In Situ Shear Test for Revealing the Mechanical Properties of the Gravelly Slip Zone Soil

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Article In Situ Shear Test for Revealing the Mechanical Properties of the Gravelly Slip Zone Soil

Zongxing Zou 1, Qi Zhang 1, Chengren Xiong 1,*, Huiming Tang 1, Lei Fan 2, Fang Xie 1, Junbiao Yan 1 and Yinfeng Luo 1

1 Three Gorges Research Center for Geo-Hazards, China University of Geosciences, Wuhan 430074, China; [email protected] (Z.Z.); [email protected] (Q.Z.); [email protected] (H.T.); [email protected] (F.X.); [email protected] (J.Y.); [email protected] (Y.L.) 2 Changjiang River Scientiﬁc Research Institute, Wuhan 430074, China; [email protected] * Correspondence: [email protected]

Received: 8 October 2020; Accepted: 10 November 2020; Published: 15 November 2020

Abstract: Slip zone soil is usually composed of clay or silty clay; in some special geological environments, it contains gravels, which make the properties of the slip zone soil more complex. Unfortunately, in many indoor shear tests, gravels are removed to meet the demands of apparatus size, and the in situ mechanical property of the gravelly slip zone soil is rarely studied. In this study, the shear mechanical property of the gravelly slip zone soil of Huangtupo landslide in the Three Gorges Reservoir area of China was investigated by the in situ shear test. The test results show that the shear deformation process of the gravelly slip zone soil includes an elastic deformation stage, elastic–plastic deformation stage, and plastic deformation stage. Four functions were introduced to express the shear constitutive model of the gravelly slip zone soil, and the asymmetric sigmoid function was demonstrated to be the optimum one to describe the relationship of the shear stress and shear displacement with a correlation coeﬃcient of 0.986. The comparison between the in situ test and indoor direct shear test indicates that gravels increase the strength of the slip zone soil. Therefore, the shear strength parameters of the gravelly slip zone soil obtained by the in situ test are more preferable for evaluating the stability of the landslide and designing the anti-slide structures.

Keywords: in situ shear test apparatus; mechanical property; sliding zone; shear constitutive model; Huangtupo landslide

1. Introduction The shear mechanical properties of slip zone soil play a key role in the deformation and failure of landslides [1–3]. In some geological conditions, the parent rocks, such as the limestone and argillaceous limestone, near the slip zone are not completely weathered during the process of formation and evolution of the landslide, and gravels formed in the slip zone [4–6]. The existence of gravel signiﬁcantly changes the properties of the slip zone soil [7]; therefore, it is of great signiﬁcance to investigate the shear mechanical properties of slip zone soil-containing gravels for evaluating the stability and analyzing the evolution process of the landslide. At present, the research methods of the shear mechanical properties of the slip zone soil are mainly indoor tests, including the direct shear test, ring shear test, and triaxial test in the laboratory. The direct shear test is widely used in obtaining the strength parameters of soil because of its convenience and quickness, and a lot of experience has been accumulated in engineering [8–10]. Landslides usually experience a process of large deformation [11]; however, the large shear displacement of the slip zone soil cannot be implemented in the direct shear test, and the ring shear test was thereby developed to study the shear mechanical properties of the slip zone soil with large displacement [12–18]. Compared with

Sensors 2020, 20, 6531; doi:10.3390/s20226531 www.mdpi.com/journal/sensors Sensors 2020, 20, 6531 2 of 16 the direct shear test and the ring shear test, a triaxial test has the advantage of well controlling the stress condition and drainage condition, and the shear plane of soil in the triaxial test is not set artificially, which is more consistent with the actual engineering situation. Therefore, triaxial tests are often adopted to study the mechanical properties of slip zone soil [19,20]. However, the soil is usually remolded in the above laboratory tests, which destroy its original structure. Moreover, the sizes of the test samples are limited by the test equipment, which fails to study the mechanical properties of the slip zone soil containing gravels. Consequently, the laboratory test results are usually inconsistent with the actual mechanical properties of the gravelly slip zone soil. With the improvement of large-scale engineering and conditions, in situ tests are becoming more and more popular [21], as they can overcome the structural disturbance of the slip zone soil and realize the test of soil containing gravel. However, the in situ shear tests are rarely implemented to study the shear mechanical properties of the gravelly slip zone soil. Therefore, it is significant to carry out the in situ shear test for the gravelly slip zone soil to reveal their in situ shear mechanical properties. The objective of the study is to investigate the in situ shear mechanical properties using an in situ shear test apparatus. In this study, an in situ direct shear apparatus was designed and applied to investigate the shear mechanical properties of the gravelly slip zone soil of the Huangtupo landslide in the Three Gorges Reservoir area. On the basis of the tests, constitutive models of the gravelly slip zone soil were proposed to describe the relationship of the shear stress and shear displacement. The in situ test was further compared with the laboratory test, and the differences of the obtained mechanical properties of the slip zone soil by two methods were analyzed.

2. Materials and Methods

2.1. Site Description To investigate the mechanical property of the gravelly slip zone soil, a group of in situ shear tests were conducted in the sliding zone of the Huangtupo landslide, which is volumetrically the largest reservoir landslide in the Three Gorges Reservoir area, having a volume of approximately 70 million m3 [22,23]. The Huangtupo landslide is located in Badong County of Hubei Province in China (Figure1a) on the south bank of the Yangtze River about 72 km upstream from the Three Gorges dam (Figure1b). It has been quite active since reservoir impoundment, and it posed a great threat to a community with more than 16,000 people [24]. For this reason, a billion RMB was invested to relocate the residents from the Huangtupo landslide. The Huangtupo landslide consists of four sub-landslides (Figure1c), including the Riverside Slump I# (northwest part), Garden spot sub-landslide (southwest part), Riverside Slump II# (northeast part), and Substation sub-landslide (southeast part) [25]. The stability of this large landslide, especially Riverside Slump I#, is of great concern regarding the safety of the Yangtze River main channel. Therefore, a large in situ experimental station was constructed for monitoring, evaluating, and researching this crucial landslide. This experimental station consists of underground tunnels (Figure1) and a series of multi-filed monitoring systems. The tunnels include a 908-m-long main tunnel, five branch tunnels with lengths of 5–145 m, and two testing tunnels of about 10-m length [26]. The tunnel groups not only reveal the spatial structure of the landslide, but also expose the sliding zone, which can provide powerful site conditions for the in situ shear test of the slip zone soil. The Riverside Slump I# has an average thickness of 69.4 m, an average N–S length of 770 m, a width of 475 m in the W–E direction, and a volume of approximately 23 million m3. The slide mass of the Riverside Slump I# mainly contains cataclastic rock, rock block, and rock fragments with clay [27]. The sliding zone is composed of silty clay with gravels and has a thickness of 0.5–1 m. The slide bed mainly consists of argillaceous limestone. In this study, in situ shear tests were conducted in the testing tunnel that connects to the No.3 branch tunnel (Figure1c,d). The moisture content and density of the undisturbed slip zone soil sample taken from the testing site were tested (Figure1e). The natural density, moisture content, and dry density of the slip zone soil are 2.31–2.43 g/cm3, 12.9–18.1%, 2.09 g/cm3, Sensors 2020, 20, 6531 3 of 16 respectively. The slip zone soil is typically composed of fine-grained soil (silty clay) with a significant fraction of coarse-grained particles (gravels), constituting approximately 20–50 wt % (weight percent) of the material [6]. Sensors 2020, 20, x FOR PEER REVIEW 3 of 18

Figure 1. (a,b) show the location of the Huangtupo landslide; (c,d) are the map and profile section, Figure 1. (a,b) show the location of the Huangtupo landslide; (c,d) are the map and proﬁle section, respectively, of the Huangtupo landslide; (e) is the face of the testing tunnel where the in situ shear respectively, of the Huangtupo landslide; (e) is the face of the testing tunnel where the in situ shear tests were conducted. tests were conducted.

The Huangtupo landslide consists of four sub-landslides (Figure 1c), including the Riverside Slump I# (northwest part), Garden spot sub-landslide (southwest part), Riverside Slump II# (northeast part), and Substation sub-landslide (southeast part) [25]. The stability of this large landslide, especially Riverside Slump I#, is of great concern regarding the safety of the Yangtze River main channel. Therefore, a large in situ experimental station was constructed for monitoring, evaluating, and researching this crucial landslide. This experimental station consists of underground tunnels (Figure 1) and a series of multi-filed monitoring systems. The tunnels include Sensors 2020, 20, x FOR PEER REVIEW 4 of 18 a 908-m-long main tunnel, five branch tunnels with lengths of 5–145 m, and two testing tunnels of about 10-m length [26]. The tunnel groups not only reveal the spatial structure of the landslide, but also expose the sliding zone, which can provide powerful site conditions for the in situ shear test of the slip zone soil. The Riverside Slump I# has an average thickness of 69.4 m, an average N–S length of 770 m, a width of 475 m in the W–E direction, and a volume of approximately 23 million m3. The slide mass of the Riverside Slump I# mainly contains cataclastic rock, rock block, and rock fragments with clay [27]. The sliding zone is composed of silty clay with gravels and has a thickness of 0.5–1 m. The slide bed mainly consists of argillaceous limestone. In this study, in situ shear tests were conducted in the testing tunnel that connects to the No.3 branch tunnel (Figure 1c,d). The moisture content and density of the undisturbed slip zone soil sample taken from the testing site were tested (Figure 1e). The natural density, moisture content, and dry density of the slip zone soil are 2.31–2.43 g/cm3, 12.9–18.1%, 2.09 g/cm3, respectively. The slip zone soil is typically composed of fine-grained soil (silty clay) with a significant fraction of coarse-grained particles (gravels), constituting approximatelySensors 2020, 20, 6531 20–50 wt % (weight percent) of the material [6]. 4 of 16

2.2. Principle of In Situ Shear Test 2.2. Principle of In Situ Shear Test The principle of the in situ shear test is basically the same as the indoor shear test (see Section 2.5 forThe details). principle The of obvious the insitu advantage shear test of isthe basically in situ thetest sameis that as the the samples indoor shear in the test in (seesitu Sectionshear test 2.5 canfor details).avoid disturbance The obvious and advantage keep the original of the in structure situ test isof that the theslip samples zone soi inl. theUnder in situ the shearcondition test canof avoid disturbance and keep the original structure of the slip zone soil. Under the condition of keeping keeping the soil in its original state, a normal load (Fn) is firstly applied in the normal direction to F generatethe soil in the its normal original stress. state, aAfter normal the loadnormal ( n) deformation is firstly applied induced in the by normal the normal direction load to is generate relatively the normal stress. After the normal deformation induced by the normal load is relatively stable, a thrust stable, a thrust (Fs) is to be slowly applied to produce shear stress on the preset shear surface F (Figure( s) is to 2). be When slowly the applied sample to steps produce into shearthe failure stress state, on the an preset ultimate shear steady surface shear (Figure stress2). is When acquired the sample steps into the failure state, an ultimate steady shear stress is acquired and is named the shear and is named the shear strength (τf). Various ultimate shear strengths are obtained by conducting a seriesstrength of in (τ f).situ Various shear ultimatetests with shear different strengths normal are obtained stresses. byTaking conducting the normal a series stress of in as situ an shear abscissa tests valuewith diandfferent the shear normal strength stresses. as Takingan ordinate the normal value, stress the slope as an and abscissa intercept value of and the thestraight shear line strength can be as obtainedan ordinate by value, linearly the fitting slope the and d interceptata with of the the least straight square line method. can be obtained The intercept by linearly value fitting obtained the representsdata with the the least value square of the method. cohesion; The the intercept value ofvalue the obtained arctangent represents of the slope the value is the of value the cohesion; of the the value of the arctangent of the slope is the value of the internal friction angle of the slip zone soil. internal friction angle of the slip zone soil. The equation of τf -σn can be expressed as follows: The equation of τf -σn can be expressed as follows:

 tan  c (1) τfnf = σn tan ϕ + c (1) wherewhere ττff isis the the shear shear strength strength of of the the slip slip zone zone soil; soil; σσnn isis the the normal normal stress; stress; and and φϕ andand cc areare the the internal internal frictionfriction angle angle and and the the cohesion cohesion of of the the slip slip zone zone soil, soil, respectively. respectively.

FigureFigure 2. 2. DiagramDiagram showing showing the the in in situ situ shear shear test test.. 2.3. In Situ Shear Test Apparatus

The in situ shear tests of the slip zone soil are carried out in the field and usually in remote mountainous areas, so the in situ shear test apparatus needs to be convenient to carry. The shear test apparatus designed in this experiment has the advantage of being detachable, portable, and easy to install. This in situ shear apparatus is composed of loading devices, force transmitting devices, reaction walls, a shear box, and measurement instrumentations (Figure3). The loading devices include one hydraulic jack providing force on the normal direction of the sample ( 5 in Figure3) and two hydraulic jacks providing force on the shearing direction of the sample ( 8 in Figure3). The normal hydraulic jack that provides normal force on the sample can produce a maximum working force of 500 kN, and it has an effective action area of 80 cm2. To prevent the shear box from deflection, two tangentially arranged hydraulic jacks are adopted to load the thrust in the shear direction. Both the tangentially arranged hydraulic jacks can produce a maximum working pressure of 300 kN and have an effective action area of 60 cm2. These hydraulic jacks have a 100-mm stroke, implying that the maximum shear and normal displacement of the jacks are 100 mm. Sensors 2020, 20, x FOR PEER REVIEW 5 of 18

2.3. In Situ Shear Test Apparatus The in situ shear tests of the slip zone soil are carried out in the field and usually in remote mountainous areas, so the in situ shear test apparatus needs to be convenient to carry. The shear test apparatus designed in this experiment has the advantage of being detachable, portable, and Sensorseasy to2020 install., 20, 6531 This in situ shear apparatus is composed of loading devices, force transmitting5 of 16 Sensors 2020, 20, x FOR PEER REVIEW 5 of 18 Sensors 2020devices,, 20, x FOR reactionPEER REVIEW walls, a shear box, and measurement instrumentations5 of 18 (Figure 3).

2.3. In Situ Shear 2.3.Test In Apparatus Situ Shear Test Apparatus The in situ shearThe tests in ofsitu the shear slip testszone ofsoil the are slip carried zone outsoil inare the carried field andout inusually the field in remoteand usually in remote mountainous areas,mountainous so the in situareas, shear so the test in appa situ ratusshear needs test appa to beratus convenient needs to to be carry. convenient The shear to carry. The shear test apparatus designedtest apparatus in this designed experiment in thishas experimentthe advantage has of the being advantage detachable, of being portable, detachable, and portable, and easy to install. Thiseasy into situinstall. shear This apparatus in situ shear is composed apparatus of isloading composed devices, of loading force transmitting devices, force transmitting devices, reactiondevices, walls, a reactionshear box, walls, and ameasurement shear box, and instrumentations measurement instrumentations (Figure 3). (Figure 3).

Figure 3. TheThe composition composition of of the the in in situ situ shear shear test test apparatus. apparatus. ① Reaction Reaction wall wall (as (asshown shown in ( ina)); ( a②)); Figure 3. The composition of the in situ shear test apparatus. ① Reaction wall (as shown in (a)); ② Figure 3. The compositionsteel backing of the inplate; situ shear ③ transfertest apparatus. column ① Reaction (as shown wall (as inshown (b));b in ④(a)); guiding② part; ⑤ the normal hydraulic steel backing plate;③ transfer column (as shown④ in ( ));⑤ guiding part; the normal hydraulic steel backing plate;steel ③ backingtransfer columnplate; (as transfer shown column in (b)); (as④ shownguiding in part; (b)); ⑤ the guiding normal part; hydraulic the normal hydraulic jackjack (as (as shown shown in in (d ());d⑥)); ⑥steel steel rollers rollers (as shown (as shown in⑦ (e )); in (eshear));⑧ ⑦ box; shear tangentially box; ⑧ tangentially arranged hydraulic arranged jack (as shown injack (d)); (as ⑥ shown steel rollers in (d)); (as shown steel rollersin (e)); (as ⑦ shown shear box;in (e ⑧)); tangentially shear box; arranged tangentially arranged ⑨ ⑩ ⑪ hydraulic jacks;jacks;hydraulic ⑨hydraulic steel steelstick; jacks; jacks; stick;⑩ normal ⑨steel steelnormal stick;displacement stick; displacementnormal ⑩gauge displacementnormal (as shown gauge displacement gaugein (as(f)); (as shown⑪ shown horizontal gauge inin ((ff ));)); (as horizontal horizontal shown in displacement (f)); ⑪ horizontal gauge ⑫ displacement gauge(asdisplacementdisplacement shown (as shown in in gauge ( fgauge(f));)); ⑫ (as pressure shown(as shown in gauge. (f)); gauge. in pressure (f)); ⑫ gauge. pressu re gauge.

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The3) andnormal to jack the two th hydraulic sampleat provideshydraulic jack smoothly normal th atjacks provides force onproviding on thenormal the othersample force force hand.can on theon Thesample the force-transmitting shearing can direction devices of the produce a maximum working force of 500 kN, and it has an effective action area of 80 cm2. To produce a maximumincludesample aworking(⑧ steel in backingforceFigure of 5003). plate, kN,The transferand normal it has column,an hydraulic effective guiding action jack area part,that of 80p androvides cm2 steel. To rollersnormal ( 2force, 3 ,on4 the, 6 samplein Figure can3, prevent the shearprevent box from the sheardeflection, box fromtwo tangendeflection,tially two arranged tangen hydraulictially arranged jacks hydraulicare adopted jacks to are adopted to 2 load the thrustrespectively).produce inload the theshear athrust maximum direction. The in the transfer Bothshear working the direction.column tangentially Both force is assembledthearranged of tangentially 500 hydraulic kN, by arranged a and seriesjacks it canhydraulic has of produce short an jacks effective steela can columns produce action a with area a length of 80 of cm 40. cm To 2 maximum workingtoprevent meetmaximum pressure the the working requirement shearof 300 kN pressure box and from ofhave of di300 an ff deflection, erent kNeffective and testing have action an two areaeffective tunnel tangentiallyof 60 action sizes.cm2. These area In ofarranged orderhydraulic 60 cm to. These prevent hydraulic hydraulic the force jacks transfer are adopted column to jacks have a 100-mm stroke, implying that the maximum shear and normal displacement of the jacks have afromload 100-mm the deflexion stroke, thrust implying in as the the that shear result the maximumdirection. of the shearshear Both and movement the normal tangentially displacement of the samplearranged of the during hydraulic the shearjacks test,can produce two steel a jacks are 100 mm.jacks are 100 mm. 2 Force-transmittingbackingmaximumForce-transmitting platesdevices working andtransmit apressure devices setthe ofjack steeltransmit pres of sure300 rollers the to kN jackthe areand reactionpres set surehave wall between to thean on reactiontheeffective one the wallhand normal action on and the hydraulicone area hand of and 60 jack cm and. These the hydraulic slip zone transmit thesoiljacks jacktransmit pressure sample. have the to a The thejack 100 sample pressure size-mm ofsmoothly stroke,to the the steelsample on implying the backing smoothlyother hand. that plateson Thethe the otherforce-transmitting is maximum40 hand. cm The40 force-transmitting devices shearcm, and and the normal devices steel rollers displacement are arranged of the to include a steel backing plate, transfer column, guiding part, and steel× rollers (②, ③, ④, ⑥ in include a steelformjacks backing aare size 100plate, of mm. 30transfer cm column,30 cm. guiding Force-transmitting part, and steel rollers devices (②, ③, should ④, ⑥ in have a large stiffness to reduce the Figure 3, respectively).Figure 3,The respectively). transfer column ×The transferis assembled column by isa assembledseries of short by asteel series columns of short with steel a columns with a length of 40deformation cm lengthtoForce meet of the-40transmitting ofcmrequirement theto meet devices the of devices differrequirement duringent transmittesting the of differ loadingtunnel theent sizes. testingjack process, In pressuretunnelorder soto sizes. prevent as to toIn the order reduce the reaction to prevent the deformationwall the on the one measurement hand and force transfererrortransmit columnforce of transferfrom the the sample.deflexion jack column pressure as from the deflexionresult to theof theas sample theshear result movement smoothly of the shear of the onmovement sample the other during of the hand.the sample The during force the -transmitting devices shear test, two steel backing plates and a set of steel rollers are set between the normal hydraulic shear test, twoinclude steelMeasurement backing a steel plates backing and instrumentations a set plate, of steel transferrollers are include column,set between two guiding the pressurenormal part hydraulic, gauges and steel ( 12 rollersin Figure (②, 3③) and, ④, eight⑥ in jack and the slipjack zone and soil the sample. slip zone The soilsize sample. of the steel The backingsize of the plates steel is backing 40 cm × plates 40 cm, is and 40 cmthe × steel 40 cm, and the steel rollers are arrangeddisplacementFigurerollers to 3, formare respectively). arranged a size gauges of to 30 form cm ( The×10 a 30 size cm. andtransfer of Force-transmitting 30 11cm ×incolumn 30 cm. Figure Force-transmitting devicesis3 ).assembled Theshould pressure have devices by a alarge shouldseries gauges have of shorta are large adopted steel columns to measure with a stiffness to reducethelengthstiffness pressure the of deformation to40 reduce cm of theto the of meet normalthedeformation devices the and requirementduring of the tangentially thedevices loading during of process, different arranged the loading so as testing to hydraulicprocess, reduce tunnel sothe as jacks. to sizesreduce Displacement. theIn order to prevent gauges arethe deformation measurement error of the sample. deformationemployedf measurementorce transfer to error measure column of the sample. the from shearing deflexion displacement as the result and of normal the shear displacement movement during of the the sample shearing during process. the Measurement instrumentationsMeasurement instrumentations include two pressure include gaugestwo pressure (⑫ in gaugesFigure 3)(⑫ and in eightFigure 3) and eight displacementTheshear gaugesdisplacement precision test, (⑩ andtwo of gauges⑪ steel the in Figure(pressure ⑩backing and 3). ⑪ The gaugeinplates pressureFigure isand 3). 0.01gauges The a MPa. setpressure are of adopted Thesteel gauges precision torollers measureare adopted are of the theset to shearmeasurebetween and the the normal normal displacement hydraulic pressure ofgauges jackthepressure normaland are the ofand 0.01 slipthe tangentially mm normalzone and soiland 0.001arranged sample.tangentially mm, hydraulic The respectively. arranged size jacks. of hydraulic Displacementthe steel jacks. backing gaugesDisplacement platesare gaugesis 40 cmare × 40 cm, and the steel rollersThe are inner arranged size ofto theform shear a size box of 30 is 50cmcm × 30 in cm. length, Force 50-transmitting cm in width, devices and 40should cm in have height a large ( 7 instiffness Figure to3). reduce There the are fourdeformation observation of the holes devices in the during side ofthe the loading shear box,process, and so steel as to sticks reduce ( 9 thein Figuredeformati3) areon inserted measurement via these error holes, of the providing sample. access for measuring the displacement of the slip zone soil sample.Measurement instrumentations include two pressure gauges (⑫ in Figure 3) and eight displacementThe reaction gauges wall (⑩ iscasted and ⑪ in in site Figure to fit the 3). Theposture pressure of the gauges transfer are column adopted and to the measure shear box, the ensuringpressure that of the the forcenormal produced and tangentially by the hydraulic arrange jacksd hydraulic is smoothly jacks. transmitted Displacement to the tunnel gauges wall. are The reaction wall ( 1 in Figure3) was casted with fast hardening concrete in this experiment to shorten the forming time of the building reaction wall.

2.4. In Situ Shear Test Process The general process for the in situ shear test of the slip zone soil is shown in Figure4. Sensors 2020, 20, x FOR PEER REVIEW 6 of 18

employed to measure the shearing displacement and normal displacement during the shearing process. The precision of the pressure gauge is 0.01 MPa. The precision of the shear and normal displacement gauges are 0.01 mm and 0.001 mm, respectively. The inner size of the shear box is 50 cm in length, 50 cm in width, and 40 cm in height (⑦ in Figure 3). There are four observation holes in the side of the shear box, and steel sticks (⑨ in Figure 3) are inserted via these holes, providing access for measuring the displacement of the slip zone soil sample. The reaction wall is casted in site to fit the posture of the transfer column and the shear box, ensuring that the force produced by the hydraulic jacks is smoothly transmitted to the tunnel wall. The reaction wall (① in Figure 3) was casted with fast hardening concrete in this experiment to shorten the forming time of the building reaction wall.

2.4. In Situ Shear Test Process Sensors 2020, 20, 6531 6 of 16 The general process for the in situ shear test of the slip zone soil is shown in Figure 4.

Figure 4. Flow chart for the in situ shear test of slip zone soil.soil.

2.4.1. Sample P Preparationreparation Sliding zones are exposed by the tunnels in many places of the Huangtupo landslide ﬁeldfield experimental station, and the most observable one one is in the east of No.3 branch tunnel; therefore, this sitesite waswas selected selected for for the the in in situ situ shear shear test. test. The The bedrock bedrock exposed exposed at this at this location location dips dips 52◦ toward 52° toward 346◦ (Figure346° (Figure1d). The 1d). shear The testshear samples test samples were made were in made the sliding in the zone. sliding The zone. shear The direction shear ofdirection the sample of the is consistent with the bedrock tendency. The sample is a cuboid, having a size of 50 cm 50 cm 45 cm sample is consistent with the bedrock tendency. The sample is a cuboid, having a size× of 50 ×cm × 50 (length width height), as shown in Figure5. Gravels with larger diameter on the surface of the cm × 45× cm (length× × width × height), as shown in Figure 5. Gravels with larger diameter on the samplesurface are of removed the sample to make are theremoved surface to of makethe sample the surfacesmooth, ofand the the sample corresponding smooth dents, and were the treatedcorresponding with slip dents zone were soil, sotreated that the with samples slip zone can soil better, so contact that the with samples the apparatus. can better Six contact samples with were the preparedapparatus.Sensors 20 and20 Six, 20 marked ,samples x FOR PEER as were S01–S06 REVIEW prepared from theand outside marked to as the S01 inside–S06 offrom the the testing outside tunnel to the (Figure inside57). ofof 18 the testing tunnel (Figure 5).

Figure 5. Prepared samples for the in situ shear test in the No.3 testing tunnel in the Huangtupo landslide. Figure 5. Prepared samples for the in situ shear test in the No.3 testing tunnel in the Huangtupo 2.4.2.landslide Apparatus. Installation

2.4.2. ApparatusThe installation Installation procedures of the in situ shear test apparatus can be divided into the following steps: The first step was to install the shear box. After the sample was made, we slowly applied pressure The installation procedures of the in situ shear test apparatus can be divided into the following and pressed the shear box into the sample until the distance between the lower edges of the shear box steps: The first step was to install the shear box. After the sample was made, we slowly applied pressure and pressed the shear box into the sample until the distance between the lower edges of the shear box and the bottom of the sample was about 5 cm. This 5-cm thick soil was preserved for the shearing space (Figure 2), which is greater than the maximum gravel size in the sliding zone. The gap between the shear box and ground was filled with slip zone soil. Then, we leveled the soil sample at the top of the shear box to facilitate the installation of subsequent devices. The second step was to install the normal loading devices and force-transmitting devices. A 40 cm × 40 cm steel plate was placed on top of the sample. Then, the steel rollers (Figure 3e) were placed on the steel plate to prevent the eccentricity of a normal load in the shearing process. We put another steel plate on the steel rollers and installed the hydraulic jack. Then, we put the steel plate on the back of the hydraulic jack and installed the short steel columns behind the steel plate until the top of the steel column reached the chamber wall, and finally, we cast the reaction wall. The third step was to install the tangentially arranged direction loading devices. A steel plate was placed on the side of the shear box as the force transfer plate of the tangential jack, and then the hydraulic jack, transfer column, and reaction wall were installed successively. When the location of the sample was close to the testing tunnel wall, the transfer column could be omitted, and only reaction walls were retained. The fourth step was to install the measurement instrumentation. There were two observation holes on both sides of the shear box, 10 cm away from the bottom. Steel sticks were inserted into the soil sample through the observation holes, and normal and horizontal displacement gauges were installed on the steel sticks. The full view of the installed apparatus for the in situ shear test in the testing tunnel of the Huangtupo landslide is shown in Figure 6. Sensors 2020, 20, 6531 7 of 16 and the bottom of the sample was about 5 cm. This 5-cm thick soil was preserved for the shearing space (Figure2), which is greater than the maximum gravel size in the sliding zone. The gap between the shear box and ground was filled with slip zone soil. Then, we leveled the soil sample at the top of the shear box to facilitate the installation of subsequent devices. The second step was to install the normal loading devices and force-transmitting devices. A 40 cm 40 cm steel plate was placed on top of the sample. Then, the steel rollers (Figure3e) were × placed on the steel plate to prevent the eccentricity of a normal load in the shearing process. We put another steel plate on the steel rollers and installed the hydraulic jack. Then, we put the steel plate on the back of the hydraulic jack and installed the short steel columns behind the steel plate until the top of the steel column reached the chamber wall, and finally, we cast the reaction wall. The third step was to install the tangentially arranged direction loading devices. A steel plate was placed on the side of the shear box as the force transfer plate of the tangential jack, and then the hydraulic jack, transfer column, and reaction wall were installed successively. When the location of the sample was close to the testing tunnel wall, the transfer column could be omitted, and only reaction walls were retained. The fourth step was to install the measurement instrumentation. There were two observation holes on both sides of the shear box, 10 cm away from the bottom. Steel sticks were inserted into the soil sample through the observation holes, and normal and horizontal displacement gauges were installed on the steel sticks. The full view of the installed apparatus for the in situ shear test in the testing tunnel of the Huangtupo landslide is shown in Figure6. Sensors 2020, 20, x FOR PEER REVIEW 8 of 18

Figure 6. 6. Full view of the i installednstalled apparatus of the in situ shear test in the testing tunnel of the Huangtupo landslide.

2.4.3. Stress L Loadingoading The maximum normal stress is estimated accordingaccording to the thickness of the overburden layer of the slidingsliding mass.mass. Then, the normal pressure applied on eacheach soilsoil samplesample isis determined.determined. During the shearing process, the normal pressure of the test samplesample waswas keptkept stablestable byby continuouslycontinuously adding pressure to to the the hydraulic hydraulic jack. jack. The The normal normal stress stress was was loaded loaded step step by step by throughstep through three tothree four to levels. four Thelevels. meter The reading metershould reading be conducted should be every conducted three minutes every inthree each minutes level. When in each the di level.ﬀerence When between the thedifference two consecutive between the readings two consecutive of the normal readings displacement of the normal was less displacement than 0.01 mm, was indicating less than that0.01 themm, deformation indicating that is relatively the deformation stable, the is nextrelatively level loadstable, was the applied. next level After load the was last applied. level of loadAfter was the applied,last level the of normal load was deformation applied, the should normal be measured deformation and recorded should be at measured an interval and of 5 recordedmin. When at the an accumulatedinterval of 5 min normal. When displacement the accumulated of two normal consecutive displacement 15 min of wastwo consecutive less than 0.01 15 min mm, was the less normal than deformation0.01 mm, the normal was considered deformation to be was stable, considered and then to be the stable, shear and load then was the applied. shear load was applied. According to the estimated maximum shear strength, the shear load was applied step by step through 8–12 levels, and the load of each level was applied every 10 min. When the shear displacement increased sharply or reached 1/10 of the sample size (50 cm), the shearing surface cannot bear more stress, it was considered to be damaged, and the test was terminated. The readings of all gauges were recorded immediately before and after each load was applied.

2.4.4. Post-Processing

The normal stress (σn) and shear stress (τ) can be calculated by following equations:

Fn  = (2) A0

Fs  = (3) A0

where Fn and Fs are the normal load and shear load acting on the shear plane, respectively; A0 is the area of the shear plane of the slip zone soil sample. In this test, the pressure readings of the final normal hydraulic jack (Pn) are 2.1, 2.5, 4.4, 5.9, 7.9, and 9.9 MPa, respectively. The cross-sectional area of a normal jack cylinder (A1) is 80 cm2. The size of the shear box is 50 cm × 50 cm × 40 cm (length × width × height), so the normal stress area of the slip zone soil sample (A0) is 2500 cm2; consequently, the normal stress of the sample can be estimated by the following formula: Sensors 2020, 20, 6531 8 of 16

According to the estimated maximum shear strength, the shear load was applied step by step through 8–12 levels, and the load of each level was applied every 10 min. When the shear displacement increased sharply or reached 1/10 of the sample size (50 cm), the shearing surface cannot bear more stress, it was considered to be damaged, and the test was terminated. The readings of all gauges were recorded immediately before and after each load was applied.

2.4.4. Post-Processing

The normal stress (σn) and shear stress (τ) can be calculated by following equations:

F σ = n (2) A0 F τ = s (3) A0 where Fn and Fs are the normal load and shear load acting on the shear plane, respectively; A0 is the area of the shear plane of the slip zone soil sample. In this test, the pressure readings of the ﬁnal normal hydraulic jack (Pn) are 2.1, 2.5, 4.4, 5.9, 7.9, 2 and 9.9 MPa, respectively. The cross-sectional area of a normal jack cylinder (A1) is 80 cm . The size of the shear box is 50 cm 50 cm 40 cm (length width height), so the normal stress area of the slip × ×2 × × zone soil sample (A0) is 2500 cm ; consequently, the normal stress of the sample can be estimated by the following formula: Fn PnA1 σ = = = 0.032Pn (4) A0 A0 Therefore, the ﬁnal normal stresses of six samples are estimated as 0.067, 0.080, 0.141, 0.189, 0.253, and 0.317 MPa, respectively. Two tangentially arranged direction hydraulic jacks were installed for providing the shear load. 2 The cross-sectional area of each jack cylinder (A2) was 60 cm . The two jacks were pressurized by the same hydraulic system, and their pressures were displayed by the same pressure gauge. During the shearing process, the shear area decreased with the increase of shear displacement. Since the total shear displacement was relatively small compared with the size of the slip zone soil sample. Therefore, 2 the area of the shear plane was estimated to be the cross-sectional area of the sample A0 (2500 cm ). The shear stress τ can be calculated by the following formula:

Fs 2PsA2 τ = = = 0.048Ps (5) A0 A0

The strain-controlled direct shear apparatus (Figure7) was adopted for this indoor direct shear test. The remolded soil sample had a diameter of 61.8 mm and a height of 20 mm. The gap between the upper and lower shear boxes is about 0.1 mm. It was sheared at a shear rate of 0.8 mm/min. Four samples were sheared under normal pressures of 100, 200, 300, and 400 kPa, respectively.

2.5. Indoor Direct Shear Test In order to compare with the shear strength parameters (c, ϕ) of the slip zone soil obtained by the in situ shear test, a series of the indoor direct shear test of the remolded slip zone soil was carried out. The location of the soil samples taken in the laboratory test was the same as that of the in situ test. The large particles in soil samples were removed during the test. To meet the requirements of the test instrument for the particle size of soil, a 2-mm-sieving was used to sieve the slip zone soil. Then, a group of screened soil samples with moisture content of 13.63% was prepared by the dynamic compaction method. The dry density and target density of the prepared sample are 2.07 g/cm3 and 2.37 g/cm3, respectively. Sensors 2020, 20, x FOR PEER REVIEW 9 of 18

FPAn n 1  = = 0.032P (4) AA n 00

Therefore, the final normal stresses of six samples are estimated as 0.067, 0.080, 0.141, 0.189, 0.253, and 0.317 MPa, respectively. Two tangentially arranged direction hydraulic jacks were installed for providing the shear load. The cross-sectional area of each jack cylinder (A2) was 60 cm2. The two jacks were pressurized by the same hydraulic system, and their pressures were displayed by the same pressure gauge. During the shearing process, the shear area decreased with the increase of shear displacement. Since the total shear displacement was relatively small compared with the size of the slip zone soil sample. Therefore, the area of the shear plane was estimated to be the cross-sectional area of the sample A0 (2500 cm2). The shear stress τ can be calculated by the following formula:

FPAs2 s 2  = = 0.048Ps (5) AA00

The strain-controlled direct shear apparatus (Figure 7) was adopted for this indoor direct shear test. The remolded soil sample had a diameter of 61.8 mm and a height of 20 mm. The gap between Sensorsthe upper2020, 20and, 6531 lower shear boxes is about 0.1 mm. It was sheared at a shear rate of 0.8 mm/min.9 of 16 Four samples were sheared under normal pressures of 100, 200, 300, and 400 kPa, respectively.

Figure 7. Indoor direct shear test apparatus.

3.2.5. Results Indoor Direct Shear Test

3.1. DeformationIn order to ofcompare the Sample with during the shear Shear strength Test parameters (c, φ) of the slip zone soil obtained by the in situ shear test, a series of the indoor direct shear test of the remolded slip zone soil was The test results of six samples under different normal stresses are shown in Figure8. carried out. The location of the soil samples taken in the laboratory test was the same as that of the It can be seen from the normal displacement–shear displacement curves (u –u curves) in Figure8a in situ test. The large particles in soil samples were removed during n thes test. To meet the that during the shearing process of sample S01, the normal displacement of the soil sample increases requirements of the test instrument for the particle size of soil, a 2-mm-sieving was used to sieve the firstly and then decreases; that is, the soil sample rebounds in the later stage. This may be related slip zone soil. Then, a group of screened soil samples with moisture content of 13.63% was prepared to the rotation of the larger gravels near the shear plane during the shearing process (Figure8m). During the shearing process, the normal displacement of samples S02 and S03 increases gradually in the early stage, and then, it tends to be stable (Figure8b,c). The normal displacement of sample S04 increases steadily with the increase of shear displacement, and the normal displacement does not converge obviously (Figure8d). In the process of the shear test, the normal displacement of sample S05 increases slowly; then, it accelerates and finally slows down, and the total displacement is very small, about 1.2 mm (Figure8e). The normal displacement of sample S06 increases slowly at first; then, it increases rapidly, and finally, it decreases again (Figure8f), which is similar to sample S01. The rebound in the normal direction may be attributed to the movement mode of large gravels on the shear plane (Figure8r). Figure8 shows that the shape of shear stress–shear displacement curves ( τ–us curves) are different. Since the in situ slip zone soil is inhomogeneous, the structure and composition of the sample are not exactly the same. The slope of the τ–us curve of S01, S02, and S03 decreases, increases, and then decreases again with the increase of shear displacement (Figure8g,h,i). While in the samples S04, S05, and S06, the slope of the curves shows a gradual decrease and the curves eventually tend to be horizontal (Figure8j,k,l). The inhomogeneity of the material makes the test data irregular. For example, the normal stress of the S01 sample is 0.067 MPa, which is smaller than that of sample S02 (0.080 MPa), but its shear strength is larger than that of sample S02. Sensors 2020, 20, 6531 10 of 16

Sensors 2020, 20, x FOR PEER REVIEW 11 of 18

Figure 8. (a–f) Shear stress with the increase of shear displacement under various normal stress conditions. (g–l) Normal displacement of the samples with the increase of the shear displacement under various normal stress conditions. (m–r) Shear plane surface of the samples after the shear test. Sensors 2020, 20, 6531 11 of 16

In general, the τ–us curves exhibit that with the increase of shear displacement, the slope of each curve gradually decreases, and curves finally tend to be stable. According to the deformation and failure feature of the slip zone soil, τ–us can be divided into three stages, and it is described as follows: (1) Elastic deformation stage: In this stage, the sample just begins to be stressed and deformed, and the shear stress–shear displacement curve is approximately linear. Tiny deformation is generated at the bottom of the sample, and no complete shear plane is formed. (2) Elastic–plastic deformation stage: In this stage, the internal fracture of the sample begins to occur, but the propagation speed of the fracture is relatively slow. Some irregular change of the τ–us curve may appear in this stage (such as S01, S02, S03). The slope of the curve of τ–us decreases gradually. (3) Plastic deformation stage: With the continuous increase of shear displacement, the shear plane has basically formed in this stage; the plastic flow occurs, and the shear stress tends to be stable. The characteristic of the gravelly slip zone soil is different from that of the slip zone soil without gravels. Previous study shows that the slip zone soil without gravels exhibits a phenomenon of strain softening. The typical full process of shear deformation and failure of the slip zone soil can be divided into five stages, including the pore shear compaction stage, the elastic deformation stage, the plastic hardening stage, the strain softening stage, and the residual strength stage [2].

3.2. Shear Strength of the Slip Zone Soil by In Situ Shear Test According to the curve of shear stress–shear displacement (Figure8g–l), the shear strength values (the maximumSensors 2020, value20, x FOR in PEER the curve) REVIEW under various normal pressures are determined. Under the13 normal of 18 stresses of 0.067, 0.080, 0.189, 0.317, 0.141, and 0.253 MPa, the shear strengths are 0.059, 0.044, 0.109, can0.138, be 0.084,drawn and (Figure 0.122 9 MPa,), and respectively. the linear fitting Furthermore, equation the is relationshipobtained according curve of toτ– σthen can least be square drawn method:(Figure9 ), and the linear ﬁtting equation is obtained according to the least square method:

2 τ = 0.363σn + 0.0294 (R2= 0.9239) (6) =0.363n +0.0294 (R =0.9239) (6)

FigureFigure 9. 9. ShearShear strength strength of of the the slip slip zone zone soil soil of of the Riverside Slump Slump I# I# of of the Huangtupo landslide landslide obtainedobtained by by in in situ situ shear shear tests. tests.

Consequently,Consequently, the the shear shear strength strength parameters parameters of of slip slip zone zone soil soil of of the the Riverside Riverside Slump Slump I# I# of of the the HuangtupoHuangtupo landslide cancan bebe obtained. obtained. The The cohesion cohesion is 29.4is 29.4 kPa, kPa, and and the internalthe interna frictionl friction angle angle is 19.9 is◦ , 19.9°which, which is the is arctangent the arctangent of 0.3626. of 0.3626.

3.3.3.3. S Shearhear C Constitutiveonstitutive M Modelodel of of S Sliplip Z Zoneone S Soiloil with with G Gravelsravels TheThe constitutive constitutive model model (quantitative (quantitative relationship relationship between between stress stress and displacement) and displacement) of slip zone of slip soil zoneis an important soil is an basis important for analyzing basis for the analyzing mechanical the behavior mechanical of landslides. behavior Currently, of landslide manys. stress–strain Currently, manyconstitutive stress– modelsstrain constitutive have been developed models have based been on laboratory developed tests; based however, on laboratory few stress–displacement tests; however, fewconstitutive stress–displacement models [2] and constitutive few shear models constitutive [2] and models few shear based constitutive on in situ tests models have based been developed.on in situ tests have been developed. It can be seen from the shear stress–shear displacement curves (Figure 8) that the initial curve is steep in the elastic stage; with the increase of shear displacement, the slope of the curve gradually decreases, and finally, the shear stress value tends to be stable. Considering the above characteristics of the curves, four kinds of equations, namely asymmetric sigmoid function, symmetric sigmoid function, exponential function, and power function, are introduced to describe the shear mechanical behavior of slip zone soil, and the shear stress–shear displacement constitutive model is thereby established. The asymmetric sigmoid function is expressed as follows:

A  =-A p u (7) 1 s u0

The symmetric sigmoid function is expressed as follows:

A  = (8) 1+ek us0 u 

The exponential function is expressed as follows:

uu/  =Aa - e s0 (9) Sensors 2020, 20, 6531 12 of 16

It can be seen from the shear stress–shear displacement curves (Figure8) that the initial curve is steep in the elastic stage; with the increase of shear displacement, the slope of the curve gradually decreases, and ﬁnally, the shear stress value tends to be stable. Considering the above characteristics of the curves, four kinds of equations, namely asymmetric sigmoid function, symmetric sigmoid function, exponential function, and power function, are introduced to describe the shear mechanical behavior of slip zone soil, and the shear stress–shear displacement constitutive model is thereby established. The asymmetric sigmoid function is expressed as follows:

A τ = A p (7) − 1 + us u0

The symmetric sigmoid function is expressed as follows:

A τ = (8) k(us u ) 1+e− − 0 The exponential function is expressed as follows:

us/u τ = A a e− 0 (9) − · The power function is expressed as follows:

p τ = a us (10) ·

A, u0, p, k, a are the relative parameters of the above equations. Each equation has two to three parameters. Figure 10 shows the fitting results of in situ shear test data of the slip zone soil of the Riverside Slump I# of the Huangtupo landslide using the above formulas. On the whole, the test data are well fitted by the equations mentioned above, as demonstrated by the correlation coefficient of each group of curves with values above 0.9 (Figure 11). Among the four formulas, the asymmetric sigmoid function shows the best fitting, with an average correlation coefficient of 0.980, followed by a power function, exponential function, and symmetric sigmoid function, with average correlation coefficients 0.963, 0.948, and 0.907, respectively (Figure 11). As can be seen from Figure 10b, most of the fitting curves of the symmetric S-curve do not pass through the initial point and maximum point, and the fitting curve changes greatly from the actual point position when the normal stress σn = 0.253 MPa. Figure 10c shows that although the fitting degree of the curve is high, the change from the steep part to the gentle part is too sudden to express the elastic–plastic stage. Figure 10d shows that data are well fitted, but it cannot reflect the feature of shear stress tending to be stable in the plastic stage, and the curve cannot converge; therefore, it cannot be adopted. The curves in Figure 10a fit the characteristics of each stage well. To sum up, the asymmetric sigmoid function not only has a high fitting degree with the test results, but it also defines the relationship between shear stress and shear displacement in the process of the direct shear test, which can be applied to analyze the change of shear stress with shear displacement in each stage of the direct shear test. The asymmetric sigmoid function is the best for constructing the shear constitutive model of the gravelly slip zone soil. The parameters in this proposed shear constitutive model have clear physical meanings. The value of A in Equation (7) represents the yield strength of slip zone soil, and parameters a and u0 jointly control the shape of the constitutive curve. The above three parameters change with the normal stress value of the slip zone soil. The presented model describes the stress–displacement relations. It is significant for landslide stability analysis, because the shear stress varies as the slip surface displaces. Utilizing the shear–displacement model, the dynamic stability of the landslide can be evaluated [2]. Moreover, Sensors 2020, 20, x FOR PEER REVIEW 14 of 18

The power function is expressed as follows:

p  =au s (10)

A, u0, p, k, a are the relative parameters of the above equations. Each equation has two to three parameters. Figure 10 shows the fitting results of in situ shear test data of the slip zone soil of the Riverside Slump I# of the Huangtupo landslide using the above formulas. On the whole, the test data are well fitted by the equations mentioned above, as demonstrated by the correlation coefficient of each groupSensors of2020 curves, 20, 6531 with values above 0.9 (Figure 11). Among the four formulas, the asymmetric13 of 16 sigmoid function shows the best fitting, with an average correlation coefficient of 0.980, followed by a powerthe shear function, strength exponential parameters function, can be derived and symmetric from the shear sigmoid constitutive function, model; with therefore, average thiscorrelation shear coefficientsconstitutive 0.963, model 0.948, is of and great 0.907, use inrespectively practice. (Figure 11).

2 (a) σn=0.317MPa，R =0.9707 (b) =A-A/[1+(us/uo)^p] -k(us-u0) 2 2 =-A/[1+e ] n=0.317MPa，R =0.9180 n=0.253MPa，R =0.999 0.12 0.12 2 2 n=0.253MPa，R =0.9109 n=0.189MPa，R =0.9488

2 (MPa)  =0.189MPa，R =0.9119 2  n (MPa)  =0.141MPa，R =0.9913  n 0.08 0.08  =0.141MPa，R2=0.8883 2 n n=0.067MPa，R =0.9768

2 2 =0.067MPa，R =0.8966 0.04 n=0.080MPa，R =0.9932 0.04 2

n=0.080MPa，R =0.9179 Shear stress,Shear Shear stress,Shear Parameter Asymetric sigmoid function Parameter Symmetric sigmoid function σ (MPa) n 0.067 0.080 0.189 0.317 0.141 0.253 σn (MPa) 0.067 0.080 0.189 0.317 0.141 0.253 0.00 A 0.092 0.081 0.248 0.345 0.148 0.157 0.00 A 0.054 0.042 0.112 0.128 0.081 0.112

u0 18.84614.66356.82258.08315.023 4.373 u0 8.191 2.739 9.753 4.975 3.948 2.525 p 0.682 0.488 0.646 0.530 0.440 0.568 k 0.199 0.533 0.141 0.291 0.354 0.587 0 10 20 30 40 50 0 10 20 30 40 50

Shear displacement, us(mm) Shear displacement, us(mm)

0.12 2 0.12 2 n=0.253MPa，R =0.9540 n=0.253MPa，R =0.9679

2 2

n=0.189MPa，R =0.9345 n=0.189MPa，R =0.9512

(MPa) (MPa)  0.08  0.08 2 2 n=0.141MPa，R =0.9801 n=0.141MPa，R =0.9341

2 2 n=0.067MPa，R =0.9687 n=0.067MPa，R =0.9663 0.04 2 0.04 n=0.080MPa，R =0.9481 2 n=0.080MPa，R =0.9585 Parameter Exponential function

Parameter Power function Shear stress,Shear Shear stress,Shear σ (MPa) 0.067 0.080 0.189 0.317 0.141 0.253 n σ (MPa) 0.067 0.080 0.189 0.317 0.141 0.253 0.00 A 0.058 0.043 0.130 0.138 0.081 0.113 0.00 n a a 0.055 0.038 0.114 0.123 0.071 0.101 0.015 0.018 0.025 0.039 0.035 0.048 p 0.369 0.298 0.421 0.388 0.272 0.266 u0 11.714 4.413 19.726 8.121 4.612 3.640 0 10 20 30 40 50 0 10 20 30 40 50 Shear displacement, u (mm) Shear displacement, u (mm) s s FigureFigure 10. 10. FittingFitting curves curves withwith four four functions functions for basedfor based on in on situ in shear situ test shear data. test (a) Asymmetricdata. (a) Asymmetric sigmoid sigmoidSensorsfunction; 20function;20 (b, )20 symmetric, x FOR(b) symmetricPEER sigmoid REVIEW sigmoid function; function; (c) exponential (c) exponential function; ( dfunction;) power function.(d) power function. 15 of 18

R2 1.00 Average value Standard deviation

0.98

n e

i 0.96

o c

0.94

a l

e 0.92

o C 0.90

0.88 1 2 3 4 Model number

Figure 11. 11. CorrelationCorrelation coe coefficientsﬃcients for di forﬀ erent different functions. functions. 1 Asymmetric 1 Asymmet sigmoidric function;sigmoid 2 function; symmetric 2 symmetricsigmoid function; sigmoid 3 function; exponential 3 exponential function; 4 function; power function. 4 power function.

As can be seen from Figure 10b, most of the fitting curves of the symmetric S-curve do not pass through the initial point and maximum point, and the fitting curve changes greatly from the actual point position when the normal stress σn = 0.253 MPa. Figure 10c shows that although the fitting degree of the curve is high, the change from the steep part to the gentle part is too sudden to express the elastic–plastic stage. Figure 10d shows that data are well fitted, but it cannot reflect the feature of shear stress tending to be stable in the plastic stage, and the curve cannot converge; therefore, it cannot be adopted. The curves in Figure 10a fit the characteristics of each stage well. To sum up, the asymmetric sigmoid function not only has a high fitting degree with the test results, but it also defines the relationship between shear stress and shear displacement in the process of the direct shear test, which can be applied to analyze the change of shear stress with shear displacement in each stage of the direct shear test. The asymmetric sigmoid function is the best for constructing the shear constitutive model of the gravelly slip zone soil. The parameters in this proposed shear constitutive model have clear physical meanings. The value of A in Equation (7) represents the yield strength of slip zone soil, and parameters a and u0 jointly control the shape of the constitutive curve. The above three parameters change with the normal stress value of the slip zone soil. The presented model describes the stress–displacement relations. It is significant for landslide stability analysis, because the shear stress varies as the slip surface displaces. Utilizing the shear– displacement model, the dynamic stability of the landslide can be evaluated [2]. Moreover, the shear strength parameters can be derived from the shear constitutive model; therefore, this shear constitutive model is of great use in practice.

4. Discussion The indoor direct shear test results are shown in Figure 12. The shear strength parameters measured by the indoor direct shear test are 16.5 kPa for cohesion and 7.2° for internal friction angle. Compared with the results of the in situ direct shear test and laboratory test, it can be found that the cohesion decreases by 43.9%, and the internal friction angle decreases by 63.8%. The strength of the slip zone soil is underestimated by the indoor direct shear test, which may be attributed to the following reasons. Firstly, the structure of the slip zone soil was disturbed in the remolded soil sample. More importantly, gravels in the slip zone soil were removed due to the size limitation of the indoor direct shear test apparatus. In the in situ test, the content of coarse particles in the soil sample is higher, and the interaction and friction between the particles are more severe than those in the indoor direct shear tests. So, the internal friction angle of the soil sample is greatly Sensors 2020, 20, 6531 14 of 16

4. Discussion The indoor direct shear test results are shown in Figure 12. The shear strength parameters measured by the indoor direct shear test are 16.5 kPa for cohesion and 7.2◦ for internal friction angle. Compared with the results of the in situ direct shear test and laboratory test, it can be found that the cohesion decreases by 43.9%, and the internal friction angle decreases by 63.8%. The strength of the slip zone soil is underestimated by the indoor direct shear test, which may be attributed to the following reasons. Firstly, the structure of the slip zone soil was disturbed in the remolded soil sample. More importantly, gravels in the slip zone soil were removed due to the size limitation of the indoor direct shear test apparatus. In the in situ test, the content of coarse particles in the soil sample is higher, and the interaction and friction between the particles are more severe than those in the indoor direct shearSensors tests. 20 So,20, the 20, x internal FOR PEER friction REVIEW angle of the soil sample is greatly higher than that of the16 indoor of 18 direct shear test. It further reveals the important role of the gravels played in the slip zone soil strength. higher than that of the indoor direct shear test. It further reveals the important role of the gravels Since the shear strength parameters are the basic data for evaluating the stability of the landslide and played in the slip zone soil strength. Since the shear strength parameters are the basic data for designing the anti-slide structures for landslides, the shear strength parameters obtained by the in situ evaluating the stability of the landslide and designing the anti-slide structures for landslides, the test are more preferable. shear strength parameters obtained by the in situ test are more preferable.

FigureFigure 12. 12.Indoor Indoor directdirect shear test test results results of of the the 2 2-mm-sieved-mm-sieved slip slip zone zone soil soil of ofthe the Riverside Riverside Slump Slump I# I#of of the the Huangtupo Huangtupo landslide landslide with with water water content content of of 13.63% 13.63%.. (a ()a ) Shear Shear stress stress vs. vs. displacement displacement under under variousvarious normal normal stress; stress; ( b(b)) shear shear strength strength vs.vs. normalnormal stress.stress.

TheThe inin situsitu shear test test has has many many advantages advantages such such as aspreserving preserving the theoriginal original structure structure of the of slip the slipzone zone soil soil and and allowing allowing large large shear shear displacement displacement;; however, however, it it is is time time-consuming-consuming and and expensive. expensive. Nevertheless,Nevertheless, the the in in situ situ shear shear testtest isis aaffordableﬀordable andand preferablepreferable inin manymany importantimportant projects.

5.5. Conclusions Conclusions UtilizingUtilizing thethe large-scalelarge-scale ﬁeldfield comprehensivecomprehensive landslidelandslide experimental base base in in the the Three Three Gorges Gorges ReservoirReservoir area, area, a groupa group of of in in situ situ shear shear tests tests of theof the gravelly gravelly slip slip zone zone soil ofsoil the of Riverside the Riverside Slump Slump I# of theI# Huangtupoof the Huangtupo landslide landslide were conducted were conducted by using theby using designed the portabledesigned apparatus. portable apparatus. The shear mechanicalThe shear propertiesmechanical of p theroperties gravelly of slipthe zonegravelly soil wereslip zone analyzed. soil were The analyzed. following The conclusions following are conclusions reached: are reached:(1) In the later stage of the shear test, rebound may occur along the normal direction of the sample. The rebound(1) In the phenomenon later stage is of relatedthe shear to thetest, rotation rebound of the may gravels occur onalong the the shear normal plane. direction of the sample.(2) The The shearing rebound process phenomenon of the gravelly is related slip to zonethe rotation soil of theof the Huangtupo gravels on landslide the shear revealed plane. by the in situ(2 shear) The test shearing can be process divided of into the three gravelly stages slip including zone soil an of elastic the Huangtupo deformation landslide stage, elastic–plastic revealed by deformationthe in situ shear stage, test and can plastic be divided deformation into three stage. stages including an elastic deformation stage, elastic– plastic deformation stage, and plastic deformation stage. (3) Four types of functions were introduced to describe the relationship of the shear stress and shear displacement of gravelly slip zone soil. The asymmetric sigmoid function is demonstrated to be the optimal one and the parameters of the function have clear physical meaning. (4) The shear strength of the slip zone soil obtained by the indoor direct shear test is underestimated compared with the in situ shear test result. The gravels have critical influence on the mechanical property of the slip zone soil, and the in situ shear test is preferable to obtain the shear strength of the gravelly slip zone soil for reliable landslide stability evaluation and anti-slide structure design.

(3) Four types of functions were introduced to describe the relationship of the shear stress and shear displacement of gravelly slip zone soil. The asymmetric sigmoid function is demonstrated to be the optimal one and the parameters of the function have clear physical meaning. (4) The shear strength of the slip zone soil obtained by the indoor direct shear test is underestimated compared with the in situ shear test result. The gravels have critical inﬂuence on the mechanical property of the slip zone soil, and the in situ shear test is preferable to obtain the shear strength of the gravelly slip zone soil for reliable landslide stability evaluation and anti-slide structure design.

Author Contributions: This work was carried out as a collaboration among all the authors. Conceptualization, Z.Z., C.X. and H.T.; Data curation, L.F. and F.X.; Funding acquisition, Z.Z., C.X. and H.T.; Methodology, Z.Z.; Resources, C.X. and H.T.; Writing—original draft, Z.Z. and Q.Z.; Writing—review and editing, J.Y. and Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Key Research and Development Program of China (No. 2017YFC1501303); the National Natural Science Foundation of China (Nos.41827808, 41502290). Acknowledgments: The authors would like to thank all the scholars helping in the discussion and realization of this paper. This work was supported by the National Key Research and Development Program of China (No. 2017YFC1501303); the National Natural Science Foundation of China (Nos.41827808, 41502290). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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