Experimental Studies of Scale Effect on the Shear Strength of Coarse-Grained Soil

Shear strength is an essential index for the evaluation of soil stability. Test results of the shear strength of scaled coarse-grained soil (CGS for short) are usually not able to accurately reflect the actual properties and behaviors of in situ CGS due to the scale effect. Therefore, this study focuses on the influence of the scale effect on the shear strength of scaled CGS, which has an important theoretical significance and application for the strength estimation of CGS in high earth-rock dam engineering. According to previous studies, the main cause of the scale effect for scaled CGS is the variation of the gradation structure as well as the maximum particle size (dmax), in which the gradation structure as a characteristic parameter can be expressed by the gradation area (S). A total of 24 groups of test soil samples with different gradations were designed by changing the maximum particle size dmax and gradation area S. Direct shear tests were conducted in this study to quantitatively explore the effect of the gradation structure and the maximum particle size on the shear strength of CGS. Test results suggest that the shear strength indexes (i.e., the cohesion and internal friction angle) of CGS present an increasing trend with the improvement of the maximum particle size dmax, and thus a logarithmic function relationship among c, φ, and dmax is presented. Both cohesion (c) and internal friction angle (φ) are negatively related to the gradation area (S) in most cases. As a result, an empirical relationship between c, φ, and S is established based on the test results. Furthermore, a new prediction model of shear strength of CGS considering the scale effect is proposed, and the accuracy of this model is verified through the test results provided by relevant literature. Finally, the applicability of this model to different types of CGS is discussed.

Seepage and Dynamic Characteristics of Carbonaceous Mudstone Coarse-Grained Soil Embankment with Different Gradations

This study investigates the seepage and deformation characteristics of carbonaceous mudstone coarse-grained soil embankment with different gradations under the action of dynamic load and rainfall. An indoor geotechnical test is conducted, and the mechanical parameters of carbonaceous mudstone coarse-grained soil with different gradations are analyzed. A numerical calculation model of seepage and dynamic characteristics of carbonaceous mudstone coarse-grained soil embankment is established on the basis of the test data. The different effects of rainfall infiltration and vehicle load are evaluated. The seepage, settlement, and slope stability evolution characteristics of graded carbonaceous mudstone coarse-grained soil embankment slope are studied. Results show that under the condition of the same rainfall time, the greater the nonuniformity coefficient, the faster the decrease in pore water pressure of the coarse-grained soil embankment at the same monitoring point. The seepage velocity vector in the embankment is concentrated below the soil shoulder. The smaller the saturated permeability coefficient and saturated water content, the larger the seepage velocity vector. The greater the nonuniformity coefficient, the larger the coarse-grained soil embankment under vehicle load. The smaller the embankment settlement, the lower the safety factor of embankment. The safety factor decreases slowly at first and then decreases rapidly in the whole study period under the effect of dynamic wetting. The research results provide a theoretical reference for the practical engineering application of carbonaceous mudstone coarse-grained soil embankment in rainy areas.

A simplified distillation-based sulfur speciation method for sulfidic soil materials

Speciation of inorganic sulfur species, mainly pyrite and metastable iron sulfides by operationally defined methods, is widely used for risk assessment of acid sulfate soils by quantifying the acidity producing elements, as well as for general characterisation of marine sediments and subaqueous soils. “Traditional” sulfur speciation methods commonly use highly specialised glassware which can be cumbersome for the operator, or, require long reaction times which limit the usability of the method. We present a simplified method which has a sufficiently low limit of detection (0.002%) and quantitation (0.006%) required for the analysis of sulfidic sulfur in acid sulfate soil materials. Commercially available sulfide reagents were used for determining reproducibility and the method was assessed on natural sulfidic soil materials, including fine to coarse grained soil materials as well as sulfide bearing peat, with a large variation of metastable sulfide and pyrite content.

Mechanism of the Breakage of Spherical Gypsum Particles under 3-Point Contact Conditions

Particle breakage has a great influence on the mechanical properties of coarse-grained soil materials. In the structure, a particle usually contacts with several surrounding particles when breakage occurs. The crushing mechanism of spherical particles under three-point contact conditions was investigated theoretically and experimentally. In the theoretical analysis, the contact force required for particle breakage is solved by using a stress superposition method based on the ball–ball contact model. To verify the theory, particle contact tests of gypsum spheres under three-point contact conditions were carried out. The experimental results are consistent with the theoretical prediction. Different from the ball–ball contact test, the rupture surface after breakage is a fixed plane passing through all three contact points under three-point contact conditions. Under multi-point contact conditions, the size of the conical core depends on the normal force on the contact point at the moment of particle breakage. Multi-point contact makes particle breakage more difficult, and the stronger the constraint of surrounding spheres, the more difficult it is for the particle to break. Both the theory and the experiment provide evidence that the arrangement of particles affects the overall strength of the coarse-grained soil structure.