Studying the Failure Behavior of Cement-fiber-treated Sand under Triaxial Direct Tension Tests

The main goal of this research is to study the failure behavior of cement-fiber-treated sand under triaxial direct tension condition tests. Thus, a new loading system and triaxial cell was designed and built for tensile loading. Samples were prepared with content cement of 3 and 5% (dry wt.) of the sand, while two types of polypropylene fibers 0.024 m in length and 23 μm and 300 μm thick were added at 0.0% and 0.5% (dry wt.) of the sand and cement mixture. After a seven-day curing period, the samples were loaded under triaxial direct tension tests under confining pressures of 100, 200, and 300 kpa in drained conditions. Stress-strain behavior, changes in volume and energy absorbed by cement-fiber reinforced sand were measured and compared with the results of other studies. Adding fibers resulted in reduced peak deviatoric stress and increased residual deviatoric stresses of the cement-fiber reinforced sand, with changes from brittle to ductile behavior. The initial stiffness and stiffness at 50% maximum tensile stress of the samples is decreased with the addition of fibers and with an increase in fiber diameter, the reduction rate of this stiffness is more evident. The absorbed energy for fibers with a thickness of 23 μm is less than fibers with a thickness of 300 μm. The effect of adding fibers to strength parameters showed that the cohesion intercept decreases, while the internal friction angle increases.

Gas Permeability Change With Deformation and Cracking of a Sandstone Under Triaxial Compressiongas Permeability Change With Deformation and Cracking of a Sandstone Under Triaxial Compression

In this paper, a THMC (Thermal-Hydrological-Mechanical-Chemical) multi-field coupling triaxial cell was used to systematically study the evolution of gas permeability and the deformation characteristics of sandstone. The effects of confining pressure, axial pressure, and air pressure on gas permeability characteristics were fully considered in the test. The gas permeability of sandstone decreases with increasing confining pressure. When the confining pressure is low, the variation of gas permeability is greater than the variation of gas permeability at high confining pressure. The gas injection pressure has a significant effect on the gas permeability evolution of sandstone. As the gas injection pressure increases, the gas permeability of sandstone tends to decrease. At the same confining pressure, the gas permeability of the sample during the unloading path is less than the gas permeability of the sample in the loading path. When axial pressure is applied, it has a significant influence on the permeability evolution of sandstone. When the axial pressure is less than 30 MPa, the gas permeability of the sandstone increases as the axial pressure increases. At axial pressures greater than 30 MPa, the permeability decreases as the axial pressure increases. Finally, the micro-pore/fracture structure of the sample after the gas permeability test was observed using 3D X-ray CT imaging.

Investigation on the Influence of Temperature and Confining Pressure on the Hydraulic Conductivity of the Integrated and Fractured Jurassic Conglomerates

This paper presents an experimental investigation on the properties of hydraulic conductivity and permeability of conglomerates under different temperatures and confining pressures with integrated samples and samples with shear failure. Constant head tests were carried out in a temperature-controlled triaxial cell with samples obtained from the Zhuxianzhuang Coal Mine. Five levels of temperatures (10°C, 20°C, 28°C, 35°C, and 50°C) and three levels of confining pressures (3 MPa, 5 MPa, and 7 MPa) were chosen for the tests. The results show that there is a negative relationship between hydraulic conductivity and confining pressure with both original and shear failure samples. An inflection point of 35°C is found in the relationship between the flow rate and temperature. However, with increasing temperature conditions, hydraulic conductivity first increases and then decreases at 50°C with the intact sample, while hydraulic conductivity first decreases from 20°C and then increases with the shear failure sample. Finally, nonlinear regression equations of hydraulic conductivity and temperature were obtained under different confining pressures.

Laboratory simulations of rainfall-induced landslide reactivation mechanisms following the 2016 Kaikōura Earthquake

<p><b>Increases in rainfall-induced landsliding following large earthquake are well documented but the time frames over which this heightened hazard persists in the land scape remains poorly understood. Whilst it is well known that the presence of failed and partially slopes after earthquakes significantly reduces the rainfall thresholds required to activate slope movement, their failure susceptibility during specific storms and how this changes through time remains poorly studied. To improve knowledge in this field requires well documented slope failures following earthquakes and a detailed understanding of their potential failure mechanisms when pore pressures are elevated in the slope. The 2016 Mw 7.8 Kaikōura earthquake provides a unique opportunity to study how rainfall events following the earthquake may impact the timing and mechanisms of landslide reactivation. </b></p><p>This study conducted a suite of specialist triaxial cell experiments, designed to replicate varying rainfall scenarios on remoulded samples collected from two sites where numerous earthquake-induced landslides were recorded in similar Late Cretaceous to Neogene sediments with similar physical properties (the Leader Dam Landslides (LDL) and the Limestone Hill landslide (LHL)). In each experiment rainfall events were simulated using a series of different pore pressure scenarios (increases and decreases in mean effective stress) at representative field stress conditions whilst monitoring material deformation behaviour. </p><p>The results demonstrate that both the deformation behaviour and pore pressure required to generate failure were influenced by the previous changes in pore pressure. Samples subjected to stepped increases in pore pressure were subject to greater pre-failure deformation (dilation) and subsequently failed at lower pore pressures (higher mean effective stress) when compared to samples subjected to linear increases in pore pressure. In addition, increases in the rate of pore pressure also increased the amount of pre-failure deformation allowing failure to occur when pore pressures were lower. In contrast a sample subjected to both increases and decreases in pore pressure underwent pre-failure densification and subsequently required a larger increase in pore pressure to fail. The results demonstrate that landslide reactivation is influenced by a number of factors including the amount and rate of previous changes in pore pressure and the slope drainage history. </p><p>The results provide new insights into why landslide susceptibility may remain elevated for prolonged periods of time (e.g. decades) in the landscape as well as why the rainfall thresholds for site specific failures during storms may be difficult to predict. </p>

Laboratory simulations of rainfall-induced landslide reactivation mechanisms following the 2016 Kaikōura Earthquake

<p><b>Increases in rainfall-induced landsliding following large earthquake are well documented but the time frames over which this heightened hazard persists in the land scape remains poorly understood. Whilst it is well known that the presence of failed and partially slopes after earthquakes significantly reduces the rainfall thresholds required to activate slope movement, their failure susceptibility during specific storms and how this changes through time remains poorly studied. To improve knowledge in this field requires well documented slope failures following earthquakes and a detailed understanding of their potential failure mechanisms when pore pressures are elevated in the slope. The 2016 Mw 7.8 Kaikōura earthquake provides a unique opportunity to study how rainfall events following the earthquake may impact the timing and mechanisms of landslide reactivation. </b></p><p>This study conducted a suite of specialist triaxial cell experiments, designed to replicate varying rainfall scenarios on remoulded samples collected from two sites where numerous earthquake-induced landslides were recorded in similar Late Cretaceous to Neogene sediments with similar physical properties (the Leader Dam Landslides (LDL) and the Limestone Hill landslide (LHL)). In each experiment rainfall events were simulated using a series of different pore pressure scenarios (increases and decreases in mean effective stress) at representative field stress conditions whilst monitoring material deformation behaviour. </p><p>The results demonstrate that both the deformation behaviour and pore pressure required to generate failure were influenced by the previous changes in pore pressure. Samples subjected to stepped increases in pore pressure were subject to greater pre-failure deformation (dilation) and subsequently failed at lower pore pressures (higher mean effective stress) when compared to samples subjected to linear increases in pore pressure. In addition, increases in the rate of pore pressure also increased the amount of pre-failure deformation allowing failure to occur when pore pressures were lower. In contrast a sample subjected to both increases and decreases in pore pressure underwent pre-failure densification and subsequently required a larger increase in pore pressure to fail. The results demonstrate that landslide reactivation is influenced by a number of factors including the amount and rate of previous changes in pore pressure and the slope drainage history. </p><p>The results provide new insights into why landslide susceptibility may remain elevated for prolonged periods of time (e.g. decades) in the landscape as well as why the rainfall thresholds for site specific failures during storms may be difficult to predict. </p>

Elastic waves in particulate glass-rubber mixtures

We investigate the propagation of waves in dense static granular packings made of soft and stiff particles subjected to hydrostatic stress. Physical experiments in a triaxial cell equipped with broadband piezoelectric wave transducers have been performed at ultrasound frequencies. The time of flight is measured in order to study the combined effect of applied stress and rubber content on the elastic properties of the mixtures. The bulk stiffness deduced from the wave speed is nonlinear and non-monotonic with the increasing percentage of rubber with a more prominent effect at higher pressures. Moreover, in the frequency domain, a spectral analysis gives insights on the transition from a glass- to a rubber-dominated regime and the influence of rubber particles on the energy dissipation. Mixtures with rubber content below 30% show enhanced damping properties, associated with slightly higher stiffness and lighter weight.

Gas Permeability Change with Deformation and Cracking of a Sandstone under Triaxial Compression

In this paper, a THMC multi-field coupling triaxial cell was used to systematically study the evolution of gas permeability and the deformation characteristics of sandstone. The effects of confining pressure, axial pressure and air pressure on gas permeability characteristics were fully considered in the test. The gas permeability of sandstone decreases with increasing confining pressure. When the confining pressure is low, the variation of gas permeability is greater than the variation of gas permeability at high confining pressure. The gas injection pressure has a significant effect on the gas permeability evolution of sandstone. As the gas injection pressure increases, the gas permeability of sandstone tends to decrease. At the same confining pressure, the gas permeability of the sample during the unloading path is less than the gas permeability of the sample in the loading path. When axial pressure is applied, the axial stress has a significant influence on the permeability evolution of sandstone. When the axial pressure is less than 30 MPa, the gas permeability of the sandstone increases as the axial pressure increases. At axial pressures greater than 30 MPa, the permeability decreases as the axial pressure increases. Finally, the micro-pore/fracture structure of the sample after the gas permeability test was observed using 3D X-ray CT imaging.

Freezing Pressurized Water into a Standard Cylindrical Ice Sample in a Triaxial Cell

The mechanical characteristics of high-pressure frozen ice are a basis for the design of deep underground frozen walls, the drilling of thick permafrost and ice sheets, and the probing of extraterrestrial ice. The continuous control of the sample stress state from freezing to testing is essential for the experimental study of in situ mechanical response of high-pressure frozen ice. In the context, we developed a preparation technique for freezing pressurized water into a standard cylindrical ice sample in a triaxial cell. Through theoretical analysis, a cylindrical water sample with precise dimensions and strong sealing was fabricated using heat shrinkable tubing, sectional end caps, and an assembly cylinder. A mounting device was designed to insert the water sample into the triaxial cell without deformation. In order to deal with the lateral surface irregular of the resulting ice sample caused by freezing expansion, we proposed a pressurization method in which the volume of the confining medium is controlled to restrict the radial deformation of the sample, and the axial pressure on the sample is kept constant; thus, the freezing expansion will develop along the height direction through releasing the expansion pressure. Based on the analysis of sample deformation and finite element numerical simulations, the control method of the temperature fields of the sample and the confining medium was obtained, and the standard cylindrical ice sample which satisfies the geometric accuracy requirements was produced. The comparison of ice samples frozen by different freezing methods showed that the control of the confining medium mean temperature and the sample unidirectional freezing is necessary to improve the dimensional precision of the ice sample.

A Laboratory Study on the Shear Strength Behavior of Two Till Deposits from Northern Germany

This paper presents the findings of a laboratory study of the shear strength and yielding behavior of two glacial till soil deposits from the area of Heiligenhafen, northern Germany. The tests were conducted on reconstituted forms of the soils using a triaxial cell capable of controlling the temperature of the specimens. The experimental program included a series of multi-stage consolidated drained (CD) compression triaxial tests at temperature ranges between 20 and 60 °C. For the temperature range considered in this study, a mild reduction in the effective friction angle of the two till soils of less than 1° was observed due to an increase in temperature from 20 to 60 °C. All the results were carefully assessed in view of the intrinsic soil behavior and fabric, and existing trends are highlighted. The findings of this study provide valuable insights into the shearing properties of till deposits, and can contribute to the enhancement of existing soil constitutive models as well as the development of new models that are particularly suited to the behavior of glacial tills under elevated temperatures.