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Abstract

In the Asse salt mine, a system with relatively small pillars and stopes was excavated between 1909 and 1964. From 1965 until 1978, low- and intermediate-level radioactive wastes were disposed there permanently. Most of the chamber volume (∼3.5 million m3) was exposed to free convergence until 1995, when a backfilling campaign was started using pneumatic transportation of granular salt material. The barrier to the overburden rocks is formed by rock salt that has a minimal thickness of only 15 m in the upper part. The flank dips ∼70°SW. The Asse site has been monitored for decades by displacement observations, stress and strain measurements in the pillars, and recording of the backfill pressure built up in the chambers. Softening and damaging in the pillars and stopes of the mining horizon have led to stress redistributions into the overburden rocks, where rupture processes have occurred. Hence, because of the small dimension of the bearing elements on the southern flank and the close distance to the overburden, far-reaching geomechanical interactions exist.

Introduction

Salt deposits are used worldwide for exploitation of the minerals they contain, for the storage of gaseous and liquid hydrocarbons, and for the long-term disposal of dangerous waste.

The Asse mine was exploited for potash and rock salt from 1909 to 1964. GSF, the National Research Centre for Environment and Health, GmbH, Munich (as of 1 January 2008 the Helmholtz Center Munich—German Research Center for Environmental Health [GmbH]) has owned the former potash and rock salt mine Asse since 1965. From 1965 until 1978, low- and intermediate-level radioactive wastes were disposed there permanently. Research and development projects on behalf of the Federal Republic of Germany continued there until mid-1995. Due to the long period that excavations were kept open, pillars and stopes in the excavation fields are characterized by creep deformation, strain softening, and rupturing. Backfilling of the Asse mine started in 1995 and is scheduled for completion by 2017.

Federal mining law governs the closure of the Asse. The regulatory agency must approve a schedule for closure activities (official name: Abschlussbetriebsplan) that includes an assessment of safe long-term performance of subsequent waste isolation capability. A key part of that assessment is an evaluation of the stability of underground openings and overburden. The principal approach to such an evaluation is described by the recommendations of the working group “Salzmechanik” of the German Geotechnical Society (DGG). These include the initial condition of the rock mass at the Asse site as determined by the geological, tectonic, and hydrogeological setting, the primary stress field, and the mechanical behavior of the overburden and the salt. Site-specific data have been used to develop models that describe the initial status, including a geological-tectonic model, a hydrogeological model, and a rock mechanical model.

The geological-tectonic and the hydrogeological models describe the geological system containing the mine workings. The rock mechanical model describes the mechanical behavior of the excavations, of the ground support system, and their interactions with the host rock. The geological-tectonic model is the baseline for calculating all rock mechanical models. Each model was adapted to the requirements of the different work phases and updated on that basis. This effort included extensive mine surveys during mine development, exploratory drilling, and geophysical investigations underground and from the surface.

Location

The Asse mine is located southeast of Braunschweig in the state of Lower Saxony, in northern Germany. Regionally, the geology of the Asse is associated with the western Subhercynian Basin area, which extends between the Harz Mountains in the southwest and the Flechtinger Höhenzug (ridge) in the northeast (Fig. 1). The basin is characterized by lines of emergence and narrow anticlines.

Figure 1.

Structural overview of the northern foreland of the Harz Mountains and the Asse structure (after H. Jordan in E.-R. Look, 1984). This figure shows a view from the region around the Asse structure with typical structures of synclines (Mulde, —x—) and anticlines (Sattel, —⋄—) between Flecht-inger Höhenzug and Harz Mountains. Mulde—syncline; Sattel—anticline; M.—mulde (syncline).

Figure 1.

Structural overview of the northern foreland of the Harz Mountains and the Asse structure (after H. Jordan in E.-R. Look, 1984). This figure shows a view from the region around the Asse structure with typical structures of synclines (Mulde, —x—) and anticlines (Sattel, —⋄—) between Flecht-inger Höhenzug and Harz Mountains. Mulde—syncline; Sattel—anticline; M.—mulde (syncline).

The Asse represents the western end of a line of 25-km-long salt diapirs, the so-called Asse-HeesebergZug (ridge). The 8-km-long Asse ridge extends as a narrow anticline between the broader structures of Elm in the northeast and Großer Fallstein in the south. Its north flank is thrust against the south flank. In the NE and SW, the Asse is bounded by the Cretaceous rim synclines of Schöppenstedt and Remlingen.

The Asse ridge is composed of three major tectonic elements (Table 1), striking from NW to SE. Its core consists of a Zechstein salt diapir that was formed during the Late Cretaceous.

Table 1.

Tectonic Elements of the Asse Ridge

ElementRock typeLevel
SuprasalinarTriassic to QuaternaryUpper
SalinarZechstein 2–4Middle
SubsalinarZechstein 1, PaleozoicLower
ElementRock typeLevel
SuprasalinarTriassic to QuaternaryUpper
SalinarZechstein 2–4Middle
SubsalinarZechstein 1, PaleozoicLower

The upper tectonic element, called Suprasalinar, consists of nonsaliferous Zechstein sediments and overlying Triassic, Lower and Middle Jurassic, Lower Cretaceous through Tertiary, and Quaternary sediments. The middle element is formed by the Zechstein salt sequence (in the Asse mine, Zechstein 2 through Zechstein 4). The lower element, the so-called Subsalinar, consists of folded Paleozoic beds including volcanics and red beds, and nonsalt sediments of Zechstein 1 and lower Zechstein 2. Each of the three major tectonic elements plays a distinct role in the development of the Asse structure.

Exploitation of the Asse

From the start of mining to closure of the Asse mine, planned for 2017, three different work phases can be distinguished that are relevant to the development of the geologic-tectonic model: mining phase (1909–1964), research phase (1965–1994), and closure phase (1995–2017). During each of these phases, the geological situation has been investigated according to the specific requirements.

Mining Phase (1909–1964)

In the beginning, geological exploration focused exclusively on the detection of mineable evaporites and their production. In 1893, deposits of potash salts were discovered in the subsurface of the Asse by exploration wells Asse I and II (Fig. 2). Three additional deep wells were drilled in 1894 and 1895. The production of potash from the Staßfurt seam (K2) on the northern flank of the Asse saddle started in 1909, after the Asse 2 shaft was drilled.

Figure 2.

Topographic map of the Asse showing positions of deep wells 1–9 (Tiefbrl.), Asse shafts 1–4 (Schacht Asse 1 to Schacht Asse 4), hydrogeological wells (Bhrg.), and cross section 2.

Figure 2.

Topographic map of the Asse showing positions of deep wells 1–9 (Tiefbrl.), Asse shafts 1–4 (Schacht Asse 1 to Schacht Asse 4), hydrogeological wells (Bhrg.), and cross section 2.

During the mining phase, the geological system of the Asse was explored by boreholes. Fulda (in Wollstedt, 1931) reported that the Asse anticline is asymmetrical. The northern flank is higher than the southern flank. Mesozoic sediments cover the northern and southern flanks of the structure. The beds of the south flank dip steeply, and some are nearly vertical.

The mining phase was characterized by the opening of the three mining fields for extraction of the minerals (Table 2). Carnallite mining on the northern flank of the Asse saddle produced ∼900,000 m3 between 1909 and 1925. Most of that space was backfilled as part of the mining cycle.

Table 2.

Mining Fields for Extraction of the Minerals

AreaRock typeVolume excavated (million m3)Year
Northern flankCarnallite0.901909–1925
Southern flankHalite (Na3)3.501916–1964
Central regionHalite (Na2)0.451927–1963
AreaRock typeVolume excavated (million m3)Year
Northern flankCarnallite0.901909–1925
Southern flankHalite (Na3)3.501916–1964
Central regionHalite (Na2)0.451927–1963

From 1916 until 1964, rock salt was mined upward from the southern flank of the Asse saddle, mainly in the Leine series (Na3), but also in the central part of the saddle. About 3.5 million m3 of salt was produced between 1916 and 1964. These excavations were left open. Mining in the central region of the Asse excavated 450,000 m3 between 1927 and 1963. These workings were also left open. In total, 10 million tons of raw salt had been produced by 1964.

Figure 3 shows a three-dimensional (3-D) perspective of the three mining areas viewed roughly from west to east. The oldest area is the potash field (shown in pink), which encompasses the northern flank at approximately the 750 m level. From 1909 until 1925, 900,000 m3 of material was excavated from this section. Salt mining started along the southern flank of the saddle in 1916 (shown as light blue, Na3). This was the largest mine section, and >3 million m3 of rock salt was extracted from the 750 m level up to the 490 m level.

Figure 3.

Mining sections of the Asse from 1909 until 1964. This figure shows a view roughly from west to east of the three working sections. The oldest working section is the potash section (lower left) on the northern flank at about the 750 m level.

Figure 3.

Mining sections of the Asse from 1909 until 1964. This figure shows a view roughly from west to east of the three working sections. The oldest working section is the potash section (lower left) on the northern flank at about the 750 m level.

Rock salt mining of the Staßfurt series (dark blue) started in 1927 in the central region of the Asse and stopped in 1963, after excavation of more than 450,000 m3 of material.

Research Phase (1965–1994)

In 1965, GSF acquired the Asse mine. Surface and underground infrastructure was adapted for research and development projects for the disposal of radioactive waste in deep geological formations.

In 1967, large-scale experiments of waste disposal were undertaken. The mine was adapted to research and development purposes by refitting shaft 2, sinking shaft 4, constructing a cavern by conventional mining at a depth of ∼1000 m, and driving a spiral road from the 490 m level to the 800 m level (shown in part in Fig. 3). By the end of 1978, 124,497 casks of low-level and 1293 casks of intermediate-level radioactive waste had been handled and stored within the Asse mine. Since 1979, in situ tests for radioactive waste have been conducted.

During the research phase, additional questions arose regarding the characterization of the host rock and the safety of embedding radioactive waste in deep geological formations. They included the nature of the inner structure of the salt saddle and its external boundary, and the need to obtain further details on petrographic character, thickness, bedding conditions, and lithostratigraphy of the overlying units.

An extensive geological and hydrogeological program was implemented to address new geological questions with regard to mining safety and long-term isolation of radioactive waste materials. The aim of this program was to develop a new and detailed geological characterization of the Asse structure and overburden rock units.

To determine the best positions for a geological drilling program, a seismic-reflection profile was recorded from the Elm across the Asse saddle and most parts of the northern and southern adjacent synclines. Interpretation of the 10-km-long profile shows two N-dipping faults with a throw of several tens of meters in the flat-lying basal Zechstein beds, ∼2070 m below sea level.

Deep wells Remlingen 5 and 6 (Fig. 4) and hydrogeological wells were drilled into the Mesozoic strata. In addition, information about the morphology and locations of fault structures was obtained from seismic-reflection profiles.

Figure 4.

Geological profile of Harz-Elm (after Baldschuhn et al., 1996). This figure shows a cross section through the salt structures between Harz and Elm and their geologic-tectonic attributes.

Figure 4.

Geological profile of Harz-Elm (after Baldschuhn et al., 1996). This figure shows a cross section through the salt structures between Harz and Elm and their geologic-tectonic attributes.

The results of this exploration were incorporated into the geologic-tectonic atlas of Lower Saxony. The regional geology and tectonic setting between Harz and Elm are represented in the cross section after Baldschuhn et al. (1996) (Fig. 4).

The Zechstein series 3 and 4 (Leine and Aller series) extend from the Elm under the northern syncline (Schöppenstedter syncline) to the north flank of the Asse. Most of the Zechstein series 2 (Staßfurt) salt has migrated into the Asse structure by viscoplastic creep. Interpretation of the seismic-reflection data shows that the Zechstein salt is wedged into the southern flank of the Asse.

During salt diapirism, the Mesozoic rocks were compressed from north to south; therefore, the northern flank was raised, and the lower and middle Buntsandstein were driven as a spur under the salt anticline. The north margin of the Harz Mountains functioned as an abutment.

Until recently, the available information was not sufficient to develop a three-dimensional picture of the overburden in the vicinity of the Asse. Model predictions of mining safety, based on the geological body of knowledge, differed considerably from in situ measurements. Calculated displacements in excavations and overburden were significantly lower than the measured displacements obtained from mine surveys. The differences between calculated and measured displacements indicated that the model did not properly represent the real conditions of the Asse site. It appeared that the high mobility of the overburden on the south flank of the Asse was caused by an intensive tectonic stress field.

Closure Phase (1995–2017)

Closure of the Asse mine must take the embedded radioactive waste into account. In 1992, GSF was compelled to finish all research and development projects. The closure of the mine had to be planned according to federal mining law. The aim of closure was the safe isolation of embedded radioactive and chemotoxic waste from the biosphere.

The required documents included a project plan, a technical description, a safety analysis, and an assessment of safe long-term performance of the subsequent waste isolation capability.

The proof of long-term safety describes the safe isolation of the disposed radioactive waste in the Asse mine from the biosphere. Protection of the overburden against mining-induced damage, as well as seismic and groundwater monitoring in compliance with the atomic and groundwater protection laws are essential preconditions for a safe closure of the Asse mine.

Figure 5.

Reinterpreted SW-NE cross section in the middle of the E-W extension of the mine (cross section 2 through the Asse shaft 2 [Schacht 2]). Also, this figure shows the deep wells 5 and 6 (vertical lines toward center of figure).

Figure 5.

Reinterpreted SW-NE cross section in the middle of the E-W extension of the mine (cross section 2 through the Asse shaft 2 [Schacht 2]). Also, this figure shows the deep wells 5 and 6 (vertical lines toward center of figure).

Figure 6.

Geological map of the Asse mine. Basis of the map: topographic map, Blatt 3829 and 3830, scale 1:25,000 (Woldstedt, 1931). The distance between features (coordinates) on map is 1 km both horizontally and vertically. Please note that the legend matches that in Figure 5.

Figure 6.

Geological map of the Asse mine. Basis of the map: topographic map, Blatt 3829 and 3830, scale 1:25,000 (Woldstedt, 1931). The distance between features (coordinates) on map is 1 km both horizontally and vertically. Please note that the legend matches that in Figure 5.

The prognosis of mining safety until the end of the operating phase is crucially important. This prognosis is created by modeling, based on the knowledge of the geological and geomechanical situation in the repository horizon and the overburden.

Long-term safety can be proven only by modeling based on the local conditions. The knowledge at the site includes the geological system, stress patterns, hydrogeology, geomechanical behavior, seismology, and the inventory of disposed radioactive waste.

Computer simulations were used to evaluate the geoscientific long-term prognosis. For that, a model validation using actual in situ measurements was necessary. The result of the validation showed that the knowledge of the overburden did not correspond to the real situation.

Interpretation of Measurements and Observations of Mine Surveys

The Asse site is monitored by a geotechnical, geophysical, and hydrogeological measuring program. The southern flank of the anticline and the top of the structure in the vicinity of the Asse mine were surveyed by seismic reflection in 1997 (Bauer et al., 1998). In total, 37 km of seismic-reflection profiles in an orthogonal grid were surveyed over an area of ∼7 km2, and seismic-refraction profiles and borehole seismic measurements were interpreted within three boreholes and integrated.

Adequate seismic reflections could be observed to depths greater than 2000 m. These horizons could be traced along the flank regions almost without interruption. Nearvertical tilting of the Mesozoic strata produced by the flowing salt, and collapse caused by subrosion, result in tectonic disruptions in the high flank regions and the apex of the saddle. These vertical units manifest themselves as missing or discontinuous reflections.

Interpretation of the seismic reflection in combination with drill-hole data, seismic borehole, surface geology, and correlation with data from nearby profiles produced a detailed fault inventory.

Faults that parallel the strike of the thrust-fault system are the dominant structural fabric. The locations of transverse faults, although previously mapped, were displayed insufficiently in the widely spaced seismic grid because of their steep dip (>80°). To further improve the model, old data (Woldstedt, 1931) were reevaluated and incorporated into new cross sections together with the new data (Klarr, 1981). The modified cross section 2 through Asse shaft 2 (Fig. 5) represents the results of the extensive seismic program. The surface rock is considered to be a discontinuity.

The basis for the geoscientific long-term prognosis and the proof of long-term safety—the geological model—is now available. The structure of the Asse is determined by the main thrust fault that separates the northern from the southern flank of the structure. The peak fault dips steeply northeast. Gently NE-dipping Lower Buntsandstein of the north flank abuts against very steeply SW-dipping Upper Buntsandstein and Muschelkalk of the southern flank (Fig. 5). Figure 6 shows the tectonic structure in plan view.

Measurements confirm the previous conceptual model of the structure of the Asse as follows: the north flank of the structure was thrust over the south flank. Lower and Upper Buntsandstein of the south flank separated, and the overlying beds tilted steeply. The northeast flank is characterized by relatively moderate dips, ranging from 30° to 50°, while the southeast flank dips steeply from 40° to 75°. Dominant NW-oriented folding and faulting are results of SW-vergent thrusting. NE-SW compression produced faulting along both upper flanks and the crest of the Asse structure. This deformation was influenced by subsequent mobilization and redistribution of salt (into the core of the structure).

North of the NE-dipping crest fault that separates Lower Bunt-sandstein from Upper Buntsandstein and Lower Muschelkalk, there are faults in the Lower Buntsandstein of the northeast flank that dip southwest (antithetic). They make the block nearer the structure appear to be thrust over the block farther away.

A similar picture presents itself on the southwest flank or footwall. Here, the faults dip northeast toward the structure. In this way, a parallel graben-style feature developed along the axis of the main thrust and above the salt table and the cap rock.

Assuming that displacement on these antithetic and synthetic faults is taken up by detachments within and at the boundaries of the salt layers, dip-slip faults far away from the structure were probably caused by compression forces modified by the mobilization of the salt. Today, they resemble overthrusts (high-angle reverse faults). Above the top of the salt structure lies a zone of complex structural blocks of primarily Lower Buntsandstein that are intensively faulted and tilted against each other (typical keystone structure along tight folds).

The faulting pattern of the Asse overburden was revealed by the deep drill holes Remlingen 5 and 6 (Fig. 2). A fracture analysis of cores from these holes showed a definite peak of antithetic fractures that dip 30°NE. These joints were opened in the south and north flanks by salt migrating and backing up. Above the salt structure, there are faults that dip toward the axis of the structure. These were probably formed by the collapse of Triassic beds overlying Zechstein salt that was removed by subrosion.

Viscoplastic creep of Zechstein salt, triggered by deformation in the Subsalinar, started the flow of the salt into the structure. Migration of the 450-m-thick Staßfurt halite is responsible for much of the tectonic pattern of the overburden. Except for small residues, the salt beds of Zechstein 2–4 migrated from adjacent synclines into the Asse structure. During that process, the invading Staßfurt salt lifted the northern overburden layers as in a salt-pillow structure.

The Zechstein beds above the Staßfurt extend from the Elm below the Schöppenstedt syncline into the north flank of the Asse. During their migration, the 30-m-thick brittle anhydrite (A3) broke up in places and, for the most part, remained in the deeper horizons. Within the Asse excavations, it is found only in allochthonous blocks of up to 2000 m3. The southern flank stayed more or less horizontal at first, but then the salt invaded the overlying beds like a wedge and tilted the overburden layers on the south flank ever steeper, even overturning them in places (based on core analysis and underground exposures).

Figures 5 and 6 show clearly how the overburden is divided into blocks by faults parallel to and across the structure. This block pattern to a large extent determines the geomechanical behavior of the overburden. Depending on the stress field and the hydrogeological regime within the overburden, faults to a large extent determine its ability to deform. It follows that deformation in the overburden consists mostly of movement along faults that results in block patterns. The consequence of that finding is that in all further geomechanical considerations and models, the actual tectonic structure of the overburden must be expressed as a discontinuous rock mass and not, as happened until recently, a continuum or “smeared” discontinuum.

For the geotechnical safety analysis of the operating phase and the assessment of safe long-term performance of the Asse mine, the primary stress field in the overburden is a fundamental parameter to describe the starting conditions of the rock mass (aside from the analysis of the geological system). The local stress field of the Asse structure and overburden is determined by (or it is a subset of) regional stress conditions. Investigations of stress conditions of the Asse region are based on the results from Müller et al. (1992), investigations of the stress conditions in east Europe (Bankwitz et al., 1992; Becker and Paladini, 1990), and on results from investigations in the western region of northern Germany (Grote, 1998). As shown on the stress map of Germany (Fig. 7), the stress situation in the area of the Asse was unknown until recently. Evaluation of the existing deep-drilling and exploration-drilling data near the Asse and in the region appeared to be the most promising method for determining the stress field (Lempp and Röckel, 1999).

Figure 7.

The 2005 (Reinecker et al.) release of the world stress map (Germany) with sH- maximal horizontal compressive stress. Regime: NF—normal faulting; SS—strike-slip faulting; TF—thrust faulting; U—unknown regime. Quality: A: 10–15°; B: 15–20°; C: 25°.

Figure 7.

The 2005 (Reinecker et al.) release of the world stress map (Germany) with sH- maximal horizontal compressive stress. Regime: NF—normal faulting; SS—strike-slip faulting; TF—thrust faulting; U—unknown regime. Quality: A: 10–15°; B: 15–20°; C: 25°.

The method used to determine the stress state was based on the evaluation of data from the deep “Remlingen” drill holes on the southern flank of the Asse saddle (Fig. 2). The cores were reevaluated in combination with drilling data from borehole tests to interpret indicators for the stress field in the overburden (Lempp and Röckel, 1998).

The resulting qualitative and quantitative description of the stress is illustrated in Figure 8, a 3-D illustration of fault planes that reconstructs stress directions. This graphic shows the main element affecting the tectonics of the Asse overburden—the tectonic regime in the upper major tectonic element “Suprasalinar,” the overburden of the salt diapir (Table 1). In the overburden, the stress condition is variable. Stress directions are unstable and vary between Hercynian (NW-SE) and Rhenish (SSW-NNE) directions. At depth, the greatest horizontal stress direction is Hercynian (Lempp and Röckel, 1998). The arrows in Figure 8 show the three major stresses, σ1, σ2, and σ3, in the Suprasalinar. The vertical stress, σ v, is larger than the horizontal stress, σ H, in the direction of strike (NW/SE), and σh (NNE/SSW), the other horizontal stress perpendicular to σH, shows the smallest values. The major horizontal stress direction is in good agreement with the known stress field in northern Germany (Fig. 7).

Figure 8.

Stress field in the Suprasalinar of the Asse structure.

Figure 8.

Stress field in the Suprasalinar of the Asse structure.

Looking at the stress field of the Subsalinar, we now know that the horizontal stresses are twisted by 90°. This shows that the stresses of the Subsalinar and Suprasalinar are decoupled by the salt diapir. The stress condition in the Subsalinar is north and south, based on the results of core analysis, boring reports, and geological field analysis (Lempp and Röckel, 1999). The estimated values of the gradients for the Suprasalinar are 25 MPa/km for σ v and 15 MPa/km for σ h or σ3 (Lempp and Röckel, 1998).

The qualitative and quantitative evaluation of the stress field of the Suprasalinar of the Asse mine establishes the basis for further model calculations with regard to long-term performance assessment. Another important characteristic of this stress evaluation is that because of the tectonic dissection of the south flank of the Asse (Fig. 8), this flank reacts to movements with some degree of mechanical flexibility. The tilt of the south flank beds exposed underground is almost perfectly parallel to the tilt of beds exposed at the surface.

Two fundamental elements of modeling determine the geomechanical behavior and associated load-bearing behavior of the overburden: (1) the overburden is characterized by intensive fault deformation that partitions the overburden into numerous discrete structural blocks. (2) the stress field in the Asse anticline and the surrounding region could be quantified and is characterized by the distinction between Suprasalinar, Salinar, and Subsalinar. A specific stress direction dominates each of these levels. Relative stresses vary in each case in a characteristic manner.

The stress monitoring stations in the intact pillars show the stress field of the Asse with σ1, the largest value of stress, cutting across the strike of the Asse saddle. These measurements show the load direction from the overburden over the flank of the saddle into the load-bearing assembly of the south flank.

Due to the geometry of the chamber arrangement on the southern flank, which consists of small pillars and stopes, the load-bearing assembly is a pliable system. After decades of operation, this system is characterized by creep fracturing, strong plastic deformation, and rupturing (Fig. 9).

Figure 9.

View of the broken contour zone of a pillar in the upper part of the Na3 section.

Figure 9.

View of the broken contour zone of a pillar in the upper part of the Na3 section.

With backfilling, the pillar deformation rates have stabilized and have recently shown a decreasing trend. Pillar stabilization is confirmed by the decrease of microseismic activity in the mining section on the southern flank. Figure 10 presents a picture of increasing activity in the adjacent overburden rocks. This shows stress redistribution from the mine to the overburden, which leads to fissuring there. As long as the overburden displacement persists, these geomechanical interactions will continue.

Figure 10.

Microseismic activity in the mining section on the southern flank. The dimension of circle is approximate proportional to the magnitude of microseismic activity (-3 to 0).

Figure 10.

Microseismic activity in the mining section on the southern flank. The dimension of circle is approximate proportional to the magnitude of microseismic activity (-3 to 0).

Summary and Conclusion

Because the owner of the Asse mine has no further research needs, the mine is now being prepared for closure according to the regulations of the federal mining law (Bundesberggesetz). Because of the long operating period during which excavations were kept open, pillars and stopes are characterized by creep deformation, strain softening, and rupturing.

The major aim of the mine closure concept is the continuing safe isolation of embedded radioactive waste from the biosphere. The starting point of the assessment of safe long-term performance is the rock mechanical condition at the end of the operating phase. It establishes the proper preconditions for analysis and assessment of current and future load-bearing behavior of the overburden.

Extensive new investigations and geophysical measurements form the basis for more accurate and detailed geological, geotechnical, and hydrogeological models. These geoscientific models are necessary for long-term prognostic calculations and the analysis of consequences. This paper shows that new geoscientific methods can supply high-quality data and analyses for safety prognoses. The recharacterization of the Asse mine is more comprehensive and better tied into the regional geology as a result of the integration of new data sources, such as seismic profiles.

The Asse mine is structurally complex and dominated by block faulting due to salt diapirism and compression of the Mesozoic strata associated with salt mobilization and deformation within an overall thrust fault regime. The overburden is more deformed and structurally less competent than earlier studies had suggested.

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Gregersen
,
S.
Pavoni
,
N.
Stephanson
,
O.
Ljunggren
,
C.
1992
,
Regional pattern of tectonic stress in Europe
:
Journal of Geophysical Research
  v.
97
, no.
B8
p.
11783
11803
.
Reinecker
,
J.
Heidbach
,
O.
Tuigay
,
M.
Sperner
,
B.
Müller
,
B.
2005
,
The 2005 release of the world stress map (Europe)
 :
University of Karlsruhe, International Lithospheric Program
,
Karlsruhe
.
Roth
,
F.
et al
1998
,
Spannungsmessungen in Osteuropa—Orientierungsdaten aus Nordostdeutschland
,
Weißrussland und der Westukraine
 : in
ICDP/KTB Kolloquium Bochum, Wissenschaftliches Programm und Abstracts
.
Schwerdt
,
L.-O.
1984
,
Abschlussbericht über Reflexionsseismische Messungen Asse 1983
:
Wathlingen
:
Unveröffentlichter Bericht im Auftrag der GSF
 ,
10
p.
Woldstedt
,
P.
1931
,
Erläuterungen zur Geologischen Karte von Preußen und benachbarten deutschen Ländern, Blatt 2095 (=3830)
Schöppenstedt, mit Beiträgen von E. Fulda und G. Görz
 :
Berlin
.
64
p,
2 Abbildungen, 1 Karte
,
Berlin
.

Figures & Tables

Figure 1.

Structural overview of the northern foreland of the Harz Mountains and the Asse structure (after H. Jordan in E.-R. Look, 1984). This figure shows a view from the region around the Asse structure with typical structures of synclines (Mulde, —x—) and anticlines (Sattel, —⋄—) between Flecht-inger Höhenzug and Harz Mountains. Mulde—syncline; Sattel—anticline; M.—mulde (syncline).

Figure 1.

Structural overview of the northern foreland of the Harz Mountains and the Asse structure (after H. Jordan in E.-R. Look, 1984). This figure shows a view from the region around the Asse structure with typical structures of synclines (Mulde, —x—) and anticlines (Sattel, —⋄—) between Flecht-inger Höhenzug and Harz Mountains. Mulde—syncline; Sattel—anticline; M.—mulde (syncline).

Figure 2.

Topographic map of the Asse showing positions of deep wells 1–9 (Tiefbrl.), Asse shafts 1–4 (Schacht Asse 1 to Schacht Asse 4), hydrogeological wells (Bhrg.), and cross section 2.

Figure 2.

Topographic map of the Asse showing positions of deep wells 1–9 (Tiefbrl.), Asse shafts 1–4 (Schacht Asse 1 to Schacht Asse 4), hydrogeological wells (Bhrg.), and cross section 2.

Figure 3.

Mining sections of the Asse from 1909 until 1964. This figure shows a view roughly from west to east of the three working sections. The oldest working section is the potash section (lower left) on the northern flank at about the 750 m level.

Figure 3.

Mining sections of the Asse from 1909 until 1964. This figure shows a view roughly from west to east of the three working sections. The oldest working section is the potash section (lower left) on the northern flank at about the 750 m level.

Figure 4.

Geological profile of Harz-Elm (after Baldschuhn et al., 1996). This figure shows a cross section through the salt structures between Harz and Elm and their geologic-tectonic attributes.

Figure 4.

Geological profile of Harz-Elm (after Baldschuhn et al., 1996). This figure shows a cross section through the salt structures between Harz and Elm and their geologic-tectonic attributes.

Figure 5.

Reinterpreted SW-NE cross section in the middle of the E-W extension of the mine (cross section 2 through the Asse shaft 2 [Schacht 2]). Also, this figure shows the deep wells 5 and 6 (vertical lines toward center of figure).

Figure 5.

Reinterpreted SW-NE cross section in the middle of the E-W extension of the mine (cross section 2 through the Asse shaft 2 [Schacht 2]). Also, this figure shows the deep wells 5 and 6 (vertical lines toward center of figure).

Figure 6.

Geological map of the Asse mine. Basis of the map: topographic map, Blatt 3829 and 3830, scale 1:25,000 (Woldstedt, 1931). The distance between features (coordinates) on map is 1 km both horizontally and vertically. Please note that the legend matches that in Figure 5.

Figure 6.

Geological map of the Asse mine. Basis of the map: topographic map, Blatt 3829 and 3830, scale 1:25,000 (Woldstedt, 1931). The distance between features (coordinates) on map is 1 km both horizontally and vertically. Please note that the legend matches that in Figure 5.

Figure 7.

The 2005 (Reinecker et al.) release of the world stress map (Germany) with sH- maximal horizontal compressive stress. Regime: NF—normal faulting; SS—strike-slip faulting; TF—thrust faulting; U—unknown regime. Quality: A: 10–15°; B: 15–20°; C: 25°.

Figure 7.

The 2005 (Reinecker et al.) release of the world stress map (Germany) with sH- maximal horizontal compressive stress. Regime: NF—normal faulting; SS—strike-slip faulting; TF—thrust faulting; U—unknown regime. Quality: A: 10–15°; B: 15–20°; C: 25°.

Figure 8.

Stress field in the Suprasalinar of the Asse structure.

Figure 8.

Stress field in the Suprasalinar of the Asse structure.

Figure 9.

View of the broken contour zone of a pillar in the upper part of the Na3 section.

Figure 9.

View of the broken contour zone of a pillar in the upper part of the Na3 section.

Figure 10.

Microseismic activity in the mining section on the southern flank. The dimension of circle is approximate proportional to the magnitude of microseismic activity (-3 to 0).

Figure 10.

Microseismic activity in the mining section on the southern flank. The dimension of circle is approximate proportional to the magnitude of microseismic activity (-3 to 0).

Table 1.

Tectonic Elements of the Asse Ridge

ElementRock typeLevel
SuprasalinarTriassic to QuaternaryUpper
SalinarZechstein 2–4Middle
SubsalinarZechstein 1, PaleozoicLower
ElementRock typeLevel
SuprasalinarTriassic to QuaternaryUpper
SalinarZechstein 2–4Middle
SubsalinarZechstein 1, PaleozoicLower
Table 2.

Mining Fields for Extraction of the Minerals

AreaRock typeVolume excavated (million m3)Year
Northern flankCarnallite0.901909–1925
Southern flankHalite (Na3)3.501916–1964
Central regionHalite (Na2)0.451927–1963
AreaRock typeVolume excavated (million m3)Year
Northern flankCarnallite0.901909–1925
Southern flankHalite (Na3)3.501916–1964
Central regionHalite (Na2)0.451927–1963

Contents

References

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1992
,
Regional pattern of tectonic stress in Europe
:
Journal of Geophysical Research
  v.
97
, no.
B8
p.
11783
11803
.
Reinecker
,
J.
Heidbach
,
O.
Tuigay
,
M.
Sperner
,
B.
Müller
,
B.
2005
,
The 2005 release of the world stress map (Europe)
 :
University of Karlsruhe, International Lithospheric Program
,
Karlsruhe
.
Roth
,
F.
et al
1998
,
Spannungsmessungen in Osteuropa—Orientierungsdaten aus Nordostdeutschland
,
Weißrussland und der Westukraine
 : in
ICDP/KTB Kolloquium Bochum, Wissenschaftliches Programm und Abstracts
.
Schwerdt
,
L.-O.
1984
,
Abschlussbericht über Reflexionsseismische Messungen Asse 1983
:
Wathlingen
:
Unveröffentlichter Bericht im Auftrag der GSF
 ,
10
p.
Woldstedt
,
P.
1931
,
Erläuterungen zur Geologischen Karte von Preußen und benachbarten deutschen Ländern, Blatt 2095 (=3830)
Schöppenstedt, mit Beiträgen von E. Fulda und G. Görz
 :
Berlin
.
64
p,
2 Abbildungen, 1 Karte
,
Berlin
.

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