Safety concept and criteria for hazardous waste sites

Engineering Geology, 34 (1993) 159-167

159

Elsevier Science Publishers B.V., Amsterdam

Safety concept and criteria for hazardous waste sites M. L a n g e r

Bundesanstalt fiir Geowissenschaften und Rohstoffe, Postfach 510153, Hannover, Germany (Revised version accepted December 3, 1992)

ABSTRACT Langer, M., 1993. Safety concept and criteria for hazardous waste sites. In: M. Arnould, T. Furuichi and H. Koide (Editors), Management of Hazardous and Radioactive Waste Disposal Sites. Eng. Geol., 34: 159-167. The problem of waste disposal in Germany has been solved by using a combination of above-ground and underground disposal. Site selection criteria and precise criteria for the performance assessment of various types of waste disposal are available. In view of long-term safety of disposal, it is necessary to include geological and hydrogeological viewpoints in addition to purely engineering viewpoints. In particular, the geotechnical site-specific safety assessment is described, as defined by the government in "Technical Regulations on Wastes" (TA-Abfall) in the section "Underground Disposal". This safety assessment must cover the entire system comprising waste, cavern/mine and surrounding rock. For this purpose geo-mechanicalmodels have to be developed. According to the multi-barrier principle, the geological setting must be able to contribute significantly to isolation of the waste over longer periods. The assessmentof the integrity of the geologicalbarrier can only be performed by making calculations with validated geomechanical models. Various engineering geologicaldata are required for the selection of a site, for the design and construction of a repository, and for a safety analysis for the post-operational phase. These data can only be attained by the execution of a comprehensive site-specificgeomechanicalexploration and investigation program. The planning and design of an underground repository in rock salt layers are described, as an example for the various steps of this type of safety assessment.

1. Objectives and principles Economic and technical developments, together with basic changes in consumer habits, have led to considerable amounts of wastes in the world. This situation has, in turn, increasingly emphasized the importance of waste management in terms of the conservation of natural resources and energy. Solving the problem of waste has, therefore, become one of the central tasks of environmental protection. A decisive step for the development of waste management in G e r m a n y was the "Waste Management Program" of the Federal Government in 1975. This program had three objectives: (1) Reduction of the production of wastes; (2) Increased recycling of wastes; and (3) Disposal of wastes without damage to the environment.

The possibilities for disposal sites in the F R G , either above or below ground, are limited. Therefore, it is necessary to tighten the requirements placed on the production and treatment of wastes. This is true not only for ground-level facilities, but also for disposal underground, which is often thought of as the "last refuge". The Technical Manual for Waste Management (Annex to the fourth amendment of the Waste Disposal Law, 1990) provides uniform regulations with respect to the disposal methods and types of disposal site appropriate for specified types of waste, The past has shown us that the requirements placed on the disposal of waste were not strict enough. The problems resulting from old disposal sites, problems that present great technical, legal, organizational and financial difficulties, justify these new efforts. The objective of all these scientific, technical and legal activities is to minimize the risks to our

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environment that result from the disposal of hazardous wastes. The environment is a very complex system. To protect all parts of the environment, the air, soil and water bodies must be viewed as one single system. When a hazardous waste facility is set up, protection of natural resources, including water, is very important, both during and after the operations phase. 2. The

barrier

concept

and

long-term

safety

The objective of a hazardous waste repository (i.e., prevention of hazardous substances from entering the biosphere), is attained, in principle, through the use of a system of natural and manmade barriers, whereby the artificial barriers (systems of seals) are more important for aboveground sites and natural barriers (host rock) are more important for underground repositories. The effectiveness of the barriers can be increased by the following measures: - - choice of site, - - deposition of only certain types of waste at a specific site, - - waste treatment, - - monitoring of seals, - - leakage treatment, --long-term surveillance after the end of operations, and - - analysis of possible disturbances to the system and measures to avoid them. The public acceptance of a waste repository depends to a large degree on the assurance that the natural and artificial barriers are sufficient to provide the necessary protection. A safety analysis, therefore, is of main importance to the planning and authorization of a repository, and thus the question arises as to the principles on which such an analysis is based. Natural geological barriers are an important part of a multiple-barrier system. Thus, the loadbearing capacity of the rock (expressed, for example, as settlement or cavern stability), its geological and tectonic stability (e.g., mass movement or earthquakes), and its geochemical and hydrogeological development (e.g., groundwater movement and the potential for dissolution of the rock) are important aspects of the safety analysis. The safety

analysis is, therefore, not a purely engineering problem, but must include geological factors. The normal stability parameters and safety factors of engineering are not sufficient. A site-specific modelling of geological, hydrogeological, geochemical and geo-mechanical features and processes is needed (Fig. 1). The safety analysis must be based on a safety concept that includes the possibilities of failures (failure scenarios) that could occur during the excavation, operation and post-operation phases, as well as measures to avoid such failures. Monitoring is also a part of the safety concept. As described in Fig. 2, our safety analysis includes the following steps (Langer, 1985): (a) Analysis of the effectiveness of each barrier with analytical methods appropriate for each type of barrier system. For example: --probability analysis for technical systems (e.g., the form of the waste itself and the container); - - stability analysis for geotechnical systems (e.g., back fill, seals, bulkheads); and --forecasts of the geochemical, hydrogeological and tectonic processes that may occur in the geological systems. (b) Analysis of the physical, geochemical and hydrogeological processes that occur as the result of the influence of the different barrier systems on each other, with emphasis on their influence on the transport of hazardous substances in the near and far fields. For example: - - redistribution of stress in the rock around the cavern,

SAFETY ASSESSMENT

REAL LOADING

- stress - strain - temperature

l

J

by M O D E L L I N G

SYSTEM

C

WASTE-ROCKCONSTRUCTION (LIMITING STATES~

Fig. I. Safety assessment by modelling.

IENGINEERED

- disposal - mine - components

SAFETY CONCEPT AND CRITERIA FOR HAZARDOUSWASTESITES

161

SAFETY ANALYSIS ~i (MULTIPLE BARRIERS) II

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I ROEKMECHANICAL

TECHNICAL SYSTEMS

SYSTEMS I

J PROBABILISTIC l RISKANALYSIS

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PROOF GEO- TY TECHN EALofSTABL

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PROGNOSTI GEOLOGY C I

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SCENARIO ANALYSIS LLCONSEO,UENCE ANALYSIS jjil Fig. 2. Safety concept based on the principle of multiple barriers.

- - changes in the hydraulic equilibrium in the rock resulting from the excavation, and --physical and chemical reactions between the stored wastes and the rock or the formation water. (c) A comprehensive analysis of the safety of the repository in which the interaction of all of the barriers is evaluated for certain theoretical events (accidents, failure of one or more barriers) that could release hazardous substances into the environment, i.e., identification of possible release paths and the resulting effects (failure analysis). For example: - - collapse of the cavern during operations of the repository, - - a geological or tectonic event (e.g., earthquakes), and - - accidents during operations. Some experts may consider these requirements for a safety analysis too excessive. It must be kept in mind, however, that a repository must provide a special protective function over a very long period of time. It must also be borne in mind that uncertainties are unavoidable in geo-engineering and that these uncertainties must be taken into consideration during planning, designing and construction. Uncertainties are, for example: - - variation of material properties with respect to space and time, - - uncertainties in the determination of the load,

of the model (simulation of the physical and geological conditions), and - - errors of omission and unexpected events. Geological and geotechnical uncertainties can be mastered by a method of calculated risk ("Geoengineering Confidence Building", see Fig. 3). The main part of this method is the handling and validation of models. --inexactness

3. G e o t e c h n i c a l

requirements

and site criteria

(barrier evaluation)

A repository as an engineered structure in a geological medium requires interdisciplinary cooperation. Thorough geoscientific and technical studies are necessary for each case to work out a concept in which site, deposition methods, properties of the host rock and the geological situation are so coordinated that natural and artificial barriers are optimally used to protect man and environment. An underground repository can be considered to be a permanent repository only if it is constructed in a geological environment in which hydrogeological and geochemical transport processes occur so slowly that the deposited materials can be viewed as permanently isolated from the biosphere. Geological barriers must also be taken into consideration for disposal facilities above ground because in the case of failure (e.g., in the

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SAFETY CONFIDENCE BUILDING

TYCONC

HAZARDS EVALUATION (LIMITING STATES)

GEOLOGICAL l MODEL

PARAMETERS EXACTNESS

STATIC (NUMERICAL MODEL

SAI~I-y CRITERIA

NUMERICAL CALCULATION

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PARAMETER ANALYSIS

SUBMODELS -

STRUCTURES GEOMECHANICS HYDROGEOLOGY GEOCHEMISTRY -

MODEL VERIFICATION

PROBABILISTIC TREATMENTS

MODEL VALIDATION

ON SITE CONFIRMATION QUALITY ASSURENCE (CONTROL)

Fig. 3. Geo-engineering safety confidence building.

liner system or in drains) they serve as the last part of the multiple barrier system. The necessary geotechnical and geoscientific studies are derived from this concept (Fig. 4). First, the best method of deposition is determined on the basis of the type and amount of waste and the geological situation, and its feasibility is assessed. For this purpose a suitable geological structure is ENGN I EERN I G GEOLOGY FOR WASTE DISPOSAL

I SITECHARACTERIZATION J ~

SAFETYANALYSIS

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SAFETYCONCEPT I

SITESELECT,ON I I

1 SITEINVESTIGATION J [

SAFETYCRITERIA

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J

I I

H 0 DE L L I N G i---{GEOTECHNICALSTABILITY] GEOLOGY HYDROGEOLOGY E fi OMECHANICS GEOCHEMS I TRY

Ir I--1

SCENARIOANALYSIS I

WASTEDESI REPOSITORY GN I CONSTRUCTIONOPERATION Fig. 4. Engineering-geololgical site and safety analysis.

sought (e.g., a salt dome or thick clay sediments). The first site selection is made on the basis of geological maps, of an evaluation of archived material, particularly borehole data, seismic studies, and other deep sounding methods, and the infrastructure of the area is considered, e.g., population density and transportation connections. Competing interests such as groundwater protection areas and natural resources protection areas should also be taken into consideration at this early stage. Moreover, recommendations for geoscientific studies of the site are to be made on the basis of all available data. In the following investigation of the site, all project relevant parameters must be evaluated for the concluding assessment of the technical feasibility and long-term safety of the repository. The first task of the site study is to define the present situation: --determination of the geological sequences, fissure systems, and faults through drill holes and geophysical studies (e.g., seismics and well logging); --hydrogeological inventory (e.g., nature of the aquifer, its division into various levels, cover layers, hydraulic contacts, area, pumping tests, permeability tests in wells); -sampling of groundwater for chemical analysis, and if possible for age determinations; - - rock samples for determining the properties of the rocks (e.g., mineral content, pore volume and permeability);

SAFETY CONCEPT AND CRITERIA FOR HAZARDOUS WASTE SITES

studies (e.g., laboratory determinations of the stability and deformability of rock samples); and - - interpretation of seismic records to determine the regional earthquake activity (geodynamic risk). The evaluation of these studies provides the basis for the geo-mechanical and geohydraulic simulation models used to test the reaction of the system to engineering activities. The chemical analysis of the formation water, and the mineralogical analysis of the rocks, yield information about the origin of the water and thus the future development of the disposal site. At the same time, the leaching capacity of this water is obtained and, hence, important information for predicting reactions between the wastes and the foundation soil. Geological, hydrogeological and geotechnical monitoring is also necessary during the construction and operation phases of a repository project. The objective of such monitoring is to continually test the assumptions and calculations in the plans, in order to change the plans to fit any unexpected condition. --geomechanical

4. D i s p o s a l o f h i g h l y h a z a r d o u s w a s t e s i n r o c k s a l t layers

Purpose and aims Compared with other geological media, rock salt seems to be the most suitable medium for constructing waste disposal cavities, at least for a large range of waste types. Therefore, the Working Group "Salt Mechanics" of the German Society for Soil Mechanics and Foundation Engineering have given recommendations concerning the engineering geological and geotechnical aspects of underground disposal of hazardous wastes in salt (Langer, 1990). These recommendations apply to the planning, construction, operation and postoperational management of salt caverns used for underground disposal of hazardous wastes. In particular, geotechnical site-specific safety verification, as required by the governmental Technical Regulations on Wastes (TA-Abfall) in the section entitled "Underground Disposal", is described. Highly toxic waste has to be disposed off in underground facilities. The objective is to provide

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(depending upon the properties of the waste in question) permanent isolation from the biosphere (principle of complete inclusion, isolation), or the emplacement of the wastes in such a manner eliminating the risk of contamination of or detrimental changes occurring to the groundwater compared with the geogenic status (principle of neutral immission deposition). This definition of waste management targets leads to the division of underground disposal facilities into the following two types: (a) Underground cavities in salt rock (caverns, mines); and (b) Underground cavities in other rocks. The salt cavern model has the following characteristics: - - geological barrier is salt; - - t h e r e exists permanent separation from the biosphere; - - seal of access is well in the groundwater-bearing overburden; - - the cavern is inaccessible; - - the wastes are non-retrievable; --emplacement of bulk goods and slurry with in situ solidification; - - separate storage of wastes is not possible; - - convergence of host salt rock mass causes gradual inclusion of wastes; and - - permanent dry storage of wastes by engineered plugging of cavern throat and access well. It is stipulated that the suitability of the rock mass for the construction of an underground disposal facility is to be documented by a site-specific safety proof. This site-specific safety verification should consider the waste, the subsurface construction and the surrounding rock mass as a single, whole system. In addition, geomechanical models are to be prepared to allow an appraisal of the reaction of the entire system to the interference of the operations. A safety proof is based on the analysis of pertinent potential risks during the construction, operational and post-operational phases (see sections 2 and 3).

Engineering geological investigations (exploration) The primary objective of engineering geological investigations at some preselected site is to explore

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the relevant geological phenomena such that the geological data as outlined in the specifications are available for the construction project. In the case of underground disposal caverns this includes in particular those data needed for the geotechnical safety proof for the construction and operation of the cavern and also for the safety considerations for the post-operational phase. The comprehensiveness, intensity and results of the investigations should enable the preparation of a rock mechanic model for the relevant geological mass. A correctly managed exploration programme provides a basis for confirming the general geological prognosis. The outline of the salt dome and its external structures are recorded using a series of geophysical methods, preferentially gravimetrics and reflection or refraction seismic. The internal structures are explored by drilling. In addition to the recovery of cores, down-hole logs are run in order to acquire data on the geological environment in the immediate proximity of the boreholes and in well sections whithout cores. The scale of the exploration is determined by safety and technical parameters, together with this specific complex problem. The target here is to minimize the perforation of the overburden and the top of the salt due to the drilling. Nondestructive geophysical exploration should have a high priority. In order to fully meet the safety requirements for disposal caverns, it must be an objective of the exploration to achieve maximum areal and spatial coverage. To meet these targets the exploration work is concentrated on determination of: --the spatial location and limits of the salt formation; - - the depth and features of the caprock; --the spatial location and thickness of potash beds (hard salt, carnallitite, sylvinite, etc.), anhydrite beds and claystone beds if deemed necessary; --homogeneous areas within the saline structure; and - - the recovery of representative cores for laboratory testing. The exploration programme of a saltdome often has the following break down: -Depending upon the knowledge of the

M. L A N G E R

regional geology, stratigraphy and tectonics of the salt formation in question, one or more exploratory wells are drilled, possibly used at a later stage as cavern wells. - - In cases where existing knowledge of the location in the salt-dome area is sufficient, it is possible to eliminate the preliminary exploration phase. - - The most important geological data is gained by coring the wells from caprock down to total depth in salt. As a minimum condition, cores should be recovered for those sections relevant for the safety verification. The core recovery programme is complemented by a down-hole logging programme. This logging programme simplifies the correlation of layers. As far as the overburden is concerned it should be sufficient to take mud samples together with a down-hole logging programme to allow a lithological profile to be drawn up.

Determ&ation of geotechnical characteristic values The load-bearing properties of the salt rock mass and the hazardous waste deposited in the cavern (referred to in the following as the cavern fill) is primarily determined in the operational and post-operational phases by the material properties of the rock mass and the cavern fill, and their interaction. One of the bases for the geomechanical calculations described in the next section ("Geotechnical safety considerations") consists thus of the qualitative and quantitative determination of the thermomechanical properties, both for the relevant rock mass layers, and for the cavern fill. In some cases it is conceivably necessary that the cavern fill makes some contribution to the load-bearing behaviour of the rock mass by providing some support. In these cases it would be necessary to determine the requirements of the cavern fill depending on the local conditions and to incorporate the requirements during the facility operation. Depending on the material laws applied in the geotechnical safety verification, it is necessary to determine the relevant characteristic values with which the local rock and rock mass qualities can

SAFETY CONCEPT AND CRITERIA FOR HAZARDOUS WASTE SITES

be quantified, and similarly the existing and demanded qualities of the cavern fill. It should be pointed out that conditioning of hazardous waste to form a cavern fill displaying predefined qualities is not yet standard procedure on an industrial scale. The recommendations formulated here are, therefore, not based on actual experience in the discipline of "waste mechanics", but are rather, and for the most part, derived from associated areas ( e.g., soil mechanics, rock mechanics, building material testing) and adapted accordingly. In contrast to most other hard rocks, the behaviour of salt rocks displays pronounced non-linear stress, temperature and time-dependent deformation. In addition the strength characteristics are also significantly dependent upon the stress levels current at any time. This special and complex material behaviour places high demands on the test technique s (equipment levels, measurement techniques, time and cost), and requires a tight correlation between the material law expressions found in the verification concept selected and the test to be carried out in the laboratory (type, length), as well as, on the material characteristic values to be determined. The determination of the thermal and mechanical material parameters includes: - - b a s i c s (general; core material and documentation; specimen preparation, test equipment, measured value recording and measured value processing), - - petrographic characteristic values, - - thermal characteristic values, - - m e c h a n i c a l characteristic values (deformation characteristics values; strength characteristic values), and - - field tests. It is then possible to determine the material behaviour and the material characteristic values of the surrounding rock using the standard laboratory and field tests of rock mechanics depending upon the geomechanical requirements. When filling the cavern with solid waste materials, a differentiation is made between granulates with loose rock qualities, and suspensions with binding agents which solidify in situ in a material with hard-rock qualities. The material introduced

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into the cavern as cavern fill represents a considerable factor of influence on the load-bearing capacity of the disposal cavern, because of its interaction with the rock mass. It is thus necessary, with regard to the reliability of the geomechanical calculations, that the cavern fill displays defined, quantitatively determined material properties which allow the theoretical assessment of its mechanical behaviour together with that of the rock mass. In this sense the cavern fill has to be regarded as a building material on which quality demands are made and which is subject to some form of quality control. In addition to the demands described in the preliminary comments, other requirements exist which under some circumstances represent opposed demands of the cavern fill, e.g., with respect to injection density, deformability, strength and binding additive, all of which are to be determined on a site-specific basis. To determine the properties of the cavern fill, it is possible to apply the procedures which exist in the guidelines and recommendations of the disciplines of soil mechanics, rock mechanics, material studies, bulk mechanics, backfill technologies and conditioning. It is then, in this case, possible to fall back on procedures with a long track record and which are for the most part standardized. The investigations necessary can refer to one or all of the following: hazardous waste, binder, additives, liquids, freshly injected cavern fill or compacted/solidified cavern fill. Which tests are required has to be decided on a project-specific case-by-case basis. In order to ensure that the quality demanded for the cavern fill is also maintained, it is also necessary to set up a multi-layer monitoring system, in which the individual layers consist of a suitability test, a quality test and a follow-up test. In the above, the suitability tests are intended to facilitate the development of mix formulae and conditioning processes such that the hazardous waste to be disposed of achieves the required cavern fill qualities, whilst bearing in mind the conditioning, transport and disposal techniques. Quality tests are designed to monitor the properties of the cavern fill during the industrial scale conditioning phase. As a result measurement methods must be applied which can be carried out quickly,

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frequently and with little effort or expense. The follow-up tests are intended to determine whether or not the deposited cavern fill meets the requirements set.

Geotechnical safety considerations

Caverns should be designed, operated and abandoned with the prerequisite that, at all times during the construction, operation and post-operative phases (after completion of filling), they are stable and represent no risk to the host rock, the environment or the groundwater quality as a result of either deformation or leakage. The geotechnical safety verifications required during the planning and design of the caverns are to be determined for various specified, usually threshold, cases, using theoretical/mathematical methods. They are based on the results of laboratory tests and supported by experience gained from in situ measurements. Under the umbrella of the geotechnical safety assessment, a particularly high priority has to be placed on the stress-deformation states. Possible risks determine the influence on the load-bearing capability and capacity by carrying out sensitivity analyses and hence be in a position to assess and verify the cavern integrity. Stability questions and dimensioning for sub-surface cavities differ from the equivalent tasks in general civil engineering for several reasons, including the following: - - The host rock provides the actual loadbearing function in securing the cavity by countering the rock stress changes caused during the construction of the cavern. The properties of the rock mass determine both the impinging forces (e.g., lithostatic pressures) as well as the countering forces (e.g., rock reaction). - - It is necessary to investigate the host rock mass with regard to its structure and its material properties, but also the natural spread of these variables at the location in order to be in a position to design realistic calculation models. The spread of material properties is larger in natural rock than compared to material used in standard civil engineering. In order to specify the conditions to be studied, the following must be known:

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- - the results of the geological and rock mechanical tests in laboratory and in situ; - - the draft design of the cavern, including geometry, access ways, seals, etc.; - - the solution mining method chosen; - - t y p e , properties, injection method, short- and long-term behaviour of the cavern fill; and --planned operational and final state of the disposal cavern. As far as the more important states during the construction, operational and post-operational phases are concerned, the more important limiting situations are to be specified. Such limiting situations, initially only described on a verbal basis, could be, e.g., questions on the following: (1) Whether or not during and after the construction of the cavity non-permitted deformations are predicted of the cavern itself or the ground level which could affect the functionality of the cavern for the safety of the buildings at surface. (2) Whether the load-bearing behaviour of the rock mass is sufficient to avoid sudden or gradual collapse of the subsurface space. (3) Whether the injected materials (waste, backfill) have a stabilizing effect over a longer term. (4) Whether the deformations due to the convergence in the salt rock mass and in the overburden cause unacceptable changes to the hydrological conditions, e.g., groundwater flows. Primary causes are to be allocated for the limiting situations with extreme values of possible spread and accompanying secondary conditions with average values of their spreads. The influence of spread bands on the safety statements is to be assessed within the terms of a parameter analysis. The safety plan is to be established as early as possible in the planning, specifying the limiting states to be borne in mind overall and the required measures and verifications. Theoretical verifications are the norm since in situ measurement results are not available during the planning stage and since such measurements generally only measure the actual status, but do not register the limiting situation. Suitable verification models are to be developed for the limiting situations requiring study and which comprise the following individual steps:

SAFETYCONCEPTANDCRITERIAFOR HAZARDOUSWASTESITES

of the limiting situation to be studied; - - influential calculation model; - - material model for the rock mass behaviour; - - calculations for safety-relevant status variables; - - review and assessment of calculation results; - - safety verification; and - - i n s i t u measurements with interpretation. As a rule, the calculation assumptions should be conservative in nature. --description

Conclusions

Two important steps for establishing a waste repository are site selection and site investigation. These investigations do not only produce the necessary data for safety analysis, but also for the planning and operation of the repository. From a technical viewpoint, an underground repository is not too different from a normal mine, the underground openings being much more limited, however, and specifically designed for long-term stability and isolation, taking the natural (geological) barrier into account. A number of geological formations have been studied over the last decades to assess their potential as host rocks for deep underground repositories. The formations studied include evaporites (generally rock salt), hard crystalline rocks (e.g., granite, gneiss, basalt), and argillaceous formations (clay, shale). Considerable consideration has been given to the possible use of some of these formations and are included in repository design, waste emplacement techniques and long-term safety assessments. From an engineering geological point of view, any disposal site for highly toxic wastes must be so chosen that the transport of dangerous quanti-

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ties of toxic particles into the biosphere via circulating groundwater can be avoided. In principle, all final disposal concepts should be safeguarded by a system of parallel or interlocking natural and technical barriers (multi-barrier principle), although the effectiveness of such technical and natural barriers may receive different weighting in different disposal concepts (Langer, 1989, 1991). Many of the actual environmental problems can only be solved by a close cooperation of geoscientists and civil or mining engineers. For instance, the suitability of a geological medium for final disposal can only be demonstrated if a comprehensive safety analysis has shown that the interaction of the system "waste product/final disposal facility/overall geological situation" can maintain the predetermined protection aims. The product to be disposed of and the engineering concept of the deposit facility on the one hand, and the condition of the geological formations on the other hand, mutually interact and simulaneously place requirements on each other. References Bundesminister fiir Umwelt, Naturschutz und Reaktorsicherheit, 1990. Technische Anleitung zur Lagerung, chemisch/physikalischen und biologischen Behandlung und Verbrennung von besonders iiberwachungsbediirftigen Abf'~llen (TA-Abfall). GMBI, 41: 866-896. Langer, M., 1985. Safety criteria required for waste disposal. In: K. Cidlinsky (Editor), Geoenvironment and Waste Disposal. UNESCO, pp. 203-215. Langer, M., 1990. Empfehlungen des Arbeitskreises "Salzmechanik" der Deutschen Gesellschaft fiir Erd- und Grundbau e.V. zur Geotechnik der Untertagedeponierung von Sonderabf'allen im Salzgebirge--Ablagerung in Kavernen. Bautechnik, 67: 91-95. Langer, M., 1989. Engineering geology and environmental protection. In: E. Bliicher LTDA (Editor), De Mello Volume. Sao Paulo, pp. 251-258. Langer, M., 1991. Ingenieurgeologische Arbeiten zum Umweltschutz. Geol. Jahrb., A 127: 101-125.