A number of soil test service providers are willing to take shortcuts due to short deadlines and underbudgeting from the client or due to greed and malpractice. A hasty and poor quality soil test leads to extremely expensive failures and lengthy delays in bridge construction, as highlighted by the cases of Jabdighat and Thimura
Development of road networks and bridges in Nepal started around 60 years ago. During the initial years, most of the major works were completed through the technical and financial assistance of donor countries. Later, the Department of Roads (DoR) and other departments like DoLIDAR took up the task. Today, around 250-300 bridges are built annually by DoR alone, and of a similar range by the provincial and local governments.
Some of these constructions fail at different stages before their design life span. However, the failure rate of bridges in Nepal has increased tremendously in the last five years. During 2017-2021, at least 50 bridge failures have been highlighted in the national media.
This year alone, national newspapers have reported the collapse of 21 motorable bridges in different parts of the country. Most of them have collapsed during construction or just after their completion.
Eleven of them failed during the construction period and the remaining after completion of the structural works. The cost of these bridge failures is around Rs 2 billion, as per DoR. More importantly, the series of bridge failures has created an environment of fear among the users whose livelihood (both social movement and business) may be affected.
The probable cause of bridge failure can be broadly categorised into improper construction methodology, inferior quality of works, faulty design and poor maintenance.
Some bridges failed during construction due to washout of the shuttering support as in the Mahesh Khola in Dhading. And some completed bridges collapsed after the support settled during floods as in the Kamala and Jabdighat.
Some bridges were overflooded as in Bhoteodar and Melamchi. At the same time, some bridges in the Chure region are affected by excessive deposition, impending flood damage as in Dolalghat.
Different considerations must be made to ensure the safety of a bridge at three distinct stages, namely design, construction and maintenance. We shall discuss aspects of the design stage in this article. Before starting the detailed design, the bridge site must be studied thoroughly – the topography, hydrology and geomorphology of the river, geotechnical condition, roadway alignment and ecological concerns.
Bridge site selection is one of the most important steps towards its safety. A bridge constructed at the bend of a river must take into account different considerations than at a straight reach. Our design approach is mostly based on straight flow assumption, which can be risky for a bridge at a river's bend.
Most of the bridges are built without analysing the possible extreme morphology of a river. Likewise, study on the extreme hydrology of the streams is quite preliminary. Ample good quality data is the biggest challenge to hydrologists in Nepal.
Some bridges suffer from poor hydraulic design.
There is no practice of considering sedimentation under the bridges in Nepal, which is commonly seen in the bridges at the Chure foothills. Moreover, bridges in Nepal are designed for 100 years of floods whereas in a developed country, it is designed for 150 years. Additionally, to minimise the cost of a bridge, there is a practice to reduce water-way from the normal channel width. This may cause changes in the morphology of the rivers, such as bed and bank erosion and upstream deposition.
Another major challenge to bridge design in Nepal has been the quality of geotechnical investigation. A number of soil test service providers are willing to take shortcuts due to short deadlines and underbudgeting from the client or due to greed and malpractice. A hasty and poor quality soil test leads to extremely expensive failures and lengthy delays in bridge construction, as highlighted by the cases of Jabdighat and Thimura.
At the same time, structural design of bridge components has been on the over-safe side in most of the recent bridges in Nepal.
Whether it is due to poor construction supervision and quality control practice in Nepal or plain lack of knowledge in design, designers often adopt oversafe size of bridge parts.
Additional concrete poured without need is a waste of valuable resources on the one hand and poor economics on the other.
Unnecessary conservatism in design also leads to uncomfortable approach roads joining bridges, as seen in several bridges in urban areas as well as the Tarai. In order to tackle the lack of knowledge in design, a two-pronged approach is proposed. One is capacity building of design professionals through trainings and workshops.
Another is vetting all designs through capable third-party checkers. Design costs are generally 1-2 per cent of construction cost. So small addition of the proof-checking cost (like 0.5%) can result in substantial savings from out of proportion sizing of bridge elements, and additional safety from some factors overlooked by the design team.
Lastly, another design issue that leads to failure of bridges during construction is the failure of temporary supports or "scaffolding".
Design of temporary works used to support the bridge during construction falls on the contractor, but it is rarely requested by the client or submitted by the contractor. However, the major issue with temporary support in Nepal is not its structural strength, but its stability inside the river.
Where diversion of the river is not possible, sufficient waterway must be ensured between/under the prop supports. Problem of insufficient waterway under the supports was aptly highlighted by the failure of the Badigad Bridge in Gulmi twice in two years. Design of temporary supports requires input from not only the structure engineer, but also the hydraulics and geotechnical engineer. The more the time spent and considerations made during design, the less will be the issues faced during the subsequent stages of bridge management.
Jha and Chaudhary are engineers by profession
A version of this article appears in the print on July 20 2021, of The Himalayan Times.