Triaxial Apparatus Research Articles

Experimental Study on the Liquefaction Mechanism of Iron Ore Fines Cargoes

The liquefaction of Iron Ore Fines (IOF) during maritime transportation has got more attention in recent years. To study its liquefaction mechanism, the undrained monotonic loading tests on saturated IOF were conducted by using compression tri-axial apparatus. The static liquefaction behaviors of IOF under different densities and confining pressures were investigated. And different phenomenon has been observed on the IOF specimen. For the loose IOF specimen, slight dilate was observed and the phase transformation occurs in its stress-path curve under a high confining pressure. There is no phase transition in the intermediate density IOF sample, which shows more complex behavior. For the dense IOF specimen, a contractive behavior appears firstly then a long dilate starts. In view of these abnormal behaviors, the flow pattern failure mechanism of IOF is discussed, which provides a basis for the safe transportation of this kind of goods.

Deformations and stability of granular soils: Classical triaxial tests and numerical results from an incremental model

Abstract This article presents a modified incremental model describing pre-failure deformations of granular soils under classical triaxial conditions. The original shape of equations has been proposed by Sawicki and Świdziński [40, 41]. A new form of equations that are consistent with the proposed definitions of deviatoric loading and unloading is suggested. Triaxial tests necessary for calibrating the proposed model have been performed. The modified model is used to simulate the deformations and stability of sand for every pre-failure loading path and makes it possible to describe the behaviour of granular soil under both drained and undrained conditions. A comparison of experimental and numerical results is presented. All investigations were performed in a classical tri-axial apparatus.

Experimental study to evaluate the effect of particle size on the small strain shear modulus of coarse-grained soils

Abstract Coarse-grained soils are found commonly in nature and are extensively used for a wide range of engineering applications, such as slopes, earth dams, and railway foundations. This study presents an experimental investigation on the effect of mean particle size D50 on the small strain shear modulus Gmax of coarse-grained poorly-graded and well-graded soils. Combined with particle morphology analysis, a series of shear wave velocity tests were performed on pure sand, pure gravel and sand-gravel mixture specimens by taking advantage of a torsional vibration measurement device in a large scale triaxial apparatus. The results demonstrate that Gmax is insensitive to D50 for uniform sand and gravel specimens. On the other hand, Gmax increases gradually with D50 for well-graded sand-gravel mixture specimens. The discrepancy in the relationship between Gmax and D50 for poorly- and well-graded soils is related to the degree of size polydispersity in the specimens.

Resilient and permanent deformation of unsaturated unbound granular materials under cyclic loading by the large-scale triaxial tests

The road base and subbase are normally situated above the underground water table and thus in the unsaturated state. To study the cyclic behavior of unbound granular materials (UGMs) in the road base and subbase courses under controlled matric suction conditions, a series of cyclic tests on the unsaturated UGMs were conducted by the large-scale triaxial apparatus. The large-scale triaxial apparatus was upgraded using an unsaturated module to control the matric suction by the axis-translation technique. Emphases were placed on the effects of matric suction and cyclic stress amplitude on the accumulated axial strain and resilient modulus of UGMs. Test results showed that the increase in matric suction reduced the development of accumulated axial strain exponentially under different cyclic stress amplitudes. Based on the shakedown theory, both the plastic shakedown and plastic creep limits were determined, which tended to grow linearly with the increase in matric suction. A new empirical model was proposed to predict the accumulated axial strain of unsaturated UGMs. Moreover, the increase in both matric suction and cyclic stress amplitude led to the increase in resilient modulus, and linear relationships existed between the resilient modulus and matric suction.

Stress-strain Relationship in Homogeneous and Two-layered Triaxial Test Specimens

The stress-strain relationship of a homogeneous specimen, obtained from triaxial compression test, allows to determine stiffness parameters for numerical-method based analyses in common geotechnical software. Stiffness parameters are defined as the ratio of stress to strain along an axis. However, when a heterogeneous specimen is tested, the equivalent elastic modulus that represents a simplification of the nonlinear behavior is complex. This paper presents a study intended to contribute to the debate about the degree to which conventional soil mechanics approaches can be applied to layered specimens. Triaxial compression tests were carried out on both homogeneous and two-layered specimens under a low effective confining pressure of 30 kPa. The triaxial apparatus was chosen since the applied stress and specimen boundary conditions are well defined, and the repeatability of the test method is good. The behavior of both specimens was studied in terms of the stress-strain relationship and stiffness. The main differences were crucial to understanding the composite soil-aggregate interaction, which is discussed and compared. The results indicate that the interface between composite soil and aggregate is important to keep the stability of the layer of aggregate over the soft composite soil, and practical methods of achieving that are suggested.

Resilient Modulus of Expansive Soils at High Suction Using Vapor Pressure Control

Abstract Robust design of pavement sections underlain by high-plasticity subgrade soils requires determination of subgrade resilient modulus over a wide range of suction to account for seasonal and diurnal variation of moisture. A laboratory experimental apparatus capable of performing suction-controlled repeated load triaxial tests at high suction (5 to 600 MPa) has been developed by integrating automated vapor pressure control to the specimen gas phase within a cyclic triaxial setup. The system eliminates limitations on applicable upper-bound suction that exist in more commonly adopted axis-translation approaches. This article describes the experimental setup, its working principle, and test procedures. Results are presented for a high-plasticity clayey soil to demonstrate testing capability for suction ranging from 5 to 100 MPa. Results show repeatability having an average coefficient of variation of less than 6 % for all loading sequences.

Microstructure Evolution of Hydrate‐Bearing Sands During Thermal Dissociation and Ensued Impacts on the Mechanical and Seepage Characteristics

AbstractIn this study, a microfocus X‐ray computed tomography‐based triaxial testing apparatus was used to observe and quantify the microstructure evolution of hydrate‐bearing sands during thermal dissociation of hydrate. Three triaxial shear tests with X‐ray computed tomography were conducted to study the influence of hydrate dissociation on the mechanical behavior of hydrate‐bearing sands. The results show that hydrate covering the sand particle surface dissociates first and then at the menisci between sand particles. The secondary hydrate formation mainly occurs on the menisci between sand particles and the surfaces of the hydrate shells where hydrates already exist. Hydrate dissociation could cause fabric changes in hydrate‐bearing sands, resulting in a more isotropic orientation distribution of sand particles. The failure behavior of the hydrate‐free sand specimen is similar to that of the specimen after hydrate dissociation, which shows an obvious drum‐shaped failure pattern with X‐shaped shear bands. However, the hydrate‐bearing sand specimen exhibits a shear band with a determined thickness and inclination angle. Hydrate dissociation could cause a significant loss of supporting cementation, resulting in a decline in the stiffness and failure strength. The failure strength of hydrate‐free sand specimen is slightly higher than that of the specimen after hydrate dissociation; this may due to that the homogeneous orientation frequency distribution can enhance the stability of the force chain between sand particles. The secondary hydrate formation causes sediment deformation resulting in a decrease in absolute permeability due to pore blockage.

Individual Effect of Matric Suction on Soil Microstructure and Hydraulic Conductivity

The reduction of the soil micro-pore volume and hydraulic conductivity by unsaturated infiltration have been widely recognized. Among other factors responsible for this phenomenon, matric suction plays a key role in such a process. It is quite difficult to figure out the individual contribution of matric suction on soil microstructure and hydraulic conductivity change during unsaturated infiltration due to the complexity of the infiltration process. In this paper, individual effect of matric suction on soil microstructure and hydraulic conductivity was quantified using lab tests and Kozeny-Carman equation. A series of experiments with different matric suction actions were performed using unsaturated triaxial testing apparatus without axial stress. Nuclear magnetic resonance test equipment was used to quantify the imperceptible effect of the matric suction action on the soil micro-pore change. Results show that: under different matric suction actions, porosity of the specimens decreases gradually with the increase of matric suction and the matric suction action frequency. Soil is gradually compressed under matric suction action. Hydraulic conductivity of remolded silty clay was predicted based on the test data and Kozeny-Carman equation. With the increase of matric suction and the matric suction action frequency, hydraulic conductivity can be decreased by up to 25%.

Effect of Shredded Rubber on Undrained Shear Strength of Fine-Grained Sands

In many urban cities due to the enormous growth in the number of vehicles, scrap rubber disposal has been a critical environmental problem. In recent years, important research efforts have been dedicated to investigating the use of scrap rubbers in civil engineering applications, like recycling or reuse of scrap rubbers is the preferred possibility from a waste management perspective. This study furthers the knowledge of the undrained shear strength in clean and mixed well-sorted sands (different shape properties) with waste sand-sized rubber. For this purpose, shredded rubber content varies from 0 to 25% and initial effective consolidation stress of 100 to 300 kPa. Thirty-six undrained shear tests (CU) were conducted using a triaxial apparatus. On randomly mixing sands with shredded rubber, a dilative and strain-hardening and softening behavior governed, and undrained strength generally decreased. The results indicated that undrained shear strength is augmented with the growth of the effective mean stress. Peak index and Young’s modulus decreased, and flow potential increased with shredded rubber content increment in composite specimens. The undrained shear strength parameters (qpeak and qf) across 200 and 300 kPa effective mean stress were higher in the sand with a large angularity (low rr and rs). Further, a decrease in particle shape scale (rs and rr) ratios caused an increase in peak index and elasticity modulus and reduced occurrence in flow potential.

Effect of stress paths on the particle crushing

In previous studies, almost all they had carried out the crushing tests under usual tri-axial compression, Ko and simple shear conditions. However, it is necessary to perform the crushing test under general stress conditions in ground with controlled each principal stresses individually to find the mechanical characteristics of the particle crushing. Therefore, the purpose of this study is to confirm the effect for particle crushing under various stress paths in the combination of principal stresses. Authors plan the particle crushing tests with combinations of principal stresses operating the mean stress p, deviatoric stress q and Lode angle θ up to the Shear Failure Line (SFL) on the pie-plane. These particle crushing tests were carried out using the high pressure true tri-axial compression apparatus under the planned stress paths. As a result, the degree of Lode angles has little traction to the progress of the particle crushing and authors found that deviatoric stress q has essential effect on the particle crushing. In addition, the degree of particle crushing are little effected by the various monotonic stress paths on p-q plane when the maximum values of p and q are same.

A miniature triaxial apparatus for investigating the micromechanics of granular soils with in situ X-ray micro-tomography scanning

The development of a miniature triaxial apparatus is presented. In conjunction with an X-ray microtomography (termed as X-ray μCT hereafter) facility and advanced image processing techniques, this apparatus can be used for in situ investigation of the micro-scale mechanical behavior of granular soils under shear. The apparatus allows for triaxial testing of a miniature dry sample with a size of 8 mm × 16 mm (diameter × height). In situ triaxial testing of a 0.4–0.8 mm Leighton Buzzard sand (LBS) under a constant confining pressure of 500 kPa is presented. The evolutions of local porosities (i.e., the porosities of regions associated with individual particles), particle kinematics (i.e., particle translation and particle rotation) of the sample during the shear are quantitatively studied using image processing and analysis techniques. Meanwhile, a novel method is presented to quantify the volumetric strain distribution of the sample based on the results of local porosities and particle tracking. It is found that the sample, with nearly homogenous initial local porosities, starts to exhibit obvious inhomogeneity of local porosities and localization of particle kinematics and volumetric strain around the peak of deviatoric stress. In the post-peak shear stage, large local porosities and volumetric dilation mainly occur in a localized band. The developed triaxial apparatus, in its combined use of X-ray μCT imaging techniques, is a powerful tool to investigate the micro-scale mechanical behavior of granular soils.

Capability of Triaxial Apparatus with Respect to Evaluation of Nonlinearity of Soil Stiffness

Capability of Triaxial Apparatus with Respect to Evaluation of Nonlinearity of Soil Stiffness

Ethylcellulose-based drug nano depots fabricated using a modified triaxial electrospinning

New strategies based on advanced technologies are highly desired for expanding the applications of biological macromolecules in the applied scientific fields. In the present study, a new kind of core-shell nano depots were designed, in which the shell section was a drug-polymer composite and the core section was a drug reservoir. With ethyl cellulose and ketoprofen as a filament-forming polymeric matrix and a model drug, respectively, a triaxial electrospinning apparatus was developed to conduct both coaxial and triaxial processes, by which monolithic nanofibers F1 and core-shell nano depots F2 were successfully prepared. Although both of them had the same double components, their different nanostructures generated considerable differences in providing drug sustained release profiles. The core-shell nanofiber depots F2 were able to provide a better zero-order drug release profile: no initial burst release, smooth sustained release effect, and smaller tailing-off release for a nice zero-order drug release kinetics. The release percentage (Q) can be linearly manipulated through the release time (t) according to the equation Q2 = 9.40 + 4.74 t (R = 0.9936), providing opportunity for precise administration. The developed strategy and advanced electrospinning technique exhibit a new way for constructing process-structure-performance relationships at nano scale and for expanding the potential applications of biological macromolecules in the applied fields.

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.

Shear strength and permeability of the cement-casing interface

The shear strength and hydraulic permeability of the interface between well cement and casing was investigated using a triaxial direct shear apparatus. For the first time, these experiments provide measurements under controlled stress conditions with fluid flow measurements along the interface. The low cohesion (1.1 ± 1.1 MPa) and the high friction angle (43.4 ± 2.0°) indicates that the shear strength of the interface is provided by friction. This implies that the state of stress of the cement is critical to well integrity. The hydraulic aperture of the undamaged cement-steel samples was 6.8 ± 1.0 microns. Shear damage to the interface caused a decrease (-20 %) in hydraulic aperture for samples aged up to 1 month, and an increase (+300 %) for samples cured for two years. We performed numerical simulations to estimate the leakage potential from a carbon storage operation. This model predicts negligible leakage amounts (47 tonnes) in a shear-damaged well for the modeled injection of ∼1.26 million tonnes of CO2. Thus, our measurements indicate that the cement-casing interface is not a significant leakage pathway in its intact or damaged state, and that shear-driven failure scenarios for this interface are not a significant risk to CO2 storage security.

Acoustic Emission Behavior of Tropical Residual Soil

emission technique has extensively been used for a wide variety of engineering materials, but less attention has yet been paid on the application of this non-destructive technique to investigate the fundamental behaviors of geomaterial. In the present study, acoustic emission method was adopted in conjunction with the conventional stress-strain-time measurement to investigate the mechanical behaviors of a selected tropical residual soil. A systematic acoustic emission instrumentation setup, which was devised in a monotonic triaxial shear apparatus, was evidenced to provide reasonable experimental results. From the isotropic consolidation results, it was realized that the Kaiser’s effect was observable and the pre-stressed level as induced by compaction could be determined through acoustic emission. In undrained shearing, the acoustic emission response was found to be corresponding with the axial strain measurement. Initial soil yielding, which was mobilized at small strain range, was also able to be determined. The acoustic emission response of the studied tropical residual soil also showed good similarity with the reported soils constituting considerable fines content.

Stabilisation of Stiffer Rail Track Substructure Using Artificial Inclusion

The railway transport system in many countries plays a significant role in the passage of bulk freight and passengers. However, increased train speeds and higher freight loads (large dynamic wheel loads) accelerate the deterioration of rail track substructure. This problem is more critical in isolated rail track locations where the track substructure is much stiffer than the regular surface track assembly such as track at the bridges and tunnels. Ballast is a key track foundation material placed underneath the sleepers which provides structural support against high cyclic and impact stresses caused by moving trains. Inclusion of rubber mats called under ballast mats (UBMs) placed between the ballast and stiffer base layer is one of the measures to minimise the ballast deterioration. In this study, cyclic loads representing fast and heavy haul trains were simulated on stiffer track foundation condition using a large-scale process simulation prismoidal triaxial apparatus to investigate the mitigation of strain, stress and degradation characteristics of ballast stabilised with UBM. These UBMs were locally manufactured from recycled tyre wastes. The results show that ballast on a stiff foundation substructure stabilised with UBM experienced significantly less vertical and lateral deformation, ballast interface and inter-particle stresses and degradation. This study also confirmed that the recycled tyre UBMs used in this study had adequate damping to absorb the energy transmitted to the moving train to the track, thus preventing excessive plastic deformation and degradation of the ballast layer.

A high-precision and user-friendly triaxial apparatus for the measurement of permeability of gassy coal

The calculation method of coal seam gas permeability is the key starting point for studying a series of mine safety issues. However, the existing apparatuses are complex in operation, inconvenient in sample replacement, single in function, inaccurate in deformation measurement and poor in sealing effect. In this paper, a high-precision and user-friendly triaxial apparatus is developed. The apparatus consists of triaxial pressure and deformation measurement unit, gas filling and control unit, hydraulic loading unit and flow control and acquisition unit. The apparatus has excellent sealing performance and can accurately measure the axial and circumferential deformation of the sample simultaneously. In addition, the apparatus is user-friendly and integrated with the rapid replacement of the sample module. The permeability test of gassy coal in the whole process of deformation and failure is carried out and the results are consistent with previous research results, which proves the validity and reliability of the apparatus.

True Triaxial Experimental Study of Disturbed Compound Dynamic Disaster in Deep Underground Coal Mine

With the increase of coal extracting depth, a considerable number of dynamic disasters display the co-occurrence and coupling effect of rockburst and coal–gas outburst, which is defined as the disturbed compound dynamic disaster. In this study, two loading modes were adopted to investigate disaster characteristics using true triaxial apparatus. The stress states of one freed and the other five stressed faces were introduced to simulate actual stress conditions. Two high-speed cameras were used to capture the disaster process. The mechanical and strength properties, failure modes, and ejection kinetic energy were analyzed. Results showed that the compound dynamic disaster mainly exhibited local grain ejection, fragment spalling, large-scale grain ejection, plate bending, and ultimately failure. The strength of the sample first increased and then decreased slowly with the increase in the intermediate principal stress. After failure, a V-shaped coal-burst pit was formed, which was approximately parallel to the intermediate principal stress and perpendicular to the free face. The grain ejection exhibited obvious characteristics of spatial sorting, and the grain size decreased with the distance from the free face. The kinetic energy showed little change with increase in the intermediate principal stress in the displacement loading mode; whereas, it first increased and then decreased in the stress loading mode. The pressurized gas promotes the development of coal cracks and fully fractures the coal rock. Under the combined actions of the elastic energy stored in coal mass and the internal energy of pressurized gas, compound dynamic disasters may occur. Gas extraction and coal seam elastic softening techniques can effectively reduce and prevent the occurrence probability of compound dynamic disasters.

Effect of fine particles on cyclic liquefaction resistance of sands

ABSTRACT This paper presents an experimental study that investigates the influence of the fines on the initiation of instability of sands under cyclic loading. The materials used for this study and the experimental device are first presented, then the results of typical liquefaction tests for both loose and medium dense specimens are presented with emphasis on the exhibited behaviour. Loose samples have shown a total liquefaction phenomenon however the cyclic mobility phenomenon was initiated for medium dense samples. The results of a series of cyclic undrained tests that were done with the aid of a triaxial apparatus for different percentages of fines are also presented and analysed. The results reveal that the increase in the non-plastic fines content lead to an increase in the liquefaction resistance of sands whereas the increase in the plastic fines lead to a decrease in the resistance to liquefaction.