Elsevier

Engineering Geology

Volume 274, 5 September 2020, 105725
Engineering Geology

Strain-softening failure mode after the post-peak as a unique mechanism of ruptures in a frozen soil-rock mixture

https://doi.org/10.1016/j.enggeo.2020.105725Get rights and content

Abstract

A large number of traffic systems in China pass through a large area of frozen soil, most of which contains frozen soil–rock mixtures (FSRMs). For a profound understanding of the mechanical behaviours and damage mechanisms of geotechnical and geoenvironmental projects, it is necessary to understand the stress–strain characteristics and failure modes of FSRMs. This study focuses on the uniaxial compression strength (UCS), Young's modulus, and complete stress–strain curve under loading, considering the post-peak behaviour. More than 250 FSRM specimens were tested for UCS evaluations, while considering the effects of different freezing temperatures (−5, −10, −20, −30, and − 40 °C), initial water contents (15.0, 25.0, and 30.0%), and volumetric block proportions (0, 30, 40, 50, and 100%). The stress–strain curve of the FSRM was characterised by six distinct stress levels and deformation stages, different from those of rock or soil in the normal state. Particularly, the turning point from volume compression to expansion was observed after the post-peak, at which superficial cracks were initiated. Ice crystals provided ductility of the FSRM and maximised its load capacity to prevent premature cracks on the FSRM surface. The block stone hindered the linear propagation of cracks. The failure patterns of the FSRM for the UCS evaluation could be classified into four types. The UCS was influenced by mainly the strength of the mixture composed of ice crystals and soil particles and friction and occlusal strength between blocks. Notably, the gradual thawing of the FSRM might cause the largest deformation at the bottom of the active layer in summer and autumn, according to the experimental results. Thus, it is very important to understand the strength characteristics, deformation stages, and failure patterns associated with the geomechanical behaviour of the FSRM for engineering design and applications in a permafrost region.

Introduction

The soil–rock mixtures (SRMs) are widely distributed in the nature and are used in engineering projects such as slopes, foundations, subgrades, and tunnels. The SRM is composed of mainly stone, fine-grained soil, water, and pores. The specific internal structure of the SRM makes it inhomogeneous, discontinuous, and nonlinear on the mesoscale. Its deformation and failure mechanisms are more complex than those of rock on the same scale, which has varying behaviours with different petrographic and mineralogical properties under loads (Coggan et al., 2013; Undul, 2016). This may lead to challenging problems during the design and construction of structures. The geomaterials defined as SRMs (You and Tang, 2002) can also be referred to as bimsoils/bimrocks (Medley, 1994), which implies blocks in matrix soils or in matrix rocks, e.g., mélange (Hsu, 1988), coarse-grained soil (Indrawan et al., 2006; Rahardjo et al., 2008; Uday et al., 2013; Zhang et al., 2014), granular soil (Vallejo and Mawby, 2000; Xiao et al., 2016; Hu et al., 2017; Pegah et al., 2017), and gravelly soil (Chang and Phantachang, 2016; Dong et al., 2017). Typical specimens and outcrops of SRMs are illustrated in Fig. 1.

In the Tibet plateau area, six of the eight large landslides from Linzhi to Songze section (with an accumulated mileage of 427 km along the Sichuan–Tibet highway) belonged to SRM landslides (Shang et al., 2001). The main material composition of the SRM migrated from cold areas at high altitudes is referred to as glacial till from a genetic perspective. Approximately 3.576 × 107 km2 of the Earth's area is covered by permafrost, 24% of the global land area (Lai et al., 2007). Variations in seasonal soil freeze/thaw states are important indicators of climate change and influence the ground temperature, hydrological processes, surface energy, moisture balance (Peng et al., 2016), and freezing heaving/thawing in engineering infrastructures in the Tibet plateau. A large number of traffic systems (Sichuan–Tibet railway, national highway 318, etc.) pass through a large area of frozen SRM (FSRM), most of which consists of frozen glacial till. The ice leads to essential differences between the FSRM and unfrozen soil including the SRM. The FSRM can generally be described as a four-phase material consisting of stone, fine-grained soil, ice crystals, gas, and unfrozen water. The frost action in SRM layers of a subgrade or slope is often ignored, because these materials are usually considered unsusceptible to frost. However, fines in the SRM can modify the frost susceptibility and cause an intensity change leading to more freeze–thaw cycle events, depending on factors such as the fines' content and mineralogy, degree of particle breakage, water availability, and duration of freezing (Konrad and Lemieux, 2005).

The research on pure soil and rock affected by freeze–thaw cycles is comprehensive (Prick, 1995; Chen et al., 2000; Liu et al., 2016). Most of the studies on frozen soil have been limited to inhomogeneous coarse-grained sediments. Some studies have been carried out on the mechanical properties of FSRMs and influencing factors, such as the ice content, temperature, and freezing–thawing cycles. Tests have been carried out to qualify the impacts of gradation, temperature, compaction, and initial moisture content on the formation of pore ice in frozen coarse-grained soils (Fourie et al., 2007). In fine-grained soils, the pore water starts to freeze at −5 °C and strong-binding water can be maintained below −70 °C. In contrast, in the SRM, the free water starts to freeze below 0 °C (Andersland and Branko, 2013). The unfrozen water in the frozen coarse-grained soils is in the middle of the pore space, compared to the unfrozen water film immediately surrounding fine-grained saline or nonsaline soil particles (Arenson and Sego, 2006). The SRM rarely has membrane water. In addition, bound water migration does not occur. Thus, the freezing expansion of the SRM was mainly caused by the change of pore water into solid ice (Wang, 1986). The ice causes essential differences between the FSRM and SRM. When the mixture is frozen under a certain temperature, fine particles and water act as cementing materials, similar to the blocks in matrix rocks (bimrocks). The ice changes the relationship between the components of the SRM, so that the mechanical properties of the SRM largely change upon the freezing. Pore ice can significantly increase the strength of frozen coarse granular debris under normal load (Nickling and Bennett, 1984). The freeze–thaw action has large influences on the hydraulic behaviour, deformation, and strength characteristics of crushable volcanic coarse-grained soils, even if the soil is a non-frost-susceptible geomaterial (Ishikawa and Miura, 2011; Ishikawa et al., 2016). Fractures in the frozen ground self-organise into networks through interactions between sequentially emplaced fractures, tensile stress, and developing fracture pattern (Plug and Werner, 2001). The changes in pore water pressures of the coarse-grained sandy soil during open-system step-freezing and thawing tests depend on the freeze–thaw history, degree of saturation, and temperature (Zhang et al., 2014). Freeze–thaw triaxial experiments, combined with the Particle Flow Code, have demonstrated the relationships between macroparameters and mesoparameters of the SRM (Zhou et al., 2019).

Previous studies have been focused on the strength and deformation characteristics of normal SRM and frozen soil. However, no extensive studies have been carried out on the failure characteristics, failure mechanisms, and geomechanical behaviours of FSRMs (e.g., frozen glacial till). When freeze–thaw disasters occur, the internal structure of the FSRM is destroyed to some extent and its mechanical properties and failure mechanisms are different from those of the SRM in the normal state. Therefore, for a profound understanding of the mechanical behaviours and damage mechanisms of geotechnical and geoenvironmental projects, it is necessary to understand the stress–strain characteristics and failure modes of FSRM. This study was carried out to determine the relationships between the peak strength, failure strength, secant Young's modulus (E50), ice content, and volumetric block proportion (VBP) of the FSRM at different freezing temperatures. The following sections present the main experimental results for the FSRM, focused on the uniaxial compression strength (UCS), E50, and complete stress–strain curve under a uniaxial compression loading, while considering the post-peak behaviour. The findings are important to understand the progressive motion law, damage process, and disaster prevention of frozen glacial till landslides in permafrost areas as well as to study the rheologies of FSRM materials for a detailed investigation of the morphology and flow characteristics of features such as talus cones and rock glaciers in alpine environments (Nickling and Bennett, 1984).

Section snippets

Specimen preparation

The SRM specimens were prepared using a fine-grained silty clay, coarse-grained quartz, and water (Fig. 2). The cumulative curve of grain size grading of the silty clay is shown in Fig. 3, which was sampled from a SRM slope in-situ, located along national highway 318 in Xigaze, Tibet. The maximum particle diameter was smaller than 2.0 mm. The liquid limit of the silty clay was 31.3%, while its plastic limit was 14.5%. The quartz particles had a density of 2.59 g/cm3. The average UCS of the

Failure patterns for pure ice

The UCSs of pure ice specimens were evaluated at −5, −10, −20, −30, and − 40 °C. In this study, the ice crystal type was columnar ice. The uniaxial compression rate of the ice was 1.67 × 10−4 /s, equal to that of the FSRM. The average UCS of the pure ice was 2.63 MPa (Table 1). Three types of failure patterns of the ice specimens for the UCS measurement were observed, shear failure, attributed to the brittleness at a high strain rate, bifurcation failure, attributed to the ductility at a lower

Stress–strain relationship for the conventional rock or soil

The rock failure process was characterised by several distinct deformation stages (Brace et al., 1966). Martin et al. (2001) and Cai et al. (2004) have reported the stress–strain relationship showing the stages of crack development. It is important to evaluate the stress levels associated with these deformation stages for engineering design and applications (Cai et al., 2004). In the stress–strain curve (Martin, 1993; Martin et al., 2001; Cai et al., 2004), σcc is the crack closure stress

Conclusions

This study was focused on the UCS, E50, and complete stress–strain curve under the uniaxial compression loading, considering the post-peak behaviour. The relationships between UCSP, UCSF, E50, ice content, and VBP at different freezing temperatures of the FSRM were analysed in detail. The conclusions of this study can be summarised as follows.

(1) The strain-softening curve of the FSRM at a strain rate of 10−4 /s during a uniaxial compression test was characterised by six distinct stress levels,

Acknowledgements

This research is financially supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (Grant No. 2019QZKK0904), Chinese Academy of Sciences Key Deployment Project (Grant No. KFZD-SW-422), Youth Innovation Promotion Association CAS (Grant No. 2017092), the National Natural Science Foundation of China (Grant No. 41672316), the Science and Technology Project of Yunnan Provincial Transportation Department (Grant No. 2016-160) and the International Cooperation

Declaration of Competing Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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