There are 42 piers in the Padma Bridge. Each pier is supported on a group of 6 steel piles, each 3 metres in diameter and 120 metres deep. The piles were arranged in a battered (inclined) fashion for resistance of combined vertical loads and horizontal seismic loads. To facilitate design of bridge pile foundations, soil investigation is carried out at each pier site to determine the soil type and properties. During design phase, soil investigation was carried out at only 13 pier locations instead of all 42 locations. However, during construction, extensive soil investigation was carried out at all 42 pier locations.
Padma is one of the mightiest rivers in the world, second only to the Amazon River in terms of high river current during the monsoon season. It was estimated the river scour was estimated to be 49 meters deep, considering a 100-year event. Another 13 metres of soil would lose strength due to liquefaction during a design seismic event. The load bearing capacity of a pile is composed of 2 components: skin friction and end bearing. The skin friction is the load resistance provided by friction between outer surface of steel pile and the surrounding soil. The end bearing is the load resistance provided by soil at the bottom end of the pile. Thus out of total 120-metre pile length, 62 metres are not considered for skin friction capacity.
The capacity of the piles was increased by cement grouting of soil around the piles and at the bottom end of the piles. These operations had challenges on their own and had to be overcome. About 60% of the total load is transferred by skin friction and remaining 40% by end bearing. Design capacity of each pile was assumed to be 10 Mega Pascal (MPa). However, pile load testing revealed a capacity of only 7 MPa. Pile capacity was inadequate due primarily to the presence of clay type of soil instead of usual dense sands. Clayey soils have poor load bearing capacity compared of sandy soils. This created a major challenge during construction of the bridge. Relevant experts were engaged to develop a solution. Based on additional analysis, it was decided to add a 7th pile vertically and at the center of the pile group under 22 piers. To drive these piles multiple hammers were used and one of the hammer was Menk 3500 KJ which is the largest of its kind in the world built by German company Menk. This process caused a construction delay of almost two years.
Another problem arose in 2019. It was related to the bearings under the main steel superstructure. Friction Pendulum Bearings (FPB) are used at the support between the steel superstructure and the concrete pier foundation. These bearings help dissipate seismic forces on the bridge during earthquakes. One of the bearings showed problem during installation. These bearings are the largest in world in bridge application. It is used for the first time in Bangladesh. The Benicia-Martinez Bridge in San Francisco first used this type of bearing in 2002 for seismic retrofit. Now it is used all over the world.
These bearings were manufactured in Wuhan, China. The manufacturer has a very good testing facility and they tested all the bearings of the Padma Bridge in their facility. One of the bearing was damaged during installations in 2019. The bearings were modified and tested again in the Wuhan lab. As a part of independent quality assurance (QA), one of the bearings was tested at the University of California San Diego (UCSD) Seismic Device testing Lab in January 2020. This is the only lab in world that can test in multi directional seismic load. Other labs can only perform unidirectional test. The bearings are modified and tested again in Wuhan lab. The bearing passed the testing.
With the visionary leadership of late National Professor Jamilur Reza Choudhury as chairman of the International Panel of Experts and his relentless pursuit of perfection, and the hard work of BBA engineering management team, Bangladesh is getting a world class bridge over the mighty Padma River. In this regard, I would like to point out that one of the key issues for successful completion of the project was having the same Project Director for entire duration of the project. It kept continuity as the project went through many challenges during its construction. We know that some projects in Bangladesh see their project director change many times during course of the project. That is detrimental to a project’s successful completion.
We learned a lot during construction of the project. With numerous testing of the soil we acquired tremendous amount of knowledge about the deep layers of soil in Bangladesh. With the knowledge learned during the construction of the Padma Bridge, Bangladeshi engineers now can build future bridges in significantly lower time. In future projects, a small portion of the budget should be allocated to train young local engineers. Bangladeshi engineers are now working on mega projects in all over the world. So our local engineers can build large projects in Bangladesh if we can train them properly and also via technology transfer.
According to data available, the cost of the Padma Bridge was estimated at USD 1.6 billion in 2006 price (Ref. Request for Proposal, Padma Multipurpose Bridge Design Project, 25 March 2008). The project is being completed in June 2022 at an approximate cost of USD 4 billion. Considering the extra work during pile installation, the seismic bearings, separating the 400KV electrical line and overall inflation during the 16 year period, this cost is considered quite reasonable. As a comparison, I was part of the engineering team for construction of the new eastern span of the San Francisco Oakland Bay Bridge (“the Bay Bridge”). Initially its construction cost was estimated at USD 1 billion in 1999. Construction of the Bay Bridge started in 2002 and was completed in 2013. The final cost of this project rose to USD 6.5 billion due to multiple challenges during its construction.
Mega projects always face difficulties during construction. Some of these challengers are unique due to local site conditions, massive size of the various components and large forces involved. Sometimes research and development is needed to find appropriate solutions. It can be emphasized that the people of Bangladesh got a world class bridge with a reasonable cost. Having overcome all the challenges, including financial and technical, the Padma Bridge will be an iconic symbol and a great source of pride for Bangladesh for the generations to come.
* Dr. Mohammad Abdul Awal is Member, International Panel of Experts, Padma Multipurpose Bridge Project and Bridge Engineer, San Francisco, California, USA