In this work, 3D printing (3DP) technology is applied to study rock fracturing behaviors in Brazilian disc tests. First, uniaxial compression tests were performed to identify the most suitable 3DP material from five available 3DP materials, i.e., ceramics, gypsum, PMMA (poly methyl methacrylate), SR20 (Acrylic copolymer) and resin (accura® 60), to simulate hard and brittle rocks. The experimental results demonstrated that the transparent resin produced via Stereolithography (SLA) was the best 3DP material for mimicking rocks. Then, static and dynamic Brazilian disc tests were carried out on the resin-based 3DP rocks and the corresponding prototype rocks. The testing results show that the fracturing behaviours of the 3DP rocks agreed well with those of the prototype rocks, which confirms the feasibility and validity of using 3DP to study rock fracturing behaviors in tensile tests. This work facilitates the application of 3DP to rock mechanics.
Rock fracturing has been traditionally studied in the laboratory using natural rocks. However, at present, the experimental study of rocks is encumbered by three problems: (1) rocks are both heterogeneous and unrepeatable; (2) rock cores collected from deep underground are difficult and expensive; and (3) manmade rock specimens with internal structures are difficult to fabricate. To solve these problems, identifying and developing some alternative materials and techniques are needed.
Three-dimensional printing (3DP), also called additive manufacturing, may help to address the rock sample preparation problem. 3DP has advantages over conventional manufacturing in fast and flexible preparation, high repeatability, and preparation of complex internal defects . Due to these benefits, 3DP has been widely applied in biomedicine  and materials science  etc.
However, the application of 3DP in rock mechanics is in its infancy. By combination of the CT scan, 3D reconstruction and 3DP technologies, Ju et al.  produced a physical model to replicate natural coal rock. Jiang and Zhao  produced 3DP samples with polylactic acid (PLA) using fused deposition manufacturing (FDM) technique. Fereshtenejad and Song  studied the means of enhancing the compressive strength of the 3D printed gypsum samples. However, in these studies, the 3DP samples failed/yielded with low compressive strength, i.e., between 1 to 30 MPa, exhibiting ductile behavior. Therefore, a more brittle and strengthened 3DP material should be used to effectively mimic hard and brittle rocks.
The estimation of the mechanical properties of rock joints is crucial in terms of safety when it comes to design of slopes in open pit mines or caverns used for the storage of hazardous materials, for instance - nuclear waste. Photogrammetry provides a simple, objective method for joints roughness assessment, without the need for expensive and time consuming laboratory tests or subjective empirical methods. In this study, a new photogrammetric method was used to estimate the roughness, shear strength and friction angle of a discontinuity of 2 m by 1 m fresh rock joint. The estimation was done by analyzing the profiles of digital models of joint surface. Surface Length and Slope Measurement methods were used to calculate the values of Joint Roughness Coefficient (JRC) of analyzed surfaces. Next, the shear strength and friction angle of the rock discontinuity were obtained experimentally with multistage pull testing. The results obtained with both methods were analyzed and compared. JRC values from photogrammetrically created digital models of the joint surface were overestimated due to the low density of the models, which resulted in high noise to signal ratio. Shear strength obtained with photogrammetrically created models were overestimates in relation to the results of the pull test by approximately 45 %. The errors made during this research are analyzed in the article and recommendations on how to improve reliability of the results are made. Main error in photogrammetric prediction was low density of the point clouds and in laboratory test too low stiffness of the test arrangement. The alternative methodology for photogrammetric studies used in previous stage of the research project was tested during this study and was proven to give significantly higher accuracy of generated digital models. The stiffness of the testing machine and proper positioning of the sample halves on top of each other were identified as the most sensitive aspects of methodology of big scale pull test when it comes to the reliability of results.
The determination of shear strength of rock discontinuities is an object of research since the middle of the last century, yet the developed models and failure criterions are based on simplifications which are the topic of ongoing discussion in the field of rock mechanics . The reason for that is the multiplicity and complexity of the parameters affecting the value of the shear strength of a joint. Those parameters include joint surface condition (dry, wet, submerged, weathered, unweathered), roughness of the joint surface, matedness (matching) of the opposites of a joint, compressive strength of a joint, normal load which the joint is subjected to and the mineral composition of the jointed rock, which determines its basic friction angle . The parameter which is the most challenging to quantify is the roughness of a joint surface. That is mainly due to the anisotropic character of natural joints. Directional variation in the joint roughness results in the different shear strength of the same joint depending on the direction of shearing . Therefore, most commonly used method of determining the roughness - Joint Roughness Coefficient (JRC) profiling , ISRM suggested method  is considered subjective by significant amount of investigators, since it only quantifies the roughness in one direction and involves a human decision on where to measure the shape of the profile, and then to match the obtained profile with a reference [3, 6, 7].
The nuclear disposal concept is based on compacted bentonite barrier placed into a borehole in the host rock. In crystalline rock, one of the issues is a non-uniform saturation of bentonite caused by inhomogeneity of the rock, typically the flow being concentrated to fractures. We solve a hydro-mechanical model problem in bentonite in the general engineering multiphysics simulation software ANSYS. The elasto-plastic transition as result of saturation is represented by a special non-linear stress- strain curve of elastic model (saturation-dependent weakening). The model is further extended to a contact-problem formulation, where the swelling is possible either as free swelling until filling the gap or as confined state after. It is a synthetic case of one horizontal fracture crossing a vertical borehole. We demonstrated that for large time period, the saturation is limited to a part of the volume, producing a non-uniform stress field in the whole volume. The stress is also controlled by the gap width.
Numerical modelling is one of the tools for analysis of the processes in barriers of the spent nuclear fuel disposal concept. Our work is motivated by a potential damage of disposal barriers due to non-uniform swelling of bentonite caused by inhomogeneous hydration. In the repository, the interaction of the bentonite and the host rock (fractured granite considered) is equally important, requiring the joining of the two disciplines of soil and rock mechanics.
Recent studies deal with modelling of thermo-hydro-mechanical processes and also compare results from full scale experiments, e.g. [8, 11]. Additionally, we developed a non-linear hydro-mechanical (HM) method for solution of bentonite hydration with relatively simple implementation which can be easily-defined in a number of commonly used simulation tools. It offers an alternative to complex models of hydraulic or mechanic behaviour of bentonite, e.g. [1, 9]. It extends the previous work  on hydraulic-only models of non-uniform saturation process based on data from field experiments.
Coal burst is a sudden and violent rock/coal failure that occurs in underground coal mines. It is considered to be a highly catastrophic phenomenon which can cause significant damage to mine workings and equipment as well as result in multiple fatalities. Throughout the history of underground pillar design, the relation between the post-peak behavior of pillars and stiffness of the surrounding strata has been extensively studied. These two concepts play an important role in determining the failure mode of the coal pillars and the amount of potential energy that can be converted to kinetic energy, which is the cause of coal burst. In this paper, the post-peak behavior of pillars and surrounding strata stiffness are reviewed and the criterion developed to investigate the instability of the pillar failures is explained. It is concluded that, as the pillar width to mining height (w/h) ratio increases, its post-peak modulus increases; and a pillar exhibits different failure modes for various w/h ratios.
Experiences in both hard rock and coal mines reveal that pillar failures are still one of the major hazards in underground mining and forecasting the mode of the pillar failure can be vital. Therefore, it is essential to fully understand the mechanics behind these failures in order to predict and control them. As suggested by Tincelin and Sinou , pillar failures can be classified into two categories:
i. Slow, progressive deterioration of the pillars that causes relatively delayed surface subsidence and even damage if the pillars fail,
ii. Sudden, violent collapse of pillar causing immediate surface damage and mostly associated with fatal accidents.
The first type of pillar failures is also named as controlled pillar failures which occur gradually and typically over long periods of time (i.e. pillar spalling). These pillar failures are also termed as creep and squeeze . Uncontrolled pillar failures, on the other hand, take place in a sudden and violent manner and fall into the second group of pillar failures as described above. Since the uncontrolled pillar failures occur rapidly and may not be preceded by any deterioration of the pillars, they cause significant health and safety risks (e.g. coal burst, entrapment, windblast etc.) .
Drilling directional and horizontal wellbores plays an important role in the exploitation of hydrocarbons. In the case of long operated and unconventional fields the importance of this type of drilling is fundamental. The horizontal and directional drilling technologies are also applicable in other fields. Directional and horizontal holes are among other various types of drilling under buildings, microtunnels, injection and storage holes. This technology is also used in obtaining coal-bed methane and in underground coal gasification methods.
Compared to holes with vertical axes, directional drilling is a much more complex issue. The curvature of the wellbore’s axis, passing through the horizontally stratified rock mass, causes the conditions around its cross section to be strongly anisotropic. Not infrequently, the same rock material of each layer exhibits anisotropic properties. Ensuring the stability of the well is very difficult under such conditions.
The proper determination of the state of stress, strain and displacement of the rock mass around the hole is critical in the design of directional holes. Numerical analysis tools which take into account the structure of the rock mass and the mechanical properties of rocks along the well trajectory are used for this purpose.
The article presents numerical simulations of changes of the rock mass state around a directional well. The calculations were conducted with various material models of rock: the linear-elastic model, the linear-elastic with regard to pore fluid flow model, the elastic – plastic model, the Drucker-Prager cap model with pore fluid flow, the concrete damage plasticity model (CDP), and the cracked material model. The impact of primary pressure, lamination and anisotropy of the rock mass are also included in the calculation. Based on the results of the calculations we analyze the impact of the behavior of rocks on the wellbore’s stability.
Directional wells have become common in hydrocarbon exploitation mainly due to increased contact with the deposit. They are used especially for thin and low-pressure reservoirs. Drilling more horizontal lateral holes renders a project even more profitable.
In the present work, dynamic stress-strain response of Himalayan quartzite is tested under high loading rates using split Hopkinson pressure bar (SHPB) device for the first time in the literature. The physical and static mechanical properties of quartzite e.g. dry and saturated density, specific gravity, static compressive strength and elastic modulus values are also determined. Petrological studies of quartzite are carried out through X-ray diffraction (XRD) test and scanning electron microscope (SEM) test. In the SHPB tests, it is observed from the stress-strain response that the dynamic peak stress increases with increasing strain rate whereas the elastic modulus does not show any clear trend with increase in strain rate. Dynamic force equilibrium at the incident and transmission bar ends of the rock samples is attained in all tests till the failure of the rock samples. Dynamic increase factor (DIF) for the rock is determined at a particular strain rate by comparing the dynamic to static peak compressive stress. Correlation equation for dynamic strength increase factor with respect to strain rate has been proposed herein.1. Introduction
Design, development and building of civil infrastructure in the mountainous regions involve many complexities in terms of diverse geological and geomorphological features of the region - the Chenab river bridge in the Himalayas, the Gotthard Base tunnel in the Alps are to name a few. The young mountain ranges of the Himalayas and the Alps contain joint planes, shear seams, active fold, and fault zones. Moreover high in-situ stresses and high level of seismicity in these regions pose severe challenges to the construction of infrastructure. In addition to this, unanticipated loads caused by natural hazards, e.g. landslide, earthquake and manmade hazards, e.g. blast and projectile penetration add to the difficulties already existing therein. It may be noted that the loads caused by hazardous events like earthquake and blast are highly transient in nature generating high strain rates in rock and strain rate caused by blast may reach up to 104∙s-1 [1, 2] which in turn affects both the stiffness and the strength properties of the rocks. Thus, in order to ensure sustainable design of civil infrastructure in the mountains, it becomes necessary to characterize the rocks under static and dynamic loading conditions.
He, Manchao (State Key Laboratory for Geomechanics & Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining & Technology) | Zhang, Xiaohu (State Key Laboratory for Geomechanics & Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining & Technology) | Zhao, Shuai (State Key Laboratory for Geomechanics & Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining & Technology)
In China, engineering geological disasters are frequently encountered in the underground mining sites where room and pillar method is used. This paper presents a novel method for developing the gateway tunnel without any excavation using directional tension blasting (DTB) in the mine panels. The mining-induced stress transfer will be blocked by cutting the roof stratum with a slit produced by DTB. The method has the following advantages: 1) forming the gateway tunnel without any excavation work; 2) de-stress, and 3) reducing the number of the pillars significantly. The DTB has been applied in many giant coal mining corporations in China and proved to be very helpful in 1) increase in productivity and reduction in costs dramatically, and 2) elimination of engineering geological disasters induced by the coal pillar and mining stress.
Room and pillar method is the major mining way in china. The shallow coal resources are gradually reducing along with the continuously mining, which leads to the rising of mining depth. But engineering geological disasters, especially large deformation problem of surrounding rock, which is much more serious than that in shallow mines, are frequently encountered in deep underground mining sites. According to incomplete statistics, around 80 %~90 % accidents occur at the deep gate road [1, 2], and 80 %~90 % among them happen in roadway along gob.
In order to reduce tunnel excavation, non-coal pillar mining with reused roadway method was developed with the improvement of surrounding rock control theory and support technology. It can reuse the last sublevel of mining roadway for the next sublevel along the edge of gob. And this technology can eliminate the island mining face, alleviate the continuous tension of workface, reduce the amount of roadway excavation, realize reciprocating mining, improve the coal recovery rate , and it was widely used in coal mine enterprises at home and abroad. At present, the main research is based on the caving method, which focuses on the wall supporting form and resistance, the determination of filling materials and the optimization design of supporting parameters and so on [4-7].
The program for the final disposal of low and intermediate level radioactive waste was established by Paks Nuclear Power Plant, Hungary. Preparation of final disposal has been done as part of a national program since 1993. The Central Nuclear Financial Fund and the Public Limited Company for Radioactive Waste Management (Puram) have been established to coordinate organizations and activities for all tasks in connection with nuclear waste treatment. The project was started with a geological screening in order to find the most suitable geological formation for radioactive waste repository. The selected potential host rock is a granite complex in the Mórágy Granite Formation in the south-western part of Hungary, close to the village of Bátaapáti.
In the underground facility different measuring systems have been used (extensometers, inclinometers, convergence measurements), requiring a transformation into radial and tangential displacements. Until now not much emphasis was laid on the 3D geodetical displacement monitoring measurements or on the use of advanced methods such as the evaluation of displacement vector orientations.
The final survey results are ensured by the Mecsekerc Ltd. Department of Geodesy as a daily actualized database with Unified National Projection system (EOV) coordinates. Upon request the surveyors provide data in local coordinates (they provide relative and absolute coordinates simultaneously), allowing a meaningful analysis. The aim of this paper is to shortly introduce the surveying process and interpretation of the 3D optical displacement measurement methodology. Our research is related to geodetic measurements which were carried out in the repository chambers and some exploratory tunnels. The mentioned exploratory tunnels crossed through fault zones. We have studied the effect of the fault zones on the measured 3D displacements, using the common evaluation and displacement prediction methodologies, and we show the results obtained during the control of the „normality” of the displacements from the underground spaces which were excavated full face and at larger sections by top heading and bench method. The results of the presented case studies show that the 3D optical displacement monitoring enables the prediction of the geotechnical conditions ahead of the face and the influence of fault zones located outside of the excavated space.
Konecny, Pavel (Institute Institute of Geonics of the CAS, Planetarium Ostrava, Faculty of Mining and Geology, VSB-Technical University of Ostrava) | Hagi, Abdirahman (Somaliland Ministry of Energy & Minerals) | Plevova, Eva (Institute Institute of Geonics of the CAS) | Vaculikova, Lenka (Institute Institute of Geonics of the CAS) | Murzyn, Tomasz (The Strata Mechanics Research Institute of the Polish Academy of Sciences)
The results of physico-chemical characterization of limestone samples come from the Mesosoic limestone deposit areas in the region of Berbera (Republic of Somaliland) were reported in this paper. The chemical analysis was carried out by X-ray fluorescence analysis, FT-IR spectroscopy and thermal analysis. The physical characterization of limestone samples were performed in laboratory conditions. This investigation provided necessary data for subsequent utilization and exploitation of limestone in this region. The tested material is a good quality and purity. From the chemical composition point of view the tested limestone has a high content of about 96% CaCO3, with small amount of clay minerals and with traces of quartz, which corresponds to the medium purity limestone. It was confirmed, that this limestone is suitable for cement production. The density and porosity measurement indicated that the medium to high density limestone is very compact with porosity lower than 4%. These data very good corresponds to relatively high Young's modulus up to 48 200 MPa and maximum strength of 204 MPa.
Republic of Somaliland (Northwestern Somalia) is situated on the northern side of the Horn of Africa. From the geological point of view the country is comprised mainly by Mesozoic and Tertiary continental-margin and marine sedimentary rocks deposited unconformably on Precambrian metamorphic and igneous rocks . The Berbera plain is a wide down-faulted sector, parallel to Somaliland coast along the Gulf of Aden, limited towards the south by high fault of the Precambrian crystalline basement forming the high mountains . The basement in Berbera plain is covered by a thick Meso-Cenosoic sedimentary succession, including marine limestone facies (Fig. 1).
In the early 1980s, a cement plant became operational at Berbera in Somaliland, however, the lack of proper maintenance, shortage of spare parts, lack of managerial staff and skilled workers during the civil war caused the closure of this plant and termination of cement production . National cement consumption was estimated to be about 250 000 metric tons in 2012 , in view of this fact some plan to restart the country's only cement plant at Berbera in Somaliland appears in recent time.
The present work focuses on the characterization of the geometry of the microstructure of porous oolitic rocks. These rocks are constituted by an assemblage of porous grains (oolites), pores and inter-granular crystals. X ray 3D Computed Tomography is used to identify the different components of these rocks by applying an algorithm based on grayscale values. This analytical method allows the characterization of the porous network (size, spatial distribution, and volume fraction), oolites and inter-oolitic crystals. The microstructure of these porous rocks has a significant effect on their macroscopic behavior. This micro-macroscopic relationship is taken into account in micromechanical models developed within the framework of the homogenization theory (e.g., Maxwell scheme) of random heterogeneous media. X ray tomography images showed that pores have irregular shapes, so the micromechanical modeling based on analytical solution is not relevant. Then, pores are approximated by ellipsoids using principal components analysis (PCA) method, which allows us to obtain the geometrical properties such as length of semi-axes and orientation of ellipsoids. To validate mechanically this approximation, we compared the contribution of irregularly shaped 3D pores and ellipsoidal pores to the effective elastic properties. The relative error due to this ellipsoidal approximation is then estimated. The compliance contribution tensors of irregular 3D pores are evaluated numerically using finite element method while those of approximated ellipsoidal pores are obtained analytically and numerically. The same procedure of approximation is applied on oolites. Shape and spatial parameters, such as the volume, radius and center of each oolite are also determined. The sphericity of the approximated oolites is calculated. The obtained values are close to 1, so oolites can be reasonably approximated by spheres.
In this paper, we characterize the geometry of porous oolitic rocks that are modeled as a heterogeneous material composed by an assemblage of porous grains (oolites), pores and inter-granular crystals (cement). We used a simplified model within the framework of Maxwell homogenization scheme described in . Three scales are then identified. First is the microscopic scale that corresponds to the intra-oolitic pores. Second is the mesoscopic scale that corresponds to the oolites, the inter-oolitic cement and pores. Third is the largest scale and corresponds to the representative elementary volume (REV) which is considered large compared to the oolites size, the intra and inter-oolitic pores. The first homogenization step concerns intra-oolitic pores of spherical or ellipsoidal shape within oolites using Self Consistent Approximation . It allows the transition from the microscopic to the mesoscopic scale. The second step allows the transition from the mesoscopic to the macroscopic scale using Maxwell homogenization scheme. As in , at the mesoscopic scale we consider that the heterogeneous medium is formed by three phases: the porous oolites approximated by spheres, inter-oolitic cement and irregularly shaped pores approximated by ellipsoids with randomly distributed orientation. This Maxwell model and its reformulation based on contribution tensors are discussed in [3, 4, 5, 6]. Indeed, this method has been used by  for the calculation of effective parameters of poroelastic composite materials. This method has also been extended in  to the case of viscoelatic microcracked materials. In our study, we considered an oolitic limestone as heterogeneous material. To verify the homogenization method, we approximate the rock components by the PCA method presented in  as follows: oolites are approximated by spheres and interoolitic pores are approximated by ellipsoids. The microstructure of the material was observed using 3D X-ray nano-computed tomography and scanning electron microscopy. We analyze then the X-Ray images using image processing software in order to distinguish the different components of the material. The procedure for the geometry analysis of oolites and pores is presented in section 2. To validate the approximation, we evaluate the contribution of irregularly shaped 3D pores and ellipsoidal pores to the effective elastic properties. Our work is based on the previous calculation of compliance contribution tensors of 3D pores of irregular shape in carbon/carbon composites . Analytical method based on Eshelby solution for ellipsoidal shapes are usually used to evaluate the contribution of pores to effective elastic properties. However, in our case, this solution is not relevant due to the high irregularity of the pores, so numerical methods such as finite element method (FEA) is used. The FEA procedure for the evaluation of the contribution tensors of irregular shape pores and their corresponding ellipsoids is presented in section 3. Then, we evaluated analytically the contribution tensors of the ellipsoids and we estimate the relative error due to this approximation.