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Man-made deposits - recent and ancient (state of the art and future objectives)

ECSMGE Session 1

Professor Heinz Brandl graduated in civil engineering from the Technical University in Vienna in 1963, from where he also obtained the degree of Dr. techn in geotechnical engineering with honours in 1966.

From 1963 to 1966 Brandl progressed from assistant at the Institute for Ground Engineering and Soil Mechanics at the Vienna Technical University, to assistant professor then head of the soil mechanics laboratory. In 1971 he was appointed associate professor before leaving academia to pursue a career as a freelance consultant.

In 1977 he became professor for soil and rock mechanics and foundation engineering (including tunnelling) at the Technical University of Graz. From 1978 to 1981 he was head of the Geotechnical Institute in Graz, and since 1981 he has been professor at the Technical University of Vienna, chairing the Institute for Soil Mechanics and Geotechnical Engineering. Brandl was vice president of the ISSMGE from 1997 to 2001 and has been chairman of the Austrian Society for Soil Mechanics and Geotechnical Engineering since 1972.

Man-made deposits may be agricultural, industrial, residential or recreational; they may be temporary or permanent. Each has particular characteristics, with different mechanical, physicalchemical and biological behaviour.

Ground conditions may be polluted, damaged or contaminated.

Wide transition zones exist because none ofthese terms has distinctive characteristics to identify sites uniquely.

Site characterisation for geoenvironmental purposes and risk analyses allows development of plans for brownfield restoration and contaminated land remediation with regard to ancient deposits, and construction, operation and aftercare of recent waste deposits.

In many cases politics, public opinion and local self-interest are also relevant, not only with respect to new deposits but also to remediation, where temporary impact on surroundings is unavoidable.

Start with siting Risk analyses, quality assurance, and regulations concerning waste deposits or contaminated land start with siting.

This is greatly influenced by politics and public opinion. The 'not-in-my-backyard' syndrome seems to confront every landfill owner or developer, whether public or private. All too often, the issues opposing a facility are nontechnical - ie political, social, conceptual and emotional.

However strong these arguments, ifengineered landflls are not built, waste, some ofit potentially hazardous, will continue to just 'disappear' into the landscape, or be placed elsewhere.

Investigation zones Investigations for contaminated land and for new waste disposal facilities are broadly similar.

The site should comprise a control zone, the waste deposit itself, a central monitoring zone and the security zone (Figure 1). The security zone is the area where there is potential for groundwater contamination and where pollution should be contained. Its length is a product ofgroundwater flow velocity and reaction time, the latter depending on investigation techniques and monitoring frequency. About three months ofcontinual monitoring ofthese parameters is suf ficient as a design value for the reaction time.

This shows site investigation should always extend beyond the direct boundaries ofa waste deposit or contaminated land.

New techniques for geoenvironmental investigations, including the many additional CPT sensors and (the increasingly accepted) geophysical methods, have their particular merits and constraints. What has been used successfully in one location may not necessarily work as well, or at all, in another.

Site characterisation and risk analysis for new waste deposits should also consider containment. Ignoring the latter is a method typically adopted by those 'experts' who want to prevent a project going ahead.

Site specification and characterisation, waste properties and disposal (ie risk potential), containment quality and intensity of monitoring, and repair options in case offilure should be considered and evaluated as one.

Risk analyses Risk analyses are essential to assess the risk potential and the environmental compatibility of ancient, recent or future manmade deposits. They should also consider how quickly, easily, and to what extent rehabilitation ofa waste deposit and its surrounding environment is possible in the case offailure. The extent ofenvironmental impact depends on:

lThe effectiveness of the components ofa multi-barrier system lThe quantity, the chemo-physical properties and the migration (in space and time) ofa leakage lThe dilution ofa leakage along possible flow paths lThe object(s) affected by the failure and their sensitivity towards leaking substances.

Consequently, siting a new waste disposal facility should be based on three classifications:

lSite classification: characterisation ofthe geotechnical and hydrological properties lClassification of the risk potential ofthe waste lLandfill design classification, involving the quality ofthe technical measures (barriers, leachate removal, monitoring, possibilities ofrepair).

Risk analyses, quality assurance and regulations in environmental geotechnics refer not only to the siting, design, construction, operation, aftercare, monitoring and so forth of waste disposal facilities, but also to contaminated land remediation.

From a philosophical point of view, Figure 2 illustrates the most important statement both from theory and from practice:

An 'absolute' site cleaning in a strict physical/chemical sense is not practically possible.

Cost effective measures achieve an effectiveness of 70% to 80% (90)%. Cleaning ofmore than 90% (95)% commonly leads to an excessive increase ofcosts. From a pragmatic point ofview, it is therefore preferable to remediate more sites at a lower level than to spend the available money on just one site - the question 'how clean is clean' still remains.

Figure 2 also illustrates that '100% barrier effectiveness' cannot be achieved, even with highly engineered waste disposal facilities, unless the waste exhibits a very low risk potential or is properly pretreated. Hence, waste pretreatment should have priority over complicated design and construction.

Design ofnew waste disposal facilities favours reasonable regulations which require negligible impact for a prescribed period.

This should be at least 100 years after closure of the landfill, which corresponds widely to the design-life commonly assumed in structural engineering.

It is not satisfactory to have 50 or even 30 years design life and to pass the impact ofimproperly designed or maintained facilities on to future generations.

On the other hand, regulations requiring negligible or even no impact in perpetuity represent unrealistic restrictions and might, therefore, cause delay in constructing urgently needed new facilities. The term 'negligible effect' on the groundwater and air should be quantified on the basis ofchemistry, biology, and health-related implications.

Tailings dams Tailings dams represent the largest man made deposits on the planet, some covering many hundreds ofhectares with ponds holding sludge and other materials.

They are usually built by mine operators to provide waste storage. While this has the advantage ofslow construction, control is often poor, resulting in an unacceptable number ofimpoundment failures, some 10 times higher than conventional embankment dams.

Many ofthese filures have resulted in massive damage, with severe economic and environmental impact and in several cases, loss oflif. This has led to increased awareness and emphasis on safety of tailings basins in recent years.

Communication is vital between companies, public regulators, operators, management, designers, consultants and reviewers. There should be feedback to the designers to ensure design expectations are being met. Instrumentation should be installed during construction instead ofafer, and it should be simple, robust and reliable. It is also important to document and report changes in operating procedures and the physical performance ofa tailings facility.

Operational and performance monitoring is essential for reliable assessment ofrisk and safty factors. Risks increase when a mining operation is prematurely closed; requiring immediate change from temporary to permanent measures. Finally, a good operating manual and an up-todate emergency plan can compensate, to some extent, for the 'human factor'.

Recent innovations in this area are the thickened tailings technique and roller-integrated continuous compaction control for supporting embankment dams.

Paste and thickened tailings make non-segregated high density tailings possible. These can be transported to a disposal facility, giving the opportunity for building stable landforms from the outset ofa project. The primary disadvantage ofthis technology is cost (AB Fourie).

Roller-integrated continuous compaction control enables optimisation, control and documentation ofeach embankment layer.

This method has significant advantages over random compaction control, thus increasing quality and safety factors.

Future of waste deposits Disposal facilities of municipal and household waste, industrial waste and especially ofhazardous waste should be designed and built according to the 'multibarrier system'.

The term originates in nuclear engineering and originally defined a security system consisting ofseveral protective measures, or barriers, which act independently. It has been extended in waste disposal terminology and has been widely used in Germany and Austria ever since.

The multi-barrier concept comprises natural and man-made (technical) barriers. In a broader sense, multi-barrier systems may also include the deposited material itself. Pretreatment of waste and operation ofthe disposal facility (influencing behaviour of the deposit and immobilising hazardous material emissions) therefore play a significant role within the framework of a safe, wellmanaged deposit.

The costs for construction, operation, maintenance and aftercare of a waste disposal facility fall with the degree of waste pretreatment. Lower levels mean higher security requirements for the facility and the multi-barrier system (Figure 3).

Moreover, pretreatment commonly reduces the contaminating life-span of the waste. Depositing waste which is not pretreated means passing on the environmental impact to future generations. This refers not only to hazardous waste but also to municipal waste.

Figure 4 shows the possible envelope ofthe time-concentration curves ofdissolved organic carbon in the leachate oflandflls consisting ofuntreated municipal waste and similar light commercial waste where it is deposited without being separated.

The earlier a deposit is covered by a tight capping, the longer the time until the leachate becomes environmentally compatible.

From this point ofview, an initial leaching ofthe waste by water infiltration (eg rain, surface water etc) seems to be preferable to early sealing.

Furthermore, the bottom liner and drainage system is certainly more reliable in the early phase ofa landfl than decades (or centuries) after closure. An undetected leak in the capping system may facilitate entry of seepage water later which would activate contaminants in the waste at times when aftercare and monitoring will have most probably stopped.

On the other hand, excessive pore water pressures within the landfill or liquefaction of the waste must be avoided ifwater infiltration is favoured during operation and some years after closure, otherwise slope stability would decrease dramatically.

Consequently, waste management should focus increasingly on waste pretreatment, recycling, and re-use to reduce the necessary volume oflandflls and the risk ofenvironmental impact in the case offailure.

Aftercare of landfills is especially minimised by incineration or multi-phase biological treatment (composting, fermentation) ofmunicipal waste befe depositing the residual material.

A radical change ofwaste management practice is certainly not possible, but an incremental improvement must be the goal of waste management worldwide.

In Austria, landfill owners pay fees which subsequently are used for the remediation of abandoned waste or 'brownfield land'; the fee depending on waste properties and on the quality of waste containment and gas collection.

From 2004, waste disposal will only be allowed ifthe total organic carbon ofthe waste is 5% of the maximum or ifthe lower caloric value is 6,000kJ/kg.

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