Engineering and Structural Geology Evaluation of Khansar-Boien Miyandasht Tunnel
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Engineering and Structural Geology Evaluation of Khansar-Boien Miyandasht Tunnel Ghazaleh Edrisi M.Sc. Structural Geology and Tectonics The University of Damghan, Semnan, Iran e-mail: [email protected] Rassoul Ajalloeian Associate Professor The University of Isfahan, Isfahan, Iran e-mail: [email protected] ABSTRACT Lack of geological and tectonical knowledge in a region causes hazard in project implementation. There are many examples related to this issue in Iran and the world. Main purpose of this research is to analyze the fractures because of their importance and effect on the implementation of engineering and civil engineering projects such as Khansar-Boien Miyandasht tunnel. For this purpose, the process and density of the fractures and their spatial-geometric position were investigated. Therefore, joints and faults in the Khansar syncline area and the site of Khansar–Boien Miyandasht tunnel were collected, then processed by interpreted in the software such as Stereo32 and Georient. According to the result, a group of joints can be related to pre-tectonics, another one can be related to syn-tectonics(folding and faulting) and some fractures are related to the post-tectonics. Geomechanically, syn-tectonic fractures are extensive in depth and these issues are very important, so it should be considered in implementing the project of Khansar tunnel. Results of the geological engineering study such as, uniaxial strength test, point loading, Schmidt hammer, and ultra-sonic Test were showed high-resistance massive orbitolina limestone , and alternation of shale , limestone , medium- resistance limestone and black slates include low resistance that lead to apply the supports with higher safety factor. Considering the tunnel direction (NNE-SSW), high slope fractures and tunnel tensions, it is possible to create sliding wedges in the left wall and left ceiling of the tunnel. Therefore, tunnel drilling should be performed cautiously by blasting method. KEYWORDS: Faults, Joints, Tunnel, Syn-tectonic, Post-tectonic, Slope, Sliding wedges, Geological Engineering, INTRODUCTION Khansar-Boien Miyandasht area is located at 160 km northwest of Isfahan. This area has been exposed to severe deformation because it is located in The Sanandaj-Sirjan zone. This zone has different names [20, 12, 10, 21, 8, 23]. Falken[10], Brou and Riko[8], Hinz and McQuillan[7], Farhoodi[8]and Alavi[9] Were considered the zone as a subzone of the Zagros orogeny due to structural trend conformity, structural pattern uniformity, thrust dominance especially the acceptance of standard pattern of orogenic regions in concurrence zones. Mohajjel and Sahandi[15] believed the Sanandaj-Sirjan zone consists of large scale composite duplex structures with oriental northern slope that cause tens or hundreds kilometers displacement of sheets from phanerozoic rocks units[1]. According to Mohajel et al. [16, 17], internal Sanandaj–Sirjan zone has completely been deformed - 1751 - Vol. 20 [2015], Bund. 7 1752 and it includes passive marginal sequence of Paleozoic–Mesozoic in which Khansar is located. According to Nabavi[18], fractures systems and faults that are attributed to Simrian and Katangaie orogenic movements are like central Iran and they cut Zagros orientation. In terms of tectonics, this zone consists of large- scale composite duplex structures or over thrusts whose trusts slope is toward oriental north. These successive thrusts lead to a complete stratigraphy sequence of different formations can't be found in Sanandaj–Sirjan zone and most contacts, faults and rocks are discrete and crushed (Figure 1). Figure 1: Continues on the next page. Vol. 20 [2015], Bund. 7 1753 Figure 1: Generalized structural and tectonic map of Iran (adapted from Stöcklin, 1968[22] and Berberian, 1981[5]). Study area has been shown with yellow color lines. Structural Geology Geological history of the study are related to map1/100000 Golpaiegan. This has been prepared by Mohajel and Eftekharnejad[14]. Nadimi and Konon[19] have studied strike–slip faults in the central part of Sanandaj–Sirjan zone in Zagros orogenic belt and active terminals of this faults in Zayandehrood river displacement. As total orientation of Sanandaj–Sirjan zone shows, the study area has many faults and fractures and folds which are in the same orientation as Sanandaj–Sirjan zone. The mountainous region along the Khansar-Boien Miyandasht road is considered as a syncline axis and the Major faults of the regions, are approximately stretched in the same orientation. These faults have Reverse and strike-slip components of movements. These findings reveal that the orientation of exerting forces were initially NE-SW. After that, these structures have become folded again because of exerting forces with other orientations. Therefore, recumbent and inclined synclines have been made in some parts. A group of the fractures are almost N–S; and another group follows Zagros trend and has NW-SE trend. The area of the fractures in this region has NE-SW trend, almost perpendicular to Zagros direction, because this region is under pressure of Arabic platform. Another group of the fractures follow Zagros trend. A group of the faults and the joints do not follow mentioned trends and they are affected by significant folding in the region. Lithology of the tunnel direction Firstly, there is unit K1 or limestone. This unit is thick in the study area and it is as a mass in some place. It seems that it has good compressive strength. However, because this unit is from limestone, karstic and soluble cavities should be evaluated carefully in order to reduce the possible problems during drilling. A main point about this unit is that it has shale inter layers in the study area; and it Vol. 20 [2015], Bund. 7 1754 causes problems in drilling direction. Shales with active clay are expanded when they become wet. In addition, during drilling and removing tension, they become expanded and cause joints, fractures and fissures. In some cases, shale layers do not allow ground water to pass because of their impermeability and therefore when they are removed, groundwater, existing water in karstic cavities, leaks into the tunnel and it becomes problematic. Thus, careful investigation of these layers and geo- mechanical characteristics are necessary. Second unit is Ksh-l. It includes limestone shales with inter layers from thick layer limestone. The third unit is Kdo or buff- colored sand dolomite and final unit belongs to older rocks, i.e. black slates and Jurassic serzite schists. Dynamic analysis of the study area In dynamic analysis of an area with specific tectonic conditions, two parameters play main roles including: 1) Main tesnsion direction 2) Ellipcity and ellipsoid orientation of the tension [18].Figure 2, shows the position of the normal and inverse faults and geometrical lineation, maximum and minimum main strains orientations on the structural map 1/40000 prepared from Geological map of Golpayegan 1/100000. Existing tensions of region based on the progressive geometry show the fractures of the under study region that follow specific geometry of shearing zone. A group of these fractures follows Zagros trend (west north-East south) such as Dalan and Khansar faults that act as dextral inverse; another group has northern-southern trend. They are fractures R'(dark green) with position 322/50 that act in opposite orientation of dextral shear, i.e. sinistral shear. Third group includes fractures in shear direction with a difference of φ/2 from fractures R' , which are called fractures R(yellow) and is located in 275/60. The fourth group, which are compressive fractures includes fractures P (blue) which is located in 355/36. The final group includes fractures T (black) which is located in 62/78 (which shows tensile fractures). Faults and joints represent these fractures. Figure 2: Position of the faults and lineation on map 1/100000 Golpaiegan (parallelogram shows strain and arrows show maximum and minimum main strain orientation). Vol. 20 [2015], Bund. 7 1755 General pattern of the joints orientations In the field studies about 994 joints were collected. These joints were analysed by in the software stereo32. Figure (3a, b) shows the total characteristics of the collected joints as contour diagram and distributions of the joints show three high frequency points. The highest orientation belongs to NE (figure 3a). These joints indicate high to medium dips and are perpendicular to bedding (layering) or they are oblique. The second orientation is related to SE which has high dips. The third orientation is related to NW and these joints have the high dips too. Rosette diagram related to joints in part indicates the joints frequency based on dip direction. The dip direction 225, 290, 210 and 310 are high, respectively. Therefore, it is concluded that direction 30 to 35, direction 20 to 25 and direction 310 to 320 indicate first, second and third frequencies, respectively. a b c d e f Figure 3: Continues on the next page. Vol. 20 [2015], Bund. 7 1756 g h Figure 3: General pattern of all joints orientations a, b; in Jurassic lithology c , d; in lower Cretaceous lithology, e, f; in upper Cretaceous lithology, g, h Orientation pattern of the joints in Jurassic lithology indicates (c, Contour diagram), dominant direction of NW-SE that has medium dip. Part d shows rosette diagram based on dip direction that is dominant orientation N20E. Because Jurassic layers are from black slates, there are many crushing. Figure 3e shows contour diagram based on dip direction of the joints from lower Cretaceous rocks. Joints frequency has been shown in three regions on contour diagram. The most frequency is related to dip direction 115 to 120. These joints have high dip. Second frequency is related to dip direction 300 and last frequency belongs to dip direction 35. Rosette diagram shows four frequencies in dip direction NW and SW. Figure 3 g, h shows contour diagram and rosette diagram based on dip direction of joints from upper Cretaceous rocks.