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The Influence of Regional Freeze–Thaw Cycles on Loess

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The Influence of Regional Freeze–Thaw Cycles on Loess water Article The Influence of Regional Freeze–Thaw Cycles on Loess Landslides: Analysis of Strength Deterioration of Loess with Changes in Pore Structure Zuyong Li * , Gengshe Yang and Hui Liu School of Architecture and Civil Engineering, Xi’an University of Science and Technology, Xi’an 710054, China; [email protected] (G.Y.); [email protected] (H.L.) * Correspondence: [email protected] Received: 25 September 2020; Accepted: 28 October 2020; Published: 30 October 2020 Abstract: The loess landslide in Gaoling District of Xi’an, Shaanxi in China is closely related to the seasonal freeze–thaw cycle, which is manifested by the destruction of pore structure and strength deterioration of the loess body under freeze–thaw conditions. In order to study the relationship between macro-strength damage and pore structure deterioration of saturated loess under freeze–thaw conditions and its influence on the stability of landslides, this paper explores the effect of freeze–thaw cycles on the strength of saturated undisturbed loess through triaxial compression test, and explores the micro-microstructure changes of saturated undisturbed loess through scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). This is to analyze the evolution of the pore structure and strength loss evolution of saturated loess during the freeze–thaw process, and to describe the freeze–thaw damage of saturated undisturbed loess through the change of porosity and strength deterioration. Then, the internal correlation expression between the porosity change and the strength degradation is established to realize the verification analysis of the test data based on the correlation model. The research results show that: (1) As the number of freeze–thaw cycles increases, the peak strength loss rate gradually increases, and the strength deterioration of saturated loess becomes more and more obvious. (2) The freeze–thaw cycle will lead to the development of pores and cracks in the sample, accompanied by the generation of new cracks, which will cause the deterioration of the pore structure of the sample as a whole. (3) The response of strength damage and porosity deterioration of saturated undisturbed loess is roughly similar under the freeze–thaw cycle. The change in porosity can be measured to better reflect the strength deterioration of saturated loess. Therefore, the change of pore structure of undisturbed loess under freeze–thaw cycle conditions is tested by field sampling and indoor tests to reflect the phenomenon of strength deterioration, thereby analyzing the stability of loess slopes. Keywords: freeze–thaw damage; scanning electron microscopy; nuclear magnetic resonance; porosity; strength degradation; landslide 1. Introduction Loess is characterized by looseness, porosity, and easy water seepage. It is easily eroded by flowing water to form gullies, which can easily cause subsidence and collapse [1–4]. Loess is widely distributed in Northwestern China, as shown in Figure1. Due to the special geographical location and climatic conditions, the annual temperature difference is large about 35 ◦C forming a seasonal frozen soil area with regional characteristics. The region lies in a process of freezing and melting cycles all the year round, and is sensitive to thermal disturbances in the external environment [5]. The freeze–thaw cycle is part of the main factors that cause the deterioration of the physical and Water 2020, 12, 3047; doi:10.3390/w12113047 www.mdpi.com/journal/water Water 2020, 12, 3047 2 of 18 Water 2020, 12, x FOR PEER REVIEW 2 of 18 mechanical propertiesproperties ofof loess, loess, which which aggravates aggravates the the landslide landslide hazard hazard problem problem in thein the loess loess area. area. In the In projectthe project under under construction construction and proposed, and proposed, the yellow the yellow soil foundation soil foundation and slope and are slope prone are to instabilityprone to instabilityand damage and under damage the e ffunderect of seasonalthe effect freeze–thaw of seasonal cycles, freeze–thaw which will cycles, endanger which the will construction endanger andthe constructionassociated safety and [associated6,7]. For example, safety [6,7]. during For theexam occurrenceple, during of athe slope occurrence failure, theof a freeze–thawslope failure, cycle the playsfreeze–thaw a catalytic cycle role. plays When a catalytic frozen, role. the When ice crystal frozen, frost the expansion ice crystal force frost destroys expansion the force soil structure.destroys Thethe soil deformation structure. is The unrecoverable deformation during is unrecoverable melting, resulting during in melting, a decrease resulting in strength in a anddecrease spalling in strengthunder the and softening spalling action under of waterthe softening and the action catalysis of ofwater ice. Theand freeze–thawthe catalysis cycle of ice. will The cause freeze–thaw relatively cycleintense will freeze–thaw cause relatively erosion intens on thee freeze–thaw loess, changing erosion the soil on structurethe loess, and changing causing the uneven soil structure settlement and of causingthe foundation. uneven Therefore,settlement inof seasonallythe foundation. frozen Therefor soil areas,e, in the seasonally freeze–thaw frozen cycle soil is areas, closely the related freeze– to thawthe instability cycle is closely of loess. related So, it to is the necessary instability to furtherof loess. explore So, it is the necessary effects ofto freeze–thawfurther explore cycles the effects on the physicalof freeze–thaw and mechanical cycles on the properties physical of and loess. mechanical properties of loess. Figure 1. SpatialSpatial distribution distribution of of landslides landslides in in the the Loess Plateau of the Yellow River Basin [[8].8]. The freeze–thaw cycle is extremelyextremely destructive, which can change the internal structure of the loess and weaken its mechanical properties, leading to a decrease in the bearing capacity of the loess and causing engineering disasters. Xu et al. [[9]9] studied the meso-structuralmeso-structural characteristics of loess after freeze–thaw cycles, such as equivalent diameter, diameter, particle orientation, circularity, circularity, and pore area ratio, by comparing remolded loess with undisturbed loess.loess. Wang etet al. [[10]10] studied the rate of change of loess volume before and after freeze–thaw by changingchanging the blending ratio, in order to determine the reasonable reasonable ratio ratio so so that that the the loess loess can can maintain maintain its its microstructure microstructure during during the the freeze–thaw freeze–thaw cycle. cycle. Li etLi al. et al.[11] [11 aimed] aimed at the at thestrength strength degradation degradation and and structural structural degradation degradation of compacted of compacted loess loess under under the dualthe dual effects effects of wet of wet and and dry dry and and freeze–thaw. freeze–thaw. Li Liet etal. al. [12] [12 used] used nuclear nuclear magnetic magnetic resonance resonance (NMR) (NMR) technology toto studystudy the the degradation degradation characteristics characteristics of sandstone of sandstone microstructure microstructure in freeze–thaw in freeze–thaw cycles, cycles,and used and fractal used theoryfractal totheory calculate to calculate the fractal the dimension fractal dimension of rock poreof rock development pore development after diff erentafter differentfreeze–thaw freeze–thaw cycles. Tian cycles. et al. Tian [13 ]et studied al. [13] the studied freeze–thaw the freeze–tha propertiesw properties of three soilof three with soil diff erentwith differentcompositions compositions based on nuclearbased on magnetic nuclear resonancemagnetic (NMR)resonance proton (NMR) spin proton relaxation spin time relaxation (T2) distribution time (T2) distributionand free induction and free decay induction (FID) measurements.decay (FID) measurements. Testing the strength of freeze–thaw cycle loess is of great importance for evaluating the stability of slopes and and foundations foundations in in loess loess areas. areas. In In order order to to explore explore the the changes changes of ofmechanical mechanical properties properties of loessof loess under under the the action action of freeze–thaw of freeze–thaw cycles, cycles, it is itfound is found that that the change the change of water of water content content and andthe rate the ofrate freezing of freezing have have a great a great influence influence on onthe the strength strength change change [9,11,14]. [9,11,14 ].Yan Yan et et al. al. [15] [15] studied the unconfinedunconfined compressive strength strength and and pore pore distributi distributionon characteristics characteristics of of lime lime fly fly ash ash loess loess through through a seriesa series of of experiments experiments under under freeze–thaw freeze–thaw cycles. cycles. Xu Xu et et al. al. [16] [16] carried carried out out freeze–thaw tests, direct Water 2020, 12, 3047 3 of 18 Water 2020, 12, x FOR PEER REVIEW 3 of 18 shearshear test test and and scanning scanning electron electron microscope microscope on on loess loess to to study study the the strength strength characteristics characteristics of of loess loess after after freeze–thawfreeze–thaw cycles. cycles. AtAt present, present, there there are are few few studies studies on on the the correlati correlationon between between loess loess strength strength and and pore pore structure. structure. ThisThis paper paper considers considers predicting predicting the
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