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

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infrastructures 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
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