The service performance of frozen soil is one of the important factors that needs to be considered in designing and assessing the safety of artificial ground freezing projects. We conducted shear tests on ice-containing frozen soil and assessed soil performance and damage characteristics of the ice-frozen soil interface. On the basis of experimental results, we further investigated the damage of ice-containing frozen soil numerically using the finite-discrete element method. Experimental and numerical results show that temperature, the normal load, and moisture content are the primary factors influencing the mechanical properties of the ice-frozen soil interface. The effects of these parameters on shear strength, shear modulus, cohesion, and angle of internal friction were analyzed and discussed. There was a transition from ductile to brittle behavior at the ice-frozen soil interface with decreasing temperature. Transition occurred at higher temperatures in soils with higher moisture content. Because ice and sand differ in terms of stiffness, fractures appeared first at the ice-frozen sand interface. Under continued loading, the specific form of damage and maximum load-bearing capacity varied as a function of the location of the maximum shear stress zone and the ice in the soil. Our research findings provide valuable theoretical insights for the design and evaluation of the safety of artificial ground freezing engineering projects.

True triaxial tests were conducted on artificially frozen sand. The effects of the intermediate principal stress coefficient, temperature and confining pressure on the strength of frozen sand were studied. The stress-strain curves under different initial conditions indicated a strain hardening. In response to increases of either the intermediate principal stress coefficient or the confining pressure or to a decrease of temperature, the strength typically increased. Furthermore, a new strength criterion was proposed to describe the strength of artificially frozen sand under a constant b-value stress path, combining the strength function in the p-q and pi planes. Considering the low confining pressure, the strength criterion in the p-q plane fitted the linear relationship in the parabolic strength criterion well. The strength criterion in the pi plane was combined with stress invariants, and a new strength criterion was established. This criterion considers unequal tension and compression strength, and integrates temperature. Test results indicated its validity. All parameters of the strength criterion could be easily determined from the triaxial compression and triaxial tensile tests.

The artificial ground freezing (AGF) method is a frequently-used reinforcement method for underground engineering that has a good effect on supporting and water-sealing. When employing the AGF method, the mesoscopic damage reduces the strength of the frozen sandy gravel and consequently affects the bearing capacity of the frozen curtain. However, a few studies have been conducted on the mesoscopic damage of artificial frozen sandy gravel, which differs from fine-grained soil due to its larger gravel size. Therefore, based on triaxial compression tests and CT scanning tests, this paper investigates both the mesoscopic damage mechanism and variations in artificial frozen sandy gravels. The findings indicate that there are contact pressures between gravel tips within the frozen sandy gravel, with damage primarily concentrated around these gravels during incompatible deformation within a four-phase medium consisting of ice, water, soil, and gravel. Furthermore, numerical simulation validates that failure typically initiates at delicate contact surfaces between gravel and soil particles. For instance, when the axial strain reaches 8%, the plastic strain at the location of gravel contact reaches 4.6, which significantly surpasses most of the surrounding plastic strain zones measuring around 1.3. Additionally, the maximum local stress within the soil sample is as high as 48 MPa. This failure event is distinct from viscoplastic failure observed in frozen fine-grained soil or brittle failure seen in frozen rock. The findings also indicate that the mesoscopic damage is about 0.3 when the axial strain is 10%. The study's findings can serve as a valuable guide for developing finite element models to assess damage caused by freezing in sandy gravel using AGF method.

As temporary support in geotechnical and tunneling scenarios, frozen soil bodies are often subjected to varying stress states during different construction stages and techniques and, thus, exhibit stepwise loading and unloading, leading to multi-stage creep. However, experimental and numerical investigations on frozen soil creep behavior have focused primarily on monotonic loading, i.e., single-stage creep. This study expands an existing experimental database on stepwise loaded creep and introduces a unique test series focusing on the uniaxial creep behavior of frozen sand under stepwise unloading and load-unload cycles. Here, similar to stepwise loaded creep, the minimum creep rate is found to remain mostly independent of the loading history, while the corresponding frozen soil lifetime depends on the latter. In contrast to equivalent single-stage creep scenarios, the lifetime becomes longer for stepwise loaded creep and shorter for stepwise unloaded creep. To consider multi-stage creep in the geotechnical design of frozen soil bodies, based on our experimental database and literature data, we test the ability of two versions of an advanced constitutive model to capture the frozen soil creep behavior under varying stress states. Comparison of the extended version, called EVPFROZEN, with the original highlights the advantages of EVPFROZEN in consistently capturing the creep rate evolution and the practically important frozen soil lifetime under complex loading histories. Combining the insights from the novel experimental database with testing and validation of the advanced constitutive model EVPFROZEN advances the efficient and sustainable design of frozen soil bodies in geotechnical applications under multi-stage loading conditions.

The stability performance of the frozen curtain formed under standpipe freezing is closely associated with the weak zone penetrated by thermal gradient-related fracture (TGF). The TGF-rich zone further affects the liquid phase flow when the frozen curtain is thawed. However, there is a lack of studies on the TGF-rich zone within the frozen curtain. To address this gap, a simplified and practical 2D bonded particle model-based numerical simulation strategy was developed to identify the possibility of acquiring field characteristics of the TGF-rich zone by conducting numerical tests on samples considering size effects. The results, validated by the experiment, indicated that the influence of size on crack localization zone was comparable to that of the parameter gradient but had a weaker characteristic on crack orientation, which represents the orientation of TGF. In particular, the characterization result of the TGF-rich zone using crack localization zone in the simulation closely matched that using lateral strain localization zone both in simulation and experiment. Regarding the size effects of the TGF-rich zone revealed in the simulation, the estimated field length of the TGF-rich zone accounted for approximately 30% of the zone width characterized by a horizontal thermal gradient, with maximum orthotropic deformation occurring at about 10% of the zone width. These observations validate the existence of TGF within the frozen curtain and contribute to the development of a precise grouting technique to mitigate subsidence within soil deposits subjected to freeze-thaw.

Constitutive models in the literature for creep of frozen soil are based on the direct use of time counted from the onset of creep. An explicit time dependence in a constitutive equation violates the principles of rational mechanics. No change in stress or temperature is allowed for during creep, using the time-based formulations. Moreover, the existing descriptions need much verification and improvement on the experimental side as well. Creep behaviour of artificially frozen sand was evaluated experimentally. Novel testing methods were used, and new insights into the creep behaviour of frozen soil were gained. Creep rate under uniaxial compression was examined with different kinds of interruptions, like unloadings or overloadings. Experimental creep curves were presented as functions of creep strain. They were brought to a dimensionless form which describes the creep universally, despite changes in stress or temperature. Possible anisotropy of frozen soil was revealed in the creep tests on cubic samples with changes of the loading direction. Using the particle image velocimetry (PIV) technique, information on the lateral deformation and the uniformity of creep were obtained. Volumetric creep of unsaturated frozen soil under isotropic compression was demonstrated to be due to the presence of air bubbles only.

Due to the presence of ice and unfrozen water in pores of frozen rock, the rock fracture behaviors are susceptible to temperature. In this study, the potential thawing-induced softening effects on the fracture behaviors of frozen rock is evaluated by testing the tension fracture toughness ( K IC ) of frozen rock at different temperatures (i.e. - 20 degrees C, - 15 degrees C, - 12 degrees C, - 10 degrees C, - 8 degrees C, - 6 degrees C, - 4 degrees C, - 2 degrees C, and 0 degrees C). Acoustic emission (AE) and digital image correlation (DIC) methods are utilized to analyze the microcrack propagation during fracturing. The melting of pore ice is measured using nuclear magnetic resonance (NMR) method. The results indicate that: (1) The K IC of frozen rock decreases moderately between - 20 degrees C and - 4 degrees C, and rapidly between - 4 degrees C and 0 degrees C. (2) At - 20 degrees C to - 4 degrees C, the fracturing process, deduced from the DIC results at the notch tip, exhibits three stages: elastic deformation, microcrack propagation and microcrack coalescence. However, at - 4 degrees C-0 degrees C, only the latter two stages are observed. (3) At - 4 degrees C-0 degrees C, the AE activities during fracturing are less than that at - 20 degrees C to - 4 degrees C, while more small events are reported. (4) The NMR results demonstrate a reverse variation trend in pore ice content with increasing temperature, that is, a moderate decrease is followed by a sharp decrease and - 4 degrees C is exactly the critical temperature. Next, we interpret the thawing-induced softening effect by linking the evolution in microscopic structure of frozen rock with its macroscopic fracture behaviors as follow: from - 20 degrees C to - 4 degrees C, the thickening of the unfrozen water film diminishes the cementation strength between ice and rock skeleton, leading to the decrease in fracture parameters. From - 4 degrees C to 0 degrees C, the cementation effect of ice almost vanishes, and the filling effect of pore ice is reduced signi ficantly, which facilitates microcrack propagation and thus the easier fracture of frozen rocks. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).