Multistage Triaxial Compression Tests Research Articles

Application and comparison of multistage triaxial compression test procedures on reconstituted Ankara clay

The ability to conduct conventional triaxial compression tests on multiple identical specimens is restricted by available sample quantity, sample homogeneity, as well as testing duration. Multistage triaxial testing is an alternative method to tackle these issues by using a single specimen sheared under different confining stresses to attain the strength parameters. Although there are widely accepted procedures to decide when to stop each shearing stage and proceed to the next stress level, the applicability of these procedures on different soil types is still a question. This study examines the applicability of combinations of two multistage triaxial testing procedures (Rational Procedure and Minimum Slope) under two different deviator stress conditions (Sustained or Cyclic) during confining stress increase. The outcome is compared to conventional triaxial test results for undrained and drained shearing of reconstituted specimens of high-plasticity Ankara Clay. Out of the four options, the rational procedure with cyclic deviator loading and minimum slope with cyclic deviatoric loading conditions, are found to give the most accurate strength parameters in reference to single-stage test results. The maximum number of the shearing-reconsolidation sequences applicable before strength loss is also investigated for each multistage triaxial testing procedure.

BARIK: laboratory programme within the framework of the development of an extended Hoek–Brown-based anisotropic constitutive model for fractured crystalline rock

Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behaviour of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the “containment effective rock area” in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. This contribution provides an overview and first results of the laboratory programme, which was performed as a part of the research project. The aim was to generate a fundamental and extensive dataset for an anisotropic material used for verification and validation purposes of the newly developed constitutive model. The rock material for the test programme was “Freiberger gneiss” because of its pronounced anisotropic properties regarding deformation and strength as well as the good access to obtain a larger amount of sample material. A wide range of basic tests have been performed, e.g. determination of density and porosity, measurement of ultrasonic wave velocities to get dynamic elastic properties, Brazilian tests, and uniaxial compression tests to obtain strength data. Additionally, a large number of more complex multi-stage triaxial compression tests with examination of the post-failure region and hydromechanical coupled triaxial compression tests have been conducted or are in progress. The hydromechanical part, in particular, plays an important role in examining and quantifying the evolution of the micromechanical damage process, which changes the permeability and Biot coefficient and therefore the effective stresses inside a saturated rock material. Tests were conducted with different orientations of the structural planes in relation to the loading directions.

Measurement of multi-stage triaxial compression test and static elastic parameters of shale samples from the Talangakar formation for a feasibility study of shale gas in the South Palembang sub-basin, South Sumatra

Shale gas is an unconventional gas source that has great potential to be developed and produced in the future. One area that has great potential in Indonesia is in the South Sumatra basin. For good hydrofracturing planning, we used samples from core rock specimens from field well B in the Talangakar formation at a depth of 2289 m with shale lithology located within the South Palembang sub-basin in the South Sumatra basin. We tested the sample by multi-stage triaxial (MST). MST is a compression test requiring only one rock specimen to be tested in three cycle stages with different confined pressures From this test, the curve of Stress-Strain, yield point, elastic zone and plastic zone is obtained. Then it can also be generated Mohr-Coulomb curve, elastic parameters, i.e: Young’s modulus, Poisson’s ratio, etc. The results of this measurement are also useful for predicting the minimum horizontal stress which is very important in the design of hydraulic fracturing in shale gas.

Recycled Glass Blends with Recycled Concrete Aggregates in Sustainable Railway Geotechnics

This research investigates the static and cyclic characteristics of recycled glass (RG) as a supplementary material with recycled concrete aggregate (RCA) in the rail track capping layer. RG was blended by-weight with RCA in 10% increments up to 50% RG content. A performance-based laboratory testing scheme was designed according to the field loading conditions of capping layers in rail tracks. Basic geotechnical properties of RG + RCA blends were evaluated through their particle size distribution, compaction properties, and California bearing ratio. Effect of flooding was assessed with one-dimensional static and cyclic compression tests. Multistage triaxial compression tests were performed to determine the effect of RG content on shear strength parameters. A new repeated loading triaxial testing protocol was introduced for railway capping layer materials to assess the stiffness of RG blends to cyclic loading. Long term performance of samples also was evaluated through multistage cyclic permanent deformation tests. The shakedown concept was used to assess the permanent deformation results of RG + RCA samples. Results indicated that RG can be used effectively as a supplementary geomaterial in construction of rail track substructure.

Investigation of Fluid-Injection-Induced Coal Stiffness Alteration using a Single-Core Multistage Triaxial Test

Nonlinear stress-strain behavior and failure envelopes are critical rock mechanical data for gas reservoir evaluation and development, especially when the basic rock properties are measured under actual reservoir conditions that include pore pressure. For coalbed methane (CBM) reservoirs, the mechanical properties of coal may be different under actual reservoir conditions because the in situ coal is fluid saturated. Multistage triaxial testing of coal can generate a full failure envelope using a single core instead of a set of destructive core tests—as is the case with single stage testing. This multistage technique is necessary because of the scarcity of actual reservoir specimens. Consequently, in this study, a series of multistage triaxial compression tests on a fluid-saturated coal specimen was conducted to determine the mechanical properties of reservoir coal formations as a function of confining stress and fluid compositions and pressures. An oven-dried coal core 50 mm in diameter and 100 mm in length was prepared. Different fluids were employed in the test, including the non-sorbing gas He and sorbing gases, such as N2, CH4 and CO2, and also H2O. Coal sorption behavior was investigated during the triaxial compression test, and the peak stresses under various pore pressures and confining pressure conditions were measured. In addition, the competition between the fluid-density-induced hardening and adsorption-induced softening of the coal core is discussed. The coal strength under gas-saturated conditions was sensitive to the confining pressure. The coal under a higher confining pressure exhibited a stiffer response and the peak load increased with higher confining pressure. Strength weakening due to gas adsorption was observed for N2, CH4, CO2, and water injection. Both CH4 and CO2 had a strong influence on coal stiffness alteration mainly because the higher adsorption capacity of coal for them. Stress-strain relationships behaved in a non-linear when a sorbing-gas was involved. The softening effect of CO2 injection was complicated by a CO2 phase change at higher pressure conditions. This study is applicable to a wide range of engineering applications, including reservoir stress response, permeability evolution, hydraulic fractures, coal mining, and borehole stability, and fault slipping.

Cyclic behavior of semi-rigid recovered plastic blends in railway track substructure

Plastic deformation of granular layers in railway tracks under cyclic loading is a key factor controlling the substructure’s stability. In particular, the granular subballast capping layer placed between the ballast and subgrade can undergo permanent deformations due to the moving wheel load of trains. This study was conducted to evaluate the cyclic behavior of recovered plastic (RP) blended at different percentages with construction and demolition (C&D) aggregates as railway subballast capping layer. Recycled concrete aggregate (RCA) and crushed brick (CB) were used as the parent C&D aggregates. Multistage triaxial compression tests were initially undertaken to determine the shear strength and the effect of RP inclusion on strength characteristics. The results of multistage triaxial compression tests were also used to find a link between static shear strength and cyclic deformation of the granular recycled materials. The plastic deformability of the RP + C&D blends was also assessed through multistage cyclic triaxial tests at three confining pressures and incremental deviator stress at each confinement. The shakedown theory was employed to characterize the changes in permanent axial and radial deformations, resilient axial strain, and permanent axial strain rate. Inclusion of RP in the RP + C&D blends was found to increase the sample’s deformability particularly in the secondary phase of deformation, reduced the stiffness, and most importantly increased the rate of permanent strain accumulation.

Evolution of shear bands and cracks in multi-stage triaxial tests with water-saturated sandstone: A study of micro-tectonics with a fractal perspective

A series of multi-stage triaxial compression tests revealed quite chaotic reactions of water-saturated Permotriassic sandstones on changing pore water pressures. Not before focussing the various test stages with respect to occurring critical phenomena, which strike the fabric structures due to their fractal nature during any triaxial testing, eventually an approach became tractable: Pore systems with cracks and shear bands can be widened in a quasi-static way, but only up to a stability limit. Thereafter the fabric collapses with a seismogenic contraction of the pore system causing a rise of the pore water pressure. Changes of structural patterns occur as formation either of fissures up to collapse with acoustic emission, or of a rather fractal pattern of crossing shear bands generated without audible noise but with fine grit due to comminution. Dilation and contraction of the pore system are visible from overall volume changes. Both modes of pattern formation can be understood as critical phenomena of the mineral solid. Scale-independent features of these changes of fabric patterns can be transferred to the meso- and macro-scales in the field and in the lithosphere, respectively. However, questions of scale-dependent features related to cohesion should be further investigated.

Investigation of static and dynamic bulk moduli in a carbonate field

Bulk modulus which relates seismic attributes to rock and fluid properties is considered as an important parameter for seismic reservoir characterisation workflows, and can provide useful information for such studies. However, its measurement and modelling approaches are yet to be sufficiently addressed and discussed especially for carbonate rocks. In this research, in order to provide more information about bulk modulus, nine samples from a carbonate oilfield were subjected to static and dynamic investigations under dry and saturated conditions. Then, the dynamic modulus was calculated using well log data and ultrasonic measurements. In order to obtain static data, multi-stage triaxial compression tests were performed on the vertical plugs extracted from conventional cores taken at different selected depths. The results showed that static and dynamic bulk moduli follow similar trends, although the value of dynamic modulus is usually higher. Furthermore, several empirical relationships based on simple linear regression and forward and backward stepwise multiple linear regressions were developed to relate measured static and dynamic bulk moduli for the given field, then proposed equations were evaluated by ANOVA analysis. Besides, dry and saturated bulk moduli were modelled using Xu and Payne rock physics model. Here, saturated bulk modulus was modelled using either dynamic or static dry bulk moduli within the Gassmann’s theory. It is observed that Gassmann’s equation gives a more accurate result using dynamic data rather than static ones. The poor Gassmann’s equation prediction under static condition could be attributed to microcracks in the samples and the uncertainty associated with this theory for complex pore geometry. The outcomes of this study can, furthermore, provide the necessary information and relationships for rock physics (e.g. shear wave estimating and fluid substitution modelling) and geomechanical (e.g. CO2 injection and compaction forecast) studies for exploration and development programs.

Wetting effects and strength degradation of swelling shale evaluated from multistage triaxial test

The multistage triaxial compression test was mainly utilized in this research to investigate the degradation in the strength of swelling shale under different wetting conditions. Shale samples were tested in four different conditions: (1) under their natural moisture content denoted as intact, and after soaking in, (2) water, (3) polymer solution, and (4) bentonite solution, separately. The experimental results showed that a considerable decrease in the shale’s strength was noted after soaking in water and the lubricant fluids (LFs). The polymer solution caused the least decrease in the strength compared to other fluids. Strength envelopes of the tested shale were constructed for intact and soaked states. These envelopes showed fair agreement between the single stage and the multistage triaxial compression test results, indicating suitability of the multistage triaxial compression test for the tested swelling shale.

Deformation and Damage Failure Behavior of Mudstone Specimens Under Single-Stage and Multi-stage Triaxial Compression

In tunnel engineering, due to the effect of excavation disturbance, the surrounding rock mass can produce an excavation damage zone with different damage extents. Therefore, knowledge of rock deformation and damage behavior is especially significant for the design of deep tunnel support. However to date, a few experiments and numerical simulations have been conducted to investigate the deformation and mechanical failure behavior of damaged rocks. Therefore, in this research, multi-stage triaxial compression test was used to investigate the mechanical behavior of mudstone specimens with different damage extents by experiment and two-dimensional particle flow code. First, a group of micro-parameters was calibrated by single-stage triaxial compression experiments of mudstone, and the numerical results agree very well with the experimental results. Then, multi-stage triaxial compression experiment and discrete element modeling of mudstone specimens were carried out. The more axial strain the specimens sustained, the less strength they had (because the degree of damage increased). A damage variable was defined by the ratio of the area of micro-cracks to the total area of the specimen. As the post-stress reducing ratio increases, the damage variable increases rapidly until the post-stress reducing ratio reaches 0.4; then, it remains constant. The force field were analyzed to reveal the damage evolution mechanism in the mudstone specimens under multi-stage triaxial compression.

Geomechanical characterization of Newberry Tuff

In this paper we present the results of a petrophysical and geomechanical characterization program focusing on the rock mechanical properties of welded tuffs from Newberry volcano, Oregon. The rock samples are from drill cores obtained from three wells in the vicinity of the Newberry EGS namely, GEO N-2, GEO N-1, and OXY 72–3. As part of the rock characterization process, petrographic thin sections were prepared and used to describe rock texture and mineralogy. High resolution X-ray CT scanner was also used to provide 3D images of the rock pore structure. Rock porosity and permeability were measured using the gas expansion method and a pulse permeameter, respectively. Triaxial compression tests were performed on the specimens extracted from whole core to determine their Young's modulus, Poisson's ratio, and failure envelopes. In particular, multistage triaxial compression tests were carried out to determine deformation and failure properties, and to establish relationships between petrophysical and mechanical/failure properties. In addition, multistage triaxial shear tests were performed to determine the mechanical properties and shear strengths of the fractures developed in triaxial compression tests. Joint roughness coefficient (JRC) and Joint Wall Compressive Strength (JCS) were obtained through analysis of the shear tests. Joint normal stiffness and shear stiffness were also estimated and it was observed that a higher confining pressure results in higher joint shear stiffness. These studies provide insight into the effects of mineralogy and texture and structure on the strength and deformation properties of the tuff samples tested. Mineral composition and the presence of phenocrysts, argillitic materials, and natural fracture were noted to impact rock mechanical properties. Natural fracture and vein mineralogy also influence fracture development in the samples as well as frictional and shear strength properties of the joints tested. Rock samples with higher clay mineral content were found to be relatively more ductile with minimal increase, or even decrease, in permeability under failure in compression tests. The data set obtained in this study is very useful for stimulation design and interpretation of observed micro-seismicity in relation to permeability enhancement.

Experimental study of microstructure and rock properties of shale samples

The study of shale mechanical properties is very important for well completion and stimulation in shale gas reservoirs. Thus,the geomechanical properties of Paleozoic marine shale in Lower Yangtze Basin were studied.With the application of argon ion polisher,high-solution CT and scanning electron microscope,the shale samples were processed and their microstructures were observed. Besides, mineral compositions of this shale were analyzed by XRD technique. In order to get the compression characteristic of shale, the scratch and uniaxial experiments were performed. In addition, the loading and unloading situations were simulated under different effective stress conditions in order to reduce negative effects of natural fissures and rock relaxation. Finally,the multi-stage triaxial compression deformation experiment was performed on the triaxial rock testing system for calculating cohesive strength and inherent friction angle. The study of shale mechanical properties is veryimportant for well completion and stimulation in shale gas reservoirs. Thus, a systematic experiments was performed on Paleozoic marine shale samples collected from Lower Yangtze Basin, and the microscopic structure,mineral compositions and the geomechanical properties of marine shale samples were studied. Testing result shows that except for shale sample with natural fractures,both the values of Youngs modulus and Poissons ratio increase with the increase of confining pressure. But in the unloading condition, the Young s modulus increases with the decrease of confining pressure,whereas the Poisson s ratio is greater than that in its loading condition. The inherent friction angle is within approximately 40°and the shale sample with high brittleness index tends to crack as fracture network according to the multi-stage triaxial compression test. This research is greatly helpful for the study of shale mechanical properties,and provides technical parameters for well completion design and stimulation.

Spatial acoustic emission evolution of red sandstone during multi-stage triaxial deformation

Multi-stage triaxial compression tests for cylindrical red sandstone specimens (diameter of 50 mm, height of 100 mm) were carried out with a rock mechanics testing system and spatial acoustic emission (AE) locations were obtained by adopting an AE monitoring system. Based on spatial AE distribution evolution of red sandstone during multi-stage triaxial deformation, the relation between spatial AE events and triaxial deformation of red sandstone was analyzed. The results show that before peak strength, the spatial AE events are not active and distribute stochastically in the specimen, while after peak strength, the spatial AE events are very active and focus on a local region beyond final microscopic failure plane. During multi-stage triaxial deformation with five different confining pressures, the spatial AE distribution evolution in the red sandstone was obtained. The obtained spatial AE locations of red sandstone at the final confining pressure agree very well with the ultimate failure experimental mode. Finally, the influence of confining pressure on the spatial AE evolution characteristics of red sandstone during triaxial deformation was discussed. The AE behavior of red sandstone during multi-stage triaxial deformation is interpreted in the light of the Kaiser effect, which has a significant meaning for predicting the unstable failure of engineering rock mass.

Shear strength of microfine cement grouted sands

Unconsolidated–undrained (single and multi-stage) triaxial compression tests were conducted to evaluate the shear strength of microfine cement grouted sands. Microfine cements of three different types were obtained by pulverising ordinary cements produced in Greece. Multi-stage triaxial compression tests can be used dependably for determination of the shear strength parameters of cement grouted sands. It has been observed that the Mohr–Coulomb failure criterion represents adequately the behaviour of the grouted sands. Grouting with microfine cement suspensions improves the strength of sands significantly, and the improvement is primarily controlled by the water-to-cement (W/C) ratio of the suspensions. The positive effect of microfine cement grouting on the shear strength of sands is mainly the addition of cohesion, which is substantial even at a distance of 1·2 m from the injection point. Grouting with suspension, using W/C = 1 provides the sand with cohesion of about 2·6 MPa. The shear strength parameters vary with axial strain, and cohesion attains a maximum value well before failure.

Effect of chemical additives on the strength of sandstone

Chemical alteration of the strength of sandstone has been investigated to establish the fundamental knowledge for chemically assisted fracturing. If the rock strength can be chemically lowered, this technology would be useful to raise the fracturing efficiency. The purpose of the present study is to evaluate the effect of chemical solutions on the strength. To evaluate experimentally the effect, Brazilian tests and multi-stage triaxial compression tests have been performed on Gosford sandstone saturated with solutions of dodecyltrimethyl ammonium bromide (DTAB), polyethylene oxide (PEO) and aluminium chloride (AlCl 3 ). The tensile strength varied with the concentration of chemical additives, became the lowest at certain concentrations which are consistent with the zero zeta-potential concentrations. AE activity is the most active in dry specimen, and the least AE activity is found in the specimen saturated with chemical solution. The uniaxial and triaxial strengths did not vary significantly with concentration of the chemical solutions. In case of compression test where failure mode is dominated by shear failure, there is little effect of chemical solutions on the compressive strength.

Influence of stope geometry and blasting patterns on recorded overbreak

Chemical alteration of the strength of sandstone has been investigated to establish the fundamental knowledge for chemically assisted fracturing. If the rock strength can be chemically lowered, this technology would be useful to raise the fracturing efficiency. The purpose of the present study is to evaluate the effect of chemical solutions on the strength. To evaluate experimentally the effect, Brazilian tests and multi-stage triaxial compression tests have been performed on Gosford sandstone saturated with solutions of dodecyltrimethyl ammonium bromide (DTAB), polyethylene oxide (PEO) and aluminium chloride (AlCl 3 ). The tensile strength varied with the concentration of chemical additives, became the lowest at certain concentrations which are consistent with the zero zeta-potential concentrations. AE activity is the most active in dry specimen, and the least AE activity is found in the specimen saturated with chemical solution. The uniaxial and triaxial strengths did not vary significantly with concentration of the chemical solutions. In case of compression test where failure mode is dominated by shear failure, there is little effect of chemical solutions on the compressive strength.

Results and interpretation of multistage triaxial compression tests : Soranzo, M Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P353–362. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)

Results and interpretation of multistage triaxial compression tests : Soranzo, M Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P353–362. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)