An improvement in the method of correcting indirect radial strain measurements during triaxial strength tests in rocks.

The present article provides an improvement in the method to correct indirect strain measurements in triaxial compressive strength tests through axial displacement and hydraulic fluid volume change measurements. The improvement focused on reducing the parameters of the formula proposed for indirect volumetric strain in the original method, thereby facilitating the development of a simpler formula in which the radial strain depends on only two parameters: the initial volume of the rock specimen and the volume changes of the hydraulic fluid for each instant. The comparison between the improvement proposed, and original method resulted in a mean absolute difference of 0.003.•This improvement does not depend on the axial strain, unlike the original method, which requires correcting the indirect axial strain measurements before correcting the indirect radial strain measurements.•This improvement can be useful for research on the stress-strain behavior of intact rock under laboratory conditions, such as in the study of the post-peak state.

Comparative study on mechanical performance of silty soil and expansive soil below canal structures in cold regions.

Silty soil was widely used as filling soil materials for the replacement of expansive soil in cold regions. This paper presents a straightforward approach for the effects of wetting-drying-freezing-thawing cycles on mechanical behaviors of silty soil and expansive soil by laboratory tests. The results showed that the silty soil and expansive soil after 7th wetting-drying-freezing-thawing cycles presented the decreases of elastic modulus, failure strength, cohesion and angel of internal friction by 8.9 %∼12.0 %, 7.7 %∼9.0 %, 7.9 %, 4.5 % and 17.6 %∼37.0 %, 20.5 %∼29.4 %, 43.2 %, 13.0 %, respectively, indicating that wetting-drying-freezing-thawing cycles had little impact on mechanical property of silty soil and a great influence on that of expansive soil. Among them, the mechanical property attenuation ratio in the first three wetting-drying-freezing-thawing cycles accounted for over 90 % of the total. In the meantime, the micro-structure damage, surface crack characteristics and grain size distribution variations of expansive soil were all more significantly than these of silty soil exposed to wetting-drying-freezing-thawing cycles, which brought insight into the causes of the differences in mechanical properties for silty soil and expansive soil. It is found that the silty soil properties were more stable than expansive soil properties, and the silty soil is very effective for replacing the expansive soil below canal structures in cold regions.

Assessing continuum plasticity postulates with grain stress and local strain measurements in triaxially compressed sand.

Critical state and continuum plasticity theories have been used in research and engineering practice in soil and rock mechanics for decades. These theories rely on postulated relationships between material stresses and strains. Some classical postulates include coaxiality between stress and strain rates, stress-dilatancy relationships, and kinematic assumptions in shear bands. Although numerical and experimental data have quantified the strains and grain kinematics in such experiments, little data quantifying grain stresses are available. Here, we report the first-known grain stress and local strain measurements in triaxial compression tests on synthetic quartz sands using synchrotron X-ray tomography and 3D X-ray diffraction. We use these data to examine the micromechanics of shear banding, with a focus on coaxiality, stress-dilatancy, and kinematics within bands. Our results indicate the following: 1) elevated deviatoric stress, strain, and stress ratios in shear bands throughout experiments; 2) coaxial principal compressive stresses and strains throughout samples; 3) significant contraction along shear bands; 4) vanishing volumetric strain but nonvanishing stress fluctuations throughout samples at all stages of deformation. Our results provide some of the first-known in situ stress and strain measurements able to aid in critically evaluating postulates employed in continuum plasticity and strain localization theories for sands.

Experimental study on dynamic behavior of polyacrylamide-reinforced tailings.

Earthquakes are a significant factor that contributes to tailings dam failure. Generally, the seismic stability of a tailings dam can be increased by improving the dynamic properties of tailings. The dynamic properties of tailings can be improved effectively using polymers. In this study, the dynamic properties of polyacrylamide-reinforced tailings were investigated via a sequence of dynamic triaxial tests. The content of polyacrylamide in the test sample was 0.3%. Test results show that the cyclic liquefaction resistance, initial dynamic shear modulus, dynamic shear modulus, and dynamic shear modulus ratio of polyacrylamide-reinforced tailings were slightly greater than those of unreinforced tailings. The damping ratio of polyacrylamide-reinforced tailings was lower than that of unreinforced tailings when the dynamic shear strain exceeded 0.038%. The increase in the dynamic pore water pressure of polyacrylamide-reinforced tailings during cyclic loading decelerated significantly compared with that of unreinforced tailings. The revised Zeng model can effectively described the changes in dynamic pore-water pressure of unreinforced and polyacrylamide-reinforced tailings. The polyacrylamide can improve the structural stability of the tailings specimen and also improve the dynamic properties of the tailings, thereby enhancing the seismic stability of the tailings dam.

Mitigation of sand liquefaction under static loading condition using biogas bubbles generated by denitrifying bacteria.

Nitrogen bubbles that is generated by microbial denitrification process is a new pollution-free clean material for mitigation of sand liquefaction. The current study aims to assess sustainability as well as distribution character of biogas bubbles in sand column under the condition of hydrostatics along with its performance of mitigating sand liquefaction under static loading. A series of laboratory experiments were conducted, and test results indicate that biogas bubbles has excellent sustainability in sand pores, and after 92 weeks, an increase of saturation from 84.5 to 85.1% marking only 0.6% rise. The volume of biogas generated by bacteria increases linearly with decrease of depth. Under undrained condition, if saturation of sample decreased from 100% to around 92.4%, strain softening behavior will transfer to strain hardening, and undrained shear strength can be increased by approximately two times in both of compression and extension tests. The excess pore water pressure ratio and liquefaction potential index have significant reduction with the decrease of saturation, and the magnitude of impact on compression is comparatively bigger than the extension tests. This study validates that as a new material, biogas bubbles are very stable in soil, desaturation using nitrogen bubbles is an effective method for mitigating the liquefaction of sand under static loading conditions. Moreover, the study provides support for the desaturation mitigating static liquefaction of sand to prevent geological disasters and reveals its potential engineering practical value.

Effect of Anisotropy on the Resilient Behaviour of a Granular Material in Low Traffic Pavement.

Granular materials are often used in pavement structures. The influence of anisotropy on the mechanical behaviour of granular materials is very important. The coupled effects of water content and fine content usually lead to more complex anisotropic behaviour. With a repeated load triaxial test (RLTT), it is possible to measure the anisotropic deformation behaviour of granular materials. This article initially presents an experimental study of the resilient repeated load response of a compacted clayey natural sand with three fine contents and different water contents. Based on anisotropic behaviour, the non-linear resilient model (Boyce model) is improved by the radial anisotropy coefficient γ3 instead of the axial anisotropy coefficient γ1. The results from both approaches (γ1 and γ3) are compared with the measured volumetric and deviatoric responses. These results confirm the capacity of the improved model to capture the general trend of the experiments. Finally, finite element calculations are performed with CAST3M in order to validate the improvement of the modified Boyce model (from γ1 to γ3). The modelling results indicate that the modified Boyce model with γ3 is more widely available in different water contents and different fine contents for this granular material. Besides, based on the results, the coupled effects of water content and fine content on the deflection of the structures can also be observed.

A Constitutive Model for Soft Clays Incorporating Elastic and Plastic Cross-Anisotropy.

Natural clays exhibit a significant degree of anisotropy in their fabric, which initially is derived from the shape of the clay platelets, deposition process and one-dimensional consolidation. Various authors have proposed anisotropic elastoplastic models involving an inclined yield surface to reproduce anisotropic behavior of plastic nature. This paper presents a novel constitutive model for soft structured clays that includes anisotropic behavior both of elastic and plastic nature. The new model incorporates stress-dependent cross-anisotropic elastic behavior within the yield surface using three independent elastic parameters because natural clays exhibit cross-anisotropic (or transversely isotropic) behavior after deposition and consolidation. Thus, the model only incorporates an additional variable with a clear physical meaning, namely the ratio between horizontal and vertical stiffnesses, which can be analytically obtained from conventional laboratory tests. The model does not consider evolution of elastic anisotropy, but laboratory results show that large strains are necessary to cause noticeable changes in elastic anisotropic behavior. The model is able to capture initial non-vertical effective stress paths for undrained triaxial tests and to predict deviatoric strains during isotropic loading or unloading.

Characterization of the constitutive behavior of municipal solid waste considering particle compressibility.

This paper presents a characterization of the mechanical behavior of municipal solid waste (MSW) under consolidated drained and undrained triaxial conditions. The constitutive model was established based on a deviatoric hardening plasticity model. A power form function and incremental hyperbolic form function were proposed to describe the shear strength and the hardening role of MSW. The stress ratio that corresponds to the zero dilatancy was not fixed but depended on mean stress, making the Rowe's rule be able to describe the stress-dilatancy of MSW. A pore water pressure reduction coefficient, which attributed to the compressibility of a particle and the solid matrix, was introduced to the effective stress formulation to modify the Terzaghi's principle. The effects of particle compressibility and solid matrix compressibility on the undrained behavior of MSW were analyzed by parametric analysis, and the changing characteristic of stress-path, stress-strain, and pore-water pressure were obtained. The applicability of the proposed model on MSW under drained and undrained conditions was verified by model predictions of three triaxial tests. The comparison between model simulations and experiments indicated that the proposed model can capture the observed different characteristics of MSW response from normal soil, such as nonlinear shear strength, pressure dependent stress dilatancy, and the reduced value of pore water pressure.

Seismic characterization and dynamic site response of a municipal solid waste landfill in Bangalore, India.

Seismic design of landfills requires an understanding of the dynamic properties of municipal solid waste (MSW) and the dynamic site response of landfill waste during seismic events. The dynamic response of the Mavallipura landfill situated in Bangalore, India, is investigated using field measurements, laboratory studies and recorded ground motions from the intraplate region. The dynamic shear modulus values for the MSW were established on the basis of field measurements of shear wave velocities. Cyclic triaxial testing was performed on reconstituted MSW samples and the shear modulus reduction and damping characteristics of MSW were studied. Ten ground motions were selected based on regional seismicity and site response parameters have been obtained considering one-dimensional non-linear analysis in the DEEPSOIL program. The surface spectral response varied from 0.6 to 2 g and persisted only for a period of 1 s for most of the ground motions. The maximum peak ground acceleration (PGA) obtained was 0.5 g and the minimum and maximum amplifications are 1.35 and 4.05. Amplification of the base acceleration was observed at the top surface of the landfill underlined by a composite soil layer and bedrock for all ground motions. Dynamic seismic properties with amplification and site response parameters for MSW landfill in Bangalore, India, are presented in this paper. This study shows that MSW has less shear stiffness and more amplification due to loose filling and damping, which need to be accounted for seismic design of MSW landfills in India.