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Article Development Mechanism and Characteristics Based on Comprehensive Exploration along ,

Shangqu Sun 1,2, Liping Li 1,2,*, Jing Wang 1,2, Shaoshuai Shi 1,2 , Shuguang Song 3, Zhongdong Fang 1,2, Xingzhi Ba 1,2 and Hao Jin 1,2

1 Research Center of Geotechnical and Structural Engineering, University, Jinan 250061, China; [email protected] (S.Sun); [email protected] (J.W.); [email protected] (S.Shi); [email protected] (Z.F.); [email protected] (X.B.); [email protected] (H.J.) 2 School of Qilu Transportation, Shandong University, Jinan 250061, China 3 School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, China; [email protected] * Correspondence: [email protected]

 Received: 28 August 2018; Accepted: 19 September 2018; Published: 21 September 2018 

Abstract: Jinan is the capital of Shandong Province and is famous for its water. Water conservation has become the consensus of Jinan citizens and the government and the community. The construction of metro engineering in Jinan has lagged behind other cities of the same scale for a long time. The key issue is the protection of spring water. When metro lines are constructed in Jinan karst area, the water-inrushing, quicksand, and piping hazards can easily occur, which can change the seepage environment and reduce spring . Therefore, we try to reveal the development conditions, mechanism, and mode of karst area in Jinan. In addition, we propose the comprehensive optimizing method of “shallow-deep” and “region-target” suitable for exploration of karst areas along Jinan metro, and systematically study the development characteristics of the karst areas along Jinan metro, thus providing the basis for the shield to go through karst areas safely and protecting the springs in Jinan.

Keywords: Jinan metro; water conservation; karst exploration; development mechanism; shield tunnel

1. Introduction With the rapid development of cities and urban population, traffic congestion has become one of the greatest problems in most cities in China. However, metro construction makes full use of underground space and reduces congestion on the ground. Therefore, metro construction has become the main form of urban infrastructure and traffic in China in the 21st century [1–7]. Until now, 53 cities have won the approval of rail transit planning, with a total length of about 8600 km. By the end of 2020, a total of 177 rail lines will be in construction, with a total operating distance of about 6100 km. Due to the complex urban geological conditions, a large number of metro run through karst geological areas. The existence of karst along the planning metro line can have great effect on the safety of tunnel design, construction, and operation [8]. Although several research on shield tunnelling in soft and sand and gravels have been conducted by many scholars [9–15], shield tunnel excavation in karst regions is rarely discussed. geology is widely distributed throughout China. Moreover, seven primary karst areas can be concluded in China, each of which has its own characteristics. The common karst types include subterranean -type caves, sink hole-type caves, through caves, phreatic caves along river valleys,

Sustainability 2018, 10, 3383; doi:10.3390/su10103383 www.mdpi.com/journal/sustainability Sustainability 2018, 10, x 3383 FOR PEER REVIEW 22 ofof 2121 valleys, vadose caves, and caves [16]. During the metro construction in karst areas, caves that vadose caves, and tufa caves [16]. During the metro construction in karst areas, caves that have have developed or are developing are often encountered, which pose great difficulties in tunneling developed or are developing are often encountered, which pose great difficulties in tunneling by shield by shield machine. As most of the karst caves are in non-filled, semi-filled, or rich muddy water-filled, amachine. number Asof hazards most of theincluding karst caves sink areholes, in non-filled, karst collapse, semi-filled, and water or rich inrush muddy can water-filled,easily occur ain number metro tunnelof hazards projects including when they sink pass holes, through karst collapse, karst areas and [17,18]. water inrush can easily occur in metro tunnel projectsJinan when City they is located pass through in Eastern karst China areas and [17 ,is18 famous]. for its springs with complex groundwater network.Jinan Jinan City isoverlooks located inthe Eastern Mount China Tai to and the issouth famous and for across its springs the Yellow with complexRiver to groundwaterthe north, as indicatednetwork. in Jinan Figure overlooks 1. The structural the Mount unit Tai of tothe the northern south foothills and across of Mount the Yellow Tai, gradient River to from the north,south toas north, indicated is accelerative in Figure1. Thefor formation structural unitof spring of the wa northernter. After foothills the atmospheric of Mount Tai, rainfall gradient infiltrates from south into theto north, soluble is carbonate accelerative rocks, for formationthe water flows of spring from water. south Afterto north the along atmospheric the topographic rainfall infiltratestrend, gathers into inthe the soluble connecting carbonate area rocks, of the the monoclinic water flows structure from south and the to northnorthern along plain, the topographicand then crops trend, out gathers of the surface,in the connecting forming a area rising of thespring. monoclinic The comprehensiv structure ande conditions the northern of strata, plain, andgeological then crops structure out of and the hydrogeologysurface, forming lead a risingto concealed spring. karst The comprehensivecaves, which can conditions cause ground of strata, surface geological settlement structure and karst and collapse, water leadinrush, to concealedmud inrush, karst and caves, other which hazard cans. The cause metro ground lines surface under construction settlement and in Jinan karst willcollapse, inevitably water inrush,go through mud inrush,karst areas, and other which hazards. contain The rich metro groundwater lines under constructionand a large innumber Jinan will of concealedinevitably karst go through caves. karstKarst areas,caves whichwill threaten contain the rich safe groundwater construction and and a large stability number of shield of concealed tunnel, especiallykarst caves. when Karst considering caves will the threaten complexity the safe of constructiongroundwater andin Jinan stability spring of shieldareas and tunnel, the appeal especially for “protectingwhen considering the spring”. the complexity Therefore, of safe groundwater constructi inon Jinan is particularly spring areas important and the appeal in karst for areas “protecting where Jinanthe spring”. Metro runs Therefore, through. safe construction is particularly important in karst areas where Jinan Metro runs through.

Figure 1. OverviewOverview of of Jinan Jinan city [19]. [19].

This paper mainly mainly focuses focuses on on the the karst karst areas areas where where R1 R1 and and R2 R2 lines lines of of Jinan Jinan Metro Metro go go through. through. The planning map of main metro lines is shownshown inin FigureFigure2 .2. AfterAfter revealingrevealing thethe conditionsconditions andand development mechanism mechanism of of karst karst area area in Jinan, we propose the comprehensive detection method of “shallow-deep” and “region-target” suitable for exploration of karst areas, and systematically studies thethe development scalescale andand characteristicscharacteristics of of karst karst areas areas along along Jinan Jinan metro, metro, thus thus providing providing the the basis basis for forthe the shield shield tunnel tunnel to go to throughgo through karst karst areas areas safely. safely. Sustainability 2018, 10, 3383 3 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 3 of 21

FigureFigure 2.2. JinanJinan metrometro planplan mapmap [[19].19]. 2. Karst Development Conditions and Mechanism along the Jinan Metro 2. Karst Development Conditions and Mechanism along the Jinan Metro The karst development is mainly controlled by the solubility of rocks, geological structures The karst development is mainly controlled by the solubility of rocks, geological structures (including rock strata, faults, fissures, etc.), and groundwater [20–25]. Among them, the fluidity of (including rock strata, faults, fissures, etc.), and groundwater [20–25]. Among them, the fluidity of water is the most active factor. According to the special hydrogeological conditions in Jinan, the factors water is the most active factor. According to the special hydrogeological conditions in Jinan, the affecting karst development can be divided into four categories: topography, formation lithology, factors affecting karst development can be divided into four categories: topography, formation groundwater action, and geological structure. lithology, groundwater action, and geological structure. 2.1. Topography 2.1. Topography The terrain of Jinan is high in the south and low in the north, with small mountain ranges in the east andThe westterrain sides, of Jinan forming is high a unique in the semi-basinsouth and low landform. in the north, The southern with small part mountain of Jinan is ranges a hilly in area, the whileeast and the west northern sides, part forming is near a unique the Yellow semi-basin River. Thelandform. urban The area southern in Jinan ispart located of Jinan in theis a piedmonthilly area, plain.while Thethe northern highest elevation part is near of thethe southernYellow River. mountains The urban is approximately area in Jinan 1108.4is located m, whilein the the piedmont lowest elevationplain. The in highest the north elevation is about of the 5 m, southern leading mounta to the elevationins is approximately difference of 1108.4 about m, 1100 while m. the Owing lowest to gravity,elevation the in groundwater the north is flowsabout from5 m, southleading to north.to the Inelevation addition, difference severalmountains of about 1100 are locatedm. Owing along to thegravity, east-west the groundwater direction, blocking flows from the flow south and to diffusionnorth. In addition, of groundwater. several Asmountains a result, theare located groundwater along runsthe east-west from three direction, directions, blocking accelerating the flow its and flow diff andusion exacerbating of groundwater. the karstification. As a result, the groundwater runsAccording from threeto directions, the geomorphological accelerating it characteristicss flow and exacerbating of Jinan, the the city karstification. is divided into classification I containingAccording plain to andthe geomorphological mountain, and classification characteristics II, whichof Jinan, contains the city five is divided more specific into classification categories, asI containing shown in Tableplain1 and. The mountain, terrain turns and fromclassification high to low II, which from south contains to north. five more The typesspecific of categories, landform genesisas shown are in low Table mountain 1. The terrain area, monadnock-hilly turns from high area,to low alluvial-diluvial from south to plainnorth. area, The alluvialtypes of plain landform area, andgenesis dissolved are low monadnock mountain area, area, monadnock-hilly as shown in Figure area,3. alluvial-diluvial plain area, alluvial plain area, and dissolved monadnock area, as shown in Figure 3. Table 1. Terrain classification. Table 1. Terrain classification. Classification I Classification II Classification I Classification II Alluvial Plain Plain Accumulation Alluvial Plain Plain Accumulation Alluvial-Diluvial Plain Alluvial-Diluvial Plain Monadnock, Hilly Area Monadnock, Hilly Area Mountain Erosion Low Mountain Mountain Low Mountain Karst Dissolved Monadnock Karst Dissolved Monadnock SustainabilitySustainability 20182018, ,1010, ,x 3383 FOR PEER REVIEW 44 of of 21 21

FigureFigure 3. 3. GeomorphologicGeomorphologic map map of of Jinan Jinan city city [19]. [19].

2.2.2.2. Formation Formation Lithology Lithology ItIt can can be be seen seen from from Figure Figure 33 that that the the mountainou mountainouss region region is located is located mainly mainly in the in thesouth south and and the plainthe plain region region is situated is situated mainly mainly in the in north the northof Jinan of City. Jinan Based City. on Based the lithology, on the lithology, the hilly the area hilly can area be dividedcan be divided into three into subareas, three subareas, including including metamorphi metamorphicc rocks, carbonatite rocks, carbonatite rocks, rocks,and magmatite and magmatite rock. Therock. first The subarea first subarea distributes distributes in the insouthwest the southwest of Zhangqiu of Zhangqiu city, southeast city, southeast of Changqing of Changqing County. County. The secondThe second subarea subarea is located is located mainly mainly around around the thefirst first subarea. subarea. The The third third subarea subarea is issituated situated in in the the northeastnortheast of of Zhangqiu Zhangqiu city. city. In In the the second second subarea, subarea, the the minerals of of carbonate carbonate rocks rocks in in the the study study area area areare mainly mainly calcite and and , dolomite, and and the the chemical chemical components components are are mainly mainly CaO CaO and and MgO, MgO, which which are are soluble.soluble. The The mineral and and chemical chemical compositions compositions are are the the basic basic conditions conditions to to determine determine to to what what extent extent thethe karst karst develops. develops. In In accord accord with with the the topography, topography, th thee plain plain region region can can be be divided divided into into two two categories: categories: thethe Huangfan Huangfan Plain Plain and and the the piedmont piedmont sloping sloping plain. plain. Huangfan Huangfan Plain Plain is is situated situated mainly mainly along along Yellow Yellow RiverRiver and and the the thickness thickness becomes becomes deeper deeper from from south south to to north. north. The The main main stratigr stratigraphyaphy consists consists of of silty silty sand,sand, clay, clay, sandy sandy loam, loam, and and mucky mucky . soil. The The piedmont piedmont sloping sloping plain plain is is located located in in the the center center of of Jinan, Jinan, whichwhich belongs belongs to to the the piedmont piedmont alluvial alluvial plain. plain. The The stratigraphy stratigraphy includes includes clay, clay, sandy sandy loam, loam, and and gravel gravel withwith mud. mud. Several Several small buttes can bebe foundfound onon thethe piedmontpiedmont sloping sloping plain, plain, caused caused by by the the intrusion intrusion of ofgabbro gabbro in in the the Yanshan Yanshan period. period. 2.3. Hydrogeological Environment 2.3. Hydrogeological Environment The hydrogeological map of Jinan City is shown in Figure4. Four groups can be found The hydrogeological map of Jinan City is shown in Figure 4. Four aquifer groups can be found in the study area, including a Quaternary porous aquifer group (AqP), a karst aquifer group (AqK), in the study area, including a Quaternary porous aquifer group (AqP), a karst aquifer group (AqK), an interbedded karst-fissure aquifer group (AqKF), and a fissure aquifer group (AqF). The four aquifer an interbedded karst-fissure aquifer group (AqKF), and a fissure aquifer group (AqF). The four groups are connected to each other in the whole groundwater circulation. In the karst aquifer group, a aquifer groups are connected to each other in the whole groundwater circulation. In the karst aquifer wide range of carbonate rocks are deposited, including limestone and dolomite formed in the Cambrian group, a wide range of carbonate rocks are deposited, including limestone and dolomite formed in and the Ordovician period. Therefore, the karst area is -developed. Soluble carbonatites can be the Cambrian and the Ordovician period. Therefore, the karst area is well-developed. Soluble dissolved and corroded, which will result in a large number of pores, fissures, grikes, and karst caves. carbonatites can be dissolved and corroded, which will result in a large number of pores, fissures, grikes, and karst caves. Sustainability 2018, 10, 3383 5 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 5 of 21

TheThe soluble soluble rock rock can can be bedissolved dissolved by bythe the solution solution of water. of water. Water’s Water’s ability ability to dissolve to dissolve is derived is derived from bicarbonate (HCO3−), which− is formed through combination of (CO2) and water, from bicarbonate (HCO3 ), which is formed through combination of carbon dioxide (CO2) and andwater, can anddissolve can dissolvecarbonate carbonate rocks under rocks long-term under long-term reactions. reactions. In addition, In addition, due to duethe toflow the of flow groundwater,of groundwater, transportation transportation will willexpand expand the thescale scale of karst of karst development. development. Among Among the the chemical chemical componentscomponents of karst of karst water water near near the the exposed exposed spring spring water, water, the the cation cation is ismainly mainly Ca Ca2+, 2+with, with content content of of 80~123 mg/L, followed by Mg2+, 2+with content of 15~24 mg/L; the anion is mainly HCO3−, which− is 80–123 mg/L, followed by Mg , with content of 15–24 mg/L; the anion is mainly HCO3 , which is 171~338 mg/L, followed by SO42−, 2with− content of 3–107 mg/L. The mineralization is 492~699 mg/L; 171–338 mg/L, followed by SO4 , with content of 3–107 mg/L. The mineralization is 492–699 mg/L; the total hardness is 262~405 mg/L; the PH value is between 7.1~7.8; HCO3− water− is the only chemical the total hardness is 262–405 mg/L; the PH value is between 7.1–7.8; HCO3 water is the only chemical typetype of ofthe the karst karst water, water, which which has has good good quality, quality, therefore, therefore, can can pres presentent overall overall characteristics characteristics of ofthe the karstkarst water. water.

FigureFigure 4. Hydrogeological 4. Hydrogeological map map of Jinan of Jinan [19]. [19 ]. 2.4. Structural Influence 2.4. Structural Influence Jinan area is generally a north-dipping monoclinic structure dominated by Paleozoic strata, with a numberJinan area of east-west is generally small a north-dipping folds and faults. monoclinic There are alsostructure many dominated structural jointsby Paleozoic and cracks strata, influenced with a numberby quartz of veins. east-west The erosionsmall fold of waters and on faults. rockmass There generally are also begins many with structural fissures, jointsand and the cracks karst is influenceddeveloping by withquartz the veins. crack The expansion. erosion Therefore,of water on the rockmass development generally degree begins and extensionwith joint directionfissures, of andfissures the karst usually is developing control the extentwith the and crack direction expansion. of karst Therefore, development. the development degree and extensionIn thedirection northern of fissures part of usually the monoclinic control the structure, extent and there direction are extensive of karst magmatic development. activities. In the southernIn the northern part of the part monoclinic of the monoclinic structure, structure, the crystalline there are extensive of magmatic the Archean activities. gneiss isIn widely the southernexposed, part with of Paleozoicthe monoclinic strata. structure, There are developedthe crystalline regional basement faults of but the few Archean folds and gneiss magma is widely activities. exposed,The dip with angle Paleozoic is gentle. strata. In the There monoclinic are deve structure,loped aregional number faults of large-scale but few Northfolds andand Northwestmagma activities.trending The faults dip grow,angle togetheris gentle. with In the Northeast monoclin trendingic structure, faults. a number The North of andlarge-scale Northwest North trending and Northwestfaults are trending distributed faults roughly grow, together equidistantly with No fromrtheast east trending to west, faults. dividing The the North monoclinic and Northwest structure trendinginto several faults faultare distributed blocks, which roughly play importantequidistan rolestly from in the east hydrogeological to west, dividing conditions the monoclinic in the area. structure into several fault blocks, which play important roles in the hydrogeological conditions in Sustainability 2018, 10, 3383 6 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 6 of 21 theAs shownarea. As in shown Figure in5, theFigure four 5, typical the four faults typical are faults F1 (Mashan are F1 Fault),(Mashan F2 Fault), (Qianshan F2 (Qianshan Fault), F3 (DongwuFault), F3 (DongwuFault), and Fault), F4 (Wenzuxin and F4 (Wenzuxin Fault). Fault).

Figure 5. Jinan city geological map (modified after [19]). Figure 5. Jinan city geological map (modified after [19]). 2.5. Karst Development Mechanism along the Metro Lines 2.5. Karst Development Mechanism along the Metro Lines Instead of a single reaction, the multiple factors influencing karst development affect each other andInstead result in of variousa single feedbacks.reaction, the The multiple interaction factors mechanism influencing andkarst mutual development feedback affect system each ofother the andmultiple result factors in various have beenfeedbacks. summarized The interaction according mechanism to the karst conditionsand mutual of Jinanfeedback Metro, system as shown of the in multipleFigure6. factors The karst have water been in summarized Jinan mainly according comes from to atmosphericthe karst conditions precipitation. of Jinan The Metro, climatic as factorsshown inincluding Figure 6. temperature, The karst water amount in ofJinan rainfall mainly and evaporationcomes from allatmospheric affect the precipitation. precipitation. They The replenishclimatic factorsthe groundwater including temperature, system directly. amount Additionally, of rainfall the and chemical evaporation action all of affect soluble the rockprecipitation. is generated They by replenishprecipitation. the groundwater The terrain of system Jinan, which directly. is higher Addition in theally, south the andchemical lower inaction the north,of soluble promotes rock theis generatedoverall flow by ofprecipitation. karst water. The The terrain geological of Jinan, structures, which is including higher in the the fault south zone, and fault,lower fracturein the north, zone promotesand the development the overall flow degree of kars andt orientationwater. The ofgeological the rockmass structures fissure,, including determine the to fault what zone, extent fault, the fracturekarst develops. zone and The the combined development action degree of the and factors orientation mentioned of the above rockmass determine fissure, the determine flow velocity to what and extenthydraulic the karst characteristics develops. of The karst combined water in action Jinan area,of the the factors result mentioned of which affects above karstification determine the in flow turn, velocitythus forming and hydraulic the overall characteristics interaction relationship of karst andwater development in Jinan area, mechanism the result of karst of which in Jinan affects area. karstification in turn, thus forming the overall interaction relationship and development mechanism of karst in Jinan area. Sustainability 2018, 10, 3383 7 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 7 of 21

Figure 6. Karst development mechanism along the metro lines.

3. Karst Development Mode along the Metro Karst is a general term for grooves, fissures,fissures, and caves caused by the chemical and physical effects of water on soluble rock formations, includingincluding carbonate strata ( and dolomites), sulfate strata (), and and haloge halogenn strata strata (rock (rock salt), salt), and and phenom phenomenaena produced produced by by these these effects. effects. The nature ofof karstkarst isis chemicalchemical action,action, that that is, is, chemical chemical erosion erosion of of water water on on rock. rock. Karst Karst development development is isthe the evolution evolution under under the comprehensivethe comprehensive conditions conditions of geological of geological structure, structur groundwatere, groundwater action, solubleaction, solublerock, and rock, topography. and topography. Jinan area is dominated by karst water. The ka karstrst groundwater mainly comes from the southern mountainous area,area, wherewhere the the karst karst landforms landforms develop, develo thusp, thus forming forming karst karst erosion. erosion. Typical Typical karst formskarst formsinclude include karst caves, karst karstcaves, depressions, karst depressions, groundwater groundwater channeling, channeling, and solution and cracks.solution The cracks. southern The southernmountainous mountainous area is exposed area is limestoneexposed limestone with some with parts some having parts thin having overlying thin soil.overlying Moreover, soil. Moreover, developed karst fissures are widely distributed. Thus, karst water channels and water storage spaces are easily formed. In addition, the special terrain of Jinan can promote the flow of karst Sustainability 2018, 10, 3383 8 of 21

developedSustainability karst2018, 10 fissures, x FOR PEER are widely REVIEW distributed. Thus, karst water channels and water storage spaces8 of 21 are easily formed. In addition, the special terrain of Jinan can promote the flow of karst water from southwater tofrom north south along to thenorth strata along and the tectonic strata and belts. tect Theonic urban belts. area The is urban mainly area composed is mainly of composed the inclined of piedmontthe inclined plain piedmont and a plain small and part a small of exposed part of limestoneexposed limestone area. Compared area. Compared with the with southern the southern hilly area,hilly thearea, overlying the overlying Quaternary Quaternary strata strata are thicker are thicker and theand depththe depth of some of some local local area area can can reach reach 100 100 m. Them. The atmospheric atmospheric precipitation precipitatio entersn enters the underlying the underlying limestone limeston stratume throughstratum porethrough water, pore leading water, to karstleading dissolution to karst dissolution and erosion, and which erosion, produce which karst prod water.uce karst At the water. bottom At of the the bottom Quaternary of the strata,Quaternary there arestrata, obvious there redare loessobvious and red mucky loess clay and water-resistingmucky clay wa layer,ter-resisting which layer, has the which characteristics has the characteristics of confined water.of confined Therefore, water. the Therefore, karst development the karst development models in Jinan models can be in divided Jinan can into be direct divided karst, into indirect direct karst, karst andindirect confined karst karst. and confined karst. DirectDirect karst karst refers refers to theto the phenomenon phenomenon that atmosphericthat atmospheric precipitation precipitation directly directly supplies supplies the exposed the limestoneexposed limestone area, where area, a shallowwhere a karstshallow zone karst is formed zone is due formed to fracture due to fracture development development and crush and caused crush bycaused long-term by long-term weathering and dissolution, and dissolution, and that an thed karstthat the infiltrates karst infiltrates through limestonethrough limestone fissures, leadingfissures, to leading karst , to karst as channel, shown as in Figureshown7 a.in InFigu there limestone 7a. In thestrata, limestone theclosed strata, fissuresthe closed have fissures poor permeability,have poor permeability, while the open while cracks the open have cracks high have permeability. high permeability. The open The cracks open are cracks directly are exposed directly toexposed air and to precipitation, air and precipitation, intensifying intensifying the chemical the dissolution, chemical dissolution, thus resulting thus in karstresulting depressions, in karst which,depressions, to some which, extent, to changes some extent, the direction changes of the karstification. direction of Inkarstification. addition, owing In addition, to the special owing terrain to the ofspecial Jinan terrain area, the of karstificationJinan area, the presents karstification a vertical presen directionts a vertical and a direction horizontal and direction a horizontal from direction south to north,from south as shown to north, in Figure as shown7b. The in Figure directkarstification 7b. The direct is karstification mainly located is mainly in the southernlocated in hilly the southern areas in Jinan.hilly areas The generalized in Jinan. The model generalized is shown model in Figure is 7shownc. The karstin Figure water 7c. here The has karst the characteristicswater here has of the a phreaticcharacteristics layer. of a phreatic layer.

(a)

Figure 7. Cont. Sustainability 2018, 10, 3383 9 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 9 of 21

(b)

(c)

FigureFigure 7. The7. The evolution evolution of of direct direct karst karstdevelopment development in Jinan: ( (aa)) Primary Primary stage; stage; (b ()b Evolution) Evolution process; process; (c)( Simplifiedc) Simplified model. model.

TheThe indirect indirect karst karst strata strata mainly mainly include include (I)(I) overlyingoverlying Quaternary strata strata and and (II) (II) soluble soluble limestone limestone strata,strata, and and its its development development evolution evolution is is shown shown inin FigureFigure8 8.. AsAs revealed in in Figure Figure 8a,8a, the the atmospheric atmospheric precipitationprecipitation directly directly replenishes replenishes the the overlying overlying QuaternaryQuaternary strata and and is is collected collected as as pore pore water water into into thethe underlying underlying limestone limestone strata. strata. According According to the to fissure the fissure development development and the and permeability, the permeability, limestone stratalimestone can be strata divided can into be divided closed fractureinto closed low-permeation fracture low-permeation zone and openzone fractureand open quick-permeation fracture quick- permeation zone. Water is extremely easy to be stored in the open fracture. Therefore, the karst zone. Water is extremely easy to be stored in the open fracture. Therefore, the karst dissolution and dissolution and erosion are more intense, accelerating karst water seepage, resulting in karst erosion are more intense, accelerating karst water seepage, resulting in karst , see Figure8b. depression, see Figure 8b. The fracture opening of limestone strata and the permeability decreases The fracture opening of limestone strata and the permeability decreases with the increase of depth, with the increase of depth, except for the widened infiltrating main fracture and fault. As a result, except for the widened infiltrating main fracture and fault. As a result, surface precipitation is easy surface precipitation is easy to store in shallow limestone fissure zone, especially during the rainy to storeseason. in The shallow special limestone terrain of fissure Jinan zone,and karst especially depressions during often the rainychange season. vertical The karstification special terrain into of Jinanhorizontal. and karst The depressions overall direction often change of karstification, vertical karstification however, intoremains horizontal. vertical Theand overallhorizontal direction from of karstification,south to north. however, Karstification remains also vertical leads to and typical horizontal formations from including south to karst north. caves, Karstification dissolution also cracks, leads to typicaland groundwater formations channeling. including karstThe generalized caves, dissolution model of cracks, indirect and karst groundwater development channeling. is shown in The generalizedFigure 8c. modelThe limestone of indirect strata karst receive development precipitation is shown from the in Figureoverlying8c. TheQuaternary limestone strata, strata and receive the precipitationinfiltrating frommain thechannel overlying is formed Quaternary in the fracture strata, andconcentrating the infiltrating zone mainwith larger channel opening is formed or fault in the fracturestructure. concentrating Then, the zonewater with head larger difference opening ca orused fault by structure. the topography Then, the enhances water head horizontal difference causeddevelopment by the topography of karst, forming enhances karst horizontal channels development in the fracture of development karst, forming zone, karst which channels has inthe the fracturecharacteristics development of a phreatic zone, which layer. Indirect has the characteristicskarst is mainly located of a phreatic in the layer.inclined Indirect piedmont karst plain. is mainly located in the inclined piedmont plain. Sustainability 2018, 10, 3383 10 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 10 of 21

(a)

(b)

(c)

FigureFigure 8.8.The The evolution evolution of indirectof indirect karst karst development development in Jinan: in Jinan: (a) Primary (a) Primary stage; ( bstage,) Evolution (b) Evolution process; (process,c) Simplified (c) Simplified model. model. Sustainability 2018, 10, 3383 11 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 11 of 21

AsAs forfor thethe confinedconfined karst,karst, therethere areare waterwater resistingresisting layerslayers withwith redred loessloess andand muckymucky clayclay inin thethe overlyingoverlying QuaternaryQuaternary strata strata and and limestone limestone strata, st cuttingrata, cutting hydraulic hydraulic connection connection between thebetween overlying the poreoverlying water pore and water the underlying and the underlying karst water, karst as shown water, inasFigure shown9 a.in Consequently,Figure 9a. Consequently, the karst water the karst has thewater property has the of confinedproperty water.of confined Only in water. some locations,Only in some water-proof locations, aquifer water-proof misses, forming aquifer skylight, misses, whereforming the skylight, pore water where is inthe close pore hydraulic water is in relationship close hydraulic with therelationship karst water. with The the confinedkarst water. karst The is mainlyconfined distributed karst is mainly in alluvial-diluvial distributed in plain alluvial-diluvial areas of Jinan. plain Such areas karst of water Jinan. is Such blocked karst by water igneous is rocksblocked in theby igneous north, forming rocks in horst the north, structure, forming leading hors tot springstructure, water leading groups. to spring The generalized water groups. model The is showngeneralized in Figure model9b. is shown in Figure 9b.

(a)

(b)

FigureFigure 9. 9. TheThe evolution evolution of confined of confined karst development karst development in Jinan: ( ina) Evolution Jinan: ( aprocess;) Evolution (b) Simplified process; (model.b) Simplified model.

4.4. Karst Development Characteristics along Jinan MetroMetro AtAt present,present, R1 lineline ofof JinanJinan metrometro hashas beenbeen successfullysuccessfully completed,completed, and R2 line are underunder construction.construction. Characteristics of of the karst developme developmentnt areas of the above lines have been counted, summarized,summarized, and analyzed in this paper, inin orderorder toto establishestablish mechanismsmechanisms ofof formingforming thethe karstkarst areaarea wherewhere JinanJinan metrometro runsruns through.through. Sustainability 2018, 10, x FOR PEER REVIEW 12 of 21 Sustainability 2018, 10, 3383 12 of 21 4.1. Number of Karst Caves at Various Heights 4.1. NumberAs for R1 of Karst line, Cavesthere are at Various 71 drill Heights holes exposed by karst caves among 145 exploratory holes, the proportion of which is 47.7%, and the total number of karst caves exposed is 129. Among them, 79 As for R1 line, there are 71 drill holes exposed by karst caves among 145 exploratory holes, the karst caves are smaller than 1 m and 50 larger than 1 m; 29 karst caves are larger than 2 m, 23 karst proportion of which is 47.7%, and the total number of karst caves exposed is 129. Among them, 79 karst caves larger than 3 m; 12 karst caves are larger than 5 m. During the section between Renjiazhuang caves are smaller than 1 m and 50 larger than 1 m; 29 karst caves are larger than 2 m, 23 karst caves Station and Lashan Station in R2 line of Jinan metro, there are 97 drill holes exposed with limestone, larger than 3 m; 12 karst caves are larger than 5 m. During the section between Renjiazhuang Station and 26 drill holes exposed with karst caves, with the proportion of 26.8%. The overall height of karst and Lashan Station in R2 line of Jinan metro, there are 97 drill holes exposed with limestone, and caves are shown in Table 2. 26 drill holes exposed with karst caves, with the proportion of 26.8%. The overall height of karst caves are shown in Table2. Table 2. Number of karst caves at various heights.

Range of Height(m)Table 2. Number Numbers of karst cavesof Karst at various Caves heights. Proportion (%) Range of Height(m)0–1 Numbers of Karst114 Caves60.32 Proportion (%) 1–2 34 17.99 0–1 114 60.32 1–22–3 3410 5.29 17.99 2–33–5 1016 8.47 5.29 3–55–7 168 4.23 8.47 5–77–9 83 1.59 4.23 7–9 3 1.59 9–11 2 1.06 9–11 2 1.06 11–1311–13 11 0.53 0.53 13–1513–15 11 0.53 0.53 In TotalIn Total 189 189 100.00 100.00

TheThe diagramdiagram ofof thethe karstkarst cavecave heightheight waswas drawn,drawn, asas shownshown inin FigureFigure 1010.. ItIt cancan bebe seenseen thatthat thethe cavescaves inin thethe karstkarst areaarea alongalong thethe Jinan metro areare mainlymainly small-small- andand medium-sized ones,ones, andand thatthat karstkarst cavescaves withwith heightheight lessless thanthan 22 mm accountaccount forfor 78.31%,78.31%, andand withwith heightheight betweenbetween 33 mm toto 55 mm accountaccount forfor aboutabout 10%.10%. In addition, a small number of large-sized cavescaves areare developed,developed, andand thethe maximummaximum heightheight ofof exposedexposed karstkarst cavecave isis 13.213.2 m.m.

Figure 10. Number of karst caves at various heights. Figure 10. Number of karst caves at various heights.

Sustainability 2018, 10, 3383 13 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 13 of 21

4.2. Filling Types of Karst Taking the exposed karst caves of R1 line as an example, there are 129 karst caves exposed by drill holes. Among Among them, there are 117 filled filled and semi-filled semi-filled caves, accounting for 90.7%, and 12 caves without fillings,fillings, accounting for about 9.3%. Thus, the majority of karst caves are filled filled and semi-filled semi-filled ones. The The fillings fillings are mainly cohe cohesivesive soil, earth and rubble. Amon Amongg them, them, 44 44 caves are filled filled with cohesive soil, accounting for 34.11%; 60 caves with earth soil, accounting for 46.51%; 13 caves with rubble, accounting for 10.08%; 12 caves with water or nothing, accounting for 9.3%. Karst caves with typical fillingsfillings andand without without fillings fillings can can be be seen seen in Figurein Figure 11. The11. The non-filled non-filled and water-filledand water-filled karst caveskarst cavesare mostly are mostly developed developed in deep in limestonedeep limestone strata. strata The water-filled. The water-filled karst caveskarst arecaves full are of full confined of confined water. water.The flow The of flow groundwater of groundwater accelerates accelerates the erosion the oferos karstion fissure of karst water, fissure greatly water, affecting greatly the affecting stability the of stabilityshield tunnels. of shield tunnels.

(a) (b)

(c) (d)

Figure 11. TypicalTypical fillings fillings of of karst karst caves: caves: (a) ( aCohesive) Cohesive soil soil filling; filling; (b) (Earthb) Earth filling; filling; (c) No (c) Nofilling; filling; (d) Water(d) Water filling. filling.

5. Study on Spatial Distribution Characteristi Characteristicscs of Karst Based on Bo Boreholerehole and Integrated Geophysical Exploration Geophysical Exploration 5.1. Exploration of Abnormal Karst along Jinan Metro 5.1. Exploration of Abnormal Karst along Jinan Metro In terms of conventional shallow karst area explorations, drilling exploration has obvious advantagesIn terms for of lithological conventional discrimination, shallow karst stratum area structure, explorations, andkey drilling area checking.exploration The has geophysical obvious advantagesexploration for has lithological large-scale discrimination, detecting area stratum and relatively structure, high and precision key area owing checking. to various The geophysical methods explorationcomplementing has eachlarge-scale other. Mostdetecting karst area areas and along relatively the Jinan high metro precision are covered owing with to Quaternaryvarious methods strata, complementing each other. Most karst areas along the Jinan metro are covered with Quaternary strata, with limestone developed strata and abundant groundwater below. In light of this, comprehensive with limestone developed strata and abundant groundwater below. In light of this, comprehensive selection of “shallow-deep” and “region-target”, which is suitable for exploration of karst areas selection of “shallow-deep” and “region-target”, which is suitable for exploration of karst areas along along Jinan metro, has been proposed. Firstly, Ground Penetrating Radar (GPR) method, which Jinan metro, has been proposed. Firstly, Ground Penetrating Radar (GPR) method, which has high has high precision and fast efficiency for close exploration, was adopted to detect the soil hole precision and fast efficiency for close exploration, was adopted to detect the soil hole and stratum distribution in the shallow overlying Quaternary strata. Secondly, considering the intense Sustainability 2018, 10, 3383 14 of 21

Sustainability 2018, 10, x FOR PEER REVIEW 14 of 21 and stratum distribution in the shallow overlying Quaternary strata. Secondly, considering the intensekarstification karstification of limest ofone limestone and rich andwater rich along water the alongJinan themetro Jinan lines, metro the high lines, density the high resistivity density resistivitymethod and method transient and transient electromagnetic electromagnetic method method were wereused useddue dueto the to the differences differences in in electrical conductivity and resistivity between surrounding rocksrocks and karst caves. The high density resistivity method is obviously effective on unfilled unfilled or water-filledwater-filled karst caves which have significantsignificant electrical difference with surrounding rock. The The rock rock resist resistivityivity is is determined determined by, by, apart from the mineral composition of of the the rock, rock, factors factors including including water water co contentntent and and temperature. temperature. When When there there are are cracks cracks or orfault fault zones zones in inthe the underground underground rock rock and and water-ri water-richch strata strata with with water-conducting channels, the resistivity of rocks in these areas will be significan significantlytly reduced, and obvious electrical differences with the surrounding strata will be presented. Since ea eachch geophysical method has its own advantages and corresponding disadvantages, disadvantages, it it is is often often difficult difficult to to meet meet the the requirements requirements of of accuracy accuracy by by applying applying a asingle single method. method. Nevertheless, Nevertheless, a combination a combination of multiple of multiple geophysical geophysical methods methods can complement can complement each eachother. other. The transient The transient electromagneti electromagneticc method method has obvious has obvious advantages advantages in finding in finding low-resistance low-resistance targets targetsamong amonghigh-resistance high-resistance geological geological bodies. bodies. The Theelectrical electrical and and electromagnetic electromagnetic methods methods were incorporated to define define abnormal karst areas. Finally, Finally, in the crucial karst areas identified identified by the above methods, the cross-hole resistivity CT method and laserlaser scanning were adopted for detailed detection. The comprehensive detectingdetecting methodmethod ofof “shallow-deep”“shallow-deep” andand “region-target”“region-target” isis shownshown inin FigureFigure 12 12..

Figure 12. Comprehensive exploration system along Jinan metro.

Taking thethe section section between between Wangfuzhuang Wangfuzhuang Station Station and and Dayangzhuang Dayangzhuang Station Station along along Jinan metroJinan R1metro line R1 as anline example, as an example, general general detection detection by methods by methods of GPR, high-densityof GPR, high-density resistivity resistivity and transient and electromagnetictransient electromagnetic was first conducted.was first conducted. Then, after Then, initial after interpretation initial interpretation of detecting of detecting results, detailed results, explorationdetailed exploration was carried was out carried for some out key for areas some with key cross-hole areas with resistivity cross-hole CT resistivity method and CT laser method scanning. and Thelaser corresponding scanning. The geophysicalcorresponding results geophysical can be seen results in Figure can be 13 a–e,seen respectively.in Figure 13a–e, Figure respectively. 13a shows theFigure potential 13a shows cave the through potential the cave GPR, through but the detectionthe GPR, but depth the candetection only reach depth about can only 10 m reach due toabout the absorption10 m due to effect the absorption in overlying effect soil. in According overlying tosoil. the According interpretation to the result interpretation of transient result electromagnetic of transient apparentelectromagnetic resistivity, apparent the karst resistivity, in the the area karst is strongly in the area developed, is strongly with developed, depth basically with depth below basically 15 m, highlybelow 15 coincident m, highly with coincident buried depthwith buried of the tunnel.depth of The the main tunnel. areas The of main karst areas development of karst development and zone are circledand zone by redare dashedcircled lineby red in Figure dashed 13 b.line Additionally, in Figure 13b. it is inferredAdditionally, through it theis inferred distribution through features the distribution features of apparent resistivity of five measuring lines that there are east-west karst and fracture zone in the middle of the detection area, as shown from AB-CD range in Figure 13c. In order to further determine the development size of the karst cave in the potential karst development zone, the detailed detection by cross-hole resistivity CT method was carried out, and Sustainability 2018, 10, 3383 15 of 21 of apparent resistivity of five measuring lines that there are east-west karst and fracture zone in the middle of the detection area, as shown from AB-CD range in Figure 13c.

SustainabilityIn order 2018 to, 10 further, x FOR PEER determine REVIEW the development size of the karst cave in the potential 15 karst of 21 development zone, the detailed detection by cross-hole resistivity CT method was carried out, and five abnormalfive abnormal karst karst caves caves were found.were found. Combined Combined with the with corresponding the corresponding geological geological and drill and hole drill analysis, hole itanalysis, is deduced it is thatdeduced the abnormalities that the abnormalities of R1-1, R1-2, of R1-1, R1-3, R1-2, and R1-3, R2-1 areand water-rich R2-1 are water-rich caves or fractured caves or areas,fractured and areas, that R2-2 and isthat a semi-filledR2-2 is a semi-filled cave, asshown cave, as in shown Figure in 13 Figured. The 13d. previous The previous geophysical geophysical survey conductedsurvey conducted 2d detection 2d fromdetection the whole from and the local whole areas. and In local order toareas. obtain In theorder three-dimensional to obtain the spatialthree- distributiondimensional ofspatial the karst distribution caves and of provide the karst more ca accurateves and guidanceprovide more to the accurate construction guidance department, to the CALSconstruction (Cavity department, Auto Scanning CALS Laser (Cavity System) Auto made Scanning by MDL Laser company System) made was adopted by MDL to company carry out was 3d laseradopted scanning to carry of theout caves3d laser through scanning two of , the caves andthrough the location two boreholes, of the and the waslocation marked of the in Figureborehole 12 d.was The marked specific in karstFigure cave 12d. morphology The specific are karst shown cave in morphology Figure 13e. Comparedare shown within Figure transient 13e. electromagneticCompared with detecting transient results, electromagnetic the cross-hole detectin resistivityg results, CT and the scanning cross-hole method resistivity achieves detailedCT and detectionscanning method of water-rich achieves caves. detailed In terms detection of overall of wate distributionr-rich caves. of resistivity,In terms of compared overall distribution with the CT of results,resistivity, the compared left low-resistance with the area CT results, is larger the than left the low-resistance right, and the area buried is larger depth than of karst the right, development and the isburied shallow depth on theof karst left and development deep on the is right. shallow The on results the left shows and that deep multiple on the detectionright. The methods results shows are in goodthat multiple agreement. detection methods are in good agreement.

(a)

(b)

Figure 13. Cont. Sustainability 2018, 10, 3383 16 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 16 of 21

(c)

(d)

Figure 13. Cont. Sustainability 2018, 10, 3383 17 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 17 of 21

(e)

Figure 13. ResultsResults (a) ( aGPR) GPR method; method; (b) (bCross-section) Cross-section of high-density of high-density electrical electrical method; method; (c) (Comprehensivec) Comprehensive interpretation interpretation of oftransient transient electromagnetic electromagnetic method; method; (d (d) )Cross-section Cross-section of of cross-hole resistivity CT method; (e) Scanning results.

5.2. Depth of Karst Caves 5.2. Depth of Karst Caves The depth of karst caves along Jinan metro lines are shown in Table3, and the corresponding The depth of karst caves along Jinan metro lines are shown in Table 3, and the corresponding curve was drawn, as shown in Figure 14. As can be seen from the figure that most depths of karst curve was drawn, as shown in Figure 14. As can be seen from the figure that most depths of karst caves of Jinan metro R1 line are in the range of −25–0 m, accounting for 91.46%, and that depths caves of Jinan metro R1 line are in the range of −25~0 m, accounting for 91.46%, and that depths below below −25 m account for 8.54%, indicating that development mainly took place in shallow karst. −25 m account for 8.54%, indicating that development mainly took place in shallow karst. To be To be specific, karst development concentrates on depths between −20 m–−15 m, accounting for specific, karst development concentrates on depths between −20 m~−15 m, accounting for 31.78%, 31.78%, followed by the range of −15 m–−10 m, which accounts for 23.26%. The proportions of karst followed by the range of −15 m~−10 m, which accounts for 23.26%. The proportions of karst caves caves with depth between −10 m and −5 m, and −25 m and −20 m are basically the same, while with depth between −10 m and −5 m, and −25 m and −20 m are basically the same, while the percentagethe percentage of caves of caves developed developed in the in Quaternary the Quaternary strata strata and deep and limestone deep limestone areas is areas extremely is extremely small. Thesmall. development The development regularity regularity of karst ofcaves karst along caves R2 along line shows R2 line the shows same the trend same with trend that with of R1 that line. of ThisR1 line. is mainly This is because mainly becausethe erosion the erosioneffect of effect groundwater of groundwater activity activity is intense is intense in shallow in shallow karst areas. karst − − Theareas. depths The depths of shield of shield tunnel tunnel are basically are basically between between −20 m20 mand and −10 10m, m,falling falling into into the the range range with concentrated karstkarst development.development. Therefore, Therefore, under under the the long-term long-term groundwater groundwater action, action, the scalethe scale of karst of karstcave maycave bemay further be further expanded, expanded, affecting affecting the stability the stability of shield of shield tunnel. tunnel.

Table 3. Number of karst caves in various depths. Table 3. Number of karst caves in various depths. − Depth RangeDepth ( Rangem) (−m) Number Number of of Karst Karst Caves Caves Proportion Proportion (%) (%) 0–50–5 22 1.06 1.06 5–10 25 13.23 5–10 25 13.23 10–15 34 17.99 15–2010–15 4734 17.99 24.87 20–2515–20 2847 24.87 14.81 25–3020–25 1128 14.81 5.82 30–3525–30 711 5.82 3.7 35–40 6 3.17 In Total30–35 1897 3.7 100.00 35–40 6 3.17 In Total 189 100.00 Sustainability 2018, 10, 3383 18 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 18 of 21

Figure 14. Number of karst caves in various depths. Figure 14. Number of karst caves in various depths. 5.3. Spatial Relationship between Caves and Shield Tunnels 5.3. Spatial Relationship between Caves and Shield Tunnels According to situation of karst caves exposed along the Jinan Metro, the analysis of the positional relationshipAccording between to situation the tunnel of karst and caves the cavesexposed was along carried the Jinan out and Metro, the datathe analysis in detail of can the bepositional seen in Tablerelationship4 and Figure between 15. Thethe tunnel distribution and the is listedcaves aswas follows: carried out and the data in detail can be seen in Table 4 and Figure 15. The distribution is listed as follows: (1) There are 29 exposed karst caves above the runnel roof, accounting for 22.48% of the total number. Among them,Table 20 karst 4. Statistics caves are of withinrelative the position range of of karst 0.5 time caves of and the shield cave diametertunnels. above the tunnel roof; 7 karst caves are within 0.5 to 1 time of the cave diameter; 2 karst caves are within 1 to 2 Distance between Caves and Tunnels Size of Caves Relationtimes ofbetween the cave Caves diameter. and Tunnels There are 21 exposed karst caves less than 1 m above the tunnel roof, Time of Cave Diameter Number Height (m) Number 8 caves larger than 1 m, 3 caves larger than 2 m, and 2 caves larger than 3 m. ≤0.5 D 20 ≤1 21 (2) There are 65 exposed karst caves within the0.5~1 range D of tunnel, accounting 7 for 1~2 50.39% of the 5 total Caves above tunnel number. Among them, 34 exposed karst caves1~2 are D less than 1 m, 31 2 caves larger 2~3 than 1 m, 19 1 caves larger than 2 m, and 15 caves larger than 3 m.>2 D 0 >3 2 (3) There are 35 exposed karst caves below the tunnel floor, accounting for 27.13%≤ of1 the total number.34 1~2 12 AmongCaves them, in 18 tunnel karst caves are within the range of - 0.5 time of the cave diameter below the tunnel floor; 11 karst caves are within 0.5 to 1 time of the cave diameter; 4 karst2~3 caves are4 within 1 to 2 times of the cave diameter; 2 karst caves are within 2 or more times of>3 the cave diameter15 ≤0.5 D 18 ≤1 24 below the tunnel floor. There are 24 exposed karst caves less than 1 m below the tunnel floor, 0.5~1 D 11 1~2 4 Caves below tunnel 11 caves larger than 1 m, 7 caves larger than1~2 2 m, D and 6 caves larger 4 than 3 m. 2~3 1 >2 D 2 >3 6 Table 4. Statistics of relative position of karst caves and shield tunnels.

Distance between Caves and Tunnels Size of Caves Relation between Caves and Tunnels Time of Cave Diameter Number Height (m) Number ≤0.5 D 20 ≤1 21 0.5–1 D 7 1–2 5 Caves above tunnel 1–2 D 2 2–3 1 >2 D 0 >3 2 ≤1 34 1–2 12 Caves in tunnel - 2–3 4 >3 15 ≤0.5 D 18 ≤1 24 0.5–1 D 11 1–2 4 Caves below tunnel 1–2 D 4 2–3 1 >2 D 2 >3 6 SustainabilitySustainability 20182018,, 1010,, x 3383 FOR PEER REVIEW 19 of 21

FigureFigure 15. 15. RelativeRelative position position of of karst karst caves caves and and shield shield tunnels. tunnels. 6. Conclusions and Discussion (1) There are 29 exposed karst caves above the runnel roof, accounting for 22.48% of the total number.Based on Among the systematic them, 20 analysis karst caves of the are hydrogeological within the range characteristics of 0.5 time of the of Jinancave diameter area, this above paper studiesthe thetunnel karst roof; development 7 karst caves conditions are within and 0.5 mechanismsto 1 time of the in cave areas diameter; where Jinan 2 karst Metro caves runs are through. within We try1 to to 2 propose times of a the comprehensive cave diameter. exploration There are system21 exposed for karst karst area caves and less reveal than the 1 m karst above development the tunnel scale,roof, filling 8 caves types, larger and spatialthan 1 distributionm, 3 caves larger features than along 2 m, theand Jinan 2 caves Metro larger lines. than 3 m. (2) ThereThe topography are 65 exposed and geomorphology, karst caves within lithology the range distribution, of tunnel, hydrogeology, accounting for and 50.39% tectonic of influencethe total of Jinannumber. are systematically Among them, analyzed.34 exposed With karst reference caves are to less the than geological 1 m, 31 characteristics caves larger than of Jinan 1 m, and 19 the karstcaves development larger than 2 conditions,m, and 15 caves the karst larger development than 3 m. conditions and feedback mechanism along (3)Jinan There Metro are lines 35 wereexposed analyzed. karst caves Furthermore, below the formation tunnel floor, and developmentalaccounting for mechanism27.13% of the of directtotal karst,number. indirect Among karst and them, confined 18 karst karst caves were are proposed.within the Accordingly,range of 0.5 time when of Jinanthe cave Metro diameter run through below the indirectthe tunnel karst floor; area, 11 itkarst needs caves tobe are noted within that 0.5 karst to 1 time collapse of the can cave cause diameter; great damage 4 karst tocaves surface are buildings,within roads, 1 to 2 andtimes pedestrians. of the cave In diameter; the area of2 pressure-bearingkarst caves are within karst distribution,2 or more times due of to the a certain cave waterdiameter pressure below in the the cave, tunnel special floor. attention There are should 24 exposed be paid karst to protecting caves less the than thickness 1 m below of the the rock tunnel wall duringfloor, the 11 shield caves tunneling. larger than The 1 geologicalm, 7 caves analysislarger than can 2 provide m, and a6 geologicalcaves larger basis than to 3 the m. planning and design of metro line and the safety of shield tunnel in Jinan. 6. ConclusionsThe karst and development Discussion scale and filling type characteristics along Jinan Metro are analyzed. The results show that the karst caves are mainly small- and medium-sized. It needs to be noted that Based on the systematic analysis of the hydrogeological characteristics of Jinan area, this paper there are several large-scale and water-filled caves along the subway. Due to confined water and studies the karst development conditions and mechanisms in areas where Jinan Metro runs through. weakening effect on rocks, it can easily cause water inrush hazard during shield tunneling. We try to propose a comprehensive exploration system for karst area and reveal the karst According to karst development characteristics in areas where Jinan Metro runs through, development scale, filling types, and spatial distribution features along the Jinan Metro lines. a “shallow-deep” and “region-target” comprehensive exploration system of karst caves was proposed. The topography and geomorphology, lithology distribution, hydrogeology, and tectonic The system solves the problem that a single method cannot meet the detection precision needs influence of Jinan are systematically analyzed. With reference to the geological characteristics of Jinan especially in complex geological conditions. The system makes full use of the advantages of multiple and the karst development conditions, the karst development conditions and feedback mechanism methods. Therefore, the potential karst caves in the shallow and regional along the Jinan subway along Jinan Metro lines were analyzed. Furthermore, formation and developmental mechanism of line can be effectively explored. Furthermore, for the key areas, the refined exploration method has direct karst, indirect karst and confined karst were proposed. Accordingly, when Jinan Metro run been adopted to achieve the quantitative acquisition and visual characterization of karst cave. The through the indirect karst area, it needs to be noted that karst collapse can cause great damage to results demonstrate that laser scanning is an effective “target” method to reconstruct the shape of surface buildings, roads, and pedestrians. In the area of pressure-bearing karst distribution, due to a irregular karst caves based on the “area” exploration, which can determine the accurate parameters certain water pressure in the cave, special attention should be paid to protecting the thickness of the including position, depth and volume, and so on. These parameters can be beneficial for the efficient rock wall during the shield tunneling. The geological analysis can provide a geological basis to the and high-quality treatment of karst caves. Compared to single method, the system proposed in this planning and design of metro line and the safety of shield tunnel in Jinan. Sustainability 2018, 10, 3383 20 of 21 paper can improve the detection efficiency and accuracy, which will provide guidance for similar metro projects. Based on the comprehensive exploration system, the spatial positional relation between karst development elevation and the shield tunnel was statistically analyzed. The results show that karst caves in the range of tunnel structure account for about 50.39%, that karst caves above tunnel structure account for about 22.48%, and that karst caves below tunnel structure account for about 27.13%, respectively. When the karst cave is overlying above the tunnel, it is easy to cause karst collapse. When the cave is situated under the tunnel, excessive settlement of the karst foundation is likely to occur. Water inrush hazard can occur when the rock thickness between the tunnel face and the cave is too small. Therefore, based on the statistics of the positional relationship between the cave and the tunnel, we can take preventive measures to ensure the safety of the project in advance accordingly.

Author Contributions: Methodology, S.S. (Shangqu Sun); Writing-Original Draft Preparation, L.L.; Investigation, J.W.; Validation, S.S. (Shuguang Song); Data Curation, Z.F. and X.B.; Writing-Review & Editing, S.S. (Shaoshuai Shi) and H.J. Funding: This research was funded by [National Science Fund for Excellent Young Scholars] grant number [51722904]; [National Natural Science Foundation Item] grant number [51679131] and [51479106]; [National Natural Science Foundation of China] grant number [51809158] and [51709160]; [Shandong Province Science Foundation for Distinguished Young Scholars] grant number [JQ201611]; [China Postdoctoral Science Foundation] grant number [2018M630780]; [Shandong Provincial Natural Science Foundation, China] grant number [ZR2017QEE009] and [ZR2018BEE045]. Conflicts of Interest: The authors declare no conflict of interest.

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