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The construction of the Chenani-Nashri tunnel, also known as Patnitop Tunnel and Dr. Syama Prasad Mookerjee Tunnel, started in 2011 and was opened for traffic in 2017.
The project involved the construction of two parallel tunnels, a bi-directional main traffic tunnel and an escape tunnel, between the villages of Chenani and Nashri, in Jammu and Kashmir. Both the tunnels were constructed simultaneously from the South and North end of the Chenani and Nashri villages, respectively.
Keeping in view the inclement winters around the Patnitop range, where the roads get blocked due to heavy snowfall, the National Highway Authority of India (NHAI) was entrusted with the responsibility for the development of this 9-km-long Chenani-Nashri tunnel to by-pass Patnitop. After construction, the distance between Chenani and Nashri was reduced from 41 km to 9 km.
Planning and development of the tunnels through complex and non-uniform geographical conditions with overburden in excess of 1050 m was a difficult task. The geology of the tunnel area and the design methodology used to construct the Chenani-Nashri tunnel have been discussed in this article.
1. Geology of the Chenani-Nashri Tunnel Site
The Chenani-Nashri tunnel region lies in the Western Himalayan zone in an area of collisional belt known as the Sub-Himalayas. This structural area is limited toward the north by the main boundary thrust and in the south by the Himalayan frontal thrust.
These primary thrusts, just as the majority of the belts and units of the north-west region of Himalaya orogeny, show a strike towards the north-west to south-east with moderate to mild plunges either towards the south or the north.
The following points describe the geology of the Chenani-Nashri tunnel:
- Most of the rocks present along the axis of the tunnel belong to the Lower Murree formation.
- Sedimentary rocks are classified under Murree Structural Belt, which shows excellent strength in crushing. Also, the sedimentary rocks are bounded on the north by a complex of thrusts regionally called the Main Boundary Thrust (MBT) and on the south by the Main Frontal Thrust (MFT).
- Both MBT and MFT were developed from the metamorphic succession.
- A group of argillaceous and arenaceous rocks are mostly confined in the Muree formation. At the Chenani-Nashri tunnel, interbedded sandstones, siltstones, and claystone are available in the argillaceous and arenaceous rock groups. The thickness of sandstone beds ranges from few meters to 10 m.
- The dip directions are unfavorable for the tunnel excavation at the Chenani end, while it is the opposite at the Nashri end. Generally, dipping along the tunnel axis represents favorable condition for excavation.
- The strike direction at the Chenani end is parallel to the tunnel axis, whereas, in the Nashri end, the strike direction is perpendicular to the tunnel axis.
- Several bands of sandstone, claystone, and siltstone of differing thickness were often found during excavation.
- The bands of mixed rocks, for example, intermix siltstone, sandstone, , and claystone, were additionally encountered during tunnel excavation.
- Uniaxial compressive strength (UCS) of the sandstone was in the range of 65-110 MPa, whereas the UCS for siltstone and claystone was in the range of 30-50 MPa and 9-16 MPa, respectively.
- Low UCS of claystone represents that whenever the claystone is left exposed to the environment, it will disintegrate into small pieces in about a week or thereabouts.
- The siltstone joints have erodible mud fillings.
- Three number of joint sets were encountered on the claystone, whereas for both sandstone and siltstone, one or two number of joint sets were found.
|S.No.||Type of rock||Rock mass rating (RMR)||Rock mass quality (Q –value)||N-value|
|4||Mixture of sandstone and siltstone||48||1.60||4|
|5||Mixture of siltstone and claystone||43||0.54||2.5|
As per the table-1, RMR, Q-value, and N-value for claystone are the lowest among the other rocks. Thus, the roof supports for the claystone had required heavy steel supports. Whereas, the sandstone had required fewer roof supports.
2. Construction Details of the Chenani-Nashri Tunnel
NHAI was given the responsibility by the government of India for restoring, reinforcing, and expanding the NH-1A section to four lanes. Construction of the Chenani-Nashri tunnel was taken up on an urgent basis as the whole section of NH-1A at Patnitop hill gets blocked during the winters due to snowfall. Thus, the connectivity of the Jammu and Kashmir states with the rest of India gets hampered.
The Chenani-Nashri tunnel project included the construction of an underground tunnel, two bridges, toll plazas, 1.3 km of approach road toward the south portal, 0.6 km of approach road towards the north portal, all project facilities, and spoil dumps.
A 9-kmlong main tunnel was constructed together with a separate parallel escape tunnel. The main tunnel was designed as a bi-directional tunnel with two lanes in each direction. The escape tunnel was designed for emergency use during any accident.
Also, the escape tunnel is used to divert the traffic from the main tunnel during maintenance. The escape and main tunnel are connected at an interval of 300 m for pedestrian crossing and 1200 m for vehicular crossing.
The width of the main tunnel is 11.75 m, out of which 9.35 m width is provided for the carriageway, and 1.2 m width is provided for the walkways on both sides. The geometry of the tunnel was defined in such a way that the provisions for the fully transverse ventilation system over the entire section of the tunnel could be provided. The Chenani-Nashri tunnel is India’s first tunnel equipped with a fully transverse ventilation system (exhaust air and fresh air ducts).
On the contrary, the geometry of the escape tunnel was defined based upon the availability and size of equipment with the contractor because the construction of the main tunnel had to be advanced from the escape tunnel. After a detailed study on the construction equipment, such as, jumbos, backhoes, and unloaders, a 5 m width of escape tunnel was finalized. Escape tunnel is designed as fully motorable with footpaths on both sides.
Cross passages between the escape tunnel and main tunnel were used for the transits of machines and gears. The extra face for the excavation of the main tunnel was provided through the cross passages. Thus, a faster rate of excavation was achieved because more than two faces were available for the excavation of the main tunnel.
The drill and blast tunneling method was used for the construction of the Chenani-Nashri tunnel. The section was mainly sub-divided into heading (top portion of the tunnel) and benching (bottom portion of the tunnel) for excavating through hard rock faces. In contrast, the mechanical excavators were used for excavating through the shear zones and weak rock mass.
The cross-section for the excavation of the main tunnel was varying between 130 m2 and 170 m2 because, in poor rock mass conditions, heavier supports (enclosing more area) were required and vice-versa. Shotcrete, lattice girder, and rock bolts were used as primary lining support to stabilize the rock mass immediately after excavation.
The final lining was designed to withstand the anticipated long-term live loads, hydraulic loads, and seismic loads. Between the primary and final lining, a waterproofing membrane combined with geotextile protective felt was provided around the entire periphery of the tunnel to the concrete foundation beams.
3. Tunneling Method Used to Construct the Chenani-Nashri Tunnel
The medium-to-high strength rocks were available on most of the stretch of the Chenani-Nashri Tunnel. Thus, the drill and blast method was adopted to construct both the main and the escape tunnel. Also, the drill and blast method can be used for a wide range of rock conditions and provides more stand-up time for the stabilization of the face. The use of tunnel boring machines (TBMs) was not adopted because of difficulty in transporting them to the Himalayas and the high initial investment cost.
The typical cycle of construction of tunnel using drill and blast method is described below:
- Marking the pattern of blasting on the excavation face
- Drilling the marked pattern of blasting up to a required depth
- Loading the drilled holes with the explosives
- Detonating the explosives, followed by ventilation to remove blast fumes
- Removing the excavated rock through the blast
- Installing the primary lining supports
- Monitoring the displacements and installing the waterproofing membranes
- Finally, installing the permanent lining
Mostly, the roads on NH-1A at Patnitop hill remain closed during the winter season for quite a long time due to heavy snowfall. Thus, it was decided to construct the Chenani-Nashri tunnel to by-pass the Patnitop hill.
The construction of the Chenani-Nashri tunnel was started in 2011. Both the main and escape tunnels were constructed simultaneously from the south and north end of the Chenani and Nashri village, respectively and construction was completed in 2016.
Interbedded sandstones, siltstones, and claystone were encountered during the construction of the Chenani-Nashri tunnel.
The Chenani-Nashri tunnel is India’s first tunnel equipped with a fully transverse ventilation system (exhaust air and fresh air ducts).
The medium-to-high strength rocks were available on most of the stretch of the Chenani-Nashri Tunnel. Thus, the drill and blast method was adopted to construct both the main and escape tunnel.