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Environmental issues of geotechnical engineering

ICSMGE Plenary session 5

Masashi Kamon is Professor of environmental geotechnics at the Department of Global Environmental Engineering at Kyoto University in Japan. He has spent his entire academic career at the university after obtaining his doctorate in civil engineering in 1973. He was director of the Japan Society of Civil Engineers from 19992001 and is director of the Japan Society of Material Science and council member of the International Geosynthetic Society. His main areas of research are environmental geotechnics, ground improvement, physical-chemical properties of soils and geo-hazards. Kamon has authored over 200 papers and has written books on soil mechanics, ground improvement and environmental geotechnics.

Recent awareness of the green agenda has given rise to the need to develop better technical knowledge of geo-environmental issues. Since the term 'environment'has different meanings to many geotechnical engineers, the definition of environmental geotechnics should be clearly drawn.

The theme lecture focuses on one of the most important environmental issues of geotechnical engineering, the minimisation of the impact of waste. This is tackled by techniques such as the reduction of waste generation, development of suitable intermediate techniques, re-use and recycling, proper containment and remediation of polluted sites. Emphasis is placed on the establishment of safe and proper waste management from the geotechnical point of view.

Defining a role 'Environmental geotechnics' is set to enjoy comparable development to that of its parent discipline. This does not mean that environmental geotechnics will be the future of all geotechnical activities, it is only a facet of our profession.

Environmental geotechnics contributes in an inter-disciplinary context and in the early stage of a civil engineering project. Work aims to identify the possible effects caused by the project and to point out ways and means to avoid, compensate or limit negative environmental impact.

The main research objectives of environmental geotechnics are:

l The creation of better environment l The prevention of environ- mental risks to human activities l The prevention of dangers to human life caused by natural hazards.

Risk management The identification and development of appropriate technologies for environmental site assessments is discussed in the lecture, along with the obstacles to using environmental site assessment as a tool for protecting and restoring environmental quality, and the associated costs of assessment.

The objective of any risk assessment is to determine the risk to groundwater, other aquatic systems and ecosystems through discharge of substances from a landfill. Risk assessment can be considered as a tool that accounts for uncertainties in a rational manner. In relation to contaminated land and pollution control, it can be applied to both the migration of gases, the movement of leachate from a landfill or the leaching of contaminants from a contaminated site. Risks are calculated as a function of the probability of the failure of the system and the loss resulting from adverse affects on the environment and human health.

Environmental regulations are affected by a number of factors, which include local perception of risk (cost/benefit as well as socio-economic issues that vary greatly even within developed countries).

As a result, the field is quite diverse.

Waste management should be based on an environmental geotechnical approach. Large amounts of waste are generated from industry and human activity. The hierarchy in management is waste minimisation, proper treatment, reuse/recycling and energy recovery. Natural resources should be preserved and the need for landfill minimised.

Re-use of waste The lecture examines examples of potential use of the following waste materials:

l Surplus soil and waste slurry l Waste concrete and waste rock powder l Coal ash l Iron, steel, and other slags l Mining waste l Municipal solid waste incinerated ash (MSWIA) l Sewage sludge incinerated ash l Paper sludge l Waste tyres l Waste plastics and other similar materials When using recycled materials, such as industrial and municipal wastes and surplus soils, the potential for pollution must be assessed under the environment of a given application. Solidification techniques using cement are often used to stabilise waste materials to attain suitable properties for geotechnical applications.

One use of cement considered was to immobilise toxic chemicals in waste. However, it was recently revealed that Portland cement might have a risk of leaching of six-valent chromium (Cr6+). Leaching tests for the cement-stabilised soils were carried out to clarify the level of contamination risk. That from Cr+6 leachate was significant, restricting the type of soils that can be treated by this method.

Waste containment Waste containment is one of the most important issues in waste management practice. The major objectives in the safe disposal and containment of any type of waste include:

l The construction of liners, floors, walls and cover (capping) systems that adequately limit the spread of pollutants and the infiltration of surface water l The containment, collection and removal of leakage from landfills l The control, collection, and removal or use of landfill gases l The maintenance of landfill stability l Monitoring to ensure that the necessary long term performance is being achieved.

Various types of bottom liner systems have been proposed, regulated and used, with selection based on the risk levels of site conditions.

Sophisticated bottom liner systems usually consist of barrier (liners) overlaid by collection layers. Collection layers remove the leachate generated from waste and reduce the hydraulic gradient across the liner by reducing the head of water.

The performance of a liner system is directly affected by the boundary conditions. The most common scenario for new landfills is diffusion with positive advection, where the compacted clay liner is placed above the water table. Diffusion with negative advection is the most effective containment system but it is not always used, due to practical problems of continuous dewatering. There are many numerical solutions for the advectivedispersive-reactive equation to describe the behaviour of containment systems.

Vertical cut-off walls (barriers) are also appropriate for surrounding landfills and contaminated sites to cut off the path between source of emission and its target. Cut-off walls can be constructed using various foundation engineering processes.

To avoid toxic leachate, effective design is needed not only on a bottom liner and a vertical barrier but also to establish a capping system. This will prevent infiltration of precipitation and surface water to the waste layer and minimise the generation of waste leachate. Recently, the strategy of re-using sludge materials to form barrier layers has been examined.

Sites contaminated with heavy metals and/or organic contaminants, leakage from underground storage tanks, surface water contamination, uncontrolled landfills, and agrochemical contamination are some of the more common environmental problems. From the remediation standpoint, the most important factors are likely to be a definition of the clean-up target, technical and cost feasibility, natural recovery estimates and the ability to distinguish and/or control continuing contamination.

Different remediation methods for contaminated sites are available.

Providing containment by cutting off migration pathways by:

l capping systems l vertical cut-off walls l basal lining systems l active pneumatic or hydraulic measures l immobilisation.

Decontamination of contam- inated land by l physical treatment l thermal treatment l microbiological treatment l soil washing.

Replacement of contaminated soil and waste: this method should not be used since the contaminated soil and waste still need to be properly disposed of.

Many remediation techniques are based on established technologies or recent innovations. But others are still at the experimental stages. One such, the solidification/ stabilisation technique has the potential to stabilise or treat soils, which would permit safe containment.

A permeable reactive barrier (PRB) consists of a permeable curtain containing reactive materials, generally built to intercept the path of a contaminant plume. Much work on PRB work has been done and the technology has been applied to or proposed for a wide range of common groundwater contam- inants including metals, chlorocarbons, and petroleum hydrocarbons. The major conceptual advantage of the PRB is its cost effectiveness in remediating contaminated groundwater over pump and treat systems.

Dioxins are some of the most toxic and harmful pollutants produced artificially into environment.

The chloro-organic compounds are a byproduct of waste incineration and are also generated during natural disasters such as volcanic eruptions and forest fires: any burning of organics in combination with chlorine can potentially generate dioxin. Dioxin originating from incinerator ash is almost immobile, is not biodegradable and insoluble in water. If dioxin is associated with oil, it is much more likely to spread into the groundwater.

Although dioxins are not considered a typical soil contaminant, there are a number of cases where the chemical is found with other contaminants, eg high concentrations of PCB etc. Remediation can be carried out in two main ways: off-site or on-site treatment.

Off-site treatment means the removal by excavation and treating material remotely in a variety of ways:

l thermal, or solvent reduction l removal by a physico-chemical process l destroying dioxin by incinerating or vitrifying the contaminated soil l storage of the contaminated soil in a safe place.

Special attention is needed to avoid the generation of other toxic organics or dioxins during the dechlorination process.

Conclusions This review of recent developments in environmental geotechnics has focused on waste management, re-use of waste and remediation of polluted soils. Engineers' responsibilities are discussed in regards to developing proper waste management technology for a better environment and the methodology of cleaning up contaminated sites to avoid severe environmental risk.

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