This paper presents the case study of a stability analysis of an over-tilted (inverse) slope in an ornamental granite quarry. Based on traditional small quarrying practices and due to space constraints, the SW slope was carved following the occurrences of highly persistent joints that were dipping around 80° counter-slope. Experience dictates that this sort of slopes tends to be unstable, at least in case of average to low quality rock masses. In this way, a stability analysis was due to analyze slope stability. The good quality granite rock mass in the slope was characterized, joint data was recorded and laboratory testing was done to estimate the main significant parameters involved in the study and the geometry of the slope. A stability analysis of the slope was performed by means of the calculations of safety factors against toppling. These calculations have been done contemplating various possibilities regarding the occurrence of joints and its spacing, which was found to be the most relevant parameter controlling stability.
This paper presents the case study of a stability analysis of an over-tilted (inverse) slope in a granite quarry. Due to space constraints of the quarry located in a hill slope, the SW end of the quarry happens to be parallel to a set of highly persistent joints that were dipping around 80° counter-slope. Therefore, as shown in Fig. 1, large parts of the slope are overt-tilted, that is, the slope is inverse –dips more than 90°– in these areas. In such situations, the local mining regulations enforce the owner to carry out a slope stability study appropriately justifying the stability of the slope. In this paper, the authors presents the most relevant topics of this geomechanical study, together with some considerations regarding the actual occurrence of instability toppling phenomena in rock slopes.
1.1. Brief description of the quarry and mined stone
This paper applies a modified ubiquitous joint plane model (Modified Ubi) to describe the mechanical behavior of layered rock masses. The constitutive model is an implicit-continuum based one where the joints are smeared across the rock mass. This modified model concerns not only the strength anisotropy but also it integrates the elastic stiffness anisotropy. Thus, the elastic stress increments and the plastic corrections from the original ubiquitous joint model have been altered. Now, it is possible to simulate the elastic and plastic behavior of transversely isotropic rocks. Modified Ubi is applied to simulate the behavior of the transverse isotropic rock samples in uniaxial compressive loading and triaxial loading tests. The results out of the modified model were compared to the analytical solution from Jaeger and the CANISO model from FLAC 8.0.
Determining the rock mass properties and expecting its behavior are matter of the modelling of the discontinuities in the rock continua. Because of the complex mechanism of the rock mass which may not be directly predicted using the conventional ways of modeling (such as: closed form solutions or physical modelling); numerical modelling is one of the most trending means to model behavior of rock mass . The condition of the rock mass (such as: excessively fractured or reasonably fractured or intact massive) and the scale of the engineering application (i.e.: layering is significantly smaller or bigger than the scale of the application) affect strongly the optimum approach selection of the applied numerical model, either continuum-based approach or discontinuum-based one . Modelling of layered rock mass is still a point of interest between different computational tools which either consider the joint implicitly (i.e.: FLAC - Ubiquitous joint model) or use the explicit representation of the discontinuities (i.e.: UDEC - Discreet element model) . However, both the computational power and time consumed in calculation limit the usage of such discrete approaches. The accuracy of different implicit joint models has been investigated between both the ubiquitous joint model and Cosserat model . This paper adopts the continuum-based approach to implicitly model an isotropic rock mass (i.e.: smeared joints).
In Japan, natural gas that has been dissolved in water is mainly produced by gas lifted through a perforated casing. Since abnormal formation pressure causes abnormal cross flow among production sections, a technique for sealing the sections with abnormal formation pressures is required, in order to increase natural gas production. In this study, we propose a new drilling technique for the fresh sealing cement using waterjet. Preliminary laboratory drilling tests were conducted under air condition to clarify the fundamental drillability of a waterjet for the purpose of sealing cement. The main results obtained in this study can be summarized as follows: (1) The waterjet has a good drilling performance for the fresh sealing cement. (2) The suitable aging time before sealing cement for drilling should be determined based on the mechanical properties required for sealing the sections with abnormal formation pressures.
In the South Kanto natural gas fields of Japan, natural gas that has been dissolved in water is mainly produced by gas lifted through a perforated casing that serves as production tubing. In this gas field, the typical length of a production zone in a production well is several hundred meters. Since abnormal formation pressures cause abnormal cross flows  among production zones, it is important to select the appropriate production zone to prevent the gas flow from developing cross flow . However, such cross flow often occurs during the lengthy gas-production process. Therefore, a technique to seal the sections with abnormal formation pressure is required, both to prevent cross flow and to increase the production of natural gas.
The conventional sealing technique calls for cementing the target formation that has caused the cross flow. After the cement has cured, the cured cement is drilled out with a drill bit, an operation that requires a large drilling facility. A lighter drilling system suitable for use with sealing cement is needed. In this study, we propose a new drilling technique for fresh sealing cement in using a waterjet. Preliminary laboratory drilling tests were conducted under air conditions to clarify the fundamental drillability of a waterjet for the fresh sealing cement. The drilling performance related to the cement sealing and the suitable curing time for the sealing cement needed before drilling are discussed in this paper.
The purpose of this review is to present the state-of-the-art methods, problems and potential future design techniques used for rock engineering in frozen ground, with particular regard to the design of mine openings in rock masses that may be subject to thawing. The impetus for this work is the recognition that civil and mining infrastructure is extending into such frozen material, and the design procedures currently available may not be optimal. Additionally, civilian infrastructure in frozen ground in alpine regions (e.g. central Europe) is also becoming subject to thawing conditions. In any such scenario, increased heat exchange with an exposed rock mass surface will cause thawing to occur. In general, a rock mass is stronger in frozen conditions than in dry conditions (i.e. without ice or water), but a rock mass will be least strong at the time ice-filled discontinuities are thawing, resulting in a significant shear strength reduction. In this review, we will discuss the various existing design methods in the context of the phenomena involved in the formation of frozen rock and the behaviour of thawing rock. We will show the deficiencies of these methods and highlight potential developments that will lead to future robust design protocols.
Although research on permafrost began in the first half of the eighteenth century, the study of permafrost as an applied science (known as geocryology) began in the late nineteenth century when industrial development was accelerating in northern Russia. Applied research in permafrost was required to develop railways, mining operations, as well as to protect civil structures from freeze-thaw damage . One of the earliest documented cases of permafrost engineering was in 1876 on the use of artificial freezing, which was applied at Brunkeberg tunnel in Stockholm .
Since the beginning of the twentieth century, the study of permafrost was further accelerated by the energy industry as well as different military research institutions, most originating in Russia [1-5]. The second World War gave impetus to some of the first formal geotechnical investigations for mining and civil structures in frozen ground . Furthermore, during the Cold War, permafrost research was of great interest for American and Russian military researchers, so much so that a permafrost tunnel was excavated and operated by the U.S. Army Cold Regions Research and Engineering Laboratory (CCREL) in Alaska. The CCREL were tasked with investigating mining techniques and how to maintain underground excavations in perennially frozen ground .
In material sciences, it is a well-known fact that linear or linearized theory based on Hooke’s law does not offer a satisfactory description of solids in special regimes, which include e.g. too high strains under large uniaxial stresses. Therefore, in general, the response to biaxial or triaxial loading cannot be obtained as superposition of uniaxial load responses. Striking paper book example of material demonstrating such behavior is rubber subjected to uniaxial or isotropic compression. Despite this fact, linear mechanical moduli, being secant or differential, determined through standard rock-mechanics tests, mostly from the uniaxial compression, are still widely used for description of deformational behavior of rocks. Without doubt, an appropriate interpretation of these effective quasi-elastic or stiffness moduli can give useful information about mechanical properties of the rocks, especially in comparative sense. However, for more reliable constitutive modeling of any solid materials, paricularly rocks, an experimental investigation of deformational responses to uniaxial, biaxial and triaxial loading or unloading regimes is very useful. This contribution presents the results of an experimental case study on homogeneous sandstone exposed to isotropic triaxial, and equi-biaxial or uniaxial loading regimes. The measured deformational response of this rock is compared with behavior of elastic solid materials. Finally, benefit of the experimental testing for constitutive modeling based on phenomenological description is briefly discussed.
1.1. Triaxial experiment for study of mechanical behavior of rocks in variable stress conditions
Investigation of mechanical behavior of rocks under applied external stress fields constitutes necessary base for understanding many natural geophysical processes in the Earth’s crust, or for solutions of engineering problems connected with a wide spectrum of human activities. For these tasks, simple or very sophisticated numerical models can be adopted. However, for a satisfactory prediction of mechanical evolutions, these models need experimental data in the form of basic mechanical parameters. Rocks usually represent the complicated aggregates of mineral grains of variable size, chemical compositions, or magnitude of binding forces. Such microscopically heterogeneous materials, when subjected to increasing external stress, from the start of loading, undergo reversible elastic deformation, as well as permanent deformation originating preferably from microfractures.
Standard material engineering can successfully adopt generalized Hook’s law, describing elastic and mostly linear response through constant elastic moduli, for majority of construction materials at sufficiently low strains. However, in rock mechanics, the mechanical moduli determined from laboratory experiments play the role of linearized effective coefficients rather than constant intrinsic material parameters. For ideal material described through linear relations between strain and stress, a deformational response to general loading conditions can be determined through superposition of uniaxial loads. Therefore, the simplest uniaxial compression/extension laboratory test may be often sufficient to estimate the deformational response in other loading regimes including general triaxial stress state. This consideration is widely used also in geomechanical modeling, i.e. Young moduli and Poisson ratios determined from uniaxial tests on rocks as secant or linearized coefficient are accepted as inputs. However, such attempt is not fully satisfied, and laboratory investigation of the deformational behavior of rocks under alternative triaxial loading regimes is necessary.
Vavro, Leona (Institute of Geonics of the CAS) | Soucek, Kamil (Institute of Geonics of the CAS) | Kytýr, Daniel (Institute of Theoretical and Applied Mechanics of the CAS) | Fíla, Tomás (Institute of Theoretical and Applied Mechanics of the CAS) | Kersner, Zbynek (Brno University of Technology) | Vavro, Martin (Institute of Geonics of the CAS)
The article deals with the use of computed X-ray radiography to visualize the development of the fracture process zone in the rock samples. For radiographic observations during the three-point bending loading, glauconitic sandstone from the Řeka quarry (sometimes also referred to as Godula sandstone) was used. The chevron-notched cylindrical specimens with the diameter of 29 mm and 120 mm nominal length were prepared from the sandstone blocks. These specimens were subjected to the Chevron Bend (CB) test carried out in accordance with the ISRM suggested methodology; the span was 94 mm. The evolution of the fracture process zone was continuously scanned using X-ray radiography during the realized Mode I fracture toughness tests (FTT). The scanning was conducted using an industrial X-ray micro-tomographic inspection system equipped with a flat panel X-ray detector of 4,000 × 4,000 pixels and micro-focus X-ray source with reflection target, which are very suitable for obtaining highly detailed radiographic images during the FTT tests. Three-point bending tests were carried out using an in-house designed table-top loading device, construction of which allows precise control of the loading during testing. Continuous X-ray examination and subsequent radiographic image analysis enable investigation of the crack initiation and the process zone development during FTT and represents a useful tool for a better understanding of failure behavior of the rock material during the loading process.
The process of crack formation in quasi-brittle materials leads to the damage of the material and to energy consumption in the direct proximity of the crack, i.e. in the fracture process zone (FPZ). The shape of the fracture process zone during the crack growth varies, and simultaneously the consumption of energy needed for crack propagation changes. The damage area in quasi-brittle materials macroscopically differs from the plastic zone developing at the crack tip in ductile materials (e.g. metals) . Advanced computational tools to analyze fracture behavior of elements and structures of quasi-brittle materials (cement composites, concrete) have recently been created . The FPZ in concrete is currently being studied using experimental techniques, such as high speed photography, acoustic emission testing, scanning electron microscopy (SEM) and laser-speckle interferometry . Numerous numerical analyses of FPZ in concrete specimens have also been performed in this scientific field.
Muñiz-Menéndez, M. (Laboratorio de Geotecnia, CEDEX) | Perucho-Martínez, A. (Laboratorio de Geotecnia, CEDEX) | Rodríguez-Peces, M. J. (Universidad Complutense de Madrid) | Cano-Linares, H. (Laboratorio de Geotecnia, CEDEX)
Interpretations of cavity expansion tests (pressuremeter, radial jack, etc.) are based (in most occasions) on the analysis of the deformability of a cylindrical cavity in a continuous, isotropic, and homogeneous medium. However, many rock masses show an anisotropic behaviour due to the presence of discontinuity planes of different origins. Cavity expansion tests in these media have been studied here with an empirical approach. Several tests have been simulated in a three-dimensional, anisotropic and discontinuous medium—using 3DEC by Itasca—and their deformation has been analyzed in order to establish the principal factors that control the behaviour of the rock mass in these situations. Based on this analysis, it has been developed a new method for the interpretation of the cavity expansion tests carried out in laminated rock masses, which allows estimating the principal deformation moduli of the rock mass (maximum and minimum). This method can be used for any dip of the discontinuity planes.
The deformation modulus of the rock mass is one of the most important parameters for a geotechnical engineering project. The determination of this parameter is an issue not completely solved, neither from a theoretical nor from a practical point of view [1, 2].
Most of the existing field testing methods for the measure of the deformability are expensive and hard to place. Among these methods, pressuremeter test is the easiest and the least expensive one.The interpretation of this test is based on the cavity expanding theory in a continuous and isotropic medium but, most of rock masses are anisotropic and discontinuous media due to the presence of bedding or joint planes. Because of this, the usual interpretation of pressuremeter tests must be questioned, reviewed and, probably, a new methodology must be proposed.
1.1. Theoretical background
As it has been mentioned, the habitual interpretation of pressuremeter tests supposes a homogeneous, continuous and isotropic medium with a radial symmetry. According to the cavity expanding theory, the pressuremeter modulus (EP) can be obtained from the analysis of the load-deformation diagram:
The Telfer underground sub-level cave (SLC) operations are located approximately 600 to 1000 m below the west wall of Main Dome open pit at the Telfer Gold Mine, Australia. The Telfer SLC commenced caving in 2006 and broke through to the surface in 2009 forming a large subsidence zone that has progressively developed above the SLC operation. Surface deformations within the subsidence zone continue at a rate of 500 mm to 2000 mm per month and are dependent to the rate of underground mining in the SLC operation. Underground and open pit operations have continued concurrently with the progressive enlargement of the SLC subsidence zone. The stage 4 open pit orebody was solely accessible via the West Ramp, which is located adjacent to the SLC subsidence zone with notable steady state ductile deformation occurring on ramp. Risks were managed by understanding the cave propagation mechanism through modelling and monitoring, and the use of comprehensive TARPs (trigger-action-response-plans) and an array of real-time surface and sub-surface deformation monitoring instruments. This paper discusses the monitoring systems used to track the cave subsidence and monitor ground deformation. These include slope stability radar, automatic total stations with survey prisms, shape-accel arrays, time-domain reflectometers, seismicity monitoring arrays, networked SMART markers, surface extensometers, automated crackmeters and monthly aerial surveys. These monitoring systems were used to determine the cave subsidence zone shape, location, rate of propagation and the direct response in the slopes adjacent to the West Ramp. The monitoring systems and TARPs are critical for identifying approaching hazards and taking proactive risk mitigation measures.
Underground sub-level cave (SLC) operations commenced in 2006 and caving began to propagate towards the surface from approximately 800 m below the pit . In preparation for the SLC operation, a large bench was established in preparation for the initial cave breakthrough area (i.e. ‘Breakthrough Bench’ or predicted subsidence crater extents) as illustrated in Figure 1a. In order to reduce the likelihood of future high wall instability above the subsidence crater, the Breakthrough Bench was backfilled with 1.1Mt of rock fill (Figures 1b & 1c). The life of concurrent open pit and SLC operations was anticipated to be a further five years.
Rock bursts are defined as sudden, violent failures of rock mass that are of such a magnitude that they expel large amounts of coal and rock into the face area during longwall or pillar extraction in sedimentary rocks. In an attempt to develop tools for assessing stress bump potential, the first author initiated a comprehensive study using site specific information from 25 case studies undertaken in U.S. mines. This effort builds on an initial study while expanding the data base and including additional variables and analyses. Multiple linear regression and numerical modeling analyses of geological and mining conditions were used to identify the most significant factors contributing to stress bumps in coal mines. Twenty-five factors were considered initially (mechanical properties of strata, stress fields, face and pillar factors of safety, joint spacings, mining methods, and stress gradients, among others). Allowances were made for favorable local yielding characteristics of mine roof and floor in reducing damage severity. Pillar and face factors of safety were calculated using displacement-discontinuity methods for specific geometries in case studies having experienced both violent and nonviolent failures. This work identified the most important variables contributing to coal bumps and violent failure of near seam strata. These are  mechanical properties of strata, including local yield characteristics of a mine roof and floor,  gateroad geometry and/or gate pillar factors of safety,  stress gradients associated with the approach of mining to areas of higher stress concentrations such as abutment stresses from multiple seam mining, and  roof beam thickness, joint spacing, and stiffness characteristics, which influence cave conditions and dynamic loading. The latter variables, combined together to form a new variable called “strata rigidity-cavability”, reflect some of the most important aspects of violent failure, i.e. having massive and stiff near-seam stratigraphic units capable of absorbing high strain energy, forming high stress on mine structures and poor cave conditions, and thus the potential for dynamic loads.
In the Mediterranean area, cliff slopes represent widespread high-risk landforms as they are highly frequented touristic places often interested by landslide processes. Malta represents a significant case study as several cliffs located all around the island are involved in instability processes, as evidenced by wide block-size talus distributed all along the coast line. These diffused instabilities are related to the predisponding geological setting of Malta Island, i.e. the over-position of grained limestone on plastic clay deposits, that induces lateral spreading phenomena associated to falls and topples of different-size rock blocks and is responsible for a typical landscape with stable plateau of stiff rocks bordered by unstable cliff slopes.
The ruins of Għajn Ħadid Tower, the first of the thirteen watchtowers built in 1658 by the Gran Master Martin de Redin, stand out in the Selmun area. Currently the safety of this important heritage site, already damaged by an earthquake on October 12th 1856, is threaten by a progressive moving of the landslide process towards the stable plateau area. During autumn 2015, a field-campaign was realized to characterize the jointed rock mass. A detailed engineering-geological survey was carried out to reconstruct the geological setting and to define the mechanical properties of the rock mass. Based on the surveyed joint spatial distribution, 58 single-station noise measurements were deployed to cover both the unstable zone and the stable area. The obtained 1-hour records were analyzed in the frequency domain for associating vibrational evidences to different instability levels, i.e. deriving the presence of already isolated blocks by the local seismic response.
The here presented results can be a useful contribute to begin to asses defense strategies for the Selmun Promontory, in the frame of managing the landslide risk in the study area and preserving the local historical heritage.
Cliff slopes are high-risk landforms in the Mediterranean area due to the diffused landslide processes that affect sites of touristic relevance as well as buildings which are part of the cultural heritage. Malta Island represents a significant case study as its geological setting, i.e. the over-position of grained limestone on plastic clay deposits, predisposes large lateral spreading processes associated to falls, slides and/or topples of different-size rock blocks. These instabilities interest countryside areas, e.g. the rock slabs where the city of Mdina and Citadel are built , as well as sea cliffs all along the coast line, especially in the north-west part of Malta and in Gozo island.