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Environmental issues in geotechnical engineering

ICSMGE Plenary session C

Although the exact date when geotechnical engineers began to address environmental issues is uncertain, such issues became a primary concern in geotechnical engineering around 1980.

Thus, geotechnical engineers have been dealing with environmental issues on a formal basis for at least a quarter of a century.

As a result, the time is appropriate for an assessment of some environmental issues of interest in geotechnical engineering, as well as some issues that are likely to be important in the future.

Seven current and/or future environmental issues are examined in this session:

long-term performance of waste containment systems

acceptance of alternative barriers and barrier materials

need for innovative barriers and innovative barrier materials

emergence of new waste forms

increasing signifi cance of biological processes

the role of modelling

the importance of professional identity and the need for consistent professional terminology.

Long term performance of waste containment systems

A relatively recent review of the state of the practice of modern waste containment systems illustrates that such systems are to date a success, in terms of minimising the potential for groundwater contamination.

However, given the relatively young age of modern waste containment systems, and the periodic modifi cation of the regulations governing the disposal of wastes, continuous assessment of facilities will be required to ensure that the long term performance continues to protect groundwater quality, the environment, and human health.

In this regard, the report will give two examples of recent data that illustrate the potential importance of this issue: the relatively recent occurrence of volatile organic compounds (VOCs) in collection lysimeters beneath liners in a number of landfills in Wisconsin, USA, and the long term hydraulic conductivity of geosynthetic clay liners subjected to extended permeation with inorganic salt solutions.

Alternative barriers and barrier materials

Alternative barriers can be defi ned as barriers, such as liners and covers for waste containment systems, used in place of commonly regulated or prescribed barriers (eg compacted clay liners, geomembranes and geosynthetic clay liners).

Alternative barrier materials refer to materials used as alternative barriers or as components of alternative barriers, and may include non-soil or non-polymer materials or unusual soils that may not be capable of achieving typically prescribed properties (eg, k 10 -9 m/s).

In terms of covers, interest in the use of alternative earthen final covers (AEFCs) relative to regulated or prescriptive covers has been growing.

AEFCs are comprised entirely of soils, designed on the basis of water balance principles to perform as well as, if not better than, the prescriptive counterparts and typically with greater durability and/or lower cost.

The advantages of AEFCs relative to more traditional, prescribed cover systems are delineated in the report and examples of some of the measured data for AEFCs at two sites located in semi-arid climates and one site in a sub-humid climate illustrate some of the key factors involved in designing and evaluating their performance.

Innovative barriers and barrier materials Innovative barriers and barrier materials are distinguished from alternative barriers and barrier materials in that the former are still in the fundamental stage of research and development whereas the latter have reached the demonstration stage for regulatory approval and application.

The objective in developing innovative barriers and barrier materials is to produce a barrier that is more effi ient and/or less costly than existing ones.

In this regard, the report focuses on two types of innovative barriers and barrier materials - clay membrane barriers, and polymer clay nanocomposites.

The potential benefi t of clay membrane barriers is illustrated, followed by data that indicates the relevance of membrane behaviour in bentonite-based barrier materials.

Polymer-clay nanocomposites are polymer materials containing a small fraction of layered silicates (usually less than a few percent by weight) dispersed within the polymer matrix.

The report illustrates the potential benefi ts from their use for environmental applications, such as in liners and covers for waste containment where migration of gases (VOCs, hydrogen sulfi de, oxygen, radon) is an important consideration.

Emerging waste forms

Another potentially important environmental issue facing the geotechnical engineering community is how to deal with the disposal and containment of emerging waste forms that recently have been recognised or, as yet unseen, forms that will appear in the future.

The potential environmental issues from two emerging waste forms derived from animal wastes and nanowastes are presented to illustrate the importance of emerging waste forms in geotechnical engineering.

Significance of biological processes

Also of signifi cant environmental concern to geotechnical engineers is the recognition of the increasing importance of biological processes in governing the performance of geotechnical engineered systems.

In terms of bioremediation, recent emphasis has been placed on stimulating native bacteria (biostimulation) in low permeability soils contaminated with organic compounds.

These are subjected to biodegradation by driving nutrients into the soils via elecrokinetics under DC electrical fi elds, as well as the use of sulfate reducing bacteria in permeable reactive walls for insitu treatment of acid mine drainage.

Interest in bioreactor landfills has also re-emerged due to advances in environmental regulations.

Examples of these applications are described to illustrate the importance of biological processes in environmental applications of geotechnical engineering and some of the issues arising from this importance.

The role of modelling

The ability of models to predict accurately fi eld performance of engineered systems has been and will continue to be an issue facing geotechnical engineers.

This is particularly important when dealing with environmental problems because models are often used as a tool to predict the future impacts and resulting risk from engineering activities related to environmental protection, such as new waste disposal facilities and existing sources of contamination (eg waste dumps and piles and accidental chemical spills).

Within this context, predictions made with existing models generally cannot be considered reliable until and unless these predictions are verifi d by comparison with fi eld data.

Results of simulations using four existing unsaturated flow models (HYDRUS-2D, LEACHM, UNSAT-H, and VADOSE/W) to predict the water balance of AEFCs are compared with the field measured water balance for two field test covers. Conclusions are drawn regarding the ability of the models to accurately predict the field performance.

Professional identity

There is a plethora of terminology used to represent the general fi eld of environmental issues related to geotechnical engineering, including: waste geotechnics, environmental geotechnology, environmental control, geotechnical engineering for waste disposal, geotechnical practice for waste disposal, geotechnics of waste disposal, environmental geotechnics and geoenvironmental engineering.

This extensive list emphasises the importance of professional identity in the environmental industry.

The report argues that the two most commonly used terms, environmental geotechnics and geoenvironmental engineering should be used.

Environmental geotechnics refers to a sub-discipline of geotechnical engineering and geoenvironmental engineering refers to an interdisciplinary fi eld that includes geotechnical engineering.

The rationalisation for this distinction is provided, as well as an example to illustrate why such a distinction is important.

Charles Shackelford is a professor in the department of civil engineering, Colorado State University, US. He has 18 years of experience in geoenvironmental aspects of waste management and environmental remediation, is a registered professional civil engineer in California and Colorado, and has served as an expert on waste disposal issues.

Shackelford is also director of the Rocky Mountain Regional Hazardous Substance Research Center, which focuses on remediation of mine waste sites.

His research focuses on evaluating flow and transport of hazardous liquids and contaminants through soil and geosynthetic containment barriers and vertical (soil) cutoff walls.

Shackelford is past chair and a current member of the geoenvironmental engineering committee of ASCE's GeoInstitute, is co-chair for the environmental geotechnics committee TC5 of ISSMGE, served on the technical coordination council of ASCE's Geo-Institute, and was an elected board member of the US University Council on Geotechnical Education & Research.

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