applied sciences

Article Strength Characteristics and Slope Stability Analysis of Expansive Soil with Filled Fissures

Zhangjun Dai 1,* , Jianhua Guo 1,2, Hongming Luo 1, Jian Li 1 and Shanxiong Chen 1

1 State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China; [email protected] (J.G.); [email protected] (H.L.); [email protected] (J.L.); [email protected] (S.C.) 2 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected]; Tel.: +86-13667217486



Received: 4 June 2020; Accepted: 1 July 2020; Published: 3 July 2020


Abstract: Fissured expansive soils were widely distributed in the South-to-North Water Transfer Project. Most of the fissures were filled with clay, which controlled the stability of the slope. With the method of layered filling—bevel cutting—refilling and a modular design idea, the sample with a filled fissure preparation device for triaxial test was designed. After setting the filled fissures of gray-green clay in the expansive soil, triaxial tests were carried out for the samples with no filled fissures and with filled fissures with inclination angles of 15◦, 30◦, and 45◦. Then, considering the spatial distribution and the strength of the filled fissures in the slope, the stability analysis method for the expansive soil slope with filled fissures was proposed. The stability of a typical slope in Nanyang was analyzed. The results show that the c of expansive soil with filled fissures was about 12 to 15 kPa and the ϕ was 3◦ to 6◦. Filled fissures had an attenuation effect on the strength of the expansive soil. The larger the inclination of filled fissures, the more significant the effect of soil strength attenuation. The fissured slope stability was controlled by the filled fissures. The sliding surface was affected by the vertical fissures on the top of the slope and the slow-inclined long-large fissures in the slope, and the shape of the sliding surface was a broken line, which was basically consistent with the actual landslide.

Keywords: expansive soil; filled fissure; triaxial test; sample preparation device; strength characteristics; slope stability; sliding surface

1. Introduction In expansive soil slope engineering, it has always been known that it will slide every time after cutting [1–3]. The mid-route of the South-to-North Water Transfer Project in China has a total length of over 1472 km, among which expansive soil areas will cover about 340 km. Expansive soils are mainly Middle Pleistocene alluvial and flood sedimentary clay, Lower Pleistocene flood sedimentary clay, and Neogene hard clay. Strong expansive soil is mainly distributed in the Xingtai-Handan area and Nanyang Basin. The instability of the expansive soil slope will seriously affect engineering construction and safe operation [4–7]. Fissures are one of the three major characteristics of expansive soil, and they are also the most intuitive and obvious feature in expansive soil engineering. The development of fissures complicates the strength characteristics of expansive soil [8–10]. The strength of the expansive soil is comprehensively controlled by the distribution, density, tendency, inclination, extension, and filling of the fissures, and the directional arrangement of the fissures causes the anisotropy of the strength of the expansive soil [11–13]. Due to the significant effect of fissures on the stability of expansive soil slopes, research on fissures in expansive soil has been carried out repeatedly [14–17]. A digital image acquisition system was

Appl. Sci. 2020, 10, 4616; doi:10.3390/app10134616 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 17 Appl. Sci. 2020, 10, 4616 2 of 16 Due to the significant effect of fissures on the stability of expansive soil slopes, research on fissures in expansive soil has been carried out repeatedly [14–17]. A digital image acquisition system employed to capture the evolution and propagation of fissures in the soil specimen subjected to was employed to capture the evolution and propagation of fissures in the soil specimen subjected to desiccation [18]. The effect of the drying environment, i.e., temperature and relative humidity, on the desiccation [18]. The effect of the drying environment, i.e., temperature and relative humidity, on the engineering properties and the development of fissures of an expansive soil was investigated [19]. engineering properties and the development of fissures of an expansive soil was investigated [19]. Suction measurements, free shrinkage tests, constrained shrinkage tests, and splitting tensile strength Suction measurements, free shrinkage tests, constrained shrinkage tests, and splitting tensile strength tests (STTs) were undertaken to investigate the shrinkage cracks in expansive soils [20]. The influential tests (STTs) were undertaken to investigate the shrinkage cracks in expansive soils [20]. The factors such as soil matric suction on the formation of surficial fissures and the stability of unsaturated influential factors such as soil matric suction on the formation of surficial fissures and the stability of soil slopes were studied [21]. The researchers divided the strength of expansive soil into the soil strength unsaturated soil slopes were studied [21]. The researchers divided the strength of expansive soil into and the fissure surface strength, carried out direct shear and triaxial tests [22–24], and introduced the soil strength and the fissure surface strength, carried out direct shear and triaxial tests [22–24], CT scanning [23], image recognition and other methods [24] to quantitatively analyze the effect of and introduced CT scanning [23], image recognition and other methods [24] to quantitatively analyze fissures on the strength of expansive soil. These studies obtained some useful conclusions. Expansive the effect of fissures on the strength of expansive soil. These studies obtained some useful conclusions. soils swell in the presence of the water and shrink in its absence, thereby producing fissures which Expansive soils swell in the presence of the water and shrink in its absence, thereby producing significantly alter their mechanical and hydraulic performance [18]. Fissures decrease the soil’s strength fissures which significantly alter their mechanical and hydraulic performance [18]. Fissures decrease and increase its hydraulic conductivity and compressibility [20]. However, there are still deficiencies. the soil’s strength and increase its hydraulic conductivity and compressibility [20]. However, there Most studies have not systematically classified and studied the fissure itself. Fissures in different sizes are still deficiencies. Most studies have not systematically classified and studied the fissure itself. and states in the expansive soil have different effects on the engineering properties of expansive soil. If Fissures in different sizes and states in the expansive soil have different effects on the engineering the fissures contain fillers, the engineering properties of the fillers also have a great influence on the properties of expansive soil. If the fissures contain fillers, the engineering properties of the fillers also strength of the expansive soil. The shape of the fissures in the expansive soil is random, and the shear have a great influence on the strength of the expansive soil. The shape of the fissures in the expansive plane in the direct shear test is difficult to extend completely along the fissure surface. In addition, soil is random, and the shear plane in the direct shear test is difficult to extend completely along the when using undisturbed soil to make triaxial samples, the fissures are easily disturbed, and the shape fissure surface. In addition, when using undisturbed soil to make triaxial samples, the fissures are of the fissures is not easy to control. easily disturbed, and the shape of the fissures is not easy to control. The limit equilibrium method [25,26] and finite element method [27] are commonly used to The limit equilibrium method [25,26] and finite element method [27] are commonly used to analyze the stability of unsaturated soil slope, which are suitable for the analysis of the stability analyze the stability of unsaturated soil slope, which are suitable for the analysis of the stability of expansiveof expansive soil slopes. soil slopes. Numerical Numerical analysis analysis using the using finite the element finite element method methodwas conducted was conducted to analyze to analyze frequent shallow slope failures on highway embankments of expansive soil in the North Texas frequent shallow slope failures on highway embankments of expansive soil in the North Texas region [2region8]. The [28 finite]. The element finite elementmethod methodwas used was to usedstudy to the study effect the of e rainfallffect of rainfallon the factor on the of factor safety of of safety fill slopesof fill slopesconstructed constructed with expansive with expansive Yazoo Yazoo clay soil clay in soil Mississippi in Mississippi [29]. A [29 numerical]. A numerical exercise exercise was performedwas performed on a slope on a in slope Regina, in Regina, Canada, Canada, using a using program a program that implemented that implemented the constitutive the constitutive model intomodel infinite into infinite slope formulation slope formulation to reflect to reflect the influence the influence of the of the stress stress regime regime change change and and associated associated softeningsoftening on on unsaturated unsaturated expansive expansive soil soil slope slope stability stability [30]. [30]. Major Major landslides landslides that that occurred occurred on on the the midmid-altitude-altitude slopslopeses of of Mount Mount Elgon, Elgon, eastern eastern Uganda, Uganda were, were studied studied through through the analyses the analyses of soil ofparticle soil particlesize distribution, size distribution, Atterberg Atterberg limits, shear limits, strength shear strength and the factorand the of safetyfactor [of31 ].safety In the [31]. stability In the analysis stability of analysisexpansive of expansive soil slopes, soil the slopes, soil with the fissures soil with was fissures usually was homogenized usually homogenized [32]. In the analysis,[32]. In the the analysis, strength theparameters strength parameters obtained from obtained the experiment from the experiment were used were as the used overall as the strength overall parametersstrength parameters of the slope of thesoil. slope On soil. this basis,On this the basis, stability the stability of the slope of the was slope analyzed was analyzed by reducing by reducing the strength the strength parameters parameters to reflect tothe reflect influence the influence of the fissures of the fissures [33]. The [33]. calculated The calculated sliding sliding surface surface of the slopeof the wasslope often was arc-shapedoften arc- shaped(Figure (Figure1a), and 1a), after and surveying after surveying the 19 the excavated 19 excavated channel channel landslides landslides in the in Nanyangthe Nanyang section section of the of theSouth-to-North South-to-North Water Water Transfer Transfer Project, Project, the the landslides landslides werewere mainlymainly controlled by by various various fissures. fissures. TheThe sliding sliding surface surface was was controlled controlled by by the the vertical vertical fissures fissures at at the the trailing trailing edge edge and and the the slow slow-inclined-inclined longlong-large-large filled filled fissures. fissures. The The shape shape of of the the sliding sliding surface surface was was basically basically a broken a broken line line (Figure (Figure 1b).1b).

Strata boundary Strata boundary

Q4 Taupe clay Q4 Taupe clay Q2 Brown clay Arc-shaped sliding surface Q2 Brown clay Broken line sliding surface

Q2 Brown clay with Q2 Brown clay with gray-green clay gray-green clay

Q1 Gray-green clay Q1 Gray-green clay Fissures

(a) (b)

FigureFigure 1. 1. ConventionalConventional generalized generalized sliding sliding model model and and actual actual sliding sliding model model of of expansive expansive soil soil slope slope.. (a(a) )Arc Arc-shaped-shaped sliding sliding surface; surface; (b (b) )broken broken line line sliding sliding surface. surface. Appl. Sci. 2020, 10, 4616 3 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 17

ItIt is is impossible impossible to to objectively objectively consider consider the the characteristics characteristics of of the the fissures fissures in in expansive expansive soil, soil, especiallyespecially the the spatial spatial distribution distribution and and strength strength of of the fissures, fissures, when equivalent homogeneous soil soil was used used to to calculate the the slope slope stability, stability, and the the effect effect of fissures fissures on slope stability stability was was reflected reflected by by strengthstrength attenuation. attenuation. It It was was difficult difficult to to an analyzealyze how how these these fissures fissures c couldould constitute potential sliding surfacessurfaces [34 [34,,35]35].. Even Even if if the the strength strength parameter parameter used used in in the the calculation calculation was was the the lowest lowest strength strength of of the the fissurefissure surface, for the the actual actual slope, slope, it it was was only only correct correct when when the the sliding sliding surface surface was was along along the the fissure fissure surface,surface, which which often often resulted resulted in in the the calculation calculation parameters parameters being being too conservative too conservative and the and safety the safety factor factorwas too was small. too small. InIn this paper, aa samplesample preparation preparation device device for for preparing preparing triaxial triaxial test test samples samples with with filled filled fissures fissures was wasdeveloped. developed. Using Using the gray-greenthe gray-green strong strong expansive expansive soil in soil the in Nanyang the Nanyang section section of the of South-to-North the South-to- NorthWater TransferWater Transfer Project asProject the filling as the material filling andmaterial the brown and the medium brown expansive medium soil expansive as the matrix soil as of the the matrixexpansive of the soil, expansive triaxial samples soil, triaxial with filled samples fissures with were filled prepared. fissures wByere carrying prepared. out triaxialBy carrying tests, out the triaxialstrength tests, properties the strength of expansive properties soils ofwith expansive filled fissuressoils with are filled studied. fissures On are this studied. basis, a slopeOn this stability basis, aanalysis slope stability method that analysis could method reflect the that impact could of reflect filled fissures, the impact with of calculation filled fissures conditions, with whichcalculation were conditionsclose to the which true state were of close the fissuredto the true expansive state of soilthe fissured slope, was expansive proposed, soil and slope the, was stability proposed, analysis and of thethe stability fissured analysis expansive of soilthe slopefissured was expansive carried out. soil This slope method was carried can provide out. This a reference method forcan solving provide the a referenceslope stability for solving problem the in slope the expansive stability problem soil distribution in the expansive area. soil distribution area.

2.2. Morphology Morphology and and Filling Filling Characteristics Characteristics of of Fissures Fissures in in Expansive Expansive Soil Soil BasedBased on on the the geological geological survey survey of of the the Nanyang Nanyang section section of of the the South South-to-North-to-North Water Water Transfer Transfer Project,Project, the the most most typical typical feature feature of of the the expansive expansive soil soil wa wass that that the the f fissuresissures wer weree extremely extremely developed, developed, andand the the fissuresfissures containcontaineded a large amount of filler.filler. The The fissure fissuress of of Nanyang expansive expansive soil soil wer weree basicallybasically filled filled fissure fissuress (Figure 22))..

Filled fissure Filled fissure

(a) (b)

FigureFigure 2. 2. FilledFilled fissures fissures in in the the expansive expansive soil. soil. ( (aa)) Primary Primary fill filleded fissures; fissures; ( (bb)) secondary secondary fill filleded fissures. fissures.

ExpansiveExpansive soil soil fissures fissures were widely distributed. According According to to the the causes causes of of fissures, fissures, combined with the distribution and and the the shape, shape, they they could could be be divided divided into into primary primary fissures fissures (Figure (Figure 2 2a)a) and and secondarysecondary fissures fissures (Fi (Figuregure 22b).b). The primary fissuresfissures werewere formedformed duedue toto thethe uneven shrinkage stress inin the the soil. soil. Since Since the the shrinkage shrinkage stress stress was was proportional proportional to the to amount the amount of water of loss, water the loss, stress the gradually stress graduallydecreased decreased from the surface from the to thesurface deep to part the ofdeep the part soil. of The the primary soil. The fissures primary were fissures often widewere atoften the widetop and at the narrow top and at the narrow bottom. at Theythe bottom. were basically They were filled basically with secondary filled with clay secondary and the fissure clay and surface the fissurewas smooth. surface The was secondary smooth. The clay secondary was the clay clay deposited was the clay from deposited the original from site the by original wind andsite by water wind to andthe distantwater to location. the distant It was location gray-green. It wa clays gray in this-green area, clay which in this was area, deposited which inwas the deposited primary fissures in the primaryunder the fissures action ofunder a series the of action natural of forces,a series forming of natural the fillingforces, clay. forming Secondary the filling fissures clay. refer Secondary to tensile fissuresor shear refer fissures, to tensile which orwere shear formed fissures, due to which changes were in formed the external due and to changes internal in stress the externalconditions an ofd internalthe soil. stress Shear conditions fissures and of tensilethe soil. fissures Shear fissures were widely and tensile distributed fissures and were dense. widely These distributed fissureswere and dense.irregular, These short, fissures and narrow. were irregular, There was short, no obviousand narrow. directionality, There was the no fissure obvious surface directionality, was relatively the fissurerough, andsurface it was was often relatively filled with rough, gray-green and it was clay. oft Theen fillingfilled claywith contained gray-green more clay. clay The minerals, filling suchclay contained more clay minerals, such as montmorillonite and illite, and had a more dense structure, Appl. Sci. 2020, 10, 4616 4 of 16 as montmorillonite and illite, and had a more dense structure, higher content of fine particles, lower permeability and higher plasticity. The expansion of the filling was stronger than that of the original expansive soil, and the engineering properties were extremely poor. The secondary fissure gradually developed into the long-large fissure under the influence of atmospheric camping forces and other factors, and its influence on the slope gradually approached that of the original fissure. The groundwater activity in the expansive soil area was frequent, and most of the fissures in the soil were filled with gray-green clay, and the rest of the fissures were filled with calcium and iron-manganese. The number of unfilled fissures was very small. During the deposition process, the soil crept repeatedly under the action of expansion. Because the creeping effect had a sorting and rounding effect on the soil particles, the fissure surface was often very smooth and had a waxy luster, which was the natural weak surface in the soil. In addition, due to these special deformation characteristics, such fissures were often active areas of water movement, and soils often appeared gray-green after leaching. The filling was formed by the ion exchange or mineral deposition of the clay minerals such as montmorillonite and illite in the expansive soil during the migration of groundwater through the fissures. The content of clay particles and hydrophilic minerals in the filler was extremely high, and the filling gray-green clay was very delicate, and the moisture content was large. The investigation showed that the filled fissures of weak expansive soil accounted for 64.3% to 83.9% of the total fissures. The filled fissures of gray-green clay in the medium expansive soil area had vertical zoning characteristics, and the filled fissures within 6 m depth account for about 80% of the total fissures. However, the development of filled fissures in strong expansive soils was more significant, accounting for more than 90% of the total fissures. The filling was irregularly distributed in the soil in a net shape. The filling thickness was generally 2 to 5 cm. This was in the form of a film or lens, and the local thickness was more than 10 cm. Lu [36] conducted engineering geological mapping and statistics of structural surface morphology on the channel slopes of the Nanyang section of the South-to-North Water Transfer Project, and found that the fissures had a certain direction, and the fissures were clear and smooth, with waxy luster and scratches, and were mostly filled with clay. At the foot of some landslides, long-large fissures were exposed along the slope with a slow dip angle. The dip angle was 7◦ to 17◦. The length were generally more than 20 m and the thickness was even more than 40 cm. Zhao [37] made statistics on the occurrence of fissures in Nanyang expansive soil, and found that the soil fissures were dominated by slow and medium dip angles, both of which accounted for 96.7% of the total fissures, while steep dip angle fissures were very few in number, accounting for only 3.3% (Table1). At the same time, the inclination of the fissure had a correlation with its extension scale. The longer the extension of the fissure, the higher the proportion of the slow inclination fissure.

Table 1. Statistical table of fissure dip angle of Nanyang expansive soil [37].

Slow Dip Angle Medium Dip Angle Steep Dip Angle Fissure Classification ( 30 ) ≤ ◦ (30 –60 ) ( 60 ) ◦ ◦ ≥ ◦ Count Proportion Count Proportion Count Proportion Long-large fissure 917 66.7% 449 32.7% 8 0.6% Large fissure 345 37.2% 528 56.8% 56 6.0% Small fissure 15 14.0% 78 72.9% 14 13.1% Total 1277 53.0% 1055 43.7% 78 3.3%

Long-large fissures often extended more than 2 m in length, and the longest reached nearly 100 m. Many long-large fissures were rich in low-strength filled clay and formed weak structural planes. They often directionally penetrated the slope with a small inclination angle, which controlled the stability of the expansive soil slope. When this kind of filled fissure was developed in the slope and the tendency was the same as that of the slope, the impact on the slope stability was very significant. A large number of on-site landslide investigations showed that the expansive soil landslides were basically cut out along the fissures at the leading edge, forming the lower edge of the sliding surface. Appl. Sci. 2020, 10, 4616 5 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 17

3. Experimental Study on Strength Characteristics of Expansive Soil with FilledFilled Fissures

3.1. Sample Preparation Device At present, in order to study the mechanical characteristics of the filled filled fissures fissures of the expansive soil, the the triaxial triaxial test test was was usually usually carried carried out using out using the original the original sample. sample. In actual In operation, actual operation, the following the deficienciesfollowing deficiencies still existed. still existed. 1. TheThe success rate rate of of cutting cutting the the sample sample by using by using the original the original sample sample was low, was the low, adhesion the adhesion between betweenthe expansive the expansive soil and soil the and filled the fissure filled wasfissure weak, was and weak, it was and easy it was to easy break to along break the along fissure the fissureduring during the cutting the cutting process. process. 2. TheThe sample preparation processprocess had had a a large large disturbance disturbance to to the the filled filled fissures fissure ofs the of expansivethe expansive soil, soil,the fillingthe filling material material was easywas toeasy break to break and fissure, and fissure, and the and bonding the bonding conditions conditions and closed and stateclosed of statethe in of situ the filledin situ fissures filled fissures were changed. were changed. 3. ItIt was was difficult difficult to control the spatial distribution of the filled filled fissures.fissures. Since a group of triaxial teststests often often required required 3 3 to to 4 4 parallel samples, the the filled filled fissure fissure distribution of each sample was difficultdifficult to to keep keep consistent, consistent, resulting resulting in test errors. In view of thethe aboveabove deficiencies,deficiencies, based on the conventional triaxial test sample preparationpreparation method, the mold was improved, the strong expansion fissurefissure fillerfiller was manually placed in the sample, and the thickness and the inclination angle of the filled filled fissurefissure were controlled. The layered filling—bevelfilling—bevel cutting—refillingcutting—refilling waswas thethe basicbasic ideaidea ofof thethe samplesample preparationpreparation devicedevice andand method.method. A sample preparation device for triaxial test sample containing a filled filled fissure fissure was designed (Figure(Figure3 3).). TheThe samplesample preparationpreparation devicedevice includedincluded aa basebase andand aa moldmold fixingfixing component,component, aa samplesample pushing and positioning component, and a sample mold. The base of the device was a gate-shapedgate-shaped structure weldedwelded byby steelsteel plates.plates. TheThe center center of of the the top top plate plate was was provided provided with with a holea hole as aas channel a channel for pushingfor pushing the the sample. sample. The The threaded threaded pushing pushing screw screw was was concentric concentric with with the hole,the hole, and and there there were were two screwstwo screws on each on side each of side the top of the surface top of surface the base of to the fix base the mold. to fix A the positioning mold. A positioningruler was provided ruler was on theprovided inner sideon the of theinner base, side together of the base, with toge a locatorther with fixed a atlocator the bottom fixed ofat the pushingbottom of screw the pushing to form thescrew positioning to form the component, positioning the component, driving wrench the driving was fixedwrench at was the bottomfixed at ofthe the bottom pushing of the screw, pushing and thescrew, pushing and the screw pushing was screw moved was up moved and down up and by down twisting by thetwisting driving the wrenchdriving towrench push to the push sample. the Thesample. sample The moldsample was mold composed was composed of upper of generalupper general mold, lowermold, generallower general mold, mold, upper upper inclined inclined mold andmold lower and lower inclined inclined mold withmold an with inner an diameterinner diameter of 39.1 of mm. 39.1 The mm. device The device adopted adopted the modular the modular design, anddesign, it could and it control could thecontrol inclination the inclination of filled fissuresof filled byfissures changing by changin the mold,g the which mold, was which simple was and simple easy toand operate. easy to operate.

Upper Upper general inclined mold mold

Lower Lower inclined general mold mold

Positioning ruler Filled fissure Pushing screw θ Locator Driving wrench Prepared sample

Figure 3. SampleSample preparation preparation device for triaxial sample with fill filleded fissure.fissure. Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 17

3.2. Sample Preparation Method The sample preparation steps were as follows. 1. Applied petroleum jelly on the inner sides of all the molds and installed the lower general mold to form a cylindrical mold with an inner diameter of 39.1 mm. 2. According to the volume of the mold and the moisture content and density of the expansive soil, the amount of the expansive soil was calculated. Filled the lower general mold with the expansive soil matrix and compacted it layer by layer, and scraped the top surface. At this time, the height of the sample was the maximum height of the sample under the filled fissure (Figure 4a). 3. Installed the lower inclined mold, twisted the driving wrench to push the expansive soil matrix in the lower general mold to the top surface flush with the highest point of the lower inclined mold, and cut the soil along the top surface of the lower inclined mold to form the samples with a certain inclined top surfaces (Figure 4b). Appl.4. Sci.Install2020ed, 10 ,the 4616 upper inclined mold and the upper general mold, and filled the fissure filling6 ofsoil 16 in the mold and compacted it so that the sample height exceeded the height from the bottom of the sample to the top of the filled fissure (Figure 4c). 3.2.5. SampleTwisted Preparation the driving Method wrench to move down the thickness of the filled fissure, pressed the sample Thedown sample to the preparationbottom of the steps mold, were remove as follows.d the upper inclined mold and the upper general mold, and cut the sample along the top surface of the lower inclined mold to form a filled fissure with 1. aApplied certain petroleum inclination jelly (Figure on the 4d). inner At sides this oftime, all the the molds preparation and installed of the the filling lower fissure general and mold the expansiveto form a cylindricalsoil matrix moldat the with bottom an innerof the diameter sample was of 39.1 completed. mm. 2.6. InAccordingstalled the to upper the volume inclined of the mold mold and and the the upper moisture general content mold, and and density filled of the the mold expansive with soil,the expansivethe amount soil of thematrix expansive and compact soil wased calculated. it so that the Filled top thesurface lower of general the sample mold reache with thed the expansive sample height,soil matrix i.e., and80 mm compacted (Figure 4e) it layer. by layer, and scraped the top surface. At this time, the height 7. Twistof theed sample the driving was the wrench maximum to push height out ofthe the sample sample to undercomplete the the filled sample fissure preparation. (Figure4a).

39.1mm

Fissure filling soil Expansive soil h’ > t t matrix h2

θ t θ h1 h1 h

θ 1 80mm h1 Expansive soil Expansive soil θ matrix matrix Expansive soil h1 Expansive soil matrix matrix Expansive soil matrix (a) (b) (c) (d) (e)

Figure 4. The sample preparation steps. ( (aa)) E Expansivexpansive soil matrix filled filled and compact compacted;ed; ( b) e expansivexpansive soil matrix with a certain inclined top surfaces; ( c) fissurefissure fillingfilling soilsoil filledfilled andand compactedcompacted until the height exceededexceeded the top of the filledfilled fissure;fissure; (d) sample cuttingcutting to form a filledfilled fissurefissure with a certaincertain inclination;inclination; ((e) expansiveexpansive soilsoil matrix filledfilled andand compactedcompacted toto completecomplete thethe sample.sample.

3.3.3. TestInstalled Equipment the lower and Materials inclined mold, twisted the driving wrench to push the expansive soil matrix in the lower general mold to the top surface flush with the highest point of the lower inclined Taking the thickness and inclination of the filled fissure as control conditions, the triaxial tests mold, and cut the soil along the top surface of the lower inclined mold to form the samples with a of the expansive soil with filled fissure were carried out. The TSZ30-2.0 strain control triaxial certain inclined top surfaces (Figure4b). apparatus produced by Nanjing Soil Instrument Factory was used. The diameter of the triaxial 4. Installed the upper inclined mold and the upper general mold, and filled the fissure filling soil in sample was 39.1 mm, the cross-sectional area was 12 cm2, and the height was 80 mm. The matrix of the mold and compacted it so that the sample height exceeded the height from the bottom of the the expansive soil in the sample was the brown medium expansive soil in the Nanyang section of the sample to the top of the filled fissure (Figure4c). South-to-North Water Transfer Project, and the fissure filling soil was the scraped gray-green clay in 5. Twisted the driving wrench to move down the thickness of the filled fissure, pressed the sample the primary fissures of the strong expansive soil in the Nanyang section. The basic physical properties down to the bottom of the mold, removed the upper inclined mold and the upper general mold, of the sample are shown in Table 2. and cut the sample along the top surface of the lower inclined mold to form a filled fissure with a

certain inclination (Figure4d). At this time, the preparation of the filling fissure and the expansive soil matrix at the bottom of the sample was completed. 6. Installed the upper inclined mold and the upper general mold, and filled the mold with the expansive soil matrix and compacted it so that the top surface of the sample reached the sample height, i.e., 80 mm (Figure4e). 7. Twisted the driving wrench to push out the sample to complete the sample preparation.

3.3. Test Equipment and Materials Taking the thickness and inclination of the filled fissure as control conditions, the triaxial tests of the expansive soil with filled fissure were carried out. The TSZ30-2.0 strain control triaxial apparatus produced by Nanjing Soil Instrument Factory was used. The diameter of the triaxial sample was 39.1 mm, the cross-sectional area was 12 cm2, and the height was 80 mm. The matrix of the expansive soil in the sample was the brown medium expansive soil in the Nanyang section of the South-to-North Water Transfer Project, and the fissure filling soil was the scraped gray-green clay in the primary fissures Appl. Sci. 2020, 10, 4616 7 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 17 of the strong expansiveTable soil in 2. the Basic Nanyang physical section.properties The of the basic soil physicals in the sample. properties of the sample are shown in Table2. Particle Soil Moisture Density Liquid Plastic Free Expansion Composition (%) Type Content (%Table) 2.(g/cmBasic3) physical properties of the soilsLimit in (% the) sample.Limit (%) Rate (%) Silt Clay Particle Composition (%) Liquid Plastic Free Expansion SoilFilled Type Moisture Content (%) Density (g/cm3) 31.56 1.93 7 93 90.11 Limit (%) 29.46Limit (%) Rate110 (%) fissure Silt Clay Filled Soil 31.56 1.93 7 93 90.11 29.46 110 fissure 22.12 1.92 19 81 57.32 23.05 70 Soilmatrix matrix 22.12 1.92 19 81 57.32 23.05 70

3.4.3.4. TestTest MethodMethod InIn thisthis test,test, thethe mechanicalmechanical propertiesproperties ofof expansiveexpansive soilsoil with filledfilled fissuresfissures ofof didifferentfferent inclinationinclination anglesangles underunder aa certaincertain thicknessthickness werewere studied.studied. Samples were manually prepared withwith filledfilled fissuresfissures withwith inclinationinclination anglesangles ofof 1515◦°,, 3030◦°,, andand 4545◦°,, andand withwith aa thicknessthickness ofof 77 mm,mm, andand thethe samplesample withoutwithout filledfilled fissuresfissures waswas preparedpreparedwith withthe thesame samemedium mediumexpansive expansive soil soil for for comparison. comparison. AfterAfter sampling sampling on on site, site, the the soil soil was was dried, dried, ground, ground, and passed and passed through through a 0.05 mm a 0.05 sieve, mm prepared sieve, accordingprepared according to the measured to the moisturemeasured content, moisture placed content, in a placed closed in container a closed and container left to standand le forft to 24 stand h to ensurefor 24 h that to ensure the soil that moisture the soil was moisture evenly was distributed. evenly distributed. The filling soil The had filling a moisture soil had contenta moisture of 32% content and aof wet 32% density and a wet of 1.93 density g/cm of3, and1.93 theg/cm soil3, and matrix the hadsoil matrix a moisture had contenta moisture of 28%content and of a wet28% density and a wet of 1.92density g/cm of3. 1.92 g/cm3. FourFour samples were were taken taken from from each each group group and and tested tested at atconfining confining pressures pressures of 100, of 100, 150, 150,200, 200,and and250 250kPa. kPa. The samplesThe samples were were saturated saturated during during the test, the test, and andthe shear the shear rate ratewas was 0.015 0.015 mm/ mmmin./min. The Theconsolidated consolidated undrained undrained (CU) (CU) triaxial triaxial test test was was conducted conducted toto study study the the effect effect of of filled filled fissures fissures withwith didifferentfferent inclinationinclination anglesangles onon thethe strengthstrength characteristicscharacteristics of of the the expansive expansive soil. soil.

4.4. AnalysisAnalysis ofof StrengthStrength CharacteristicsCharacteristics ofof ExpansiveExpansive SoilSoil withwith FilledFilled FissuresFissures

4.1.4.1. AnalysisAnalysis ofof Stress-StrainStress-Strain CurveCurve andand DeformationDeformation CharacteristicsCharacteristics FigureFigure5 5 shows shows the the typical typical failure failure modes modes of of triaxial triaxial sample sample without without a a filled filled fissure, fissure, and and triaxial triaxial samplessamples withwith aa filledfilled fissurefissure withwith anan inclinationinclination angleangle ofof 1515◦°,, 30 30◦°and and 4545◦°.. TheThe stress-strainstress-strain curvecurve ofof eacheach samplesample isis shownshown inin FigureFigure6 .6.

(a) (b) (c) (d)

FigureFigure 5.5. FailureFailure modesmodes ofof thethe triaxialtriaxial samples.samples. (a) Without aa filledfilled fissure;fissure; ((bb)) filledfilled fissurefissure withwith anan inclinationinclination angleangle ofof 1515◦°;(; (cc)) filledfilled fissurefissure withwith anan inclinationinclination angleangle ofof 3030◦°;(; (dd)) filledfilled fissurefissure withwith anan inclinationinclination angleangle ofof 4545◦°..

The failure mode of the soil without a filled fissure was a typical triaxial test failure mode, and shear failure occurred at an angle of approximately 45◦ + ϕ/2 to the horizontal direction of the sample. The deformation of the expansive soil was strain hardening. The shear stress increased rapidly after Appl. Sci. 2020, 10, 4616 8 of 16 the test started. When the strain reached about 5%, the growth rate gradually slowed down, the sample began to fail, and the shear stress quickly reached the peak value. After that, the shear stress remained basically stable, and the deformation of the soil appeared as plastic sliding along the shear surface. This strain hardening characteristic became more and more significant as the confining pressure increased. The failure of a sample with a filled fissure of 15◦ inclination angle was similar to that of a sample without a filled fissure, but bulging deformation occurred near the filled fissure. The reason for this was that the strength of the fissure filling clay was lower than that of the soil matrix. When the sample was subjected to axial partial stress, a large deformation first occurred in the weak part. When this deformation developed to a certain extent, the upper and lower expansive soil matrix gradually approached, and the filling clay lost the space for lateral deformation. The deformation characteristics of the sample gradually approached the state of sample without fissure, and the failure mode of the conventional triaxial test appeared. The shear plane did not develop along the 15◦ slow-inclined fissure, but remained at the theoretical 45◦ + ϕ/2, and then the failure characteristics of shear plane cutting and filling fissures appear. This shows the failure characteristics of shear plane cutting the filled fissure. The stress–strain curve showed a certain degree of downward movement, indicating that the filled fissure had an attenuating effect on the strength of the soil. Under low confining pressure, the downward movement amplitude was small, which was basically the same as that of no filled fissure. With the increase in confining pressure, the downward movement was getting larger and larger. At the load level of 250 kPa, the ultimate strength of the sample without a filled fissure was close to 300 kPa, while the strength of the sample with 15◦ inclination was close to 200 kPa. The deformation was still strain hardening. After the sample with a 30◦ inclination filled fissure was destroyed, its shear plane was fully developed in the 7 mm-thick filled fissure. The shear plane was not parallel to the 30◦ inclined plane, but developed from the upper section at one end of the filled fissure, and passed through the fissure diagonally, cut out from the lower section of the filled fissure, and its inclination was slightly greater than 30◦, and it was closer to the inclination of the shear plane of the theoretical analysis. Figure7 shows the shear failure characteristics of the sample along the diagonal of the filled fissure. Compared with the result of the sample with a 15◦ inclination filled fissure, the stress–strain curve of the sample with a 30◦ inclination filled fissure moved downwards overall, but it still showed a lower downward amplitude under low confining pressure. As the confining pressure level increased, the amplitude of the curve downward increased. At the load level of 250 kPa, the ultimate strength of the sample with a 15◦ inclination filled fissure was close to 300 kPa, while the strength of the sample with a 30◦ inclination filled fissure was close to 150 kPa. The deformation of the samples still showed strain hardening. For the sample with a 45◦ inclination filled fissure, the shear plane still retained the failure characteristics of the sample with a 30◦ inclination filled fissure, but at this time, the inclination angle of the filled fissure was already large and the shear plane was basically parallel to the filling fissures, and the actual shear plane inclination was greater than 45◦. Compared with the test results of the previous samples, the stress–strain curve changed significantly, showing a change from strain hardening to strain softening, and also showing a certain brittleness. In addition, the peak strength was greatly improved, which was almost close to the test results of the sample with 15◦ inclination filled fissure, but it still showed a decay trend in the later stage of deformation compared with the previous groups of tests, and the strength in the later stage was stable between 100 and 150 kPa. Appl. Sci. 2020, 10, 4616 9 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 17

300 300

250 250

200 200

/ / kPa / kPa 3

3 150 150

σ σ

- -

1 1 σ σ 100 100 σ₃=100kPa σ₃=100kPa σ₃=150kPa σ₃=150kPa 50 50 σ₃=200kPa σ₃=200kPa σ₃=250kPa σ₃=250kPa 0 0 0 5 10 15 20 25 0 5 10 15 20 25 Axail Strain / % Axail Strain / %

(a) (b)

300 300

σ₃=100kPa 250 250 σ₃=100kPa σ₃=150kPa σ₃=150kPa σ₃=200kPa σ₃=200kPa 200 200

σ₃=250kPa σ₃=250kPa

/ / kPa / kPa 3

3 150 150

σ σ

- -

1 1 σ Appl. σ Sci.100 2020, 10, x FOR PEER REVIEW 100 9 of 17

50 50 and larger. At the load level of 250 kPa, the ultimate strength of the sample without a filled fissure was close0 to 300 kPa, while the strength of the sample with0 15° inclination was close to 200 kPa. The 0 5 10 15 20 25 0 5 10 15 20 25 deformation was still strain hardening. Axail Strain / % Axail Strain / % After the sample with a 30° inclination filled fissure was destroyed, its shear plane was fully developed in the 7 mm-(thickc) filled fissure. The shear plane was not parallel(d )to the 30° inclined plane, but developed from the upper section at one end of the filled fissure, and passed through the fissure Figure 6. Stress–strain curve of the triaxial test. (a) Without a filled fissure; (b) filled fissure with an diagonallyFigure ,6. cut Stress out– fromstrain thecurve lower of the section triaxial of test the. ( filleda) Without fissure, a filled and fissureits inclination; (b) filled was fissure slightly with angreater inclination angle of 15◦;(c) filled fissure with an inclination angle of 30◦;(d) filled fissure with an thaninclination 30°, and itangle was ofcloser 15°; (toc) filledthe inclination fissure with of an the inclination shear plane angle of of the 30 °theoretical; (d) filled fissure analysis. with Figure an 7 inclination angle of 45◦. showsinclination the shear angle failure of 45 characteristics°. of the sample along the diagonal of the filled fissure.

The failure mode of the soil withoutTriaxial a filled sample fissure was a typical triaxial test failure mode, and shear failure occurred at an angle of approximately 45° + φ/2 to the horizontal direction of the sample.

The deformation of the expansive soil was strain hardening.A The shear stress increased rapidly after the test started. When the strain reached about 5%, the growth rate gradually slowed down, the sample began to fail, and the shear stress quickly reached the peak value. After that, the shear stress remained basically stable, Shearand the plane deformation of the soil appearedFilled fissure as plastic sliding along the shear surface. This strain hardening(A→ B) characteristic became more and more significant as the confining pressure increased. B Expansive soil matrix The failure of a sample with a filled fissure of 15° inclination angle was similar to that of a sample without a filled fissure, but bulging deformation occurred near the filled fissure. The reason for this was that the strength of the fissure filling clay was lower than that of the soil matrix. When the sample Figure 7. The shear failure characteristics of the sample alon alongg the diagonal of the filledfilled fissure.fissure. was subjected to axial partial stress, a large deformation first occurred in the weak part. When this deformation4.2. StrengthCompared Fitting developed with Method the to result anda certain Strength of the extent, sample Characteristic the with upper a Analysis 15 and° inclination lower expansive filled fissure, soil matrix the stress gradually–strain approached,curve of the sampleand the withfilling a 30clay° inclination lost the space filled for fissure lateral moved deformation. downwards The deformation overall, but characteristics it still showed According to the stress–strain relationship curve of the triaxial test results, the peak stress (σ1f –σ3f ) ofa the lower sample downward gradually am approachedplitude under the state low confiningof sample without pressure. fissure, As the and confining the failure pressure mode of level the of the specimen at failure was obtained. According to the static equilibrium conditions, Equations (1) conventionalincreased, the triaxial amplitude test of appeared. the curve The downward shear plane increased. did not At developthe load level along of the 250 15 kPa,° slow the- inclinedultimate and (2) were used to calculate the normal stress σn and shear stress τ in the filled fissure when the fissure,strength but of theremained sample at with the atheoretical 15° inclination 45° + filledφ/2, and fissure then was the close failure to 300characteristics kPa, while ofthe shear strength plane of specimen failed [38]. cutting and filling fissures appear. This shows the failure characteristics of shear plane cutting the the sample with a 30° inclination filled fissure+ was close to 150 kPa. The deformation of the samples filled fissure. The stress–strain curve showedσ1 f σ3 af certainσ1 f degreeσ3 f of downward movement, indicating still showed strain hardening. σn = + − cos 2α (1) that theFor filled the sample fissure with had aan 45 attenuating° inclination2 effect filled on fissure, the 2 strength the shear of the plane soil. still Under retained low the confining failure pressure,characteristics the downward of the sample movement with a 30amplitude° inclination was filled small, fissure, which but was at basically this time, the the same inclination as that ofangle no filledof the fissure. filled fissure With thewas increase already inlarge confining and the pressure, shear plane the was downward basically movement parallel to wasthe fillinggetting fissures, larger and the actual shear plane inclination was greater than 45°. Compared with the test results of the previous samples, the stress–strain curve changed significantly, showing a change from strain hardening to strain softening, and also showing a certain brittleness. In addition, the peak strength was greatly improved, which was almost close to the test results of the sample with 15° inclination filled fissure, but it still showed a decay trend in the later stage of deformation compared with the previous groups of tests, and the strength in the later stage was stable between 100 and 150 kPa.

4.2. Strength Fitting Method and Strength Characteristic Analysis

According to the stress–strain relationship curve of the triaxial test results, the peak stress (σ1f– σ3f) of the specimen at failure was obtained. According to the static equilibrium conditions, Equations (1) and (2) were used to calculate the normal stress σn and shear stress τ in the filled fissure when the specimen failed [38].

(휎1푓 + 휎3푓) (휎1푓 − 휎3푓) 휎 = + cos 2훼 (1) 푛 2 2

(휎1푓 − 휎3푓) 휏 = sin 2훼 (2) 2

where σn is the normal stress on the shear plane, τ is the shear stress on the shear plane, σ1f is the large peak principal stress, σ3f is the small peak principal stress, and α is the angle between the shear plane and the horizontal plane. Appl. Sci. 2020, 10, 4616 10 of 16

  σ1 f σ3 f τ = − sin 2α (2) 2 where σn is the normal stress on the shear plane, τ is the shear stress on the shear plane, σ1f is the large Appl.peak Sci. principal 2020, 10, xstress, FOR PEERσ3f REVIEWis the small peak principal stress, and α is the angle between the shear10 plane of 17 and the horizontal plane. AccordingAccording to to the the Mohr Mohr–Coulomb–Coulomb theory, theory, the the relationship relationship curve curve between between normal normal stress stress σσnn andand shearshear stress stress ττ couldcould be be obtained, obtained, and the shearshear strengthstrength parametersparametersc cand andϕ φof of the the filled filled fissure fissure can can be beobtained. obtained. The The fitting fitting results results are are shown shown in in Figure Figure8 and 8 and Table Table3, respectively. 3, respectively.

90 60 80 c=17.81kPa c=14.63kPa φ=8.07 50 φ=5.84 70 τ=17.81+σtan(8.07) τ=14.63+σtan(5.84) R2=0.978 R2=0.923 60 40 50

30

/ kPa / kPa / n

n 40

τ τ 30 20 20 Experimental data 10 Experimental data 10 Linear fitting Linear fitting 0 0 0 100 200 300 400 500 0 100 200 300 400 500 σn / kPa σn / kPa (a) (b) 50 35 45 c=14.22kPa c=12.04kPa φ=4.01 30 φ=2.97 40 τ=14.22+σtan(4.01) τ=12.04+σtan(2.97) 35 R2=0.998 25 R2=0.997 30 20

25

/ kPa / kPa /

n n τ τ 15 20

15 10 10 Experimental data 5 Experimental data 5 Linear fitting Linear fitting 0 0 0 100 200 300 400 500 0 100 200 300 400 σn / kPa σn / kPa (c) (d)

FigureFigure 8. 8. SShearhear strength strength fitting fitting results results of of the the triaxial triaxial test test.. (a ()a Without) Without a afilled filled fissure fissure;; (b (b) )filled filled fissure fissure with an inclination angle of 15°; (c) filled fissure with an inclination angle of 30°; (d) filled fissure with with an inclination angle of 15◦;(c) filled fissure with an inclination angle of 30◦;(d) filled fissure with anan inclination inclination angle angle of of 45 45°.◦ .

TableTable 3. 3. SShearhear strength strength results results of of the the triaxial triaxial test. test.

InclinationInclination Angle Angle of of the the Fill Filleded FissureFissure ( ◦(°)) ShearS Strengthhear Strength No FilledNo FissureFilled Fissure 1515 30 30 45 45 c (kPa)c (kPa) 17.8117.81 14.6314.63 14.22 14.22 12.04 12.04 ϕ (◦)φ (°) 8.078.07 5.845.84 4.01 4.01 2.97 2.97

ItIt c canan be be seen seen that that the the homogeneous homogeneous medium medium expansive expansive soil soil matrix matrix was was used used for for the the triaxial triaxial test test,, and the strength parameter c = 17.81 kPa and φ = 8.07°. After setting the filled fissures of gray-green and the strength parameter c = 17.81 kPa and ϕ = 8.07◦. After setting the filled fissures of gray-green strongstrong expansive expansive soil soil in in the the medium medium expansive expansive soil soil matrix, matrix, it it had had a agreater greater impact impact on on the the mechanical mechanical propertiesproperties of of the the expansive expansive soil, soil, resulting resulting in in different different degrees degrees of of strength strength attenuation attenuation at at various various load load levelslevels in in the the triaxial triaxial tests. The strength attenuationattenuation was was positively positively correlated correlated with with the the inclination inclination of theof thefilled filled fissure. fissure. The The strength strength parameters parameters of of expansive expansive soils soils with with filled filled fissures fissures were were basically basically stable, stable, in in which the cohesive force c was about 12 to 15 kPa and the internal friction angle φ was about 3° to which the cohesive force c was about 12 to 15 kPa and the internal friction angle ϕ was about 3◦ to 6°. The deformation characteristics of the samples had a tendency to develop from strain hardening to strain softening after setting the filled fissures, and this was most significant when the fissure was filled with a 45° inclination angle.

Appl. Sci. 2020, 10, 4616 11 of 16

6◦. The deformation characteristics of the samples had a tendency to develop from strain hardening to strain softening after setting the filled fissures, and this was most significant when the fissure was Appl.filled Sci. with 2020, a 10 45, x◦ FORinclination PEER REVIEW angle. 11 of 17

5.5. Stability Stability Analysis Analysis of of Expansive Expansive Soil Soil Slope Slope Considering Considering Fill Filleded Fissures Fissures

5.1.5.1. Engineering Engineering Geological Geological Survey Survey of of the the Landslide Landslide TheThe landslide landslide in in Nanyang Nanyang section section of of the the middle middle route route of of the the South South-to-North-to-North Water Water Transfer Transfer ProjectProject wa wass selected selected as as an an example example to to analyze analyze the the slope slope stability (Figure (Figure 99a)a).. The Nanyang section wawass located in in the the southern southern part part of of the the South-to-North South-to-North Water Water Transfer Transfer Project Project and was and awa typicals a typical strong strongexpansive expansive soil distribution soil distribution area. The area. expansive The expansive soils in soils the in area the were area mainlywere mainly Middle Middle and Lower and LowerPleistocene Pleistocene flood sedimentary flood sedimentary clays, and clays, Neogene and Neogene lacustrine lacustrine sedimentary sedimentary clays, including clays, brown including clay brownand gray-green clay and gray clay.-green clay.

Expansive soil Distribution area

Landslide location (Nanyang)

(a) (b)

Reddish brown clay

Brown-yellow clay

Brown clay and gray- green clay

Landslide slope

Layer 1 Elevation / m

Layer 2 Excavation line θ=75°-84° Layer 3

θ=11°-28° Sliding surface Channel center line centerChannel

Distance / m

(c)

FigureFigure 9. 9. TheThe landslide landslide characteristics characteristics.. (a (a) )Site Site location location;; (b (b) )landslide landslide plan plan;; ( (cc)) landslide landslide profile profile and and landslidelandslide engineering engineering geological geological information. information.

TheThe landslide landslide wa wass located inin thethe mediummedium expansive expansive soil soil area, area, the the original original slope slope ratio ratio was wa 1:2,s 1:2, the thetop top elevation elevation of theof the slope slope was wa 145.695s 145.695 m, andm, and the the bottom bottom elevation elevation was wa 131.384s 131.384 m. m. The formation was the Middle Pleistocene alluvial (Q2al+pl) clay, and according to the detailed geological survey on site, it was found that the slope could be divided into three layers (Figure 9c). The first layer of the landslide was reddish brown clay containing a small amount of iron manganese nodules—the thickness was about 3 m. The second layer was brown-yellow clay with a thickness of Appl. Sci. 2020, 10, 4616 12 of 16

al+pl The formation was the Middle Pleistocene alluvial (Q2 ) clay, and according to the detailed geological survey on site, it was found that the slope could be divided into three layers (Figure9c). The first layer of the landslide was reddish brown clay containing a small amount of iron manganese nodules—the thickness was about 3 m. The second layer was brown-yellow clay with a thickness of about 4 m, and the content of calcium nodules and black iron manganese nodules was low. The third layer was brown clay and gray-green clay, containing some black iron manganese nodules. The sliding direction of the landslide was 130◦, and the total length of the landslide body was about 38.48 m. The trailing edge of the landslide was generally vertical, with a length of about 3.4 m. The left side of the landslide was heavily deformed and severely broken. The maximum height of the steep sill was 1.8 m, and the height of the right side was 0.2 to 0.8m. The front edge of the landslide cut out along the bedding gentle fissure at the slope foot. The landslide profile is shown in Figure9. Geology cuts were arranged on both sides of the slope and fissures were recorded. It was concluded that there were 105 fissures in the length of L > 1.0 m in the slope. The fissures developed mainly in two groups, and they were mainly low inclination fissures, followed by medium inclination. One group of along-slope fissures had the tendency of 120◦ to 140◦ and the inclination of 5◦ to 50◦, the other group of anti-slope fissures had the tendency of 310◦ to 325◦and the inclination of 3◦ to 60◦. The steep inclination fissures along the slope developed in the upper part of the slope, the inclination angle was 75◦ to 84◦, and the fissure surfaces were flat and smooth, filled with gray-green clay. At the foot of the slope, low inclination angle fissures along or against the slope developed, the inclination angle was 11◦ to 28◦, and the fissure surfaces were also flat and smooth, filled with gray-green clay.

5.2. Slope Generalization and its Stability Analysis The stability analysis of slope was carried out by using the platform Slide, which is the two-dimensional limit equilibrium process for the evaluation of the rock and soil slope safety coefficient and failure probability. Three limit equilibrium methods, namely Janbu correction, Spencer and Morgenstern, which could meet inter slice force and moment equilibrium and were suitable for a broken line slide surface, were chosen for slope stability calculation. According to the actual slope, the 2D geometry model of the slope was established. The vertical fissures at the top of the slope and the controlled fissures in the slope body were converted into thin layers in the model, and the mechanical strength parameters of the thin layers were considered to reflect the influence of the fissures on the slope stability. The thickness of the thin layer was selected as the average thickness of the measured fissure. According to the measured statistics, the thickness of the fissure was generally 3 to 8 cm. Therefore, the following methods were used to generalize the spatial distribution of fractures Firstly, the slope surface equation was established in the plane coordinate system.

y = ax + b, x (m, n) (3) ∈ For the fissure distributed in the elevation h, with the inclination angle of θ, the length of l, and the thickness of t, the coordinate at the endpoint in the slope surface was [(h b)/a, h]. In the model, − the fissure was simplified to a linear form, and the equation could be expressed as follows. !   h b  l  h b h b y = x + h x   + tan θ − ,  p  − , − (4) − a ∈ 1 + tan2 θ a a

In the actual modeling, through the preparation of the program, the fissure Equation (4) was translated upward by the fissure thickness t, and the intersection point with the slope Equation (3) is obtained to obtain the coordinates of the four vertices of the parallelogram fissure. The geometric model of slope fissure surface distribution is basically consistent. These coordinates were imported Appl. Sci. 2020, 10, 4616 13 of 16 into the slope model to establish the geometric model of the slope with filled fissures. The calculation model is shown in Figure 10. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 17

c=16kPa Groundwater level Φ=14°

c=23kPa Φ=16°

c=23kPa Φ=17° Filled fissures c=14kPa Φ=4°

FigureFigure 10.10. TheThe calculationcalculation modelmodel ofof thethe slope.slope.

InIn situsitu directdirect shearshear teststests werewere carriedcarried outout onon thethe slopeslope soil,soil, andand indoorindoor triaxialtriaxial teststests werewere carriedcarried outout onon thethe filled filled fissure. fissure. Strength Strength parameters parameters were were confirmed confirmed in ain comprehensive a comprehensive consideration consideration of the of testthe datatest data of the of undisturbedthe undisturbed expansive expansive soil andsoil theand expansive the expansive soil withsoil with filled filled fissures fissures (Table (Table4). The 4). firstThe layer first of layer reddish of reddish brown clay brown was clay hard was plastic, hard with plastic, a free with expansion a free rate expans of 42%,ion lowrate permeability, of 42%, low andpermeability, the lowest and shear the strength. lowest shear The strength. second layer The second of brown-yellow layer of brown clay- wasyellow hard clay plastic, was hard with plastic, a free expansionwith a free rate expansion of 56%, lowrate permeability,of 56%, low permeability, relatively developed relatively fissures, developed and highfissures, shear and strength. high shear The thirdstrength. layer The of brown third layer clay andof brown gray-green clay and clay gray was- hardgreen plastic, clay was with hard a free plastic, expansion with a rate free of expansion 93% and strongrate of swelling93% and characteristics, strong swelling and characteristics, shear strength and equivalent shear strength to that ofequivalent the previous to that layer. of the The previous fissures werelayer. very The developed,fissures were and very most developed, of them were and long-largemost of them fissures. were long-large fissures.

TableTable4. 4.Calculation Calculation parametersparameters ofof expansiveexpansive soilssoils inin thethe slope.slope. Location c (kPa) φ (°) Location c (kPa) ϕ (◦) 0 to 3 m0 to 3 m 1616 1414 3 to 7 m3 to 7 m 2323 1616 Below 7 mBelow 7 m 2323 1717 Filled fissureFilled fissure 1414 4 4

IncorporatingIncorporating typicaltypical filledfilled fissuresfissures intointo thethe modelmodel andand calculatingcalculating underunder thethe conditionsconditions closestclosest toto thethe real working conditions, conditions, it it could could be be found found that that the the safety safety factor factor of the of theslope slope was was about about 0.9. The 0.9. Theslope slope was was in an in anunstable unstable state. state. At Atthe the same same time, time, und underer the the same same geometric geometric model, model, soil parametersparameters andand groundwatergroundwater conditions,conditions, thethe stabilitystability analysisanalysis ofof thethe homogeneoushomogeneous expansiveexpansive soilsoil slopeslope withoutwithout fissuresfissures waswas carriedcarried out.out. ItIt waswas foundfound thatthat thethe safetysafety factorfactor ofof thethe slopeslope waswas aboutabout 1.2,1.2, andand thethe slopeslope waswas notnot unstable. unstable. As As shown shown in in Figure Figure 11 11,, in in a homogeneousa homogeneous expansive expansive soil soil slope slope without without fissures, fissures, the slidingthe sliding surface surface is arc-shaped, is arc-shaped extending, extending from thefrom top the of thetop slopeof the to slope the foot to the of thefoot slope of the (Figure slope 11(Figurea). In the11a). slope In the with slope fissures, with thefissures, shape the of theshape sliding of the surface sliding is asurface broken is line, a broken which line is comprised, which is comprised of vertical fissuresof vertical in thefissure top ofs in the the slope top andof the the slope straight and line-shaped the straight sliding line-shaped surface sliding develops surface from develop the bottoms from of thethe verticalbottom fissuresof the vertical to the footfissure of thes to slopethe foot (Figure of the 11 slopeb). The (Figure sliding 11b). surface The shapesliding is surface affected shap by thee is verticalaffected fissures by the vertical on the top fissures of the on slope the top and of the the slow-inclined slope and the long-large slow-inclined fissures long in-large the slope, fissure whichs in the is basicallyslope, which consistent is basically with theconsistent actual failurewith the characteristics actual failure of characteristics the landslide. of the landslide. Appl. Sci. 2020, 10, 4616 14 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 17

Calculation model Calculation model

Actual sliding surface

Calculated sliding surface Calculated sliding surface

Slow-inclined long-large fissure Arc-shaped sliding surface

(a) (b)

FigureFigure 11. 11.Calculate Calculate slidingsliding surfacesurfaceand and actualactual slidingsliding surface.surface. ((aa)) HomogeneousHomogeneous expansiveexpansive soilsoil slope;slope; (b(b)) expansive expansive soil soil slope slope with with filled filled fissures. fissures.

6. Conclusions 6. Conclusions Fissured expansive soils were widely distributed in the mid-route of the South-to-North Water Fissured expansive soils were widely distributed in the mid-route of the South-to-North Water Transfer Project in China. Most of the fissures were filled fissures, which were filled with the gray-green Transfer Project in China. Most of the fissures were filled fissures, which were filled with the gray- clay. The gray-green clay was a typical strong expansive soil, and its content of clay particles and green clay. The gray-green clay was a typical strong expansive soil, and its content of clay particles hydrophilic minerals was extremely high. The long-large filled fissures controlled the stability of the and hydrophilic minerals was extremely high. The long-large filled fissures controlled the stability of deep expansive soil slope. the deep expansive soil slope. The limitations of the commonly used test methods for dealing with expansive soil filled fissures The limitations of the commonly used test methods for dealing with expansive soil filled fissures were analyzed, based on the conventional triaxial test sample preparation method; the mold was were analyzed, based on the conventional triaxial test sample preparation method; the mold was improved, the strong expansion fissure filler was manually placed in the sample, and the thickness and improved, the strong expansion fissure filler was manually placed in the sample, and the thickness the inclination angle of the filled fissure were controlled. The layered filling—bevel cutting—refilling and the inclination angle of the filled fissure were controlled. The layered filling—bevel cutting— was the basic idea of the sample preparation device and method. A sample preparation device for refilling was the basic idea of the sample preparation device and method. A sample preparation triaxial test sample containing a filled fissure was designed. device for triaxial test sample containing a filled fissure was designed. Triaxial tests of the expansive soil with filled fissure were carried out. Samples were prepared with Triaxial tests of the expansive soil with filled fissure were carried out. Samples were prepared filled fissures with inclination angles of 15 , 30 and 45 , and with a thickness of 7 mm, and a sample with filled fissures with inclination angles◦ of ◦15°, 30° ◦and 45°, and with a thickness of 7 mm, and a without filled fissures for comparison. After setting the filled fissures of gray-green strong expansive sample without filled fissures for comparison. After setting the filled fissures of gray-green strong soilexpansive in the medium soil in theexpansive medium soil expansive matrix, it hadsoil a matrix, greater it impact had a on greater the mechanical impact on properties the mechanical of the expansiveproperties soil, of the resulting expansive in strength soil, resulting attenuation in strength at various attenuation load levels at various in the triaxial load levels tests. in The the strength triaxial attenuationtests. The strength was positively attenuation correlated was positively with the inclination correlated ofwith the the filled inclination fissure. The of the strength filled parametersfissure. The c ofstrength expansive parameters soils with of filledexpansive fissures soils were with basically filled fissures stable, were in which basically the cohesive stable, in force whichwas the aboutcohesive 12 toforce 15 kPa c was and about the internal 12 to 15 friction kPa and angle the internalϕ was about friction 3◦ toangle 6◦. Theφ was deformation about 3° to characteristics 6°. The deformation of the samplescharacteristics had a tendencyof the samples to develop had a from tendency strain to hardening develop from to strain strain softening hardening after to setting strain thesoftening filled fissures,after setting and itthe was filled most fissures, significant and whenit was themost fissure significant was filled when with the afissure 45◦ inclination was filled angle. with Thisa 45° providedinclination parameters angle. This for provided the slope parameters stability calculation for the slope of the stability actual project, calculation and alsoof the provided actual project, a basis forand the also analysis provided of the a strength basis for of the the analysis filled fissure of the according strength to of the the geological filled fissure survey according on site, which to the wasgeological convenient survey for on quickly site, which predicting was convenient the possibility for ofquickly landslide predicting occurrence the possibility and the type, of landslide location andoccurrence scale of and the landslide.the type, location and scale of the landslide. TheThe spatialspatial distributiondistribution ofof thethe filledfilled fissuresfissures inin thethe slopeslope waswas includedincluded inin thethe model,model, andand thethe strengthstrength parameters parameters of of the the filled filled fissures fissures were were considered. considered. The The geological geological model model of the of slope the withslope filled with fissuresfilled fissures was generalized, was generalized, and a stability and a stability analysis analysis method method for the expansive for the expansive soil slope soil with slope filled with fissures filled wasfissu proposed.res was proposed. A typical A landslidetypical landslide in the South-to-North in the South-to- WaterNorth TransferWater Transfer Project Project was selected was selected as an exampleas an example to analyze to analyze the slope the stability. slope stability. It could beIt could found be that found the safety that the factor safety of thefactor slope of wasthe slope about was 0.9. Theabout slope 0.9. was The inslope an unstable was in an state. unstable The sliding state. The surface sliding was surface affected was by affected the vertical by the fissures vertical on thefissures top ofon the the slope top of and the the slope slow-inclined and the s long-largelow-inclined fissures long- inlarge the fissure slope,s and in the shapeslope, ofand the the sliding shape surface of the wassliding a broken surface line, was which a broken was basicallyline, which consistent was basically withthe consistent actual failure with the characteristics actual failure of characteris the landslide.tics of the landslide. Appl. Sci. 2020, 10, 4616 15 of 16

Author Contributions: Conceptualization, H.L. and Z.D.; methodology, Z.D.; software, H.L.; validation, J.L., J.G. and S.C.; formal analysis, Z.D.; investigation, H.L. and J.G.; resources, J.L.; data curation, J.G.; writing—original draft preparation, Z.D.; writing—review and editing, H.L.; visualization, J.L.; supervision, S.C.; project administration, Z.D.; funding acquisition, Z.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 41702337. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Ng, C.W.W.; Zhan, L.T.; Bao, C.G.; Fredlund, D.G.; Gong, B.W. Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration. Géotechnique 2003, 53, 143–157. [CrossRef] 2. Alonso, E.E.; Gens, A.; Delahaye, C.H. Influence of rainfall on the deformation and stability of a slope in overconsolidated clays: A case study. Hydrogeol. J. 2003, 11, 174–192. [CrossRef] 3. Chen, F.H. Foundations on Expansive Soils; Elsevier: Amsterdam, The Netherlands, 1988. 4. Hou, T.-S.; Xu, G.-L.; Shen, Y.-J.; Wu, Z.-Z.; Zhang, N.-N.; Wang, R. Formation mechanism and stability analysis of the Houba expansive soil landslide. Eng. Geol. 2013, 161, 34–43. [CrossRef] 5. Ito, M.; Azam, S. Engineering properties of a vertisolic expansive soil deposit. Eng. Geol. 2013, 152, 10–16. [CrossRef] 6. Zhan, T.L.; Ng, C.W.; Fredlund, D.G. Field study of rainfall infiltration into a grassed unsaturated expansive soil slope. Can. Geotech. J. 2007, 44, 392–408. [CrossRef] 7. Cheng, Y.; Wang, S.; Li, J.; Huang, X.; Li, C.; Wu, J. Engineering and mineralogical properties of stabilized expansive soil compositing lime and natural pozzolans. Constr. Build. Mater. 2018, 187, 1031–1038. [CrossRef] 8. Chertkov, V.Y. Using Surface Crack Spacing to Predict Crack Network Geometry in Swelling Soils. Soil Sci. Soc. Am. J. 2000, 64, 1918–1921. [CrossRef] 9. Chertkov, V.Y.; Ravina, I. Modeling the Crack Network of Swelling Clay Soils. Soil Sci. Soc. Am. J. 1998, 62, 1162–1171. [CrossRef] 10. Al Fouzan, F.; Dafalla, M.A. Study of cracks and fissures phenomenon in Central Saudi Arabia by applying geotechnical and geophysical techniques. Arab. J. Geosci. 2013, 7, 1157–1164. [CrossRef] 11. Li, X.-W.; Wang, Y.; Yu, J.-W.; Wang, Y.-L. Unsaturated expansive soil fissure characteristics combined with engineering behaviors. J. Central South Univ. 2012, 19, 3564–3571. [CrossRef] 12. Li, J.; Zhang, L.; Li, X. Soil-water characteristic curve and permeability function for unsaturated cracked soil. Can. Geotech. J. 2011, 48, 1010–1031. [CrossRef] 13. Hu, X.W.; Li, Q.; Zhao, Z.S.; Kong, D.F. Mechanical properties of fissured clay. Chin. J. Geotech. Eng. 1994, 16, 81–88. 14. Chen, T.-L.; Zhou, C.; Wang, G.-L.; Liu, E.-L.; Dai, F. Centrifuge Model Test on Unsaturated Expansive Soil Slopes with Cyclic Wetting–Drying and Inundation at the Slope Toe. Int. J. Civ. Eng. 2017, 16, 1341–1360. [CrossRef] 15. Mutaz, E.; Dafalla, M.A. Chemical analysis and X-ray diffraction assessment of stabilized expansive soils. Bull. Int. Assoc. Eng. Geol. 2014, 73, 1063–1072. [CrossRef] 16. Chaduvula, U.; Viswanadham, B.V.S.; Kodikara, J.; De, A.; Reddy, K.R.; Yesiller, N.; Zekkos, D.; Farid, A. Desiccation Cracking Behavior of Geofiber-Reinforced Expansive Clay. Geo-Chicago 2016 2016, 271, 368–377. [CrossRef] 17. Zong, Y.; Chen, D.; Lu, S. Impact of biochars on swell-shrinkage behavior, mechanical strength, and surface cracking of clayey soil. J. Plant Nutr. Soil Sci. 2014, 177, 920–926. [CrossRef] 18. Chaduvula, U.; Viswanadham, B.; Kodikara, J. A study on desiccation cracking behavior of polyester fiber-reinforced expansive clay. Appl. Clay Sci. 2017, 142, 163–172. [CrossRef] 19. Kong, L.-W.; Wang, M.; Guo, A.-G. Effect of drying environment on engineering properties of an expansive soil and its microstructure. J. Mt. Sci. 2017, 14, 1194–1201. [CrossRef] 20. Al-Dakheeli, H.; Bulut, R.; Clarke, C.R.; Nevels, J.B. Hydro-Mechanical Analysis of Crack Initiation in Expansive Soils. PanAm Unsatur. Soils 2017 2018, 303, 332–341. [CrossRef] 21. George, A.M.; Chakraborty, S.; Das, J.T.; Pedarla, A.; Puppala, A.J. Understanding Shallow Slope Failures on Expansive Soil Embankments in North Texas Using Unsaturated Soil Property Framework. PanAm Unsatur. Soils 2017 2018, 302, 206–216. [CrossRef] Appl. Sci. 2020, 10, 4616 16 of 16

22. Liu, H.Q.; Yin, Z.Z. Test study of influence of crack evolution on strength parameters of expansive soil. Rock Soil Mech. 2010, 31, 727–731. 23. Hu, B.; Gong, B.W.; Cheng, Z.L. Test study of shear strength of fissure-plane in Nanyang expansive soil. Rock Soil Mech. 2012, 33, 2942–2946. 24. Bao, C.G.; Liu, T.H. Shear strength of swelling soil in Nanyang district, Henan province. J. Yangtze River Sci. Res. Inst. 1990, 7, 1–8. 25. Bishop, A.W. The use of the Slip Circle in the Stability Analysis of Slopes. Géotechnique 1955, 5, 7–17. [CrossRef] 26. Chen, Y.; Lin, H. Consistency analysis of Hoek–Brown and equivalent Mohr–coulomb parameters in calculating slope safety factor. Bull. Int. Assoc. Eng. Geol. 2018, 78, 4349–4361. [CrossRef] 27. Li, Y.; Qian, C.; Fu, Z.; Li, Z. On Two Approaches to Slope Stability Reliability Assessments Using the Random Finite Element Method. Appl. Sci. 2019, 9, 4421. [CrossRef] 28. Khan, M.S.; Hossain, M.; Ahmed, A.; Greenwood, K.; Shishani, A. Parametric Study on Slope Stability Using Recycled Plastic Pin. Geo-Risk 2017 2017, 283, 226–236. [CrossRef] 29. Khan, M.S.; Nobahar, M.; Khan, M.S.; Amini, F. Rainfall Induced Shallow Slope Failure over Yazoo Clay in Mississippi. PanAm Unsatur. Soils 2017 2018, 302, 153–162. [CrossRef] 30. Qi, S.; Vanapalli, S.K. Hydro-mechanical coupling effect on surficial layer stability of unsaturated expansive soil slopes. Comput. Geotech. 2015, 70, 68–82. [CrossRef] 31. Mugagga, F.; Kakembo, V.; Buyinza, M. A characterisation of the physical properties of soil and the implications for landslide occurrence on the slopes of Mount Elgon, Eastern Uganda. Nat. Hazards 2011, 60, 1113–1131. [CrossRef] 32. He, P.; Li, S.; Xiao, J.; Zhang, Q.-Q.; Xu, F. Shallow Sliding Failure Prediction Model of Expansive Soil Slope based on Gaussian Process Theory and Its Engineering Application. KSCE J. Civ. Eng. 2017, 22, 1709–1719. [CrossRef] 33. Oh, W.T.; Vanapalli, S.K. Influence of rain infiltration on the stability of compacted soil slopes. Comput. Geotech. 2010, 37, 649–657. [CrossRef] 34. Li, R.J.; Yan, R.; Liu, J.D.; Shao, S.J. A new practicable approach to the stability analysis of unsaturated expansive soil slope. Disaster Adv. 2013, 6, 33–43. 35. Zuo, C.-Q.; Chen, J.-P. The stability analysis of expansive slope in Jing-Yi Expressway. J. Zhejiang Univ. A 2007, 8, 270–274. [CrossRef] 36. Lu, D.J. Geo-engineering property and slide mechanism of highly expansive soil of Nanyang on mid-route of South-North Water Transfer Project. Master’s Thesis, University of Chinese Academy of Sciences, Beijing, China, 2013. 37. Zhao, L. Study on fissure characteristics and its impact on slope stability of expansive soil. Ph.D. Thesis, Yangtze River Scientific Research Institute, Wuhan, China, 2012. 38. Zhang, J.J.; Gong, B.W.; Hu, B.; Zhou, X.W.; Wang, J. Study of evolution law of fissures of expansive clay under wetting and drying cycles. Rock Soil Mech. 2011, 32, 2729–2734.

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