Triaxial Specimens Research Articles

Development and testing of a novel vibratory compaction instrument considering its natural frequency.

This study aims to develop a novel vibratory compaction instrument designed for the preparation of large triaxial soil specimens, offering an efficient alternative to the traditional manual specimen preparation process. However, when the motor frequency closely aligns with the natural frequency of the structure, severe resonance phenomena may occur. This paper employed finite element modal analysis, experimental modal analysis, and operational modal analysis techniques to conduct a multi-objective optimization of the structure, aiming to improve the dynamic characteristics. First, the natural frequency and mode shapes of the vibratory compaction instrument were determined via finite element technology. Subsequently, the optimization of this instrument was conducted by integrating the Kriging surrogate model with the Non-dominated Sorting Genetic Algorithm II (NSGA-II). Finally, the steel column underwent a modal test using impulse stimulation to ascertain its natural frequency, and the stochastic subspace identification (SSI) approach was employed to conduct an operational modal analysis of the vibratory compaction instrument to determine its modal characteristics. The experimental findings corroborated the conclusions obtained from the finite element analysis, validating the precision of the finite element modal analysis and the rationality of the optimized structure. The research results are of significant importance for improving the preparation efficiency of large triaxial specimen, contributing to advancements in soil mechanics testing.

Modeling of Drain Consolidation in the Quick Triaxial Test and Its Analytical Solution

ABSTRACTSand columns have been widely used to accelerate drainage and then improving the mechanical properties of soft soil foundations. The sand column has also been introduced into the triaxial test by researchers, in the center of the cylindrical specimen, to greatly accelerate drainage and consolidation process. The objective of this paper is to evaluate the consolidation properties of the triaxial cylindrical specimen considering the presence of a sand column, and then to propose a consolidation model that simulates the consolidation process of the triaxial test. The consolidation equations were derived considering the drainage of the specimen with a sand column composed of both vertical and double‐radial flows. Then the analytical solution of the model was obtained based on specific initial and boundary conditions. The comparison between the consolidation model and the laboratory tests yielded highly consistent. The case study demonstrated that the proposed consolidation model accurately simulates the evolution of average pore pressure and degree of consolidation in triaxial specimens containing a sand column. The studies on the consolidation parameters showed that there were different effects on the drainage rate for the diameter of specimen, the permeability coefficients of specimen and sand column, as well as the radius of the sand column.

Triaxial test analysis and discrete element simulation of CFBC

In order to explore the mechanical behavior and crack propagation of cement fly ash stabilized brick-concrete gravel (CFBC) base mixture under three-dimensional compression, firstly, the triaxial loading tests of three different proportions of mixtures were carried out through laboratory tests. The axial load is controlled by a load control loading system, the sampling frequency is 5 Hz, and the triaxial specimen is loaded at a rate of 0.1 MPa /s.The stress–strain curves of the mixture under four different confining pressures were obtained, and the failure characteristics and strength characteristics were analyzed. Secondly, the SEM analysis of the specimens under different confining pressures was carried out, and the microstructure parameters of the mixture under four different confining pressures of 0 MPa, 0.5 MPa, 1.0 MPa and 1.5 MPa were studied based on the PCAS analysis software. On this basis, a three-dimensional discrete element model was established to numerically reproduce the triaxial test of the specimen. The results show that the failure mode of the specimen is mainly related to the confining pressure, and the crack shape has the characteristics of top-down in the direction of principal stress.Through the stress–strain curve, it can be seen that with the increase of confining pressure, the peak stress of the specimen is also increasing. In the BCA1 ratio, the peak stress of the 1.5 MPa confining pressure is increased by 23 % compared with the non-confining pressure. In addition, the cement fly ash stabilized brick-concrete gravel mixture specimen has a larger bearing capacity than other concrete brittle materials.PCAS pore analysis showed that the main controlling factors affecting the porosity of the mixture were brick content and confining pressure, which in turn affected the shear strength and peak stress of the mixture, and the porosity of BCA1 < BCA3 < BCA5; the numerical simulation results show that the failure mode and stress–strain curve of the specimens with the shear strength error of less than 3 % and the friction angle error of less than 6 % are in good agreement. The maximum displacement particle clusters appear on the upper and lower edges of the specimen, and the sliding zone angle of the specimen is about 45° under all stress paths except the horizontal sliding zone under the uniaxial stress state, which reflects that the numerical model is consistent with the mechanical response at the macro scale. The use of three-dimensional discrete element models in the field of construction engineering can simulate and optimize the use of the above materials in construction sites to minimize waste and maximize efficiency.

EFFECTS OF MICROGRID REINFORCEMENT ON SOIL STRENGTH

In geotechnical engineering geogrid materials are in use to reinforce the soil recently. When these materials are used together with the soil, the strength parameters of the soil increases. The usage of geogrids is also very profitable to improve the soil because the material is very economical, easy to apply and durable. Engineers needs some design parameters about this materials when used withsoils for geotechnical designs. Therefore, experimental studies that will enable these parameters to be determined are of great importance. In this study, triaxial compression tests were carried out under four different cell pressures(50, 100, 150, 200 kPa) in labarotory by using microgrids, which showss similar functions with geogrid. Soil samples were prepared without microgrid, single layer microgrid, two layers microgrid and three layers microgrid and each of them were individually tested in triaxial compression. 2 mm aperture sized grid reinforcement configurated in 50*100 mm in silty soil triaxial specimen as in layer form of one to three. In the experimental study, it was determined that the strength values of the soil samples using geogrid were higher than those without geogrid. It was also found that the most efficient reinforcement was the use of two layers of microgrid. One of the important findings was taht with the use of microgrid reinforcement, the strength parameters of the soil could be increased by % 307.

Investigation of deformation coordination between optical fibre and borehole sand backfill

Land subsidence has threatened the safety of municipal infrastructures and even that of inhabitants. As one of the deformation monitoring methods, distributed optical fibre sensing (DOFS) technology has been developed for the investigation of land subsidence. The deformation coordination between the optical fibre and soil (DCf-s) under different conditions is critical for land subsidence monitoring with DOFS. In this paper, a medium-sized triaxial apparatus was modified for testing the DCf-s. Consolidated drained triaxial tests were conducted to investigate the effect of sand types on the DCf-s. By linearly fitting the deformation of the sensing cable with that of the triaxial specimen, the other factors that affect the DCf-s, such as the confining pressure, dry or wet state of the soil and cyclic variation of the loads, can be discussed. The experiments reveal that better DCf-s comes with a larger particle size, poor gradation of sand and larger confining pressure. The DCf-s of wet sand is better than that of dry sand. The DCf-s coefficient tends to be stable with an increase of loading cycles. The DCf-s obtained and its dependence on influencing factors can be used to modify the measured cable strain in practical land subsidence monitoring with DOFS.

Wave Reflection Effects on Bender Element Test of Very Stiff Clay

A common source of error in bender element testing lies in the determination of the shear wave arrival time arising from the near-field effect, which is attributed to shear wave–like motion of the compression wave obscuring the arrival of the shear waves, and can be mitigated by using a sufficiently high travel-distance-to-wavelength (L/λ) ratio. However, the L/λ ratio also is relevant for signal interference due to side reflections, and its role in this important mechanism is far less understood. Stiff materials such as cement-treated soils, which have a very short shear wave travel time, are likely to exacerbate these errors. This paper examined the causes of travel time errors in cement-treated soil using bender element tests of standard-sized triaxial specimens, large block samples, and three-dimensional finite-element analyses. The results show that the reflection of shear waves from sample boundaries generates a second pulse in the received signal, resulting in signal distortion. This can be ameliorated at sufficiently high L/λ ratios. The reflection of the compressional waves generates spurious prearrivals and distorts the profile of the direct shear wave even at high L/λ ratios. Shear and compressional wave reflections can be mitigated by increasing the specimen diameter to decouple the latter from the direct shear wave arrival.

Research on micro-influence mechanism of fine particles on dynamic characteristics of graded macadam materials in heavy-haul railway subgrade beds

Fine particle content is an important physical state parameter of graded macadam materials used in heavy-haul railway subgrade beds. Understanding the micro-influence mechanism of the fine particle content on the filler’s dynamical characteristics is of great significance. In this study, the granular system generation logic in PFC3D software was modified to enhance efficiency and quality, and the three-dimensional discrete element numerical model of graded macadam materials dynamic triaxial specimen was established. The servo program was used to simulate the dynamic triaxial numerical test confining pressure and dynamic loading conditions. A series of laboratory tests were performed using a GCTS large-scale cyclic triaxial test system to calibrate and verify the numerical model. Finally, dynamic triaxial numerical tests were conducted for graded macadam materials with different fine particle contents. These tests had the aim of determining the variation law for the contact force chain distribution, mechanical coordination number, average rotational angular velocity, and energy dissipation of the specimens under cyclic loading with the increase of fine particles and then revealing the influence mechanism of fine particles on the dynamic characteristics of graded macadam materials used in heavy-haul railway subgrade beds from a microscopic perspective. The study’s findings serve as a framework for describing the mechanism of subgrade bed disease occurrence and proposing effective remediation measures.

Method for measuring the membrane penetration of triaxial specimens based on an external circular array of laser displacement sensors

In triaxial tests, membrane penetration is an important factor influencing the accuracy of volume change and pore pressure measurement. To measure and evaluate the membrane penetration of specimens made of gravelly sandy soils, a method using a newly developed measurement system based on an external circular array of laser displacement sensors is proposed in this paper. A series of experimental studies were conducted on the effects of several factors on membrane penetration. The result shows that there is a linear correlation between the unit membrane compliance and the effective confining pressure on a semilog plot. In addition, as the gravel content increases, the membrane penetration increases, but this trend weakens with increasing relative density. Notably, when the characteristic particle size is the same, the membrane penetration will be greater for more broadly graded soils. Moreover, the error caused by the thickness of the membrane increases as the diameter-to-thickness ratio decreases.

Modeling of Sand Triaxial Specimens under Compression: Introducing an Elasto-Plastic Finite Element Model to Capture the Impact of Specimens’ Heterogeneity

This paper follows up on a series of reference papers that inspired MDPI’s Topic “Stochastic Geomechanics: From Experimentation to Forward Modeling”, where global and local deformation effects on sand specimens are fully described from high-resolution boundary displacement fields, as supported by a comprehensive experimental database (which includes varying degrees of specimen’s heterogeneity) that is available to the scientific community for further study. This paper presents an elasto-plastic comparative analysis of different finite element models reproducing different sand specimen heterogeneity configurations as follows: loose, dense, and half-dense half-loose specimens. The experimental conditions for these specimens’ heterogeneity configurations were simulated with an axisymmetric finite element model. To characterize the stress-strain response obtained from the experiments, an elasto-plastic constitutive model with strain-hardening and softening laws was adopted to reproduce the sand specimens’ mechanistic response. An expert-based calibration of the numerical models accounted for both global and local effects by making use of global observations captured by the triaxial point sensors (i.e., axial force and displacement) and by local observations captured by 3D digital image correlation analysis (i.e., 3D boundary displacement fields). Results show that predictions of the proposed numerical models are in good agreement with the experimental observations, both global and local responses. The combined use of global and local observations to calibrate sand triaxial specimens sets the basis for a more comprehensive parameterization process. For the first model set, three experiments were assumed with homogeneous materials. While both dense and loose models showed good agreement with the experiments, the displacement field prediction of the half-dense half-loose layered model identified limitations in reproducing heterogeneous configurations. Afterward, the second set compared and analyzed the half-dense half-loose layered models by implementing a heterogeneous model, showing significantly better model predictions (i.e., after the implementation of the heterogeneous model, which accounts for a transition zone between the upper and lower segments).

Microstructural evolution of saturated normally consolidated kaolinite clay under thermal cycles

This study aims to quantify the microstructural evolution of saturated clay during one thermal cycle. For this purpose, five saturated normally consolidated kaolinite clay triaxial specimens were consolidated under the same conditions. One specimen was subjected to a mechanical consolidation only; while the other four specimens were subjected to different thermal paths: freezing (F), freezing-thawing (FT), freezing-thawing-heating (FTH), and freezing-thawing-heating-cooling (FTHC). After completing the mechanical and thermal stages, thin disks were cut from the bottom of each specimen. The microstructures of these disks were preserved using flash freezing and freeze-drying. The evolutions of the microstructure of the specimens during the thermal cycle was assessed using mercury intrusion porosimetry, gas adsorption methods, and processing of scanning electron microscopy (SEM) images. The results show that (1) freezing increased the modal throat size and the specific surface area (SSA); (2) freezing-thawing decreased both modal throat and pore sizes, while the SSA recovered to its initial value; (3) freezing-thawing-heating reduced the pore and throat sizes and SSA even further; and (4) freezing-thawing-heating-cooling caused permanent reduction in SSA and throat and pore size distributions. Finally, the shape of the pores became more circular after temperature changes, while the number of pores increased during the thermal cycle with the highest number corresponding to the end of heating (i.e., the third stage in the thermal cycle).

Thermal Behavior of Saturated Sand at Different Relative Densities

Abstract With the proposal of a dual carbon goal in China, shallow geothermal energy as a kind of clean energy has been gradually promoted and applied. At the same time, more and more geotechnical workers have gradually paid attention to the influence of temperature on the mechanical characteristics of rock and soil mass. Existing experimental research has shown that the thermodynamic behavior for cohesive soil such as clay and silt is relatively mature but is relatively less mature for noncohesive soil, especially for sand. Based on the hollow cylinder triaxial specimen, a series of temperature-controlled triaxial tests have been carried out on saturated Fujian sand under different initial relative density and temperature conditions to capture the change of axial and volumetric strains for saturated sand specimen with increasing temperature. In addition, a bulk volumetric coefficient of thermal expansion of the sand specimen has been put forward, and the relationship between this coefficient and initial relative density also has been established. Then, the thermal deformation mechanism of saturated sand specimen has been revealed. After that, based on the test data during undrained shearing, the stress–strain relationship, deviatoric stress, and pore water pressure at peak state for loose and medium-dense saturated sand specimens have been explored, which can be used to provide some theoretical guidance for shallow geothermal energy and other temperature-related engineering applications.

Experimental Study on Liquefaction Characteristics of Saturated Sands Mixed with Fly Ash and Tire Crumb Rubber

The liquefaction of saturated sandy soils during dynamic loading can inflict excessive damage on the structures, leading to significant human and economic losses. Recycling and reusing industrial waste materials may offer a sustainable and economic solution to this problem. This study investigates the influence of two waste materials, namely, recycled fly ash and tire crumb rubber, on the liquefaction characteristics of sand. For this purpose, loose and medium-dense triaxial specimens were prepared using sand–fly ash mixtures containing 0–40% of fly ash and sand–tire crumb rubber mixtures containing 0–30% of crumb rubber. The liquefaction characteristics of the specimens were examined through a series of stress-controlled, undrained, dynamic triaxial tests. The tests were conducted at 1 Hz loading frequency and under initial effective confining stresses of 50 and 100 kPa. The experimental results showed that, at a similar relative density, the liquefaction resistance of the sand–fly ash specimens decreased as the fly ash content (FA) increased up to about 20%; then, it slightly increased until FA reached 40%. Sand-only specimens showed greater liquefaction resistance than sand–fly ash specimens. The liquefaction resistance of the sand–tire crumb rubber specimens was enhanced by increasing the rubber content (RC) in the mixtures. It was found that the increasing liquefaction resistance of sand with the addition of tire crumb rubber was more noticeable under higher confining stresses.

Novel sustainable base material for concrete slab track

The use of scrap rubber in ballasted tracks has shown promising results in terms of vibration reduction, enhancement of damping ratio, and so on, but its use in slab tracks has not been explored yet. The present study investigates the use of scrap rubber in the form of ground and granulated rubber mixed with soil and treated with Polyurethane Foam Adhesive (PFA). The optimum content of rubber and PFA were obtained from static direct simple shear test. This optimum rubber and PFA dosage were then used to prepare triaxial specimens and tested cyclically to assess the performance of the treated and untreated soil. A novel base layer for a slab track comprising soil, scrap rubber and PFA is proposed. The novel base material shows superior performance in terms of the reduced settlement, decreased excess pore water pressure and enhanced damping ratio.

Gerilme Kontrollü Dinamik Üç Eksenli Deneyler ile Temiz Kumun Sıvılaşma Davranışının Belirlenmesi

Loosely packed cohesionless soils may suffer partial or complete liquefaction during seismic loading, causing significant structural damage. The dynamic behavior of liquefiable soils is widely investigated through element testing under controlled cyclic loading in undrained conditions. In this work, a total of 20 stress-controlled dynamic triaxial experiments were conducted on saturated specimens of clean sand to improve the understanding of the liquefaction phenomenon. The triaxial specimens were prepared at different relative densities in the range of 38 to 90% and subjected to varying cyclic stress ratios (CSR) with loading frequencies of 0.1 and/or 1 Hz. The experimental results indicated that under similar test conditions, the number of cycles needed for liquefaction was greater at 1 Hz than at 0.1 Hz, revealing that sand specimens exhibited higher liquefaction strength at higher loading frequencies. Furthermore, regardless of the cyclic loading frequency, the liquefaction resistance of sand increased with increasing densities.

Effects of prior non-liquefying undrained cyclic loading on sand liquefaction resistance via discrete element analysis

Earthquake cases demonstrate that prior earthquakes have critical impacts on the liquefaction resistance of sandy soils during subsequent earthquakes. Most previous studies focused on the influence of previous liquefaction events on soil reliquefaction resistance, while few have investigated the effects of prior non-liquefying undrained cyclic loading history on the liquefaction resistance. In this study, discrete element method (DEM) was employed to simulate the liquefaction behaviors of triaxial specimens reconsolidated from non-liquefying states during prior undrained cyclic tests with various cyclic stress ratios (CSRs). Results showed that prior non-liquefying loading could increase or decrease the sand liquefaction resistance, which significantly depended on the CSR of the prior loading and the excess pore pressure ratio (ru) caused during the prior loading. Specimens that were previously loaded under larger CSRs had stronger anisotropy and fewer interparticle contacts after reconsolidation and thereby exhibited lower liquefaction resistance, while specimens that were reconsolidated from prior tests with smaller CSRs had weaker anisotropy and more interparticle contacts and thus exhibited obviously higher liquefaction resistance. Sand liquefaction resistance after a prior non-liquefying loading was well correlated with initial anisotropic degree and mechanical average coordination number.

Representative elementary volume analysis of E-B constitutive model parameters of rockfill materials using DEM

The engineering application of granular materials is based on continuum mechanics formulated at the macroscopic level. Triaxial compression tests are usually used to determine the equivalent mechanical properties of rockfill materials. However, rockfill materials have a wider range of gradation and the particles are more easily crushed, which is not observed for finely graded materials (e.g. sand, grain), except under high stress levels. So both particle arrangement and particle crushing must be considered for rockfill materials, which makes it more complicated to study the effect of specimen size on rockfill materials and find the representative elementary volume (REV). To investigate this topic, the effect of specimen size on the mechanical behavior of basalt rockfill materials was studied by DEM simulation. First, a series of laboratory single-particle crushing tests were conducted to analyze the crushing characteristics of basalt particles and to determine the particle crushing strength parameters required for DEM simulations. Then the REV of the E-B constitutive model parameters of the rockfill materials is assessed by a series of triaxial compression simulation tests with different specimen sizes, particle arrangements and confining pressures. The results show that for basalt rockfill materials with easily broken particles and a wide range of gradation, particle arrangement and specimen size both impact mechanical behavior. The mean value and standard deviation of each mechanical parameter decrease as specimen size increases. The sizes of the REV with different mechanical parameters are not all the same, and the REV for the E-B constitutive model parameters is 360 × 720 mm (diameter D × height H). It is suggested that the triaxial specimen size should not be less than 6 times the maximum particle size when assessing the E-B constitutive model parameters of the rockfill materials.

Characteristics of particle breakage and constitutive model of coarse granular material incorporating gradation evolution

Single particle crushing tests were carried out to research the breakage characteristics of natural rockfill particle. The results showed that the size distribution of broken particles can be described by the normal distribution, and the size distributions of all size fractions converge to a uniform normal distribution after being normalised by the size of the parent particle. Based on the breakage characteristics of rockfill particles, a gradation evolution model was proposed to predict the evolution of the gradation curve of triaxial specimen. This model adopts the loading manner of incremental stress. The correlation among the relative breakage indexes was analysed in terms of the ratio of areas enclosed by the gradation curves and it was found that there is a linear relationship between different indexes. This indicates that there is no essential difference among these indexes from the mathematical point of view. Finally, an elastoplastic constitutive model incorporating the gradation evolution model was developed. This model can predict the stress–strain behaviour and gradation evolution of triaxial specimens of limestone rockfill under the conditions of consolidation, drained and quasi-static shear loading.

Suggested Method of Specimen Preparation for Triaxial Tests on Partially Saturated Sand

Abstract In geotechnical engineering, triaxial testing is widely adopted to evaluate the mechanical behavior of sand. Methods of specimen preparation for triaxial tests on dry and completely saturated sand are well established in the literature, whereas very little guidance exists on the preparation of partially saturated sand at relatively high degrees of saturation (typically Sr &amp;gt; 80 %). The purpose of this study is to elucidate the suitable method of specimen preparation for partially saturated sand using sodium percarbonate and investigate the effects of partial saturation on the undrained cyclic behavior of sand. Loosely and densely packed specimens are prepared in a dynamic triaxial apparatus through dry pluviation and tamping of sand. For tests on fully saturated sand, dry specimens are flushed with carbon dioxide and deaired water and saturated applying back pressure. For tests on partially saturated sand, sodium percarbonate is used in either fine powder or aqueous solution form to create oxygen bubbles in the voids, reducing the degree of saturation of specimens. The results suggest that not only the degree of saturation but also the way partially saturated specimens are prepared affects the liquefaction resistance. At the same level of saturation and principal stress difference, specimens prepared with the aqueous solution exhibit higher liquefaction resistance than those prepared with the powder. The solution of sodium percarbonate proves to be a more reliable and repeatable technique for preparing partially saturated triaxial specimens with relatively high degrees of saturation.

Design of Triaxial Tests with Polymer Matrix Composites.

Multiaxial testing in composites may generate failure modes which are more representative of what occurs in a real structure submitted to complex loading conditions. However, some of its main handicaps include the need for special facilities, the correct design of the experiments, and the challenging interpretation of the results. The framework of this research is based on a triaxial testing machine with six actuators which is able to apply simultaneous and synchronized axial loads in the three space directions. Then, the aim was to design from a numerical point of view a triaxial experiment adapted to this equipment. The methodology proposed could allow for an adequate characterization of the triaxial response of a polymer-based composite with apparent isotropic behaviour in the testing directions. The finite element method (FEM) is applied in order to define the geometry of the triaxial specimen. The design pursues to achieve homogeneous stress and strain states in the triaxially loaded region, which should be accessible for direct measurement of the strains. Moreover, a fixing system is proposed for experimentally reproducing the desired boundary conditions imposed on the numerical simulations. The procedure to determine the full strain tensor in the triaxially loaded region is described analytically and with the help of FEM virtual testing. The hydrostatic component and the deviatoric part of the strain tensor are proposed for estimating the susceptibility of the polymer-based composite to fail due to the triaxial strain state imposed. Then, the loading scenarios that cause higher values of the deviatoric components in the triaxially loaded region are considered to be more prone to damage the region of interest. Nevertheless, the experimental failure is expected to be produced in the arms of the specimen which are uniaxially loaded, since in all of the loading cases the simulations show higher levels of stress concentration out of the triaxially loaded region. Thus, although the triaxial strength could not be accurately determined by the proposed tests, they can be utilized for observing the triaxial response before failure.

Static and dynamic characterization of fiber reinforced sand: A numerical investigation

For the characterization of fiber-reinforced soil under static loading conditions, there is a wealth of literature. However, study on the dynamic behavior of fiber-reinforced soil is very limited. Now that earthquakes are occurring frequently around the world, the dynamic soil analysis has become important for all classes of geotechnical engineering problems. Through numerical simulation of triaxial specimens, the current study explores the behaviour of fiber-reinforced cohesionless soil. The numerical model was validated using existing laboratory triaxial compression testing literature. The stress–strain response of fiber-reinforced sand has been investigated using static and cyclic triaxial testing, as well as other combinations of fiber contents. Fiber-reinforced sand is tested for bulk modulus, shear modulus, and damping values. Effects of fiber contents on static and dynamic stress–strain response of fiber-reinforced soil are highlighted. It has been observed from results that, with an increase in fiber content modulus of elasticity, bulk modulus and shear modulus values increase while damping coefficients decrease for the same. It is believed that the highlighted numerical approach will be an alternative to laboratory experiments to determine the dynamic properties of fiber-reinforced soil.