Conventional Triaxial Apparatus Research Articles

A novel method for imitating true-triaxial stress path with conventional triaxial apparatus

The stress paths of the cylindrical specimen in the p–q stress space by controlling the ratio of the axial and the radial loading is guaranteed to be consistent with the cuboid specimen, a novel method for imitating true-triaxial stress path by conventional triaxial apparatus was presented. Under the condition that p and q were variables and b was constant, the true-triaxial stress paths were realized by conventional triaxial apparatus strictly and easily. Under the condition that b and p were invariants, the b was used to control the ratio of axial and radial loading to ensure p constant, the method can be used to measure the strength on the π plane. If the tests were conducted at the different p with the same b, the critical state line of different b could be obtained. Under the condition that p and q were constant, the proposed method of nonlinear loading with b as a parameter could be used to design the various stress paths of true-triaxial under the condition of deviatoric stress consolidation, and which could be used to determine the deformation and the plastic flow of soil in 3D space. The proposed method could be used to achieve the equivalent stress path in the p–q stress space to obtain the 3D mechanical properties, and the stress path controlled by stress, strain, and a hybrid of stress and strain. Once the software of conventional triaxial apparatus was developed by the novel method, the measuring range of stress paths could be expanded greatly.

Glass Beads Test with True Triaxial Stress Path Achieved by Conventional Triaxial Apparatus

The impact of fabric anisotropy, fractal dimension, and breakage on the strength and deformation of granular materials were diminished by uniform-size spherical glass beads. Triaxial drained and undrained tests were performed on glass beads based on a novel method to substitute true triaxial stress paths with conventional triaxial apparatus equivalents with varying intermediate principal stress coefficients (b-values). The result indicates that all specimens manifested a noticeable strain-softening phenomenon. The peak strength decreased with increasing b-value, and the specimens showed more pronounced dilatancy. This pattern is similar to the results of the true triaxial test in current research. Compared to the undrained test, the peak friction angle in the drained test displayed a greater variation with varying b-values, which indicated that the mechanical response of glass beads is sensitive to water. This difference provides experimental evidence for comprehending effective stress in granular materials with constant friction coefficients. The experiments reflect the effect of b-value changes on the p-q stress path, as well as on the peak stress ratio, the state transition stress ratio, and the critical state stress ratio. The specimens exhibited a distinct shear band at different b-values ranging from 0.2 to 0.6, which is different from observations in conventional triaxial tests for granular materials.

The Generalized Mohr-Coulomb Failure Criterion

With the construction of supertall buildings such as high earth dams, the linear envelope of the Mohr-Coulomb (M-C) failure criterion fitted to lower confined pressure would significantly underestimate the loading capacity of foundations, causing a huge increase in the amount of earthwork. Given that the M-C criterion has dominated in the stability analysis of geotechnical structures, it is proposed in this study that the M-C criterion remain invariant in form but the cohesion c and the frictional factor f be related to the coefficient of intermediate principal stress b, called the Generalized Mohr-Coulomb (GMC) criterion. In other words, c and f are both functions of b, written as c(b) and f(b). In the simplest way, the GMC criterion for soils, a true three-dimensional failure criterion, can be established by using a piece of conventional triaxial apparatus. The GMC has a non-smooth strength surface like its conventional version. However, we prove from true triaxial tests and the characteristic theory of stress tensors that the failure surfaces in the stress space should be non-smooth per se for b = 0 or 1. Comparisons with other prominent failure criteria indicate that the GMC fits the test data best.

Large diameter laterally loaded piles in sand: Numerical evaluation of soil stress paths and relevance of laboratory soil element testing

This paper uses 3D numerical analyses to investigate the stress path experienced by soil elements around large diameter piles in sand subjected to monotonic drained lateral loading. Inspection of the loading-induced stresses in the soil revealed the multiaxial nature of these stress paths, which are characterised by rotation of one or more principal stress axes. Based on the outcome of the finite element analyses, typical stress paths for different soil elements around the piles are extracted. Such stress paths are then evaluated against those enabled by conventional and advanced laboratory soil element testing. It is found that a combination of tests in the Hollow Cylinder Torsional Apparatus (HCTA) can reproduce most features of the numerically identified stress paths for soil elements around the pile. Unavoidable limitations in laboratory testing are discussed as well as the major challenge in replicating the loading direction with respect to the material axes. Some guidance for the experimental implementation of these stress paths in the HCTA are provided as well as a discussion on the use of conventional experimental equipment, such as the conventional triaxial or simple shear apparatus.

HP-TACO: A high-pressure triaxial compression apparatus for in situ x-ray measurements in geomaterials.

Triaxial compression experiments are commonly used to characterize the elastic and inelastic behavior of geomaterials. In situ measurements of grain kinematics, particle breakage, stresses, and other microscopic phenomena have seldom been made during such experiments, particularly at high pressures relevant to many geologic and man-made processes, limiting our fundamental understanding. To address this issue, we developed a new triaxial compression device called HP-TACO (High-Pressure TriAxial COmpression Apparatus). HP-TACO is a miniaturized, conventional triaxial compression apparatus permitting confining pressures up to 50 MPa and deviatoric straining of materials, while also allowing in situ x-ray measurements of grain-scale kinematics and stresses. Here, we present the design of and first results from HP-TACO during its use in laboratory and synchrotron settings to study grain-scale kinematics and stresses in triaxially compressed sands subjected to 15 and 30 MPa confining pressures. The data highlight the unique capabilities of HP-TACO for studying the high-pressure mechanics of sands, providing new insight into micromechanical processes occurring during geologic and man-made processes.

A New Apparatus for Shear-Seepage Testing at the Clayey Soil-Structure Interface

Abstract In high earth-rockfill dams, large shear displacement exists in the contact area between the earth core and the concrete cushion at the bottom and on both sides of the banks of a river valley. Exposed to a high hydraulic gradient, seepage erosion is likely to occur in this area. A practical measure is to compact a thin layer of clayey soil with high plasticity such that it lies between the earth core and the concrete cushion. It is crucial to evaluate the capabilities of a clayey soil for this thin layer. A new apparatus modified from a conventional triaxial apparatus is developed to test the seepage characteristics of the clayey soil-structure interface under large relative shear conditions. Compared to the previous device, the new device can achieve a larger relative shear displacement that is closer to the actual condition while providing stress and a seepage state with a simple testing procedure. This apparatus consists of a soil-structure model, a vertical loading system, two back-pressure systems, and a monitoring system. The vertical loading system allows the soil-structure displacement to develop at a constant speed. Two back-pressure systems can simulate the high hydraulic gradient and the consolidation state of the specimen. The monitoring system automatically records the force load, the volume of water outflow, and the volume change in the pressure chamber. Basic tests and shear-seepage tests were conducted using this device to validate its efficiency and reliability. The results indicate that the permeability of the soil-structure interface decreases with the shear displacement over a large range. A finite element (FE) analysis was carried out to reveal the strain-stress and seepage state inside the specimen to explain the seepage mechanism.

Strength and Deformation Characteristics of Reconstituted Sand under Different Stress Paths in True Triaxial Tests

To improve the geotechnical stress–strain analysis, the stress–strain behavior of geomaterial under general three-dimensional stress conditions prevailing in the field need to be captured. The true triaxial apparatus is an enhanced version of the conventional triaxial apparatus, which allows to simulate stresses by applying loadings independently in 3 orthogonal directions. This study evaluated the strength and deformation behavior of Bangka sand under true triaxial test conditions. The test specimens were prepared by means of the multi-sieve sand pluviation method. Various true triaxial test stress paths were applied under axial compression, lateral extension, axial extension, and lateral compression with the objective of understanding and developing the empirical correlation of coarse-grained soil strength parameters in axial compression stress paths related to other stress paths. The test results showed that an increase in the value of b, the parameter used to quantify the relative magnitude of the intermediate principal stress to the other principal stresses, resulted in an increase of the internal friction angle and a decrease of the peak stress ratio. In addition it was observed that the Lade-Duncan failure criterion fitted the results of this study better than other failure criteria, namely the extended von Mises, Mohr-Coulomb, and Matsuoka-Nakai failure criteria.

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ∼0.006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities >1 µm s−1 in the coal sample under vacuum-dry conditions but at >10 µm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Post-liquefaction pore pressure dissipation in sand under cyclic stress triaxial testing

Soil liquefaction has conventionally been studied as a pure undrained condition subject to cyclic shearing in the laboratory. However, in the field, ground settlement and expulsion of pore water in the form of sand boils are frequently observed, which conventional triaxial and simple shear apparatus are unable to replicate. In this study, a novel insight of coupled pore pressure generation and dissipation in liquefiable clean sand is discussed with the use of a modified cyclic triaxial testing set-up. Stress-controlled cyclic triaxial tests with controlled pore water drainage far lower than the soil's permeability were carried out to investigate ‘near-perfect undrained’ conditions, which are more representative of field conditions. Comparison of results from three sets of stress-controlled cyclic triaxial testing were carried out: (a) conventional shearing under an undrained condition; (b) shearing under controlled drainage; and (c) undrained shearing until liquefaction, and thereafter continual shearing under controlled drainage. The critical cyclic shear stress ratio where excess pore pressure dissipation dominates excess pore pressure generation is identified from the coupled cyclic shear loading and pore water drainage tests. In addition, this study also noted a critical void ratio where the soil is no longer susceptible to liquefaction even under considerable shearing amplitudes. These findings may offer guidance on the tolerable cyclic shear stress and minimum degree of densification to avoid occurrence of soil liquefaction in the field.

Seismo‐Mechanical Response of Anisotropic Rocks Under Hydraulic Fracture Conditions: New Experimental Insights

AbstractUnconventional hydrocarbon resources found across the world are driving a renewed interest in mudrock hydraulic fracturing methods. However, given the difficulty in safely measuring the various controlling factors in a natural environment, considerable challenges remain in understanding the fracture process. To investigate, we report a new laboratory study that simulates hydraulic fracturing using a conventional triaxial apparatus. We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry. We use anisotropic Nash Point Shale (NPS) from the early Jurassic with high elastic P wave anisotropy (56%) and mechanical tensile anisotropy (60%), and highly anisotropic (cemented) Crab Orchard Sandstone with P wave/tensile anisotropies of 12% and 14%, respectively. Initiation of tensile fracture requires 36 MPa for NPS at 1‐km simulated depth and 32 MPa for Crab Orchard Sandstone, in both cases with cross‐bedding favorable orientated. When unfavorably orientated, this increases to 58 MPa for NPS at 800‐m simulated depth, far higher as fractures must now traverse cross‐bedding. We record a swarm of acoustic emission activity, which exhibits spectral power peaks at 600 and 100 kHz suggesting primary fracture and fluid‐rock resonance, respectively. The onset of the acoustic emission data precedes the dynamic instability of the fracture by 0.02 s, which scales to ~20 s for ~100‐m size fractures. We conclude that a monitoring system could become not only a forecasting tool but also a means to control the fracking process to prevent avoidable seismic events.

Influence of Shear on Permeability of Clayey Soil

AbstractThe variation of stress–strain state induces changes in the permeability of clayey soil and then affects the properties of seepage and consolidation. In previous studies, variations of the permeability of soil in shear process were mainly neglected, and there were no reasonable descriptions of the permeability in such a condition. To study the variation of the hydraulic conductivity of clayey soil in shear process, new triaxial seepage equipment was designed by changing the conventional triaxial apparatus. Based on a series of tests under different confining pressures carried out by this equipment, the influences of the stress–strain state of clayey soil on hydraulic conductivity were investigated. Furthermore, a mathematical model for the hydraulic conductivity of clayey soil under the condition of large shear deformation was proposed. The comparison between the seepage test results and the calculated values of the proposed model indicates that the mathematical model can calibrate the effects of ...

The Dynamic Response of Unsaturated Clean Sand at a Very Low Frequency

A series of cyclic triaxial tests at very low frequency was carried out on unsaturated clean sand in order to quantitatively investigate the influence of the degree of saturation on dynamic response. The conventional triaxial testing apparatus, which is usually used on saturated soil, was employed to test the unsaturated soil with the additional pore air pressure controller. During the series of tests, four different degrees of saturation level (Sr = 55%, 70%, 85%, 98%) were applied to the soil specimen based on a single value of effective confining pressure (σ’3). The results revealed that the application of cyclic loading at a very low frequency occurring continuously triggered the decrease of soil resistance. For degree saturation, Sr = 55% revealed that the resistance of soil was stronger in comparison to another level. Furthermore, the experimental results confirmed that applied cyclic loading induced a change in saturation level before and after testing. In addition, at a certain level of saturation, a phenomenon of settlements was likely to occur and the soil specimen then underwent liquefaction.

A New Triaxial Testing System for Unsaturated Soil Characterization

Suction-controlled triaxial tests have been widely used to characterize unsaturated soils. However, this type of test requires sophisticated and therefore expensive equipment, and is very time consuming because of the low permeability of unsaturated soils. Only a few research universities can afford the equipment, which limits the advancement and implementation of unsaturated soil mechanics. This paper proposes a new triaxial testing system for unsaturated soils based upon minor modifications on the conventional triaxial test apparatus for saturated soils. Instead of controlling suction, high-suction tensiometers are adopted to monitor matric suction variations during constant water content triaxial testing. Also, a photogrammetry-based method is used to measure volume changes of unsaturated soil specimens during triaxial testing. To evaluate the capabilities of the proposed testing system, a series of constant water content triaxial tests were performed on unsaturated soils with different moisture content. Matric suction and volume variations during testing were monitored by the high-suction tensiometers and the photogrammetry-based method, respectively. New methods were also proposed to analyze the test results. Analysis results showed that the proposed system is cost effective and efficient for unsaturated soil characterization.

Experiment Study on Rheological Model of Soft Clay

This paper adopts the conventional tri-axial apparatus to respectively conduct tri-axial rheological tests on soft clay of different confining pressures by multi-stage loading vertical stress, specifically on the rheological behavior of the soft clay in Beijing area. The experimental results show that the deformation of this kind of clay has obvious time effects and nonlinear characteristics. According to the results, the test is to propose generalized Kelvin three unit and five unit model. Power function creep model and time hardening experiential models are used to describe the clay rheological behavior. The results show that, time hardening creep model is an advanced model with high accuracy, simple equations and fewer parameters and establishes a typical clay rheological model that fits Beijing area.

A photogrammetry-based method to measure total and local volume changes of unsaturated soils during triaxial testing

Triaxial tests have been widely used to evaluate soil behaviors. In the past few decades, several methods have been developed to measure the volume changes of unsaturated soil specimens during triaxial tests. Literature review indicates that until now it remains a major challenge for researchers to measure the volume changes of unsaturated soil specimens during triaxial testing. This paper presents a non-contact method to measure the total and local volume changes of unsaturated soil specimens using a conventional triaxial test apparatus for saturated soils. The method is simple and cost-effective, requiring only a commercially available digital camera to take images of an unsaturated soil specimen during triaxial testing from which accurate 3D model of the soil specimen is reconstructed. In this proposed method, the photogrammetric technique is utilized to determine the orientations of the camera where the images are taken and the shape and location of the acrylic cell, multiple optical ray tracings are employed to correct the refraction at the air-acrylic cell and acrylic cell–water interfaces, and a least-square optimization technique is applied to estimate the coordinates of any point on the specimen surface. The paper first discusses the theoretical aspects of the proposed method. An image analysis on a caliper was then used to evaluate the accuracy of photogrammetric analysis in the air. A series of isotropic compression tests on a stainless steel cylinder were used to demonstrate the procedure and evaluate the accuracy of the proposed method, while triaxial shearing tests on a saturated sand specimen were used to exam the capacity of the proposed method for measuring the total and localized volume changes during triaxial testing. Results obtained from the validation tests indicate that the accuracy for the photogrammetry in the air is about 10 µm. The average accuracy for single point measurements in the triaxial tests ranges from 0.056 to 0.076 mm with standard deviations varying from 0.033 to 0.061 mm. The accuracy for total volume measurements is better than 0.25 %.

Improved Isotropically Consolidated Undrained Triaxial Test Method for Non-Self-Supporting Materials

Abstract The non-self-supporting materials that are often encountered in practice include shield tunnel grout, cement mortar, mine tailings, and dredged sludge. These materials typically have characteristics of high moisture content and high flowability. So far, a complete mechanical process (e.g., consolidation and shearing) of the non-self-supporting materials has not been well characterized because the materials of high flowability cannot be tested directly in conventional triaxial test equipment. This paper presents an improved triaxial test method capable of preparing the flowable specimens and testing them in a conventional triaxial apparatus. The procedure includes: (1) a flowable specimen prepared in the split mold and forced to be erective through a small confining pressure applied to the holes built in the split mold, (2) the split mold was detached from the specimen using the specially designed components and remained in the triaxial cell, and (3) the specimen was then subjected to the consolidation and shearing test. The improved triaxial test method was used to test the shield tunnel grout, dredged sludge, and the dry sand in an isotropically consolidated undrained shear condition. The test results indicated the improved triaxial method was used successfully to simulate the stress–strain behavior of the non-self-supporting materials in a holistic manner.

Measurement of small strain behavior of a local soil by fiber Bragg grating-based local displacement transducers

The small strain stiffness of soils has a significant role in geotechnical design and analysis. In laboratory, the small strain behavior is examined by different techniques including the bender element method for measuring the maximum shear modulus and local displacement transducer (LDT) for measuring Young’s modulus at small strains. However, most commonly used LDTs are based on electrical components and have limitations, such as electromagnetic interference and possibility of short circuit in the presence of water. To overcome these limitations, in the present study, a conventional triaxial apparatus has been modified to measure and study the stress–strain behavior of soils at small strains. The major modifications include addition of piezoceramic sensors (bender elements) for measuring the maximum shear modulus and fiber Bragg grating (FBG) sensor-based local displacement transducers (FBG–LDTs) for measuring the stress–strain behavior at small strains. The modified triaxial apparatus has been used to conduct several tests on a completely decomposed granite soil from a site in Hong Kong. The stress–strain behaviors measured by the newly developed FBG–LDTs and an external displacement transducer are compared and discussed. In particular, the shear modulus values obtained using the bender elements, FBG–LTDs, and the external displacement transducer are also compared, discussed, and evaluated. The comparison and assessment show that the FBG–LTDs are more suitable for measuring the small strain behavior of soils.

Summary of collapsible behaviour of artificially structured loess in oedometer and triaxial wetting tests

This paper summarizes experimental studies on wetting-induced collapsibility in loess using single-oedometer, double-oedometer, and triaxial wetting tests. Artificially structural loess samples with interparticle bonding calcite (CaCO3) and a large void ratio were tested in the laboratory to avoid sampling disturbance of natural loess. The comparison between the single- and double-oedometer tests confirms that the wetting-induced deformation is independent of the sequence of wetting and loading. The conventional triaxial apparatus was enhanced for investigating the collapse deformation in response to different water content increments when subjected to different stress levels. The wetting-induced strain subjected to high confining pressure develops in two steps. It increases with increasing water content and reaches a relatively stable plateau, and then increases rapidly again until a final stable state is reached. The initial collapse surface was found by plotting the wetting-induced strain vectors observed in triaxial wetting tests. The wetting-induced strain is negligible when a specimen is wetted at a stress point inside this surface, while it becomes significant when wetted beyond this surface.

Universal Calibration Device for Fluid and In-Soil Calibration of Pressure Transducers

Evaluation of stresses generated at soil-structure interface or within the soil mass influences the design decisions of substructures. Though many analytical techniques are available, actual measurement of these stresses is of prime importance from analytical and construction view point. The measurement of stresses also provides valuable information for validating constitutive theories of geo-material behaviour and computational techniques for examining soil-structure interaction problems. Calibration of pressure transducer involves application of known pressure to the transducer and obtaining the relationship between applied pressure and pressure cell output. In absence of such relationship, interpretation of data obtained using transducers is highly questionable. Present study describes details of indigenously developed universal pressure transducer calibration device, developed by modifying the conventional triaxial apparatus. It allows demanding task of fluid and in-soil calibration of pressure transducers, which was not possible with existing calibration devices. Performance of the developed device checked using three different types of transducers revealed accurate and repeatable results.

Technique for Undrained Triaxial Tests on Unsaturated Soils Using Active Control of Pore-Air Pressure

Abstract Triaxial testing apparatus and procedures for unsaturated soils have become common along with the development of equipment and sensors. Although many drainage conditions are related to the pore-air and pore-water phase, the drained pore-air condition is usually adopted for tests of unsaturated soils. The main reason is that it is difficult to keep the pore-air undrained because the air diffuses easily through a rubber membrane. However, pore-air and pore-water are sometimes not allowed to drain under ground deformation problems. Therefore, undrained conditions in which pore-air and pore-water are not allowed to flow in and out of the soil are achieved in the experiments described herein. First, problems of a conventional triaxial apparatus for unsaturated soils were investigated to produce the undrained conditions. Then, triaxial testing methods under the undrained conditions were developed for overcoming the problems. Finally, the performance of the proposed testing procedure was demonstrated in comparison with results obtained using conventional testing.