Effect of freeze-thaw cycles on shear strength of unsaturated bentonite modified clay

Highlights

• The strength degradation mechanism of clayey soil were studied from microscopic perspective after freeze-thaw cycles.

• The study divided the freeze-thaw damage into three parts according to the main source of water replenishment during cycles.

• Tests show that bentonite as an additive can effectively reduce the damage of soil after freeze-thaw cycles.

Abstract

Freeze–thaw cycles is a seasonal process which occurs in many north regions of soils. Understanding the effects of freeze–thaw cycle on the physico-mechanical behaviours of soils is critical for construction stabilisation and reduce the freeze-thaw damage of foundation engineering. Over the years, studies has been conducted to investigate the effect of freeze-thaw cycles to the periodic strength characteristics of clay. Additive reinforced materials are used to reduce the damages which caused by freeze-thaw cycles and has made series significant contribution to foundation engineering. In this study, bentonite would be added as an additive to investigate the strength characteristics of unsaturated bentonite modified clay after different freeze–thaw cycles. Specimens of natural clay were mixed with different saturation, dry density and bentonite content. The results indicated that dry density, saturation and bentonite content give significant effect on shear strength of bentonite modified clay after freeze–thaw cycles. Meanwhile, the cohesion and internal friction angle of soils gradually attenuated with the increase of dry density, freeze–thaw cycles and saturation. It is found that during the freeze–thaw cycles, the reduction of internal friction angle and cohesion is most obvious at the first freeze-thaw cycle, and gradually stabilized after 7th freeze-thaw cycle. Moreover, results shown that the content of bentonite can effectively improve the cohesion, and maintain positive effects to the internal friction angle. Therefore, appropriate content of bentonite can effectively improve the freeze-thaw resistance of clay.

Introduction

Ningxia Hui Autonomous Region is located in the northern of China's Loess Plateau, which is the arid-semi-arid region of the northwest China. It is dry with little rainfall and extreme winter. The average annual minimum temperature is −28.4 °C, and the sub-zero temperature can last for about 107d (Dong et al., 2018), and the maximum freezing depth is 1.3m or more (Zheng et al., 2006). In Ningxia, previous studies have shown that the rises height of capillary water in clay was more than 2.0m (Xue, 2012). This serious frost-heave and freezing damage after freeze–thaw cycles often cause deformation of the roadbed, slip of the slope, crack and uplift of the lining (Zhao et al., 2015). Therefore, the study of freeze–thaw cycle damage on clay in Ningxia should be taken seriously.

From the microscopic point of view, the migration of capillary water is the main source of water supplement during the freeze-thaw process (Lu et al., 2017; Xu et al., 2016; Ghanbarian and Hunt, 2017; Watanabe and Osada, 2017; Bing et al., 2015). In the area of capillary water migration, only the part of clay which near the groundwater is saturated (S_r>85%). The middle and upper zone of clay are in an unsaturated state which are unsaturated soils. Therefore, as shown in Fig. 1, soil freeze-thaw zone can be divided into three parts. Since there is little rain during winter in Ningxia, only groundwater is considered as the source of water supply. In zone-I, it is unsaturated clay, which is far from the groundwater level, and there is no groundwater recharge during the freeze-thaw cycles, so it is a non-water supplement freeze-thaw region. In zone-III, it is saturated clay that recharged by groundwater during freeze-thaw process, and which is a freeze-thaw problem of saturated soil. Unsaturated soil, especially in zone-II, which has a larger frost-heave property and a higher proportion in the freeze-thaw zone. The freeze-thaw deformation in zone-II is the main component of foundation freeze-thaw deformation, and it is the significant part to research the freeze-thaw effect of clay, so the freeze-thaw deformation of unsaturated soil in the water supplement area (zone-II) should be considered seriously and firstly.

In order to understand the effects of freeze-thaw cycles on the physical and mechanical properties of clay, domestic and foreign scholars have conducted a series of researches in this area. Previous studies suggested that under the condition of freeze-thaw cycles, the strength and cohesion of soil showed a decreased trend, while the internal friction angle increased exponentially (Zhou et al., 2014; Wang et al., 2018; Viran and Binal, 2018). Watanabe et al. (Watanabe and Kugisaki, 2017) performed one-dimensional freeze-thaw test on describing the effects of macro-pores on the formation of frozen soil layers in unsaturated soils. Those results revealed that large pores in the soil can prevent the formation of frozen soil layers. Meanwhile, Steiner et al. (2017) have elaborated the freezing rate and shear strength will be influenced by altering and forming between capillary water and ice crystals. Some studies (Gao et al., 2016; Zhang et al., 2019) pointed that the cohesion and friction angle after freeze-thaw cycles of the clay decrease with the increase of water content and salt content, increase with the increase of dry density and degree of compaction.

According to present studies (Ghazavi and Roustaie, 2010; Ghazavi and Roustaei, 2013; Zaimoğlu et al., 2016; Aldaood et al., 2014a, 2014b; Orakoglu and Liu, 2017; Roustaei et al., 2015), many scholars have added new materials (additives) to enhance the freeze-thaw resistance of soils. Ghazavi et al. (Ghazavi and Roustaie, 2010; Ghazavi and Roustaei, 2013) verified the enhancement of geotextile layer on soil strength characteristics under the condition of periodic freeze-thaw cycles through unconsolidated and undrained triaxial compression test. The test results showed that the addition of geotextiles can effectively enhance the peak strength of the soil under periodic freeze-thaw and reduce the adverse effects of freeze-thaw process. Geotextile layer as a permeable material can transfer the water in the soil pores to the soil layer beyond the depth of the frozen soil, which can effectively reduce freeze-thaw damage. From the perspective of environmental protection and sustainable development, Zaimoğlu et al. (2016) incorporated the secondary product poultry feathers into high-plastic clay, after performed a series of triaxial and freeze-thaw cycles tests, the effect of randomly dispersed chicken feathers on the freeze-thaw cycles of high-plastic clay was studied. The results revealed that feathers can provide a acceptable resistance to slip at the interface between soil particles. Under different lateral pressure conditions, the deviatoric stress increases obviously with the increase of feather incorporation. Aldaood et al., 2014a, 2014b conducted a series of tests on open and closed freeze-thaw cycles. The tests verified that with the increase of cycles, the moisture content of samples increased, which make a decrease in the durability and strength of samples. During the open-system, the water supply and the appropriate gypsum content will have a significant impact on the strength of the soil, the addition of lime will increase the efficiency of samples on water absorption. Orakoglu et al. (Orakoglu and Liu, 2017) found that different added materials would lead to different properties of mixed samples, because different added materials would change the fracture mode of soil fracture surface. The results showed that the moisture content of soil can be effectively reduced by adding appropriate proportion of basalt and glass fiber to samples, and the yield strength can be improved under the condition of freeze-thaw cycles. Roustaei et al. (2015) studied the effect of polypropylene fiber on freeze-thaw characteristics of fine-grained soil, which shown that the reduction of elastic modulus and cohesion in freeze-thaw cycles can be effectively improved by adding polypropylene fiber, the fiber component increased the toughness and plasticity of samples, and also reduced its internal friction angle.

With the development of the economy, foundation engineering in seasonally frozen soil region is developing fast. In previous studies, the use of different additives in soil reinforcement has significant on reducing the freeze-thaw effect of soils. In order to investigate better materials to reduce the effects of freeze-thaw cycles in soils. In this study, bentonite will be used as an additive to study the cyclic freeze-thaw characteristics of clayey soil under limited water supply. Bentonite is a nonmetallic mineral with montmorillonite as its main mineral component. It has characteristics of expansion when mixed with water, and shrinkage when lost water, intensity deformation in different moisture content and outstanding ion exchange properties, which has been applied in soil reinforcement, soil impermeability improvement and soil water retention. Cui et al., 2010, 2018 carried out an experimental study on the improvement of sand permeability by combining bentonite and cohesive soil. Studies have shown that the combination of bentonite and clay can significantly reduce the permeability of sand, bentonite can effectively improve the compressive strength of clay, in a certain range, increasing the increment of bentonite can enhance the liquid limit, plastic limit, unconfined compressive strength (Wen et al., 2016). Bentonite grouts can effectively improve the engineering properties of granular soils (El Mohtar et al., 2015; Yoon and El Mohtar, 2015).

In comparative with traditional external additives, there are limited studies on the effect of freeze-thaw cycles on strength characteristics of unsaturated bentonite modified clay. In order to fill the gap of research in this area and reduce the impact of freeze-thaw damage, based on previous research experience and theoretical analysis, an orthogonal test scheme was designed based on the content of bentonite, dry density and saturation. Through visual analysis, variance analysis and regression analysis and other methods to find some conclusions which are beneficial to reduce the freeze-thaw effect of cohesive soil by compare with the unreinforced clay.