Static Triaxial Tests Research Articles

Shakedown response of recycled rubber–granular waste mixtures under cyclic loading

Understanding and quantifying the long-term deformation behaviour of granular materials under repeated loads is imperative for ensuring the longevity of railway tracks. One of the most relevant characteristics of granular materials under repeated cycles of loading and unloading is their ability to achieve a relatively stable state (shakedown) after being subjected to initial compression. The shakedown response of blended rubber–granular waste mixtures under triaxial test conditions has been investigated in past studies highlighting the influence of the rubber content, confining stress and cyclic loading amplitude. However, a clear methodology for estimating shakedown yield limits of these granular mixtures has not been discussed in detail. The current study highlights the influence of the peak shear strength of these mixtures under static loading on their shakedown response in cyclic loading conditions. It is observed that the variation of static shear strength with rubber contents and confining stresses is found to affect the shakedown response. A unified method of estimating the shakedown limit is proposed by analysing permanent axial strains with normalised cyclic stress ratio at different loading cycles. The proposed method is validated through two independent sets of drained cyclic triaxial test data on coal wash–rubber crumb mixtures and rail ballast.

Improving Mudstone Materials in Badland in Southwestern Taiwan by Increasing Density and Low-Cement Amount

The southwestern foothills in Taiwan contain abundant mudstone, which softens or slakes easily when contacting water. Thereafter, this condition causes high rates of erosion during heavy rain, eventually leading to the formation of badlands. To increase land use value in Taiwan, this study aimed to offer a new solution with the use of compaction techniques and also adding a small amount of cement as soil amendment. We examined the feasibility of this concept by performing a series of tests, including the basic physical property test, compaction test, unconfined compression test, static triaxial CU and UU tests, consolidation test, California bearing ratio (CBR) test, and triaxial permeability test. The specimens were created based on the controlled moisture at the wet side along the compaction curve. Three clods, three preparation methods, and two types of cement content were used for specimen preparation. The test results revealed that clod will affect the results of unconfined compression strength and soil permeability. For mudstone with added cement, its unconfined compression strength increased 7- to 10-fold and its shear strength increased 2- to 3-fold. The results of consolidation and the CBR test showed that improvement is possible by using low amounts of cement. Overall, the present method not only adheres to low-carbon and environmental protection requirements, but also verifies the feasibility of using compacted mudstone as an engineering material.

Experimental study on seepage-induced concentrated leak erosion in sand under triaxial testing conditions

Experimental study on seepage-induced concentrated leak erosion in sand under triaxial testing conditions

Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand–Bentonite Mixture

The bentonite is commonly used mixed with soils for groundwater retention and waste contaminant facilities. The incorporation of bentonite could significantly reduce hydraulic conductivity. In this study, the effects of bentonite content, hydration time and effective confining pressure on the static properties of a sand–bentonite mixture were studied using experimental and numerical methods. Firstly, a large number of drainage static triaxial tests on the sand–bentonite mixture with various bentonite contents were conducted. The test results show that the increase in bentonite content and hydration time leads to a slight decrease in shear strength and initial tangent modulus of the sand–bentonite mixture. The presence of bentonite reduces the shear shrinkage and dilatancy trend of the mixture. The cohesion of the mixture increases with the increase in bentonite content and hydration time, but the internal friction angle decreases correspondingly. The hydration of bentonite on the surface of sand particles changes the contact form between particles. The bentonite slurry between pores of the sand skeleton also affects the mechanical behavior of the sand–bentonite mixture. Then, a series of 3D discrete element models were established for numerical simulations of drainage static triaxial tests. The numerical model parameters were calibrated by experimental results. The meso-mechanism of bentonite content affecting the mechanical behavior was revealed according to the contact force distribution between particles. The research results are helpful to understand further the mechanism of bentonite on the mechanical properties of the sand–bentonite mixture.

Characterisation of Geogrid and Waste Tyres as Reinforcement Materials in Railway Track Beds.

The engineering behaviour of ballast is an important factor to determine the stability and safety of railway tracks. This paper examines the stress–strain, shear strength, peak deflection stress and reinforcement strength ratio of different reinforcement materials and reinforcement locations in ballast track bed layers based on large scale static triaxial shear tests. The results show that geogrid and waste tyre reinforcement have a significant effect on the peak deviator stress of railway track bed layers and the stress–strain relationship is strain-hardened. The peak deviator stress and shear strength of geogrid reinforcement are greater under the same conditions compared with waste tyres. The reinforcement of geogrid and waste tires increases the shear strength of the track bed significantly. The more layers of geogrid reinforcement, the more energy is required for the deformation of the track bed. The energy required for deformation is greater in the centre of the waste tyre than in the other reinforced forms, and the energy required for deformation is minimal in the fully reinforced form. Excessive tyre reinforcement changes the stiffness of the track bed layer, leading to an increase in the settlement rate. The reinforcement strength ratio between geogrid and waste tyre increases significantly with the increasing of the confining pressure and reinforcement layers. Moreover, the reinforcement strength ratio of the geogrid is significantly higher than that of the waste tyre.

Study on the Factors Affecting the Design of Deep Backfill Foundation Pit and the Factors Affecting Soil Construction Disturbance

Water is an indirect construction disturbance factor, which has important influence on the stability evaluation and analysis of foundation pit slope engineering. The study is mainly aimed at the deep backfill, the mechanical parameters are not uniform, and the effect of water on its mechanical parameters is great. The mechanical parameters of the backfill under different moisture content were measured by static triaxial shear test. The results show that the cohesion, internal friction angle and shear strength of the backfill are attenuated with the raise of the water content, the cohesion and internal friction angle of the attenuation law showed a linear re-gentle trend, and the shear strength of the decay trend gradually slowed down. The simulation results show that the mechanical parameters of backfill are reduced with the increasing number of saturation-dehydration cycles. The first degree of decay is the most significant, and finally tends to a certain value. The connection between the deterioration coefficient and the cycles follows the negative exponential function, and the mathematical expression of water deterioration was obtained finally. The study analyzes the damage mechanism of water in the construction disturbance, which has some theoretical support for the stability evaluation and disaster mitigation of the foundation pit.

Experimental Study on the Strength and Deformation Characteristics of Concrete under True Triaxial Compression after Sulfate Attack

To explore the mechanical properties of concrete under true triaxial static compressive load after sulfate attack, uniaxial static compression test and true triaxial static compression test at four stress ratios were carried out on concrete specimens immersed in 15% sulfate solution for 0–120 days by the integrated true triaxial static and dynamic load testing system, and the variation of performance indicators such as the strength and deformation of concrete under the coupling action of sulfate attack and complex stress state was analyzed. The results show that the uniaxial compressive strength of concrete increases at the beginning and then decreases with the increase of sulfate attack time and reaches the peak on the 30th day, with an increase rate of 16.57%; the strength of concrete under triaxial compression increases significantly, and the maximum triaxial compressive strength is 3.18 times of uniaxial compressive strength under the combination of 0-day sulfate attack and 0.2 : 0.8 stress ratio; and the deterioration of concrete under sulfate attack is more prominent at high confining pressure, and as the sulfate attack worsens, the sensitivity of triaxial compressive strength of concrete to lateral compressive stress is reduced. In conclusion, triaxial compression can significantly enhance the ductility of concrete by playing a role in restraining the deformation and cracking of concrete after sulfate attack.

Valuation of hydraulic fracturing potentials of organic-rich shales from the Anambra basin using rock mechanical properties from wireline logs

This study was carried out to determine the rock mechanical properties relevant for hydrocarbon exploration and production by hydraulic fracturing of organic rich shale formations in Anambra basin. Shale samples and wireline logs were analysed to determine the petrophysical, elastic, strength and in-situ properties necessary for the design of a hydraulic fracturing programme for the exploitation of the shales. The results obtained indicated shale failure in shear and barreling under triaxial test conditions. The average effective porosity of 0.06 and permeability of the order of 10-1 to 101 millidarcies showed the imperative for induced fracturing to assure fluid flow. Average Young’s modulus and Poisson’s ratio of about 2.06 and 0.20 respectively imply that the rocks are favourable for the formation and propagation of fractures during hydraulic fracking. The minimum horizontal stress, which determines the direction of formation and growth of artificially induced hydraulic fractures varies from wellto-well, averaging between 6802.62 to 32790.58 psi. The order of variation of the in-situ stresses is maximum horizontal stress>vertical stress>minimum horizontal stress which implies a reverse fault fracture regime. The study predicts that the sweet spots for the exploration and development of the shale-gas are those sections of the shale formations that exhibit high Young’s modulus, low Poisson’s ratio, and high brittleness. The in-situ stresses required for artificially induced fractures which provide pore space for shale gas accumulation and expulsion are adequate. The shales possess suitable mechanical properties to fracture during hydraulic fracturing. Application of these results will enhance the potentials of the onshore Anambra basin as a reliable component in increasing Nigeria’s gas reserves, for the improvement of the nation’s economy and energy security.
 Key Words: Hydraulic Fracturing, Organic-rich Shales, Rock Mechanical Properties, Petrophysical Properties, Anambra Basin

Determining Soil Moisture Content and Material Properties with Dynamic Cone Penetrometer

This study utilised static triaxial and dynamic cone penetration tests to examine the identification of changes in strength in soil materials as a result of an increase in moisture content. The applicability of a light dynamic cone penetrometer device in railway environments was also studied. On a broader scale, the aim was to find an investigation method suited to field locations that identify low-quality or persistently moist materials directly from the structure. The triaxial tests found an apparent increase in shear strength when the water content dropped below 7%. Based on the series of laboratory tests, the dynamic cone penetrometer reacted strongly to material density, but the impact of moisture content was also evident. Furthermore, the results showed that dynamic cone resistance is a reasonably unfeasible metric for assessing the structural quality of materials consisting primarily of sand, due to the number of factors affecting the resistance. In the laboratory tests, the lowest dynamic cone resistances were measured in the material with the highest structural quality.

Exploring the relationship between particle shape and critical state parameters for granular materials using DEM

This paper establishes the relationship between particle shape and critical state parameters for granular materials using discrete element method (DEM). A series of DEM simulations are performed under drained triaxial test conditions considering a varying elongation parameter (η). The elongated particles are generated using a commonly employed multi-sphere approach. From the analysis of results, it is found that the macroscopic response of elongated particles exhibits post-peak strain-softening behaviour while reaching a critical state at an axial strain of ~ 40%. The critical state strength of granular materials is found to be dependent on η. The microscopic behaviour in terms of coordination number (CN) shows an exponential decay against axial strain. In the early stage, CN decreases with increasing η, while the steady-state CN is found to increase with an increase in η values. Furthermore, quantitative relationships are established among the shape parameters and critical state parameters, thus providing a way to develop macro-scale constitutive models with intrinsic micro-scale built-in properties of granular materials.

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.

Weakening Characteristics of Soft Clay Foundation under Earthquake

The dynamic triaxial test is carried out by the dynamic cyclic shear equipment GDS to determine the strength of soil after earthquake cyclic loading. The consolidation pressure is determined according to the in-situ stress state of soil samples. The samples are loaded with sinusoidal periodic loads of different amplitudes at a fixed frequency, with the failure standard of achieving the same final residual strain. Compared with the static triaxial test results, we can determine the strength weakening of soft clay foundation with different depths under earthquake load, which can provide a reliable reference for engineering problems such as the bearing capacity of pile foundation in soft clay.

Effects of freeze-thaw cycles on the moisture sensitivity of a compacted clay

This study investigates the influence of freeze-thaw (FT) cycles on the (i) microstructure, (ii) soil water characteristics, and (iii) sensitivity of the volumetric strain and mechanical properties to moisture content for a compacted clay. The mechanical properties herein include resilient modulus (MR), unconfined compression strength (qu), and reloading modulus (E1%) and stress (Su1%) at 1% strain. Specimens were compacted at optimum moisture content and subjected to different FT cycles (i.e. 0, 1, 3, and 10 cycles). Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were performed to reveal the evolution of the microstructure during FT cycles. Specimens were then wetted or dried to different suction (s) and degree of saturation (Sr) values to obtain the (i) soil water characteristics using filter paper method and (ii) MR, qu, E1%, and Su1% using cyclic and static triaxial tests. Experimental results demonstrate that FT cycles induce cracks that are tens of microns in dimension, which reduce specimens' water retention capacity and the volumetric strain upon wetting and drying. Mechanical properties also reduce significantly and become less sensitive to the s and Sr after FT cycles. A model was used to predict the variation of the MR, qu, E1% and Su1% with s and Sr for specimens subjected to different FT cycles. An attenuation coefficient χFT was proposed to describe the FT-induced degradation in the qu, E1%, Su1%, and MR. It was found that the χFT for different mechanical properties are consistent. The evolution of their χFT with FT cycles can be described by a uniform empirical equation. The study presented in this paper is useful for the rational understanding and prediction of the hydromechanical behaviors of compacted clay taking account of the influence of FT cycles.

Laboratory assessing of the liquefaction potential and strength properties of Sand soil treated with mixture of nanoclay and glass fiber under dynamic and static loading

This study evaluated an improvement method based on static and dynamic tests for dune sand soil on the coast of Bandar Anzali city. This soil has very poor and insufficient mechanical performance for many construction projects as well as the potential for liquefaction. Short glass fibers have been used in this improvement method to reinforce the soil modified with nanoclay. The effects of simultaneously adding glass fibers and modified nanoclay at optimal weight percentages on the sand specimen were evaluated through static triaxial tests at consolidated drained conditions. The results showed that the simultaneous addition of glass fibers and nanoclay at lower confining pressures had more significant effects and was more useful for soil modification. The results indicated increases of more than 31% and about 23% in the peak deviatoric stress of the modified sand specimen at the confining stresses of 50 and 100 kPa, respectively, compared to the pure sand specimen. Furthermore, the modified sand specimen had a 20% reduction and a 50% increase in failure axial strain, compared to the pure sand specimen, under the static triaxial test at the confining pressures of 50 and 200 kPa, respectively; however, its failure axial strain did not change at the confining pressure of 100 kPa compared to the pure sand specimen. The results of static triaxial tests showed that the simultaneous addition of glass fibers and nanoclay at the optimal weight percentages increased the cohesion of the sand specimen and did not change its internal friction angle. To investigate the effect of this improvement method on dune sand under dynamic loading, the cyclic liquefaction potential of pure and modified sand specimens was evaluated through cyclic triaxial tests under consolidated undrained conditions. The results showed a significant effect of sandy soil improvement on reducing its liquefaction potential. The number of loading cycles causing liquefaction was increased, by more than 5.5 times at the cyclic stress ratio of 0.27, by about 3 times at the cyclic stress ratio of 0.3, and finally, by more than 2 times at the cyclic stress ratio of 0.34 for the modified sand specimen compared to the pure sand specimen. By defining the cyclic stress ratio corresponding to liquefaction in the twentieth loading cycle as the specimen cyclic strength, the cyclic strength of the modified sand specimen was increased by more than 13% compared to the pure sand specimen. Based on the results of a series of different experiments conducted in this study, it was concluded that the simultaneous addition of glass fibers and modified nanoclay at their optimal weight percentages (0.5% and 1% by total dry weight of specimen, respectively) significantly improved many different resistance characteristics of dune sand soil from the Bandar Anzali coast, and using this composition can be considered as an effective and efficient way to improve this very special type of soil.

Evaluation of reclaimed asphalt pavement as base/subbase material in Egypt

The application of reclaimed asphalt pavement (RAP) has become a common practice in road construction as a substitute to natural aggregate. The significance of RAP amount on the resilient modulus behavior, shear strength and hydraulic conductivity characteristics of unbound granular base materials were investigated in this research. RAP was blended with crushed aggregate by different percentages ranging from 0% to 100% by the blend weight. The laboratory testing program includes modified compaction, California Bearing Ratio (CBR), permeability, and repeated and static triaxial tests. A descriptive statistical analysis was conducted on all testing results. Furthermore, the X-ray computed tomography (CT) scanning technique was applied to investigate the internal (micro) structure of specimens. It was noted that with the addition of more RAP to the blend the resilient modulus increased and the coefficient of permeability decreased. The apparent cohesion of the RAP blends increased almost linearly and the friction angle decreased as the RAP replacement level increased. The CT scanning results indicated that the virgin crushed aggregate has a higher void ratio than the RAP blends, which interpreted the lower permeability and higher resilient modulus of RAP blends. In conclusion, blending RAP with virgin aggregate produces superior quality material for road bases.

Static and cyclic characterization of fouled railroad sub-ballast layer behaviour

This paper investigates the static and cyclic characteristics of a fouled sub-ballast material using static and cyclic triaxial tests. Static triaxial tests were conducted first to determine the peak strength, which was used as the applied stress in the cyclic tests. Moreover, the results of the displacement-controlled static triaxial test were used to assess the shear stiffness and strength during the cyclic stress-controlled loading cycles. Isotropically consolidated drained (CD) compression cyclic triaxial tests were conducted to simulate the long-term response of fouled railway substructure under progressive repetitive loading and unloading cycles. The accumulated plastic strains of the fouled railroad sub-ballast were determined under cyclic loading and unloading conditions. The obtained results indicated that the soil plasticity and effective confining pressure, and relative density are not the only important factors in shaping the damping curves but also the loading rate, and particle size. Both parameters play a major role in determining the behaviour of fine contaminated sub-ballast. The railway operational speed limits were deduced from the stress-strain behaviour of the fouled sub-ballast interlayer zone. It is recommended that the maximum speed of trains in fouled railroad segments be limited to a maximum of 32 km/h. The developed guidelines are aimed to prolong the service lifespan of the fouled track segments, while maintaining safety standards to avoid train derailments, track deterioration, and high maintenance costs.

Three-dimensional discrete element simulation of the runaway vehicle deceleration process on the arrester bed of truck escape ramps.

Due to imperfect design norms and guidelines for China's truck escape ramp, previous studies have not been able to reflect the effect of wheel subsidence process on the deceleration of runaway vehicles. A discrete element method was used to establish an aggregate discrete element and a wheel discrete element. The three-dimensional discrete element model for an aggregate-wheel combination was established based on a particle flow code in three dimensions on a software platform using the "FISH" language. The microscopic parameters of the aggregate discrete element particles and wheel discrete element particles were calibrated using a simulated static triaxial compression test and real vehicle test data, respectively. Four sets of numerical simulation tests were designed for analyzing the influence of the aggregate diameter, grade of the arrester bed, truckload, and entry speed on the wheel subsidence depth and stopping distance of runaway vehicles. The results indicate that the smaller the aggregate diameter and entry speed and the greater the truckload and grade of the arrester bed, the more easily the wheel falls into the gravel aggregate, the better the deceleration effect, and the smaller the stopping distance. As the wheel subsidence depth increases, the speed at the unit stopping distance decreases more quickly. The maximum subsidence depth mainly depends on the truckload. The research results can provide a theoretical basis for the design of the arrester bed length and the thickness of the aggregate pavement in a truck escape ramp.

Triaxial behavior of cement-stabilized organic matter–disseminated sand

Organic matter–disseminated sand (OMDS) is a widely distributed problematic soil in coastal areas of Hainan province, China. Its existence makes the installation of piles difficult and has the risk of insufficient bearing capacity. OMDS is different from other organic soils such as peat in terms of formation, mineral component, organic content, and forms of organic matters. In this study, 20% (w/w) cement together with 7.5% (w/w) lime at a water–cement ratio of 0.45 was mixed with OMDS to improve its mechanical performances. A series of unconsolidated undrained static triaxial test was conducted on the stabilized OMDS to investigate the failure mode, stress–strain relationship, maximum and residue deviator stress, axial strain at failure, and elastic modulus under various confining pressures (0, 200, 300, 400 kPa) and curing time (7d, 14d, 28d). The test results showed that higher confining pressure and longer curing time in general led to higher maximum and residue deviator stress, larger axial strain at failure, and larger secant elastic modulus of cement-stabilized OMDS. The maximum and residue deviator stress of cement-stabilized OMDS increased with curing time and ranged from 500 to 2180 kPa and from 250 to 1800 kPa, respectively. Under elevated confining pressure, maximum deviator stress increased substantially, irrespective of curing time. Secant elastic modulus (E50) increased with confining pressure at these three curing time, from 29 to 42 MPa. Due to the existence of organic matters, the strength of cement-stabilized OMDS was lower than cement-stabilized non-organic sand, regardless of confining pressure and curing time. This study provided new insight into the shear strength behavior of cement-stabilized OMDS under different confining conditions. This will facilitate the design and construction of foundations in this type of soil.

Microscopic Mechanism of the Macroscopic Mechanical Properties of Cement Modified Subgrade Silty Soil Subjected to Freeze-Thaw Cycles

In order to study the effects of the microstructure parameters of cement modified subgrade silty soil (CMSS) in a frozen area under freeze-thaw (F-T) cycles on the macroscopic mechanical properties, the static triaxial test, scanning electron microscopy (SEM), and grey relation analysis (GRA) were implemented on silty soil modified with 0% and 2% cement at optimum moisture content from the northwest in Jilin Province in China. The results showed that the shear strength, the cohesion of 0% and 2% CMSS, decreased with the increase of F-T cycles, while the internal friction angle was not obviously changed. The shear strength and its parameters of 2% CMSS doubled compared to that of 0% CMSS. The micro-parameters, representing the particle morphological characteristics, particle arrangement, and pore characteristics of CMSS, changed differently under F-T cycles. If the cement was not added, the cohesion and the internal friction angle were most sensitive to the average particle diameter (Dp) and the average particle abundance (C), respectively. When the cement content was 2%, the cohesion was chiefly affected by the particle size fractal dimension (Dps), while the internal friction angle was mainly related to the average pore diameter (Dh). The main principle of cement improvement was to decrease Dh of soil under F-T cycles.

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.