Land Degeneration Due to Water Infiltration and Sub-Erosion

sustainability

Article Land Degeneration due to Water Inﬁltration and Sub-Erosion: A Case Study of Soil Slope Failure at the National Geological Park of Qian-an Mud Forest, China

Xiangjian Rui, Lei Nie, Yan Xu * and Hong Wang Construction Engineering College, Jilin University, Changchun 130026, China * Correspondence: [email protected]

Received: 7 August 2019; Accepted: 26 August 2019; Published: 29 August 2019

Abstract: Sustainable development of the natural landscape has received an increasing attention worldwide. Identifying the causes of land degradation is the primary condition for adopting appropriate methods to preserve degraded landscapes. The National Geological Park of Qian-an mud forest in China is facing widespread land degradation, which not only threatens landscape development but also endangers many households and farmlands. Using the park as a research object, we identiﬁed the types of slope failure and the factors that contribute to their occurrence. During June 2017, a detailed ﬁeld survey conducted in a representative area of the studied region found two main types of slope failure: soil cave piping and vertical collapse. Physicochemical properties of the soil samples were measured in the laboratory. Results show that soil slope failure is controlled by three factors: (1) the typical geological structure of the mud forest area represented by an upper layer of thick loess sub-sandy soil and the near-vertical slope morphology; (2) particular soil properties, especially soil dispersibility; and (3) special climate conditions with distinct wet and dry seasons.

Keywords: landscape; mud forest; land degeneration; water inﬁltration; sub-erosion; soil slope failure

1. Introduction Recently, the sustainable development of natural landscapes has received an increasing attention worldwide. Sustainability studies of landscapes have evolved into a vibrant research ﬁeld, especially since 2004–2006 [1]. The landscapes of a territory are the consequence of its history [2] and the result of biodiversity distribution in accordance with physiographic and structural traits [3,4]. It is important to explore the nature of landscape change [5]. Aiming for landscape sustainability science to move forward, it needs to integrate landscape sustainability into other disciplines. Emphasizing both “linking knowledge to action” and “understanding human-environment interactions,” is the essence of sustainability science [1]. Land degradation occurs in all types of landscapes over the world [6]. Concerning the face of mounting changes and threats to landscapes [7], we should be keenly aware of this challenge [8]. The threat to sustainable development caused by land degradation was explicitly recognized at the 1992 Earth Summit and 2002 World Summit on Sustainable Development [9]. Land degradation involves deterioration in soil properties related to crop production, infrastructure maintenance, and natural resource quality [10,11]. It also is associated with the decline in the productivity of ecosystems over time [12]. Approximately 60% of the world’s land area is regarded as degraded and land degradation, including soil erosion, is one of the greatest challenges for land managers [11,13]. Soil erosion is not only a geomorphological but also a land degradation process that may cause environmental damage aﬀecting people’s lives [14,15]. Ebabu et al. [16] regarded soil erosion as a major

Sustainability 2019, 11, 4709; doi:10.3390/su11174709 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 4709 2 of 17 cause of land degradation in different regions of the world. Chalise et al. [11] pointed out that land degradation, particularly soil erosion, is currently a major challenge for Nepal, and rainfall infiltration is thought to accelerate land degradation. Soil erosion by water is also one of the major threats to soils in the European Union, with a negative impact on ecosystem services, crop production, and drinking water [17]. Similarly, land degradation due to soil erosion is a major challenge in Africa [18]. Although land degradation occurs in all kinds of landscapes over the world, the drivers of land degradation vary from region to region [6]. Generally, soil erosion results in the loss of nutrients and fine particles as well as desertification in most semi-arid areas [19–21]. The loess region of China also has suffered from serious soil erosion for years [19]. Found in loess areas, mass movement is very typical and characteristic. The occurrence of mass movement is controlled by a series of internal and external factors, such as rainfall [22–24]. Dry loess can sustain near-vertical slopes; however, a loess area can rapidly disaggregate when locally saturated by rainfall [25] and, thus, a loess slope is highly prone to mass movement processes [26]. Mass movements are common and typical and in loess areas, especially following intense rainfall or prolonged rainfall [27]. Mass movement involves multifarious movement types, and several classification systems for mass movements have been developed [28]. Varnes [29] developed a mass movement classification based on the material (mud, soil, earth, rock, and debris) and movement type (falls, topples, slides, lateral spreads, and flows). Varnes [29] also proposed a further movement type, which he named complex, this type is a combination of two or more principal types of movements [30]. Based on geometry and movement mechanisms, mass movement also can be divided into four categories: bedrock contact landslides, palaeosol contact landslides, mixed landslides, and slides within loess [31]. Xu et al. [32] suggested a systemic classification of loess landslides, including slides, flows, and combined loess and bedrock landslides. Abramson et al. [33] recognized five types of mass movement: falling, toppling, sliding, spreading, and flowing, however, there are few researches on the type of mass movement caused by sub- erosion. Sub-erosion generally refers to various forms of erosion caused by groundwater below the surface [34,35]. Li et al. [35] pointed out that under the action of sub-erosion, the loess soil slope failure with caves mainly is divided into three types: the formation of a cave or cave system and its subsequent deformation and failure; the whole or partial deformation of slope due to the existence of caves; and the subsequent transformation of newly created caves on the slope failure due to rainfall after the overall slope failure. The National Geological Park of Qian-an mud forest in China is used as the research object in this study. Qian-an mud forest was officially approved as a national geological park in 2009. Due to its unique landform (Figure1) caused by sub-erosion, this landscape has become the only protected “mud forest” site in China with these geological features, giving the area a high aesthetic and scientific investigation value. Soil slope failure is a direct and main form of land degradation in the mud forest area. Recently, soil slope failure has become increasingly serious, and the destruction trend is evident. Observed in this area, sub-erosion is the main process of soil slope failure, causing damage to its unique and beautiful landscape and leading to soil mass failure. The destruction scope of the soil slope reaches over 5 km. Soil mass failure threatens numerous households and farmlands in the vicinity (Figure2), as well as the development of the mud forest geological park. This limits the sustainability of the mud forest landscape and local agriculture. The mud forest landscape has distinctive features. It is rare at home and abroad. It is a geological landscape of great ornamental and research value [34]. To make it provide landscape services in a long and stable way, we must understand the causes of landscape degradation. Agricultural income is the main source of income for local residents. Accompanying the expansion of the destruction range of the soil slope, the cultivated land area decreases. Timely suppression of land degradation promotes local sustainability. Several studies have been conducted on the mud forest area, most of which focused on the formation of the mud forest landscape. Take, for example, Zhou et al. [36] who considered the area a peculiar geological landscape formed by the combined action of various geological factors. They proposed the view of protecting the landscape. Chi et al. [37] presented that the mud forest formation is not only related to internal and external dynamic geological processes, but also closely related to the soil composition. Zhu et al. [38] divided Sustainability 2019, 11, x FOR PEER REVIEW 3 of 18 Sustainability 2019, 11, x FOR PEER REVIEW 3 of 18 Sustainability 2019, 11, 4709 3 of 17 related to internal and external dynamic geological processes,processes, butbut alsoalso clclosely related to the soil composition. Zhu et al. [38] divided the formation of mud forest landforms into four periods. Few studies,the formation however, of mud have forest focused landforms on the slope into four failure periods. of the Fewmud studies, forest at however, its current have condition. focused During on the 2013,slope failurecertain ofcontrol the mud measures forest atwere its current implemented condition. in locations During 2013, that certainposed controlthe highest measures threats were to residentialimplemented area, in such locations as cutting that posed slopes, the spraying highest concrete, threats to and residential anchorage, area, however, such as these cutting measures slopes, ultimatelyspraying concrete, failed because and anchorage, the influence however, of sub-erosion these measures has ultimately been neglected. failed because Slope failure the influence was not of controlled,sub-erosion and has the been local neglected. landscape Slope was failure damaged was not severe controlled,ly. Thus, and the the causes local of landscape soil slope was failure damaged must beseverely. clearly Thus, investigated. the causes of soil slope failure must be clearly investigated.

Figure 1. Mud forest sub-erosion landscape. Figure 1. Mud forest sub-erosion landscape.

Figure 2. SoilSoil slope slope failure failure threaten threatenss large areas of farmland. Figure 2. Soil slope failure threatens large areas of farmland. Two typical types of slope failure are found in the mud forest area, namely, soil cave piping and Two typical types of slope failure are found in the mud forest area, namely, soil cave piping and verticalTwo collapse. typical types According of slope to thefailure classiﬁcation are found mode,in the themud former forest isarea, characterized namely, soil by cave the subsequentpiping and vertical collapse. According to the classification mode, the former is characterized by the subsequent transformationvertical collapse. of According newly created to the cavesclassification on the mo slopede, failurethe former due is to characterized rainfall after by the the overall subsequent slope transformation of newly created caves on the slope failure due to rainfall after the overall slope failure, failure,transformation while the of newly latteris created characterized caves on by the sliding. slope fa Weilure realize due to that rainfall the after failure the factor overall of slope soil slope failure, is while the latter is characterized by sliding. We realize that the failure factor of soil slope is closely Sustainability 2019, 11, 4709 4 of 17

Sustainability 2019, 11, x FOR PEER REVIEW 4 of 18 closely related to geological structure, topographic conditions, soil physicochemical properties, and related to geological structure, topographic conditions, soil physicochemical properties, and climatic climaticconditions. conditions. 2. Materials and Methods 2. Materials and Methods 2.1. Description of the Study Area 2.1. Description of the Study Area The National Geological Park of Qian-an mud forest (123 36 123 42 E, 44 45 44 50 N, covering The National Geological Park of Qian-an mud forest (123°36◦ 0−′−123°42◦ 0′E, 44°45◦ ′−0−44°50◦ ′N,0 covering an area of more than 110 km2) is located in the northwest part of Jilin Province, China (Figure3). an area of more than 110 km2) is located in the northwest part of Jilin Province, China (Figure 3). The Themud mud forest forest region region is distributed is distributed in the in western the western alluvial alluvial plain of plain the Songnen of the Songnen plain, where plain, the where terrain the terrainis inclined is inclined from east from to eastwest toand west from and south from to north south due to to north the influence due to the of neotectonic inﬂuence of movement. neotectonic movement.This inclination This inclinationresulted in resultedthe absence in the of absencemodern rivers of modern on the rivers Qian-an on theterrace Qian-an plain, terrace however, plain, however,numerous numerous depressions, depressions, lakes, and lakes, marshes and marshes left over left overfrom from ancient ancient rivers rivers formed formed relatively relatively independentindependent closed closed ﬂow flow areas areas as theyas they permeated permeated and and supplied supplied each each other. other. Preliminary Preliminary statistics statistics shows thatshows nearly that 700 nearly lakes 700 exist lakes within exist an within area of an over area 6 of km over2, including 6 km2, including the large the Dabusu large Lake,Dabusu which Lake, is a typicalwhich and is a representative typical and representative inland lake. inland lake.

Figure 3. Location of the study area. Figure 3. Location of the study area. The Qian-an mud forest region is distributed mainly in the secondary terrace of the Dabusu Lake The Qian-an mud forest region is distributed mainly in the secondary terrace of the Dabusu Lake basinbasin (negative (negative topography) topography) (Figure (Figure4 4).). TheThe trendtrend ofof thethe lake basin basin is is high high and and steep steep in in the the east east but but lowlow and and gentle gentle in in the the southwest, southwest, with with aa relativerelative heightheight difference diﬀerence of of 30 30 m. m. The The secondary secondary terrace terrace elevationelevation is approximatelyis approximately 150–160 150–160 m, m, whereas whereas the the ﬁrst first is is approximately approximately 140140 m,m, andand the lowest point point is approximatelyis approximately 119.1 119.1 m. Based m. Based on the onregional the regional stratigraphic stratigraphic sequence, sequence, the geologicalthe geological age ofage this of regionthis is veryregion late is very Pleistocene. late Pleistocene. The upper The layer upper comprises layer comprises thick loess thick sub-sandy loess sub-sandy soil. soil. Sustainability 2019, 11, x FOR PEER REVIEW 5 of 18 Sustainability 2019, 11, 4709 5 of 17

Figure 4. The Dabusu National Nature Reserve geomorphological map [39]. Figure 4. The Dabusu National Nature Reserve geomorphological map [39]. The study area has a typical temperate continental monsoon climate with four distinct seasons whereThe drought study and area semi-drought has a typical coexist. temperate The continenta annual averagel monsoon precipitation climateis with approximately four distinct 400 seasons mm, nearlywhere 70% drought of which and issemi-drought concentrated coexist. in June, The July, annual and August. average The precipitation annual average is approximately evaporation 400 is approximatelymm, nearly 70% 1800 of mm, which which is concentrated is 4.5 times higher in June, than July, the and amount August. of annual The annual average average precipitation evaporation [40]. Theis approximately annual average 1800 temperature mm, which is is 4.6 4.5◦ Ctimes with higher extremely than highthe amount temperatures of annual varying average from precipitation 30 ◦C to 35[40].C andThe lowannual temperatures average temperature ranging from is 4.630 °C Cwith to extremely23 C. high temperatures varying from 30 °C ◦ − ◦ − ◦ to 35Vegetation °C and low in temperatures the study area ranging is characterized from −30 by°C meadowto −23 °C. steppes. The current vegetation types are mainlyVegetation from the in stipethe study grass area group, is characterized white thorn community, by meadow reed steppes. community, The current erigeron vegetation community, types andare crops.mainly from the stipe grass group, white thorn community, reed community, erigeron community, and crops. 2.2. Field Survey and Sampling 2.2. Field Survey and Sampling A detailed ﬁeld survey was conducted in a representative part within the study area that experiencesA detailed severe field erosion. survey Observations was conducted show in thata representative soil slope failure part iswithin a main the process study area of land that degradationexperiences and severe the extenterosion. of slopeObservations damage is show serious. that The soil failure slope types failure of the is slopea main are process classiﬁed. of Theland slopedegradation also was and observed, the extent and of its slope height damage was recorded is serious. using The afailure tape measure. types of the According slope are to classified. the drill disclosure,The slope wealso drew was theobserved, drill column and its as height shown was in Figure recorded5. The using loess a sub-sandytape measure. soil hadAccording a thickness to the ofdrill 14 m,disclosure, with a top we of drew 1m and the a bottomdrill column of 15 m.as shown Each sample in Figure was 5. collected The loess every sub-sandy two meters, soil forhad a a totalthickness of eight of samples, 14 m, with namely a top C1–C8, of 1m respectively.and a bottom The of location15 m. Each of the sample sampling was point collected can beevery seen two in Figuremeters,4. Meanwhile,for a total of a eight sample samples, of underground namely C1–C8, water respectively. was taken. The location of the sampling point can be seen in Figure 4. Meanwhile, a sample of underground water was taken. Sustainability 2019, 11, 4709 6 of 17 Sustainability 2019, 11, x FOR PEER REVIEW 6 of 18

Figure 5. Drill column near the sampling location. 2.3. Laboratory Test

Soil samples were analyzed for grain size, physical properties, such as moisture content, density, porosity, liquid limit moisture content, and plastic limit moisture content. Grain size composition was determined through the densitometer method, whereas moisture content was calculated through the oven drying method. The densityFigure of5. samplesDrill column was near measured the sampling through location. the cutting ring method. Porosity was determined through the mercury injection method, and the limit moisture content was measured 2.3. Laboratory Test through the combined method of liquid and plastic limits. Qualitative mineralogy was measured using a D2700Soil disamplesffractometer. were analyzed The crystalline for grain composition size, physical was properties, estimated such quantitatively as moisture through content, a density, method thatporosity, combines liquid chemical limit moisture and di ffcontent,ractometric and dataplasti [c41 limit,42]. moisture Regarding content. terms ofGrain soil size chemistry, composition major elementswas determined were determined through the via densitometer X-ray fluorescence method, spectrometry whereas moisture following content the was analytical calculated procedure through of Franzinithe oven etdrying al. [42 method.] and Leoni The and density Saitta of [ 43samples]. This was method measured uses powder through pellets the cutting and is ring based method. on the + fullPorosity matrix was correction determined method. through Total the volatile mercury components injection method, (H2O and the CO 2limit) were moisture determined content as was loss onmeasured ignition through (LOI) at the 950 combined◦C on powders method dried of liquid at 105 and◦C[ 44plastic–46]. limits. The sample Qualitative C8 was mineralogy selected as was the testmeasured object, using its PH a value D2700 and diffractometer. soluble salt content The crysta werelline measured. composition PH was was determined estimated using quantitatively a PHS-3C instrument.through a method The type that and combines amount chemical of cations and and diffractometric anions present data in [41,42]. the sediment Regarding pore terms water of were soil assessed.chemistry, Sediment major elements at natural were moisture determined content wasvia X-ray mixed fluorescence with sufficient spectrometry distilled water following to reach the liquidanalytical limit. procedure This sediment of Franzini water et systemal. [42] wasand Leon left toi and stand Saitta overnight [43]. This to establishmethod uses equilibrium. powder pellets Then, 10–25and is mLbased of poreon the water full wasmatrix extracted correction from method. the saturated Total sedimentvolatile components paste. This saturation(H2O+ and extractCO2) were was + + 2+ 2+ 2 analyzeddetermined for as the loss main on cations ignition (Na (LOI),K at, Ca 950 ,°C and on Mg powders) and anionsdried at (Cl 105−, HCO°C [44–46].3− and SOThe4 sample−) through C8 Dionexwas selected Dx-120 as ion the chromatograph. test object, its PH SAR, value PS, andand TDS soluble were salt then content calculated were using measured. the soluble PH was salt contentdetermined to analyze using a the PHS-3C soil dispersion. instrument. The The chemical type and composition amount of of cations the water and sample anions was present tested. in the sediment pore water were assessed. Sediment at natural moisture content was mixed with sufficient distilled water to reach the liquid limit. This sediment water system was left to stand overnight to establish equilibrium. Then, 10–25 ml of pore water was extracted from the saturated sediment paste. This saturation extract was analyzed for the main cations (Na+, K+, Ca2+, and Mg2+) and anions (Cl-, Sustainability 2019, 11, x FOR PEER REVIEW 7 of 18

3.1. Types of Soil Slope Failure in the Mud Forest Area The failure mode of soil slope is classified in accordance with their different failure mechanisms.

SustainabilityTwo main2019 types, 11, 4709 of soil slope failures were found in the study area: soil cave piping (Figure 6) and7 of 17 vertical collapse (Figure 7).

3. Results3.1.1. Soil and Cave Discussion Piping Failure of Slope Soil cave piping failure of a slope is particularly common in the mud forest area. During the 3.1. Types of Soil Slope Failure in the Mud Forest Area rainy season, surface water collects in negative landforms and penetrates downward along the verticalThe failure joints. modeSurface of water soil slope infiltration is classiﬁed is the insource accordance of piping with failure. their Accompanying diﬀerent failure erosion, mechanisms. the Twoground main surface types of gradually soil slope forms failures geomorphic were found features, in the such study as moniliform area: soil cave sinkholes piping (Figures (Figure 6a,b)6) and verticaland soil collapse bridges (Figure (Figure7). 6c).

FigureFigure 6. Moniliform 6. Moniliform sinkhole sinkhole and and soil soilbridge. bridge. Moniliform Moniliform sinkholes sinkholes can canbe seen be seen in the in the(a) and (a,b ),(b and), and soil soil Sustainabilitybridgesbridges can can 2019be seen be, 11 seen, xin FOR the in PEER( thec). ( cREVIEW). 8 of 18

FigureFigure 7.7. Vertical collapse failure. failure.

During the transfixion of sinkholes, surface water stops penetrating the soil at a certain depth and discharges toward the free face together with groundwater. Meanwhile, horizontal suffosion caves or channels are formed at the foot of the slope (Figure 8), leading to slope failure.

Figure 8. Horizontal suffosion caves or channels formed at the foot of the slope.

3.1.2. Vertical Collapse of Slope Vertical collapse is another common form of slope failure in the mud forest area (Figure 7). Surface water infiltrates along the vertical joints (Figure 9a), and vertical cracks are wide and deep. The water infiltration reduces the shear strength of the soil and cutting forms vertical layered or columnar soil (Figure 9b). Occurring at a certain depth, surface water stops penetrating the soil and discharges toward the free face together with groundwater. Water seepage infiltrating along the Sustainability 2019, 11, x FOR PEER REVIEW 8 of 18

Sustainability 2019, 11, 4709 8 of 17

3.1.1. Soil Cave Piping Failure of Slope Soil cave piping failure of a slope is particularly common in the mud forest area. During the rainy season, surface water collects in negative landforms and penetrates downward along the vertical joints. Surface water infiltration is the source of piping failure. Accompanying erosion, the ground surface gradually forms geomorphic features,Figure such 7. Vertical as moniliform collapse failure. sinkholes (Figure6a,b) and soil bridges (Figure6c). During thethe transfixiontransfixion of of sinkholes, sinkholes, surface surface water water stops stops penetrating penetrating the the soil soil at a at certain a certain depth depth and anddischarges discharges toward toward the free the face free together face together with groundwater. with groundwater. Meanwhile, Meanwhile horizontal, horizontal suffosion suffosion caves or caveschannels or channels are formed are at formed the foot at ofthe the foot slope of the (Figure slope8), (Figure leading 8), to lead slopeing failure. to slope failure.

Figure 8. HorizontalHorizontal suffosion suffosion caves or channels formed at the foot of the slope. 3.1.2. Vertical Collapse of Slope 3.1.2. Vertical Collapse of Slope Vertical collapse is another common form of slope failure in the mud forest area (Figure7). Surface Vertical collapse is another common form of slope failure in the mud forest area (Figure 7). water infiltrates along the vertical joints (Figure9a), and vertical cracks are wide and deep. The water Surface water infiltrates along the vertical joints (Figure 9a), and vertical cracks are wide and deep. infiltration reduces the shear strength of the soil and cutting forms vertical layered or columnar soil The water infiltration reduces the shear strength of the soil and cutting forms vertical layered or (Figure9b). Occurring at a certain depth, surface water stops penetrating the soil and discharges columnar soil (Figure 9b). Occurring at a certain depth, surface water stops penetrating the soil and toward the free face together with groundwater. Water seepage infiltrating along the vertical cracks discharges toward the free face together with groundwater. Water seepage infiltrating along the promotes the movement of loose particles near the foot of the slope, which forms a seepage channel. Horizontal seepage forms hollowed zones at the bottom of the layered or columnar soil. Finally, a staggered collapse occurs due to the sheer force of free face soil and the self-weight of the overlying soil (Figure9c). Following the free face of the soil slope collapse (Figure9d), a new free face is exposed, which results in cyclical slope failure. While the accumulation of soil gradually is carried away by gully river scouring, vertical collapse of slope continues to occur. Sustainability 2019, 11, x FOR PEER REVIEW 9 of 18 vertical cracks promotes the movement of loose particles near the foot of the slope, which forms a seepage channel. Horizontal seepage forms hollowed zones at the bottom of the layered or columnar soil. Finally, a staggered collapse occurs due to the sheer force of free face soil and the self-weight of the overlying soil (Figure 9c). Following the free face of the soil slope collapse (Figure 9d), a new free face is exposed, which results in cyclical slope failure. While the accumulation of soil gradually is Sustainability 2019, 11, 4709 9 of 17 carried away by gully river scouring, vertical collapse of slope continues to occur.

Figure 9. Formation process of vertical slope failure in the mud forest area. (a) Surface water infiltrates Figure 9. Formation process of vertical slope failure in the mud forest area. (a) Surface water infiltrates along the vertical joints; (b) vertical cracks are wide and deep, with vertical layered or columnar soil along the vertical joints; (b) vertical cracks are wide and deep, with vertical layered or columnar soil formed by cutting, and a hollowed zone forms at the bottom of layered or columnar soil by horizontal formed by cutting, and a hollowed zone forms at the bottom of layered or columnar soil by horizontal seepage; (c) staggered slope begins to occur; (d) vertical collapse of slope occurs. seepage; (c) staggered slope begins to occur; (d) vertical collapse of slope occurs. 3.2. Factors Controlling Slope Failure 3.2. Factors Controlling Slope Failure The mud forest landscape not only has the aesthetic experience of landscape, but also can increase the geologicalThe mud knowledgeforest landscape for tourists. not only It is ahas rare the geological aesthetic landscape experience in the of world.landscape, However, but also from can the increaseperspective the of geological the ecological knowledge environment for andtourists. geological It is disasters,a rare geological it is very fragilelandscape [36]. in The the landscape world. However,is suffering from from the twoperspective failure forms of the mentioned ecological above. environment To develop and the geological geological disasters, landscape it resources is very fragileof mud [36]. forest The sustainably, landscape it is is suffering necessary from to figure the two out howfailure to coordinateforms mentioned natural above. geological To develop phenomena the geologicalwith geological landscape problems. resources The slopeof mud failure forest has sust reducedainably, the it area is necessary of cultivated to landfigure which out how has ledto coordinateto a reduction natural in land geological use effi phenomenaciency, which with threatens geological local problems. economic The development. slope failure Accordinghas reduced to thethe area research, of cultivated we found land that which the failure has led factor to a reduction of soil slope in land closely use isefficiency, related to which geological threatens structure, local economictopographic development. conditions, climaticAccording conditions, to the research, and soil physicochemicalwe found that the properties. failure factor of soil slope closely is related to geological structure, topographic conditions, climatic conditions, and soil physicochemical3.2.1. Geological properties. Structure and Topography Approximately 20,000 years ago, in the early to late Pleistocene, the function and influence of neotectonic movements caused the ground to rise gradually and tilt toward the northwest. The Huolin River gradually moved toward the north, and with lake basin areas shrinking and differentiating, the Dabusu Lake was formed as a residual lake. During the neoid period, the earth’s crust rose several times, especially tilted toward the northwest with numerous fault activities. Thus, the banks of the Dabusu Lake basin had asymmetric topography characteristics, with the gully on the east bank further Sustainability 2019, 11, 4709 10 of 17 developed with strong erosion and sub-erosion. This phenomenon was the basis of the formation of the mud forest area and also was the geological structure factor of the soil slope failure. Geotectonically, the mud forest area belongs to a depressed part of the Songnen plain. Since the Miocene era, this basin-like plain has been subsiding and accumulating thick Mesozoic sediments continuously. Drilling revealed that the upper part of the Songnen plain is loess sub-sandy soil with large pores and abundant vertical cracks, and the visible thickness of this layer is greater than 21 m. Field observation and measurements reveal that, on the surface, there are many negative terrains, such as pits. These negative terrains are conducive to rainfall accumulation. Subsequently, the accumulation of rainwater promotes the occurrence of sub-erosion. This accelerates the process of soil cave piping failure. The occurrence of mass failure depends on the characteristic of landforms, which can be represented by slope gradient, slope height and slope surface morphology [47]. Generally, the larger the slope gradient and the higher the slope height, the more likely vertical collapse will occur. The angle of the soil slope in the mud forest area is nearly vertical, and the slope height ranges from 15 m to 21 m. This creates topographic conditions for the vertical collapse of the slope. Generally, terrain slope profiles may be grouped into three types: plane, concave, and convex [47]. Most of the soil slopes in the mud forest are concave, which are prone to runoff accumulation and erosion. The especially developed vertical crack of the soil slope becomes an important reason for the vertical collapse of slope. These vertical cracks provide channels for precipitation infiltration. Following the infiltration of rainfall, the crack expands. Consequently, soil shear strength decreases, and vertical collapse occurs. Vertical fissures and beaded drop holes are evident on the free face of the slope, and the failure range of soil slope increases annually in an apparent trend. 3.2.2. Climatic Conditions Rainfall plays an important role in stimulating the destruction of soil slopes [48]. Given that rainfall is concentrated into short periods of time, most geomorphic work (e.g., soil erosion) occurs in short temporal intervals [49]. Occurring in the mud forest region, the dry season lasts for 8 months. The total annual average precipitation is 404 mm (Table1), over 80% of which falls during the wet season (from June to September). Thus, the precipitation in this area is particularly concentrated. The total annual average evaporation is approximately 1800 mm. The concentrated precipitation in a short period makes the surface soil vulnerable to erosion and the long drought season facilitates the development of abundant vertical joints and cracks in the soil material surface [50]. The cracks in the soil gradually enlarge and widen under the action of dry–wet circulation and erosion.

Table 1. Total annual average precipitation and evaporation in Songyuan, Jilin province. The data include the average data of precipitation and evaporation from 2014–2018 [51].

Month 1 2 3 4 5 6 7 8 9 10 11 12 Total Precipitation (mm) 1.19 1.70 5.93 15.68 25.78 63.59 136.19 94.25 37.60 14.59 5.16 2.35 404.05 Evaporation (mm) 18.8 32.5 91.1 233.9 344.0 287.9 242.1 198.8 162.2 125.0 51.3 22.1 1798.5

The rain soaks the soil for a long time, in the concave surface terrain, and the soil underneath is eroded at the same time, on the one hand. The infiltrating surface water carries away the fine particles in the soil and weakens its cementation strength, which expands the pore volume of the soil. The overlying soil body collapses and, thus, the process of soil cave piping failure is accelerated. Rainwater enters the inner soil body via cracks along the numerous vertical joints and tends to weaken the soil consistence, on the other hand. The vertical joints expand due to rainwater infiltration and sidewall soil body cutting. The soil body absorbs part of the rainwater entering the cracks, increasing the sidewall weight, which becomes an important reason for its sliding. The sidewall slide finally leads to the vertical failure of the slope. The rainy season is also a high temperature period. The cycle of rainfall and evaporation occurs. It is also a wet and dry cycle. Dry–wet circulation leads to soil fracture expansion, which is more conducive to rainfall infiltration [50]. This is also an important reason for slope vertical collapse. Sustainability 2019, 11, 4709 11 of 17

3.2.3. Soil Physicochemical Properties Certain physicochemical properties of the clays were found to influence the different erosion processes fundamentally [52]. The textural data in Table2 and Figure 10 demonstrate that all samples are sandy–clayey silt with a high sandy fraction. Grain size analysis shows that (a) no particles are larger than 0.5 mm, (b) approximately 70% of particles are smaller than 0.075 mm, and (c) a clay fraction <0.005 mm represents that all samples are approximately 25%. Soil particles are dominated by silt with weak hydrophilic, which is easy to erode by precipitation and groundwater during the rainy season. The data in Table3 demonstrate that all samples have a high porosity greater than 50%. These pores provide channels for rainfall infiltration and migration of fine particles, accompanied by the process of physical sub-erosion [53]. The higher porosity accelerates the sub-erosion process. Fine particles migrate through pores to the free face along with water. The soil cave piping failure of the slope is accelerated. Table3 shows several physical soil properties, including the important soil plasticity index. The plasticity index of the soil samples is small; thus, the plasticity of the soil sample is poor and easily suffers irreparable damage when exposed to external forces. Furthermore, the liquid indices of the soil samples are less than 0, indicating that the soil samples have no plasticity [54].

Table 2. Grain size composition.

Grain Size Composition (wt. %) Sample Depth (m) (2–1) (1–0.5) (0.5–0.25) (0.25–0.075) (0.075–0.005) <0.005 >2 mm mm mm mm mm mm mm C1 1 - - - 0.87 23.80 51.35 23.98 C2 3 - - - 0.44 25.00 47.95 26.60 C3 5 - - - 0.63 28.57 41.89 28.92 C4 7 - - - 1.77 24.60 46.66 26.97 C5 9 - - - 1.17 24.20 52.53 22.10 C6 11 - - - 0.12 30.23 45.68 23.97 C7 13 - - - 0.68 28.46 48.14 22.72 SustainabilityC8 2019, 11, x 15 FOR PEER REVIEW - - - 1.73 29.83 40.45 27.9812 of 18

Figure 10. Grain size composition of the plot samples on the ternary diagram.

Table 3. Physical properties of soil.

Plastic Liquid Density Moisture content Plastic Liquidity Sample limit limit Porosity (g/cm3) (%) index index (%) (%) C1 1.23 10 20.19 24.71 4.52 −3.47 55.65 C2 1.30 10.15 19.46 22.19 2.73 −2.25 57.60 C3 1.59 15.71 18.80 24.87 6.07 −0.51 58.38 C4 1.35 15.71 18.30 26.17 7.87 −0.33 53.21 C5 1.36 16.73 17.90 28.67 10.77 −0.11 54.47 C6 1.51 17.22 18.90 24.80 5.90 −0.28 50.48 C7 1.33 16.65 20.91 24.84 4.61 −1.84 56.69 C8 1.26 16.65 20.91 24.84 3.93 −1.08 56.43

Table 3. Physical properties of soil.

Density Moisture Plastic Liquid Plastic Liquidity Sample Porosity (g/cm3) Content (%) Limit (%) Limit (%) Index Index C1 1.23 10 20.19 24.71 4.52 3.47 55.65 − C2 1.30 10.15 19.46 22.19 2.73 2.25 57.60 − C3 1.59 15.71 18.80 24.87 6.07 0.51 58.38 − C4 1.35 15.71 18.30 26.17 7.87 0.33 53.21 − C5 1.36 16.73 17.90 28.67 10.77 0.11 54.47 − C6 1.51 17.22 18.90 24.80 5.90 0.28 50.48 − C7 1.33 16.65 20.91 24.84 4.61 1.84 56.69 − C8 1.26 16.65 20.91 24.84 3.93 1.08 56.43 −

Tables4 and5 show the mineralogical and chemical compositions of the samples, respectively. The mineral composition of sediments is relatively uniform; quartz, potash feldspar, plagioclase, and calcite are the main non-clay components, and dolomite components are only present in the C8 sample. The clay mineral assemblage is characterized by a mass of illite/smectite mixed layer, minerals, with a small number of chlorites. Illite/smectite mixed layer minerals have a weak connection, it is easy for the water molecule to infiltrate, and the shear strength is low. The shear strength of soil decreases after water absorption, which is a reason for soil slope failure. The mixed layer minerals not only have a strong hydrophilicity, but also have the property of expansion and contraction. Its cementation and adhesion often change reversibly with the change of water content. That is to say, the cementation is strong in the dry climate, and weak in the precipitation infiltration condition [23]. Therefore, after rainfall infiltration, soil cementation strength decreases. Further, soil shear strength decreases. The soil cannot bear the shear stress of the sidewall soil and vertical collapse occurs. The chemical composition of the studied samples (Table5) is uniform and closely reflects their mineralogy. The chemical compositions of SiO2, Al2O3, and CaO are the highest, which is consistent with the main mineral compositions of quartz and feldspar.

Table 4. Mineral composition of soil.

Sample Qz Phf Pe Cc Dol Chl Ism C1 30 26 10 9 - 2 23 C2 28 29 7 9 - - 27 C3 32 26 6 6 - - 30 C4 30 26 8 8 - - 28 C5 34 22 9 9 - - 26 C6 30 33 8 7 - - 22 C7 39 26 8 8 - - 19 C8 38 28 9 5 4 - 16 Note: Qz = quartz; Phf = potash feldspar; Pe = plagioclase; Cc = calcite; Dol = dolomite; Chl = chlorite; Ism = illite/smectite mixed layer minerals.

Table 5. Chemical composition of soil (major element, in wt. %).

SiO2 Al2O3 Fe2O3 FeO CaO MgO K2O Na2O TiO2 P2O5 MnO LOI C1 73.73 9.70 1.86 0.36 3.63 0.89 2.78 1.57 0.36 0.05 0.07 4.60 C2 70.25 11.36 1.88 0.23 3.85 0.95 2.66 1.99 0.44 0.05 0.07 5.77 C3 68.80 12.96 2.01 0.45 3.97 1.12 2.81 1.67 0.39 0.06 0.07 5.34 C4 71.23 11.95 2.66 0.47 4.04 1.08 2.95 1.86 0.55 0.07 0.08 3.01 C5 69.54 10.88 2.36 0.39 4.34 1.05 2.88 2.11 0.48 0.06 0.09 5.78 C6 65.60 12.56 2.89 0.54 5.27 1.36 2.87 2.01 0.57 0.07 0.09 6.16 C7 67.78 12.89 2.56 0.49 4.22 1.21 2.77 2.21 0.57 0.07 0.07 5.14 C8 68.48 12.02 2.48 0.49 4.29 1.16 2.96 2.24 0.58 0.07 0.07 4.74 Sustainability 2019, 11, 4709 13 of 17

Monovalent cations, particularly sodium and lithium, promote dispersion, whereas di- and trivalent cations favor ﬂocculation [55]. Table6 shows the content of monovalent cation (Na + and K+) evidently is higher than that of divalent cation (Mg2+ and Ca2+). The diﬀusion layer formed by sodium ions is relatively thick, and the high content of sodium ions reduces the cohesion of soil particles, which is conducive to their dispersion. An alkaline environment for soil dispersion was provided because the PH is 8.2 (Table6), which provides an environment for the occurrence of chemical sub-erosion.

Table 6. Soluble salt content (meq/l) and related parameters controlling clay dispersivity.

+ + 2+ 2+ a b c Na K Ca Mg SO2–4 Cl− HCO-3 TDS PS SAR PH Content 7.844 0.014 1.896 0.301 1.623 4.011 4.865 10.055 78.011 7.484 8.2 a TDS = total dissolved salts = Na+ + K+ + Ca2+ + Mg2+ (in meq/L); b PS = percentage sodium = [Na+ (meq/L)/TDS (meq/L)] 100; c SAR = sodium adsorption ratio = Na+ (meq/L)/[(Ca2+ + Mg2+)/2]1/2. × Clay dispersibility is a good indicator of the dispersion vulnerability of soil and, therefore, of the associated risks of soil erosion [56]. Sherard et al. [57] regarded SAR, TDS, and PS as the dispersion functions and discussed the relationship (Figure 11). They showed that the clays of zone A have a high tendency for spontaneous dispersion, the materials of zone B are ordinary erosion-resistant clays, and the sediments of zone C may be dispersive or nondispersive. Table6 presents the results of SAR, TDS, and PS. The sample plots fall under zone A (Figure 11), which indicates its high dispersivity. Clay dispersibilitySustainability results 2019, 11 in, x FOR the PEER disruption REVIEW of soil stability and fragmentation [58–61] and increased14 of 18 soil water erosion [59,62]. Furthermore, Sherard et al. [57] pointed out that dispersive clays most often dispersivity. Clay dispersibility results in the disruption of soil stability and fragmentation [58–61] developand large increased pipes soil and water erosion erosion tunnels [59,62]. through Furthermore, rapid Sherard enlargement et al. [57] of pointed small out cracks that dispersive and fissures as a consequenceclays most of the often spontaneous develop large dispersion pipes and erosion of clays tu liningnnels through the fissure rapid walls enlargement when theseof small come cracks in contact with rainwater.and fissures Thus, as a theconsequence parent materials of the spontaneous of the mud dispersion forest of area clays have lining a greatthe fissure propensity walls when to produce a colloidalthese dispersion come in contact when with saturated rainwater. by Thus, rainwater. the parent The materials dispersion of the mud of cementing forest area have material a great destroys the cementationpropensity ofto soil,produce thus a the colloidal particle dispersion size of soilwhen decreases. saturated Fineby rainwater. particles The are dispersion easier to beof carried cementing material destroys the cementation of soil, thus the particle size of soil decreases. Fine away by rainfall and groundwater. Found in concave terrains, due to the migration of fine particles, particles are easier to be carried away by rainfall and groundwater. Found in concave terrains, due the internalto the poremigration volume of fine of soilparticles, increases the internal then gravitypore volume erosion of soil occurs. increases Under then gravity the action erosion of gravity, the overlyingoccurs. Under soil collapses, the action formingof gravity, caves.the overlying Regarding soil collapses, the vertical forming slope, caves. fine Regarding particles the vertical are easy to be carriedslope, due to fine the particles strong are dispersion easy to be of carried soil,which due to leadsthe strong to the dispersion decline of in soil, the cementationwhich leads to strengththe of soil. Thisdecline promotes in the cementation the occurrence strength of verticalof soil. This collapse. promotes the occurrence of vertical collapse.

Figure 11.FigureSherard’s 11. Sherard’s suggested suggested diagnosis diagnosis shows shows that the the sample sample has has a dispersivity a dispersivity (expressed (expressed through through the PS, TDS,the PS, and TDS, SAR and SAR parameters parameters deﬁned defined in in Table Table3 3))[ [57].57].

Underground water composition plays a vital role in landform development. The relative amounts of monovalent and di- or trivalent cations in the underground water (and then in the adsorbed complex) significantly influence the clay’s physical properties, in particular, their tendency for spontaneous colloidal dispersion (dispersivity) [47]. Table 7 shows that the water sample is white and turbid, indicating the dispersion of soil substances. Sodium ion, bicarbonate ion, and chloride ion contents in the water sample are very high, forming a hydrochemical type of HCO3·Cl-Na. This finding is consistent with the results of soluble salts in soil. When the soil is dry in its natural condition, cemented connection and strong adsorbed water connection is the dominant position. However, this type of connection has weak water resistance, especially in high Na ion content, and thick water film forms when waters meet. Thus, the soil particles are extremely easy to disperse.

Table 7. Results of water sample analysis (mg/l).

Test Test item Value Value Test item Value Test item Value item Sustainability 2019, 11, 4709 14 of 17

Table 7. Results of water sample analysis (mg/L).

Test Item Value Test Item Value Test Item Value Test Item Value Visible White and Zn2+ 0.002 SO 2 99.1 NO (N) 0.003 substances turbid 4 − 2− Water 15 Cd+ 0.00082 Cl 589.9 HCO 696.1 temperature − 3− 3+ 6+ + PH 7.01 Cr /Cr 0.0001 NO3−(N) 0.30 Na 481.96 ORP 127.1 As3+/As5+ 0.0085 Fe2+/Fe3+ 0.814 K+ 0.707 2+ 2+ 4+ 2+ MnO4− 4.10 Hg 0.0049 Mn /Mn 0.0001 Ni 0.0047 + 2+ 2+ 2+ NH3/NH4 0.148 Ca 272.1 Cu 0.0204 Mg 39.68 Water Total dissolved F- 0.52 842.8 Pb2+ 0.00087 2184 hardness solids

When the soil is dry in its natural condition, cemented connection and strong adsorbed water connection is the dominant position. However, this type of connection has weak water resistance, especially in high Na ion content, and thick water ﬁlm forms when waters meet. Thus, the soil particles are extremely easy to disperse.

4. Conclusions Land degradation in the mud forest area threatens the lives and property of hundreds of people and numerous farmlands. A comprehensive knowledge of the cause–effect relationship between sub-erosion and soil slope failure is the basis for finding methods of curbing land degradation. This condition is primary for preserving degraded landscapes. Two typical types of slope failure: soil cave piping failure and vertical collapse of slope, were found. Sub-erosion caused by surface water infiltration is highly important for slope failure. Soil in the study area was loose and porous. Due to the high soluble salt content, chemical sub-erosion played a significant role. Once the cementing material was destroyed, fine particles were transported along the rich pore and accompanied by the process of physical sub-erosion. Therefore, this area is the result of the combined effect of physical and chemical sub-erosion. The cause of soil slope failure is closely related to geological structure, topographic conditions, soil physicochemical properties, and climatic conditions. Among these influencing conditions, geological and topographic factors are the root, and climatic conditions are the external factors. The special property of the loess sub-sandy soil with high silt component and high soluble salt is easy to disperse, which is the fundamental factor for slope failure. Soil particles and water solutions rich in sodium ions form an ionic diffusion layer, and the high content of sodium ions thickens the water film of the diffusion layer and reduces the intergranular attraction, which is the essential factor of soil slope failure. Accelerative factors for soil cave piping failure are: (1) Concave relief; (2) Concentrated precipitation; (3) Developmental pores in the soil; and (4) Dispersed soil. Accelerative factors for vertical collapse of slope are: (1) Near-vertical slope morphology; (2) Developmental vertical cracks; (3) Precipitation and dry–wet circulation; (4) Rich illite/smectite mixed layer minerals; and (5) Dispersed soil. Sustainability 2019, 11, 4709 15 of 17

Author Contributions: Data curation, X.R. and L.N.; Formal analysis, H.W.; Funding acquisition, Y.X.; Investigation, X.R.; Methodology, X.R. and L.N.; Writing—original draft, X.R.; Writing—review & editing, Y.X. Funding: This research received financial support from the Natural Science Foundation of China (grant no.41702300), (grant no.41502270), and (grant no. 41572254). Acknowledgments: The authors are grateful for financial support from the Natural Science Foundation of China (grant no.41702300), (grant no.41502270), and (grant no. 41572254). And we received funds to cover publication costs. We wish to thank the testing science experiment center of Jilin University for supporting experiment. We would also like to thank the Jilin Dabusu National Nature Reserve Administration for supporting our research work. Conflicts of Interest: The authors declare no conflict of interest.

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