For a landlocked country like Bhutan, road network system forms the life line for its socio-economic development. Bridges form vital links on any road network system especially in a country like Bhutan where bridges have to span deep river gorges and fast flowing rivers. Bridges in fact form the life line of the road system. The fragile slopes and the deep river gorges that mainly dominate the topography of our mountainous country, pose unique challenges to the engineers involved in design and construction of bridges. The relatively young construction industry in our country is continuously being challenged by the need to match up to the required know-how both in terms of experience and expertise to implement major bridge projects.
Of the number of major bridge projects being undertaken by Department of Roads, Amochu Bridge on Samtse – Phuentsholing Highway was one of the most challenging ones. At 175m span, the bridge is the longest permanent bridge in the country. In addition, in a bid to enhance lateral stability against wind and seismic forces and to improve overall aesthetic appearance, the main structural arch members of the bridge take the shape of a three dimensional parabola. A 175m long bridge spanning across a river gorge more than 30 m deep compounded by a relatively complex structural form made construction of Amochu Bridge a challenging task indeed.
Amochu Bridge at Tabaramtey located around 8km from Phuentsholing is the critical link on Samtse – Phuentsholing Highway. The Highway forms a part of the southern East – West Highway planned to ultimately connect Sipsu in the west with Jomotsangkha in the east of the country. In addition, the Highway connects Samtse Dzongkhag with the rest of the country by internal motorable road which otherwise remains literally cut-off from the rest of the country when there are strikes or other such disruptions in the neighboring Indian state of West Bengal.
The construction of the Samtse – Phuentsholing highway commenced in the 9th FYP (2005) from both Samtse and Phuentsholing sides with funds from the Royal Government of Bhutan.
The detailed sub-soil investigation & design of the Amochu bridge was done by the Joint venture firm of M/s Green Place Pvt. Ltd., Thimphu and M/s Civil Park International (CPI), Bangkok, Thailand in FY 2006-07. The initial estimated cost of the bridge and its approaches was Nu 161.107 million at 2007 price level.
The contract for construction of the 175m span Amochu bridge (Semi-through Type Steel Arch Bridge) was awarded to the lowest evaluated bidder i.e. the JV firm of M/s Bhutan Builders, Thimphu & M/s DK Engineering & Construction Pvt. Ltd., Jalpaiguri at its lowest evaluated bid price of Nu 175.339 million. The contract agreement was signed on April 2, 2009 with a contract duration of 30 months till September 30, 2011. The construction supervision contract was awarded to M/s Gyaltshen Consultancy, Thimphu.
Rational for Adopting Current Structural Form of the Bridge
The Amochu river is perennial and maintains considerable flow volume all year round and is not fordable. The flow is also highly seasonal with huge variation between dry and wet season flow volumes. The average height of the bridge deck from the riverbed is more than 28m and the deck level is controlled by the ruling gradient of the approach road on Samtse side. The characteristics of the site eliminate the feasibility of a multi-span bridge since providing one would mean having to build massive piers with height in excess of 30m. In addition, the intermediate piers will have to be designed to resist the fury of the massive flood the river brings every year. Therefore, it was necessary to adopt a bridging solution that connects the banks directly in single span without interfering with the water course. The total economical span between the banks, in consideration of the aforementioned factors, was determined to be 175m.
Each bridge type has an economical span range and beyond this range the bridge type becomes less competitive cost-wise. For instance, the economical span range for Reinforced Concrete (RC) solid slab bridge is up to 12m and beyond this span and up to about 30m, the slab has to be either pre-stressed or other bridge types such as the RC T-beam or the Steel-Concrete composite type has to take over.
After considering all the possible types of bridge, steel arch bridge was chosen as the most suitable bridge type for Amochu Bridge. Furthermore, “Semi-through type 3D steel arch” was chosen for its final structural form. If the arch is fully under the road way (deck type), it becomes flatter and this adversely affect the load carrying capacity of the arch. If the arch is placed fully above the road way (through-type), the rise to span ratio increases meaning the arch becomes unnecessarily very high making it more expensive and difficult to construct. The horizontal parabolic profile of the arch ribs helps improve lateral stability of the bridge system against seismic and wind loads. The golden rule of seismic design is that the structure, to the extent possible, should be able to resist most of the seismic forces by its structural form and shape. The 3D arch by virtue of its form and shape enhances the performance of the bridge against seismic forces and therefore economizes the design. The 3D arch form also enhances the aesthetic appeal of the bridge. Amochu Bridge, besides being a long span bridge, is expected to last for over a century and therefore due consideration was given on the aesthetic appearance of the bridge.
Challenges and Breakthroughs
The first contract with the JV firm of M/s Bhutan Builders & DK Engg. & Construction Pvt. Ltd. had to be terminated towards the end of the initial 30 months contract duration in August 2011 due to unreasonable delays. The physical progress of works at the time of contract termination was around 50% only. Despite the contract termination, the Contractor was allowed to complete the on-going steel fabrication work as it is a specialized work which cannot be taken over without complete fabrication and trial assembly. It took substantial time to complete fabrication, trial assembly and settle payment upon termination.
On termination of the first contract, a Project Management Unit (PMU) was established and it was planned to take up the balance works departmentally except the erection of the bridge super-structure which was tendered out. Tendering process for the bridge super-structure erection works could be initiated only at the end of October 2011. However, there were no bidders, which led to re-tendering in the beginning of December 2011. Two prospective bidders submitted their bids and after detailed evaluation, only one bidder qualified. The qualified bidder was required to make a technical presentation on the erection methodology for the superstructure to the MLTC in mid of January 2012. However, the technical presentation was not convincing, thus resulting in the decision to re-tender the works.
Re-tendering was initiated at the end of February 2012 and with a month time extension for submission, the bids were opened on April 30, 2012. There were four prospective bidders. The bid price varied from Nu 115.752 million to Nu 225.918 million.
After reviewing the bid evaluation report, the MLTC decided to award the contract to the lowest evaluated bidder, which was again required to make a technical presentation on the erection methodology, which was done on June 11, 2012. Finally, the contract for erection of bridge super-structure and associated works was awarded to the Joint Venture firm of M/s Gaseb Construction & Kalika Construction, Nepal for a contract price of Nu 115.752 million with a contract duration of 13 months starting from August 1, 2012 to August 31, 2013. With genuine hindrances occurring during the contract duration, the contract duration was extended till November 2013. However, the work was not completed even after expiry of Liquidated Damage (LD) period and due to complexity of the works, the Contractor was allowed to continue the work beyond LD period. As can be seen from above, it took one full year on the tendering & re-tendering processes before the contract could be awarded to the second contractor.
There were other specialized works and procurements to be done such as for the hanger cables and their installation. The tendering process started at the beginning of December 2012 and ended in mid-March 2013 with the decision to re-tender the works for supply and installation of the hanger cables. After the limited bidding process, the contract for supply and installation of the hanger cables was finally awarded to the JV of M/s Juli Sling Ltd., China & M/s Durga Enterprise, Thimphu in April 2013.
Due to the massive flood onJuly 1, 2015, the temporary erection tower collapsed and hit one of the main members of the bridge and ripped off the main bracing member, which further delayed the completion of the superstructure. The damaged parts had to be removed, re-fabricated and re-installed. This was a time consuming process. Also there were no experienced contractors. The work was finally awarded to M/s Santalall and Brothers, a Siliguri based Indian firm in February 2016 and work completed in June 2016.
On completion of the fabrication and re-erection of the K-girder, the concreting of the deck started in January 2017. Halfway through the deck concreting, there was another problem of excessive deflection/sagging of the K-Girder on both the banks and cracks developed at its welded joints on Samtse side. It needed to be strengthened to keep the deflection within the permissible limit. It was another complicated and arduous task.
The Chief Designer for the bridge was from M/s Civil Park International, Bangkok and since the consultancy contract for design was completed in 2006, it was extremely difficult to obtain necessary information from the designer. It took lot of time to re-analyze the design drawings and finalize the measures to strengthen K-girders. Once the design drawings were finalized there was no specialized Contractor to take up the works. It was only by June 2017 that the work for fabrication and erection of the temporary towers, working platform and strengthening of the main bridge elements awarded to M/s Santalall& Brothers, Siliguri and M/s B.B. Engineering Works, Jaigoan. The final strengthening works which includes external post-tensioning works was awarded to M/s Karma Enterprise, Guwahati in March 2018 and work completed by May 2018. After casting of the bridge deck, the bridge was successfully load tested on June 9, 2018 for its intended load carrying capacity of IRC Load Class 40R.
Reflecting after the completion of the bridge, there are many lessons to be learnt. Following the termination of the contracts, there were many contractual issues and experiences learnt for future bridge projects. The project completion cost had almost doubled from its initial contract amount of Nu 175.339 million. One innovative construction method being adopted at Amochu Bridge that is worth emulating at other project sites with similar conditions is the use of “alpha truss” slabs for casting of the deck slab. The alpha truss serves as a formwork during the construction stage and become the permanent part of the bridge once concreting is done. No propping or temporary support is required for deck casting which otherwise for Amochu Bridge would be very expensive and difficult to provide. The design of the bridge itself was too complicated, the first of its kind in the region and with no experienced contractors to take up specialized works. Unless aesthetics is a big consideration, 3D arches may rather be avoided for future bridge projects. There may be some saving in the construction material due to added lateral stability provided by the 3D form, but this is more than offset by the additional effort, time and cost required to build the 3D arch form.
It was quite unfortunate that the construction of Amochu bridge took 9 years to complete despite all the efforts. The Contractors, Consultants and more importantly all the engineers and the technicians involved during construction of bridge faced lots of challenges due to long bridge spanning across deep river gorge and its complex structural form. After the work was taken up departmentally since September 2011, many engineers and technicians from DoR worked tirelessly for the project. Some worked equally or even more than the labourers doing odd laborious jobs. They were always at the forefront leading the inexperienced labourers such as to climb on top of the arch members, getting inside the arch segment, underneath the deck slab, etc. The sacrifices and the hardships faced by them are commendable. Although the Department lacked experienced engineers and equipment to supervise and carry out the complicated construction works, a full time bridge engineer was deployed at the site from DoR, HQ and all the small details of work discussed, deliberated and resolved at site for successful completion of the bridge. The PMU were also very hard on the machines and equipment hiring agents, transporters and suppliers especially with regard to their performance, punctuality, quantity and the quality. At times it so happened that many hiring agents, transporters and suppliers refused to work for Amochu bridge construction project.
During the critical stage of its construction, especially after excessive deflection observed on K-girder and cracks developed on its welded joints in January 2017, many were skeptical on the performance of the bridge. The news about this technical glitch reached far and wide. Many claiming to be experts in bridge design, construction, rehabilitation and retrofitting works visited the Department offering their expertise. We were advised that the whole design and construction works should be re-evaluated and re-analyzed. We were also warned of the impending catastrophic disasters if such evaluations and analysis were not carried out and bridge opened to traffic. Despite all, the Department stood by its principle and intuitions that the designs and construction works were carried out as per the standard practices. The issue was taken up as an isolated case and matters resolved in consultation with the Chief Designer after facing lots of challenges and hardships. Despite numerous problems, the bridge has finally been completed and today it stands to be the longest and one of the most beautiful and aesthetically pleasing permanent bridges in the country.
The Department of Roads is thankful to all the contractors, design consultants and the people involved in various activities at different stages of the construction of the bridge for their contribution in successful completion of the bridge. The Department would like to offer our sincere apology to the people of Bhutan and especially to the people of Samtse Dzongkhag for the delayed services. The lessons and the experiences gained from this project would be utilized in constructing better road and bridge infrastructures in future.
Contributed by Karma Wangdi Chief Engineer, DoR and MN Lamichaney Specialist, DoR