ABSTRACT: The applicability of a new test method involving semi-circular (truncated) Brazilian disc specimens is examined in this study to effectively eliminate the unwanted high shear stresses at the loading points, as well as the multiple cracking that commonly occurs in the testing of standard Brazilian dicks comprising hard and brittle materials. A series of tests with a wide range of hard and brittle materials, including various types of rocks and ceramics, was carried out. In all tests performed, a single straight plane of fracture was observed. Through finite element numerical modelling and high-speed photography of the fracture process, the rupture mechanism was further confirmed to be the result of an indirect tensile stress induced inside the specimens. Based on the results, it is concluded that truncated Brazilian specimens can conveniently be adopted to determine the tensile strength of a target material, rather than the standard disc profiles in which catastrophic local cracking makes the procedure unreliable.
The Brazilian (splitting) test has been studied extensively in the literature since it was first introduced in the early 1940s. It is now a popular and preferred indirect method to measure the tensile strength of rock, pavement, concrete, and similar building materials. The test simply involves splitting a thin solid disc loaded by a compressive line load to generate a compression-induced tensile stress inside the disc until it fails across its loaded diameter into almost two equal hemi-cylinders (ISRM, 1978; ASTM, 2016). However, despite the test being widely accepted in geomechanics, test results with rock-like geo and construction materials having a large brittleness index (i.e. a high ratio of compressive to tensile strength, as in hard rocks, ceramic, alloys, and diamond composites) have been extensively criticised and proved not to be generally reliable (Fairhurst,1964; Hudson et al., 1972; Cranmer and Richerson, 1988; Yu et al., 2009; Swab et al., 2011; Serati, 2014). The main reason for the lack of reliability in testing such materials is the concentration of unwanted cracks developed in the vicinity of the contacts, which adversely interfere with the tensile breakage of disc specimens through the formation of triple-cleft fractures, inverse conical shear plugs, and multiple cracking (Ovri and Davies, 1987; Fahad, 1996; Serati et al., 2015). Therefore, the flexural tensile strength measured at the centre of a Brazilian disc often tends to overestimate the actual tensile strength in such hard and brittle engineering or natural materials. Nevertheless, this failure to achieve the standard breakage in the Brazilian test has been ignored in most cases since the reasons responsible for such catastrophic crushing are not yet fully understood.
ABSTRACT: When anhydrite dissolves upon contact with water, the sulphate and calcium ions in the pore water can lead to precipitation of gypsum. This anhydrite to gypsum transformation (AGT) results in an increase in the solid volume by 61% and in a decrease or increase in the pore volume. On a macroscopic scale the so far insufficiently understood phenomenon “swelling of anhydritic rock” is observed, which is known to cause massive damage to underground constructions, such as tunnels. This contribution focuses on one specific aspect of AGT, the precipitation of gypsum directly on the surface of the dissolving anhydrite, which creates a diffusion barrier for the dissolving ions and slows further anhydrite dissolution down (hereafter referred to as “self-sealing”). The development of such gypsum layers on anhydrite is investigated experimentally by storing natural rock specimens consisting of 96%-99% anhydrite in water and observing the thickness of the developing gypsum layer optically via photography during the tests and via microscopic analysis after drying of the samples. The measured thicknesses corresponded well to the amount of precipitated gypsum determined post test via thermogravimetric analysis and to predicted values which were calculated with an existing kinetic model. The plausibility of this model could thus be verified so far.
1. INTRODUCTION AND CONCEPTUAL MODEL
As anhydrite comes into contact with water it will begin to dissolve under most circumstances below 40°C (at atmospheric pressure). As the saturation concentration with respect to anhydrite (ceq,A) is higher than that of gypsum (ceq,G), anhydrite dissolution results in oversaturation with respect to gypsum, thus triggering growth of gypsum crystals, whereby the solid volume of gypsum is 61% higher than that of anhydrite due to the additional water molecules bound in the crystals. The chemical reaction from anhydrite (CaSO4) to gypsum (CaSO4 • 2H2O) will be referred to here as AGT (Anhydrite – Gypsum Transformation) and can be formulated in short as CaSO4 + 2H2O → CaSO4 • 2H2O.
ABSTRACT: Non-destructive ultrasonic evaluation (NDE) is commonly used for assessment of civil infrastructure and characterization of construction materials. Among the acoustic methods the impact echo, ultrasonic pulse velocity (UPV), and surface waves can be distinguished. In this paper, we focus on the UPV method as an ASTM standard test method for concrete specimens. UPV method has different applications such as the assessment of the relative quality of concrete, the detection of voids and cracks, and the evaluation of the effectiveness of repairs. UPV measurements can be also used for monitoring changes in the condition of a specimen. In spite of the simplicity of the method, its results highly depend on the type of transducers, the coupling transducer-specimen, and the specimen dimensions. In this article, we investigate the effects of the sensor and the specimen dimensions. The results for UPV tests on 9 mortar specimens of different heights and diameters are presented. The specimens are tested with 54 kHz and 850 kHz resonant frequency (fc) transducers and a state-of-the-art 5MHz laser vibrometer.
Concrete is a popular structural material used in Civil Engineering applications. As for any material, condition of concrete may be affected by the quality of design, manufacturing, loads applied to a structure, character of the loads, environmental deterioration, or aging. Condition of concrete plays a key role for safety of structures (Kim et al., 2005). Non-destructive ultrasonic evaluation (NDE) is commonly used for assessment of civil infrastructure and characterization of construction materials. Among the acoustic methods the impact echo, ultrasonic pulse velocity (UPV), and surface waves analysis can be distinguished (McCann and Forde, 2001), (Popovics, 2003). The latest trends focuses more on attenuation of wave front (Aggelis et al., 2005), (Kirlangiç et al., 2015) and more sensitive methods for detecting changes in velocity (e.g. Coda Wave Interfefometry (Dai et al., 2013), (Planes and Larose, 2013), (Snieder, 2006)). Wave velocity depends on the medium properties, therefore UPV method is a very popular technique used in NDE in Civil Engineering. Propagation velocity of the longitudinal (P-wave) through the material (VP) can be calculated as:
Gam, Min Ju (Seoul National University) | Jang, Beom Seon (Seoul National University) | Lee, Dong Beom (Seoul National University) | Park, Sung Gun (Daewoo Shipbuilding & Marine Engineering Co., Ltd.) | Chun, Min Sung (Samsung Heavy Industries Co., Ltd.) | Koo, Bon Yong (Korea Energy Technology Center, American Bureau of Shipping)
The subject of this study is a sea water caisson attached to FPSO. The caisson is exposed to Morison load and global load simultaneously. Drag term in Morison load has nonlinearity due to relative velocity squared. Due to this nonlinearity, analysis should be conducted in time domain. However, time for calculation in time domain is much longer than that in frequency domain. Therefore, development of a simplified method is necessary to get a result which is similar to an exact solution easily and quickly in frequency domain. In this paper, calculation procedure for the exact solution about floating structure like the caisson is suggested. Several techniques are used to get the exact solution in time domain; superposition of response, stretching method, and expression for response in complex plane.
Generally cylinder-type offshore structure attached to a large hull such as a sea water caisson attached to FPSO is subject to Morison load from wave and global load from hull at the same time. Therefore, local stress and global stress should be considered together in structural safety assessment. If Morison load is assumed as linear in calculating fatigue damage ratio of offshore structure, the results may be underestimated in some part and overestimated in other part (Lee, et al., 1999). Thus, nonlinear wave load should be used instead of the linear wave load to prevent this phenomenon. Normally, structural safety assessment can be done in two domains; frequency domain and time domain. In frequency domain, the time used for calculation is short, but the calculation is only available under an assumption that the applied load is linear. Whereas, in time domain, the nonlinear wave load can be considered, but the required time for the calculation becomes much longer, which makes it difficult to be used in early design stage. Hence, combining advantages of both domains is needed. That is to say, it is important suggesting a new method to get a result exactly and simply.
This study has two levels as show in figure 1. Level 2 is performed in time domain to get the exact solution. This is used to verify the result of the new method. Level 1 is carried out in frequency domain, and the simplified method is developed in level 1. Level 2 is usually more complicated and time-consuming than level 1. Both level 2 and level 1 start from motion analysis to get RAOs (Response Amplitude Operator) fluid particle velocity for each depth, 6 DOF (Degree of Freedom) motion, and global stress. In level 2, time history of combined stress is generated from these RAOs and it is used to calculate fatigue damage ratio. In level 1, result can be obtained conducting spectral method for combined stress. Combined stress spectrum is generated by the sum of local stress and global stress spectrum. To get a local stress spectrum, linearization of Morison equation is performed using stochastic method and equivalent method. This paper only covers level 2; analysis in time domain.
Frolova, Anna O. (Far Eastern Federal University) | Bugaev, Viktor Grigorievich (Far Eastern Federal University) | Mamontov, Andrey I. (Far Eastern Federal University) | Antonenko, Sergey Vladimirovich (Far Eastern Federal University) | Shmelev, Alexander M. (Far Eastern Federal University) | Tsimbelman, Nikita Ya. (Far Eastern Federal University)
This research is based on the rigid-plastic material model of the hull (Norsok Standard, A.6.6) and the Mohr-Coulomb model for the soil. The coefficient of friction of the hull on a sand-gravel seabed equals μ = 0.4. Calculations of the pressure on the hull have made using three methods. Thereafter, the following two calculations are made, on the basis of the theory, to describe the process of indentation, of the hull, into the soil. A triangular prism, with a shape designed to model the cross section of the real bow, is used during this calculation.
The plastic deformation experienced during the operation of the ship is a direct consequence of the external loads causing the stress to exceed the yield strength of steel (Galor, W. , 0 Samuli Hänninen. ). Due to the plastic deformation the bow plate acquires a residual deflection and shape is changed from the initial geometry. The plate passes into a plastic state stretched membrane condition. As result of the changes of the initial geometry, the bearing capacity of the plate is increased (Norsok Standard, A.6.9.2). The shape of deformed plate corresponds to the strength, which balances the external pressure. Knowing the residual deflection, the pressure which caused it can be calculated.
The other calculations for the external pressure definition are based on proposed theory of interaction between the seabed and the bow.
According to these calculations, mooring pressure on a ship’s bow is determined by the ratio of the bottom to the square of the area in contact as well as the load-carrying capacity of ground and the hull bow configuration. In order to define pressure on the bow, a formula that takes into account the statics of the loose grounds is offered.
Some equations are given for the calculation of forces, pressures and depths of the seabed ploughing by a ship mooring onto an unequipped (or conventionally equipped) shore. The balance of the equations for the kinetic and potential energies of a moving ship are applied in the solution as are the equations of fluctuation of an impulse and a passive soil pressure is also applied.
Smooth core samples for the laboratory tests cannot always be prepared particularly from weak rocks. Although some simple indirect test methods have been developed to estimate the mechanical properties of rocks, the specimen preparation from soft rock for some indirect tests is still difficult. In order to close this gap, the needle penetration test has been used recently. This study investigates the predictability of physico-mechanical properties of pyroclastic rocks from the needle penetration index (NPI). The block samples of pyroclastic rocks were collected from six different locations of Erciyes (Kayseri) region of Turkey. The needle penetration, the uniaxial compressive strength and tensile strength, density, and porosity tests were carried out on the specimens in the laboratory. The NPI values were correlated to the physico-mechanical properties. Very strong correlations were obtained between the NPI values and rock properties. It was concluded that the physico-mechanical properties of pyroclastic rocks can be predicted from the needle penetration index.
The physico-mechanical properties of rocks are used for designing surface and underground structures in rock mass in mining and civil engineering projects. Smooth core samples suggested by standards for the laboratory tests cannot always be prepared particularly from weak and clay-bearing rocks. Some simple indirect test methods such as point load test and block punch index tests have been developed to estimate the mechanical properties of rocks. However, the specimen preparation from weak and clay-bearing rocks is still difficult for these indirect tests. In order to close this gap, the needle penetration test which is a simple and non-destructive index test was developed.
The needle penetration test is a non-destructive index test which is applicable both in the field and laboratory and does not require any special sample preparation . The test has been used for the estimation of physical and mechanical properties of weak or soft rocks. Although some researchers have suggested some correlations between the physico-mechanical rock properties and the needle penetration index (NPI) for weak rocks, the studies are limited on this area. In this study, the predictability of physico-mechanical properties of pyroclastic rocks from the NPI was investigated.
Liu, Bo (China University of Mining and Technology-Beijing) | Zhang, Gong (Beijing Uni-Construction Group co, Ltd) | Xu, Wei (China University of Mining and Technology-Beijing) | Liu, Lulu (China University of Mining and Technology-Beijing)
Artificial freezing has been proved to be an effective method and technology to prevent water seepage and shaft flooding accidents in soft water-rich rock strata in western area of China. Many studies have been undertaken using frozen rocks to understand the mechanical and physical properties as well as their degradation mechanisms. However, the existing researches on the real-time dynamic damage process of frozen rocks are very limited, so that the damage mechanisms of frozen rock under compressive loading condition remain unknown. In this study, acoustic emission (AE) method and digital microscope were adopted to study the mechanical properties and microstructure of sandstone samples taken from Meilin Temple Mines at depth of 700 m in Ordos, China. The uniaxial compression tests were performed at four different sub-zero temperatures. The AE signals (e.g., energy) were recorded during freezing and loading processes at negative temperatures. The results showed that AE activities mainly occurred in the initial freezing phase and reduced afterwards. In the uniaxial compression test, AE signals were also observed in the initial loading stage, while no obvious results was observed from rock samples in room temperature. Based on the monitored acoustic emission data, the internal micro-crack change of frozen sandstone was revealed and the damage process was analyzed to evaluate the real state of frozen sandstone at the shaft construction site.
Coal has been the primary energy source in China for a long time, and the western region of China is one of the main production areas. In order to mine the coal in this region, vertical shafts are drilled through Cretaceous and Jurassic strata, which are characterized by high water content, low mechanical strength and loose cementation. Hence, it is of critical importance for the mining projects to study the mechanical properties of the typical rock in western areas (Liu et al., 2015).
The pdf file of this paper is in Russian.
Describing changes of stress-strain state during hydrodynamic simulation is currently a problem of great importance. In this paper we consider one of possible approaches to solve coupled hydrodynamic and geomechanical problem.
The bigger mine waste dumps create the greater the issue to encounter in situ ground conditions. Landfill rock waste makes an inadequate strength and economic issue of the reclamation work is more difficult, especially in determination of slope stability. A clear differentiation based on the lump-size rock type and soil distribution and compression in the field of reclamation. GEO5 FEM, Rocklab and Stereonet7 programs performed with four rockfill modeling and stability analysis for A1, A2, A3 and A4 rock fills models where rock fills of the dump slope were limited. Anisotropic rockfill and soil mixture models were made in laboratory scale. Heterogeneous geotechnical parameters were analyzed. Regarding the topographic maps produced in the 1/1000 scale with field work and the structural cross sections of rock fill models conducted on laboratory experiments, the physical and mechanical properties for each A1, A2, A3 and A4 Models. A1, A2 slopes were close to stable state that showed to be slight safety risk. However, GEO5 programs with limestone fill model through FEM program exhibited stability with A3 and A4.
Because of growing urbanization, reclamation of mine waste dumps is required and has to be concerned. New and bigger urbanization area may face to the reclamation issue around the civil structures. The geotechnical parameters of ground are the decisive factor regarding the type of slope stability work and its efficiency (Bieniawski 1967, Cernica 1995, Das 1994).
In the stable ground, the slope has to be actively supported in order to avoid ground settlement. Modified impact resistance of rocks makes use of the 20-30 cm lump massive rock model to provide face scale to 2-3 cm. The indentation by the drilling bit enters in-situ the massive lump rock, where the volume of rock retained can regulate through the advance rate. Depending on the resistivity of the rock volume in the dump, the indentation pressure can be controlled. The pressure can be calculated depending on the slope (bit diameter, overburden depth), geological and hydrogeological conditions and any surcharge in the area affecting the stability alignment (Görög & Török 2006, Görög & Török 2007).
The calculation of slope stability of mixed face conditions of the varied rockfills such as local porous limestone, marly limestone. The waste shale and marly shale is based on the assumption of a linear (hydrostatic) distribution of support pressure over the face, which is in equilibrium with the scaling ground and water pressures in laboratory model (Bishop 1955, Hoek 1970, Hoek 2013, Hoek & Brad 1977). However, evaluation of data is made by GEO5 FEM model program (Anonymous 2009, Anonymous 2009, Pruska 2009). Earth face in-situ measurement systems have demonstrated dynamic non-linear stress distribution with strong fluctuations at times of high tonnage lorries transfer. This showed in particular differences between the phones at the slope face and critical weakness spaces in the dumps.
When dealing with tunnels in difficult ground conditions, the knowledge of the rock mass parameters is of utmost importance for the selection of appropriate excavation methods and support measures. Sections with a high content of fault material or cataclasites form the most challenging stretches during tunneling, and a proper geomechanical characterization is imperative. However, investigating the overall properties is currently a challenging task, originating from difficulties in sample acquisition, sample preparation and laboratory testing. In order to gain insight into the overall mechanical properties of bimrocks an extensive laboratory program was carried out. Artificial bimrocks were fabricated for both direct shear tests and large oedometer tests, covering a wide range of possible block proportions and block orientations. The laboratory tests were accompanied by in-situ tests, allowing the identification of differences between the small- and large-scale tests and the determination of upscaling factors. A straightforward evaluation method is presented, highlighting the effect of block orientation and block proportion on the shear behavior, shear strength and deformation behavior of bimrocks.
Tectonic faults are usually composed of lens-shaped, relatively competent rock blocks surrounded by finely grained cataclastic material (e.g. Medley 2001 and Riedmüller et al. 2001). Hence, their properties are highly anisotropic and depend on the degree of the regularity of the block orientation, the total volumetric amount of the competent lenses as well as the properties of the matrix.
To study the principle mechanical properties of fault material an extensive laboratory program was conducted on both artificial block-in-matrix rocks and real fault material (Pilgerstorfer 2014). For the examination of the mechanical behavior direct shear tests were performed, allowing investigation of the behavior of bimrocks exposed to large strains. Another important issue is the knowledge about the stress dependency of the deformation properties, especially for TBM-advances in weak rock masses. The amount of displacements, which are expected, and the risk for a shield-TBM getting stuck should be known a priori.