Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 Geological and geotechnical challenges faced during construction of Rohtang highway tunnel - A case study Pathak*, Mridupam Geologist, PEMS Engineering Consultants Pvt. Ltd., D2-ICT JV, Rohtang Highway Tunnel Project, Manali, Himachal Pradesh, India Saini, Sandeep Kumar Geotechnical Engineer, Intercontinental Consultants and Technocrats Pvt. Ltd., D2-ICT JV, Rohtang Highway Tunnel Project, Manali, Himachal Pradesh, India *E-mail of corresponding author: [email protected]) Abstract Tunnel projects in complex geological setting like the Himalayas are very unique due to the geological uncertainties and challenges to tackle them. Detailed geological investigations are an absolute necessity for effective design and construction due to large variations in the tendered and encountered rock classes witnessed here. The Seri Nalla Fault zone was encountered unexpectedly prior to its expected location. DRESS methodology proved very effective in countering the Seri Nalla zone. To counter high deformations induced in the shotcrete and lattice girders due to the high in-situ stresses, Lining Stress Controllers (LSC) are very effective. This case study presents the geological challenges faced during construction of the Rohtang Tunnel and the subsequent efforts undertaken to tackle them. 1.0 Introduction: The 8875 m long, single tube, bi-directional, two lanes, Rohtang Highway Tunnel is presently being constructed across the Pir-Panjal ranges of the Himalayas near Manali, Dist. Kullu in Himachal Pradesh. When completed, this tunnel will provide all weather connectivity between Manali and Lahaul. The Rohtang Highway Tunnel is part of a wider project to provide an all weather road connection between Leh region and the rest of India, via the Manali-Sarchu-Leh road (NH 21). This tunnel is very strategically important since it will connect Manali with Sissu in Lahaul & Spiti district all throughout the year bypassing the Rohtang Pass, which is snow covered between November to April. Also it will reduce the road distance between Manali and Leh by 46 kms. The South Portal of Rohtang Tunnel is located at a distance of 25 km from Manali at Dhundi at an altitude of 3060 m and the North Portal is located near Teling village, Sissu in Lahaul at an altitude of 3071 m. The shape of the tunnel is modified horse-shoe. Drilling and Blasting along with New Austrian Tunneling Method (NATM) is being used for the construction of Rohtang Tunnel. 24 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 Figure 1 Location of Rohtang Tunnel The design of the Rohtang Tunnel is very unique because the Emergency Egress Tunnel is not parallel but is a part of the main tunnel, below the carriageway adding to the overall cross sectional area (137 sq. m) of the tunnel. The tunnel consists of 8 m wide carriageway and 1 m wide footpath on both sides. The thickness of final concrete lining is 500 ± 50 mm and 0.5% gradient from both portals for effective drainage. Construction of the tunnel was started on 29 th Aug, 2010 and breakthrough was achieved on 15 th Oct, 2017. 500 - 550 ARCH LINING IN PCC 2600 (M 35) VENTILLATION DUCT (SEMI TRANSVERSE VENTILATION SYSTEM) LV DUCT 200 DRAIN 8936 5636 MOUNTABLE MEDIAN BITUMINIOUS CONCRETE GRANULAR 75 GRANULAR BASE 8500 75 BASE COURSE COURSE 1360 500 1290 4000 4000 CROSS FALL 1.5% CROSS FALL 1.5% CROSS FALL 3% 250 LV DUCT 150 MM DIA 2250 3600 CROSS FALL 3% HV DUCT OF 150 MM DIA PAVEMENT DRAIN PIT 150 MM DIA PIPE 300 TECH BACK FILL OF LEAN CONCRETE PRECAST ELEMENT 2256 MM LONG GROUND WATER DRAIN 600 TO 300 MM & 250 MM DIA 11300 EGRESS PASSAGE SLOTTED PIPE ALL DIMENSIONS ARE IN MM Figure 2 Cross-Section of Rohtang Tunnel 25 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 2.0 Project Geology: The Rohtang project is located within the central crystalline group of rocks. These lithological units are collectively named as the Vaikrita Group. The Central Crystalline – zone represents an anticline that trends roughly along the axis of the Great Himalayan range, extending from east to west. The rocks mainly consist of metamorphics having undergone ductile deformations. The regional geological succession at the location of the project comprises of Tandi Formation (Permian to Jurassic), Batal Formations (Permian to Jurassic), Salkhala Group (Pre-Cambrian) and Rohtang Gneissic Complex (Pre- Cambrian). The Rohtang tunnel mainly passes through the Salkhala Group, which comprises mainly of Quartz-schist, from the South Portal. From the North Portal (Koksar), the gneissic rocks of Rohtang Gneissic Complex are being encountered. The gneissic rocks comprise mainly of migmatites with intense schistosity and frequent mica banding. The main structural characteristic is the foliation which dominates the rock mass over the entire tunnel length. The folded nature of rock is dissected by variation in the strike direction of foliation. Jointing is the subsidiary structure with significant influence on tunneling. Conspicuously three sets of joints are dominant with a fourth random set. The surface geological mapping indicates orientation of joints with respect to the tunnel alignment as only moderately favorable. Figure 3 Regional Geological Map 26 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 2.1 Major Geological Features: The field geological mapping, by surface traversing along accessible slopes and valleys augmented by remote sensing data, indicated the presence of the following major tectonic structures along the alignment. a) Seri Nalla Fault b) Chandra Kothi Structure c) Rohtang Ridge Structure Seri Nalla Fault This is a NE-SW trending fault. On the surface this fault has a width of 6 –10 m, and is also continuous. The fault is identified by the contact between Quartz-schist and Migmatites (Gneissic rock). Chandra Kothi Structure This NW-SW striking fracture zone is approximately 2-4 km wide as interpreted from satellite imagery. Rohtang Ridge Structure This NE-SW striking fracture zone is approximately 1-2 km wide as interpreted from satellite imagery. 2.2 Geology Encountered During Excavation: The excavation of the tunnel was carried out by using the drill and blast method with NATM (New Austrian Tunneling Method). The Excavation Classes of Rocks were determined using a combination of Q insitu and depth of Overburden, keeping the Stress Reduction Factor constant (SRF=1). Based on this factors the Excavation classes were divided into 9 Classes namely 1,2,3,3M,4M,4S,5,6,7 which was later modified into 7 Classes namely 1,2,3,4M,5,6,7. The rock mass assessment with respect to Q insitu value is given in Table 1. Table 1 Rock Mass Assessment Category Qinsitu Values (SRF=1) * Good >10 Fair 4-10 Poor 1-4 Very Poor 0.4-1 Very Poor 0.1-0.4 Extremely Poor 0.01-0.1 * The effect of Stress levels on excavation behavior has been taken into account in the design matrices. From the South Portal, the tunnel initially was driven through Quartzitic Schist, which is highly jointed and crossed by number of shear-zones, parallel or sub-parallel to the tunnel 27 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 axis. From the North Portal the tunnel initially was driven through Gneissic rock. The other rock types encountered during tunneling was Phyllite, Quartz Phyllite, Quartzite, Mica Schist and Migmatite. ). The excavation of the Tunnel has shown that there are significant variations in rock-classes from Tendered to Encountered. This is shown in Figure 4 and also summarized in Table 2. 7 6 5 4S 4M Encountered (m) 3M Tendered (m) 3 2 1 0 500 1000 1500 2000 2500 3000 3500 Figure 4 Variation in Encountered and Tendered Excavation Classes in metres Table 2 Tendered and Encountered Excavation Classes Rock Tendered Encountered Encountered Tendered Class (m) (m) % % 1 2253.6 225 3 25 2 785 2383.4 27 9 3 616.79 1999.8 23 7 3M 1600.1 0 0 18 4M 641 3081.08 35 7 4S 159.1 0 0 2 5 1779.6 608.12 7 20 6 297 302 3 3 7 742 275.3 3 8 Total 8875 8875 100 100 3.0 Geological And Geotechnical Challenges Encountered During Excavation: The main challenges encountered during the excavation of the Rohtang Tunnel were the Seri Nalla Fault Zone and the high stress conditions encountered due to the very high overburden cover of the tunnel. 28 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 3.1 Seri Nalla Fault Zone: According to the Tender documents, the Seri Nalla Fault was extrapolated to be encountered between Ch. 2+200m to 2+800m. But during tunneling from the South Portal side, this fault was encountered 300 m earlier at Ch. 1+900m and extending up to Ch. 2+460m. Figure 5 Geological Plan at Tunnel Grade between Ch. 1+890m to Ch. 2+000m The sheared mass kept extending from the left side towards the center. At Ch. 1+913m, half of the face was covered with sheared material. At Ch. 1+925m, almost the entire face was covered with sheared material. Minor to medium inflow of water increasing to large inflow with increasing chainage was also encountered. Seri Nalla is one of the important perennial tributaries of Beas Nalla originating from southern slopes of “Rohtang-Ridge.” The Seri Nalla is structurally controlled feature as evidenced by presence of shape slopes and ridges on both the banks.