Effects of High Shearing Rates on the Shear Behavior of Saturated Loess Using Ring Shear Tests

The shear behavior of saturated loess was examined by performing a series of ring shear tests with different shearing rates. The effects of shearing rates on the shear behavior of saturated loess with different normal stress are presented and discussed. The results showed that peak shear strength and steady-state shear strength were greater when the shearing rate was low and vice versa. Compared with high and low shearing rates, the maximum strength reduction ratios of peak shear strength and steady-state shear strength were 34.2% and 37.2%, respectively. The axial displacement during shearing was measured and was found to increase with increasing shear displacement in all tests. A comparison of sample height reduction (when the shear rate was stopped) found that the low shearing rate test sample underwent a much greater reduction than the high shearing rate test sample; however, the variation reduction range was within 4 mm. Monitoring the pore-water pressure during the shearing process revealed that it increased with shear displacement, and a higher excess pore-water pressure was generated within the shear zone during the fast-shearing process. Comparing the particle size distribution of the samples after the test and the original sample showed that the particles were crushed during the shearing process. The percentage that was finer than 0.005 mm increased with shearing rates and normal stress, and the soil structure implosion became more pronounced with increasing normal stress.

Evolution of Strain Gradient Formulation in Capturing Localization Phenomena in Granular Materials

This paper highlights the use of incorporating strain gradient into flow stress to study localization behavior in materials. Pioneered by Zbib and Aifantis in the late 1980s, the formulation enabled incorporation of length scales into continuum formulations naturally. The formulation has also evolved into being able to study the effects of microstructure and heterogeneity on localization in granular materials. A multi-slip Mohr-Coulomb type plasticity model with the flow stress in the constitutive equation modified with a higher order gradient term of the effective plastic strain is used for this purpose. The possibility of abrupt changes of mobilized friction caused by intense shearing rate often leads to particle breakage. Its effects on localization is accounted for by modifying the material properties such as mobilized friction using a scaling parameter averaged over a representative elementary area. The change of shearing rate in the integration points was monitored through quasi-statistically measure parameter called inertia number. The inertia number was set to be all the time to consider quasi static less than l.0E-3. The formulation was implemented into a finite element code and used to simulate plane strain compression tests on dry sand. The model highlights effects of confining pressure, anisotropic microstructure, the non-coaxial angle between the direction of principal stress and principal plastic strain rate directions on shear band characteristics.

Research on Direct Shear Strength Characteristics of Mechanically Biologically Treated Waste

Mechanically biologically treated (MBT) waste has significant characteristics such as high stability and low moisture content, which can reduce water, soil, and gas pollution in subsequent treatments. This pre-treatment method is environmentally friendly and sustainable, and has become a popular research topic in the field of environmental geotechnical engineering. Using a direct shear test apparatus and five shearing rates (0.25, 1, 5, 10, and 20 mm/min), the shear strength characteristics of MBT waste at the Hangzhou Tianziling (HT) Landfill were studied. The results indicate: (1). With the increase in horizontal shear displacement, the shear stress of MBT waste gradually increases without a peak stress phenomenon, which is a displacement hardening curve; (2). The shear strength increased with an increase in the shearing displacement rate, and the sensitivity coefficient was 0.64–2.66. (3). The shear strength, shearing rate, and normal stress correlation model are established, and the model has a high degree of fit with the overall experimental data; (4). Cohesion (c), internal friction angle (φ), and the logarithm of the shearing rate are linear; (5). The range of c of MBT waste is 22.32–39.51 kPa, and φ is 64.24–.68.52°. Meanwhile, the test data are compared with the test data in the literature, and the shear characteristics of HT-MBT waste are discussed. The research results of this study can provide a reference for the stability calculation of MBT landfills.

The experimental investigation of the HPAM/phenolic resin deep profile system in fractured low-permeability reservoirs

The problem of injected water channeling along fractures exists in the process of water injection in fractured low permeability reservoir, aimed at this problem, deep profile control technology applies to plug fractures to improve the recovery of low permeability reservoir. In this paper, partially hydrolyzed polyacrylamide (HPAM) is used as water-plugging/profile-modifying agent and phenolic resin as crosslinker agent. Several profile control systems are tested to find the one which is suitable for fractured low permeability reservoirs. The performances of profile control systems are evaluated, and effects of formation water salinity, that of shearing rate and that of temperature on the performance are studied. Finally, in order to study effects of this profile control system on displacing oil, flowability experiment and core displacement experiment are applied. It shows that with the increase of salinty of prepared water and the increase of the shearing rate, the viscosity of this system decreases. With the increase of temperature, the gelling time shortens, the viscosity increases, but the stability weekens. This kind of profile control system has a good effect on plugging fractures of low permeability cores. After water flooding, this kind of profile control system is injected into cores, the recovery ratio can increase 3.5%. So the profile control system composed of HPAM/ phenolic resin can apply to deep profile control in fractured low permeability reservoir to enhance oil recovery.