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.
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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
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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
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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
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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.
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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. At most of the places the slopes are degraded with evidences of physical weathering, pre-glacial and fluvial erosion. At places accumulation of slope debris material from depositional stage are seen. The rocks at higher reaches are alternate bands of phyllitic quartzite and phyllite forming sharp crest like features. The Seri Nalla Fault is the most important lineament affecting the tunnel alignment, this fault is trending N50°E-S50°W, with almost vertical to sub- vertical dip. In addition to the Seri-Nalla fault there are number of parallel or sub parallel lineaments, across the Seri-Nalla and are also crossing the tunnel alignment. The Seri Nalla along the tunnel alignment at surface starts at Ch. 2+350m and ends at Ch. 2+523m. The total width of Seri Nalla zone at surface is along the tunnel alignment is 173 meters. The dip of Seri Nalla fault is vertical to sub-vertical. The width of fault zone at surface is around 173 meter and consists of sheared phyllite with clay gouge. Along the slopes of left bank (Western Bank) of Seri Nalla the rocks are highly jointed and fractured Quartz Phyllites. Along the slopes of the right bank (Eastern Bank) of Seri Nalla moderately jointed Phyllites and Quartz Phyllites are observed.
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Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019
SERI NALLA FAULT
10-12 m wide
Picture 1 Seri Nalla Fault at Surface
Figure 6 Geological Cross Section of Seri Nalla at Ch. 2+050m
3.1.1 Muck Flow Leading To Cavity Formation:
When the excavation face reached Ch. 2+046 m, minor inflow of water was recorded from face along the foliation plane. The rate of seepage increased when the face reached Ch. 2+049 m; initial rate of inflow of water at face was around 5 l/sec and then increased
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Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 up to 30 l/sec. At Ch 2+049 m to 2+050 m, the first cavity was formed with about 400 m 3 of muck flowing out. Effort was made to control and divert the water from the face by using PU grout and also by stabilizing the face with shotcrete and additional rock bolts. The second cavity, much larger than the previous one was formed at Ch. 2+077 m where about 1000 m 3 muck flowed out. The third major cavity was formed at Ch. 2+390 m where 100 to 150 m 3 muck flowed out.
Picture 2 Cavity at Ch. 2+045m Picture 3 Cavity at Ch. 2+077m 3.1.2 High Ingress of Water: High ingress of water was first encountered at Ch. 2+045m where the flow on the first day was 5 l/sec which after 2-3 days increased to 30 l/sec. The maximum ingress of water was encountered between Ch. 2+390m to Ch. 2+410m where the inflow of water was 100-110 l\sec.
3.1.3 Encounter of River Borne Material (RBM): River Borne Materials (RBM) was encountered from Ch. 2+370m till Ch. 2+460m from the South Portal side. The RBM material comprised of rounded to sub-rounded boulders, cobbles, pebbles, gravels in clayey, silty, sandy matrix along with sand pockets and clay bands.
The first RBM/rock contact was observed at Ch. 2+442m on the western side (left corner). With the advancement in excavation of the face, the Phyllitic Quartzite rock exposure which had started from the left corner gradually started to increase in the face and at Ch.2+459 m the whole face was covered with rock which the just below the 40 th round of Pipe Roofing.
3.2 High Overburden Cover:
The Rohtang Tunnel was excavated through varied overburden cover with the peak overburden cover reaching up to 1860 m at Ch. 6+100m. Almost half of the tunnel (around 4 km) was excavated under overburden cover of more than 1000 m under very
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Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 high stress conditions. The effect of the varied overburden cover was taken into consideration while designing the supports of the excavation classes (as SRF was fixed as 1 for Q insitu calculation). The grouping of the overburden is tabulated in Table 3.
Table 3 Overburden Assessment
Assessment Factor Category
d<300
300 Depth of Overburden 600 (d) 900 1200 1500 Due to the very high overburden cover and resulting vertical high stress condition there were instances of spalling, popping, cracks in the shotcrete shell. There were also the occurrences of “Delayed Cracks” which develops after several days of initial excavation and support. 3.2.1 Cave-In And Rock Collapse Between Ch. 7+245m And Ch. 7+210m: When the heading face was at Ch. 7+133.50m, the rock mass caved in from Ch. 7+245m to Ch. 7+210m. This resulted in the detachment of shotcrete from 11 “O”clock to 1“O” clock position along with the deformation of the lattice girders and exposure of the rock bolts. The cave-in triggered considerable amount of rock mass to collapse. Picture 4 Cave-in and Rock Collapse Picture 5 Collapsed Rock Mass The rock type consisted of thick bands of Biotite Mica Schist and Gneissic rock. The joints were filled with Chlorite material indicating that the rock mass was highly stressed. These joints were sub-parallel to the tunnel axis. 32 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 Figure 7 Geological Plan from Ch. 7+235m to 7+205m The maximum vertical deformation was recorded was 575mm at Ch. 7+225m at the 1 “O” clock position, 25 days after excavation. The rate of movement within two weeks of excavation was also high, i.e. around 25 mm/day, which then gradually reduced to 18 mm/day by the 3 rd week. Figure 8 Vertical Deformation Recorded For Segment from Ch. 7+345m to Ch. 7+145m The geological evaluation indicates that the rocks to be in a highly compressed state, as evidenced by the presence of mesoscopic folds in soft ductile Mica Schist and the presence of micro faults within hard gneissic rock. The geotechnical studies indicated the deformation pattern at the North Portal to be as follows: 33 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 • Opening of cracks in shotcrete at crown. • Excessive deformation at 1 “O” clock position with deformation/buckling of lattice girders. • Snaking of tunnel profile at several locations. • Continued widening of cracks in shotcrete with subsequent rock and shotcrete falls. • Rock falls and formation of excessive over break at crown up to 2.5 m. 3.3.2 Maximum Overburden Cover at Ch. 6+100m: At Ch. 6+100m the maximum overburden cover of 1860 m was encountered. The rock type consisted of thinly foliated Phyllite at the crown and Phyllitic Quartzite towards the SPL. Vertical to sub-vertical joints were present which had medium to high persistence. This area was successfully crossed without any major incidence. 3.3.3 Occurrence of Delayed Cracks between Ch. 5+300m to Ch. 5+600m: Delayed Cracks had occurred between Ch. 5+300m to Ch. 5+600m after several days of initial excavation and installation of support. Phyllitic and Quartzitic rock was encountered in this area. Converging rock masses transfers deformation to the shotcrete. These areas which are subjected to high overburden cover induces instability due to high locked in stresses, causes the following problems: • In highly foliated rock mass, shear seams are concentrated around the contact of competent and incompetent layers. The competent rock like Quartzite exhibit rock burst/spalling conditions whereas thinly foliated Phyllite tend to show more squeezing conditions. • Overstressing of lattice girders is caused by loosening rock mass around the excavation leading to buckling of the lattice girders. • Incompetent rock like thinly foliated Phyllite leads to gradual deformation and convergence of the surrounding rock mass towards the opening. The pressure due to the incompetent rock mass around the tunnel opening under the influence of overburden load and tectonic stress leads to squeezing ground conditions and development of delayed cracks. 4.0 Remedial Measures Undertaken to Overcome the Challenges: Different remedial measures were applied to overcome the geological and geotechnical challenges encountered in the excavation of the Rohtang Tunnel which is enumerated below: 4.1 Seri Nalla Fault Zone: When the Seri Nalla was first encountered, regular systematic support was installed which consisted of lattice girders, wire mesh, shotcrete and rock bolts. But after the 34 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 occurrence of the first major cavity at Ch. 2+046m, DRESS method of tunneling was adopted. DRESS method implies to Drainage Reinforcement, Excavation and Support Solutions. This method consisted of systematic pre-drainage ahead of face (installation of long perforated pipes MS pipes of dia. 76mm) to drain out the water inside the rock mass. Reinforcement of the rock mass was done by using pipe-roofs and fore-poles to from an umbrella and subsequent grouting. Pipe-roofing was done by using MS pipes of 89mm, 76mm and 114mm diameter and later on 40mm TMT bars were also inserted inside the 114mm dia. pipes for additional reinforcement. Fore-poling was carried out by using 32mm SDRs. Excavation was done in small steps by mechanical means and finally the systematic supports were installed. The DRESS method was very useful to excavate through the soft, weak and water charged strata of Seri Nalla. In total 40 rounds of Pipe- roofs were done in this area which consisted of both single and double layers of pipe roofs. Figure 9 Tunnel Section Showing Pipe-Roofs Arrangement In addition to pipe-roofing, there were several other additional measures and methods applied to successfully tackle the Seri Nalla which are enumerated below: • Grouting: Grouting was done with cement, micro-fine cement with sodium silicate admixture and Polyurethane. • Micro-piling: Micro- piling was carried out with both 76mm and 114mm dia. MS pipes of 6 m length. In some 114mm dia. pipes, 20mm TMT bars were inserted for additional reinforcement. These were done to provide additional support to the tunnel walls and to counter the high vertical deformations. • Tunnel Seismic Profiling (TSP): Tunnel Seismic Profiling is a geophysical test which is useful in predicting advance tunneling information like recognition of fault, shear and fractured zone where rock mass characteristics change. It is also useful for predicting water bearing formations. Total 10 nos. of TSPs were carried out in this project out of which 6 nos. were done in the South Portal side and all in the Seri Nalla zone. This test is useful for computation of mechanical properties of the rock like P (Primary) and S (Secondary) velocity, vP/vS ratio, Poisson’s ratio, Rock density, Dynamic Young’s modulus, Shear Modulus. • Pilot Tunnel: This method was applied in a small stretch from Ch. 2+385.45m to Ch. 2+394.50m where a secondary tunnel smaller in diameter to the main tunnel was first excavated through the weak strata. 35 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 • Multiple Drift with Panel Excavation: Multiple drifting with Panel excavation was done in the RBM area where the heading was divided into small panels and excavated accordingly. A continuous shell was created by excavating sections in panels. Each panel was individually supported with wiremesh and shotcrete. Once peripheral arch was completed, the lattice girder was erected. This also ensures that the deformation of the rock mass/tunneling media is kept to a minimum. A face stabilization core was left at the centre of the face to act as additional support against the flowing ground condition, which had been previously supported with wire mesh, shotcrete and rock bolts. • Temporary Invert: Temporary invert was executed in very poor to extremely poor ground conditions (Excavation Class 6 &7) to act as a counter measure against upheaving of soft ground. • Deep Invert: Deep invert is proposed to be constructed across the entire Seri Nalla stretch. This is done mainly to close the tunnel ring so that there is even distribution of stresses across the soft and weak Seri Nalla zone. Picture 6 Pipe-roofing Picture 7 Pilot Tunnel at Ch. 2+385.45m Picture 8 Excavation for Temporary Invert Picture 9 Multiple Drift Excavation 36 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 Picture 10 Construction of Deep Invert Picture 11 Bridge for Logistics during Deep Invert 4.2 High Stress Zone due to High Overburden Cover: Due to the high stressing owing to the very high overburden cover, there were many instances of very high deformation leading to squeezing of the tunnel, cracks in the shotcrete and buckling of the lattice girders. To counter these problems, additional swellex bolts were installed immediately after installation of lattice girders to counter the initial deformation. The maximum deformation was observed at 1 “O” clock position from North Portal side where there was the maximum buckling of the lattice girder. To counter this, slots of 50 cm width were left in the shotcrete which were later filled after the deformations had subsided. Re- profiling was done in many locations which had witnessed high deformation resulting in squeezing of the tunnel. Additional convergence, more than specified in the design, were also allowed for avoiding re-profiling works. To counter the deformation due to the in- situ stress induced in the shotcrete and for absorbing large deformations it was decided to install yielding elements like Lining Stress Controllers (LSC). Deformation of LSC Lattice Girder Picture 12 Deformation in Lattice Girder Picture 13 LSC installed at Ch. 5+402.90m 37 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 5.0 Instrumentation: Instrumentation is an essential part of NATM tunneling and is necessary for the assessment of the tunnel stability, performance and adjustment of the support system and for determining the sequence of excavation. In Rohtang tunnel, throughout monitoring was done by Bi-Reflex Targets, few Multiple Point Borehole Extensometers and Load Cells. (A) (B) (C) Picture 14 Instruments used in Rohtang Tunnel (A: MPBX; B: Bi-reflex Targets; C: Load Cell) 6.0 Conclusions: i. In tunnel projects located in complex geological setting such as the Himalayas, the cost, feasibility and design parameters of the projects are largely influenced by geological factors. The cost of the project and the time schedule along with entire construction planning need to be based on a proper “Geotechnical Design Summary Report” (GDSR). The problems faced during the construction of the Rohtang tunnel include significant variations in rock classes from the tendered document in comparison to those actually being encountered during heading excavation. This is also attributed to the limited geological/geotechnical data available at the design and tender document preparation phase. So detail investigation is very much a pre-requisite for a project of this magnitude. But sometimes due to the topography detailed investigation is not possible. Success of tunnel construction is directly related to the prior knowledge of impending geological complexities to being prepared with pre-emptive engineering solutions, tools and resources. ii. DRESS method of tunneling is very useful in countering weak, soft and flowing ground conditions. iii. TSPs can be used for advance probing in order to access the geological conditions in advance and subsequent planning to counter them. iv. LSCs are very effective in absorbing large deformations. 38 Journal of Engineering Geology Volume XLIV, Nos. 1 & 2 A bi-annual Journal of ISEG June & December 2019 References: 1. Bhandari R.C., Jangade B.D., Saini Sandeep, Choudhary Bharat Kumar and Saleira Dr. Wesley (2013). Geological Investigations for Tunnel Projects and their Impact on Cost and Schedule Related to Project Construction with Special Reference to Highway Tunnel in Himalayas, Indorock 2013: 4 th Indian Rock Conference, 29-31 May, 2013, pp 214-225. 2. Bhandari R.C., Shukla Lt. Col. Vinod, Chowdhary R.K. and Saleira Dr. Wesley (2013). Experiences from Excavation of a Highway Tunnel across Himalayan Ranges near Manali, Himachal Pradesh, Indorock 2013: 4 th Indian Rock Conference, 29-31 May, 2013, pp 594-605. 3. Rites (1996). Feasibility Study of Highway Tunnel across Himalayan Ranges, Manali, Himachal Pradesh, Phase II Main Report, Vol-I. 39