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Soil cementation technology based on microbially or enzymatically induced calcium carbonate precipitation (MICP/EICP) is one of the hot topics in the field of geotechnical and geological engineering in recent years. In this study, a systematic review was performed on this technology, focusing on advances in the cementation mechanisms of MICP/EICP and the influence of soil pore structures, properties of bacterium, urease and cementation solution, and cementation methods on characteristics of calcium carbonate (CaCO3). The results indicate that the smaller the soil pores are, the more difficult the infiltration of microorganisms or urease is and the worse the cementation uniformity is. More contact points among soil particles will produce more deposition points for CaCO3, resulting in stronger bonding and bridging effects and better cementation effects. The generation rate and total amount of CaCO3 increase as the concentration of bacterium or urease and the activity of urease increase in a certain range. However, too high concentration or activity will induce a too high generation rate of CaCO3, resulting in clogging near the injection end. The calcium carbonate crystals obtained from low concentration cementation solution are relatively small and evenly distributed in the soil. The use of appropriate grouting saturation can increase the proportion of CaCO3 with bonding effect. Multilayer alternating injection or one-phase low pH injection can improve the distribution uniformity of CaCO3 in the sample. Based on influencing factors of CaCO3 precipitation characteristics, the improvement of cemented soil uniformity, durability verification, and the adaptability and improvement scheme of applying the laboratory test results to the field scale should be the focus points of the future research.

Water retention curve (WRC) is an important tool to study the hydraulic and mechanical properties of unsaturated soils, such as permeability, strength and deformation properties. Most of the existing WRC models fail to reflect the water retention mechanisms of unsaturated soils or they are complex in form, and these models are hard to give good performance on modelling the bimodal and multimodal WRCs. In this study, based on analyzing the water retention mechanisms of unsaturated soils, the WRC was divided into two domains that are governed by adsorption and capillary mechanisms, respectively. An adsorption water retention curve model (WRCM) was developed based on micro-pore filling theory and Kelvin’s law. A capillary WRCM was established based on the capillary condensation theory and Young-Laplace equation. Then, a new water retention curve model over the full suction range was built by superposing the adsorption and capillary WRCMs. Finally, the new model was validated through modelling the experimentally measured WRCs of six representative unsaturated soils, including Shanghai soft clay, Xi’an loess, Nanyang expansive soil, Guilin lateritic clay, Western Liaoning aeolian soil and Inner Mongolia Gaomiaozi (GMZ) bentonite. Results showed that the proposed model, which was simple in form with definite physical meaning parameters and successfully reflected the adsorption and capillary mechanisms of water retention, was able to simulate WRCs with different shapes for different types of soils under different conditions.

Aging degradation of the geomembrane (GMB) significantly influences the dynamic shear characteristics of the composite liner interface, which comprises the GMB and the compacted clay liner (CCL), potentially jeopardizing the dynamic stability of landfills. In this study, cyclic shear tests were performed on two types of aging GMB/CCL interfaces, concurrently with shear tests on the nonaging GMB/CCL interface for comparison. The results suggest that the impact of aging on the dynamic shear characteristics of the GMB/CCL interface is essentially governed by the surface roughness and brittleness of the GMB, with the effect degree of brittleness influenced by the normal stress. Under low normal stress, aging increased the vertical displacement, dynamic shear strength, and shear stiffness of the GMB/CCL interface. However, under high normal stress, the dynamic shear strength and shear stiffness of the aging GMB/CCL interface were more likely to be lower than those of the nonaging interface. As the displacement amplitude increased, the influence of aging on the shear stiffness of the GMB/CCL interface gradually diminished. Aging also reduced the damping ratio of the GMB/CCL interface. The difference in vertical displacement between the exposed GMB/CCL interface and the in soil GMB/CCL interface caused by brittleness was not significant. In practical engineering, when the overlying load on the GMB/CCL composite liner is relatively small, aging makes the GMB more susceptible to tearing under seismic loads, whereas with larger overlying loads, aging is more likely to increase the shear displacement, thereby increasing the likelihood of instability in landfill. Finally, based on classic models of soil, fitting models for the normalized shear stiffness and damping ratio of the GMB/CCL interface were established and validated. This study can provide reference for analyzing the dynamic stability of landfills during long-term use. [ABSTRACT FROM AUTHOR]

A series of injection tests with compacted Gaomiaozi bentonite of varying dry density were performed using an innovative testing apparatus for hydraulic fracturing visualization, with emphases placed on fracturing macro-morphological dynamics and micro-mechanical mechanisms. Results showed that fracturing dynamics mainly consisted of three stages, namely the hydrating stage, the cracking stage, and the fracturing stage. As dry density increased, the circular hydration zone expanded, the cracking network became more intricate, and the fracturing pattern transitioned from long and clear to short and fuzzy. Breakthrough time and breakthrough pressure increased exponentially with increasing dry density, while injection rate decreased exponentially and breakthrough water volume increased linearly. The fracturing mechanism in compacted bentonite was turning from tensile to shear mode with increasing dry density. By comparing the shear strength with the sum of swelling pressure and tensile strength, a simple criterion for determining the limit state between tensile and shear modes was established, which can be popularized in other soils without swelling potential but under different confining pressures. Finally, a novel calculation method for breakthrough pressure was proposed based on cavity expansion theory, taking into account swelling effects and fracturing failure modes. [ABSTRACT FROM AUTHOR]