air, in bioreactors or industrial waste streams, microbial agricultural and food production, pharmaceuticals and vitamin production, etc.

In recent decades the frequency and volume of manufactured materials, such as geosynthetics and grouts, introduced into the subsurface as part of geotechnical systems has increased dramatically. The addition of these materials, while beneficial from an engineering performance perspective, might not necessarily be advantageous for the environment. For example, materials for grouting, which can create artificially high in situ pH levels, have in some cases contaminated the subsurface and lead to toxic conditions.

The role of biological processes in the engineering behavior of soils, and vice-versa, are inherently interdisciplinary questions. Within the confines of the traditional academic disciplines the experts that can answer aspects these questions remain in their respective disciplines and as a result minimal progress can be made in answering these questions. A forum such as a workshop that brings these people together has the strong potential to stimulate new understanding of old problems as well as new discoveries. For example, the embankment failure of Carsington Dam (cited above) was unexplainable considering only geotechnical aspects, but can be explained when biological processes are considered as well. Similarly, soil scientists hypothesize that the surfactants that roots produce alter rates of soil water uptake, but without geotechnical understanding they have not been able to quantify if this is indeed the case and whether it affects soil strength. Furthermore, exposure to other disciplines will also expose experts to the research and analysis tools in other fields that may be useful for their research.

In a forthcoming report from the National Academy of Science committee "Geological and Geotechnical Engineering in the New Millennium: Opportunities for Research and Technological Innovation" it is anticipated that the role of biological process in geotechnical engineering will be cited as a critical research thrust and opportunity for the future. The recognition of the role of biological processes has also been recently highlighted in the ASCE G-I GeoStrata magazine, which dedicated the September/October 2005 issue to "Bio-Geo: In-situ Processes".

The emerging importance of the role of biological process in geotechnical engineering has been highlighted by a recent ASCE journal paper by Mitchell and Santamarina (2005) in which the authors establish a foundation for this new area. They sequentially define and describe "biological constituents, characteristics, and processes within a soil", provide "examples of how microbiological conditions and processes may influence engineering properties and behavior of earth materials", review "geomicrobiological processes that appear particularly promising for further study", and identify "relevant references to aid in obtaining in-depth background and understanding of many of the points". The recent publication of a paper such as this (with basic definitions) emphasizes the infancy of this field and simultaneously highlights the increasing recognition of the importance of this emerging field.

Recent laboratory studies by interdisciplinary groups have demonstrated that biological processes can be harnessed and used to modify the behavior of soil. DeJong et al. (2006), using the microorganism Bacillus pasteurii, successfully cemented a loose sand specimen with calcite. Testing of the specimens revealed that the strength of the soil had been substantially improved and that specimen collapse under shear was prevented.

Figure 1a: SEM image of microbially induced calcite cementation by Bacillus pasteurii

Figure 1b: Improved shear response of biologically treated sand