As part of Northeast Policy, the Ministry of Railways took up the project of providing rail connectivity to Imphal in Manipur state. As per the detailed survey, the rail alignment takes off from existing Jiribam station in Assam and traverses a length of 111km before connecting to Imphal, running along picturesque lush green scenery marked by hills, valleys, waterfalls and rivers.
At present, people have to travel by road on the existing National Highway (NH-37) which is 230-km long with several steep gradients, hair-pin bends and weak bridges. It takes more than 10 hours by road, whereas it will be less than 2 hours once the rail line is commissioned. The rail link has other advantages of strategic importance as it will usher economic development of Manipur with tremendous tourist potential, and, more importantly, it will form a vital link of the proposed Trans-Asian Rail Network connecting several countries.
As the rail line crosses a series of hill ranges, several tunnels and viaducts are necessary to maintain the gradient required for a railway line. One of these viaducts is spanning across a gorge of width 700m and depth 141m - which is the world’s tallest pier bridge of its category, surpassing the previous record of 139m in a rail line in Europe.
Challenges encountered and techniques developed in designing and building this world record bridge to make it safe and sustainable.
Hilly Terrain: The alignment passes through the hill ranges of Patkai region (eastern trail of Himalaya), comprising a series of tunnels and viaducts. While the high mountains are penetrated by tunnels, the deep gorges need to be spanned by tall viaducts. An alternative to viaducts is high embankments, but this is not a practical solution in the present scenario, considering their susceptibility to failure, requirement of huge earth quarries and need for large spaces, and huge maintenance. Hence, a detailed comparative study was made, and viaducts were found to be the best choice for reasons of aesthetics, environment, sustainability and economy.
The tallest of such bridges (Br.No.164) spans over a gorge of depth 141m and length 703 m. This bridge is slated to become the tallest in the world in its category from the point of view of pillar height, surpassing the existing tallest of Mala-Rijeka viaduct on Belgrade-Bar railway line in Europe where the height is 139m. Digital Terrain Model of the bridge site (not to scale) is illustrated in Fig.1a
Except for a meandering NH-37, there are no other significant roads. The alignment is interior in the hilly forests and far-flung from the NH. All construction materials like cement, steel, aggregate and equipment are to be carted from various places to the work sites along the NH. Hence, new approach roads were to be made with heavy-duty excavators from the NH to the sites - often at steep slopes and sharp curves. (Fig.1b). Undulating ground with thick vegetation poses a severe constraint of space required for stacking of the materials.
Contamination of streams by construction waste is to be minimised. The rich local culture and heritage are to be given due respect.
The salubrious environment of the Northeast with fragile flora and fauna is a treasure to be well protected with least damage during construction and operation phases.
Heavy Rainfall: The area receives almost 3500mm of rain annually. Hence, the alignment needs to be designed with adequate drainage to cater to the flash floods and sharp flows causing erosion and instability. Flash floods with landslides are a serious concern, as experienced by the NH presently, which need to be addressed now to ensure that the slopes are well protected and adequate drainage arrangements are made.
Most-severe Seismic Zone: The entire railway line, including the bridge, lies in the most severe seismic zone due to its proximity to the active fault between Indian and Eurasian tectonic plates. Static methods for bridge analysis given in the code are not representative to study the dynamic response of this massive structure. Hence, to ensure overall stability of the bridge and the trains running on it, specific site studies and detailing are essential.
Geological conditions: Being sedimentary formations, the general soil type is classified as shale interspersed with sandstone. Shale when exposed to atmosphere, loses its shear strength drastically, warranting out-of-the-box slope protection measures to stabilize the slope as well as protect the surface from erosion due to the heavy rains in this region.
Security and Law and Order issues: Militancy is a serious concern which needs to be properly addressed not only during construction but also during operation. Threat calls, armed attacks on sites, kidnap and ransom demands of engineers, are a regular phenomenon which are tackled in coordination with the local State Government.
Finalization of Bridge Configuration
Due to the complexity of the conditions and challenges enumerated above, it was decided that the total bridge system needs to be properly dealt right from design to construction to operation stage. Hence, a special team of experts – the Technical Advisory Group (TAG) - was formed. It comprised members from various fields like Bridges, Earthquake, Geotechnology, Hydrology, Environment and Security. The TAG was given full powers to deliberate and decide the best configuration for the bridge from all aspects.
Recommendations of TAG
After detailed investigations, site visits and deliberations, TAG made recommendations considering the constructability, maintainability, durability, and environmental conformity, besides meeting functional requirements of the structure. The provisions are discussed in the subsequent paragraphs.
Reference Codes & Manuals
As this bridge is the first-of-its kind in India, TAG recommended that, apart from IRS codes (Indian Railway Standard), other codes like IS (Indian Standard), IRC (Indian Road Congress), AREMA (American Railway Engineering and Maintenance-of-way Association), UIC (International Union of Railway) and Euro codes shall also be considered to the relevant extent for ensuring a robust design.
Deciding Span lengths
The total gorge length to be spanned is around 700 metres. Various options were considered: continuous spans, arches, balanced cantilever, cable stayed, and simply-supported-through-type-open-web-girder. Longer the span length, higher are the seismic forces and associated deformations, which are detrimental to the safe running of trains. On the other hand, smaller spans entail a larger number of tall piers, which enhances the cost.
To strike the right balance, it was proposed to adopt a span of 106m, as such spans are already in-vogue. The simply-supported option was most desirable in view of its advantages of simple design and maintainability, besides safety and long-term stability. Other spans of 71.5m and 30m are also adopted on the sloped approaches as per site-suitability
As desired by TAG, consultants having sufficient experience in similar designs were engaged. In addition, following IITs (Indian Institute of Technology) were associated:
- IIT/ Kharagpur: For developing the site-specific earthquake spectra, as per Colda requirements
- IIT/ Kanpur: For performing the wind tunnel tests for ensuring safety and stability against vibrations
- IIT/ Guwahati: Validation of structural designs and drawings, and value addition
- IIT/Roorkee: Validation of slope-stability studies and protection works
As per exploratory bore-logs, the general trend in soil profile is 5m of thick soft shale, followed by 12m of fractured siltstone with interspersed boulders and overlaid on hard sandstone rock. Open foundations are ruled out as the SBC of shale are extremely poor. Well foundations are practically difficult considering boulder strata. Hence, Bored cast-in-situ RCC piles are the preferred option. The pile is 30m long well-anchored into the hard sandstone to safely withstand the huge vertical and horizontal forces under most-adverse conditions. Capacity of the piles needs to be calculated based upon the subsoil characteristics available beneath the perfidious layer prone to destabilization.
As some of the piers are on the slopes, structural design of pile foundation is also influenced deeply by the extent of slope failure. In absence of lateral soil resistance, the unsupported length of pile increases simultaneously and the design moment on pile also rises. In such scenarios, the predominant design factor for pile foundation is the moment capacity of piles instead of axial or lateral capacity (Ref. Figures 2a & 2b).
Skin friction is not considered as per the Codal requirement due to soil liquefaction in high seismic zones, corroborated by cyclic pile load tests. All piles are tested ultrasonically by CHUM method to establish their integrity for full depth.
As the piers are high, the design is governed more by the top lateral deflection in view of the stability of the superstructure and safety of running trains. Three different pier types were examined: Steel trestle, Reinforced Cement Concrete (RCC) hollow cylindrical and RCC hollow tapered. As there are no other standards, it was decided to limit the lateral deflection of pier to H/500 as per the Building Code, where H is the pier height. Time-History Analysis are performed to understand maximum pier deflections and associated curvature of the track.
Steel trestle was failing in deflection criteria. Tapered hollow sections though apparently economical, pose problem during construction by slip-form. Hence cylindrical hollow RCC piers are selected which have better aesthetically appeal. Economy is achieved by reducing the thickness along the height. To provide better rigidity, diaphragms are provided at regular intervals. Construction of the pier is by slip-form technique, which does not need any support from the bottom. It is supported on series of jacks that anchor against the set concrete.
Second Order Effects on Tall Piers
Tall piers are considered slender as per the height to base dimension ratio. Hence second order analysis becomes relevant. The second order effects can be considered as supplementary action to the internal forces and corresponding deformation due to the first order effect. Second order effects are additional action or effects caused by the interaction of axial forces and deformation under load [4, 5] (Refer Figure – 3a).
The influencing factors for assessment of second order effects are considered as :
- Slenderness ratio
- Magnitude of Axial Load
- Magnitude of Moment
- Reinforcement ratio
- Flexural rigidity of cracked section
As mentioned in para 3.3 supra, maximum span length of 106m is proposed with Open-web through (OWT) steel girders which are ideal and economical. Steel elements are fabricated in workshop with controlled conditions ensuring consistent quality. Apart from normal loads, members are designed for special requirements like fatigue, erection loads and serviceability. Central camber of 165mm is provided to counter for the vertical deflection under loads.
The overall height of the OWT girder is decided as per the vertical deflection criterion, whereas width had to be decided from consideration of stability and torsional rigidity. After several alternatives, height of 12.5m and width of 8.5m are found to be satisfying the requirements. The guiding criterion to decide the width of the girder is the confirmation of the torsional requirements of UIC code 774 3R.
Apart from normal loads, members are designed for special requirements like fatigue, erection loads and serviceability. Central camber of 165mm is provided to counter for the vertical deflection under loads.
High Tensile Steel of grade E410-B0 as per BIS:800 is used for main chord members with high demand of tensile strength while E250-B0 grade is adopted for cross-girders and stringers with high demand of fatigue resistance under dynamic train loads. All steel members are fabricated from sheets and rolled sections by modern techniques of automatic CNC driven submerged-arc-welding and fully tested ultrasonically. Surface protection of steel is achieved by grit-blasting and metalizing with alumimium to a thickness of 150 microns followed by 4 layers of finishing coats.
The fabricated members are transported to the site, which is more than 1000km away, in special road trailers fit for traversing on hilly terrain roads. Each girder weights around 850tons. Launching such a heavy girder to a height of 140metres is a herculean task as no central supports from bottom can be provided due to site constraints. Hence improvised site-specific scheme of launching was developed modifying the conventional cantilever erection technique the steel girders.
The individual members are erected one-by-one by a special overhead crane and connections between members is performed with high strength friction grip bolts HSFG bolts.
Due to the long span configuration, tall piers and high order of wind and seismic forces, the magnitude of loads as well as movements to be borne by the beatings are high. Conventional roller-rocker-bearings are found inadequate, especially to counter the high uplift forces due to earthquake. Even pot bearings are found to be having limitation.
Hence spherical bearings with special arrangements are the most suited alternative to safely cater to these special requirements (Fig.3).
The bearings are equipped with claw-and-clamp arrangement to hold the superstructure down to the pier with anchor bolts. This will prevent the lifting-off of the girder in case of severe seismic event that may cause dislodging of the span and eventual collapse, with severe consequences.
In addition, lateral seismic restraining blocks made of RCC are also provided on both sides at both ends to further contain the girder from toppling down during a severe seismic event.
Final Span Configuration
Summing up the studies of various elements of the bridge, the final configuration adopted is 5 spans of 106m with 2 spans of 71.5 and one of 30.0m on the approaches.
As the bridge is situated on a deep ravine, pier heights vary from 20m to a maximum of 141m (which is going to be the World’s Tallest in this category!) The final configuration of the Bridge is shown in Fig.4. The diameter of the tallest pier is 16m with the ring thickness of 2.0m gradually. The pier is supported on 81 bored-cast-in-situ RCC piles of 1.5m diameter. Superstructure comprised of steel through girder fabricated with High grade steel.
Unique Design Features
Due to the specific challenges and locational constraints mentioned in above paras, conventional design methodology was not found adequate to make a safe, constructible and sustainable design. Certain unique design and construction features adopted for this world record bridge are explained below:
The spectra given in the seismic code (IS:1893) is general in nature and valid for the entire Zone-V uniformly. The effect of faults in the vicinity of the structure is not reflected. Hence site-specific spectra had been developed by IIT/Kharagpur. All the seismic events within a range of 350km and magnitude exceeding 3.5 Richter in the last 250 years had been considered for the study and an empirical formula developed based on regression analysis. Based on this, responses with various time periods had been developed for the bridge location using computer software and these data are utilized for plotting the site-specific response spectra. The PGA as per this spectra is 1.1 times higher than the code specified value as shown in Fig.5a. Detailed modal analysis is performed on the bridge model considered as a whole (Fig.5b).
Wind Tunnel studies
The bridge is situated in a deep gorge. The design wind speed calculated as per IS:875 is around 225kmph, duly factoring for the terrain and the pier heights.
To understand the behavior of the bridge during the high wind, it was considered necessary to perform wind tunnel studies. The services of IIT/Kanpur were utilized for preparation of a 1:325 scaled down model fitted with instruments and studying the performance in a wind tunnel (Fig.6).
A comparison of forces and moments as per theoretical analysis and wind tunnel studies was performed and the higher and more critical values were adopted for final design.
As the piers are located on sloping ground, computer analysis is made simulating the ground conditions and soil properties to establish the stability during construction phase as well as on long-term basis.
Necessary soil stabilization and slope protection measures proposed by expert consultants in consultation with IIT/ Roorkee are being provided. The scheme adopted is a combination of grouted soil anchors with wire-mesh support and green facia with coir mats as shown in Fig.7a.
While the soil anchors provide stability of slope, the facia ensures surface protection from erosion due to rains.
Due to restriction of time and working space, most of the activities right from piling to pier construction are mechanized to improve efficiency and quality. Hydraulic rigs, slip-form shuttering, concrete pumps, self-erecting lifts for carrying men and material are some of the equipment deployed. Typical slip form construction is shown in Fig.8a and 8b.
Inspection & Maintenance
During trains operation phase, the bridge needs to be inspected on regular basis and maintained. Due to the magnitude of the bridge, conventional methods are not adequate.
Hence following 3-pronged approach is proposed:
- Providing access by way of spiral ladders, lifts and walk-ways to reach the various elements of the bridge
- Structural health monitoring by instrumentation where different sensors are fixed at critical locations on the bridge which constantly measure various parameters like stresses, deflections, tilt and wind speed. Typical scheme is shown in Fig.9. The sensors are connected to a central data processing system and storage unit. Real-time alerts are flashed whenever a threshold value of a parameter is exceeded so as to regulate train services as required. The data can be stored for analysis of the performance over a period of time.
- Drones fitted with high-resolution cameras that scan the surface of piers to identify any defects like cracks are flown at regular intervals. (Fig10). They can cling to the pier surface and move continuously while recording the data. The information can be stored and compared at intervals to assess the propagation of damage if any.
A bridge of this magnitude needs to be constructed with sustainability in view. Following innovative measures are being adopted to make the project sustainable:
Soil Cement Blocks
Excess soil from the excavation is converted into blocks in terms of IS:1725-1982 by simple block making machinery. After setting and curing, the blocks are used in the project itself for slope protection. This innovation is not only economical and environment-friendly, but also encouraged local employment and use of local resources.
The concept has been appreciated by the State and Central governments and nominated for the prestigious Golden Peacock award for eco-innovation.
The bridge is a world record structure and will be a global attraction by itself. Besides, its location in a picturesque valley with rich scenic beauty makes it more attractive to tourists the world over. The local government is involving the local people to promote home-stay concept, where they will act as hosts cum guide to the visitors. This eco-tourism concept is a win-win situation for all!
Promoting local resources
To the extent feasible, local material, machinery and skilled labour is deployed like aggregate, artisans and vehicles. This benefits the local people who get involved in the project.
The project is located in sedimentary rocks rich in fossils and special formations. During excavation, several fossils are unearthed. All such fossil rocks are showcased in a museum at the tourist spots.
The project acceptability of the local people is of utmost importance. Hence, to promote a positive image, certain CSR activities have also been taken up. These include water storage tanks with piped drinking water facility, equipping schools with additional rooms, play grounds and toilet facilities for promoting hygiene, and so on.
Like necessity, challenge is also the mother of innovation! The wide spectrum of challenges in this project - ranging from technical to social, cultural, environmental and security - have given us the opportunity to innovate and introduce certain novel concepts in designing and constructing this iconic structure, to serve the nation for decades to come.
It is an honor for India that this world record bridge is entirely designed and constructed indigenously by the team headed by the author. The dedicated efforts of all stakeholders is highly commendable in realizing the vision to build this mega bridge project using state-of-the-art technology to make it sustainable and to provide rail connection to the remote Manipur state and benefitting the entire nation.
- Pile Foundation Analysis and Design, By Poulos, H.G. & Davis, E.H., John Willy & Sons, 1980.
- IS 1893 (Part – 1), 2002, Criteria for Earthquake Resistant Design of Structure, Bureau of Indian Standard.
- IS: 875 (Part3), 1987, Code of Practice for Design Loads (Wind Loads) on Buildings and Structures, Bureau of Indian Standard.
- IRC: 112-2011, Code of Practice for Concrete Road Bridges, Indian Road Congress
- IRC:SP:105-2015, Explanatory Handbook to IRC: 112-2011, Indian Road Congress.
- UIC 774-3, Track/Bridge Interaction – Recommendation for Calculations, International Union of Railways.
- Bridge Health Monitoring and Inspections Systems - A Survey of Methods, By Andrew, G., Tyler, J. & Arturo, S., Department of Civil Engineering, University of Minnesota.
- Global Trends in Sustainable Design of Long Span bridges – Saibaba Ankala, Chief Engineer, Indian Railways.