Technological Voids Research Articles

Effect of technological voids on hydro-mechanical behavior of compacted bentonite

Effect of technological voids on hydro-mechanical behavior of compacted bentonite

Bentonite homogenisation and swelling: The effect of salinity

Hazardous radioactive waste must be removed from the biosphere, and geological storage is the universally favoured option for accomplishing this. In many cases, the repository designs include a clay buffer as part of the engineered barrier system (EBS) that surrounds the individual waste canisters and seals the disposal galleries. The emplacement of the EBS around the waste will generate small void spaces that must be closed to ensure that high permeability pathways do not develop. Many of the groundwaters at proposed disposal sites are saline, and this porewater chemistry, combined with the presence of technological voids may present a technical challenge to repository designers. In this study, a suite of experiments was conducted to examine the impact of fluid salinity in combination with the presence of a void space on the swelling behaviour of barrier bentonites. Both sodium and calcium bentonites were studied and the sample lengths were varied to provide an understanding of the role of axial strain on the homogenisation and swelling pressure development in the bentonite. After 100 days of testing the clay had swelled into the void space and differential swelling pressures (difference between the maximum and minimum swelling pressure recorded at a given time) had reduced substantially, highlighting the ability of the bentonite to expand into a void. However, clay that swelled into the void never generated significant swelling pressures during the testing period and differential pressures were still significant at the end of the tests. The calcium bentonite demonstrated much smaller peak swelling pressures than the sodium bentonite under the same salinity conditions, and in all tests, the generation of significant swelling pressure in the clay in the low-density end of the sample did not occur during the testing period. These findings suggest that the suppression of clay swelling at higher salinities is likely to strongly impact the void-filling process.

Coupled thermo-hydro-mechanical model for simulating thermal history-induced voids in buffer material

The thermo-hydro-mechanical processes in a deep geological repository (DGR) influence the bentonite buffer’s long-term performance. The buffer’s temperature evolution over a repository’s service life imposes a thermal history in the DGR. The developed thermal history cause shrinkage of the buffer and generates the technological voids (i.e., annular gaps) in a DGR. The void formed compromises the intimate contact between the dissimilar materials within the DGR and thus impedes the containment ability of the buffer in the long run. Therefore, to investigate the complex coupled processes involved, the present study developed a coupled thermo-hydro-mechanical (THM) model to evaluate thermal history’s influence on buffer’s hydro-mechanical performance. The developed THM model was validated using data from an infiltration test conducted under isothermal conditions. Furthermore, the THM simulations on thermal history indicated radial and vertical shrinkage within bentonite. The formed voids were sealed due to rehydration occurring through the bottom boundary. The hydro-mechanical parameters, such as swell deformation in radial and vertical directions, porosity, permeability, water intake, and swell pressure, were assessed during the hydration process. A sudden increase in the swell pressure was observed when the void gets sealed completely due to rehydration. The rate and magnitude of swell pressure generated were proportional to the peak temperature imposed at the non-yielding boundaries and the consequent void size formed. The increased swell pressures could have impaired the structural integrity of the waste contained canister at a DGR scale. Given this, the implications were comprehended based on the outcome of numerical investigations for the plausible adverse effects inside a DGR due to thermal history.

Swelling Behavior of a Clay-Pellets Mixture Intended for the Isolation of Spent Nuclear Fuel

Deep geological repository is the most favorable option for the safe disposal of high-level nuclear waste (HLW) and spent nuclear fuel (SNF). The design of such a facility relies on the multi-barrier system, encompassing a cylindrical metallic container (encapsulating the HLW/SNF), an engineered barrier system (EBS, around the metallic container); and the host-rock (or natural barrier). The most preferred buffer material to construct the EBS is bentonite clays. They are being considered as potential buffer materials because of, amongst other reasons, their high swelling capacity provides mechanical stability to the metallic canister containing the HLW/SNF; their good thermal conductivity assists to dissipate the heat released by the HLW/SNF; their low permeability delays the flow of water and gas through the system [1].
 Bentonites are highly swelling smectites clays that exhibit significant volume increase when wetted under free-swelling conditions, and develop high swelling pressure upon soaking when the volume change is restricted. It is envisaged that the EBS will be built using blocks of compacted bentonites (Figure 1(a)), or a combination of high-density clay-pellets mixtures and blocks of compacted bentonites (i.e., where the metallic container sits, Figure 1(b)). Clay-pellets are also considered as possible seal materials to fill gaps that will be present in this type of system, e.g., the gap between the EBS and the surrounding host-rock [2]. Therefore, a better understanding of the swelling capacity and behavior of the bentonite in these two forms (i.e., compacted-clay and clay-pellets mixtures) is critical for a proper design and evaluation of the long-term performance of geological repositories for HLW/SNF.
 Most of the research conducted in this area has been mainly focused on compacted bentonites looking, e.g., at the influence of several factors on their swelling pressure capacity, namely, type of clay minerals, initial dry density, water content (or degree of liquid saturation), and type of water [3]. More recently, investigations have been conducted to study the behavior of high-density pelletized clay mixtures [4, 5], however, relatively less attention has been placed in this type of material. Clay-pellets presents a number of advantage, e.g., they are very suitable for filling (small) technological voids (Figure 1(a)), there is no need for additional in-situ compaction when they are used as a buffer/backfill material (Figure 1(b)), and it is relatively easy to manufacture them.
 The granular clay-pellets samples were manufactured, and the constant volume cells were developed at Texas A&M University to investigate the swelling behavior of expansive clays. It has been shown in figure 2 that both materials (compacted clay and pelletized clay) developed similar maximum values of swelling pressures. However, the patterns associated with the swelling pressures evolutions are different, owing to the different pore structures associated with these two materials, which impact on the kinetic of clay hydration and swelling pressure evolutions. The aim of this research is to gain a better understanding of the swelling pressure behavior of clayed materials intended as potential barriers materials for the safe isolation of the HLW/SNF.

On the evolution of hydration cracks of compacted bentonite under different boundary conditions

Bentonite is widely used as a buffer/backfill material to dispose of the high-level radioactive wastes (HLWs). The precast bentonite blocks can cause the technological voids to be widespread in the repositories, acting as the primary channels for water migration. Luckily, bentonite will swell considerably when exposed to water, making it possible to seal the voids. The self-sealing process intensifies the uneven expansive deformation of bentonite and induces the cracking phenomenon. To be close to the actual situation, various boundary conditions were considered in this paper to carry out a series of hydration tests on compacted Gaomiaozi (GMZ) bentonite. The dynamic evolution of hydration cracks was investigated in both qualitative and quantitative way. Experimental results showed that the cracks basically followed a tendency of emerging first, then extending and finally closing. The bentonite sample with radial and vertical technological voids was characterized with a greater cracking degree than the case where only a radial void existed. According to the time history curves of geometric parameters, there was a close correspondence between the increase and the decrease stages from different cracks, indicating that the existing cracks provided deformation space for the initiation and propagation of subsequent cracks by sacrificing themselves. Due to stress or suction change, a re-cracking phenomenon was reflected on some bimodal curves, which demonstrated a significant memorability of the hydration cracks. Generally, the discrepancy in cracking behavior under different boundary conditions was related to the friction effect. The existence of vertical technological voids decreased the swelling pressure, thus reducing the friction force on the boundary. The suppression effect on the crack propagation was consequently weakened, which was conducive to forming wide cracks and strengthening the mutual connection between cracks.

Bentonite homogenization with technological voids upon hydration

During the construction of the nuclear waste geological repository, the inevitable creation of technological gaps will lead to the redistribution of bentonite material along the wetting path upon water intake, thus impacting its localized hydraulic and mechanical properties. It is therefore of the utmost importance to be able to understand and predict the dry density inhomogeneous distribution throughout the buffer zone. This work proposes a microscopic dynamic force balance framework, incorporating the influence of van der Waals force, diffuse double layer force, and boundary friction force, to explore the underlying mechanism of the bentonite homogenization process upon hydration. The microscopic assumption postulates that bentonite materials are an assembly of numerous parallel thin sheets with other dimensions substantially exceeding the thickness, establishing an association between the aforementioned force and the material’s volume fraction. Particular attention is directed toward exploring the mechanism of boundary friction's effects on the inhomogeneous distribution of dry density with hydration time. The application of the framework to examine the impact of engineered voids with 10% and 20% of the total volume yields a satisfactory reproduction in terms of the spatial and temporal evolution of dry density, validating the robust prediction capacity of the established model. The investigation indicates that compacted blocks in proximity to technological voids experience a rapid reduction in dry density upon hydration, leading to the homogenization process characterized by elevated values in the initial void-affected region and diminished values in the innermost area. Whereas full homogenization is yet to be achieved within the observation period of up to 2160 h, and the greater the variability of the dry density within the specimens containing larger initial voids. Further analysis reveals that as the hydration process advances, the friction force within the portion featuring the initial void exhibits a gradual increase, while emergence gradually decreases in the inner swelling zone. Notably, the high friction factor at the bentonite and steel interface allows for the maintenance of large stress gradients and the associated density gradients.

Pore fluid chemistry effects on homogenization of compacted bentonite specimen with technological voids

Pore fluid chemistry effects on homogenization of compacted bentonite specimen with technological voids

Experimental evidence on the cracking and sealing mechanisms of compacted bentonite by using microfocus X-ray computed tomography

Compacted bentonite plays important roles in multi-barrier systems for the deep geological disposal of radioactive waste; one such role is to absorb groundwater infiltrating from the surrounding rock and to expand to seal technological voids. In the self-sealing process, the expansive deformation occurs along with a cracking phenomenon, which may lead to the performance degradation of engineered barrier. In this paper, Gaomiaozi bentonite was compacted into a dense sample to conduct a hydration test, accompanied with a simulated radial technological void. Microfocus X-ray computed tomography was used to scan the sample at various hydration moments to observe the evolutions of external appearance and internal structure. Consistent and significant results were obtained through qualitative observations and quantitative analyses. The self-sealing process underwent a gradual slowdown, manifested as a linear correlation between sample volume and hydration time on the semi-logarithmic coordinate. The sealing order of technological void was from bottom to top, consistent with the wetting path. Simultaneously, the hydration cracks developed following the same sequence. The maximum height of crack propagation was always less than that of the sample expansion. Moreover, the cracking direction of bentonite was always perpendicular to the tensile stress, resulting in a vertical strike for most cracks. However, there still existed a horizontal crack, which may be attributable to the weakness of the stratified interface. Under persistent hydration, the bentonite adjacent to the cracks was supplied with water, thereby strengthening the expansion and gradually closing the cracks. The hydration, expansion, and cracking were closely interrelated, and the relevant mechanisms are revealed in detail.

Bentonite swelling into a void under suction or water flow

In the context of the deep geological disposal of nuclear waste and to improve the understanding of the homogenisation process of bentonite barriers, in particular with regard to the filling of technological voids, tests were performed with compacted FEBEX bentonite samples hydrated under limited axial swelling conditions. The samples were saturated from the top surface using the vapour transfer technique (imposing suctions of 6 and 0.5 MPa), or with deionised water injected at a low flow rate either from the bottom surface or from the gap on top. The final water content of the samples saturated via vapour transfer was related to the suction imposed during the tests according to the water retention curve. Thus, the final water content of the samples tested under suction 6 MPa was lower and not enough to allow sufficient bentonite swelling to close the gap. In contrast, the samples saturated under suction 0.5 MPa reached higher water contents and were able to fill the gap before the equilibrium water content had been reached. In the tests performed with liquid water supplied through the gap, the samples were able to swell easily into the open void and the gap closed earlier than when hydration took place from the opposite end. In all the tests, the final water content of the bentonite was higher and the dry density lower towards the hydration surface. These gradients were more remarkable and persistent as the initial strain was larger. No completely homogeneous density or water content distribution was observed in any of the tests. These changes were also reflected in the pore size distribution. The pores of size larger than the upper limit of mercury intrusion porosimetry (~ 550 µm) were also quantified, which allowed a better representation of the pore size distribution of the more swollen samples. Although the void ratio corresponding to pores smaller than 200 nm (em) was initially higher, over time the volume of macropores (eM) increased more, giving place to an overall decrease in the em/eM ratio, which tended to be constant along the samples in the longest tests. In the first stages of hydration, the macropore void ratio and size considerably increased close to the gap in the tests with saturation from it.

Effect of sample thickness on the self-sealing and hydration cracking of compacted bentonite

In the hydration process of compacted Gaomiaozi (GMZ) bentonite with technological voids, the cracking phenomenon happens simultaneously with self-sealing. In order to investigate the effect of sample thickness, a series of hydration tests were carried out in this paper and the evolutionary process of bentonite was visualized using a custom-designed device. Corresponding digital images were acquired by time-lapse photography and analyzed by image processing techniques. Experimental results showed that the sample thickness exerted an apparent effect on both the self-sealing and the hydration cracking of compacted bentonite. Increasing thickness slowed down the self-sealing process and thus prolonged the time needed to seal the technological voids. Moreover, it was found that thickening the sample inhibited the development of hydration cracks and simplified the crack network notably, indicated by a decreasing trend of crack amount, length and area. In addition, the fractal dimension, characterizing the complexity degree of crack network, decreased with the increase of thickness. By fitting and predicting, a critical value was obtained that if the sample thickness exceeded it, no hydration cracks could be observed on the surface and no penetrating cracks could be formed in the sample. These conclusions are of great importance in the design of bentonite barriers by reducing the incidence of hydration cracks.

Processing and use of waste from the mining industry

An increase in the extraction of resources is accompanied by their losses and dilution of ores due to the technogenic impact on ore-bearing massifs. The possibility of preserving the earth’s surface is provided by technologies with filling technological voids with hardening mixtures based on leach tailings, the insufficient activity of which is increased by processing in disintegrators under the influence of high energy. Experimental proof of the possibility of controlling the state of the hardening mixture was obtained in the course of many years of use of the disintegrator as part of the filling complex. The possibility of replacing cement with activated tailings has been proven not only as inert fillers but also as binders, which makes it possible to replace commercial cement with cheaper production of commercial products. Utilization of metals from enrichment tailings of various types of minerals allows the use of secondary tailings without restrictions on sanitary requirements. The combined effect of mechanical and chemical energies allows waste-free disposal of enrichment tailings and creates, in essence, a new base of metal raw materials to meet the needs of industry.

Environmental and natural leaching processes in underground ore mining

The minimization of the negative impact of the products of natural leaching of metal ores on the ecosystems of the environment is justified. It is shown that the reason for the formation of ore leaching solutions is the use of development technologies without filling voids. The results of observations are systematized and the concept of environmental management by combining the potentials of enterprises in the region is substantiated. The efficiency of using dolomite waste to reduce the leaching front by filling technological voids with dolomite-based mixtures is shown. The results of the study of the intensity and danger of natural leaching, taking into account the zoning of mineralization within the orefield, are presented. Measures to reduce the level of natural leaching are recommended. A leaching model is proposed that describes physicochemical processes as stages of reactions occurring in the ore mass. The algorithm of substantiation of environmental technologies is given. The ecological and economic criterion of the effectiveness of the implementation of environmental measures is proposed. The processes of natural leaching of metals are adequate, which makes it possible to control their development. Natural leaching is a consequence of the insufficient level of applied development technologies and can be minimized.

Analyzing of the hydration crack evolution in compacted GMZ bentonite with consideration of technological void ratio

Technological voids widely exist around the bentonite blocks in the deep geological repository. After groundwater seeps into the voids, bentonite nearby expands to fill up the voids and completes the self-sealing gradually. The self-sealing ability is of highly importance, which prevents the technological voids from playing a preferential channel for groundwater migration and thus threatening the safety of the canister accommodating the high-level radioactive nuclear waste (HLW). So far, much of the research focus has been on the hydro-mechanical properties of bentonite after self-sealing, while the process itself is neglected. But actually, a cracking phenomenon will pop up in the interim of sealing process. In this paper, the dynamic evolution of self-sealing on compacted bentonite with different technological voids and sample thicknesses was observed by using a transparent observation apparatus. Digital image processing method combined with fractal theory was utilized to process, analyze and quantify the images captured. The results show that the self-sealing process contains the lifetime of the hydration crack, including its appearance, development and extinction. The technological void ratio evidently affects the time require for closing stage compared with the cracking stage. But in the range of 10–25%, there is no clear correlation between the technological void ratio and the geometry parameters or complexity of crack network. The fractal dimension introduced to quantify the complexity corresponds well with the visible image result, showing the feasibility and credibility of box-counting method. The cracking and closing mechanisms are also investigated.

Swelling pressure of compacted MX80 bentonite/sand mixture prepared by different methods

Compacted bentonite/sand mixture have been considered as possible sealing/backfilling material in the deep geological disposal of high-level radioactive nuclear waste. The swelling pressure of the compacted bentonite/sand mixture is of significance in the design of the geological repository, which requires good consistency between data obtained by laboratory and field measurement. In this work, a series of swelling pressure tests were performed on compacted MX80 bentonite/sand specimens prepared by methods commonly adopted in laboratory (As-compacted, Transferred) and those mimicking the real block manufacturing process (Trimmed, Inserted). Results shown that with identical dry density (especially when the dry density was larger than 1.70 Mg/m3), largest swelling pressure was obtained in specimens prepared by method Inserted, followed by method As-compacted, Transferred and Trimmed. The distinct difference between the swelling pressure could be largely attributed to the effects of residual post-compaction lateral stress. More interestingly, specimens prepared by methods Trimmed and Transferred followed a similar swelling pressure-dry density relationship. From this point of view, method Transferred without causing mass loss, change of bentonite content and possible technological voids effect was recommended in lieu of method Trimmed for specimen preparation in laboratory.

Cracking and sealing behavior of the compacted bentonite upon technological voids filling

The technological voids, which are inevitably formed during the construction of the deep geological repository for high-level radioactive nuclear waste, have negative influence on the hydro-mechanical properties of the engineered barrier. In this study, hydration tests along with image acquisition were carried out on compacted GMZ bentonite samples with simulated technological voids to investigate the self-sealing behavior and research emphasis was placed on the cracks appearing during the hydration. Results showed that with the presence of technological voids, the self-sealing process of the hydrating sample can be visually divided into cracking and closing stages, both of which were affected by the sample preparation process, the void width, and the sample height. The hydration cracks occurred at the early phase was considered as tensile cracks induced by the failure of tensile strength against the tensile stress induced by the difference of swelling strain. At the end of hydration, though the cracks were closed with the sealing of technologicalvoids, the distributions of water content and dry density along the sample diameter were found in a decreasing and an increasing trend with the increasing distance from the technological voids, respectively. By applying the image processing method, the cracking and sealing behaviors were further studied with the reciprocal verification of digital images and grey scale profiles. A good agreement between the grey scale profiles and digital images was found and the quantifiable grey values could accurately reflect the color difference on the samples which was difficult to be distinguished by the naked eye, showing the applicability and superiority of image processing technology.

Homogenization of bentonite upon saturation: Density and pressure fields

Bentonite structures are a core element of engineered barriers for geological radioactive waste repositories, preventing radionuclide release into the biosphere. These are often assemblies of elements with different densities (bricks, pellets, powder), or may contain regions of different density due to the building process (technological voids, powder segregation). The performance of these barriers is largely dependent on this material reaching a final state of homogeneous hydraulic and mechanical properties upon water intake. In order to understand the coupled processes of swelling and water transport in bentonite at play at the laboratory scale, an experimental approach was designed to characterize in detail the evolution of the state of samples saturated under isochoric conditions. Firstly, three-dimensional fields of water content, solid density and displacement of a heterogeneous powder sample submitted to unidirectional hydration were measured using X-ray microtomography, as a function of time. These fields allowed to gain insight into the phenomenology of non-monotonic water and clay transport resulting from the heterogeneous and coupled evolutions of the permeability, mechanical and friction properties. Then, samples with controlled heterogeneities such as pellets, powder, technological voids or compacted blocks of different densities - but with similar average properties - were saturated in isochoric conditions with a large set of local stress measurements. The different swelling pressure average evolutions and variability, as well as post-mortem measurements provided rich data on the effect of the initial density heterogeneities on the saturation process and the final state as well as insights into the role of wall friction. In particular, hydration kinetics were shown to depend on a complex manner on the initial heterogeneities. Regions first in contact with water were shown to exhibit a non-monotonic hydromecanical response and the development of macroporosity.

STUDY OF OVERBURDEN TUFF ROCKS AND DEVELOPMENT ON ITS BASIS OF LIGHTWEIGHT STRUCTURAL CONCRETE

Introduction. The work presents the research results on the development of technology for lightweight structural concrete based on overburden tuff rocks. To develop the technology of lightweight structural concrete, overburden tuff rock (Almaty region of the Republic of Kazakhstan) is used, which is formed during the extraction of volcanic tuff for the manufacture of facing tiles. Aim. Study of the basic physical, mechanical and chemical properties of tuff, development of lightweight structural concrete compositions. Methods. The overburden tuff rock is crushed in a jaw crusher and screened through a set of standard screens. As a result, aggregates from tuff of fractions were obtained: 0.16-5 mm, 5-10 mm, and 10-20 mm. The composition of tuff concrete was selected by calculation according to the standard procedure. After calculating the composition of tuff concrete, all raw materials: cement, aggregates, additives, and water are dosed and mixed. The concrete mixture is placed in a mold and compacted by vibro-compression on a vibrating table. Hardening of concrete samples is carried out in humid conditions for 7, 14, and 28 days and steaming in a steaming chamber for 12-16 hours. Results and Discussion. After hardening, the properties were determined: compressive strength, average density, and heat transfer coefficient of concrete samples according to standard methods. The tuff concrete grade is in the range of 15.0-20.0 MPa. The average density of tuff concrete is 1822-1910 kg/m3. The thermal conductivity coefficient (λ) is in the range of 0.66-0.75 W/m∙K. The possibility of manufacturing finishing tiles from tuff concrete by grinding and polishing its surface has been established. Based on tuff sand, with a grain size of 3-5 mm, concrete stones with dimensions of 390x190x188 mm with 2 technological voids were made by vibration pressing. The hollowness of the stone is 51% of its total volume, and the compressive strength grade is 2.5-3.5 MPa. Conclusions. As a research result, the main physical, mechanical and chemical properties of overburden tuff rocks as a filler for lightweight structural concrete were studied. The possibilities of manufacturing wall blocks and facing slabs have been established.

Effects of technological voids and hydration time on the hydro-mechanical behaviour of compacted bentonite/claystone mixture

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been considered as a promising sealing/backfilling material in deep geological repository for high-level radioactive waste in France. When the pre-compacted blocks are emplaced in the gallery, technological voids can remain. After the infiltration of groundwater from the host rock, they will swell freely, filling the technological voids, and then undergo further hydration under constant-volume conditions. From the perspective of the storage safety, it is essential to understand the effects of technological voids and hydration time on the hydro-mechanical behaviour of such pre-compacted blocks. In this work, a series of infiltration tests at various hydration times was carried out on compacted MX80 bentonite/COx claystone mixture with different technological voids. The evolutions of the swelling pressures in axial and radial directions as well as the hydraulic conductivity were monitored while wetting. After the predetermined hydration times, the dry density, water content and suction at different positions were determined, together with the microstructure investigation using mercury intrusion porosimetry. It was observed that the soils close to initial voids swelled and filled the initial voids upon contact with water, with significant increases in large-pore and medium-pore void ratios. From the variation of dry density profile with time, compression and swelling zones could be identified: in the compression zone, the soils corresponding to the initial soils were subjected to compression, with decrease in large-pore and medium-pore void ratios over time, whereas in the swelling zone, the soils with a higher dry density than the expected final one underwent further swelling, with large-pore void ratio increasing until saturation and then a slight decrease due to water redistribution in the soil. Owing to the soil density heterogeneity, the axial swelling pressure and hydraulic conductivity of samples with voids were slightly larger than those of samples without voids; contrarily, the radial swelling pressure was lower than that of the samples without voids.

ЭФФЕКТИВНОСТЬ КОМБИНИРОВАНИЯ ТЕХНОЛОГИЙ ВЫЕМКИ РУД В ПРЕДЕЛАХ РУДНОГО ПОЛЯ

Актуальность исследования. Статья посвящена актуальной проблеме совершенствования технологии разработки сложно-структурных месторождений скального типа для увеличения производства содержащихся в рудах редких и ценных металлов, дефицитность которых возрастает с развитием научно-технического прогресса. Цель исследования – определение условий совмещения систем разработки разных классов для увеличения возможностей селективизации добычных работ при использовании различных вариантов извлечения руд в смежных добычных участках. Методика исследования. Основу исследования составляет экономический расчет возможных вариантов совмещения в пределах рудного поля на основе полученных экспериментально сведений о рудовмещающем массиве. Результаты. Дана характеристика условий залегания руд на конкретном скальном месторождении сложной структуры. Произведено инженерно-геологическое районирование рудовмещающего массива с дифференцированной оценкой геолого-структурных нарушений по величине и направленности. Дан прогноз возможного по аналогии с практикой поведения горных выработок под влиянием разломных структур и макротрещин. На уровне экспертной оценки определена целесообразность применения слоевой или камерной систем разработки с закладкой технологических пустот твердеющими смесями. Выводы. Эффективность выемки руд достигается селективизацией выемки запасов в пределах одного рудного поля при комбинировании систем разработки при максимальном учете геолого-структурных факторов локализации руд и механо-физических свойств рудовмещающих пород.

Experimental study on the aeolotropic swelling behaviour of compacted bentonite/claystone mixture with axial/radial technological voids

Pre-compacted blocks of MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture are chosen as sealing/backfilling materials in the French concept of deep geological disposal for radioactive waste. Creating technological voids is normally unavoidable during installation in the disposal galleries, in the form of axial voids between compacted blocks and radial voids between the compacted blocks and the host rock. In this work, the aeolotropic swelling behaviour of compacted mixture with axial/radial technological voids was experimentally investigated. Results showed that the aeolotropy coefficient (ratio of radial to axial pressures) of samples without voids increased from 0.46 to 0.88 with the increasing dry density from 1.6 to 2.0 Mg/m3 due to the relatively concentrated macro-pores which defined a relatively lower local dry density in the side part. For the samples with axial/radial voids, their final axial swelling pressures were in agreement with the unique correlation with final dry density, regardless of the soil density heterogeneity. Samples with axial voids had a larger aeolotropy coefficient than samples without voids, due to the reduction of the orientation of clay particles upon hydration, while samples with radial voids had a smaller aeolotropy coefficient, due to the relatively lower dry density near the cell wall.