Mud loss/mud gain, known as the breathing (ballooning) effect, presents a major problem in drilling because of the difficulty in distinguishing lost circulation and kicks. Based on the analysis of the geological characteristics, the geometric model is established. Then the mathematical model is established by coupling the discrete fractures and matrix, and it solved by the finite element method. The effect of formation characteristics and drilling fluid parameters on the breathing effect is investigated and analysed systematically. The results show breathing effect is prone to observe in the sandstone reservoir with the interlayer of silty shale, which has the obvious time-dependent effect. Compared with the formation with a single fracture or matrix, formation with a fracture network will result in severe breathing effect. The existence of fractures reduces the integrity of wellbore rock and changes the distribution of pore pressure around the wellbore. Matrix with high permeability will trigger a more severe breathing effect. However, Young's modulus has little effect on breathing effect. The degree of breathing effect will not deteriorate with the further aperture increase of fracture. The trend of breathing effect in different fracture's aperture goes consistent approximately when the aperture of fracture is larger than 0.3mm. Compared with the viscosity of drilling fluid, the density of drilling fluid has a greater impact on the breathing effect. Low-density drilling fluid and moderately high viscosity of drilling fluid can reduce the impact of breathing effect.

Abstract Most rocks in nature are affected by a certain degree of anisotropy. They contain cracks mostly with certain distributions. Therefore their mechanical behaviour is dependent on the direction of the strength distribution, which is different in each direction. We can distinguish the directions in which the rock is relatively firm and conversely the directions in which it is weak. The strength anisotropy and the direction of weakening is useful information for engineering applications. Metamorphic foliation with the specific orientation of rock grains also influences the mechanical behaviour, and the mechanical properties, i.e. strength, are changed in different directions of loading. This change of strength is called the strength anisotropy. The main purpose of this research is to analyze the effect of the orientation of metamorphic foliation distinguished within the rock specimens on their strength anisotropy. In the article, the anisotropy of a mechanical property, i.e. the fracture toughness, of migmatized gneiss is discussed. For this purpose, series of semi-circular bend tests were carried out on specimens of migmatized gneiss with a metamorphic foliation. The methodology used in this research is effective for evaluating the strength anisotropy of a brittle rock. It is well known that a directional variation of rock properties is one of the key aspects of rock mechanics. It is relatively uncommon to find a rock mass with completely isotropic properties. The regular distribution of the internal fabric of rocks is mostly disturbed by the presence of planes of weakness, such as discontinuities, microstructures or any mechanical fractures. Besides, the internal fabric of the rock in many cases presents a predisposition to mechanical anisotropy [1]. Metamorphic rocks were subject to extreme loading and high temperature in the course of their development. These processes led to their particular properties, where the orientation of grains in one direction forms the specific features of the rock [2]. When mechanical properties change in different directions of loading, this is defined as strength anisotropy. The strength behaviour of rocks can be characterized with an evaluation of mechanical parameters. The uniaxial compressive strength, shear strength, and tensile strength determine the macroscopic strength behaviour. Fracture toughness is one of the most important material properties of rocks in their microscopic strength behaviour. It expresses the resistance of a rock material to the initiation and propagation of cracks. It supposes a linear strength- deformation relationship with no plastic flow in the rock mass [3]. It describes the fracture behaviour of rocks in loading conditions and it is closely related to the macroscopic strength. The mechanical properties that express the deformation behaviour of rocks are very important for rock mass classification in mines or underground constructions.

Abstract A real-time monitoring system was established for 10-channel acoustic emission (AE) in hydraulic fracturing in the laboratory, with a hydraulic fracturing test conducted on two different sizes of natural outcrop cores under unconfined pressure and confining pressure respectively. Then, a discussion was carried out on the microscopic response in rock to the formation and growth of hydraulic fracture under confining pressure and unconfined pressure. The study of AE parameters shows that the change of RA (Rise Angle) in hydraulic fracturing process is always opposite to that of AF (Average Frequency). Also, AE parameters have higher dispersion under unconfined pressure. With breakdown pressure as a dividing point, as pumping pressure changes under unconfined pressure, the dominant internal fracture mechanism of rock will turn from tensile failure to shear failure. But under confining pressure, the transformation of the dominance is reversed, namely from shear failure to tensile failure. This phenomenon indicates that despite the same hydraulic fractures in macro scale under both confining pressure and unconfined pressure, there may be a large difference in microstructure between the hydraulic fractures formed under both pressure conditions respectively. 1. Introduction Hydraulic fracturing technology has become the primary means of reservoir stimulation since its first application in North America in 1949 [1]. At present, as the conventional oil and gas resources are increasingly exhausted, and the yield of unconventional oil and gas resources rises gradually, hydraulic fracturing has grown into a more critical factor to artificial production's increase [2]. From the perspective of theoretical research, with the constant development of fracture mechanics and its application to rock engineering, scholars have come to realize that the nonlinear areas have an important influence on the mechanical behavior of a hydraulic fracture before and after its formation, while the occurrence of micro cracks is an important sign of nonlinear rock failure [3].

Abstract In the present paper, an up-scaling thermo-mechanical approach is presented for rock masses considered as anisotropic composite materials. A mathematical framework is proposed to estimate overall thermo-mechanical properties based on local geometrical considerations using the Mori-Tanaka scheme. In addition, a homogenization-based effective thermal conductivity formulation is proposed for composite materials subjected to periodic micro-scale heat fluxes and governed by Fourier's heat law and the steady state balance equations. The particular case of isotropic material is also presented for completeness. The response of a Representative Elementary Volume (REV) to external mechanical loading is modeled using finite element numerical simulation in order to conclude the effective mechanical properties and compare them with the results of analytical calculations. The porosity of the REV is varied by changing the number or the radii of the embedded inclusions. As a first result, the two homogenization techniques are compared with reference to proximity to the numerical results. Moreover, it was noticed that the curves presenting the variation of effective bulk modulus with respect to the number of inclusions for different radii are convergent. Finally, the inclusions number and radius in the RVE which give close numerical and analytical results could then be determined. 1. Introduction The constitutive laws of rock materials are usually established using phenomenological approaches based on results of tests on specimens that are assumed to be homogeneous. The response of materials to loading is thus crucial. Despite their effectiveness and wide use, such approaches are limited since the responses of natural materials can be scale-dependent. Each constituent shows different material properties and/or material orientations. Therefore, the behavior of local entities needs to be studied to estimate the overall behavior. Consequently, the phenomenological approaches of materials' characterization can be insufficient. This leads to a technique that links macro-mechanical properties to micro-structural considerations known as up-scaling or homogenization. Homogenization techniques assume that scales are separated, and an intermediate level is introduced: The Representative Volume Element "RVE". The choice of this volume, its dimensions and the number of inclusions is important. Indeed, it must be smaller enough than the size of the whole structure and bigger enough than the size of the heterogeneities. It is used to get an approximation of stress and strain fields under appropriate boundary conditions. In the work of [1], the following definition of RVE is proposed: "It is the smallest material volume element of the composite for which the usual spatially constant (overall modulus) macro-scale constitutive representation is a sufficiently accurate model to represent the mean constitutive response". It is also called DRVE, deterministic representative volume element in [2]. Several researches were made in order to find the correct RVE which gives the best mechanical and thermal results. For instance, a numerical study of the size of the (RVE) for both the heat flow and linear elasticity problems is presented in [3]. Another numerical approach to evaluate the effective properties consisted on working on the sequence of microstructure volumes with prescribed sizes smaller than the DRVE [4–7].

Abstract The design of underground structures such as road tunnels, railways tunnels, hydro tunnels etc. depends on the data gathered through the physical and mechanical parameters of rocks. These geotechnical parameters of rock play a significant role in the design, safety, stability and maintenance of underground structures. The rocks, when subjected to heterogeneous in-situ stress-fields created by overburden, tectonic and gravitational stresses, which is locally complicated by water pressure and stresses, persuade due to excavations. The physical and mechanical parameters play very important role in a precise forecast of rock behaviour under such inconsistent conditions. The mechanical properties of rocks change with density, porosity, variation of mineral assemblage, grain size, texture and effective pressures acting on them. Changes in physico-mechanical properties in metamorphic rocks lead to corresponding variations in failure pattern. A comprehensive study has been undertaken using three varieties of rocks obtained from the vicinity of Rohtang Tunnel in the Himalayan region. Attention has been paid to their petrographic and petrofabric analysis through scanning electron microscope (SEM) and thin sections, and physical and geotechnical properties. Various correlations between porosity, density, resistivity, sonic wave velocity in dry and saturated specimens of phyllitic and gneissic rock have been studied. Various relations have been produced between mineral compositions, porosity and density of rocks with the help of SEM and physical properties. Strength and deformation behaviour of the rocks under uniaxial compression, brazilian tension and oblique shear, in both dry and saturated states are also defined and compared with various properties. This paper highlights various relations analysed in conjunction with data on the physical and mechanical properties of metamorphic rocks. 1. Introduction The construction of 8.8 km long two-lane bi-directional Rohtang highway tunnel above 3000 m from MSL across Rohtang Pass near Manali in Kullu district is of strategic importance from the defence point of view as it will provide all weather connectivity between Manali and Keylong, the head quarter of Lahul-Spiti district. A comprehensive study has been undertaken using three varieties of rocks namely phyllitic quartzite (PQ), quartzitic phyllite (QP) and Migmatic gneiss (MG) obtained from the study area (south portal, Rohtang tunnel) in the Himalayan region.

Abstract Shear strength along rock discontinuities by means of direct shear strength tests were performed on granitic and clayey rocks. Both rock types represent potential host rocks of radioactive waste. The paper focuses on the effect of one parameter in particular influencing the shear strength: the angle enclosed by the plane of the sample surface and the shear plane, in the direction of shear. 3D surface roughness measurements were carried out on 18 Opalinus Claystone sample (Switzerland) and 6 on granitic rock sample (Hungary) surfaces. An interval of angle was analyzed in the direction of shear and against the direction of shear. Direct shear strength tests were carried out under constant normal loading (CNL) conditions, the influence of the angle on the shear strength values were determined. Additionally, a percentage value was calculated for how much the range of both peak and residual shear strength values change, with taking into consideration the effect of upslope and downslope shear. 1. Introduction Designing and constructing in rock masses require a detailed and sound knowledge of the mechanical properties of the rock materials involved. Failure of rocks and determination of appropriate failure envelope has been studied for a long time [1, 2], however one of the most important parameters controlling the strength of rock masses is the shear strength of the discontinuities [3]. The discontinuities are usually a weak point of the rock mass; in particular, the features of their surfaces, such as roughness, joint strength, brittleness, humidity, mineralogy, (etc.) determine its strength. Mechanical, test related factors i.e.: loading conditions, type of testing machines, (etc.) influence the shear strength as well. The mechanical parameters defining the shear strength, i.e. friction angle and apparent cohesion, are investigated by means of laboratory direct shear strength tests. The basic principles of such analyses and test methods have been already published [4, 5, 6]. In this paper one parameter influencing shear strength was investigated in particular: the angle between the sample surface and the shear surface in the direction of the shear, resulting in "upslope" or "downslope" shearing. The paper compares the results of upslope and downslope shearing for granites and claystones. Both rock types represent target formations for hosting radioactive waste in Hungary [7, 8] and in Switzerland [9, 10, 11], respectively.

Abstract Shale, a fine-grained sedimentary rock, is composed of highly compacted clay particles, nanometric porosity and silt-sized particles of other minerals. In this paper, a combined use of X-Ray micro-CT and nanoindentation tests is proposed for the qualitative and quantitative characterization of this material. The individual methods are utilized at different scales of observation: nanoindentation at nano and micro level while the X-ray microtomography at the meso scale. For the purpose of mechanical morphology evaluation, the grid nanoindentation measurements are carried out resulting in the overall histograms of indentation properties, i.e. indentation modulus and hardness. Tests are performed at different values of applied load and in two directions: parallel and normal to the plane of isotropy, x 1 and x 3 , respectively. The deconvolution technique is used for the segmentation of the material constituents with respect to mechanical properties. Based on the results obtained from nanoindentation testing at nano scale, the shale matrix composition is identified, namely: the number of constituents, their volume fractions as well as the mechanical properties. In addition, at micro scale, the homogenized parameters of the intact shale material are determined. On the other hand, based on the X-ray microtomography imaging results, the geometry of the shale structure at meso scale is investigated. The studies focus on observation of the evolution of 3D crack network in the samples of shale under uniaxial compression at different levels of load. The research includes three series of scans for eight samples, previously subjected to loading. Based on the acquired images, some statistical morphological measures are calculated, i.e. volume fraction of cracks, tortuosity and crack aperture spatial distribution. In addition, the effective parameters of anisotropic permeability and their changes induced by different levels of loading are estimated using Kozeny-Carman formula. 1. Introduction Shale, a fine-grained sedimentary rock, is composed of highly compacted clay particles, nanometric porosity and silt-sized particles of other minerals. It is a multi-phase material with multi-scale internal structure. Modern technologies as nanoindentation and computed microtomography give a possibility to investigate morphological and mechanical properties of shales at very small scales of observation. Nanoindentation method was developed by Doerner and Nix [1] and later by Oliver and Pharr [2] and it allows to determine the hardness of the investigated material as well as its elastic modulus from depth sensing indentation load-displacement data (DSI) [1]. It was successfully applied for characterization of mechanical properties of shales [3, 4, 5] and other types of rock [6, 7, 8]. Computed microtomography technique is a modern method for nondestructive 3D imaging of material internal structure. The technique was successfully used to verify the porosity of soils [9, 10] and rocks [11, 12]. A number of research concerns the analysis of failure mechanism and the impact of anisotropy on the failure of rock samples [13, 14, 15].

Abstract The old workings may still generate mining menace hazard around the Polish historic salt mines of Wieliczka and Bochnia, despite the fact that adequate protection support have been provided along the underground tourist routes. The water and collapse hazards dominate in underground salt mines. Water hazard is manifested by leaks, excessive leaching, or water disasters which may lead to catastrophic flooding, while collapse hazard is marked by cracking and rock erosion. Such processes caused ceiling collapses in previous centuries, followed by sinkholes above shallow workings. Our analysis indicated that salt-mine collapse hazard monitoring is usually reduced to the observation of the rock-mass movement on the scale of both mining area and locally. Mining area movements are observed by taking measurements of the vertical displacements of land-surface and mine levels. Local movements are observed by several methods: measurement of workings' convergence, shelf and pillar deformation, and rock-layer deformation close to walls and ceilings. The frequency of measurements depends on the assumed accuracy. This paper describes the rock-mass monitoring methods applied in the Polish underground salt mines, with presentation of specific measurement results. Our analysis demonstrated that the monitoring systems used in present-day mining practice are rather reduced to rock-mass movement recording. However, such records cannot be the grounds for determination of the collapse hazard status. First of all, the measurement results should be used for a quantitative description of the rock-mass movement caused by human activity and obtaining the values of the parameters for constitutive laws applied in geomechanical models. Only the knowledge of the phenomena which occur in the rock mass will allow us to define the indicators that are usable to determine the degree of hazard and estimate the threshold values that would suggest the necessity to start implementing proper countermeasures 1. Introduction The workings of the Polish landmark Wieliczka and Bochnia Salt Mines continue to generate certain hazards [1]. However, adequate protection systems have been provided, especially along their underground tourist routes. The main hazards existing in underground salt mines include water influx and wall or ceiling collapses, which often accompany each other [2, 3]. Rock-mass movement is usually the cause of the collapse hazard. In the case of historical salt mines, rock movements are monitored primarily by surveying methods, consisting in the observations of the displacement of selected objects [4, 5].

ABSTRACT: A series of monotonic uniaxial compression tests under quasi-static loading conditions on Hawkesbury sandstone specimens were conducted. To accurately measure pre-peak and post-peak stress-strain behaviour, lateral strain-controlled test at a prescribed constant loading rate were undertaken. To study localised deformations of specimen, 3D Digital Image Correlation (DIC) technique was used. 3D DIC is a non-contacting optical method for strain measurement that uses two-digital cameras to acquire images of the undeformed and deformed shape of an object. Field of strains and strain localisation in the rock surface were analysed by 3D DIC method and coupled with the respective stress levels applied to the rock. By using 3D DIC technique relatively large strains developed in the post-peak regime within localised zones were accurately measured. Field strain development in the rock samples suggests that strain localisation takes place progressively and develops at a lower rate in pre-peak regime, otherwise it is accelerated in post-peak regime associated with the increasing rate of strength degradation. The results show that a major shear failure plane, due to strain localisation, becomes noticeable only long after peak stress. 1 INTRODUCTION The mechanical properties of intact rock extracted from the stress-strain curve in uniaxial compression tests have major implications on civil engineering projects, mining engineering and mineral exploration related operations. In this sense, the complete stress-strain characteristics of intact rock are relevant in the understanding of the total process of rock deformation. The complete stress-strain curve for both class I and II rocks (Wawersik & Fairhurst 1970) can be successfully obtained, depending on the brittleness of the rock, by implementing a closed-loop servo-controlled loading system having as feedback signal to control the applied axial load a prescribed constant lateral-strain rate (Hudson et al. 1971). This method has been by far most widely used to investigate the post-peak behaviour of quasi-brittle materials (Hudson et al. 1971, Fairhurst & Hudson 1999); the other method is based on a linear combination of stress and strain (Okubo & Nishimatsu 1985). Improper measurement of strains in the post-peak regime can lead to improper characterisation of the post-peak behaviour of rock useful to quantify postpeak fracture energy (Bazant 1989, Markeset & Hiller-borg 1995, Jansen & Shah 1997) and rock brittleness on the ground of post-peak instability (Tarasov & Potvin 2013, Tarasov & Randolph 2011). Conventional strain measurement of rock specimen in uniaxial compression includes setting up external devices, e.g. Linear Variable Displacement Transducers (LVDT) and direct-contact extensometers or strain gauges. In this respect, a major drawback using LVDT includes bedding error measurements (Taheri & Tani 2008). Although free from bedding errors, extensometers and strain gauge measurements are yet limited to a fixed gauge length and specific points of bonding, respectively. In addition, they may not capture entirely strains found in post-peak regime. In this case, extensometer misalignment and strain gauge damage are the major issues particularly when progressively growing cracks and localisation take place in the rock surface. Therefore, accurate non-contact strain measurements, via three-dimensional digital image correlation (3D DIC), becomes relevant to study strains in particular in post-peak regime.

ABSTRACT: Weathering has several effects on the physical, strength and material behaviour characteristics of rocks. Various researchers have attempted to obtain weathering classifications using qualitative and/or quantitative approaches. In this study, material behaviour under uniaxial loading conditions, mechanical and physical properties are investigated for reliable estimation of weathering degree of granodiorites. For this purpose, representative rock samples with different degrees of weathering from Havran (Balikesir/Turkey) were collected and the necessary investigations and tests were performed. Weathering grades ranging from unweathered (W 0 ) to highly weathered rock (W 3 ) were used for classification purposes. Core samples were tested in the laboratory to determine various physical and mechanical properties for each weathering class. Also, mineralogical properties of granodiorites were investigated and described for each weathering degree. Test results were used for analyzing and constructing prediction models with the aim of indirect determination of the weathering degree of granodiorites. Finally, a monogram was proposed to distinguish different weathering degrees under uniaxial loading conditions for rock engineering purposes. 1 INTRODUCTION Rocks, being conformed to technical specifications, are generally used as aggregates (cement aggregates, ballast aggregates etc.), filling materials (road embankment, joint filling material etc.), cover slab, brick and stone wall joints aimed to decoratively purposes in industry. Thus it is expected to increase production quality with detailed geological and geotechnical studies. A clear understanding of the weathering degree of rocks provides a great convenience in engineering applications. Various researchers have attempted to obtain weathering classifications using qualitative and/or quantitative approaches. Qualitative classification schemes are based on field observations and some simple index properties. Quantitative classification schemes are tightly coupled with mineralogical and physico-mechanical properties, strength and defor-mability characteristics of rocks. The progressive weathering in rocks lowers directly the quality of the rock materials and masses (Noble 1977, Fookes et al. 1988, Oyama et. al 1998, Yohta 1999, Wyllie 1999, Topal 2002, Arikan et. al. 2007, Marques et al. 2010).

ABSTRACT: Elevated temperature causes changes in mechanical properties of rock, due to thermal effect and mineralogical transformations, which show great varieties for different lithologies. The present paper provides information on seven different sandstones. The studied predominantly quartz sandstones show different petrology from fine-grained to coarse-grained with silica, carbonate and clayey cement. The changes in rigidity as a function of temperature were documented. The analyzed data set includes the uniaxial compressive strength and indirect tensile strength measured at 22°C, 150°C, 300°C, 450°C, 600°C, 750°C and at 900°C, respectively. The test results allowed determining the changes in rigidity. Brinke number was also calculated and clearly show a trend with increasing temperature. The Brinke number of the studied sandstones was in the range of 5–20, showing the higher values at higher temperatures. Constants were calculated for equations describing the trends in changes of rigidity. Our results indicate that the rigidity of sandstone depend on thermal conditions and show different rate of changes depending on the micro-fabric and petrological characteristics of sandstone. 1 INTRODUCTION Recently, the influence of the heating on the physical and mineralogical composition of stones is in the focus of research. Due to heating shock the petrophysical constants of the rocks show some changes (e.g. Brontóns et al. 2013, Tian et al. 2012; Tian et al. 2014). Studying the relationships between the heating temperature and the rock mechanical parameters is an important issue in understanding the material behavior in fire related accidents, such as tunnel fire (Smith & Pells 2008). Many of the publications focused on the mineral and color changes of sandstones after heating (e.g. Hajpál & Török 2004, TÖrÖk & Hajpál 2005, Hajpál 2006, 2007, 2008), some recent papers dealt with other parameters, too (Kompaníková et al. 2014). In this paper we analyzed and recalculated the published results of Hajpál (2002). Both non-destructive and destructive tests were carried out by Hajpál (2002). The effect of heat was studied on seven different German sandstones. From the sandstone blocks nearly 1500 cylindrical specimens (40 mm in diameter and 40 mm to 80 mm in length) were core-drilled.

ABSTRACT: In order to stimulate the fluid flow in rock, hydraulic fracturing is being widely used in many different engineering projects including enhanced geothermal system, enhanced oil recovery, and shale gas production. In many cases, hydraulic fracturing is performed in deep sedimentary rock formation where rock is not usually isotropic. The direction of fracture propagation on hydraulic fracturing does not necessarily coincides with the direction of the maximum principal stress in anisotropic rock. In this study, the characteristics of hydraulic fracturing in transverse isotropic rock subject to biaxial loading were examined. Laboratory scale tests were conducted using artificially made rock-like cement mortar specimen. In addition, the fluids having different viscosities were injected at constant flow rate. Acoustic events during fracturing were monitored to investigate the patterns of crack growth and to classify the crack types. From the tests, the direction of fracture propagation in transverse isotropic specimen was characterized. Also, it was observed that initiation pressure, breakdown pressures, reopening pressure, shut-in pressure were affected by fluid viscosity. The crack growth and cracking pattern showed significant differences depending on the viscosity of fluid and the influence of anisotropy. 1 INTRODUCTION Hydraulic fracturing and seismic monitoring are used as a method for promoting the fluid flow in the rock and in the energy field such as geothermal development and the development of oil and gas production. To better understand the mechanism of hydraulic fracturing, many studies have been actively conducted. Hubbert & Willis (1957) have shown that hydraulically induced cracks are created by tensile fracture and extended along the direction of the maximum stress. Zoback et al. (1977) investigated an increase in the difference between the initiation and breakdown pressure with increasing fluid viscosity. They also estimated the crack initiation pressure from combined analysis of mechanical parameters of the test with Acoustic Emission (AE) obtained during hydraulic fracturing. Lockner & Byerlee (1977) demonstrated that AE, generated during hydraulic fracturing test, could be used to fracturing mapping and infer its propagation rate. Ishida et al. (2004) carried out the hydraulic fracturing test in laboratory conditions using low viscosity and higher viscosity and fracturing mechanism was identified by the fault plane solutions of AE monitored during experiments.

ABSTRACT: One of the most relevant problems in rock mechanics is to determine the complete stress-strain behavior of rocks in both laboratory and field scale. Whereas there are some suggested standard procedures to compute elastic and peak criterion parameters starting from laboratory data, post-failure behavior is still under research in order to appropriately define theoretical frameworks and parameters to represent this behavior. Arzúa & Alejano (2013) carried out a comprehensive laboratory program to study the complete stress-strain behavior of intact samples of Blanco Mera granite. We use these results to fit micro-mechanical parameters in the bonded-particle model to try to recover the observed laboratory behavior. First, we have tried with the parallel-bond model defined in Potyondy & Cundall (2004) but we have found difficult to recover the actual frictional response of rock sample. To overcome these difficulties the flat-joint model defined by Potyondy (2012) was then used for modeling and the results obtained improve the quality of the simulations, which are also able to reproduce the main post-failure behavioral trends. 1 INTRODUCTION 1.1 Background For years, many authors focused their work on studying rocks and rock masses behavior up to peak strength, i.e. their elastic and strength behavior. Nevertheless, during the last decades, some authors have begun working to try to understand how rock masses behave after the failure occurs, i.e. after peak strength is overpassed. This post-peak behavior is relevant in order to understand some relevant rock mechanics challenging issues, such as the behavior of mass mining caving processes, that of fractured reservoirs under evolving stress conditions or long-term behavior of nuclear waste repositories. The post-failure behavior of a rock includes the transition from peak strength to residual and it needs to account also for evolving volumetric strain. Hoek & Brown (1997) proposed, based on their experience in the engineering and numerical analysis of a variety of real cases, three basic types of post-failure behavior for rock masses (Fig. 1). Post-failure general trends can be classified in elastic-brittle behavior associated to good quality rock masses, elastic-perfectly plastic behavior linked to poor quality rock masses and the more general strainsoftening behavior, taking place for the case of rock masses showing average geotechnical quality (GSI values between of 30 to 60).

ABSTRACT: Chalk has long been used as a strong building material. The shallow underground room-and-pillar quarries where the chalk was extracted now lie abandoned and endure the effects of time and weathering, increasing their risk of collapse. The risk of instability is an issue for more than 10 000 towns in France. We have studied the physico-mechanical behaviour and ageing of two types of chalk in underground quarries in the Parisian Basin, France. The dolomitic chalk (Saint-Martin-Le-Nœud) exhibits variations in mechanical and physical properties with horizontal pillar depth, and both density and compressive strength appear to increase outwards from the core to the wall of the pillar. SEM analysis reveals progressive degradation and increasing homogeneity of the grains from the edge to the inside of the pillar. In contrast, in physico-mechanical tests and SEM analyses performed on the Estreux glauconitic chalk, no significant variation was observed between the pillar wall and pillar core. 1 INTRODUCTION Alteration can be defined as a modification of the physico-chemical properties of minerals and therefore rocks, by atmospheric agents, groundwater or thermal waters (Foucault & Raoult 1995). The process is dependent on climate, water and temperature, as well as on the nature and degree of fissuring of the rock. Alteration generally leads to less-coherent rocks, promoting their deterioration and eventual failure (Gupta & Seshagiri Rao 2000, Massuda 2001). Macroscopic signs of alteration include the appearance of cracks, microfractures and macroporosity, as well as the presence of hard deposits that result from chemical transformations (Oyama & Chigira 2000). The alteration of rocks by dissolution (limestones, tuffs, sandstones, crystalline rocks), with or without mineral neoformation, has been studied by numerous authors (Gupta & Seshagiri Rao 2000). The term "ageing" refers to weathering of a rock within a medium influenced by human activity. In the context of underground structures, the term is used to describe the range of mineralogical and physical modifications of the rock over time. These modifications then lead to a deterioration of the hydraulic or mechanical properties.

ABSTRACT: Antique rock structures excavated in soft tuffs in Cappadocia Region of Turkey are rare rock structures excavated by mankind. The long-term behaviour of these tuffs is an important data source in terms of rock engineering. In this study, short and long-term behaviour of the surrounding tuffs, their index and mechanical properties and effects of atmospheric conditions on rock behaviour were investigated and some mechanical models to assess the stability of antique rock structures were recommended. The analysis results mainly indicate that erosion due to wind, precipitation and freezing-thawing has important effect on the stability of the openings carved in the toe of valleys and fairy chimneys. Stability assessments based on simplified models suggest that openings next to cliffs are more likely to fail in long-term as compared with those in fairy chimneys, and the fairy chimneys may be subjected to instability problems within about 1000 years. 1 INTRODUCTION Underground space has long been used through history by humankind for the purposes of human accommodation, religious ceremony, defence, food storage, etc. Residential uses probably represent the oldest use of underground space. The vernacular underground architecture constitutes a low cost solution and a suitable answer to face up to annual thermal fluctuations. Turkey, a bridge between the Asian and European continents, is richly endowed with cultural remains of various civilizations. One of the seven sites, which was included in the World Heritage List in 1985 by UNESCO, is the Cappadocia Region of Turkey extending over 40000 km 2 . The history of the underground settlements in Cappadocia definitely extends back to more than 2500 years on the basis of archaeological evidences at Ağıllı (Topada) Hittite hieroglyphs (1200–700 BC), where one can find an underground settlement facing this stone monument. A number of antique rock structures in Cappadocia have been excavated in easily carved soft tuffs with large spacing of discontinuities and they are rare rock structures excavated by mankind. In this region, there exists 22 historical underground cities, rock-cut dwellings, churches and semi-underground cliff settlements (Fig. 1a). In addition, the intriguing landscape of towering cones called "Fairy Chimneys" is the most famous attractive morphological feature of Cappadocia (Fig. 1b). Although these rock structures generally survived and kept their original integrity for a number of centuries, the observations on some cavities showed some indications of yielding and partial collapses at several locations. These structures are of great importance not only from the viewpoints of preservation of these historical structures as assets of mankind, but also they may provide some data and insight on the long-term performance of man-made structures in rock engineering. In this study, short and long-term behaviour of the surrounding Cappadocian tuffs, their index and mechanical properties and effects of atmospheric conditions on rock behaviourwere investigated and some mechanical models to assess the stability of antique rock structures, such as openings carved in valley slopes for past settlements and fairy chimneys, were recommended.

ABSTRACT: Understanding the behavior of a rock block after detachment is essential to effectively control rockfall hazards and implement protection measures. Rigid body rockfall models allow a more realistic interpretation of rockfall events by representing rock with a real size and geometrical form. This study presents the analysis of potential rockfalls that threaten the Sumela Monastery, Turkey, which is one of the important historical and touristic places. The structure of the Monastery is located in a rocky steep cliff, surrounded by a highly fractured surface of the cliff from top. The structure has been subjected to several damages due to rock detachments from the cliff surface in the last couple of years and is considerably threatened by potential rock blocks that can fall. Field studies and discontinuity surveys are performed to determine the potential rockfall source areas. The geomechanical properties of the rock materials are determined from laboratory tests. A LIDAR measurements as well as aerial photos are used to prepare slope profiles. According to previous rockfall evidences, back analyses are performed to identify the slope surface characteristics like restitution coefficients. For each profile, different rock size and shapes are simulated and corresponding runout distance, bounce height, kinetic energy and translational velocity are recorded. It is found that, since the structure is located in an almost vertical cliff with considerable height, in all cases, the detached blocks hit the structure with large peak kinetic energy values. 1 INTRODUCTION Rockfalls are spontaneous rapid phenomena with high-energy bearing features in hilly regions, which cause significant threats to the environment as well as human lives and property. A rockfall occurs when rock boulders detach from their original locations following four basic types of motions: free-falling, bouncing, rolling and sliding. Rockfall kinematics and dynamics mainly depend on the topography, block weight and geometry, mechanical properties of the slope forming material such as friction angle, roughness, restitution characteristics and rolling resistances (Azzoni et al. 1995, Dorren & Seijmonsbergen 2003). When a rock-fall event reveals a threat to people or structures, it is essential to describe the trajectory of the falling rock along a slope in order to design and implement protection measures or to prevent hazard by an appropriate land use planning. The accuracy in the estimation of the trajectories and motion of the potential falling rocks forms the basis of a safe design and verifies the protective measures.

ABSTRACT: During the design process in rock engineering Hoek-Brown failure envelope is used for the determination of rock mass failure envelope mainly in brittle rocks. An important input parameter of the Hoek-Brown failure envelope is the Geological Strength Index (GSI ), which varies between 0 and 100, and concentrates on the description of rock structure and block surface conditions. There are several methods which define GSI but a general international standard has not been specified yet. Our aim is to analyze different methods of GSI determination on the basis of observations during the construction phase of the Bátaapáti radioactive waste repository. Examinations of the values determined on-site gave significantly different results. Different correlations were determined between the calculated GSI values. 1 INTRODUCTION The Geological Strength Index (GSI ) represents today the most widely used engineering index for the categorization of rock mass quality for obtaining input data into the continuum numerical analysis codes and closed form solutions based on the Hoek-Brown failure criterion (e.g. Marinos & Hoek 2000, Marinos et al. 2007). The exact determination of this value is very important for the exact calculation of the failure envelope or the deformation moduli of the rock mass. Ván & Vásárhelyi (2014) determined the sensitivity of the GSI based on mechanical properties, such as the Hoek-Brown equation (failure envelope of the rock mass) and the Hoek-Diederichs equation (deformation moduli of the rock mass). It was shown that sophisticated empirical equations can be highly sensitive to the uncertainties in the GSI values - even if the error of the GSI is only 5%, the relative sensitivity can reach 100%! Recently, Morelli (2015) analyzed the different calculation methods of GSI. Using Monte-Carlo simulations, his simulation results indicate that the diverse relationships may predict dissimilar values of the GSI for the same rock mass. He obtained the highest GSI value from the equations which apply the conventional RMR 1989 values, and the lowest results were obtained by using the RMi method for GSI calculation.

ABSTRACT: The geometrical parameters of 639 discontinuities that surveyed in powerhouse cavern of Rodbar Lorestan pumped storage power plant project have been investigated by scanline and areal sampling methods. As regards the processing and correction of bias types, one bedding and three joint sets are existed in the site. The tectonic activities and direction of principal stresses have caused for each of trace length and spacing characteristics, the probability distribution function of joint sets differ to each other as regards their genetic types. The calculated mean trace length by scanline and areal method are very close together for one joint set and for another one the difference is 28%. The actual intensity differences between circular and rectangle sampling windows for joint sets J 1 and J 2 are 7% and 38%, respectively. Meanwhile, the calculation of volumetric intensity by various methods shows the estimation of this characteristic is very difficult in the field. 1 INTRODUCTION Discontinuities in rock mass have profound effect on deformation, strength, stress-strain relation and failure of rock mass (Ye et al. 2012). So, besides the description of mechanical properties of discontinuities and intact rock the description geometrical properties of discontinuities is very important to investigate of rock mass behavior and to determinate interaction between rock mass and structures. The paper represents geometrical properties of discontinuities in powerhouse cavern of Rodbar Lorestan pumped storage power plant in Iran. They are investigated with using Scanline and areal sampling methods (circular and rectangular). The location of cavern in tensile zone of anticline and the area is tectonized due to the existence of high Zagros zone are one the main characteristics of the study area. Recognizing of homogenous statistical zone in study area is the first step in modeling of joint geometry in rock mass (Kulatilake et al. 1990). As regards the discontinuities surveyed locations are in different areas, the various statistic tests have been done to control the data homogeneity and to investigate of uniformity of rock mass zones. As regards the 639 surveyed discontinuities and their clustering two joint sets, bedding and another joint set as strike joint set exist in the site. According to uncertainty on surveyed data, the correction of type of errors on dip, dip direction, spacing, frequency and trace length of discontinuities have been done. On the other side, as regards the intrinsic statistical properties of discontinuities geometry, the statistical analysis of the corrected data has been done. Meanwhile, the Fisher constant, intensity (areal and volumetric) and other characteristics are calculated for each joint set.

ABSTRACT: The mechanical properties of conglomeratic rocks are difficult to study by conventional laboratory tests. On the other hand, in-situ tests are more representative to establish the mechanical properties of these materials but, in general, take a long time to execute and represent a high cost for projects. This paper presents a methodology to obtain the mechanical properties of a conglomerate, using the results of a numerical modeling study carried out in virtual rock samples. A conglomeratic rock from Costa Rica was used as a study case. To determine the validity of the estimation, the results from the numerical modeling were compared with data from in-situ tests performed in the same conglomerate. 1 INTRODUCTION Many engineering works in Costa Rica are developed in conglomerates. The engineering design of these works requires a comprehensive protocol for characterization and for evaluation of the mechanical properties of these materials. The Costa Rican Electricity Institute (ICE) is currently studying the largest hydroelectric project in Central America, El Diquís Hydroelectric Project (PHED). For this project, it is expected to have open cut sections up to 110 m high, underground excavations with a span between 6 m and 32 m, and a foundation for the 170 m high concrete face rockfill dam. The conglomeratic rocks found in the area of study are classified as unwelded bimrocks (Sonmez et al. 2009). These rocks, in essence, are a mixture of good-quality blocks, embedded in a less competent matrix, whose shear strength and deformability are mainly ruled by the volumetric block proportion (VBP) (Lindquist 1994). This paper presents some of the in situ investigations carried out to describe the conglomerate of the Paso Real Formation. Samples representative of the matrix were obtained and tested in laboratory scale for both physical and mechanical properties. Field tests were carried out in order to obtain deformability and shear strength. Jiménez (2015) presents a full description of the laboratory tests and field works. The paper also presents a methodology to incorporate blocks within a fine-grain matrix and generate the rock properties through numerical experiments. Synthetic samples are generated in order to represent the conglomerate in the field. The numerical experiments results compare well with the field test results.

Abstract Durability is one of the key functional properties of natural stones used in architecture and construction. This property is often extrapolated from index physical properties, based on the results of empirical durability tests and/or from the in situ long-term experience. In this recent study, we tested how the data obtained from common rock mechanical tests can be employed as durability estimators. Along with standard rock mechanical parameters (strength, modulus of elasticity), the stress-strain curves were evaluated to obtain crack closure, crack initiation, and crack damage thresholds of several varieties of sandstones commonly used in architectural and sculptural projects. The modulus of resilience and modulus of toughness, computed from the respective parts of the stress-strain curves, proved to have a significant correlation between resistance to weathering in the accelerated laboratory tests and/or to the known field performances of the tested stone varieties. 1 Introduction Durability is the key functional property of natural stones used in construction and for the manufacturing of architectural elements and/or sculptures. Durability is often viewed as the resistance against various weathering processes (Frohnsdorff & Masters 1980 and Lewry & Crewdson 1994); however, in a broader sense, it covers technical serviceability. Durability is not a fundamental property, it can more likely be considered to be the long-term manifestation of repeated interactions between natural stone and the surrounding environment (Soronis 1992). The time period, during which the stone can be claimed durable, is equal to its macro- and microstructural stability, which secures fulfilments of its structural and aesthetic functions (Sims 1991 and Andrew 2002). Assessment of durability is generally possible through one of five major approaches: practical experience, accelerated laboratory durability test, complex environmental testing, exposure site testing, and extrapolation from other physical properties (Prikryl 2013 and references therein). Each of these approaches has certain advantages and drawbacks. Although practical experience provides the most reliable results, it is highly impractical in terms of sufficient amounts of experimental materials (extensive sampling is often prohibited from heritage structures), time, and knowledge of all past weathering processes that have resulted in the observed decay pattern(s). Accelerated laboratory tests are commonly used for durability assessment because rapid standardized test procedures allow for comparisons between various stone types and/or weathering actions (e.g., freezing/thawing, wetting/drying, salt crystallization, etc.). However, several drawbacks influence their real explanatory power: the size and geometry of test specimens are far from the dimensions and geometries of the real artefacts or structural elements. Consequently, the intensity of damage might not be absolutely comparable. Oversimplification of the weathering process presents another negative factor of accelerated tests, which generally simulate only single weathering action; while during real in situ weathering, numerous actions interact. Generally speaking, most of the accelerated durability tests and post-test evaluation of variable physical properties probably rely on a partly erroneous paradigm.