Safety Assessment of Tunnel Portals for Site Selection Based on Spatial Information Geoprocessing

Article Safety Assessment of Tunnel Portals for Site Selection Based on Spatial Information Geoprocessing

Iau-Teh Wang

Department of Civil Engineering, R.O.C. Military Academy, Kaohsiung 83059, Taiwan; [email protected]

Received: 11 September 2019; Accepted: 7 November 2019; Published: 11 November 2019

Abstract: The evaluation of portal locations for mountain tunnels is among the most crucial considerations during route selection and structural layout planning. The development of spatial information technology has provided a more objective approach for assessing the slope stability of potential portal sites. The simulations in such studies have been performed to evaluate potential hazards and slope stability. However, potential instabilities resulting from excavation are seldom considered in these studies. Therefore, a method based on spatial information technology was developed in this study for considering the potential impact of the direction and depth of excavations on portal stability. An analysis method for an inﬁnite slope was integrated into the geographical information system for evaluating the stability of critical wedges. The proposed method provides a reasonable estimation comparable with that provided by the conventional slice method. The results of applying this method to six mountain tunnel portals where slope instability occurred during construction indicate that the actual outcomes agreed with the predicted outcomes. For potential portal site evaluation, the proposed method facilitates the rapid estimation of safety factors for various slope designations, which is useful for site selection.

Keywords: site selection; safety factors; slope stability; tunnel portal location; limit equilibrium method; potential hazards assessment

1. Introduction Portal sections connect the inside and outside of tunnels. As shown in Figure1, a portal section is typically considered a boundary location with 1–2 D (D is the tunnel diameter) of coverage from the entrance, although it varies depending on geological conditions [1–3]. A portal section is a sensitive environment typically located on a gentle slope with a thin-covering rock layer composed of weather-resistant materials. Portal stability is markedly inﬂuenced by geographical, geological, meteorological, and hydrological factors [4–8]. Wang and Huane [9] studied tunnel constructions in Taiwan and reported that landslides and slope collapses can be detrimental to the success of portal constructions. Therefore, selecting appropriate portal locations has become crucial in planning and designing mountain tunnels. The slope at the entrance may be in a natural state of equilibrium as a result of long-term exposure to external forces such as weathering, erosion, and mass movement. However, excavation works following the commencement of a project can alter the original topography, thereby unbalancing the equilibrium and inducing landslides. Simultaneously, slope collapses typically occur as a result of insuﬃcient coverage. Reasons for the aforementioned situations primarily depend on the relationship between the slip surface and the portal section. Even where tunnel portals are not proximal to the location of landslides, excavation work may lead to fractures in the stratum, thereby allowing rainwater or underground water to seep into the stratum, which can lead to project failure. Therefore, slope stability is a key factor for successful portal section construction.

Infrastructures 2019, 4, 70; doi:10.3390/infrastructures4040070 www.mdpi.com/journal/infrastructures Infrastructures 2019, 4, x FOR PEER REVIEW 2 of 11 stratum,Infrastructures thereby2019, 4 allowing, 70 rainwater or underground water to seep into the stratum, which can 2lead of 11 to project failure. Therefore, slope stability is a key factor for successful portal section construction.

FigureFigure 1. 1. SchematicSchematic diagram diagram for for standard standard portal portal section section ranges. ranges. The The portal portal sect sectionion varies varies depending depending onon the the geological geological conditions. conditions.

EntranceEntrance slope slope stability stability is is a a key key consideration consideration during during portal portal location location planning planning and and related related constructionconstruction design. design. Approaches Approaches for for evaluating evaluating slope slope stability stability can can be be classified classified as as qualitative qualitative or or quantitative.quantitative. Based onon sitesite investigationinvestigation and and survey survey results, results, qualitative qualitative evaluations evaluations utilize utilize expert expert and andengineer engineer experiences experiences to assess to assess the characteristics the characterist of aics portal of aarea. portal Quantitative area. Quantitative evaluation evaluation involves involvescalculating calculating the slope the stability slopeas stability a function as a of functi forceon or of moment force or equilibrium, moment equilibrium, and the limit and equilibrium the limit equilibriummethod (LEM) method is most (LEM) frequently is most used frequently for such used analyses. for such The analyses. LEM incorporates The LEM incorporates other methods other in methodsaddition in to theaddition conventional to the conventional slice method, slice which method, is well-established. which is well-established. The LEM method The LEM first involvesmethod firstdetermining involves the determining critical slope the section critical for conducting slope section stability for analysis, conducting and representativestability analysis, parameters and representativeare subsequently parameters obtained are through subsequently field or laboratoryobtained through tests. field or laboratory tests. Previously,Previously, preliminary preliminary planning planning for for site site selection selection was was undertaken undertaken with with the the assistance assistance of of experiencedexperienced engineers. engineers. Initial Initial site site selection selection was was established established through through field field reconnaissance reconnaissance to to obtain obtain informationinformation for for conducting conducting subseq subsequentuent analyses and design processes.processes. This This typically typically requires requires considerableconsiderable manpower manpower and and is is a a highly highly time-con time-consumingsuming task. task. To To conserve conserve time time and and manpower, manpower, toolstools such such as as aerial aerial photographs photographs and and geological geological and and topographic topographic maps maps are are currently currently employed employed for for preliminarypreliminary site site interpretation, interpretation, to to predict predict potent potentialial hazards hazards and and avoid avoid unstable unstable geological geological regions regions whilewhile selectingselecting potential potential actionable actionable engineering engineering sites. Recently,sites. Recently, the rapid the development rapid development of geographical of geographicalinformation systeminformation (GIS) technologysystem (GIS) has technology enabled the has precise enabled evaluation the precise of geologic evaluation hazards of geologic [10–13]. hazardsGIS technology [10–13]. also GIS provides technology an eff ectivealso provides analytical an tool effective for the large-scale analytical assessment tool for the of geographical large-scale assessmentinformation of to geographical conduct rapid information spatial data to analysis conduct [14 rapid,15]. Favorablespatial data results analysis have [14,15]. been attainedFavorable by resultsstudies have in which been GISattained data by were studies used in to which predict GIS specific data slopewere used failure to incidentspredict specific [16]. The slope increased failure incidentsavailability [16]. of GISThe dataincreased in tunneling availability industries of GIS also data provides in tunneling strong industries functionality also in provides data processing strong functionalityand analyses in [17 data]. However, processing these and studies analyses have [17] seldom. However, considered these studies potential have instabilities seldom considered resulting potentialfrom excavation. instabilities resulting from excavation. WeWe considered incorporating the the excavation excavation depth depth and and sliding sliding range range into mechanical analysis analysis forfor calculating calculating the the entrance entrance slope slope safety safety factor factor ( (FSFS)) after after commencing commencing excavations, excavations, and and subsequently subsequently appliedapplied this this method method for the preliminary assessment of locationlocation selection.selection. By By rapidly rapidly evaluating evaluating potentialpotential portal sites,sites, thisthis study study provides provides a GIS-baseda GIS-based method method that that integrates integrates the slice the method,slice method, which whichis potentially is potentially most widelymost widely used used for estimating for estimating slope slope stability stability during during the planningthe planning stage. stage. Based Based on onspecific specific assumptions assumptions regarding regarding the excavationthe excavation depth depth and the and potential the potential slip surface, slip surface, the safety the factor safety of factora specific of a location specific can location be calculated can be bycalculated referring by to variousreferring excavation to various orientations. excavationThe orientations. minimal safety The minimalfactor can safety then factor be applied can then and be considered applied and a critical considered value fora critical evaluating value stability.for evaluating stability. TheThe followingfollowing sectionsection introduces introduces the the proposed proposed methodology methodology to facilitate to facilitate rapid selection rapid selection of potential of potentialsites for mountainsites for mountain tunnel portals. tunnel Subsequently,portals. Subseq sixuently, portals six are portals examined are examined to validate to the validate proposed the proposedmethod. Themethod. influence The of influence crucial assumptions of crucial isassumptions also addressed is also by comparing addressed the by proposed comparing method the proposedwith the conventional method with slice the conventional method. slice method. Infrastructures 2019, 4, 70 3 of 11 InfrastructuresInfrastructures 2019 2019, ,4 4, ,x x FOR FOR PEER PEER REVIEW REVIEW 33 of of 11 11

2. Research Methods 2.2. Research Research Methods Methods A method for selecting a tunnel portal site was developed based on spatial information concepts. AA methodmethod forfor selectingselecting aa tunneltunnel portalportal sitesite waswas developeddeveloped basedbased onon spatialspatial informationinformation Theconcepts.concepts. terrain TheThe of target terrainterrain areas, ofof targettarget excavation areas,areas, depth, excavationexcavation direction, depth,depth, sliding direction,direction, range, slidingsliding and mechanical range,range, andand properties mechanicalmechanical of geologicpropertiesproperties materials of of geologic geologic are considered.materials materials are are considered. considered. 2.1. Slope-Cutting Ambit and Critical Excavation Depth of Portals 2.1.2.1. Slope-Cutting Slope-Cutting Ambit Ambit and and Critic Criticalal Excavation Excavation Depth Depth of of Portals Portals For a specific area, the slope-cutting ambit for a portal is influenced by the relative entrance ForFor aa specificspecific area,area, thethe slope-cuttingslope-cutting ambitambit forfor aa portalportal isis influencedinfluenced byby thethe relativerelative entranceentrance elevation and the slope (vertical vs. horizontal) used for cutting. The slope-cutting ambit is related elevationelevation and and the the slope slope (vertical (vertical vs vs. .horizontal) horizontal) used used for for cutting. cutting. The The slope-cutting slope-cutting ambit ambit is is related related to to to the critical excavation depth, the relative height from the cutting point height to the elevated thethe critical critical excavation excavation depth, depth, the the relative relative height height from from the the cutting cutting point point height height to to the the elevated elevated portal portal portal entrance. To construct stable mountain portals and reduce the environmental impact, it is entrance.entrance. ToTo constructconstruct stablestable mountainmountain portalsportals andand reducereduce thethe environmentalenvironmental impact,impact, itit isis recommended to minimize landslides induced by excavation. recommendedrecommended to to minimize minimize lands landslideslides induced induced by by excavation. excavation. The cutting ambit of slopes is considered for evaluating potential portal sites, and the critical TheThe cutting cutting ambit ambit of of slopes slopes is is considered considered for for eval evaluatinguating potential potential portal portal sites, sites, and and the the critical critical excavation depth is also considered in the proposed GIS-based method. For a typical double-lane excavationexcavation depthdepth isis alsoalso consideredconsidered inin thethe propproposedosed GIS-basedGIS-based method.method. ForFor aa typicaltypical double-lanedouble-lane highway tunnel with a width and height of 10–12 and 6–7 m, respectively, the critical excavation depth highwayhighway tunneltunnel withwith aa widthwidth andand heightheight ofof 10–1210–12 andand 6–76–7 m,m, respectively,respectively, thethe criticalcritical excavationexcavation is approximately 10–15 m [18]. Thus, the initial value for calculating the slope stability was set at 15 m, depthdepth is is approximately approximately 10–15 10–15 m m [18]. [18]. Thus, Thus, the the initial initial value value for for calculating calculating the the slope slope stability stability was was set set and this value can be adjusted to fit dynamic field conditions if necessary. The excavation height of atat 15 15 m, m, and and this this value value can can be be adjusted adjusted to to fit fit dynamic dynamic field field cond conditionsitions if if necessary. necessary. TheThe excavation excavation each stage was 5 m, as shown in Figure2. heightheight of of each each stage stage was was 5 5 m, m, as as shown shown in in Figure Figure 2. 2.

Figure 2. Schematic diagram for excavation slope of portal section. FigureFigure 2. 2. Schematic Schematic diagram diagram for for excavation excavation slope slope of of portal portal section. section. If the critical excavation depth is known, the slice method is used to conduct a series of calculations IfIf thethe criticalcritical excavationexcavation depthdepth isis known,known, thethe sliceslice methodmethod isis usedused toto conductconduct aa seriesseries ofof to determine the potential sliding surface with a minimal safety factor for the cutting slope. The calculationscalculations toto determinedetermine thethe potentialpotential slidingsliding surfacesurface withwith aa minimalminimal safetysafety factorfactor forfor thethe cuttingcutting potential sliding mass size is typically proportional to the excavation height (hc). For simplicity, this slope.slope. TheThe potentialpotential slidingsliding massmass sizesize isis typicallytypically proportionalproportional toto thethe excavationexcavation heightheight ((hchc).). ForFor study assumed that the sliding surface was planar and that it passed the slope toe. The distance simplicity,simplicity, this this study study assumed assumed that that the the sliding sliding surface surface was was planar planar and and that that it it passed passed the the slope slope toe. toe. between the highest cutting point and the crown of the sliding mass d is proportional to the excavation TheThe distance distance between between the the highest highest cutting cutting point point and and the the crown crown of of the the sliding sliding mass mass dd isis proportional proportional depth. The slip area of the slope is d = tan β hc. The β can be arbitrarily speciﬁed to meet ﬁeld toto thethe excavationexcavation depth.depth. TheThe slipslip areaarea ofof thethe× slopeslope isis dhcdhc=×=×tantanββ . . TheThe ββ cancan bebe arbitrarilyarbitrarily conditions, and assumed tan β= tan 45◦ = 1 for the initial calculation performed in this study, as ββ  == shownspecifiedspecified in Figuretoto meetmeet3. fieldfield conditions,conditions, andand assumedassumed tantan =tan45 =tan45 1 1 forfor thethe initialinitial calculationcalculation performedperformed in in this this study, study, as as shown shown in in Figure Figure 3. 3.

Figure 3. Schematic diagram for slip surface of slope excavation. FigureFigure 3. 3. Schematic Schematic diagram diagram for for slip slip surface surface of of slope slope excavation. excavation.

ForFor aa potentialpotential site,site, thethe portalportal directiondirection cancan bebe plannedplanned toto includeinclude sitessites withwith favorablefavorable slopeslope stability.stability. EveryEvery possiblepossible cuttingcutting directiondirection shouldshould bebe assessedassessed byby consideringconsidering topography.topography. InIn thisthis Infrastructures 2019, 4, 70 4 of 11

For a potential site, the portal direction can be planned to include sites with favorable slope stability.Infrastructures Every 2019, possible 4, x FOR PEER cutting REVIEW direction should be assessed by considering topography. In this study,4 of 11 the safety factor of cut slopes was assessed for eight directions with a 45◦ interval (north, northeast, east,study, southeast, the safety south, factor southwest, of cut slopes west, was and assessed northwest), for eight as shown directions in Figure with4 .a The 45° safetyinterval factor (north, of thenortheast, eight excavation east, southeast, directions south, was southwest, calculated, west, and and the northwest), lowest safety as factorshown was in Figure then applied 4. The assafety the representativefactor of the eight safety excavation factor. directions was calculated, and the lowest safety factor was then applied as the representative safety factor.

Figure 4. Schematic diagram of aspects of slope excavation at portal.portal. The cutting slope safety factor

(FS)) inin eighteight didifferentﬀerentdirections. directions. Each Each assessing assessing area area was was set set at at every every 45 45°◦ interval interval and and was was divided divided as as a slopea slope aspect, aspect, clockwise. clockwise. p p symbolizes symbolizes positive positive and and n n symbolizes symbolizes negative. negative. x x is is x-axis x-axis and and y y is is y-axis. y-axis.

2.2. Slope Stability Evaluation Using the Limit Equilibrium Method For analyzing the slope stability of a potential site relative to the critical excavation depth and potential planarplanar slidingsliding surface, surface, the the LEM LEM considers considers gravity gravity and and motion. motion. For For all all shear-type shear-type failures, failures, the geologicalthe geological material material can becan assumed be assumed to follow to follow the Mohr–Coulomb the Mohr–Coulomb failure failure criterion, criterion, in which in which the shear the strengthshear strength is a function is a function of cohesion of cohesion and the and friction the friction angle. angle. The FSFSfor for a a potential potential sliding sliding wedge wedge (Figure (Figure5) can 5)be ca analyzedn be analyzed using theusing ratio the between ratio between the driving the force acting on the sliding surface f and the resistant force of the plane fr. The FS calculation involves driving force acting on the slidingd surface fd and the resistant force of the plane fr . The FS resolving fd into action components parallel and perpendicular to the sliding surface. In other words, calculation involves resolving fd into action components parallel and perpendicular to the sliding the angle between the failure plane and the horizontal plane θ, the total length of the sliding surface L, surface. In other words, the angle between the failure plane and the horizontal plane θ , the total and the weight of the wedge above the sliding surface W can be applied to obtain the resisting and length of the sliding surface L , and the weight of the wedge above the sliding surface W can be driving forces on the sliding plane as follows: applied to obtain the resisting and driving forces on the sliding plane as follows:

fr =f W=×WcLcoscosθθφ ×tan tanφ + +cL,, (1) r × × (1)

fd = W=×sin θθ, (2) fWd × sin , (2) where c and φ are the apparent cohesion and friction angle of geomaterials, respectively. where c and φ are the apparent cohesion and friction angle of geomaterials, respectively. The stability of the block shown in Figure5 can be quantiﬁed based on the relationship between The stability of the block shown in Figure 5 can be quantified based on the relationship between the resisting forces (S), normal forces (N) and weight of the sliding surface (W), which is known as the the resisting forces (S), normal forces (N) and weight of the sliding surface (W), which is known as safety factor. Thus, the equation for the excavation can be expressed as: the safety factor. Thus, the equation for the excavation can be expressed as: S =SNN=×tantanφφ+ +cL cL, (3) × , (3) hc AB == hc , (4) AB cos(90◦ β) , (4) cos() 90°−− β

BE=− ABsin ()β θ , (5)

LAFAB==cos()β −+θδ BE cot (), (6) Infrastructures 2019, 4, 70 5 of 11

Infrastructures 2019, 4, x FOR PEER REVIEW BE = AB sin(β θ),5 of (5) 11 − L = AF = AB cos(β θ) + BE cot(δ), (6) −1 W=×Δγγ ABF =××1 AF × BE , (7) W = γ ∆ABF = 2 γ AF BE, (7) × 2 × × × ××+θφ+ fr fr WWcLcoscosθ tan tanφ cL FS =FS === × × (8)(8) fd fWW ×sinsinθθ d × where γ is dry unit weight of geomaterials. where γ is dry unit weight of geomaterials.

Figure 5. Schematic free body diagram for the assumed analyzed wedge based on the critical excavation Figure 5. Schematic free body diagram for the assumed analyzed wedge based on the critical depth and the potential planar sliding surface. The derived equation of the potential sliding wedge can excavation depth and the potential planar sliding surface. The derived equation of the potential be quantiﬁed by the relation between the resisting and driving forces, which is termed the FS. sliding wedge can be quantified by the relation between the resisting and driving forces, which is Figuretermed6 the shows FS. a ﬂowchart of the proposed methodology. The limit equilibrium analysis was coupled with a GIS. Based on the LEM, all factors employed in this study were transformed into Figure 6 shows a flowchart of the proposed methodology. The limit equilibrium analysis was grid spatial data using the GIS. A spatial model and a digital terrain model (DTM) in the GIS were coupled with a GIS. Based on the LEM, all factors employed in this study were transformed into grid employed to identify the excavation direction and dip inclination. The safety factor distribution map spatial data using the GIS. A spatial model and a digital terrain model (DTM) in the GIS were were converted into the vector format with the descriptions of types and units linked to the map using employed to identify the excavation direction and dip inclination. The safety factor distribution map the built-in relational database management system in the GIS software. Geometric factors of slope, were converted into the vector format with the descriptions of types and units linked to the map such as gradient and aspect, were calculated based on a DTM for Hualien, Taiwan, provided by the using the built-in relational database management system in the GIS software. Geometric factors of Institute of Aerial Survey for Agriculture and Forestry of Taiwan. The resolution of the DTM grid is slope, such as gradient and aspect, were calculated based on a DTM for Hualien, Taiwan, provided 5 5 m. by× the Institute of Aerial Survey for Agriculture and Forestry of Taiwan. The resolution of the DTM grid is 5 × 5 m. Infrastructures 2019, 4, 70 6 of 11 Infrastructures 2019, 4, x FOR PEER REVIEW 6 of 11

Figure 6. Flowchart showing the various phases of the performed methodology and all of the factors Figure 6. Flowchart showing the various phases of the performed methodology and all of the factors that were transformed into a grid-type spatial database. The safety factor distribution map and the that were transformed into a grid-type spatial database. The safety factor distribution map and the description of types and units were stored in tables linked to the map using the built in relation data description of types and units were stored in tables linked to the map using the built in relation data base management system of the GIS. base management system of the GIS. 3. Application of the Proposed Method 3. Application of the Proposed Method To validate the proposed method, the slope stabilities of six portals constructed during a tunneling projectTo werevalidate tested. the The proposed study area method, is located the inslope Eastern stab Taiwanilities of (Figure six portals7). The constructed topography during is highly a variable,tunneling and project south-facing were tested. slopes The tend study to be steeperarea is thanlocated other in slopes. Eastern Three Taiwan tunnels (Figure (Tunnels 7). 1,The 2, topographyand 3) were constructedis highly variable, for a highway and south-facing renovation slopes project. tend Among to be the steeper six portals than ofother the slopes. three tunnels, Three tunnelsthe south (Tunnels portal of 1, Tunnel 2, and 2 and3) were the south constructed and north for portals a highway of Tunnel renovation 3 suffered project. from slope Among instabilities the six portalsduring construction.of the three tunnels, the south portal of Tunnel 2 and the south and north portals of Tunnel 3 sufferedFigure from8 shows slope instabilities the geological during map construction. of the study area. The main stratum units include the Tuluanshan formation, Shuilian Conglomerate formation, Fanshuliao formation, Paliwan formation, and alluvial deposit.Table Table 1. Mechanical1 lists the parameters mechanical for parametersdistinct formation and in average study area. unit weights of these formations. The averageFormation unit weight was back-calculatedrock/soil type fromc (kPa) actual φ landslide(°) γ (kN/m areas3) in the study area and from laboratory test results [15]. The DTM was used to calculate the distribution condition of the Alluvial Deposit gravel, sand, clay 8.8 20 15 gradient and aspect of the slope. Subsequently, based on the grid resolution, the parameters of Equation Fanshuliao shale 11 25 21.62 (8) and safety factors were calculated using a database management system in the GIS software. Paliwan conglomerate 15 30 23.35 Tuluanshan conglomerate 10 35 22.13 Table 1. Mechanical parameters for distinct formation in study area. Shuilian Conglomerate conglomerate 24 29 22.78 3 Figure 8 showsFormation the geological map Rock of/Soil the Type study area.c (kPa) The mainφ (◦) stratumγ (kN units/m ) include the Tuluanshan formation,Alluvial Deposit Shuilian Conglomerate gravel, sand, form clayation, Fanshuliao 8.8 20formation, 15Paliwan formation, and alluvial Fanshuliaodeposit. Table 1 lists the shale mechanical parameters 11 and average 25 unit 21.62 weights of these formations. ThePaliwan average unit weight conglomerate was back-calculated from 15 actual 30 landslide 23.35 areas in the study Tuluanshan conglomerate 10 35 22.13 area and from laboratory test results [15]. The DTM was used to calculate the distribution condition Shuilian Conglomerate conglomerate 24 29 22.78 of the gradient and aspect of the slope. Subsequently, based on the grid resolution, the parameters of Equation (8) and safety factors were calculated using a database management system in the GIS software. Infrastructures 2019, 4, x FOR PEER REVIEW 7 of 11 Infrastructures 2019, 4, 70 7 of 11 Infrastructures 2019, 4, x FOR PEER REVIEW 7 of 11 The Tuluanshan formation is volcanic, formed from pyroclastic deposits. The lithology of this formationThe Tuluanshan varies considerably, formation although is volcanic,volcanic, it primar formedily fromcomprises pyroclasticpyroclastic andesite deposits.deposits. gravel, The tuff, lithology and tuffaceous of this formationsediments. varies The Shuilian considerably, Conglomerate although formation it primarilyprimar ilyis mainly comprises composed andesite of gravel, marine tuff, tu conglomerateff, and tutuffaceousffaceous and sediments.sandstone. TheThe The FanshuliaoShuilian Conglomerate formation primarily formation cons is istsmainly of marine composed mudstone, of marine sand, conglomerate and interbedded and sandstone.shale, with The a bottom Fanshuliao layer offormation mudstone primarily tens of mecons consiststersists thick, of marine and an mudstone, upper portion sand, comprisingand interbedded sand shale,and interbedded withwith aa bottombottom shale layer layer hundreds of of mudstone mudstone of meters tens tens of thick. of meters me Theters thick, thick,Paliwan and and an formation upperan upper portion consists portion comprising ofcomprising conglomerate, sand sand and interbeddedmudstone,and interbedded blue-gray shale shale hundreds sand, hundreds of and meters interbeddedof meters thick. The thick. Paliwanshale. The The Paliwan formation alluvial formation consists deposits of consists conglomerate,are Quaternary of conglomerate, mudstone, fluvial blue-graysediments.mudstone, sand, blue-gray and interbedded sand, and shale.interbedded The alluvial shale. deposits The alluvial are Quaternary deposits are fluvial Quaternary sediments. fluvial sediments.

Figure 7. Study area located in the southeast area of Hualien, Taiwan. Figure 7. Study area located in the southeast area of Hualien, Taiwan. Figure 7. Study area located in the southeast area of Hualien, Taiwan.

Figure 8. Geological distribution map of the study area. Figure 8. Geological distribution map of the study area. Figure 8. Geological distribution map of the study area. Table2 summarizes the results of the stability analysis of the excavation slope at the portal section. Table 2. Analysis results for stability of excavation slope at portal section. The vertical-to-horizontalTable 2. Analysis ratio of results the cutting for stability slope of isexcavation the most slope critical at portal factor section. affecting the stability of the portal. If the ratio of the cutting slope is 1:1 (vertical:horizontal),Vertical: Horizontal the cutting slopes of all the six portals can be consideredTunnel stable. Portal However, site if theVertical: cutting Horizontal slope ratio for the south portal area of Tunnel Portal site 1.0: 1.0 1.0: 0.5 1.0: 0.3 Tunnel 2 and both the south and north portals of1.0: Tunnel ≥1.0 3 1.0: is 1:0.5,≥0.5 their1.0: ≥0.3 safety factors are less than North FS 1 FS 1 FS 1 1.0. The north portal of TunnelNo. 1 2 becomes also unstableFS ≥ 1 ifFS the≥ ratio1 FS is higher≥ 1 than 1:0.3. Therefore, NorthSouth FS ≥ 1 FS ≥ 1 FS ≥ 1 No. 1 ≥ ≥ ≥ South FS 1 FS 1 FS 1 Infrastructures 2019, 4, 70 8 of 11 excavating these portal slopes may lead to instability. The cutting slope ratio is approximately 1:0.3 for both portals of Tunnel 2, and approximately 1:0.5 for both portals of Tunnel 3. The analytical results agree with the construction outcomes of the three portals. The difference in the north portal of Tunnel 2 may have resulted from underestimating the effect of the slope protection measures. For example, the terrain of target areas may have been covered with thin weather-resistant materials. Based on the results of this case study, the mechanical properties of geologic materials and the excavation gradient of slopes surrounding portals were considered to be key factors controlling the slope safety near the tunnel portal.

Table 2. Analysis results for stability of excavation slope at portal section.

Vertical: Horizontal Tunnel Portal Site 1.0: 1.0 1.0: 0.5 1.0: 0.3 North FS 1 FS 1 FS 1 No. 1 ≥ ≥ ≥ South FS 1 FS 1 FS 1 ≥ ≥ ≥ North FS 1 FS 1 FS< 1 No. 2 ≥ ≥ South FS 1 FS< 1 FS< 1 ≥ North FS 1 FS< 1 FS< 1 No. 3 ≥ South FS 1 FS< 1 FS< 1 ≥

4. Discussion Numerous factors aﬀect slope stability, including terrain, underground water level, mechanical parameters, and the unit weight of geomaterials. For rapid evaluation of slope stability, performing geodetic or geological investigations is unrealistic. To evaluate the assumptions involved in the proposed fast method, the safety factors obtained through the proposed method and conventional slice method are discussed here.

4.1. Obtained Safety Factors To elucidate the inﬂuence of the safety factor of a slope on the excavation depth and slip surface, we compared the results obtained from Equation (3) with the safety factor obtained using the STABL computer program to conﬁrm the applicability of the proposed model. STABL is a computer program written for the general solution of slope-stability problems using a two-dimensional LEM. This program is used to perform slope stability analysis. The slip surface is considered as Bishop methods series of slope stability programs [19]. Three cutting slope ratios, 1.0:1.0, 1.0:0.5, and 1.0:0.3 (vertical: 3 horizontal), were tested. The geomaterial parameters were c = 25 kPa, φ = 30◦, γ = 13.55 kN/m , 3 3 γsub = 20.50 kN/m (submerged unit weight), and γsat = 25.36 kN/m (saturated unit weight). For comparison, the conventional slice method and the failure surface provided by the Bishop method of the STABL program were used to determine the minimal safety factor. Figure9 shows the safety factor obtained using the proposed method and conventional slice method at various excavation depths. The safety factors estimated using both methods decreased slightly as the critical excavation depth increased. The safety factors estimated using the proposed method were similar to those obtained using the conventional slice method when the cutting slope ratio was 1.0:0.5, but they were slightly higher when the cutting slope ratios were 1.0:1.0 and 1.0:0.3. Further research indicated that the variations were the result of various failure surfaces. When the cutting slope was gentle (i.e., the ratio was 1.0:1.0), the failure surface used in the slice method typically passed under the slope toe into a base failure type, thereby resulting in a lower safety factor. When the cutting slope was steep (i.e., the ratio was 1.0:0.3), the failure surface used in the slice method typically passed the cutting surface with a smaller failure circle, which also resulted in a lower safety factor. The analytical results are consistent with those reported by Wang and Huane [9], and they complied with those reported by the relevant literature [18], thereby validating the accuracy and applicability of the Infrastructures 2019, 4, 70 9 of 11

Infrastructures 2019, 4, x FOR PEER REVIEW 9 of 11 proposed method. Therefore, the proposed method provides an eﬃcient and rapid analytical tool for accuracy and applicability of the proposed method. Therefore, the proposed method provides an evaluatingefficient and potential rapid portalanalytical sites. tool for evaluating potential portal sites.

5.0 4.5 cutting slope ratio 1.0:1.0, proposed method cutting slope ratio 1.0:0.5, proposed method 4.0 cutting slope ratio 1.0:0.3, proposed method 3.5 cutting slope ratio 1.0:1.0, STABL cutting slope ratio 1.0:0.5, STABL 3.0 cutting slope ratio 1.0:0.3, STABL 2.5 2.0

Safety FactorSafety 1.5 1.0 0.5 0.0 0 10 20 30 40 50 Excavation depth (m)

Figure 9. Safety factor obtained using the proposed method and the conventional slice method for variousFigure excavation 9. Safety factor depths. obtained using the proposed method and the conventional slice method for various excavation depths. 4.2. Influence of Topography 4.2. Influence of Topography Because of varying terrain features, the portal section of a tunnel is typically located on a slope. Any changeBecause in altitudeof varying during terrain tunneling features, the directly portal asectionffects the of a slope tunnel gradient is typically and located height on at a the slope. portal. Moreover,Any change the selectionin altitude of during tunnel tunneling routes and directly construction affects the planning slope gradient are directly and height affected. at the At portal. a specific Moreover, the selection of tunnel routes and construction planning are directly affected. At a specific location, the tunnel entrance topography influences both the portal slope-cutting ambit and slope location, the tunnel entrance topography influences both the portal slope-cutting ambit and slope safety factor. To select a potential portal site, existing terrain should be examined and tunnel entrance safety factor. To select a potential portal site, existing terrain should be examined and tunnel topography should be considered. The influence of the tunnel entrance topography is further discussed entrance topography should be considered. The influence of the tunnel entrance topography is as follows. further discussed as follows. FigureFigure 10 10shows shows the the influence influence of the of the tunnel tunnel entrance entrance elevation elevation with with a cutting a cutting slope slope ratio ratio of 1.0:1.0 of in the1.0:1.0 vicinity in the of vicinity the southern of the southern portal of portal Tunnel of 3.Tunnel The tunnel 3. The entrance tunnel entrance elevation elevation is 95 m, is and 95 m, the and safety factorsthe safety in most factors areas in range most fromareas 2.0–3.0 range from (Figure 2.0–3.0 10b). (Figure When the10b). tunnel When entrance the tunnel elevation entrance was elevation increased bywas 5 m increased (i.e., to 100 by m), 5 m the (i.e., safety to 100 factors m), the decreased safety factors to 1.5–2.0 decreased (Figure to 1.5–2.010c). When (Figure the 10c). tunnel When entrance the elevationtunnel entrance was lowered elevation by 5 was m (i.e., lowered to 90 by m), 5 m the (i.e., safety to 90 factors m), the increased safety factors to more increased than3.0 to more (Figure than 10 a). Infrastructures 2019, 4, x FOR PEER REVIEW 10 of 11 3.0 (Figure 10a). Figure 11 shows the influence of the tunnel entrance elevation with a cutting slope ratio of 1.0:0.3. The tunnel entrance elevation is 95 m, and the safety factors in most areas range from 1.0 to 1.5 (Figure 11b). When the tunnel entrance elevation was increased or reduced by 5 m (i.e., to 100 or 90 m), the safety factors changed to 2.0–3.0 and less than 1.0, respectively (Figures 11a,c). The tunnel entrance elevation clearly exerted a substantial effect on the slope cutting safety factor. Determining an appropriate elevation is as critical as determining the tunnel route layout. This discussion verifies that the proposed method can be used to estimate the influence of the tunnel entrance elevation rapidly and effectively. In this research, a method of tunnel portal site selection is developed based on the concepts of the GIS, and the suitable site of the tunnel portal could be evaluated with objective spatial information analyses techniques. The terrain of target areas, mechanical properties of geologic materials, and excavation gradient of slopes surrounding the portal, and the orientation of the portal of the tunnel are also taken into consideration by this method. According to the results of the case study and analyses of the safety factors, the geologic materials and the excavated gradient of slopes nearby the portals of tunnels are the key factors controlling the slope safety surrounding the portals

of tunnels. (a) (b) (c)

FigureFigure 10. 10.Inﬂuence Influence of of the the tunnel tunnel entrance entrance elevation elevation on on the th safetye safety factor factor of of slopes slopes with with cutting cutting slope slope ratioratio 1.0:1.0. 1.0:1.0. (a ()a Portal) Portal locations locations elevation elevation of of 90 90 m, m,FS FS> >3. 3. (b ()b Portal) Portal locations locations elevation elevation of of 95 95 m, m,FS FS: : 2.0–3.0.2.0–3.0. (c ()c Portal) Portal locations locations elevation elevation of of 100 100 m, m,FS FS: 1.5–2.0.: 1.5–2.0.

(a) (b) (c)

Figure 11. Influence of tunnel entrance elevation on the safety factor of slopes with cutting slope ratio 1.0:0.3. (a) Portal locations elevation of 90 m, FS: 2.0–3.0. (b) Portal locations elevation of 95 m, FS: 1.0–1.5. (c) Portal locations elevation of 100 m, FS < 1.

5. Conclusions In this study, analysis technology was combined with the GIS software to improve portal section planning. Parameters such as unit weight, effective cohesion, and effective friction angle were selected as geological material parameters; gradient and aspect of slope were selected as slope geometric parameters, at the same time, a slip plane was assumed. Combining the parameters above, an assessment model involving limit equilibrium slope stability analysis linked with the GIS was developed for slope stability assessment at portal sections of a tunnel. After comparing landslide locations and scales between actual landslides and the results from the analysis model, which were derived in this study, the results for both were shown to coincide. As the results showed, the model of this study can act as the initial stage of analysis for the slope stability of a portal; the result provides good reference data for location selection of a portal and tunnel alignment.

Funding: This research received no external funding. Infrastructures 2019, 4, x FOR PEER REVIEW 10 of 11

Infrastructures 2019, 4, 70 10 of 11

Figure 11 shows the influence of the tunnel entrance elevation with a cutting slope ratio of 1.0:0.3. The tunnel entrance elevation is 95 m, and the safety factors in most areas range from 1.0 to 1.5 (Figure 11b). When the tunnel entrance elevation was increased or reduced by 5 m (i.e., to 100 or 90 m), the safety factors changed to 2.0–3.0 and less than 1.0, respectively (Figure 11a,c). The tunnel entrance elevation clearly exerted a substantial effect on the slope cutting safety factor. Determining an appropriate elevation is as critical as determining the tunnel route layout. This discussion verifies that the proposed method can be used to estimate the influence of the tunnel entrance elevation rapidly and effectively. In this research, a method of tunnel portal site selection is developed based on the concepts of the GIS, and the suitable site of the tunnel portal could be evaluated with objective spatial information analyses techniques. The terrain of target areas, mechanical properties of geologic materials, and excavation gradient(a) of slopes surrounding the portal,(b) and the orientation of the portal (c) of the tunnel are also taken into consideration by this method. According to the results of the case study and analyses of the safetyFigure factors,10. Influence the geologic of the tunnel materials entrance and elevation the excavated on the safety gradient factor of of slopes slopes nearby with cutting the portals slope of ratio 1.0:1.0. (a) Portal locations elevation of 90 m, FS > 3. (b) Portal locations elevation of 95 m, FS: tunnels are the key factors controlling the slope safety surrounding the portals of tunnels. 2.0–3.0. (c) Portal locations elevation of 100 m, FS: 1.5–2.0.

(a) (b) (c)

FigureFigure 11. 11.Inﬂuence Influence of of tunnel tunnel entrance entrance elevation elevation on on the the safety safety factor factor of of slopes slopes with with cutting cutting slope slope ratio ratio 1.0:0.3.1.0:0.3. (a )(a Portal) Portal locations locations elevation elevation of of 90 90 m, m,FS FS: 2.0–3.0.: 2.0–3.0. (b ()b Portal) Portal locations locations elevation elevation of of 95 95 m, m,FS FS: : 1.0–1.5.1.0–1.5. (c )(c Portal) Portal locations locations elevation elevation of of 100 100 m, m,FS FS< 1.< 1.

5.5. Conclusions Conclusions InIn this this study, study, analysis analysis technology technology was was combined combined with with the the GIS GIS software software to to improve improve portal portal sectionsection planning. planning. Parameters Parameters such such as as unit unit weight, weight, e ﬀeffectiveective cohesion, cohesion, and and e ﬀeffectiveective friction friction angle angle werewere selected selected as as geological geological material material parameters; parameters; gradient gradient and and aspect aspect of of slope slope were were selected selected as as slope slope geometricgeometric parameters, parameters, at theat samethe same time, time, a slip a plane slip wasplane assumed. was assumed. Combining Combining the parameters the parameters above, anabove, assessment an assessment model involving model involving limit equilibrium limit equilibri slopeum stability slope stability analysis analysis linked withlinked the with GIS the was GIS developedwas developed for slope for stability slope stabilit assessmenty assessment at portal sectionsat portal of sections a tunnel. of Aftera tunnel. comparing After landslidecomparing locationslandslide and locations scales between and scales actual between landslides actual and landslides the results and from the theresults analysis from model, the analysis which weremodel, derivedwhich inwere this study,derived the in results this study, for both the were results shown for to both coincide. were As shown the results to coincide. showed, As the the model results of thisshowed, study canthe actmodel as the of initialthis study stage can of analysis act as the for init theial slope stage stability of analysis of a portal; for the the slope result stability provides of a goodportal; reference the result data forprovides location good selection reference of a portal data and for tunnel location alignment. selection of a portal and tunnel alignment. Funding: This research received no external funding. Acknowledgments:Funding: This researchAuthor received thanks no the external engineering funding. advisory group of the Ministry of National Defense in Southern Taiwan, R.O.C., for assisting in this study. Infrastructures 2019, 4, 70 11 of 11

Conﬂicts of Interest: The author declares no conﬂict of interest.

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