Triaxial Compressive Strength Tests Research Articles

Time-Dependence of Mechanical Property Alterations on Anthracite Coals Treated by Supercritical Carbon Dioxide

The injection of supercritical carbon dioxide (Sc-CO2) into coal seams remarkably changes the physical-chemical structures of coal mass and thereby improves coal mechanical properties. In this study, a series of unconfined and triaxial compressive strength tests were performed on anthracite coal samples under the pressure of 8 MPa at the temperature of 35°C for 24 h and 48 h, respectively. Besides, experiments with longer Sc-CO2 treatment time were carried out on coal mass without stress constraint to observe the damage mode of coal. The results show that Sc-CO2 treatment obviously alters time-dependent mechanical properties of anthracite coal. The coal samples treated with Sc-CO2 for 240 h and 960 h show different damage modes from limited fractures to a complicated fracture network. The time-dependence of the mechanical weakening model on Sc-CO2-treated coal was proposed to explain the link between CO2 flow and mechanical weakening effect by means of physical-chemical effects on time scales.

Assessing the effect of freezing-thawing cycles on the results of the triaxial compressive strength test for calc-schist rock

Freezing-thawing (F-T) is an important mechanical weathering process in mountainous areas. Therefore, evaluating the effect of this phenomenon on rock properties is required for design in mining and rock engineering projects constructed in these areas. In this research, the influences of F-T cycles on triaxial compressive strength (TCS) test results were studied for calc-schist rock samples from the Angouran open-pit mine. For this purpose, 1200 m of drilling was performed in the mine in order to prepare the required specimens. About thirty calc-schist samples were tested at F-T cycles of 0, 7, 15, 40, and 75 and lateral stress of 0, 2, 4, 6, 8, and 10 MPa using the servo-control triaxial device. Each weathering cycle was conducted in 12 h of freezing and 12 h of thawing at −20 °C to 20C°, respectively. Changes occurred in texture and microstructures of samples after weathering process were analyzed using the scanning electronic microscope (SEM). The SEM results showed the increase in micro- and macro-cracks in the samples structures and disappearance of cement in their matrix. Also, the results of TCS tests showed an exponential decrease in cohesion and internal friction angle with increasing the number of F-T cycles. As another finding of this study is that increasing the number of F-T cycles leads to a decrease in TCS values. Finally, an empirical equation is developed between TCS and number of F-T cycles for Angouran mine calc-schist.

An empirical strength criterion for the Antalya tufa rock, southern Turkey

This paper formulates an empirical strength criterion for the heterogeneous Antalya tufa rock mass assuming that the heterogeneity effect is represented by the unit weight and the porosity of the tufa rock, which is highly variable from a geomechanical point of view. Unconstrained non-linear regression analyses have been used to identify the strength criterion and material constants in terms of fitting an equation and determining its regression parameters. Prior to this approach, the fitting of the laboratory data to the failure criteria defined by Coulomb (Mem Acad R Des Sci 3:38, 1776), Bieniawski (J S Afr Inst Min Metal 74(8):312–320, 1974) and Hoek and Brown (J Geotech Eng Div ASCE 106(GT9):1013–1035, 1980) has been performed by means of constrained regression analyses. For the regression analyses, the results of the uniaxial and triaxial compressive strength tests along with the Brazilian tensile strength tests have been used. The regression analysis has been performed in two stages. In the first stage, the results of the uniaxial and triaxial compressive strength tests have been included in the analyses; in the second stage, the results of the Brazilian tensile strength tests have been added to the regression analyses performed in the first stage.

Unsaturated behavior of rammed earth: Experimentation towards numerical modelling

This study concerns the coupled hydro-mechanical behavior of rammed earth, which is a real difficulty for the development of this construction technique. Unconfined compressive strength tests on samples conditioned at different relative humidities were performed to determine the variation of mechanical capacity with suction. Both compressive strength and Young’s modulus increase with suction. Further, the effect of shear characteristics with the hydric conditions was studied by direct shear test. As suction induced cohesion contributes a significant part of strength, the apparent cohesion reduced with the reduction of suction. In addition, a considerable variation was observed in the friction angle. Unconsolidated undrained triaxial tests on unsaturated samples were performed to plot the failure envelope for a greater value of normal stress and to complete the failure envelope. These three test makes it possible to put in evidence a non-linearity in the failure envelope over all the suction range studied. Consolidated undrained triaxial tests on saturated samples were performed to determine the intrinsic cohesion and intrinsic friction angle. It was observed that Mohr-Coulomb criterion is not realistic for rammed earth and the failure envelope is non-linear for unsaturated conditions. Thus, a failure criterion was proposed in which both cohesion and friction angle are dependent on normal stress applied. The expression for Bishop’s effective stress for unsaturated soil was evaluated from the results of triaxial and unconfined compressive strength test together. The non-linear failure criterion obtained for a particular suction can be extended to other suction states through Bishop’s effective stress formulation. It can further be used for coupled hydro-mechanical modeling of rammed earth structures.

Mechanical Tests and Numerical Simulations for Mining Seafloor Massive Sulfides

With the decrease of primary resources in recent years, deep seabed mineral resources, especially the massive sulfides, are of extensive research significance. In this paper, firstly, the uniaxial compressive strength (UCS) test and triaxial compressive strength (TCS) test on the seafloor massive sulfides (SMS) samples from three different segments are conducted to obtain the key mechanical properties, including the cohesive force, internal friction angle, compressive strength, elastic modulus and Poisson’s ratio. Then, by leveraging the PFC3D code, the uniaxial and triaxial numerical simulations of SMS are performed. During this process, the micro properties in the simulation are altered through a calibration process until they match the macro properties of the SMS samples measured in the laboratory tests. Finally, the micro properties are applied to simulate the cutting process of single cutting pick and two adjacent cutting picks; meanwhile, the cutting force in the fragmentation process of SMS is monitored and collected. This research can provide some guidance for the mining simulation of SMS and effectively predicting the maximum force on the cutting pick.

Strength of massive to moderately jointed hard rock masses

The Hoek-Brown (HB) failure criterion and the geological strength index (GSI) were developed for the estimation of rock mass strength in jointed and blocky ground where rock mass failure is dominated by sliding along open joints and rotation of rock blocks. In massive, veined and moderately jointed rock in which rock blocks cannot form without failure of intact rock, the approach to obtain HB parameters must be modified. Typical situations when these modifications are required include the design of pillars, excavation and cavern stability, strainburst potential assessment, and tunnel support in deep underground conditions (around σ1/σci > 0.15, where σ1 is the major principal compressive stress and σci is the unconfined compressive strength of the homogeneous rock) in hard brittle rocks with GSI ≥ 65. In this article, the strength of massive to moderately jointed hard rock masses is investigated, and an approach is presented to estimate the rock mass strength envelope using laboratory data from uniaxial and triaxial compressive strength tests without reliance on the HB-GSI equations. The data from tests on specimens obtained from massive to moderately jointed heterogeneous (veined) rock masses are used to obtain the rock and rock mass strengths at confining stress ranges that are relevant for deep tunnelling and mining; and a methodology is presented for this purpose from laboratory data alone. By directly obtaining the equivalent HB rock mass strength envelope for massive to moderately jointed rock from laboratory tests, the HB-GSI rock mass strength estimation approach is complemented for conditions where the GSI-equations are not applicable. Guidance is also provided on how to apply the proposed approach when laboratory test data are not or not yet available.

The application of VSP in the Pilbara

The construction of geological and geotechnical models in typical Pilbara iron ore environments is vital to enable an optimized mine design for the life of the asset, while maintaining pit wall integrity and overall mine safety. Geotechnical assessments require the measurement of geomechanical properties, such as the triaxial shear, direct shear and unconfined compressive strength tests and pressure and shear wave velocities on diamond core samples. Ideally, these velocities would be measured in Reverse Circulation (RC) boreholes as their spatial density is far higher than diamond drilled holes. Unfortunately, despite its value, such data is seldom collected as a large proportion of the holes are above the water table, limiting the use of sonic-logging tools. Even if measurements are possible, damage to the borehole caused by drilling biases the resulting velocity measurements. This paper details the results of a trial using the vertical seismic profile method to directly measure in-situ seismic velocities in RC boreholes. The method was successful in determining the velocities of the formations through the entire length of the holes. The data in several boreholes was of sufficient quality for the application of more advanced processing methods, important for geological mapping and the processing and interpretation of surface seismic data. The success of this first trial has implications for future iron-ore developments in the Pilbara. The widespread acquisition of accurate seismic velocity data is likely to enable the creation of more accurate geotechnical models and could improve future development decisions.

Laboratory Investigation of Deformation and Strength Characteristics of Saline Frozen Silty Sand

A series of triaxial compressive strength tests was conducted to investigate the effect of Na (2) SO (4) and confining pressure on the deformation and strength characteristics of frozen silty sand at 6A degrees C. The initial elastic modulus increased with increases in the salt content, whereas the confining pressure had only a slight effect. An increase in the confining pressure resulted in an increase in strength. The results illustrated that the cohesion of the frozen soil initially increased and then decreased with increase in the Na2SO4 content, whereas the internal friction angle as well as the strength exhibited a decrease followed by an increase with increase in the Na2SO4 content.

Effects of coir fibres modified with Ca(OH)2 and Mg(OH)2 nanoparticles on mechanical properties of lime-treated marine clay

In this study, coir fibres were modified with nanoparticles of calcium hydroxide and magnesium hydroxide via a chemical treatment in order to increase the tensile strength of the fibres and their interaction with soil. To evaluate the modification and its effects, unconfined compressive strength tests, indirect tensile strength tests, flexural strength tests and triaxial compressive strength tests were carried out at 7, 28 and 90 days of curing age on lime-treated soil reinforced with modified and unmodified fibres. The obtained results showed that nano-modification of fibres enhanced the mechanical properties of the lime-treated clay soil due to the tensile strength of the augmented fibres. The results showed that the compressive strength, the indirect tensile strength and the flexural strength of samples treated with modified coir fibres increased by 64%, 122% and 56%, respectively compared to that of samples treated with unmodified fibres. Moreover, an increase in the effective internal friction angle and the cohesion intercept was observed. Also, the results of scanning electron microscopy and energy dispersive X-ray confirmed a desired alteration in morphology of the fibres.

제주도 현무암의 점착력과 내부 마찰각

Volcanic rocks in Jeju Island indicate the differences in geological and mechanical characteristics from region to region, and have vesicular structure caused by various environmental factors. In this study, triaxial compressive strength tests were conducted for intact rocks sampled in northeastern onshore and offshore, southeastern offshore and northwestern offshore of Jeju Island. The estimated cohesion and internal friction angle from the results of triaxial compression tests were compared and analyzed with absorption, a parameter representing the vesicular properties of basalts in Jeju Island. As a result, it was found that the relationship between cohesion and absorption could be classified clearly, considering two different linear relationships in bulk specific gravity and absorption. As the absorption increases, the cohesion decreases exponentially. In addition, the internal friction angle decreases almost linearly with increasing in the absorption, regardless of the relationships in bulk specific gravity and absorption.

Analysis of Äspö Pillar Stability Experiment: Continuous thermo-mechanical model development and calibration

The paper describes an analysis of thermo-mechanical (TM) processes appearing during the Äspö Pillar Stability Experiment (APSE). This analysis is based on finite elements with elasticity, plasticity and damage mechanics models of rock behaviour and some least squares calibration techniques. The main aim is to examine the capability of continuous mechanics models to predict brittle damage behaviour of granite rocks. The performed simulations use an in-house finite element software GEM and self-developed experimental continuum damage MATLAB code. The main contributions are twofold. First, it is an inverse analysis, which is used for (1) verification of an initial stress measurement by back analysis of convergence measurement during construction of the access tunnel and (2) identification of heat transfer rock mass properties by an inverse method based on the known heat sources and temperature measurements. Second, three different hierarchically built models are used to estimate the pillar damage zones, i.e. elastic model with Drucker–Prager strength criterion, elasto-plastic model with the same yield limit and a combination of elasto-plasticity with continuum damage mechanics. The damage mechanics model is also used to simulate uniaxial and triaxial compressive strength tests on the Äspö granite.

Permanent Deformation Model Based on Shear Properties of Asphalt Mixtures

A permanent deformation model of asphalt concrete based on shear properties is developed and calibrated. This model is extended and improved from previous work with the use of laboratory test results. Triaxial compressive strength and repeated-load permanent deformation (RLPD) tests on three types of asphalt mixtures are performed at various loads and temperatures to correlate the shear properties of asphalt mixtures with rutting performance. Test results indicate that the cohesion and friction angle of a given asphalt mixture decrease with increased temperature. Also, the results of RLPD tests indicate that loading and environmental conditions such as deviatoric stress, confining pressure, test temperature, and load frequency significantly affect the permanent deformation of asphalt mixtures and must be incorporated into the prediction model. Three versions of the permanent deformation model based on different input levels are proposed and verified by the test data. Then, the model is calibrated by an accelerated pavement testing data set conducted on a typical pavement structure at various temperatures. The proposed model can successfully predict the permanent deformation of various asphalt mixtures under a wide range of loading and temperature conditions without changing model coefficients.

Influence of Loading Rate on Deformability and Compressive Strength of Three Thai Sandstones

Uniaxial and triaxial compressive strength tests have been performed using a polyaxial load frame to assess the influence of loading rate on the strength and deformability of three Thai sandstones. The applied axial stresses are controlled at constant rates of 0.001, 0.01, 0.1, 1.0 and 10 MPa/s. The confining pressures are maintained constant at 0, 3, 7 and 12 MPa. The sandstone strengths and elastic moduli tend to increase exponentially with the loading rates. The effects seem to be independent of the confining pressures. An empirical loading rate dependent formulation of both deformability and shear strength is developed for the elastic and isotropic rocks. It is based on the assumption of constant distortional strain energy of the rock at failure under a given mean normal stress. The proposed multiaxial criterion well describes the sandstone strengths within the range of the loading rates used here. It seems reasonable that the derived loading rate dependent equations for deformability and shear strength are transferable to similar brittle isotropic intact rocks.

Characterization of Permanent Deformation of Asphalt Mixtures Based on Shear Properties

A permanent deformation model was developed in this study on the basis of shear properties of asphalt mixtures, such as cohesion and friction angle. Two types of laboratory tests—triaxial compressive strength test and repeated load permanent deformation test—for three types of asphalt mixtures with various volumetric properties were performed to correlate the shear properties with rutting performance. A rational approach was proposed to determine flow number (FN) more accurately with the Hoerl model and roughness penalty method. It was observed from the test results that the FN was not accurate enough to differentiate the rutting characteristics of asphalt mixtures. Unlike the FN, the shear stress to strength ratio accurately identified the mixture rutting performance. Because the same value of the shear stress to strength ratio produced the same level of permanent strain regardless of mixture type, the ratio could be used as the rutting criterion for a performance-based mix design. A permanent deformation model was established on the basis of the shear stress to strength ratio. The proposed model is able to successfully predict the permanent deformation of the various mixtures without changing the model coefficients and could predict the permanent deformation all the way up to failure, including the tertiary flow.

A new probabilistic approach to analysis of compressive strength test data of rock specimens

In this paper, a new probabilistic approach has been proposed for dealing with the wide scatter in laboratory values of compressive strength test (uni-axial and tri-axial compressive strength tests) data of rock specimens. This wide scatter is essentially due to randomness in number as well as orientation of micro-cracks. In the proposed methodology, Stanley's approach, which uses Weibull's theory based on the weakest link model, has been modified to analyse the compressive strength test data. Stanley's approach is applicable to poly-axial tensile stress conditions. Design of all underground excavations requires, as input data, uni-axial compressive strength and the strength under poly-axial stress conditions. Data from compressive strength tests have been analysed using Weibull's theory and the proposed approach. Corresponding cumulative distribution functions of the state variable, i.e., the applied stress level, have been obtained and goodness-of-fit tests performed to check the fitness of test data to these statistical distributions. These cumulative distribution functions have been subsequently invoked to correlate the applied stress level at failure and the associated risk of failure. The analysis finds its application in specifying the design strength of rocks or rock masses for a permissible probability of failure.

The effect of suction on the hydro-mechanical behaviour of chalk rocks

The aim of this paper is to study the effect of suction on the hydro-mechanical behaviour of a partially saturated soft rock (a porous chalk) under low confining stress. Such conditions correspond to those of the vertical chalk cliffs of the English Channel, which are subjected to spatial and temporal variations of the water table under climatic variations. We apply to this material a method usually used in soils, whose hydro-mechanical behaviour is strongly modified by changes in matric suction (or capillary suction), according to the degree of water saturation. Under controlled matric suction conditions, wet chalk undergoes volume changes due to pore infilling and drainage. Relationships between matric suction and the mechanical behaviour of the chalk are analysed on the basis of drying–wetting cycles, variations of maximum strength, secant modulus during uniaxial unconfined compression and confined triaxial tests. Mechanical and drying–wetting tests were carried out on dry, saturated and moist natural chalk samples. Drying–wetting cycles result in two types of behaviours according to the range of imposed matric suction, with irreversible paths in the [ s, S r] plane for high gravimetric water contents (i.e. matric suction values between 0.1 and 6 MPa) and reversible paths for low water contents (i.e. high values of matric suction, larger than 6 MPa). For chalk samples initially at their natural water content, the behaviour is intermediate between those of the dry and saturated chalk samples. Comparison of the behaviour of the two chalk formations highlights differences on wetting, which could be linked to the different pore size distributions from one chalk formation to the other. The results of uniaxial and triaxial compressive strength tests at various water contents show elastic–brittle behaviour. Brittle failure occurs at very low water contents. The secant modulus reaches 600 MPa for dry chalk samples and decreases to 250 MPa for quasi-saturated samples. Concerning triaxial tests, the envelopes of maximum strength (failure criteria) show a noticeable shift, with an apparent cohesion varying by a factor of 2 to 3 from dry to saturated chalk. The effect of matric suction is interpreted using an effective stress approach, based on a simple model for granular soils, by considering the expression of the intergranular forces between two particles in an idealised medium at the microscopic scale. It makes it possible to describe the behaviour of chalk in relation to the variations of suction. The capillary effect is well marked at very low values of water saturation, whereas it disappears at high water content values.

Triaxial strength of a fault breccia of weak rocks in a strong matrix

Triaxial strengths are difficult to obtain when the rock is very heterogeneous with respect to particle size and particle lithology and the samples are difficult to obtain and prepare. A fault breccia with clasts weaker than the matrix was investigated. It was found that its strength was related to the proportion of the blocks and the size of the specimen. The volumetric block proportion and texture coefficient were evaluated by core scanning and image processing. Triaxial compressive strength tests were conducted on different sized cores and different proportions of blocks. The most accurate estimations of the lower bound strength were obtained using larger diameter cores and samples with a higher volumetric block proportion.

Evaluating the geomechanical properties of Misis fault breccia (Turkey)

This study investigated the correlations between the uniaxial compressive strength ( UCS), elastic modulus ( E) or the differential stress (Δ σ) and density, P- and S-wave velocity, volumetric block proportion ( VBP), average block diameter ( D ¯ block ) , average block diameter factor ( ABD F), aspect ratio and roundness. The results were evaluated by both simple and multiple regression analysis. Simple regression analysis results show no correlation between UCS, E or Δ σ and the other examined parameters, except S-wave velocity. A correlation was, however, found between UCS and S-wave velocity for 25% and 75% VBP. Other significant models including density, P- and S-wave velocity and textural properties for the prediction of UCS and Δ σ were found by multiple regression analysis. From these results it is concluded that the strength of the Misis fault breccia cannot be defined by a single parameter such as VBP as is the case for some bimrocks. It is also concluded that the models developed by multiple regression analysis can be used to predict the UCS and Δ σ of the Misis fault breccia. The multiple regression models including two or three independent variables are the most practical equations. Using these equations to predict the UCS and Δ σ of the fault breccia is easier, faster and cheaper than conducting triaxial or uniaxial compressive strength tests.

Rock variability and establishing confining pressure levels for triaxial tests on rocks

In teaching rock mechanics, often the theoretical part of the lectures are demonstrated by laboratory sessions, which are helpful in supplementing the teaching of rock mechanics principles to the students. In these laboratory sessions, an unconfined compressive strength (UCS) test can be followed by a conventional triaxial compressive strength (TCS) test on similar rock specimens. The former test is the one used most in the characterization of rock strength and in many rock classification systems; while the latter test has been widely used for the determination of design parameters such as C and f because of the equipment simplicity and convenient specimen preparation and testing procedures. However, the experience of the authors in conducting such sessions has shown that problems are encountered while performing the conventional triaxial compressive strength tests and these can be misleading to the students. The conventional TCS test consists of applying a hydraulic pressure to the external surface of a cylindrical rock specimen and at the same time axially loading the specimen in compression with increasing increments until failure of the rock. The procedure for measuring the rock strengths has been standardized by the American Society for Testing and Materials (ASTM) [1,2] and the International Society for Rock Mechanics (ISRM) [3,4]. Neither in the ASTM nor in the ISRM

Characterization of permanent deformation of an asphalt mixture using a mechanistic approach

Viscoplastic constitutive equations offer improved prediction of the accumulated permanent deformation in comparison with linear elastic or viscoelastic models. An elastic-viscoplastic continuum model is proposed to simulate the permanent deformation of an asphalt mixture at WesTrack. After analysis of vertical compressive stress under the same loading condition and layer thickness at a WesTrack replacement section, strain rates were determined to characterize asphalt mixtures subjected to loading at a represent ative vehicle speed. Uniaxial compressive strength tests were conducted at seven different strain rates to obtain the material parameters of elastic-viscoplastic constitutive model. In addition, triaxial compressive strength tests were performed at a low strain rate and four different confinement pressures to obtain material parameters for the Drucker-Prager yield function. The test temperature was 60°C based on the critical temperature for permanent deformation at the field section. The developed model and its calculated parameters were used in finite element simulation of the Simple Shear Tests at Constant Height (SST-CH) and results were compared to experimental measurements at 60°C. Reasonable agreement was found between predicted results and the experimental SST-CH measurements indicating the validity of the model and its parameters.