Variations in strength and deformation of compacted loess exposed to wetting-drying and freeze-thaw cycles
Introduction
Loess and loess-like soils are widely distributed in the world covering about 10% of the terrestrial surface such as in Western America, Southern Russia, Northern France, Canada, Australia, Middle East, Western Europe, and Northwestern China (Fig. 1) (Pécsi, 1990; Taylor et al., 1983). In China, about 7% of the land surface is occupied by loess and loess-like soils (Fig. 2) (Pye, 1987; Sun, 2005). The typical loess is an aeolian deposit with an open and meta-stable structure, characterized by high porosity, low density, and silt-dominated particle size arranged in an open fabric supported by cementation bonds. Due to these characteristics, the loess deposit can exhibit an abrupt collapse due to wetting and overburden pressure which causes geological and geotechnical hazards such as landslides, ground subsidence, differential settlement, and surface cracks leading to the failure of man-made infrastructures built on deposits of loess soil (Li et al., 2015; Derbyshire et al., 1995).
However, loess collapsibility can be eliminated by dense compaction such as surface compaction via vibratory rollers or light tampers, vibro-compaction and dynamic compaction, pre-wetting and chemical stabilization, through the rearrangement of the loess particles, enhancement of bonding strength between particles, and destruction of its open structure (Rollins and Rogers, 1994; Sha and Chen, 2006). Therefore, since these mitigation methods are available at a reasonable cost, the loess has been extensively used as a cheaper filling material than coarse-grained materials in various man-made infrastructures such as highway, railway, airfield, and hydro-engineering and earth dam (Jefferson et al., 2005).
The increase in compaction degree can decrease the hydraulic conductivity, compression, and wetting-induced collapse deformations of remolded loess specimens (El-Ehwany and Houston, 1990; Pereira et al., 2005; Zhang and Zhang, 1992; Wu et al., 1998; Rao and Revanasiddappa, 2006; Liu and Zhang, 1999; Chen and Sha, 2010; Kim and Kang, 2013; Yang and Bai, 2015). The inter-aggregate pores (macropores) can dramatically decrease and intra-aggregate pores (micropores) can decrease at less extent with increasing compaction degree (Romero et al., 2011). However, the loess still keeps its sudden collapse behavior upon wetting when the initial compaction degree is relatively low (Lawton et al., 1992). Based on several centrifuge model tests, Zhang et al. (1998) have also pointed out that the increase in compaction degree can effectively control the settlement of loess as a subbase. When the degree of compaction reaches a specific value, the densely compacted loess can meet the requirements of strength and deformation for the construction of highway embankments. For instance, according to the Chinese highway construction specifications (MTPRC, 2006), a degree of compaction (natural dry density over maximum dry density) >0.93 is required for highway embankments.
The pre-wetting mitigation method entails the wetting of loess to trigger the collapse of its open fabric before a man-made infrastructure is built on for minimizing the impacts of ground settlement on the infrastructure maintenance and serviceability in U. S. and China (Houston and Houston, 1997; Gibbs and Bara, 1967; Li et al., 2011). Although the pre-wetting mitigation method is useful for some foundations on which the applied loads are small, pre-wetting without preloading is not enough to mitigate foundation settlement. Pre-wetting can cause collapse of loess under its existing overburden pressure. Therefore, additional loads can result in additional settlement (Rollins and Rogers, 1994). So pre-wetting has been seldom used in loess foundations.
Collapsible loess can be also stabilized or reinforced by chemical stabilization and mixing various additives such as cement, lime, fly ash, and silicates or any combination of these additives to loess to mitigate or eliminate the loess collapse. Additives can enhance the bonding strength between loess particles through their physical and chemical reactions with water and minerals which increases the overall strength and decreases the compressibility (Bell, 1996; Zhou and Yan, 2006). Chemically treated loess has been successively used around the world in the field and laboratory tests in China (Shan et al., 2010), New Zealand (Tehrani, 1988; Bell et al., 1990; George, 1963; Pengelly et al., 1997), USA (Muhunthan and Sariosseiri, 2008), Thailand (Punrattanasin and Gasaluck, 2008), Bulgaria (Jefferson et al., 2008; Antonov et al., 2015), and Czech Republic (Metelková et al., 2012). According to field and laboratory tests conducted in the former Soviet Union, pre-wetting with a 2% sodium silicate solution can noticeably reduce the compressibility and increase the strength of collapsible loess (Sokolovski and Semkin, 1984).
Although the treated loess can meet the requirements of strength and deformation as a foundation, its strength may decrease and its deformation may increase after long-term strong weathering processes such as wetting-drying (WD) and/or freeze-thaw (FT) cycles (Liu et al., 2010; Ni and Shi, 2014; Wang et al., 2016). The infrastructures built on treated loess foundations may be at risk of instability or failure because of excessive or differential deformation due to these weathering processes. On other similar soils, the WD and FT cycles have a significant deterioration effect on their geotechnical properties. These weathering processes can decrease the bonding strength between particles, increase the void ratio and permeability, and destroy the chemical stabilization (Kay and Dexter, 1992; Wan et al., 2015; Albrecht and Benson, 2001; Sumner and Loubser, 2008; Malusis et al., 2011; Hua et al., 2015; Guney et al., 2007; Stoltz et al., 2014). Wang and Wei (2014) have pointed out that the behavior of expansive soils can be characterized by marked contraction during drying and expansion during wetting. Zhao and Wang (2012) have reported that the wetting-drying weathering can induce the dissolution of cementing matters, expansion of cracks and fissures, change in pore structure, and reduction in water-holding capacity. Under the exposition of several WD cycles, more macropores can hence form in treated expansive soils and their strength can decrease.
The FT cycles are also a major weathering process in cold regions. Sometimes, this weathering process can be more severe than others (Shen, 2004). Many researchers have reported that the freezing-thawing weathering can cause large variations in the distribution of particles, Atterberg limits, dry density, permeability, and strength of fine-grained soils including the silty loess (Shen, 2004; Viklander and Eigenbrod, 2000; Li et al., 2012; Ni and Shi, 2014; Zheng et al., 2015). Zhang et al. (2016) have noted that this weathering process can induce the fragmentation of coarse particles and the aggregation of fine particles, and the particle size eventually becomes homogeneous. The FT cycles have a dual effect on void ratio of soils: loose soils become denser while dense soils become looser, and both loose and dense soils will have the same void ratio after several FT cycles (Viklander, 1998; Li et al., 2017). Moreover, after few FT cycles, the hydraulic conductivity of soils can increase by one or two orders of magnitude (Chamberlain and Gow, 1979; Eigenbrod, 1996). Zheng et al. (2015) have reported that liquid and plastic limits of silty clays increased following the FT cycles due to variations in grain-size and pore-size distributions. In addition, the contents of macro- and mid-pores, controlling the collapsibility of loess, can increase and the bonding of loess particles can partially change from face-to-face contact to point-to-point contact after few FT cycles. These changes reduce the compaction degree and might lead to the secondary collapse, meaning the coefficient of collapsibility of densely compacted loess again reach or excess its benchmark value defining the collapsibility (Mu et al., 2011; Li et al., 2012; Li et al., 2015). Pardini et al. (1996) have pointed out that the FT cycles have an obvious impact on the structure and porosity of the mud-rocks. The porosity can increase with increasing number of FT cycles due to the formation of irregular pores and large cracks and fissures. Hale and Shakoor (2003) have developed a laboratory experiment on sandstones to investigate the impacts of heating-cooling, WD and FT cycles on unconfined compressive strength. They have found a significant deterioration of the mechanical properties on sandstones during the FT cycles. Gullà et al. (2006) have studied the influence of a combination of wetting-drying-freezing-thawing cycles on the compressibility and shear strength of natural clay. They have found that both the compressibility and shear strength decreased with increasing number of weathering cycles. However, the clay fraction and Atterberg limits did not change during these cycles.
Even though numerous studies have been carried out to study the effects of wetting-drying and freezing-thawing weathering on geotechnical properties of various soils including loess, their impacts on the strength and deformation of compacted loess have not been quantitatively compared after the same number of WD and FT cycles, and under the same initial conditions such as the initial water content and dry density. The study presented herein has been designed to quantitatively compare the effects of WD and FT cycles on the strength and deformation based on a series of laboratory experiments including unconfined compression tests, direct shear tests, and oedometer tests. These laboratory results provide a basis for explaining the causes and mechanisms of the second or multi-collapse of compacted loess, and for further understanding the deterioration mechanisms of other similar structural soils.
Section snippets
Sample preparation
The loess in this study was sampled near Yongdeng county (36°36′34″ N, 103°22′05″ E), Gansu province, Northwestern China (Fig. 2). This type of loess was previously used as a primary road embankment fill when the National Lianyungang-Horgos Highway (G30) in Gansu Province was built. According to the Chinese engineering geological zoning map of collapsible loess (MCPRC, 2004), the sampling site is located in the strongly collapsible loess area where the thickness of deposits of collapsible loess
Unconfined compressive strength
According to testing methods of soils for highway engineering (MTPRC, 2007), unconfined uniaxial compression tests were carried out at a vertical strain rate of 1.25 mm per minute on 3 to 4 loess specimens exposed to different weathering intensity. The load and displacement data were recorded by a data logger at a five-second interval during each test. The corresponding stress-strain curves were then plotted as shown in Fig. 4.
For the intact loess specimens and the ones exposed to only one WD
Discussion
The wetting-drying weathering causes a decrease in strength and an increase in deformation of compacted loess. There are two main reasons to explain this mechanical behavior. On the one hand, during wetting-drying weathering, the shrinking-swelling effect of clay minerals can control the mechanical behavior of loess soils. Research finds that loess soil in this study area contains two type of hydrophilic clay minerals, i.e., gaultite (11%) and clinochlore (9%). Their swelling upon wetting can
Conclusion
Unconfined uniaxial compression tests, direct shear tests and oedometer tests on densely compacted loess specimens exposed to different intensity of wetting-drying or thawing-freezing weathering have been carried out to assess the deterioration effects of weathering on the geotechnical properties of loess soils. Some conclusions can be drawn based on these laboratory results as follows:
- 1)
Wetting-drying weathering has a significant influence on the failure modes of compacted loess from a strong
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 41672310 and 41630636), the Science and Technology Major Project of Gansu Province (Grant No. 143GKDA007), National Key Research and Development Program (2016YFC0802103), the West Light Foundation of Chinese Academy of Sciences (CAS) for Dr. G.Y. Li, Research Project of the State Key Laboratory of Frozen Soils Engineering of CAS (Grant No. SKLFSE-ZY-16), and the STS research project of the Cold and Arid Regions
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