Study on the Influence of Water–Rock Interaction on The

sustainability

Article Study on the Inﬂuence of Water–Rock Interaction on the Stability of Schist Slope

Qian-Cheng Sun 1,2, Can Wei 1, Xi-Man Sha 1, Bing-Hao Zhou 1, Guo-Dong Zhang 2, Zhi-Hua Xu 1,2,* and Ling Cao 1,*

1 Key Laboratory of Geological Hazards on Three Gorges Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002, China; [email protected] (Q.-C.S.); [email protected] (C.W.); [email protected] (X.-M.S.); [email protected] (B.-H.Z.) 2 National Field Observation and Research Station of Landslides in Three Gorges Reservoir Area of Yangtze River, China Three Gorges University, Yichang 443002, China; [email protected] * Correspondence: [email protected] (Z.-H.X.); [email protected] (L.C.)

Received: 13 July 2020; Accepted: 26 August 2020; Published: 1 September 2020

Abstract: (1) The studies on the influence of rainfall on slope stability mainly focus on rainfall characteristics and the variation of strength parameters. Few studies pay attention to the micro structure changes of rock mass under long-term rainfall conditions, and the influence of failure mode. (2) Based on nuclear magnetic resonance (NMR) and electron microscopic imaging (Emmi) technology, the micro structure changes and macro deformation characteristics of the schist, under long-term immersion in different liquids are analyzed. (3) After soaking in the deionized water, the uniaxial compression strength of the intact specimen is slightly lower than that of the untreated specimens, but the test process in the elastic compression stage is considerably prolonged, and the failure modes show both shear and slip at the same time. While after soaking in acid solution, the fracture of rock samples with initial cracks can be obviously reduced and healed, which is consistent with the change of micro pore structure. The uniaxial strength and modulus of the intact samples are significantly lower, and only slip failure mode occurred. (4) It shows that water–rock interaction is an important factor influencing the stability of slope besides the external rainfall force, which affects the structural characteristics and mechanical properties of rock.

Keywords: rainfall; nuclear magnetic resonance (NMR); schist slope; microstructural characteristics; failure mode; water–rock interaction

1. Introduction Rainfall has a significant effect on the stability of slopes, and slope deformation can occur during processes of a long period of rain. Under the most serious conditions, this can lead to the triggering of large-scale landslides and cause significant damage to infrastructure and loss of life, which is not conducive to the sustainable development of social economy. There have been numerous studies on slope stability associated with rainfall. Two approaches are commonly used to characterize the effects of rainfall on slope deformation: Numerical analysis or model testing. For numerical analysis, limit equilibrium method and strength reduction method, which are originated in the soil slope and then modified for rock slope, were commonly used. The influence of rainfall on slope is mainly through simulating infiltration or reducing strength parameters. Paul et al. [1] proposed a simultaneous reduction method for overhanging rock slopes. Mao et al. [2] used limit equilibrium method and strength reduction finite element method to evaluate slope stability under rainfall infiltration. Wu et al. [3] used intensity reduction method to analyze slope seepage and stability, considering the different rainfall intensity and duration, and concluded that

Sustainability 2020, 12, 7141; doi:10.3390/su12177141 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 7141 2 of 14 rainfall intensity and rainfall duration have the most significant impact on slope, which is consistent with Chen’s conclusion [4]. Camera [5] and Ng [6], using finite element software, studied the influence of groundwater level, pore water pressure, and response characteristics of groundwater under different rainfall patterns and durations on slope. For model testing, the influence of rainfall on slope is mainly simulated by different rainfall intensity or rainfall time, and then the deformation characteristics and failure modes of slope under rainfall conditions are analyzed. Vedie et al. [7] conducted model testing on a landslide triggered by rainfall, and the typical failure mode of slope instability in the permafrost region was revealed under different slope conditions. Ray [8] studied the stability of rock slope by modeling the impact of rainfall variations and management interventions on the groundwater. Chueasamat et al. [9] conducted model tests to investigate experimentally the effects of surface sand layer density and rainfall intensity on the slope failure due to rainfall. Experimental tests using physical models have also been conducted by Damiano and Olivares [10] to observe the slope failure mechanism, and the role of infiltration processes in steep slopes stability. By means of laboratory tests, model tests, and numerical calculations, scholars worldwide have studied the issues including rainfall characteristics (rainfall pattern) [11,12], movement characteristics and failure types of different landslides [13], influence mechanism of rainfall on different types of slopes [14], landslide hydrology [15]. In fact, the influence of rainfall on slope stability is strongly related with the composition and properties of slope rock. With the development of testing technology and testing methods, more and more scholars begin to pay attention to the influence of water–rock interaction on rock properties after rainwater enters into the rock mass. Liu et al. [16] discussed two typical flow patterns on the softening of red sandstone and pointed out that dynamic water–rock interactions have a great effect on rock softening and breaking. Xu et al. [17,18] studied the effect of periodic water circulation on rock mass by P-Wave Velocity and found that the water–rock interaction changed the porosity of the rock pores and then affected its mechanical properties. Mu [19] studied the triggering mechanism and reactivation probability of a loess-mudstone landslide induced by rainwater infiltration, and concluded that shear strength of media is very sensitive to water content because of water–rock interaction, and landslide reactivation is controlled by the water sensitivity of media, especially the original sliding zone. While mechanical properties of rock based on local microstructure [20–22] show that any macroscopic damage of rock is the ultimate embodiment of its micro damage accumulation, studying the microstructural characteristics of rock in the process of water–rock interaction is also the basis of revealing the mechanism of rainfall-induced slope instability. There is a large area of metamorphic rocks in Shiyan City, Hubei Province, according to the survey data of geological disasters released by Shiyan Municipal Bureau of Land and Resources in 2016. There have been 1889 geological disasters in Zhushan, Zhuxi, and Fangxian counties in Duhe River Basin in Hubei, including 1801 landslides. As a result of geological disasters, 31 people died, and the direct economic loss was about 225 million yuan. There are still 48,800 people and about 3.7 billion RMB worth of property safety under threat [23], as well as priceless cultural heritage resources. We have conducted a lot of investigations into this area, a thorough statistical analysis of metamorphic rock slope, a kind of rock that is distinctly anisotropic, in Shiyan is shown in Table1. These examples showed the evidence of long-time rainfall acting as a trigger for landslides. In this paper, Zhushan landslide was considered in view of the specific engineering geological conditions. Microstructural evolution characteristics of schist during water–rock interaction was revealed, using low field nuclear magnetic resonance (NMR) and electron microscopy, considering the effects of seasonal acid rain, to analyze the structural changes of schist under the action of long time soaking from the perspective of micro view. The results of this work can provide a supplementary reference for accurately analyzing and evaluating the effect of long-term rain on slope. Sustainability 2020, 12, 7141 3 of 14

Table 1. The landslide caused by rainfall in Shiyan.

Location in Hubei No. Name Incentive Lithologic Composition Province 1 Yeda landslide Yunxian county Rainfall and impoundment Quartz schist 2 Yetan landslide Yunxian county Rainfall and impoundment schist 3 Daoshiping landslide Zhushan county Rainfall Quartz-mica schist 4 Machanghe landslide Zhushan county Rainfall Quartz Muscovite schist 5 Hongjiapo landslide Zhushan county Rainfall Quartz-mica schist 6 Dujiayan landslide Fang county Rainfall and impoundment Quartz-mica schist 7 Daijiawan landslide Fang county Rainfall and impoundment Chlorite mica schist 8 Longtangpo landslide Fang county Rainfall Quartz-mica schist 9 E’ping school landslide Zhuxi county Rainfall Chlorite mica schist 10 Hongjiashan landslide Zhuxi county Rainfall Quartz sericite schist

2. Overview of Zhushan Landslide Zhushan Landslide is located on the left side of Section K155+854-K156+178 of Gucheng–Zhuxi Expressway, which is a single expressway construction project with the longest construction mileage and the largest investment scale, as well as the first engineering project to implement standardized construction management in Hubei Province. It is of great significance to the implementation of the two major strategies of the rise of the central region and the development of the western region, and the construction of the ecological and cultural tourism circle in Western Hubei. Once damaged by natural disasters, it will directly affect the contact and communication between the central and western regions and affect the traffic conditions in the mountainous areas of central and Western Hubei. During the construction of the expressway in 2012, part of the landslide collapsed, and anti-slide piles were immediately set up along the road. In 2013, a retaining wall was set between anti-slide pile and expressway. However, the creep deformation of landslide gradually accumulates in each rainy season, which has begun to reduce the function of the support structure and affect the stability of retaining Sustainabilitywalls, as shown 2020, 12 in, x FigureFOR PEER1. REVIEW 4 of 16

a a b Local extrusion of leading edge Cracking at the top of retaining wall b c d

Extrusion deformation of ditch Cracking in the middle of retaining wall Figure 1. TheThe deformation deformation of support structures. The slope is fan-shaped with a main sliding direction of 10 , the longitudinal length is about The slope is fan-shaped with a main sliding direction of 10°, the◦ longitudinal length is about 60– 60–90 m, and the transverse length is about 100–160 m (as shown in Figure2). The bedrock of the 90 m, and the transverse length is about 100–160 m (as shown in Figure 2). The bedrock of the landslide is mainly grayish green and grayish black sericite schist, with a single thickness of 1–3 mm. landslide is mainly grayish green and grayish black sericite schist, with a single thickness of 1–3 mm. The occurrence of schistosity are the strike 200 –340 with the dip angle of 21 –33 , separation state The occurrence of schistosity are the strike 200°–340°◦ ◦ with the dip angle of 21°-33°,◦ ◦ separation state with aperture distance less than 3 mm, clean without ﬁlling and poor combination, which can be with aperture distance less than 3 mm, clean without filling and poor combination, which can be deﬁned as weak structural surface. The annual average precipitation in this area is about 990 mm, defined as weak structural surface. The annual average precipitation in this area is about 990 mm, which is characterized by frequent rainstorms, continuous rainy days, and uneven spatial and temporal which is characterized by frequent rainstorms, continuous rainy days, and uneven spatial and temporal distribution. The overall terrain of the landslide area is high in the south and low in the north. A part of atmospheric precipitation is automatically discharged to the southeast in the form of slope flow. While some of it infiltrates the slope overburden and enters the bedrock fissures, and finally flows into the slope bottom. The monitoring data of borehole displacement on the landslide in Figure 3 show that the landslide will have obvious displacement after the rainy season, and there is a significant lag between the occurrence of deformation and rainfall. Therefore, this paper mainly considers the influence of time effect after rainfall on slope stability and sets long-term immersion experiment to study the influence of physical and mechanical properties of schist under water–rock interaction. Sustainability 2020, 12, 7141 4 of 14

distribution. The overall terrain of the landslide area is high in the south and low in the north. A part of atmospheric precipitation is automatically discharged to the southeast in the form of slope flow. While some of it infiltrates the slope overburden and enters the bedrock fissures, and finally flows into the slope bottom. The monitoring data of borehole displacement on the landslide in Figure3 show that the landslide will have obvious displacement after the rainy season, and there is a significant lag between the occurrence of deformation and rainfall. Therefore, this paper mainly considers the

Sustainabilityinfluence 2020 of time, 12, x eFORffect PEER after REVIEW rainfall on slope stability and sets long-term immersion experiment5 of 16 to Sustainabilitystudy the influence 2020, 12, x FOR of physical PEER REVIEW and mechanical properties of schist under water–rock interaction.5 of 16 N N

100m-160m 100m-160m 10° 10°

ZK01 ZK01

ZK04 ZK04

Figure 2. Overview of the Zhushan slope. Figure 2. Overview of the Zhushan slope. Figure 2. Overview of the Zhushan slope. Deformation / mm Deformation / mm -20 0Deformation 20 40 / mm 60 80 -20 0Deformation 20 40 / mm 60 80 -200 0 20 40 60 80 -200 0 20 40 60 80 0 0

5 5 5 5

10 10 10 10 10-11 10-11 10-1610-11 10-1610-11 15 15 10-2110-16 10-2110-16 15 15 10-2810-21 10-2810-21 Depth / m Depth Depth / m Depth 11-610-28 11-610-28 Depth / m Depth Depth / m Depth 20 11-6 20 11-6 20 20

25 25 25 25

30 30 ZK01 ZK04 30 30 ZK01 ZK04 Figure 3. The monitoring data of borehole displacement on the main landslide section. Figure 3. The monitoring data of borehole displacement on the main landslide section. Figure 3. The monitoring data of borehole displacement on the main landslide section. 3. Indoor Test and Result Analysis 3. Indoor Test and Result Analysis 3.1. Test Preparation and Test Plan 3.1. Test Preparation and Test Plan In order to study the influence of water–rock interaction on the stability of schist slope, the schist blocksIn taken order from to study the landslide the influence area of are water processed–rock interaction into samples on ofthe different stability scales,of schist and slope, the samplesthe schist withblocks obvious taken fromdefects the and landslide large discretenessarea are processed are screened into samples and removed of different by ultrasonicscales, and testing. the samples The mineralwith obvious composition defects is mainlyand large composed discreteness of quartz, are screened feldspar, andmuscovite, removed and by clinochlore. ultrasonic Thetesting. natural The mineral composition is mainly composed of quartz, feldspar, muscovite, and clinochlore. The natural Sustainability 2020, 12, 7141 5 of 14

3. Indoor Test and Result Analysis

3.1. Test Preparation and Test Plan In order to study the inﬂuence of water–rock interaction on the stability of schist slope, the schist blocks taken from the landslide area are processed into samples of diﬀerent scales, and the samples with obvious defects and large discreteness are screened and removed by ultrasonic testing. The mineral composition is mainly composed of quartz, feldspar, muscovite, and clinochlore. The natural density is 2.53 g/cm3 and the saturated density is 2.76 g/cm3. The test scheme is set as follows, and the instruments used are also shown in Figure4. Sustainability 2020, 12, x FOR PEER REVIEW 6 of 16

(1) Take out the selected rock samples after drying at 45 ◦C, put them into a dryer to cool them density is 2.53 g/cm3 and the saturated density is 2.76 g/cm3. The test scheme is set as follows, and the down to room temperature and weigh them. Repeat the drying until the difference between the instruments used are also shown in Figure 4. two adjacent masses does not exceed 0.1% of the later weight, indicating that the initial drying is(1) completed. Take out the Then selected measure rock samples and record after the drying size andat 45 wave°C, put velocity them into of a each dryer rock to cool sample, them anddown group themto to room undergo temperature different and test weigh conditions. them. Repeat the drying until the difference between the two adjacent masses does not exceed 0.1% of the later weight, indicating that the initial drying is (2) Considering the influence of acid rain as it is reported, the rock samples after initial drying are completed. Then measure and record the size and wave velocity of each rock sample, and group soakedthem in to H undergo2SO4 solution different with testpH conditions.= 4 and deionized water, respectively. (3) In(2) eachConsidering group of the tests, influence a number of acid of rain control as it is rockreported, samples the rock were samples set up, after and initial the drying samples are were takensoaked regularly in H2 everySO4 solution day after with immersion pH = 4 and fordeionized scanning water, the respectively. local surface morphology by electron microscope,(3) In each group and the of tests, quality, a number wave of velocity, control androck microsamples pore were structure set up, and of thethe samples complete were rock taken samples wereregularly tested at every the same day after time. immersion for scanning the local surface morphology by electron (4) Aftermicroscope, each day’s and immersion, the quality, thewave solution velocity, and and control micro pore group structure samples of the are complete taken to rock test the samples were tested at the same time. composition and content of mineral in liquid and solid, respectively. (4) After each day’s immersion, the solution and control group samples are taken to test the (5) The strengthcomposition test and was content carried of outmineral onthe in liquid samples and undergoingsolid, respectively. different test conditions. (5) The strength test was carried out on the samples undergoing different test conditions.

FigureFigure 4. The4. The test test scheme scheme and and mainmain inst instrumentsruments used used in inthe the test. test. 3.2. Analysis of Pore Structure in Samples 3.2. Analysis of Pore Structure in Samples AfterAfter soaking soaking in di inff differenterent solutions solutions for for the the samesame time, the the low low field field nuclear nuclear magnetic magnetic resonance resonance (NMR)(NMR) test wastest was carried carried out. out. The The NMR NMR T2 T2 spectrum spectrum is the the measurement measurement of ofhydrogen hydrogen atom atom bearing bearing waterwater in rock in rock pores. pores. The The distribution distribution of of pores pores will will aaffffectect the the test test results results and and the the change change of T2 of spectrum T2 spectrum was usedwas toused analyze to analyze the change the change of micro of micro pore structurepore structure under under different different solution solution conditions. conditions. The The shorter the relaxationshorter the time relaxation of transverse time of transverse axis, the axis, smaller the smaller the micro the micro pore pore diameter, diameter, and and the the position position of T2 of T2 spectrum peak value was related to the change of sample pore size, while the size of T2 spectrum area was related to the number of sample pores [24,25]. The T2 spectrum of typical rock samples after immersion in deionized water for a number of days is bimodal distribution, as shown in Figure 5. With the increase of soaking time, the main wave peak shifted to the left only when soaking for five days. After soaking for 10 days, the main wave peak shifted to the right by a large margin, and a new wave peak appeared at the relaxation time of 1000 ms. After soaking for 15 to 30 days, the main wave peak shifted to the right again. This shows that the pore size of the schist sample decreases during the five-day soaking process. However, the Sustainability 2020, 12, x FOR PEER REVIEW 7 of 16

pore structure of schist becomes larger with the prolongation of soaking time, and larger pore size is formed with the breakthrough of small pore. Sustainability 2020, 12, 7141 6 of 14 Figure 6 shows the change of T2 spectrum of typical rock samples soaked in the solution with pH = 4 for a number of days. It can be seen from the figure that the change of the position of the two spectrummain wave peak peaks value occurred was related in 5 todays the changeand 10 ofdays, sample respectively. pore size, whileAfter thesoaking size of for T2 five spectrum days, areathe wasposition related of the to the main number wave of peak sample did poresnot change [24,25 ].obviously, while the range of the secondary wave peakThe expanded T2 spectrum to the ofright typical side, rock wh samplesich indicated after immersionthat five days in deionized soaking in water acid forsolution a number had ofa great days isinfluence bimodal on distribution, the macropore. as shown When in Figurethe immersion5. With the time increase is prolonged, of soaking the time, main the wave main peak wave of peak the shiftedsample toshifts the to left the only right when obviously soaking after for immersio five days.n Afterfor 10 soakingdays, which for 10 indicates days, the that main the wavepores peakwith shiftedsmaller to size the in right the sample by a large become margin, larger. and However, a new wave the peak process appeared of soaking at the for relaxation 10 days timeto 30 of days 1000 does ms. AfterSustainabilitynot change soaking 2020 the for, 12position, 15 x FOR to 30 PEER of days, the REVIEW themain main wave wave peak peak of shiftedthe sample, to the but right makes again. the This secondary shows that wave the 7peak poreof 16 sizeshifts of to the the schist left, sample indicating decreases that the during long-term the five-day soaking soaking under process. acid conditions However, does the pore not structurechange the of pore structure of schist becomes larger with the prolongation of soaking time, and larger pore size is schistsmall pores becomes of the larger sample with obviously, the prolongation and it is ofeven soaking helpful time, for andthe repair larger of pore the sizelarge is pores formed to a with certain the formed with the breakthrough of small pore. breakthroughextent. of small pore. Figure 6 shows the change of T2 spectrum of typical rock samples soaked in the solution with pH = 4 for a number of days. It41 can be seen from the figure that the change of the position of the two D1 main wave peaks occurred in36 5 days and 10 days, respectively. D5After soaking for five days, the position of the main wave peak did not change obviously, while theD10 range of the secondary wave peak expanded to the right side,31 which indicated that five days soakingD15 in acid solution had a great D20 26 influence on the macropore. When the immersion time is prolonged,D25 the main wave peak of the sample shifts to the right obviously21 after immersion for 10 days, whichD30 indicates that the pores with smaller size in the sample become larger. However, the process of soaking for 10 days to 30 days does 16 not change the position of theamplitude Signal main wave peak of the sample, but makes the secondary wave peak ① 11 shifts to the left, indicating that the long-term⑩ soaking under acid conditions does not change the ⑩ small pores of the sample obviously,6 and it is even helpful① for the repair of the large pores to a certain ① extent. 1 0.01 0.1 1 10 100 1000 10000 41 Relaxation time T2/ms D1 Figure 5. The T2 36 spectrum of typical rock samples soaked D5 in deionized water. D10 31 D15 Figure6 shows the change50 of T2 spectrum of typical rock samples soaked in the solution with D1D20 26 pH = 4 for a number of days. It45 can be seen from the figure that theD5D25 change of the position of the two main wave peaks occurred in 521 days40 and 10 days, respectively. AfterD10D30 soaking for five days, the position D15 of the main wave peak did not change obviously, while the range of the secondary wave peak expanded 16 35 D20 Signal amplitude Signal D25 to the right side, which indicated30 that five① days soaking in acid solution had a great influence on the 11 D30 macropore. When the immersion25 time is prolonged,⑩ the main wave peak of the sample shifts to the ⑩ 6 ① right obviously after immersion20 for 10 days, which indicates that the pores with smaller size in the ① Signal amplitude Signal sample become larger. However,1 15 the process of soaking for 10 days to 30 days does not change the 0.01 0.1① 1 10 100 1000 10000 position of the main wave peak10 of the sample, but makes the secondary wave peak shifts to the left, ⑤Relaxation time T2/ms indicating that the long-term soaking5 under acid conditions① ⑤ does not change the small pores of the ⑩ sample obviously,Figure and 5. it The is evenT2 spectrum0 helpful of for typical the repair rock samples of the largesoaked pores in deionized to a certain water. extent. 0.01 0.1 1 10 100 1000 10000 50 Relaxation time T2/ms D1 45 D5 Figure 6. The T2 spectrum of typical rock samples soaked in acid solution. 40 D10 D15 The area of T2 spectrum after35 the first saturation is defined D20as the initial area, the ratio of T2 spectrum area to the initial area30 after each immersion is defined asD25 the relative change rate of pore D30 number. The change of the pore25 number of the sample, after immersion for different time under different solution conditions, is20 characterized by calculating the relative change rate of the pore Signal amplitude Signal number, as shown in Table 2, which15 also shows the characteristic regions of typical spectrum changes ① during the soaking process. 10 ⑤ 5 ① ⑤ ⑩ 0 0.01 0.1 1 10 100 1000 10000 Relaxation time T2/ms Figure 6. TheThe T2 spectrum of typical rock samples soaked in acid solution.

The area of T2 spectrum after the first saturation is defined as the initial area, the ratio of T2 spectrum area to the initial area after each immersion is defined as the relative change rate of pore number. The change of the pore number of the sample, after immersion for different time under different solution conditions, is characterized by calculating the relative change rate of the pore number, as shown in Table 2, which also shows the characteristic regions of typical spectrum changes during the soaking process. Sustainability 2020, 12, 7141 7 of 14

The area of T2 spectrum after the first saturation is defined as the initial area, the ratio of T2 spectrum area to the initial area after each immersion is defined as the relative change rate of pore number. The change of the pore number of the sample, after immersion for different time under different solution conditions, is characterized by calculating the relative change rate of the pore number, as shown in Table2, which also shows the characteristic regions of typical spectrum changes during the soaking process. SustainabilitySustainability 2020 2020, 12, 12, x, xFOR FOR PEER PEER REVIEW REVIEW 8 8of of 16 16

Table 2. Characteristics ofTableTable chromatogram 2. 2. Characteristics Characteristics areaof of chromatogr chromatogr of typicalamam area area rock. of of typical typical rock. rock.

RelativeRelative Change Change SoakingSoaking SolutionSolution Relative ChangeRateRate Rateof of Pore Pore of ChromatogramChromatogramChromatogram Area Area of of Rock AreaRock Samples Samples of Solution Soaking Time TimeTime Pore NumberNumberNumber Rock Samples 11 1.001.00 4141 1 1.00 1 3636 1 55 0.950.95 5 5 10 10 5 0.95 3131 1515 30 30 10 1.01 26 1010 1.01 1.01 26 deionized 2121 deionized water 15deionized 1515 0.96 0.960.96 water 1616 water amplitude Signal Signal amplitude Signal 20 1.00 11 2020 1.00 1.00 11 6 252525 0.94 0.940.94 6 1 1 3030 0.94 0.94 0.010.01 0.1 0.1 1 1 10 10 100 100 1000 1000 10000 10000 30 0.94 RelaxationRelaxation time time T2/ms T2/ms 50 1 1.00 50 11 1.00 1.00 45 45 1 40 1 40 555 0.99 0.990.99 5 10 35 5 10 35 15 30 30 15 30 101010 0.92 0.920.92 30 25 acidacid 25 acid solution 15 0.98 20 1515 0.980.98 20

solution Signalamplitude solution Signalamplitude 15 20 0.96 15 20 0.96 10 20 0.96 10 5 2525 0.97 0.97 5 25 0.97 0 0 0.01 0.1 1 10 100 1000 10000 0.01 0.1 1 10 100 1000 10000 30 0.97 Relaxation time T2/ms 3030 0.970.97 Relaxation time T2/ms

ItIt can can be be found found that that the the spectrum spectrum area area of of rock rock sample sample decreases decreases significantly significantly on on the the fifth fifth day day of of It can be found that the spectrumsoakingsoaking in in deionized deionized area of water, water, rock and and sample the the area area decreasesof of T2 T2 spec spectrumtrum significantly is is the the largest largest on on onthe the 10th the10th day fifthday of of soaking. daysoaking. of AfterAfter 20 20 days days of of soaking, soaking, the the area area of of T2 T2 spectrum spectrum almost almost rises rises to to the the initial initial state, state, and and then then the the soaking in deionized water, andspectrumspectrum the area area area decreases decreases of T2 significantly, significantly, spectrum and and is the the the relative relative largest change change on rate rate the of of pore pore 10th number number day is is ofstable stable soaking. until until 30 30 After 20 days of soaking, the areadaysdays of of soaking. T2soaking. spectrum After After soaking soaking almost in in acid acid rises solution solution to fo thefor r10 10initial days, days, the the state, pore pore number number and then decreases decreases the significantly, spectrumsignificantly, area decreases significantly, andbutbut the after after relative soaking soaking for changefor 15 15 days, days, rate it it increases ofincreases pore significantly. significantly. number isDuring During stable soaking soaking until for 30for 20–30 days20–30 days, days, of soaking. the the pore pore numbernumber remained remained relatively relatively stable. stable. After soaking in acid solution for 10 days, the pore number decreases significantly, but after soaking for 15 days, it increases significantly.3.3.3.3. Characteristics Characteristics During of of Solution Solution soaking Properties Properties forand and Minerals 20–30Minerals Content Content days, the pore number remained relatively stable. TheThe pH pH value value of of the the solution solution and and the the content content of of minerals minerals in in the the solution solution were were tested tested in in the the processprocess of of soaking. soaking. The The influence influence of of soaking soaking in in diffe differentrent solutions solutions was was analyzed analyzed by by the the fluctuation fluctuation of of solutionsolution properties properties and and minerals minerals content content in in the the soaking soaking process. process. After After immersion immersion in in deionized deionized water, water, 3.3. Characteristics of Solution Propertiesthethe contents contents of andof Mg, Mg, Minerals Al, Al, and and Si Si in Contentin the the solution solution vary vary greatly, greatly, as as shown shown in in Figure Figure 7a. 7a. After After soaking soaking for for oneone day, day, the the content content of of Mg Mg reaches reaches the the peak, peak, and and then then decreases decreases in in a a fluctuation. fluctuation. The The content content of of Si Si in in The pH value of the solutionthethe solution solution and first first the increases increases content gradually, gradually, of mineralsand and then then decreases decreases in the obviously obviously solution after after were20–25 20–25 days days tested soaking, soaking, in and theand process of soaking. The influencethenthen increase increase of soaking again. again. While While in dithe theff content erentcontent of solutionsof Al Al in in the the solution solution was has analyzedhas been been in in a a state bystate theof of fluctuation. fluctuation. fluctuation After After soakingsoaking for for 10 10 days, days, the the increase increase of of Fe Fe content content indicates indicates that that the the iron iron salts salts in in the the mineral mineral cement cement are are of solution properties and mineralslargelylargely dissolved. dissolved. content After After in soaking soaking the soakingin in acid acid solution solution process. (Fig (Figureure 7b), 7b), After the the content content immersion of of Si Si changes changes in most most deionized violently. violently. water, the contents of Mg, Al,The andThe content content Si in of theof silicon silicon solution element element varyin in the the solution greatly,solution afte afte asr rsoaking soaking shown for for inone oneFigure day day reaches reaches7a. the Afterthe peak peak soakingvalue, value, and and thenthen drops drops to to the the minimum minimum after after soaking soaking for for 10–15 10–15 days. days. It It reaches reaches a a new new peak peak value value (much (much smaller smaller for one day, the content of Mgthanthan reaches the the initial initial the peak peak peak, value) value) after and after soaking soaking then for decreases for 20 20 da daysys and and in 30 30 adays, days, fluctuation. respectively. respectively. After TheAfter soaking contentsoaking for for 20 of20 Si in the solution first increasesdays,days, gradually, the the content content of of andCa Ca increases, increases, then decreases indicating indicating that that obviously the the calcium calcium cement aftercement 20–25in in schist schist daysminerals minerals soaking, begin begin to to dissolve. and then increase again. Whiledissolve. the content of Al in the solution has been in a state of fluctuation. After soaking for 10 days, the increase of Fe content indicates that the iron salts in the mineral cement are largely dissolved. After soaking in acid solution (Figure7b), the content of Si changes most violently. The content of silicon element in the solution after soaking for one day reaches the peak value, and then drops to the minimum after soaking for 10–15 days. It reaches a new peak value (much smaller than the initial peak value) after soaking for 20 days and 30 days, respectively. After soaking for 20 days, the content of Ca increases, indicating that the calcium cement in schist minerals begin to dissolve. SustainabilitySustainability 20202020, 12, 12, x, 7141FOR PEER REVIEW 9 of8 of 16 14

50 8.0 50 8.0 Ca Si Al Mg Fe PH Ca Si Al Mg Fe PH 45 45 7.5 7.0 40 40 7.0 6.0 35 35 6.5 30 5.0 30 -1

25 4.0 PH 25 6.0 PH 20 3.0 20 5.5 15 Content / mg.kg-1 / Content 15 2.0

5.0 mg.kg / Content 10 10 5 1.0 5 4.5 0 0.0 0 4.0 1 5 10 15 20 25 30 1 5 10 15 20 25 30 Immersion time / day Immersion time / day

(a) (b)

Figure 7. The solution properties and minerals content (a) in deionized water, (b) in acid solution. Figure 7. The solution properties and minerals content (a) in deionized water, (b) in acid solution. The change of minerals content in the solution corresponds to the reaction process of schist The change of minerals content in the solution corresponds to the reaction process of schist minerals in the solution. The solution reacts with the mineral composition in the pore to form transient minerals in the solution. The solution reacts with the mineral composition in the pore to form solid or sediment, which is the main reason for the larger pore size get to be smaller. The hydrolysis of transient solid or sediment, which is the main reason for the larger pore size get to be smaller. The transient solid and the gradual dissolution of minerals, including magnesium, calcium, and silicon, hydrolysis of transient solid and the gradual dissolution of minerals, including magnesium, calcium, are the important factors for the formation of smaller new pores. It is worth noting that with the and silicon, are the important factors for the formation of smaller new pores. It is worth noting that prolongation of immersion time, the pH of acid solution increases gradually and tends to be neutral, with the prolongation of immersion time, the pH of acid solution increases gradually and tends to be while even in neutral deionized water, the pH of solution fluctuates obviously during the immersion neutral, while even in neutral deionized water, the pH of solution fluctuates obviously during the of schist samples. immersion of schist samples. 3.4. Microstructural Characteristics of Schist 3.4. Microstructural Characteristics of Schist The joints of the test rock are developed well, and quartz, mica, and other minerals are arrangedThe joints in a of certain the test direction rock are in developed the process well, of rockand quartz, metamorphism, mica, and showingother minerals obvious are anisotropic arranged incharacteristics. a certain direction The characteristics in the process of schist of parallelrock metamorphism, and vertical to jointshowing surfaces obvious were observed anisotropic after characteristics.soaking in di ffTheerent characteristics solutions, to of analyze schist parallel the eff ectand of vertical different to joint solutions surfaces on thewere morphology observed after and soakingmineral in crystal different structure solutions, of schists. to analyze The the original effect samples of different were solutions cut along on the the vertical morphology and parallel and mineraljoint surfaces, crystal structure respectively, of schists. and theirThe original microscopic samples images were werecut along shown the vertical in Figure and8. parallel The vertical joint surfaces,microscopic respectively, images and show their that microscopic the joints areimages parallel, were shown the material in Figure in the8. The layer vertical is well microscopic cemented, imagesand the show fracture that is the ladder-like. joints are Afterparallel, 2000 the times material magnification in the layer, the is well ladder-like cemented, fracture and is the mainly fracture quartz is ladder-like.intergranular After fracture, 2000 times and mica magnification, crystals are the scattered ladder-like on the fracture steps (Figure is mainly8a). quartz While theintergranular integrity of fracture,the images and parallelmica crystals to the are joint scattered direction on is the good, steps and (Figure after 2000 8a). timesWhile magnification,the integrity of lamellar the images mica parallelintergranular to the joint fracture direction can be is seengood, locally and after (Figure 20008b). times magnification, lamellar mica intergranular fractureThe can mineral be seen structure locally (Figure of the samples 8b). immersed in different solutions under single polarized light and orthogonal polarizing microscope was observed, to analyze the crystal structure characteristics of different minerals reacted with different solutions as shown in Table3. The quartz in the original sample is colorless and transparent with wavy extinction. The particle size range in 0.02 to 0.25 mm, and the distribution is directional, concentrated in strip distribution, showing fold structure. There is an angle between the directional distribution direction and the strip direction of minerals. Muscovite is flaky, colorless, and the interference color is bright grade II to III, with parallel extinction and continuous directional arrangement. Biotite is flaky too, with obvious maroon brown yellow polychromism. The interference color is grade III, with parallel extinction, and mixed with Muscovite in a strip distribution. Iron is irregular granular, black, and opaque, with particle size of 0.02–0.15 mm. After soaking in solutions of different properties, cracks can be seen clearly. After soaking in deionized water, the fracture develops mainly along the direction of joint, and there are some holes left by particle hollowing out, and occasionally filled with small particles. After soaking in the acid solution, cracks along and perpendicular to the joint surface can be seen clearly, and small holes Sustainability 2020, 12, 7141 9 of 14 can be seen locally, and some of them have been connected. No matter which solution is soaked in, theSustainability microstructure 2020, 12, x of FOR mica PEER quartz REVIEW schist will be affected, and the microcracks tend to expand along10 of the 16 edge of mica-oriented distribution, which is the main reason for slip failure mode.

×100 ×100

×500 ×20000 ×20000

(a) (b)

Figure 8. The microscopic images of original samples (a) perpendicular to joint surfaces, (b) parallel to Figure 8. The microscopic images of original samples (a) perpendicular to joint surfaces, (b) parallel joint surfaces.Sustainability 2020, 12, x FOR PEER REVIEW 11 of 16 to joint Sustainabilitysurfaces. 20202020,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 1111 ofof 1616 Table 3. Mineral structure under single-polarized and orthogonal polarized conditions. Table 3. Mineral structure under single-polarized and orthogonal polarized conditions. The mineralTable 3.State structureMineral Structures structure of the under samples under Single single-polarized immersed/Orthogonal inPolarized and different orthogonal Light solutions polarized Characteristics under conditions. single polarized light and orthogonalState polarizingStructures under microscope Single//Orthogonal was observed,Polarized Light to analyze Characteristics the crystal structure State Structures under Single/Orthogonal Polarized Light Characteristics characteristics of different minerals reacted with different solutions as shown in Table 3. The quartz Mineral grains closely in the original sampleOriginal is colorless and transparent with wavy extinction.Mineralarranged, The grains particleand closelynative size range in 0.02 to 0.25 mm,Original Stateand the distribution is directional, concentrated in striparranged,tiny distribution, cracks and can native be showing fold Mineral grains closely arranged, structure.Original There State Stateis an angle between the directional distribution directiontinytiny and crackscracksseen the cancan strip bebe direction of and nativeseen tiny cracks can be seen minerals. Muscovite is flaky, colorless, and the interference color is bright gradeseen II to III, with parallel extinction and continuous directional arrangement. Biotite is flaky too, with obvious maroon brown Crack extension is yellow polychromism. The interference color is grade III, with parallelCrack extivisiblenction, extension along orand is mixed with Muscovite in a strip distribution. Iron is irregular granular, black, and opaque,verticalvisible to along withthe jointor particle size of Acid surface,vertical tolocal the small joint 0.02–0.15 mm. solutionAfterAcid soaking in solutions of different properties, cracks Crackcan extensionbe seen is clearly. visible along After or holessurface,surface,vertical appeared, locallocal tothe smallsmall and joint surface, local soakingAcid in solution deionizedsolutionsolution water, the fracture develops mainly along the direction of joint, and there are someholessmall holes appeared, holes have appeared, andbeen and some holes have been connected some holes left by particle hollowing out, and occasionally filled with somesomesmall holesconnectedholes particles. havehave beenbeen After soaking in the acid solution, cracks along and perpendicular to the joint surface can beconnected seen clearly, and small Crack extension is holes can be seen locally, and some of them have been connected. No matterCrackvisible which extension along solution the is is soaked in, the microstructure of mica quartz schist will be affected, and the microcracksdirectionvisible ofalong tend joint the toafter expand along directionsoaking,Crack of extension joint local after is visible along the edge of mica-orientedDeioniz distribution, which is the main reason for slip failure mode. hollowedthesoaking,soaking, direction hole locallocal left of joint by after soaking, Deionized wateredDeioniz water local hollowed hole left by the hollowedthe particles, hole left by ed water particles, occasional small occasionalthethegranular particles,particles, solidsmall ﬁller can be seen granularoccasional solid small filler

granularcan be solid seen filler

can be seen 3.5. Mechanical Properties of Soaked Schist 3.5. Mechanical Properties of Soaked Schist After soaking in different solutions, some specimens with primary cracks develop well in the stress-freeAfter state.soaking Specifically, in different after solutions, immersion some in deionizedspecimens water, with primarythe extension cracks of develop primary wellcracks in andthe stress-freethestress-free initiation state.state. of newSpecifically,Specifically, cracks are afterafter obvious immersionimmersion with ininthe deionizeddeionized increase water,ofwater, immersion thethe extensionextension time, as ofof shown primaryprimary in cracks cracksFigure andand 9a. theAfterthe initiationinitiation immersion ofof newnew in the crackscracks acid solution,areare obviousobvious only withwith the thelocalthe increaseincrease block spalling ofof immersionimmersion along the time,time, primary asas shownshown fracture inin FigureFigure occurred 9a.9a. Afterin the immersion initial immersion in the acid stage. solution, After onlythe immersion the local block time spalling was prolonged, along the primarythe original fracture cracks occurred on the insurfacein thethe initialinitial are intermittently immersionimmersion stage.stage. closed AfterAfter and thetheopened, immersionimmersion as shown timetime in waswas Figure prolonged,prolonged, 9b. The macroscopicthethe originaloriginal crackscracksphenomenon onon thethe surfaceissurface almost areare consistent intermittentlyintermittently with th closedclosede microscopic andand opened,opened, porosity asas shownshown results. inin FigureFigure 9b.9b. TheThe macroscopicmacroscopic phenomenonphenomenon isis almostalmost consistentconsistent withwith ththe microscopic porosity results. Sustainability 2020, 12, 7141 10 of 14

3.5. Mechanical Properties of Soaked Schist After soaking in diﬀerent solutions, some specimens with primary cracks develop well in the stress-free state. Speciﬁcally, after immersion in deionized water, the extension of primary cracks and the initiation of new cracks are obvious with the increase of immersion time, as shown in Figure9a. After immersion in the acid solution, only the local block spalling along the primary fracture occurred in the initial immersion stage. After the immersion time was prolonged, the original cracks on the surface are intermittently closed and opened, as shown in Figure9b. The macroscopic phenomenon is Sustainability 2020, 12, x FOR PEER REVIEW 1 of 16 almostSustainability consistent 2020 with, 12, x theFOR microscopicPEER REVIEW porosity results. 1 of 16

initial 5th day 15th day 30th day initial 5th day 15th day 30th day initial 5th day 15th day 30th day initial 5th day 15th day 30th day

(a) (b) (a) (b) FigureFigure 9. The 9. The cracks cracks of of samples samples developed developed after soaking soaking (a ()a in) in deionized deionized water, water, (b) in (b )acid in acidsolution. solution. Figure 9. The cracks of samples developed after soaking (a) in deionized water, (b) in acid solution. AfterAfter immersion, immersion, uniaxial uniaxial compression compression tests tests were were performed performed on on the the intact intact specimens. specimens. The The results showresults thatAfter show the immersion, peak that the strength peak uniaxial strength of thecompression of samples the samples tests soaked soaked were in inperformed the the twotwo solutions solutionson the intactis significantly is significantlyspecimens. lower The lower thanthan thatresults that of show theof the originalthat original the peak samples. samples. strength However,However, of the samples the the strength strengthsoaked inof ofthe the thetwo sample samplesolutions soaked soaked is significantly in acid in acidsolution lower solution than that of the original samples. However, the strength of the sample soaked in acid solution decreasesdecreases more more obviously. obviously. The The soakingsoaking process process prol prolongedonged the thecompression compression process process of the sample of the samplein decreases more obviously. The soaking process prolonged the compression process of the sample in in thethe elastic elastic stage, stage, andand thethe elastic modulus modulus of of the the sample sample showed showed different different degrees degrees of influence. of influence. the elastic stage, and the elastic modulus of the sample showed different degrees of influence. AlthoughAlthough immersion immersion in deionized in deionized water water has the has greatest the greatest influence influence on the processon the process of elastic of compression, elastic compression,Although immersion the influence in deionizedon the magnitude water hasof elastic the greatest modulus influence is less than on that the in process acid solution, of elastic as the influence on the magnitude of elastic modulus is less than that in acid solution, as shown in showncompression, in Figure the 10. influence on the magnitude of elastic modulus is less than that in acid solution, as Figureshown 10. in Figure 10. 10 10 2 PH=4 1 PH=7 0 initial 9 2 PH=4 1 PH=7 0 initial 9 σ0 8 σ0 σ1 8 σ 7 1 7 6 6 E0 5 σ2 E0 5 σ2 4 E1 Stress /MPa 4 E1 Stress /MPa 3 E2 3 E2 2 2 1 1 0 00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Strain /% Strain /% Figure 10. The stress-strain curve of samples. FigureFigure 10. 10.The The stress-strain stress-strain curve of of samples. samples. Although it seems the macroscopic fracture of schist is larger and more obvious after immersion in deionizedAlthough water it seems than thethat macroscopic in acid solution, fracture the ofresults schist of is strengthlarger and test more showed obvious that after the processimmersion of elasticin deionized compression water thanof schist that samplein acid solution,was prolon theged results after of immersion strength test in showeddeionized that water, the process but the of influenceelastic compression on strength ofwas schist not greatersample than was that prolon aftegedr immersion after immersion in acid solution. in deionized It is suggested water, but that the theinfluence deionized on strengthwater is wasmore not likely greater to dissolvethan that the afte fillerr immersion between inthe acid framework solution. particlesIt is suggested of schist, that the deionized water is more likely to dissolve the filler between the framework particles of schist, Sustainability 2020, 12, 7141 11 of 14

Although it seems the macroscopic fracture of schist is larger and more obvious after immersion in deionized water than that in acid solution, the results of strength test showed that the process of elastic compression of schist sample was prolonged after immersion in deionized water, but the influence on strength was not greater than that after immersion in acid solution. It is suggested that the deionized Sustainability 2020, 12, x FOR PEER REVIEW 2 of 16 water is more likely to dissolve the filler between the framework particles of schist, resulting in more andresulting more in obvious more and macro more cracks. obvious However, macro cracks. after However, immersion after in acid immersion solution, in theacid pH solution, value the of thepH solutionvalue of itselfthe solution increased, itself and increased, the reduction and of the compressive reduction strengthof compressive was the strength largest, whichwas the indicated largest, thatwhich the indicated skeleton particlesthat the ofskeleton schist mineralsparticles wereof schist more minerals likely to were be corroded more likely after to soaking be corroded in the acidafter solution,soaking in reducing the acid the solution, strength. reducing At the the same strength time,. the At retentionthe same oftime, the the reaction retention products of the filled reaction the dissolvedproducts filled pores the again, dissolved and the pores macro again, fracture and ofthe schist macro also fracture showed of schist partial also closure. showed partial closure. OnOn thethe otherother hand,hand, thethe failurefailure characteristicscharacteristics ofof thethe specimensspecimens soakedsoaked inin didifferentfferent solutionssolutions areare didifferentfferent from from those those of of thethe originaloriginal samples.samples. UnderUnder uniaxialuniaxial compression,compression, thethe specimensspecimens withoutwithout anyany treatmenttreatment exhibitexhibit typicaltypical shearshear failurefailure mode,mode, asas shownshown inin FigureFigure 11 11a,a, andand thethe specimensspecimens immersedimmersed inin acidacid solutionsolution forfor aa longlong timetime showshow aa slidingsliding failurefailure modemode underunder uniaxialuniaxial compressioncompression asas shownshown inin FigureFigure 11 11c.c. However,However, thethe specimensspecimens immersedimmersed inin deionizeddeionized waterwater forfor aa longlong timetime containcontain bothboth shearshear andand slidingsliding failurefailure characteristics,characteristics, asas shownshown inin Figure Figure 11 11b.b.

FigureFigure 11.11. The failurefailure characteristicscharacteristics ofof thethe specimensspecimens ((aa)) untreateduntreated samples,samples, ((bb)) afterafter soakedsoaked inin deionizeddeionized water,water, ( c(c)) after after soaked soaked in in acid acid solution. solution.

4.4. DiscussionDiscussion (1)(1) ItIt hashas beenbeen provedproved thatthat forfor schistschist metamorphicmetamorphic rocks,rocks, thethe acidityacidity ofof rainwaterrainwater isis anan importantimportant factorfactor aaffectingﬀecting the the slope slope stability. stability. The eTheﬀect effect of acid of rain acid on schistrain on is notschist only is physical not only dissolution, physical butdissolution, also chemical but corrosion.also chemical Chemical corrosion. corrosion Ch causesemical the corrosion formation causes and dissolution the formation of minerals and arounddissolution quartz, of minerals mica and around other quartz, particles, mica forming and other microcracks particles, distributed forming microcracks along the graindistributed edge, whichalong the is the grain fundamental edge, which reason is the for fundamental the slip failure reason of schistfor the immersed slip failure in of acid schist solution immersed under in uniaxialacid solution stress. under For uniaxial this kind stress. of slope, For this if the kind joint of distributionslope, if the isjoint consistent distribution with is the consistent sliding with the sliding direction of the slope, it is easier to produce sliding deformation than at any other time, as this engineering example, Zhushan slope, shows. (2) The creep deformation of the slope occurs after long-term rainfall and stops after the rainy season, which indicates that the starting of the sliding process of the slope is sensitive to water with time effect. As a typical bedding slope, the sliding direction of Zhushan landslide is consistent with the direction of schist schistosity plane, which creates a decisive condition for the instability of the landslide. Rainfall infiltrates into the slope rock mass, and the water content Sustainability 2020, 12, 7141 12 of 14

direction of the slope, it is easier to produce sliding deformation than at any other time, as this engineering example, Zhushan slope, shows. (2) The creep deformation of the slope occurs after long-term rainfall and stops after the rainy season, which indicates that the starting of the sliding process of the slope is sensitive to water with time effect. As a typical bedding slope, the sliding direction of Zhushan landslide is consistent with the direction of schist schistosity plane, which creates a decisive condition for the instability of the landslide. Rainfall infiltrates into the slope rock mass, and the water content increases, which not only increases the gravity component in sliding direction, but also softens the strength of the rock on the potential slip surface. Under the water–rock interaction, the failure mode of rock on the sliding surface changes from shear failure to sliding failure (or shear and sliding failure), which further reduces the requirement of external force for inducing sliding, and it is an essential condition for the deformation of Zhushan landslide. Based on this, the protection of the slope deformation in the rainy season can be carried out from three aspects. To deal with the sliding mass in sections, transferring the risk of forward sliding. To take measures related to surface protection, reducing the surface water infiltration and fissure water infiltration. To set up a retaining structure, decreasing the probability of sliding mass cutting out from the potential shear outlet. (3) The experimental conditions designed in this study mainly consider the influence of long-term hydrostatic immersion on schist in slip zone, the pore water pressure of fracture water in schist and the dynamic water pressure of flow are ignored. However, under the condition of long-term rainfall, water enters the rock mass along the joint surface and primary micro cracks, and the granular quartz and flake mica are sandwiched between the schistose planes, which provides a good channel for the flow of water in the schistose plane. In turn, water acts as a lubricant between the particles. Once pore water pressure or hydrodynamic pressure increases, the friction and anti-force between schistosity will be reduced directly, which is not conducive to the overall stability of the slope.

5. Conclusions Analyzing the formation and occurrence mechanism of slope disasters from a micro perspective is an important link to prevention and risk management for sustainable fruition, and also an important way to achieve disaster prevention and control. The main conclusions drawn from the present study can be summarized as follow:

(1) When the schist is soaked for a long time in deionized water, the pore size and the number of pores decrease temporarily in a short time. However, the change of pore number is little different from that of the initial state undergoing continuous immersion, even it makes the pore structure larger. The short-term immersion under acidic conditions will make the pore structure larger and extending the soaking time will help repair the larger pores, but the effect of the whole process on pores number is smaller than that of the former. (2) The long-term immersion in solutions with different properties not only affects the peak strength of schists, but also changes the failure mode of schists under uniaxial loading. In particular, the peak strength of un-soaked specimens is the highest, and the failure mode of the rock mass is a typical shear failure. After soaking in acid solution for a long time, the peak strength and elastic modulus of the samples are greatly reduced, and sliding failure is the main failure mode. The long-term immersion in deionized water has little effect on the peak strength, but it greatly prolongs the test process of the elastic compression stage, which makes the failure of rock take into account the shear and slip modes. (3) Under the condition of long-period and low-intensity rainfall, the dissolution of mineral particles and cementitious materials between particles changes the pore structure of schist. In particular, the early micro-cracks formed at the edge of mica particles, and gradually expanded and Sustainability 2020, 12, 7141 13 of 14

penetrated during long-term immersion, which reduced the bearing capacity of rock mineral structure and aﬀected the mechanical properties of schist (elastic modulus, peak strength, etc.). The instability of schist slope under rainfall is the result of the accumulation of minor damage and also the reﬂection of the interaction of rock mass structure and geological environment.

Author Contributions: Data curation, Q.-C.S. and G.-D.Z.; investigation, C.W., X.-M.S., B.-H.Z. and Z.-H.X.; methodology, Q.-C.S. and Z.-H.X.; writing—original draft, Q.-C.S., Z.-H.X. and L.C.; writing—review & editing, Z.-H.X. and L.C. 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 51909136); Hubei Provincial Natural Science Foundation of China, grant number 2019CFB327 and 2019CFB332; the Open Research Fund of Key Laboratory of Geological Hazards on Three Gorges Reservoir Area (China Three Gorges University), Ministry of Education, Grant No. 2018KDZ13; the Open Research Fund of National Field Observation and Research Station of Landslides in Three Gorges Reservoir Area of Yangtze River, China Three Gorges University, grant number 2018KTL11. Acknowledgments: The authors gratefully acknowledge the support by Young Talents Development Plan of Hubei province. The authors appreciate the value added through discussion of the work with Guangliang Feng. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Schlotfeldt, P.; Elmo, D.; Panton, B. Overhanging rock slope by design: An integrated approach using rock mass strength characterisation, large-scale numerical modelling and limit equilibrium methods. J. Rock Mech. Geotech. Eng. 2018, 10, 72–90. [CrossRef] 2. Mao, C.X.; Duan, X.B.; Li, Z.Y. Numerical Computer in Seepage Flow and Program Application; Hehai University Press: Nanjing, China, 1999. 3. Li, X. Ductile Mode Cutting of Brittle Materials: Mechanism, Chip Formation and Machined Surfaces. Nano Micromach. 2010, 28, 27–43. [CrossRef] 4. Chen, R.H.; Chen, H.P.; Chen, K.S.; Zhung, H.B. Simulation of a slope failure induced by rainfall infiltration. Environ. Earth Sci. 2008, 58, 943–952. [CrossRef] 5. Camera, C.; Masetti, M.; Apuani, T. Rainfall, infiltration, and groundwater flow in a terraced slope of Valtellina (Northern Italy): Field data and modelling. Environ. Earth Sci. 2011, 65, 1191–1202. [CrossRef] 6. Ng, C.W.; Wang, B.; Tung, Y.K. Three-dimensional numerical investigations of groundwater responses in an unsaturated slope subjected to various rainfall patterns. Can. Geotech. J. 2001, 38, 1049–1062. [CrossRef] 7. Védie, E.; Lagarde, J.-L.; Font, M. Physical modelling of rainfall- and snowmelt-induced erosion of stony slope underlain by permafrost. Earth Surf. Process. Landf. 2011, 36, 395–407. [CrossRef] 8. Ray, R.K.; Syed, T.H.; Saha, D.; Sarkar, B.C. Modeling the impact of rainfall variations and management interventions on the groundwater regime of a hard-rock terrain in central India. Hydrogeol. J. 2020, 28, 1209–1227. [CrossRef] 9. Chueasamat, A.; Hori, T.; Saito, H.; Sato, T.; Kohgo, Y. Experimental tests of slope failure due to rainfalls using 1g physical slope models. Soils Found. 2018, 58, 290–305. [CrossRef] 10. Damiano, E.; Olivares, L. The role of infiltration processes in steep slope stability of pyroclastic granular soils: Laboratory and numerical investigation. Nat. Hazards 2009, 52, 329–350. [CrossRef] 11. Rahimi, A.; Rahardjo, H.; Leong, E.-C. Effect of Antecedent Rainfall Patterns on Rainfall-Induced Slope Failure. J. Geotech. Geoenviron. Eng. 2011, 137, 483–491. [CrossRef] 12. Hu, M.J.; Wang, R.; Zhang, P.C. Primary research on the effect of rainfall on landslide—Take the slope piled by old landslide in Jiangjiagou valley as example. Chin. J. Geotech. Eng. 2001, 23, 454–457. [CrossRef] 13. Li, D.Y.; Xu, Y.; Yin, K.L.; Tang, S.K.; Zhang, Y.; Lian, Z.P.; Liu, L. Simulation of high-speed movement and accumulation characteristics of rainfall-induced landslide: A case of wangjiawan landslide in ningxiang county. Geol. Sci. Technol. Inf. 2019, 38, 225–230. [CrossRef] 14. Pan, H.S.; Li, T.B.; Wu, B.Y.; Ren, Y.; Song, T. Centrifugal model tests on large-scale landslide with broken-line slip surface under rainfall. Chin. J. Geotech. Eng. 2016, 38, 696–704. [CrossRef] 15. Bogaard, T.A.; Greco, R. Landslide hydrology: From hydrology to pore pressure. Wiley Interdiscip. Rev. Water 2015, 3, 439–459. [CrossRef] Sustainability 2020, 12, 7141 14 of 14

16. Liu, Z.; He, X.; Zhou, C. Influence Mechanism of Different Flow Patterns on the Softening of Red-Bed Soft Rock. J. Mar. Sci. Eng. 2019, 7, 155. [CrossRef] 17. Xu, Z.-H.; Feng, G.-L.; Sun, Q.-C.; Zhang, G.-D.; He, Y.-M. A Modified Model for Predicting the Strength of Drying-Wetting Cycled Sandstone Based on the P-Wave Velocity. Sustainability 2020, 12, 5655. [CrossRef] 18. Xu, Z.H.; Zhang, G.D.; Sun, Q.C.; Wu, H.; Tan, T.X. Experimental Research on Strength Degradation of Red Sandstone under Dry-wet Cycle Conditions. Chin. J. Highw. Transp. 2018, 31, 226–233. 19. Mu, W.; Wu, X.; Qian, C.; Wang, K. Triggering mechanism and reactivation probability of loess-mudstone landslides induced by rainfall infiltration: A case study in Qinghai Province, Northwestern China. Environ. Earth Sci. 2019, 79, 22. [CrossRef] 20. Huang, R.; Chen, J.B.; Wang, H.Q. Influence of mineral content and distribution on shale brittleness based on meso treatment: A case study from the chang 7 member of the Yanchang formation, Ordos Basin. Sci. Technol. Eng. 2019, 19, 193–199. 21. Feng, G.-L.; Feng, X.-T.; Chen, B.-R.; Xiao, Y.-X.; Liu, G.-F.; Zhang, W.; Hu, L. Characteristics of Microseismicity during Breakthrough in Deep Tunnels: Case Study of Jinping-II Hydropower Station in China. Int. J. Géoméch. 2020, 20, 04019163. [CrossRef] 22. Feng, G.-L.; Feng, X.; Chen, B.-R.; Xiao, Y.-X.; Zhao, Z.-N. Effects of structural planes on the microseismicity associated with rockburst development processes in deep tunnels of the Jinping-II Hydropower Station, China. Tunn. Undergr. Space Technol. 2019, 84, 273–280. [CrossRef] 23. Shiyan Municipal Bureau of Land and Resources. Basic Information of Hidden Danger Points of Geological Disasters in Shiyan. 2016. 24. Barbosa, L.L.; Kock, F.V.; Almeida, V.M.; Menezes, S.M.; Castro, E.V. Low-field nuclear magnetic resonance for petroleum distillate characterization. Fuel Process. Technol. 2015, 138, 202–209. [CrossRef] 25. Yoder, J.L.; Malone, M.W.; Espy, M.A.; Sevanto, S. Low-field nuclear magnetic resonance for the in vivo study of water content in trees. Rev. Sci. Instrum. 2014, 85, 095110. [CrossRef][PubMed]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).