Chenab Bridge: Construction of the World’s Highest Rail Bridge

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The Chenab Bridge is a 1315 m long, steel arch railway bridge under construction in Jammu and Kashmir, India. The bridge is a part of the Jammu-Udhampur-Srinagar-Baramulla Rail Line (JUSBRL) project being undertaken by the Indian Railways. The Chenab bridge will have a structural height of 359 m from the riverbed level, the highest in the world.

The terrain of this railway line is mountainous and will pass through tunnels and bridges constructed in difficult Himalayan geology. The crossing of the Chenab river is one of the most challenging parts of the project.

The bridge comprises of a 530 m long approach bridge, and a 785 m long arch bridge. The main arch spans 467 m, making it one of the longest arches in the world and the longest arch for rail traffic. The deck will be 13.5 m wide and will include two tracks upon completion.

This article describes the major components of the Chenab bridge, its geotechnical details of construction, and the construction methodology.

1. Components of Chenab Bridge

The Chenab bridge consists of a single arch of 467 m over the Chenab river and will have two railway lines. Its deck will be continuous over the supports with expansion joints at S10, S70, and S180 supports.

The proposed railway bridge consists of 18 piers resting on the ground, out of which four piers are resting on the left abutment and the remaining 14 piers on the right abutment. The width of the river at the bed level (RL 492 m) is about 150 m, while the rail track on the bridge will be at RL 854.517 m.

2. Geology of Site

The Himalayas are full of geological surprises with faults, folds, shear zones, etc. present due to ongoing tectonic activities in the region. Therefore, the geology of the Himalayas should be studied before constructing any major structure at the place. The geology at the site of the Chenab bridge is described below:

1. The site comprises of Shiwaliks and pre-tertiary rocks.
2. The main sub-type of rocks comprises of dolomite and limestone.
3. Some of the pier locations comprise quartzite and shale rocks.
4. There are no significant shear zones or cavities in both the abutments.
5. Top layers of the rocks at abutments are highly fractured and represent a blocky mass. However, the bottom layers of rocks are stable and strong enough for a good foundation.
6. Both abutments have foliation joints with two sets of sub-vertical joints.
7. The spacing between the foliation joints is extremely less.
8. The joints are irregular and very rough, with no or little infilling.
9. The rock mass is dry and highly interlocked.

2.1 Rock Mass and Rock Joints Properties

Preliminary analysis of rock mass can be carried out by finding various rock mass indexes. Such indexes characterize the rock mass into four categories, i.e., good, fair, poor, and very poor rock. The characterization of the site rock mass by various indexes is listed below:

1. Rock quality designation (RQD) = 49 (Fair-Poor)
2. Rock mass rating (RMR) = 48 (Fair)
3. Q-Index = 6.13 (Fair)
4. Geological strength index (GSI) = 43 (Fair)

The rating mentioned above shows that rock mass behavior is fair. Further, several in-situ and laboratory tests were carried out for establishing the rock mass strength and deformation properties. The geotechnical properties of the left and right bank abutments of the Chenab bridge are given below in the table.

Density	2.762 gm/cc
Specific Gravity	2.81
Porosity	1.30 %
Unconfined compressive strength (UCS-dry)	160.50 MPa
UCS (saturated)	160.50 MPa
Point load index	14.12 MPa
Sonic wave velocity	4.60 km/sec
Modulus of elasticity (E)	4.41 x 104 MPa
Poisson's ratio	0.22
Cohesion (c)	22.50 MPa
Friction angle	58o
Deere Miller classification	MC/MB

Rock-mass properties		Rock-joints properties
Cohesion (c)	1.40 MPa	Cohesion (c)	0.80 MPa
Friction angle	44.42o	Friction angle	38o
Dilation angle	10o	Residual friction angle	30o
Bulk modulus	5 x 1010 Pa	Normal stiffness (Kn)	4 x 1011 Pa/m
Shear modulus	3.8 x 1010 Pa	Shear stiffness (Ks)	3 x 109 Pa/m

3. Foundation and Slope Design

The following points describe the foundation and slope design of the Chenab bridge:

1. The groundwater table is located far away from the base of the foundation. However, short-term rainwater might develop some hydrostatic pressure. Therefore, the impact of hydrostatic pressure was considered in the foundation analysis.
2. Other than the usual load combination, a higher load factor was considered for seismic forces in the foundation analysis because of the presence of the bridge in the seismic zone-V.
3. For foundation stability, 2-3 m deep trial pits were excavated for all the foundations from S-10 to S-70 for geological logging and conducting plate load tests.
4. Also, for more stability of foundations at S-40 and S-50, drifts were excavated about 8-10 m below the foundation level as foundation at S-40 and S-50 location are the most critical arch foundations. All the weight of the arch will be supported by the foundation located at these locations.
5. Isolated footing and concrete pedestal were constructed for each support on both sides of the abutments. Steel piers were bolted using a base plate above the concrete pedestal of each foundation.
6. Structural steel piers were used for the foundation construction of the 18 piers.
7. A wedge failure analysis was carried out at both left and right abutments. For this purpose, DIPS and SWEDGE software were used.
8. A preliminary analysis indicated the wedge failure at the downstream side of both left and right abutments. Thus, the risk of wedge failure was avoided by flattening the slopes from 70° horizontal to 63°.
9. Detailed analysis of wedge failure provided the required amount of anchor bolts and the degree of stabilization.
10. There was a possibility of toppling failure at S-50 and S-70 piers location; therefore, to avoid this, flattening of the slope was considered as an optimum and economical solution.

4. Slope Stabilization Measures

All the 18 piers were constructed on the slope. Piers provide the base for deck construction, and all the piers of the Chenab bridge are located on the slope. Therefore, to improve the overall stability of the slope, the following measures were taken for slope stabilization. 

1. After excavating the slopes, a 100-mm thick steel fiber-reinforced shotcrete was applied in two layers of 50 mm each to provide instant stability.
2. Further, a minimum of three to five rows of passive rock bolts of 32 mm size and 11 m length were set up in 100 mm size boreholes perpendicular to the slope. Rock bolts were provided at 2.5 m spacing.
3. Also, pre-stressed bar anchors of 625 KN capability and 33 m length were installed in five rows at S-60 pier location.
4. Pre-stressed cable anchors of 980 KN capacity and 40 m length were installed in five rows at S-50 pier location.

For ensuring long life of the rock bolts and rock anchors up to 120 years, the following measures were undertaken:

1. A corrosion inhibitor solution was applied to the rock bolts.
2. Further, a minimum grout cover of 25 mm was provided between the rock and the bolts.
3. The stratum surrounding the pre-stressed rock anchors was grouted to reduce water ingress through the surrounding material.
4. Then the pre-stressed rock anchors were provided with double corrosion protection sheets.

5. Construction Methodology and Sequence

The following points describe the construction methodology and sequence details of the Chenab bridge:

1. The structural steel parts of the bridge were fabricated in workshops constructed near the site due to the lack of proper transportation to the site. Workshops and paint shops were set up on both sides of the valley.
2. There was no electricity available near the site, and the water available in nearby channels was not suitable for concrete production. Therefore, electricity was produced at the site, and provisions were made for supplying river water from far away mountains.
3. Firstly, the foundation and slope stabilization works were started. A total of 18 piers were constructed. (More details can be found in the foundation and slope design section).
4. A cable crane will be used in the main span for the bridge erection. After the construction, the cable crane will be kept at the site to be used for repair and maintenance works in the future.
5. The bridge deck panels will be fabricated in the workshop. The size of the panels is restricted to 8 m as the welding points are located under the bridge.
6. The bridge consists of 25,000 tons of steel structures. Steel columns of 100 m length will be constructed with the help of a cable crane with a capacity to handle a maximum weight of 34 tons.

5.1 Construction Sequence of Arch

The following points describe the proposed methodology for arch construction of Chenab bridge:

1. The erection of the arch will be done using derrick crane and cable crane.
2. The process has begun with the erection of the main piers by cable crane. After that, the deck is launched up the axes of support S-40 and S-50. (See Figure-1)
3. Further, the derrick crane will be placed on top of the deck. The maximum lifting capacity of the derrick crane is 100 tons. The purpose of the derrick crane is to lower the arch segments from deck level to the erection front of the arch. (See Figure-6)
4. The erection process of the deck and arch will be proceeding simultaneously. Both the arch and the deck will be maintained up to a maximum cantilever length of 48 m.
5. When the next arch pier support would be reached, temporary cables will be set up to support the constructed arch. After that, the new arch pier will be constructed using the cable cranes on the free end.
6. The process will be continued until the last pier of the arch is constructed.
7. The superstructure and the arch will briefly form a truss girder, cantilevering out from the arch abutment.
8. The final segment of the arch will be constructed using a cable crane. Two halves of the arch will be adjusted using hydraulic jacks before the final connection.
9. Finally, the temporary cables will be removed, and the final connections will be made.

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