Large Triaxial Apparatus Research Articles

Influence of particle crushing on the critical state line of rockfill materials

The mechanical behaviour of rockfill materials is strongly affected by particle breakage, which causes a continuous variation of the grain size distribution (GSD) until a stationary condition. Although the evolution of grain crushing caused among others by the initial grading, the relative density and stress level has been extensively studied, the relationship describing the influence of the current GSD on other soil properties such as the critical state line is not uniquely defined. In this study, a series of triaxial compression tests with various stress paths under monotonic loading were performed on a rockfill to examine the effect of particle breakage on the position of the critical state lines (CSLs) in the compression plane. Specimens were prepared with the same initial grading and relative density to investigate the evolution of the GSD as a result of grain breakage determined by different stress paths applied in a large triaxial apparatus. In addition, a recently proposed simplified procedure is employed to capture the evolution of the CSLs of the tested crushable rockfill as a function of a breakage parameter related to the breakage index Bg. Experimental results obtained for the tested rockfill demonstrate that the abovementioned procedure is capable to predict the position of the CSL linked to the GSD reached at the end of the triaxial test, also when the critical state condition, defined as the ultimate condition in which shearing could continue indefinitely without changes in volume or effective stresses, is not reached.

Estimating multidirectional stiffness of soils using planar piezoelectric transducers in a large triaxial apparatus

Cohesionless granular soils display stiffness anisotropy, and accurately characterizing it is essential for numerous geotechnical applications. The inherent anisotropic behaviour of geomaterials is not comprehensively understood; hence, research on the development of novel geophysical testing methods for assessing multidirectional stiffness is important. This contribution presents an innovative arrangement of vertical and horizontal planar piezoelectric transducers to describe the stiffness anisotropy of soils. The transducer assembly can be used to measure nine different elastic wave velocities of a soil specimen in three directions of principal axes having a wide range of median particle sizes. To describe the performance of the developed transducer assembly, four different types of cohesionless granular materials: Toyoura sand, Kashima river sand, glass beads, and Oiso gravel were tested in a large-sized triaxial apparatus having a rectangular specimen of size 235 × 235 × 500 mm. All the tested materials displayed considerably stiffer responses in the horizontal direction as compared to the vertical direction. Except for the coarse gravel, natural sands and glass beads demonstrated isotropic behaviour in the horizontal direction. No significant influence of specimen dimension on the measured vertical wave velocities was detected when the large triaxial test results and medium-sized triaxial test results were compared. The paper also provides a comparison of the vertical stiffness evaluated using planar piezoelectric transducers with those measured statically by applying small-amplitude vertical cyclic loading.

Magnetic measurement system of sandy gravel specimens shape during tests in a large triaxial apparatus

As well known, rockfill behaviour is strongly influenced by both intrinsic (mineralogy and size of particles, grain shapes, soil grading etc.) and state parameters (relative density, stress state, relative humidity). To investigate their mechanical response to stresses and relative humidity (RH) loadings, a large size triaxial device (H = 410 mm, D = 200 mm) has been developed at the University of Naples Federico II (Italy), including modifications required to impose partially saturated conditions in the specimen by means of the vapour equilibrium technique. In order to evaluate local axial and radial strains and global volumetric strains in partially saturated conditions, a magnetic shape detector device has been designed and installed. The accuracy of this system has been evaluated in some isotropic compression triaxial tests on compacted sandy-gravel specimens. The experimental data clearly show the effectiveness of the magnetic system in the measurement of axial displacements while the measurement in radial direction appear to be strongly affected by the non-linearity of the complex magnetic field generated during the test and requires further checking tests.

Loading Frequency Effect on Dynamic Properties of Mixed Sandy Soils

Most of the previous studies focused on pure (clean) sands, silts and clays and less effort has been dedicated toward understanding the dynamic behaviors of natural sandy soils. The purpose of this paper is to evaluate the effect of loading frequency as one of the most important factors affects the dynamic properties especially stiffness and damping characteristics of natural sandy soils mixed with silt and gravel. For this purpose, 40 dynamic triaxial tests were carried out on the cylindrical samples prepared from three mixed sandy materials. Cyclic tests were performed using large triaxial apparatus under different confinement, waveforms and loading frequencies. Results showed that, shear modulus and damping ratio were dependent on confining pressure and loading frequency. Shear modulus and damping ratio increase as loading frequency increases. Moreover, the shear modulus increases as confining pressure increases but damping ratio decreases. However, the effect of triangle, sinusoidal and rectangle waveforms on the dynamic behavior was negligible. At the ranges of strains studied, the effects of number of cyclic loading and excess pore water pressure over and were negligible. There are considerable differences between obtained results for the tested soils and literature results, even for the almost same loading frequency.

The Effect of Stress Induced Anisotropy on Shear Modulus and Damping Ratio of Mixed Sandy Soils

Sand is rarely found clean in natural deposits and it’s usually in composition of gravel, clay and silt. The purpose of this paper is to examine the effect of stress induced anisotropy (\(kc = \sigma^{\prime}_{1} /\sigma^{\prime}_{3} \ne 1\)) as one of the most important factors affecting the dynamic properties of natural mixed (with sand and silt) sandy soils. For this purpose, 77 dynamic triaxial tests is carried out on 10 cylindrical samples prepared from two mixed sandy materials, using large triaxial apparatus in isotropic and anisotropic conditions under different confining pressures and loading frequencies. The results showed that, for the studied materials, increase in anisotropy to \(kc = 2\) cause considerable increase in shear modulus, but shear modulus decreased for greater ratios. Also, shear modulus and damping ratio were dependent to confining pressure and loading frequency so that increase in loading frequency cause increase in shear modulus and damping ratio. Moreover, increase in confining pressure increased shear modulus but decreased damping ratio. The results were remarkably different from the available literature (performed studies on standard sands) at high frequencies.

Experimental Evidences of the Effect of Fibres in Reinforcing a Sandy Gravel

Studies regarding fibre-reinforcement are restricted to clays, silty and sandy soils. No information is available on gravels. It is worth checking the effect of randomly oriented discrete fibres also on such soils to see if they can be beneficial and to get a better insight into the grain to soil interaction mechanism. In this paper, the effect of a small amount of fibres having high aspect ratio on a sandy gravel was analysed by means of tests carried out in a large triaxial apparatus. Specimens of both natural and fibre-reinforced sandy gravel were prepared by wet tamping at different relative densities, and were tested along monotonic and cyclic stress paths. The results show that the addition of a small amount of fibres causes a slight increase in peak strength and a larger increase in ultimate strength at small confining stress, with an overall more ductile behaviour. The cyclic tests at small confining stress and intermediate strain levels show that, for the lowest applied strain (of the order of 10−2%), the stiffness was larger for the reinforced specimens, with a much sharper decrease at larger strains and final values similar for the reinforced and non-reinforced materials.

Use of calibration chamber as a large triaxial apparatus

A segment of a gravel column, 1 m high and 0.53 m in diameter, was modelled in a calibration chamber. The reconstituted sample was consolidated at a given confining pressure and then subjected to axial load exerted by the upper and lower membranes. Volumetric changes in both membranes and in internal chamber were measured. Up to 2% vertical strain was reached and internal friction angle was evaluated. Secant modulus of deformation was determined at considerable strains and at intermediate strain level in the unloading–reloading cycle. For the latter several times higher modulus was found.

Evaluation of Gravel Stiffness by Pulse Wave Transmission Tests

Abstract Laboratory techniques for the measurements of very small strain stiffness parameters of coarse-grained materials include: (a) static tests using local strain transducers like LDTs and (b) wave propagation measurements performed using bender elements, shear plates, and so on. The former method has been employed for most kinds of soil, while the use of wave propagation methods has been restricted to small specimens of sand and clay. At the University of Naples Federico II (DIG), a simple device to generate and monitor compressional and shear waves has been designed to be used in large-scale triaxial apparatuses. The testing device was first used on densely compacted gravel specimens in a large triaxial apparatus in Naples and, after some further improvements, also in another large triaxial cell in Tokyo. In the tests carried out in Tokyo, dynamic and static measurements were performed simultaneously on the single gravel specimens. In this paper, the details of the new system are presented and the results are discussed. The comparison between stiffness moduli evaluated by dynamic and static measurements shows that density, stress state, and strain history effects are similar, but that the dynamically measured stiffness values are consistently larger, about two times, than the statistically determined ones. Since the stiffness of gravels has proven to be strain rate independent, possible reasons for this difference are addressed.

Some geotechnical characteristics of fragmented Queenston Shale

This paper presents the results of an experimental study to determine the stress–strain behaviour of fragmented Queenston Shale from Russell, Ontario and to investigate how its strength properties altered when the material was subjected to repeated slaking cycles. Slaking tests showed that large (cobble-size) blocks of the material degrade rapidly to a particle size of 20 mm upon exposure to water and air. Consolidated drained tests in a large triaxial apparatus under monotonic loading conditions on specimens of fresh and slaked material were performed. They indicate that the shear strength of fragmented Queenston Shale of particle size smaller than 20 mm is not affected by slaking. Key words: Queenston Shale, compaction shale, granular material, rockfill, slaking, shear strength, consolidated drained tests.

Behaviour of dry sand tested in a large triaxial apparatus : Hettler, A; Vardoulakis, I GeotechniqueV34, N2, June 1984, P183–197

Behaviour of dry sand tested in a large triaxial apparatus : Hettler, A; Vardoulakis, I GeotechniqueV34, N2, June 1984, P183–197

Behaviour of dry sand tested in a large triaxial apparatus

In this Paper the results from an extensive experimental study of the stress-strain behaviour of dry sand in triaxial compression tests are presented. The experiments were performed in a new triaxial apparatus for large specimens. Emphasis is given to sources of error such as end restraint, bifurcation and imperfection sensitivity. The behaviour at small and large strains is presented and the rigid granular model of sand, its practical significance and limitations are discussed. Dans cet article on présente les résultats d'une recherche expérimentale sur le comportement rhéologique du sable sec en compression triaxiale. L'appareil triaxial est d'un genre nouveau qui contient de très grands échantillons. Une étude détaillée des erreurs de mesures dues au frottement des plaques, à la bifurcation et à l'influence des imperfections est faite. Le comportement rhéologique est présenté pour de petites et grandes déformations; le modèle du sable avec grains rigides, sa signification pratique et ses limites seront discutés.