Study on the Control of Underground Rivers by Reverse Faults in Tunnel Site and Selection of Tunnel Elevation

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Article Study on the Control of Underground Rivers by Reverse Faults in Tunnel Site and Selection of Tunnel Elevation

Peixing Zhang 1,2,*, Zhen Huang 2, Shuai Liu 3 and Tiesheng Xu 4

1 College of Management Science and Engineering, Hebei University of Economics and Business, Shijiazhuang 050061, China 2 School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China; [email protected] 3 Hebei GEO University, Shijiazhuang 050031, China; [email protected] 4 China Railway Shanghai Design Institute Group Co., Ltd. Tianjin Branch, Tianjin 300073, China; [email protected] * Correspondence: [email protected]; Tel.: +86-18362951986

Received: 5 February 2019; Accepted: 21 April 2019; Published: 28 April 2019

Abstract: Along with the need for western economic development, the number of long tunnel projects which go through mountains is constantly on the rise. In the process of construction, various disaster-causing structures are frequently exposed, which leads to many geological disasters. The traditional idea is that the reverse fault is not easily developed for an underground river, which means that the tunnel elevation design is not considered adequately. When some tunnels cross the bottom of the river, the fractures near the fault between the underground river and the excavation space may be activated and then evolve into channels, causing serious water inrush accidents during construction and operation processes. Taking the Qiyueshan Tunnel site as an example, on the premise of the anatomy of the control mechanism of the reverse fault on the development of the underground river, based on the multiperiod typical structural traces of the tunnel and surface outcrop, it was found that stratifications, dip joints, transverse joints, and tension joints of good aperture grade are important control factors. The cut block easily loses its stability and provides space for karst development, while intermittent uplifting of regional structures provides hydrodynamic conditions for the development of the underground river, causing the hydraulic gradient to be inconsistent in the overall underground river. Finally, the rainwater dynamic monitoring and tracer connectivity are data that can be fully utilized to demonstrate that a reverse fracture can control the development of the underground river. The authors further considered the effect of the vertical zoning of the fault structure and the excavation disturbance, and, drawing on the experience of the relative location of the same site in the same field, put forward the suggestion that the construction of the follow-up tunnel in the study area should be slightly higher than the elevation of the underground river. The research results can provide useful reference for similar engineering problems in the future.

Keywords: long tunnel; reverse fault; underground river; control mechanism; structural traces; elevation

1. Introduction With the development of major infrastructure projects, such as those related to nuclear power, water conservancy, and transportation, the focus of development is gradually shifting to mountainous areas and karst areas with complex geological conditions. China has become one of the countries with the largest scale and diﬃculty of tunnel construction in the world [1,2]. In recent years, due to

Water 2019, 11, 889; doi:10.3390/w11050889 www.mdpi.com/journal/water Water 2019, 11, 889 2 of 18 the occurrence of many associated disasters in tunnel construction, relevant scholars have focused on the risk assessment of water and mud bursting disasters in karst tunnels, advanced geological prediction technology in tunnel construction, the management of water and mud bursting disasters in tunnels, and so forth [2–8]; however, the research on the main control mechanism and prevention standards for targeted disaster-causing structures is relatively rare, and the prediction and prevention of groundwater and mud inrush remain a most challenging issue [9,10]. At present, the disaster structures in southwest karst mountains can be basically classified into two categories: faults and karst [11]. Among them, the fault, as a common type of disaster-causing structure, can be further divided into subclasses, such as mud faults and water-rich faults, according to lithological conditions, fault properties, filling characteristics, and presence of water-rich conditions, and thus cause different degrees of water and mud inrush when tunnels are exposed. As a plastic (or brittle) zone with low strength, deformation, and permeability and poor water resistance, the fault and its fracture zone are characterized by the fracture of nearby rock mass, strong water conductivity, and easy karst development, which is mainly believed to be controlled by the normal fault [12,13]. When the tunnel passes through the reverse fault, the traditional concept holds that karst is often not developed. The designed tunnel will not cause water or mud inrush when crossing at a certain distance at the bottom of the karst. However, the case of the Qiyueshan Tunnel of the Yichang–Wanzhou Railway indicates that, even though there was previously considered to be a certain “safety zone” at a certain distance below the karst development zone and it was subsequently reinforced, several sudden water inrush and mud burst events can still occur. The authors believe that the main cause of disasters has an unclear mechanism. We selected the rock mass near the reverse fault in the typical strong karst area as the research object, on the premise of the anatomy of the development mechanism of the reverse fault on the development of the underground river, based on the typical structure of tracks in the tunnel and surface outcrop. The dynamic monitoring data of the rainwater and the tracer connectivity data were used to verify that the reverse fault can control the development of the underground river. Further, the effect of the vertical zoning of the fault structure and the disturbance of the excavation were considered, and we drew on other tunnel design experiences in the same area. Finally, we put forward the proposal for the elevation selection of the tunnel relative to the underground river. We expect the results to provide reference for similar projects and to provide reference samples for the development of scientific, effective, and pertinent theory and technologies for tunnel disaster control.

2. Geological Background The Qiyueshan Tunnel is located in the transition zone between the western Hunan–Hubei and eastern Sichuan tectonic belts (Figure1a). The tunnel crosses the Qiyueshan anticline, and the anticline has two asymmetrical wings. The occurrence of southeastern wing is at about 115–70◦ and the occurrence of the northwest wing is at about 320–60◦. Qiyueshan area is a huge obstacle to the control of the Enshi–Lichuan section of the Hurong Expressway and other highway and railway projects in the southwest (Hubei and Chongqing sections). It extends for 226 km and its surface elevation ranges 1200–1700 m. The geological structure is relatively complex (karst microgeomorphs such as karst gullies, karst grooves, funnels, and water holes are developed in the tunnel site). For example, during the construction of the Qiyueshan Tunnel of the Yichang–Wanzhou Railway, a branch of the underground river was exposed to water inrush accidents, resulting in many deaths and property losses. Its surface water development is not high, but the groundwater system is mainly karst water, which is mostly stored in karst pipes. Groundwater is mainly supplied by atmospheric rainfall, and, due to high and steep mountain potential, it is rapidly transferred to karst depressions and water holes to enter karst aquifers through the surface, and is eventually excreted in the form of karst springs or water discharge points through dissolution cracks and pipeline runoﬀ (see Figure1b). According to incomplete statistics, many karst water bursting accidents have occurred when crossing the mountains during construction of the Yichang–Wanzhou Railway and the Shanghai–Chongqing Expressway, Water 2019, 11, 889 3 of 18

Water 2019, 11, x FOR PEER REVIEW 3 of 18 which is a typical high-risk tunnel site [14]. The underground river studied in this paper is roughly parallel to the mountain range along the reverse fault; the strike is about 54 . It is located in the in this paper is roughly parallel to the mountain range along the reverse fault; the◦ strike is about 54°. karst aquifer group from the Qiyueshan anticline west wing; that is, Daye Formation (T1d)–Jilingjiang It is located in the karst aquifer group from the Qiyueshan anticline west wing; that is, Daye Formation (T1j), Triassic. Formation (T1d)–Jilingjiang Formation (T1j), Triassic.

(a) Relative position of tunnel sites

(b) The side of the northwest wing of a typical steep anticline (photographed at the discharge point of the Xiangshuidong)

FigureFigure 1. 1. RelativeRelative distribution distribution map map of of the the tunnel tunnel area. area.

3.3. Reverse Reverse Fault Fault Control Control Mechanism Mechanism of of Underground Underground River River Development Development TraditionalTraditional studies studies have have suggested suggested that that karst karst pipe pipelineslines and and even even dark underground rivers developed rivers developed by faults areby faultsmainly are controlled mainly controlled by tensile byfaults. tensile This faults. view Thishas long view been has longrecognized been recognized by scholars by in scholars the karst in worldthe karst [12,13]. world In the [12 ,study13]. In of the the studycontrol of effect the control of a fault eff structureect of a fault on karst structure development, on karst it development, is generally consideredit is generally that considered the fracture that in the the fault fracture zone in of thethe faultreverse zone fault of theis closed, reverse which fault is is not closed, conducive which to is thenot flow conducive and occurrence to the flow of andgroundwate occurrencer; thus, of groundwater; the karst in the thus, reverse the karstfault inzone the is reverse hardly faultdeveloped, zone is sohardly does developed,not attract enough so does notattention. attract However, enough attention. with the However,continuous with maturation the continuous of understanding maturation andof understanding experience, the and rock experience, mass near the rockhanging mass wall near of thethe hangingreverse fault wall can of the form reverse large-scale faultcan unicorn form caveslarge-scale and even unicorn underground caves and pipelines even underground under certain pipelines conditions under and certain scales conditions[15]. To this and end, scales we first [15]. giveTo this an end,new weanalysis first givefrom an the new stress analysis state of from the the reverse stress fault state formation of the reverse process fault (Figure formation 2), which process is called(Figure the2), new which four is calledbasic stages. the new According four basic to stages. Reference According [15], from to Reference the perspective [ 15], from of rock the mechanics, perspective theof rockformation mechanics, of the thereverse formation fault can of thebe divided reverse faultinto three can be basic divided stages. into In threethe first basic stage, stages. under In the the actionfirst stage, of principal under the compressive action of principal stress, compressivethe rock mass stress, first the undergoes rock mass elastic first undergoesdeformation elastic to elastoplasticdeformation deformation, to elastoplastic forming deformation, joint fissures, forming which joint is the fissures, initial shape which of is the the reverse initial fault shape (Figure of the 2a). In the second stage, the reverse fault develops and forms drag folds on the fault zone (Figure 2b), and tensile stress states occur at various locations, forming local fracture zones and interlaminar

Water 2019, 11, 889 4 of 18 reverse fault (Figure2a). In the second stage, the reverse fault develops and forms drag folds on the fault zone (Figure2b), and tensile stress states occur at various locations, forming local fracture zonesWater and 2019 interlaminar, 11, x FOR PEER fracture REVIEW zones. In the third stage, the main stress is gradually released4 of 18 and dissipated. At this time, the structure is reversed, and the reverse fault is in the state of extension fracture zones. In the third stage, the main stress is gradually released and dissipated. At this time, (Figure2c), resulting in large-scale development of the longitudinal and vertical ﬁssures at the upper the structure is reversed, and the reverse fault is in the state of extension (Figure 2c), resulting in plate, and the openness of the layer increases. In addition, if the direction of the principal stress is large-scale development of the longitudinal and vertical fissures at the upper plate, and the openness rotated,of the the layer development increases. In of addition, the dip if joints the directio and transversen of the principal joints stress will occur, is rotated, and the the development layering and the jointof formation the dip joints formed and intransverse the previous joints threewill occur, stages and will the easily layering form and a smallthe joint block formation of the cutformed subsidence in (Figurethe2 previousd), which three will stages further will aggravate easily form the a small development block of the of cut karst. subsidence Regarding (Figure the 2d), vertical which zoning will of the fault-attachedfurther aggravate rock the mass development from the of surface karst. Regardin layer tog thethe deepvertical part, zoning as proposed of the fault-attached by Wang etrock al. [16], accordingmass from to the the degree surface oflayer weathering to the deep and part, tectonic as proposed action, by Wang it is easy et al. to [16], divide according it into to three the degree structural levelsof (granularweathering structure, and tectonic block action, fracture it is easy structure, to divide and it into block three structure; structural seelevels Figure (granular2d). structure, block fracture structure, and block structure; see Figure 2d).

(a) Small deformation: joint fissure “preheating” stage

(b) Large deformation: fault squeezing dislocation and initial formation of tension joints

(d) Stress rotation: development of dip joints, and transverse joints

FigureFigure 2. Analysis2. Analysis of of karst karst developmentdevelopment mechanism mechanism in inreverse reverse fault fault control. control.

Below,Below, we we explain explain the the e ffeffectect ofof thethe abovementionedabovementioned fissures fissures on onthe thefield field survey survey and the and the structuralstructural traces. traces. First,First, natural natural stratificationsstratifications themselves themselves provide provide good storage good and storage transportation and transportation space. Dong et space. Dongal. et [17] al. found [17] found through through a field geological a field geological survey of karst survey development of karst zones development that, among zones 83 karst that, caves, among 83 karst60% caves,had undergone 60% had stratigraphic undergone stratigraphicdevelopment. development.It can be seen that, It can with be seenthe increase that, with of depth the increase (the of depthrock (the mass rock gradually mass gradually gets better gets and better there are and relati therevely are few relatively fractures), few the fractures), stratification the gradually stratification

Water 2019, 11, 889 5 of 18 gradually formed the dominant water control structure surface. Bai et al. [18] pointed out that the structuralWater 2019, 11 characteristics, x FOR PEER REVIEW of rock formations would make up for the insufficiency of soluble5 of 18 rock dissolutionformed to the a certaindominant extent, water and control even structure become thesurface. main Bai factor et al. aff [18]ecting pointed karst out development. that the structural According to thecharacteristics data of karst of caves rock informations China, the would contribution make up fo rater the of insufficiency bedding plane of soluble landslides rock dissolution and tectonic to fault structuresa certain to extent, karst bodiesand even is become much higherthe main than factor that affecting of the karst original development. rock stratifications, According to whichthe data is the mainof water-controlling karst caves in China, preferred the contribution plane [rate19]. of According bedding plane to previouslandslides research,and tectonic there fault have structures been five periodsto karst of structural bodies is superpositionmuch higher than in thethat Qiyueshan of the original area rock [20 stratifications,]. Combined which with field is the investigation main water- and analysis,controlling we believe preferred that plane the stress [19]. According action represented to previous by research, NW–SE there compression–release have been five periods is the of main controlledstructural stress superposition field (i.e., thein the controlled Qiyueshan stress area [20]. field Combined of the development with field investigation of a thrust and fault), analysis, by which manywe “knee-joint” believe that the structures stress action were represented developed by in NW–SE the lower compression–release Triassic Jialingjiang is the main Formation controlled (with a stress field (i.e., the controlled stress field of the development of a thrust fault), by which many “knee- strong dissolution horizon) of the Qiyueshan anticline west wing (Figure3). According to field surveys, joint” structures were developed in the lower Triassic Jialingjiang Formation (with a strong thesedissolution secondary horizon) structures of arethe mostlyQiyueshan developed anticline in we weakst wing strata, (Figure such 3). as Acco medium-thin-layerrding to field surveys, limestone and mudstone,these secondary which structures are interlayer are mostly parasitic developed fold in belts,weak strata, indicating such as the medium-thin-layer bedding shearing limestone during the developmentand mudstone, of longitudinal which are interlayer bending parasitic folds, resulting fold belts, in indicating layer slippage. the bedding Subsequent shearingintroduction during the of NW–SEdevelopment compression–release of longitudinal causes bending the rock folds, layer resulting to be furtherin layer disengagedslippage. Subsequent (corresponding introduction to Stage of 2b), whichNW–SE provides compression–release better conditions causes for the the formationrock layer to of be good further interlayer disengaged dissolution (corresponding (corresponding to Stage to Stage2b), 2c). which According provides to onsitebetter conditions compass measurements,for the formation the of good dip angle interlayer of the dissolution rock layer (corresponding is mostly ~20–60 ◦ (Figureto 3Stage), which 2c). According is favorable to foronsite karst compass development, measurem andents, the the layered dip angle rock of the mass rock is easilylayer is broken mostly and detached~20–60° due (Figure to stress 3), concentrationwhich is favora atble some for karst bends. development, and the layered rock mass is easily broken and detached due to stress concentration at some bends.

FigureFigure 3. The 3. The large large dip dip angle angle “knee-joint” “knee-joint” structurestructure of of the the medium-thin medium-thin layer layer of limestone of limestone in the in the JialingjiangJialingjiang Formation Formation exposed exposed by by the the large-scale large-scale trenchtrench in the the northwest northwest wing wing of Qiyueshan of Qiyueshan area. area.

Secondly,Secondly, it is it generally is generally believed believed that that the the developmentdevelopment of of fractures fractures in inthe the two two sides sides of a ofsimple a simple reversereverse fault fault is relatively is relatively small, small, especially especially in in the the fault fault footwall, footwall, wherewhere the the development development of offractures fractures is the lowestis the lowest and water and water abundance abundance is poor is poor [21 ].[21]. However, However, as as shown shown in in FigureFigure2 2d,d, when a a certain certain scale scale of thrustof faults thrust (suchfaults (such as faults as faults with with a length a length of of more more than than tenten kilometers or or even even dozens dozens of kilometers) of kilometers) are underare under the actionthe action of rotatingof rotating compressive compressive stress stress (mainly NE–SW NE–SW compression, compression, and and even even E–W E–W and and N–S compressionN–S compression superimposition superimposition [19 [19]),]), the the rockrock massmass structure structure is isunstable. unstable. We We randomly randomly selected selected fractures to measure their aperture. According to the authors of [22], the cracks at the outcrop showed fractures to measure their aperture. According to the authors of [22], the cracks at the outcrop showed good opening degrees, most of which were medium-wide to very wide (see Table 1). In addition, good opening degrees, most of which were medium-wide to very wide (see Table1). In addition, most of them had no ﬁlling—or occasionally, a soluble salt ﬁlling—in the tunnel, and thus were easy to break WaterWater 2019 2019, 11, 11, x, xFOR FOR PEER PEER REVIEW REVIEW 66 ofof 1818

mostmost of of them them had had no no filling—or filling—or occasionally, occasionally, aa solublesoluble salt filling—in the tunnel,tunnel, andand thusthus were were easy easy Water 2019, 11, 889 6 of 18 toto break break and and had had lowlow strengthstrength andand poorpoor waterwater resiresistance (Figure 4). AsAs thethe depthdepth increased,increased, thethe apertureaperture grade grade of of the the fracture fracture decreased. decreased. and had low strength and poor water resistance (Figure4). As the depth increased, the aperture grade of the fracture decreased. TableTable 1.1. ApertureAperture grade of joints.

DescriptionDescription Aperture,Aperture,Table 1. ee//mmAperturemm gradeClassification of joints. Joint Joint Type Type

Description Aperture, e/mm ClassiﬁcationDipDip Joint joint,joint, Type transverse transverse Open–Medium-wideOpen–Medium-wide 0.5–100.5–10 Dehiscence joint, tension joint Open–Medium-wide 0.5–10 Dehiscence Dip joint, transversejoint, tension joint, tension joint joint Medium-wide–VeryMedium-wide–VeryMedium-wide–Very wide 2.5–100 Dehiscence–opening Stratiﬁcation 2.5–1002.5–100 Dehiscence–opening StratificationStratification widewide

Figure 4. Sample of rock taken from the fractures in the tunnel. The soluble salt filling can be easily seen. FigureFigure 4. 4. Sample Sample of of rock rock taken taken fromfrom thethe fracturesfractures in the tunnel. The solublesoluble saltsalt fillingfilling cancan bebe easily easily seen.seen. During the outcrop survey, it was also found that the horizontal karst was not easily formed in a certain range of the surface and was easily filled with mud (Figure5a), mainly developing DuringDuring the the outcrop outcrop survey, survey, itit waswas alsoalso foundfound that the horizontal karstkarst waswas notnot easilyeasily formed formed in in vertical corrosion holes (Figure5b). In the field, we used a geological hammer to hit the tunnel, a acertain certain range range of of the the surface surface and and waswas easilyeasily filledfilled with mud (Figure 5a), mainlymainly developingdeveloping vertical vertical which would easily lead to a large amount of falling pieces and randomly mixed small dissolved corrosioncorrosion holes holes (Figure (Figure 5b). 5b). In In the the field,field, wewe usedused a geological hammer to hithit thethe tunnel,tunnel, which which would would cavities (Figure5c). As found through observation in the tunnel crossway, even in the presence of easilyeasily lead lead to to a a large large amount amount ofof fallingfalling piecespieces andand randomly mixed small dissolveddissolved cavitiescavities (Figure (Figure concrete lining, this cross-cutting mode is prone to falling blocks, poor self-stability, and severe water 5c).5c). As As found found through through observation observation inin thethe tunneltunnel crossway, even in the presencepresence ofof concreteconcrete lining,lining, seepagethis cross-cutting in some crack mode surfaces, is prone among to falling which blocks, tensile poor cracks self-stability, and bedding and faces severe have water obvious seepage strong in this cross-cutting mode is prone to falling blocks, poor self-stability, and severe water seepage in corrosionsome crack (Figure surfaces,5d). among The extensive which tensile development cracks and of bedding this bedding faces karsthave structureobvious strong provides corrosion good some crack surfaces, among which tensile cracks and bedding faces have obvious strong corrosion space(Figure conditions 5d). The for extensive groundwater development storage andof this migration. bedding From karst structural structure characteristics, provides good it canspace be (Figure 5d). The extensive development of this bedding karst structure provides good space summarizedconditions as:for thegroundwater rock mass nearstorage the surfaceand migration. of the fault From zone hasstructural a granular characteristics, structure, the it stability can be is conditions for groundwater storage and migration. From structural characteristics, it can be poor,summarized and it is easilyas: the filledrock mass (Figure near5a). the However, surface of in the the fault block zone cracking has a structuregranular ofstructure, the tunnel, the stability the rock summarized as: the rock mass near the surface of the fault zone has a granular structure, the stability massis poor, can and be self-stabilized it is easily filled and (Figure the conditions 5a). However, for hole in formationthe block cracking are good structure (Figure5 c,d):of the the tunnel, bedding the is poor, and it is easily filled (Figure 5a). However, in the block cracking structure of the tunnel, the expandsrock mass laterally can be to self-stabilized form a larger and cavity, the where conditions water for surges hole throughformation the are tunnel good quickly (Figure after 5c,d): heavy the rock mass can be self-stabilized and the conditions for hole formation are good (Figure 5c,d): the rainbedding (Figure expands5e,f). The laterally stability to form of the a blocklarger structure cavity, where is the water best, and surges generally through not the conducive tunnel quickly to the bedding expands laterally to form a larger cavity, where water surges through the tunnel quickly formationafter heavy of arain good (Figure channel. 5e,f). The stability of the block structure is the best, and generally not after heavy rain (Figure 5e,f). The stability of the block structure is the best, and generally not conducive to the formation of a good channel. conducive to the formation of a good channel.

(a) The vertical hole group formed by the intersection of the Jialingjiang Formation and the vertical and (ahorizontal) The vertical joints hole was filled.group formed by the intersection of the Jialingjiang Formation and the vertical and horizontal joints was filled. Figure 5. Cont.

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(b) Stress rotation: the original rock layer is subjected to tendency joints, cross-section cutting into blocks, and then dissolution into vertical holes.

(d) Stratification strong dissolution to form a good water-guiding channel near Xiangshuidong.

(e) Water can pour through the cavity exposed in the tunnel cross section after a short burst of rain on the surface.

Figure 5. Cont.

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(f) Water gushing from the tunnel-floor cavity.

FigureFigure 5. 5. TypicalTypical outburstoutburst developmentdevelopment in a typical strong karst karst area area in in the the tunnel tunnel site site and and site site survey survey inin the the corresponding corresponding sectionsection ofof thethe tunnel.tunnel. (f) Water gushing from the tunnel-floor cavity.

OneOne ofof thetheFigure otherother 5. Typical importantimportant outburst reasonsreasons development for in the a typical format formation strongion karst of of areasmooth smooth in the tunnelpassageways passageways site and site abovesurvey above is is the the result result ofof multiplemultiple periodsin the corresponding of of tectonic tectonic section uplift uplift of inthe in the tunnel. the region; region; this this research research area area belongs belongs to the to southwestern the southwestern part part of Hubei Province and has been affected by uplift since the Neoid period [23]. After each uplift, of Hubei ProvinceOne of theand other has important been affected reasons forby theuplift format sinceion of the smooth Neoid passageways period [23]. above After is the resulteach uplift, a arelatively relativelyof stable multiple stable period periodperiods completes of completes tectonic uplift a akarstification karstificationin the region; this process, process,research areaforming forming belongs a to amultilevel the multilevel southwestern karst karst part peneplain, peneplain, terraces,terraces, and andof Hubei correspondingcorresponding Province and multilevelmultilevelhas been affected karstkarst by caves, uplift which since the gradually gradually Neoid period form form [23]. a a Aftercomplete complete each uplift, hydrodynamic hydrodynamic a rechargerecharge channel.relativelychannel. stable The tunnelperiod completes site is mostly a karstification within process,the the level level forming 2 2 (elevation (elevation a multilevel 1200–1300 1200–1300 karst peneplain, m) m) and and level level 3 3 terraces, and corresponding multilevel karst caves, which gradually form a complete hydrodynamic (elevation(elevation 900–1100 900–1100 m)m) karstkarst peneplain,peneplain, which means it it experiences experiences strong strong hydrodynamic hydrodynamic conditions conditions recharge channel. The tunnel site is mostly within the level 2 (elevation 1200–1300 m) and level 3 fromfrom the the upper-level upper-level(elevation 900–1100 catchmentcatchment m) karst area;area; peneplain, thisthis processwhich means ma mainly itinly experiences accelerates accelerates strong the the hydrodynamic expansion expansion conditions of of runoff runo ff toto karst karst (Figure(Figure6 ).6).from the upper-level catchment area; this process mainly accelerates the expansion of runoff to karst (Figure 6).

(a) Enhanced hydraulic potential after uplift and vertical karst development.

(b) After stabilization, horizontal karst development occurs.

Figure 6. The underground river formation pattern controlled by the reverse fault. Image recreated from [24]. (b) After stabilization, horizontal karst development occurs.

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Water 2019Figure, 11, 8896. The underground river formation pattern controlled by the reverse fault. Image recreated 9 of 18 from [24].

InIn conclusion, conclusion, the the cutting cutting jointsjoints formedformed under the the influence influence of of multiple multiple periods periods of of tectonic tectonic stress, stress, especiallyespecially the the stratifications stratifications with with large large aperture aperture degrees degrees and and tensile tensile joints, joints, provide provide a gooda good channel channel for continuousfor continuous erosion erosion of the of water the water body, body, thusmeeting thus meet theing condition the condition of controlling of controlling the reverse the reverse fault to fault form ato large form underground a large underground river. At theriver. same At time,the same the Earth’stime, the crust Earth’s is uplifted crust is and uplifted rainwater and israinwater collected is at steepcollected slopes. at steep In the slopes. early stage,In the infiltratingearly stage, groundwaterinfiltrating groundwater and surface and water surface have water strong have downward strong anddownward horizontal and potential horizontal energy, potential the long-termenergy, the strong long-term hydrodynamic strong hydrodynamic action continually action continually erodes the rock,erodes and the the rock, vertical and dissolution the vertical is dissolution strengthened, is st especiallyrengthened, when especially the rock when body the in rock the hangwall body in the area nearhangwall the reverse area near fault the is carried reverse away fault by is dissolution.carried away It isby worth dissolution. mentioning It is worth that, combinedmentioning with that, the hydrologicalcombined with survey the data, hydrological the hydraulic survey gradient data, of th thee upstreamhydraulic segmentgradient (Tianyinqiao–Taojiaxinfangzi) of the upstream segment is(Tianyinqiao–Taojiaxinfangzi) extremely flat, at less than 5% is ,extremely while the flat, average at less hydraulic than 5‰, gradientwhile the ofaverage the downstream hydraulic gradient segment (Taojiaxinfangzi–Xiangshuidong)of the downstream segment (Taojiaxinfangzi–Xiangshuidong) is very steep, at more than 5%. is very The steep, uplift at process more than may 5%. cause The the undergrounduplift process river may to cause appear the incongruous underground in theriver section; to appear that is,incongruous the hydraulic in the gradient section; in thethat section is, the is veryhydraulic inconsistent. gradient In in addition, the section there is arevery relative inconsistent barriers. In (such addition, as shale, there breccia are relative limestone, barriers etc.) (such on both as sidesshale, of breccia the strong limestone, karst beds, etc.) whichon both provide sides of boundaries the strong karst that concentratebeds, which theprovide confluence boundaries of water that in theconcentrate development the confluence of the scale of and water degree in the of develop the undergroundment of the river, scale causing and degree the cuttingof the underground of a U-shaped grooveriver, causing (Figure 7the). Finally, cutting anof a unimpeded U-shaped groove underground (Figure river7). Finally, is formed an unimpeded (Figure8). underground river is formed (Figure 8).

(a) Hydrogeological distribution.

(b) The horizontal section of the Qiyueshan Tunnel of the Lichuan–Wanzhou Expressway (see Figure 7a, 1-1 section).

FigureFigure 7.7. TheThe generalized model of of the the U-shaped U-shaped pipeline. pipeline.

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Figure 8. Actual development of the horizontal undergro undergroundund river channel in the Wangjiaba section.section. Figure 8. Actual development of the horizontal underground river channel in the Wangjiaba section. 4. Hydrogeological Method VerificationVerification 4. Hydrogeological Method Verification From the perspective of hydrogeology, wewe foundfound thatthat thethe dynamic characteristics ofof the flowflow are From the perspective of hydrogeology, we found that the dynamic characteristics of the flow are closely related to rainfall, and the dynamic changes are large, which reflects reflects the excellent connectivity closely related to rainfall, and the dynamic changes are large, which reflects the excellent connectivity of thethe undergroundunderground river, river, the the relatively relatively perfect perfect development development of theof pipelinethe pipeline system, system, and theand large the large scale of the underground river, the relatively perfect development of the pipeline system, and the large (Figurescale (Figure9). This 9). may This prove may prove that the that development the development degree degree and supply, and supply, or discharge or discharge fluidity fluidity of the of river the scale (Figure 9). This may prove that the development degree and supply, or discharge fluidity of the controlledriver controlled by the by reverse the reverse fault, fault, can alsocan bealso very be very mature. mature. Large Large thrust thrust faults faults that that extend extend for for tens tens of river controlled by the reverse fault, can also be very mature. Large thrust faults that extend for tens kilometersof kilometers can can also also form form unimpeded unimpeded underground dark rivers, rivers, breaking breaking the the old old belief belief that that large-scale large-scale river of kilometers can also form unimpeded dark rivers, breaking the old belief that large-scale river systems cannotcannot bebe formedformed inin thethe karstkarst aroundaround reversereverse faults.faults. systems cannot be formed in the karst around reverse faults.

Figure 9. Fluctuation graph of flow rate at the inlet and outlet of the Deshengchang subterranean FigureFigure 9. Fluctuation 9. Fluctuation graph graph of ﬂow of flow rate rate at the at inletthe inlet and and outlet outlet of the of Deshengchangthe Deshengchang subterranean subterranean stream. Image reproduced from [25]. stream.stream. Image Image reproduced reproduced from from [25]. [25]. Finally, considering that the tracer test has become a powerful method for exploring more Finally,Finally, considering considering that thethat tracer the testtracer has test become has become a powerful a powerful method for method exploring for moreexploring mature more mature karst hydrogeology [26–28], in July 2015, we introduced a high-precision tracer test method karstmature hydrogeology karst hydrogeology [26–28], in July [26–28], 2015, wein July introduced 2015, we a high-precisionintroduced a high-precision tracer test method tracer after test heavy method after heavy rain between Wangjiaba and Xiangshuidong, and found that the karst pipeline was still rainafter between heavy Wangjiaba rain between and Wangjiaba Xiangshuidong, and Xiangshuidong, and found that and the found karst pipelinethat the karst was stillpipeline relatively was still relatively smooth, especially after the rainstorm. Although the underground river system was later smooth,relatively especially smooth, after especially the rainstorm. after the Although rainstorm. the Although underground the underground river system wasriver later system captured was later captured by the Yichang–Wanzhou Railway’s drainage hole, it became two subsystems; tracer data by thecaptured Yichang–Wanzhou by the Yichang–Wanzhou Railway’s drainage Railway’s hole, drainage it became hole, two it subsystems;became two tracersubsystems; data between tracer data between Wangjiaba and Xiangshuidong show that the underground river is still guiding water Wangjiababetween and Wangjiaba Xiangshuidong and Xiangshuidong show that the show underground that the riverunderground is still guiding river is water still (Figureguiding 10 water). (Figure 10). We further pursued the tracer data of the Yichang–Wanzhou Railway construction period We further(Figure pursued 10). We further the tracer pursued data of the the tracer Yichang–Wanzhou data of the Yichang–Wanzhou Railway construction Railway period construction [29], ﬁnding period [29], finding that the flow velocity in the direction of the dark river is 2.4 × 10−1 m/s, while the vertical [29], finding that the flow velocity in the direction of the dark river is 2.4 × 10−1 m/s, while the vertical development of the cave flow rate is 1.2 × 10−1 m/s. The cross-fault (footwall) perpendicular to the development of the cave flow rate is 1.2 × 10−1 m/s. The cross-fault (footwall) perpendicular to the

Water 2019, 11, 889 11 of 18 that the flow velocity in the direction of the underground river is 2.4 10 1 m/s, while the vertical Water 2019, 11, x FOR PEER REVIEW × − 11 of 18 development of the cave flow rate is 1.2 10 1 m/s. The cross-fault (footwall) perpendicular to the × − faultfault direction direction is is relativelyrelatively weak,weak, and its flow flow rate rate is is reduced reduced to to 4.1–6.9 4.1–6.9 × 1010−2 m/s.2 m As/s. can As canbe seen be seen from × − fromthe tracer the tracer curve, curve, it can it be can further be further inferred inferred by comparing by comparing the curvilinear the curvilinear karst karstmorphological morphological model modeldiagram diagram given givenin the in literature the literature [30]: [30 a]: body a body of ofwater water perpendicular perpendicular to to the the directiondirection ofof thethe undergroundunderground river river may may have have traveled traveled through through several several series series of of small small karst karst pipes pipes with with relatively relatively weak weak development,development, and and its its curve curve is is multimodal multimodal (Figure (Figure 10 10a);a); in in the the process process of of resupply, resupply, when when more more open open pipespipes are are encountered encountered (the (the direction direction is is the the same same as as the the underground underground river), river), the the fractures fractures may may lead lead to to flowflow distribution distribution and and some some water water bodies bodies may may be be lost, lost, such such as as shown shown by by some some other other similar similar results, results, inin that that permeabilities permeabilities measured measured parallel parallel to theto the fault fa dipult dip were were up to up 10 to times 10 times higher higher than alongthan along the fault the strikefault permeabilitystrike permeability [31]. The [31]. underground The underground river appears river appears as a single as a mature single mature large karst large pipeline karst pipeline with a singlewith peaka single curve peak (Figure curve 10 (Figureb). The 10b). vertical The permeability vertical permeability and fault permeability and fault permeability of the fault canof the be onefault ordercan be of magnitudeone order lower;of magnitude therefore, lower; it can therefore, be said that it thecan development be said that of the the development underground of river the alongunderground the strike river line ofalong the reverse the strike fault line is advantageous. of the reverse The fault analysis is advantageous. of hydrogeological The analysis data can of verifyhydrogeological the correctness data of can the verify results the most correctness directly. Therefore,of the results dynamic most monitoringdirectly. Therefore, and multiseason dynamic real-timemonitoring tracer and tests multiseason must be carried real-time out under tracer highly tests specificmust be conditions. carried out under highly specific conditions.

(a) Tracer test in the direction of vertical (b) Underground river (fault strike direction) underground river (fault); connectivity test.

FigureFigure 10. 10. CharacteristicsCharacteristics of the peak peak curve curve of of the the tracer tracer test test in inthe the tunnel tunnel area. area. Image Image recreated recreated from from[29]. [ 29].

5.5. Discussion Discussion 5.1. Selection Basis and Suggestions of Tunnel Design Elevation Relative to the Location of the River 5.1. Selection Basis and Suggestions of Tunnel Design Elevation Relative to the Location of the River The selection of elevation plays an important role in the selection of geological lines. It can be The selection of elevation plays an important role in the selection of geological lines. It can be concluded from consideration of the structural changes of the vertical rock mass of the fault structure, concluded from consideration of the structural changes of the vertical rock mass of the fault structure, and the relative position of the development of the underground river can be delineated, which and the relative position of the development of the underground river can be delineated, which can can provide a reference for the design of tunnel elevation. According to the viewpoint of Xiao’s provide a reference for the design of tunnel elevation. According to the viewpoint of Xiao’s new new structure for controlling water [32], while considering the vertical direction of the fault [16] in structure for controlling water [32], while considering the vertical direction of the fault [16] in combination with the actual survey, the information presented in Table2 was obtained. The infiltration combination with the actual survey, the information presented in Table 2 was obtained. The zone is in the shallow part below the ground, but the amount of water is often not particularly large due infiltration zone is in the shallow part below the ground, but the amount of water is often not to the effect of near-surface filling; and the runoff zone is generally between 50–60 m and 150–200 m particularly large due to the effect of near-surface filling; and the runoff zone is generally between below the ground (mainly in block fractures or block structures). The block fracture structure should 50–60 m and 150–200 m below the ground (mainly in block fractures or block structures). The block be a favorable area for the development of large pipelines and underground rivers. In the detained fracture structure should be a favorable area for the development of large pipelines and underground zone below the runoff zone, in general, the water content in the fractures is small and the mobility of rivers. In the detained zone below the runoff zone, in general, the water content in the fractures is the water is relatively poor. Under normal circumstances, it can act as a resistive water layer. small and the mobility of the water is relatively poor. Under normal circumstances, it can act as a resistive water layer.

Table 2. The zoning characteristics of the fault structure with the change of depth.

Fault structure and traversing suggestions for Basic characteristics of the rock mass structure fault rock mass and development site of the underground river Located in the shallow part below the surface of Granular structure (corresponding to the the earth, the weathering is serious, the texture is infiltration zone); passable for traversing, but weak, the fractures are mostly filled with mud, the

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Table 2. The zoning characteristics of the fault structure with the change of depth.

Fault Structure and Traversing Suggestions for Basic Characteristics of the Rock Mass Structure and Fault Rock Mass Development Site of the Underground River Located in the shallow part below the surface of the Granular structure (corresponding to the infiltration earth, the weathering is serious, the texture is weak, zone); passable for traversing, but should be at a the fractures are mostly filled with mud, the stability of depth with self-stability (Figure5a,b). the rock mass is poor, and it is easy to collapse. The rock mass is cut by joints, the rock mass is hard, Block fracture (corresponding to runoff zone); the crack opening is large, and there is no filling or less passable for traversing (Figure5c–e, Figure8). filling, which means it is basically stable (the underground river will easily develop). Block structure (corresponding to the detained zone); The strength of the rock mass is high. Although it is cut where there is a large supply of water or water source by the fractures, the fractures generally tend to close, and the fault is above the tunnel elevation, it should the rock block is hard, the fracture aperture is very be carefully crossed (refer to [33]). small, and the rock mass stability is generally better.

Throughout the tunnel construction cases, most of the above three types of structures will inevitably pass through faults in line selection and elevation design (Table2). Although the granular structure has poor characteristics, it can be stable at 22.25 m below the surface, as found through the numerical simulation study on the safety depth of the Qiyueshan Tunnel site [34]; therefore, below this depth and with the corresponding support and reinforcement measures, a tunnel can be safely constructed. Is it absolutely without risk to cross the block fracture or even the block structure below the underground river? Practice has proved that crossing the block structure in the karst area is not foolproof; for example, Deng [35] determined that the tunnel passing under the underground river, even if there is no fault, is affected by the recharge of the bedding layer and still carries the risk of experiencing great water inrush. From the actual dynamic excavation process, it can be seen from the theoretical analysis of block theory [36] that, under the tunnel excavation, a free surface is created, and that under the action of engineering and self-weight, development of the key block is started because the strength of the sliding surface was insufficient to resist the sliding power, and this may lead to interlocking multiblock instability. We plot the stereographic projections by identifying the occurrence information (according to the preliminary statistical characteristics of the appearance in Figure5c), as shown in Figure 11; this type of intersection mode is mainly prone to instability and blockage in the vault, easily producing tetrahedral pyramidal blocks, and other types of blocks are generated depending on the fracture spacing [37,38]. If located under the underground river, the excavation space causes the falling of blocks to occur sooner under the action of water pressure, providing space conditions for the karst water to enter, such as when the Qiyueshan Tunnel of the Yichang–Wanzhou Railway caused a landslide when crossing this type of area, causing a serious water inrush disaster [33]. According to further analysis of stress changes, especially in a short time after the excavation disturbance, the additional stress and the confining pressure change are actually generated [39], the additional stress may cause the fractures to be more open, and the change in the confining pressure will deteriorate the fault zone lithology (Figure 12); this can also transform the original relatively water-blocking fault zone into the dominant water-control surface. Under the huge water pressure of the upper underground river, coupled with excavation disturbance, the underside will evolve into a seepage channel from the vulnerable fractures between the underground river and the excavation, which will strengthen the capacity of the seepage channel connection after the falling of blocks, increasing the water inrush risk of the tunnel (see Figure 13). Water 2019, 11, x FOR PEER REVIEW 12 of 18

should be at a depth with self-stability (Figure stability of the rock mass is poor, and it is easy to 5a,b). collapse.

The rock mass is cut by joints, the rock mass is Block fracture (corresponding to runoff zone); hard, the crack opening is large, and there is no passable for traversing (Figure 5c,d,e, Figure 8). filling or less filling, which means it is basically stable (the underground river will easily develop). Block structure (corresponding to the detained The strength of the rock mass is high. Although it zone); where there is a large supply of water or is cut by the fractures, the fractures generally tend water source and the fault is above the tunnel to close, the rock block is hard, the fracture elevation, it should be carefully crossed (refer to aperture is very small, and the rock mass stability [33]). is generally better.

Throughout the tunnel construction cases, most of the above three types of structures will inevitably pass through faults in line selection and elevation design (Table 2). Although the granular structure has poor characteristics, it can be stable at 22.25 m below the surface, as found through the numerical simulation study on the safety depth of the Qiyueshan Tunnel site [34]; therefore, below this depth and with the corresponding support and reinforcement measures, a tunnel can be safely constructed. Is it absolutely without risk to cross the block fracture or even the block structure below the underground river? Practice has proved that crossing the block structure in the karst area is not foolproof; for example, Deng [35] determined that the tunnel passing under the dark river, even if there is no fault, is affected by the recharge of the bedding layer and still carries the risk of experiencing great water inrush. From the actual dynamic excavation process, it can be seen from the theoretical analysis of block theory [36] that, under the tunnel excavation, a free surface is created, and that under the action of engineering and self-weight, development of the key block is started because the strength of the sliding surface was insufficient to resist the sliding power, and this may lead to interlocking multiblock instability. We plot the stereographic projections by identifying the occurrence information (according to the preliminary statistical characteristics of the appearance in Figure 5c), as shown in Figure 11; this type of intersection mode is mainly prone to instability and blockage in the vault, easily producing tetrahedral pyramidal blocks, and other types of blocks are generated depending on the fracture spacing [37,38]. If located under the dark river, the excavation space causes the falling of blocks to occur sooner under the action of water pressure, providing space conditions for the karst water to enter, such as when the Qiyueshan Tunnel of the Yichang–Wanzhou Water 2019, 11, 889 13 of 18 Railway caused a landslide when crossing this type of area, causing a serious water inrush disaster [33].

Water 2019, 11, x FOR PEER REVIEW 13 of 18 Water 2019, 11, x FOR PEER REVIEW 13 of 18 According to further analysis of stress changes, especially in a short time after the excavation disturbance,According the to additional further analysis stress and of stressthe confining changes, pressure especially change in a shortare actually time after generated the excavation [39], the disturbance,additional stress the additionalmay cause stress the fractures and the toconfining be more pressure open, and change the change are actually in the generatedconfining pressure[39], the additionalwill deteriorate stress themay fault cause zone the lithologyfractures (Figureto be more 12); open, this can and also the changetransform in thethe confiningoriginal relatively pressure willwater-blocking deteriorate faultthe fault zone zone into thelithology dominant (Figure wate 12);r-control this can surface. also transform Under the the huge original water relatively pressure water-blockingof the upper underground fault zone intoriver, the coupled dominant with wate excavar-controltion disturbance, surface. Under the underside the huge willwater evolve pressure into ofa seepage the upper channel underground from the river, vulnerable coupled fractures with excava betweention disturbance,the underground the underside river and willthe excavation,evolve into awhich seepage will channel strengthen from thethe capacityvulnerable of fracturesthe seepag betweene channel the undergroundconnection after river the and falling the excavation, of blocks, whichincreasing will thestrengthen water inrush the capacityrisk of the of tunnel the seepag (see Figuree channel 13). connection after the falling of blocks, Figure 11. increasing the waterFigure inrush 11. Therisk stereographic of the tunnel projection (see Figure methodmeth 13).od ofof thethe jointjoint distribution.distribution.

Figure 12. The fracture propagation on a block formed by excavation disturbance from the Qiyueshan FigureTunnel 12. of theTheThe Lichuan–Wanzhou fracture fracture propagation propagation Railway. on a block block formed formed by by excavation excavation disturbance disturbance from the Qiyueshan Tunnel ofof thethe Lichuan–WanzhouLichuan–Wanzhou Railway.Railway.

Figure 13. Fracture activation process. Figure 13. Fracture activation process. In general, when the well-developed and confluen conﬂuentt underground river has a spatial relationship with Inthe the general, tunnel tunnel whenbetween between the the well-developed the upper upper and and lower and lower sectioconfluen sections,ns, thet underground fault the faultbetween between river the has two the a easilyspatial two formseasily relationship a formshigh- withpressure the tunnelwater-rich between fault the zone upper with and strong lower hydrau sectiolicns, recharge. the fault betweenThe tunnel the will two encounter easily forms instant a high- or pressuredelayed waterwater-rich inrush fault during zone the with construction strong hydrau of thelic tunnelrecharge. undern The eathtunnel the will underground encounter river.instant The or delayedearlier design water ofinrush the Qiyueshan during the Tunnel construction of the ofYi wanthe tunnel Railway undern througheath thethe bottomunderground of the darkriver. river The earlierhas caused design several of the large-scale Qiyueshan water Tunnel inrush of the events, Yiwan indicatingRailway through that, when the bottomthe fault of isthe present, dark river the hasnearby caused rock several mass is large-scale disturbed, waterand the inrush depth events, of influence indicating can extend that, when down the about fault 200 is m;present, therefore, the nearbycrossing rock the massrock massis disturbed, too close and to the the bottom depth ofof influencethe dark rivercan extend should down be avoided. about 200This m; requires therefore, the crossingdesign elevation the rock to mass be fully too closeconsidered to the atbottom the beginni of theng dark of the river design. should We be can avoided. choose Thisposition requires a tunnel the designat similar elevation elevations to be to fully the river;considered for example, at the beginni the Zijinshanng of the Tunnel design. is Wedesigned can choose to cross position the dark a tunnel river at similar elevations to the river; for example, the Zijinshan Tunnel is designed to cross the dark river

Water 2019, 11, 889 14 of 18 a high-pressure water-rich fault zone with strong hydraulic recharge. The tunnel will encounter instant or delayed water inrush during the construction of the tunnel underneath the underground river. The earlier design of the Qiyueshan Tunnel of the Yiwan Railway through the bottom of the underground river has caused several large-scale water inrush events, indicating that, when the fault is present, the nearby rock mass is disturbed, and the depth of influence can extend down about 200 m; therefore, crossing the rock mass too close to the bottom of the underground river should be avoided. This requires the design elevation to be fully considered at the beginning of the design. We can choose position a tunnel at similar elevations to the river; for example, the Zijinshan Tunnel Water 2019, 11, x FOR PEER REVIEW 14 of 18 is designed to cross the underground river at the same height, the necessary drainage hole is set in considerationat the same height, of the the rainstorm, necessary and drainage the drain hole hole is set is in set consideration 2 m above theof the drain rainstorm, point [and40]; the drain tunnel elevationhole is set can 2 alsom above be set the at slightlydrain point higher [40]; and the lower tunnel elevations elevation relative can also to be the set hidden at slightly river; higher for example, and atlower points elevations where the relative Wulong to the Tunnel hidden passes river; safely for example, through at the points longitudinal where the slope Wulong and Tunnel the water passes flow ofsafely the underground through the river,longitudinal the tunnel slope elevation and the was water designed flow of to the be underground 20 m above the river, elevation the tunnel of the undergroundelevation was river designed [41]; other to be examples20 m above are the given elevat byion the of Qiyueshanthe underground Tunnel river of the [41]; Lichuan–Wanzhou other examples Railwayare given and by the the Qiyueshan Qiyueshan TunnelTunnel ofof thethe MoudaoLichuan–Wanzhou Connecting Railway Road (Figuresand the Qiyueshan7a and 14), Tunnel where of the elevationthe Moudao of the Connecting tunnel was Road set on(Figures the runo 7a ffandelevation 14), where line the of elevation the underground of the tunnel river was and set the on tunnel the constructionrunoff elevation process line was of the carried underground out smoothly river an withoutd the tunnel causing construction any serious process water was inrush carried accidents. out Therefore,smoothly we without believe causing that it is any generally serious safe water and inrush economical accidents. to set theTherefore, elevation we of believe the tunnel that projects it is togenerally be constructed safe and in thiseconomical region to to be set slightly the elevation higher thanof the that tunnel of the projects river. to be constructed in this region to be slightly higher than that of the river.

Figure 14. Relative position model of underground river and tunnels (Figure7a, 1-2 Section). Figure 14. Relative position model of underground river and tunnels (Figure 7a, 1-2 Section). Image Image recreated from [29]. recreated from [29]. 5.2. The Necessity of a Reasonable Multisource Exploration Method

At present, due to the problems of surveying, design precision, and construction quality, engineering technicians often encounter various geological disasters during tunnel construction, among which water inrush and mud burst events are the most frequent. Before the construction of the Lichuan–Wanzhou Expressway Tunnel in Qiyueshan area, it was estimated that it might cross the hidden river and that there would be a risk of water inrush. However, during the excavation, the project was completed without revealing the underground river. In fact, we do not fully understand the exact location relationship between the tunnels and the underground river, which means that the current exploration techniques still cannot accurately locate the turning point of the underground river (as shown by the question mark in Figure 13). Hjerne et al. [42] conducted a long-term nuclide migration tracing test of multiscale structural planes of rock masses in the project of the famous underground engineering Aspo hard rock laboratory TRUE in Sweden, pointing out that it is necessary to identify multiscale structural planes in rock masses. There are still many technical challenges in hydrogeological parameters, and the complicated path of the joint system remains diﬃcult to understand. It is suggested that future research should aim to strengthen the accurate description of the spatial structure of the rock mass joint system. Zhang et al. [43] considered the study area to be too large, so the characteristic parameters of the fracture were identiﬁed by means of drones; in the case of outcrop steepness, the

Water 2019, 11, 889 15 of 18 size of the fault and the division of the influence zone were determined by means of geophysical and photogrammetry [44]. Furthermore, Zhang et al. [45] pointed out that when the crack path is clearly identified, the fault avoidance distance can be given more accurately, the GPS-RTK is used to trace the cracks on the flat outcrop surface, and the possible seepage range of the fracture is determined. Therefore, in the large karst areas, where the karst cavity has a strong specificity of development, more attention should be paid to the form and development degree of water channels. People are beginning to realize the importance of comprehensive interpretation of multisource technology. It has been proved by practice that a single test method can no longer meet the needs of survey accuracy at the present stage, and may even lead to serious miscalculation. However, we should not excessively pursue various technologies which would simply pile up, causing waste. The key is to develop appropriate reference standards. As Yan [46] pointed out, it is necessary to not only increase the accuracy of ground geophysical exploration at great depths, but also carry out research on the application of techniques of tunnel investigation such as airborne geophysical prospecting and horizontal directional drilling combined with borehole geophysical exploration. In addition, from the karst perspective, Ficco and Sasowsky [47] pointed out that current laws for karst aquifer protection are unsystematic and many documents presenting suitable guidelines for karst aquifer management are not legally binding, and noted the problems that arise with a lack of specialist evaluation and differences in approaches. However, relevant scholars are actively making suggestions, such as Zhang et al. [48], who pointed out that the use of tracer test after the fault zone identification can improve the success rate. It is considered that the multisource survey technology and standards in the process of field geological parameter identification should be further established and improved: Deng [49] suggested bringing the content of a special geological survey for tunnel risk into a professional standard titled code for geological investigation of railway engineering. Guo et al. [50] divided karst tunnels into different categories according to certain criteria and proposed the principles of classification management and information construction. This would also be an important direction for further research.

6. Conclusions Unlike randomly distributed cavities, caves, etc., large-scale underground rivers can be effectively controlled. Taking the Qiyueshan Tunnel site project as an example, we believe that the reverse fault also has the possibility of developing a large underground river. Among them, with the increase of depth, the faulty rock stratum near the fault core is especially related to the development of the underground river. It plays a significant role and becomes the main channel of the catchment. In addition, due to the rotation of the principal stress direction, the tendency joints, stratification, and transverse or dip joints develop into sinks, water-conducting holes, and cavities under the action of water flow dissolution. In addition, the multistage tectonic uplift provides strong hydrodynamic conditions for the runoff zone, facilitating the expansion of the horizontal and downward spaces of the horizontal channel (underground river). In terms of tunnel elevation selection recommendations, the bulk structure can be selected within a certain safe depth under certain numerical simulations; when it is selected to pass under the underground river, additional stresses are generated due to excavation disturbances and fractures are generated. The fractured rock mass near the fault and its hanging wall is likely to form a strong high-pressure water-rich channel under the action of the upper underground river. Under excavation disturbance, the rock mass in the lower detention zone of the underground river is not stable and reliable. Through the practical engineering analysis, it is shown that, when there are faults and unicorn channels, the tunnel should be designed as far as possible above the level of water in the rivers. If we have to build the tunnel under the underground river’s elevation, we conduct geophysical prospecting with a tracer to effectively identify areas of possible transmission channels, and then make the necessary identification of parameters of the rock mass structure type and the stability analysis of surrounding rock after excavation, in combination with a numerical simulation method to further demonstrate the feasibility, thus making a comprehensive, targeted excavation plan. Water 2019, 11, 889 16 of 18

The control effect of faults on karst development has many influencing factors. This article is only based on the study of previous research materials and fieldwork. The related elevation design needs to be further studied systematically.

Author Contributions: P.Z. conceived and wrote the paper; Z.H. and S.L. reviewed and made corrections to improve the paper; and P.Z., S.L. and T.X. analyzed the data. All authors read the full paper and agreed to its publication. Acknowledgments: Associate Professor Zhenhao Xu in Shandong University provided much help in the fieldwork, including internal materials to help the authors understand the regional background more deeply; for this, the authors would like to express their gratitude. The authors also appreciate the aid provided by the reviewers and editors to improve the paper. Conflicts of Interest: The authors declare no conflict of interest.

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