Triaxial Chamber Research Articles

Development and Application of the Large‐Scale Simulation Experimental System for the In Situ Long‐Term Injection and Production of EGS

With the proposal of the double carbon target, the exploration, development, and efficient utilization of geothermal energy is an important field for the energy development. Meanwhile, there also exists a series of challenges. With regard to the injection and production process of enhanced geothermal system, researchers mainly focus on the numerical simulation. As the conceptual model is developed based on several assumptions, the results are unable to truly reflect the heat transfer and seepage process of the fluid. Unfortunately, there are few reports on the large‐scale physical simulation experiments. Considering the present research situation, we have developed a large‐scale physical simulation experiment system in which the in situ long‐term injection and production process of hot dry rock (HDR) can be simulated. The experimental system includes several components: high‐temperature and high‐pressure true triaxial chamber and loading system, multiwell injection system, acoustic emission monitoring system, and flow and temperature monitoring system. By designing the experiments, the performance evaluation of the temperature variation, fluid flow, and heat energy extraction in the long‐term HDR injection and production process can be completed. Taking the granite outcrop rock sample, which is from the Gonghe Basin in Qinghai Province, as an example, we have conducted the long‐term injection and production experiment. The production temperature and flow rate were monitored and discussed. The results showed that the experimental system can simulate the heat transfer and seepage process of the fluid. More working conditions can be designed for the research of the long‐term injection and production process. Hopefully, this will provide an experimental platform for the efficient development of HDR.

Development and applications of the quasi‐dynamic triaxial apparatus for deep rocks

AbstractThe mechanical behaviors of deep rocks have always posed a challenge for the implementation and safe operation of major underground engineering projects. To this end, this study modified the existing mainstream rock mechanics instruments equipped with a dynamic disturbance loading system and developed a second‐generation TFD‐2000/D triaxial instrument. The first‐generation device is equipped with an independent disturbance system and an advanced EDC‐580 all‐digital servo controller, which can apply disturbing load independently, implement the function of cyclic disturbance, and combine dynamic and static disturbances. The instrument was found to be reliable for use in analyzing the damage process of rocks in the disturbance test of marbles. The second‐generation instrument tackles three limitations of the first‐generation instrument: (i) it upgrades the strain measurement system and uses extensometers with linear variable differential transformers to accurately measure deformation; (ii) it uses the self‐balanced chamber to replace the Hoek–Franklin triaxial cell and auto‐balancing triaxial pressure chamber; and (iii) the loading rod is independently equipped with an EDC‐580 all‐digital servo controller, which measures precise loads. The experimental findings confirmed that the second‐generation instrument can be used for rock mechanics testing under cyclic disturbance loading, the disturbance–stress relaxation cycle, and the creep–fatigue cycle. In this sense, the second‐generation instrument can be a useful addition to deep rock mechanical instruments and provide a valuable reference.

A 3D image-based method to measure soil stiffness in triaxial tests

This paper describes a new simple and inexpensive method for accurately measuring the three-dimensional deformation of soil specimens during triaxial tests. The method incorporates a cost-effective image capture system that can be applied to any triaxial apparatus as well as an automated post-image analysis approach for processing the images acquired during triaxial tests. Image capture is performed using multiple low-cost Raspberry Pi cameras and accessories placed outside the triaxial chamber. The system employs particle image velocimetry, camera calibration and stereophotogrammetry to determine the spatial variation in axial and radial sample strains as samples are loaded. It is demonstrated that axial and radial displacements can be measured with an accuracy of 1–2 μm, respectively, and it is verified by comparison with data from triaxial tests on sand samples that the full nonlinear stiffness–strain degradation of soil samples from fully elastic conditions can be determined without the need for any additional sensors.

Sand modification with a saponite clay suspension as a waste of the diamond mining industry

The results of the laboratory water permeability tests of a sand modified by a saponite clay fraction from the diamond mining industry waste are presented. The filtration of clay suspension through a landfill ground bed as a method of the additive enrichment is approved. The experimental apparatus consisted of the triaxial test chambers and semi-automatic devices for water and suspension supplying. The chambers excluded a sidewall leakage in the samples and provided required values of the vertical and horizontal stresses when measuring permeability. The samples of an alluvial fine sand and a sand mixed with 3 and 5 % a chalk and a dolomite were investigated. After preliminary saturation the rate of distilled water flow through the samples were determined at four values of hydraulic gradient. Then the filtration of a suspension containing up to 0.58-0.63 % clay particles was conducted. After that, the pore disks at the top of samples were washed by circulating water flow. At the last step of experiments the velocity of water flow was measured again at four values of hydraulic gradient. The experiments indicated that the modification of the sand by clogging the pores by the saponite clay fraction from a suspension flow is possible if a sand comprises the additives causing the aggregation of clay particles. The hydraulic conductivity of a sand with 3 % chalk decreased by 15-31 times, with 5 % - by 15-39 times at different values of relative compaction. The effect of a dolomite addition was not significant, the hydraulic conductivity decreased by a maximum of 14.3 times. To achieve the maximum depth of clogging, it is recommended to filtrate the suspension through a loose sand at a hydraulic gradient that provides a flow rate more than 3.5 m/day, after that, the sand layer should be compacted.

Механические свойства базальтового щебеночного балласта

Purpose: Study of strength and deformation properties of basalt broken stone ballast depending on its grain composition, the properties are necessary for further assessment of bearing capacity and deformability of railway track ballast layer. Methods: Determination of mechanical properties of broken stone ballast from basalt rock was carried out in a triaxial compression chamber which intense state close to real operating conditions was simulated. In the trial process, the dependences of axial and volumetric deformations from tension deviator were being determined. The methods of trial result statistical processing and regression analysis were used in the work. Results: According to laboratory test result statistical processing, the key characteristics of broken stone ballast strength as well as deformation modulus and Poisson’s ratio depending on the ballast grain composition were determined. Practical importance: The study results make it possible to elaborate recommendations regarding broken stone ballast grain composition, the ballast is from the basalt used in the areas of especially heavy traffic of trains.

Nonlinear Seepage Behaviors of Pore-Fracture Sandstone under Hydro-Mechanical Coupling

This work focused on the nonlinear seepage behaviors of flow in pore-fracture media. Natural sandstones were selected to prefabricate single-fracture specimens with different inclinations (0–90°). Seepage tests of combined media were performed under different confining pressures (8–10 MPa) and different water pressures (3–7 MPa) in a triaxial pressure chamber. The fitting analysis of experimental data showed that Forchheimer’s law described the nonlinear characteristics of flow in the pore-fracture media. Linear term coefficient a and nonlinear term coefficient b of the sandstone samples with different inclinations changed more obviously with the increased inclination. When the fracture inclination was greater than 30°, a and b values had a sudden jump. The nonlinear inertial-parameter equation of fluid flow in pore-fracture media was proposed based on non-Darcy flow coefficient β and inherent permeability k. The applicability of the following methods to evaluate Darcy’s law was discussed, including normalized hydraulic conductivity, pressure gradient ratio, and discharge ratio. The three methods were able to determine critical parameters and distinguish linear and nonlinear flow. Furthermore, it was specified for the first time that when β was negative, critical nonlinear effect E was −0.1, and Forchheimer’s coefficient F0 was −0.091. In the −∇P-Q relationship, the fitting curve was convex to the −∇P axis, and the increase of Q was higher than the linear increase, presenting the nonlinearity of overflow. On the one hand, the fractures and pores were compressed under the confining pressure due to the prefabricated fractures of different shapes and different inclinations. A higher seepage water pressure was needed to stabilize the seepage system with the excessive flow rate. On the other hand, the barrier effect of the fluid inside the rock was completely lost because the fluid expanded the seepage channel. Its permeability was changed, leading to seepage instability.

Experimental Study on the Influence of Freezing Pressure on the Uniaxial Mechanical Properties of Ice

In this study, a test technique that enables continuous control of the sample stress state from freezing to testing is proposed to investigate the influence of freezing pressure on the mechanical properties of ice under uniaxial compression. In this method, the water is frozen into the standard cylindrical ice specimen under high hydraulic pressure in a triaxial pressure chamber, and then, the temperature field and stress field of the ice specimens are adjusted to the initial state of the test; finally, an in situ mechanical test is conducted in the triaxial chamber. The uniaxial compression test of ice specimens with temperature of −20°C and freezing pressure of 0.5–30 MPa is performed in the strain rate range of 5 × 10−5−1.5 × 10−6 s−1. The results show that, as the freezing pressure increases, the ductile-to-brittle transition zone of the ice specimen during failure moves to the low strain rate range, and the failure mode of the specimen changes from shear failure to splitting failure. Further, the brittleness index of the ice specimen first increases, then decreases, and then again increases with the increase in freezing pressure. The brittleness index reaches the maximum (minimum) when the freezing pressure is 30 MPa (20 MPa). The peak stress of the ice specimen also increases first, then decreases, and then increases with the increase in freezing pressure. The maximum value is also at the freezing pressure of 30 MPa, but the minimum value is obtained at the freezing pressure of 0.5 MPa. The failure strain of the ice specimen first decreases and then increases with the increase in freezing pressure, and the maximum (minimum) value is achieved at the freezing pressure of 0.5 MPa (10 MPa). When the ice specimen exhibits brittle failure, the relationships between the residual stress and the freezing pressure and between the peak stress and freezing pressure are the same, but when the ice specimen exhibits ductile failure, there is no obvious relationship between the residual stress and the freezing pressure.

An apparatus for measuring water content of unsaturated soil based on the van der Pauw principle

Understanding water migration in soil specimens under triaxial test conditions is of great significance for investigating the mechanism of distress in railway subgrades. However, the water content distribution along the specimen height is difficult to determine with current equipment. Motivated by such limitations, we developed an apparatus relying on the van der Pauw principle, to measure the water content based on the specimen’s electrical properties. Additionally, a series of exploratory tests on unsaturated soil specimens were conducted to evaluate the performance of this apparatus. To verify the practicability of the apparatus, a water-supply dynamic triaxial test was also carried out with an unsaturated dynamic triaxial test system (GDS). The results indicate that this apparatus is stable, as the measurement accuracy is not influenced by the electrode position or the soil geometry or dimensions. In a narrow triaxial pressure chamber, this apparatus can be used to determine the distribution of water content in unsaturated specimens nondestructively and consecutively during a triaxial test. The maximum error obtained from the apparatus when measuring the gravimetric water content was 0.7%. The soil resistivity calculation function was obtained considering the dry density and gravimetric water content with high correlation and precision. This apparatus has broad applications for investigating the mechanism of water migration within some geotechnical materials.

Experimental Study on Shear Behavior of a Rock Discontinuity Under Various Thermal, Hydraulic and Mechanical Conditions

Discontinuities in rock mass behave as weak planes, and it is crucial to understand their behavior when assessing the stability of underground structures. Shear characteristics of discontinuities are usually affected by the interaction among overburden stress, tectonic stresses, water pressure by groundwater level and temperature at depth. Thus, it is necessary to evaluate the variation of the frictional properties of rock discontinuities at different conditions. In this study, a series of shear tests were carried out for three types of rocks (Daejeon granite, Goheung diorite, and Linyi sandstone) having a saw-cut surface to investigate the shear characteristics of a rock discontinuity under various thermal-hydro-mechanical conditions in a triaxial compression chamber. In addition to the tests on saw-cut specimens, the effect of surface roughness on shear characteristics was examined. Cement mortar was used to reproduce identical rough discontinuities having JRC values of 2 and 12. The testing conditions were determined considering in situ conditions at the vicinity of underground opening such as radioactive waste disposal facilities, enhanced geothermal systems, and oil reservoirs. The experimental results were analyzed based on Coulomb’s and Patton’s failure criterion. It was observed that the shear characteristics of rock discontinuities were sensitive to confining and water pressure but not to a temperature below 80 °C. XRD analysis and SEM observation were performed to identify the mechanism responsible for the change of friction angle. Clay minerals having a layer lattice structure that results in weakening of the bonding of minerals may have reduced the friction angle.

Method Improvement and Effect Analysis of Triaxial Compression Acoustic Emission Test for Coal and Rock

In the study of the acoustic emission (AE) characteristics of rock samples or coal samples under triaxial compression conditions, most scholars carry out relevant experiments by placing the AE detector on the outer wall of the triaxial chamber of the rock mechanics test system. Owing to the continuous obstruction of AE signals by hydraulic oil in the triaxial chamber and the frequent interference of external noises, the final experimental data cannot objectively and truly reflect the essential characteristics of AE of rock or coal under triaxial compression conditions. It is difficult to scientifically guide and accurately predict precursory information of rock’s or coal’s rupture and instability. Based on this, a series of improvements and optimizations were made to the original triaxial compression AE test method, which is based on the modification of the communication interface of the rock mechanics test system, a test head which can put the AE detector into the triaxial chamber and withstands high confining pressure, in order to obtain the true, comprehensive, and reliable AE signals. It is of considerable significance to the scientific determination of the precursory characteristics of rock’s or coal’s rupture and instability.

Development of a Self-balanced Rock True Triaxial Compression Instrument

Based on the analysis of the structure and the performance of similar instruments at home and abroad, a Self-balanced Rock True Triaxial Compression instrument (SRT) was developed. For SRT, complex stress realization, based on three-dimensional stress alone control and change; and the self-balanced structure is adopted on the basis of frame design, which makes the instrument simple and compact; design of lateral composite press plate with embedded rolling roller, the influence of test results on the effect of friction on the side end surface of the specimen under lateral stress constraint can be eliminate effectiveness; The design scheme of transverse double sliding rod is adopted, its validity can be designed by horizontal alignment, and the problem of eccentric compression of specimen can be partly alleviated in horizontal direction. By changing the alkali of the mothed triaxial chamber, both true triaxial and conventional triaxial compression can be tested. Test results of true triaxial and conventional compression of rocks with different lithology were carried out by SRT. The results, such as the deformation and failure characteristics of rock strength in complex stress conditions, have been obtained by SRT, which indicates that the design of SRT is feasible and feasible. The development of this instrument can promote the further development of Chinese rock triaxial testing machine.

Mechanical Consequences of Suffusion on Undrained Behaviour of a Gap-Graded Cohesionless Soil - An Experimental Approach

Fine particles may migrate in the preexisting pores of an internally unstable soil matrix caused by water flow. This migration changes the fine particle distribution and content at different zones and can affect the mechanical properties of these soils. Due to the different roles that fine particles can play in the force chains of an internally unstable soil, the available geometrical assessment methods do not predict post-erosion behavior of the soil. The fine particles may sit loose in the voids, provide lateral support for the primary soil matrix, or participate directly in stress transfer. This will depend on the fine content, particle size distribution, constriction size, relative density, stress path, and particle shape. However, to evaluate the post-erosion behavior accurately, computational modelling or experimental investigation needs to be conducted. A modified triaxial apparatus connected to a water supply system and collection tank was developed to investigate the post-erosion behavior of an internally unstable cohesionless soil under different loading patterns in undrained conditions. This system allowed all test phases to be completed, including erosion inside the triaxial chamber to remove any possible impact of specimen disturbance. The results suggest that the undrained shear strength of the eroded specimen increased at small vertical strains (0–4 %) under monotonic and cyclic loadings, whereas the initial modulus of elasticity remained unchanged. Also, the eroded specimen showed much higher resistance against cyclic loadings, whereas the non-eroded specimen was liquefied during less than five cycles of loading. This improvement was due to a better interlock between coarse particles due to erosion of fine particles. The hardening strain behavior of the non-eroded specimen changed to limited flow deformation due to a decrease in the fine content. The flow deformation of the eroded specimen at medium strain may be due to the local increase in lubrication effect of fine particles in the eroded specimen.

True Triaxial Apparatus With Rigid-Flexible-Flexible Boundary and Remolded Loess Testing

Abstract Several kinds of true triaxial apparatuses have been devised and applied to obtaining the strength and deformability behavior of soils under different stress paths. However, for these true triaxial apparatuses with flexible loading boundaries boundary interference is still not solved. This paper presents a new type of true triaxial apparatus, with rigid-flexible-flexible boundaries, which addresses the boundary interference using four division strip-platens. This apparatus applies three principal stresses on a 70-mm cuboidal specimen from two rigid platens along the vertical direction and four hydraulic rubber bags around four side faces of the cuboidal specimen. The rigid platens and hydraulic rubber bags were assembled in a true triaxial chamber and driven by three servo-hydraulic cylinders. Four division strip-platens are installed along the lateral edges of the confining side wall of the chamber to separate adjacent rubber hydraulic bags and can adjust themselves to accommodate dimensional changes in the deforming specimen. Therefore, this apparatus does not only allow independent control of stresses applied in three dimensions to a cuboidal specimen but also resolve the boundary interferences. The effectiveness of this apparatus was confirmed by tests on remolded loess under five stress paths with the intermediate principal stress parameter b = 0, 0.25, 0.5, 0.75, and 1. The test results show that the stress–strain relationship, volume strain, and failure strength of remolded loess depend on the stress path.

Effects of Different Mining Methods on the Strength Behavior of Gas Hydrate-Bearing Sediments

The mining of methane hydrate from permafrost can cause the permafrost strength decreasing, softening and straining. It will be helpful to understand effects of different mining methods on the mechanical properties of hydrate-bearing sediments. In this paper, two kinds of hydrate dissociation methods (depressurization and heating) were conducted in triaxial pressure chamber. The mechanical properties of methane hydrate-bearing sediments after dissociation were measured by a low-temperature and high-pressure triaxial compression apparatus under exhaust and non-exhaust shear test. In addition, triaxial shear test of the CO2 and CH4 hydrate-bearing sediments prior to dissociation were also carried out in the identical conditions. The experimental results under the two kinds of dissociation methods showed that the strength of the methane hydrate-bearing sediments after dissociation is less than that before. The shear strength under the consolidated exhaust and consolidated non-exhaust shear tests was also quite different.

Experimental Tests of the UHPC in Triaxial Compression

An experimental study of the ultra high performance concrete (UHPC) subjected to triaxial compression is presented in this paper. The examined ultra high performance concrete was developed at the Faculty of Civil Engineering at CTU in Prague from the components locally available in the Czech Republic without using any special mixing technique or curing procedure. The uniaxial compressive strength was determined to be 123 MPa and 149 MPa for cylinders and cubes, respectively. Behaviour of the UHPC in triaxial compression was investigated in two different ways. In the first way cylinders were tested in triaxial chamber. The cylinders were 200 mm high and 100 mm in diameter. The cylinders were tested in triaxial chamber under lateral stresses equaled to 10, 20 and 30 MPa. The second way of testing UHPC in the triaxial compression was a triaxial loading machine where cubes were tested. The size of the cubes was 100 mm. The cubes were tested under the lateral stresses equaled to 15, 30, 60 and 90 MPa. Two equations for both cylinders and cubes showing the strength development were provided using power law regressions. The difference between triaxial strength development of cubes and cylinders was slight. The results obtained in this study were in good agreement with the results gained from the literature. It was experimentally verified that the strength development of the UHPC under different levels of the confinement pressure is not linear and tends to follow the power law function. In addition it was stated that the permissible stress surface of the UHPC falls below the permissible stress surface of the normal strength concretes, especially within the higher confinement pressures.

Determination of the Small-Strain Stiffness of Hard Soils by Means of Bender Elements and Accelerometers

Direct determination of seismic wave velocities in the laboratory is becoming common practice worldwide, given its great potential in the definition of the stiffness at very small strains. One of the techniques for seismic wave measurement makes use of piezoelectric transducers, such as bender elements (BE). However, some limitations remain to the applicability of this technique, namely for stiff geomaterials, such as compacted soils, naturally or artificially cemented soils and soft or weak rocks. To overcome this issue, two accelerometers have been used in conjunction with BE. In the present paper, this combined test setup implemented on a stress-path triaxial chamber will be detailed. An application study will be presented for a hard soil, prepared by laboratory compaction and tested in triaxial compression at different isotropic stress levels. The equipments, procedures and interpretation analyses will also be described. The advantages of this setup are twofold: (1) the interpretation of the acceleration measurements is straightforward, since the signals are of the same nature; (2) these measurements can be used to verify the BE signals, and thus minimize the subjectivity of the interpretation of BE results. Additionally, the accelerometers can be used autonomously wherever the interpretation of BE becomes too complex. The results of this research enabled to validate the interpretation methods used for BE testing. Moreover, this combined setup of transducers provided a simple yet powerful tool for eliminating the subjectivity inherent to BE testing, enabling reliable measurements of small-strain stiffness for a wide range of materials.

Stress-Strain Degradation Response of Railway Ballast Stabilized with Geosynthetics

Large cyclic loading on ballasted railroad tracks is now inevitable owing to an increased demand for freight and public transport. This leads to a progressive deterioration and densification of railroad ballast and consequently to the loss of track geometry and differential settlement. Understanding these complex stress-strain and degradation mechanisms is essential to predict the desirable track maintenance cycle, as well as the design of new track. This paper presents the results of cyclic drained tests and numerical studies carried out on a segment of model railway track supported on geosynthetically reinforced railroad ballast bed. The relative performance and effectiveness of single- and dual-layer configurations of geosynthetic reinforcement was evaluated using a large-scale prismoidal triaxial chamber. Laboratory tests on unreinforced and reinforced railway track were simulated in a numerical model, and the results were then analyzed to better understand the distribution of displacements and stresses inside the railroad ballast layer. It was observed that in view of strain and breakage control, both the type of reinforcement and its layout played a vital role in improving the capacity of the track. These laboratory test findings were supported by the predictions from an advanced elastoplastic numerical analysis.

Seismic measurements in sand specimens with varying degrees of saturation using piezoelectric transducers

Small-strain seismic measurements in sand specimens were undertaken in the laboratory using piezoelectric transducers. The measurements involved determining both constrained compression wave and shear wave velocities, Vpand Vs, respectively. The piezoelectric transducers were discs (PDs) for Vpmeasurements and bender elements (BEs) for Vsmeasurements. An instrumented triaxial chamber (ITC) was developed and associated support instrumentation was assembled to perform the seismic measurements. The same PDs and BEs were installed in a combined resonant column and torsional shear (RCTS) device. The soil tested in both devices was washed mortar sand. The sand was tested in dry, unsaturated, and saturated conditions in the ITC, while only dry sand was tested in the RCTS. Development, calibration, and operation of the PDs and BEs are discussed. Example waveforms are presented, which are associated with different stress levels, saturation conditions, and frequencies of excitation for P and S waves. A critical factor in performing successful measurements is the driving of each type of piezoelectric transducer at the optimum frequency, which depends on the effective confining pressure ([Formula: see text]), degree of saturation, and soil type. The excitation frequency used for PDs was found to be important when the sand specimens were at nearly or fully saturated conditions.

Behavior of Cemented Reticulate Red Clay

Mechanical properties of cemented reticulate red clay were studied in present research. Test results show that: (1) shear strength of cemented reticulate red clay increase exponentially as the increasing of cement content; (2) shear strength of cemented reticulate red clay decrease polynomially as the increasing of water content; (3) mechanical properties of cemented reticulate red clay affected significantly by curing time and curing confined pressure. In order to study viscous properties of cemented reticulate red clay, cemented reticulate red clay specimens were compressed in triaxial chamber at different shear rate. Results of the compression tests show that it is obvious that the axial strain-deviator stress relationship of cemented reticulate red clay affects by the strain rate.

Behavior of Cemented Reticulate Red Clay

Mechanical properties of cemented reticulate red clay were studied in present research. Test results show that: (1) shear strength of cemented reticulate red clay increase exponentially as the increasing of cement content; (2) shear strength of cemented reticulate red clay decrease polynomially as the increasing of water content; (3) mechanical properties of cemented reticulate red clay affected significantly by curing time and curing confined pressure. In order to study viscous properties of cemented reticulate red clay, cemented reticulate red clay specimens were compressed in triaxial chamber at different shear rate. Results of the compression tests show that it is obvious that the axial strain-deviator stress relationship of cemented reticulate red clay affects by the strain rate.