A study of possible bank protection measures near the Meghna Bridge, Bangladesh

Abstract

In 1990, in Bangladesh, the "Meghna roads and highway bridge"was finisthed. This bridge is the first one in a series of two Bridges replacing the ferries over the Upper Meghna River. During construction, and also after the finalization of the Bridge, it was revealed that the designers of this Bridge underestimated the power of the Upper Meghna River here. This revealed itself in various aspects. The most severe ones are the slope failure just upstream from the Bridge which occurred already during the construction, the collapse of (parts of) the slope protection of one of the Bridge's abutments (eight months after the finalization) and the present danger induced by the morphological planform changes of the river just upstream of the Bridge. In the future this could lead to outflanking of the Bridge by the River. This study discusses the various possible measures which can be taken to safeguard the Meghna Bridge on the long term. Firstly, in part I of this report, the present situation at the location of the Meghna Bridge is described and analyzed. Subsequently, in part I I of this Report, a design study is conducted. Designs, solving the present problems, are elaborated onto a detailed design level. These designs are all elaborated with the specific characteristics of Bangladesh being a developing country in mind. Therefore an important aspect during the design study is the use of local resources (both technical as labour) as much as possible. The design study resulted in three final alternatives. These alternatives are evaluated by means of a Multi Criteria Analysis for their non-monetary criteria. Taking also into account the costs estimates for all alternatives resulted in the recommendation of "alternative I I I ". This "alternative III" consists of the following. A series of six groynes is foreseen in the river bend just upstream of the Bridge and a guide bank structure replaces the damaged abutment of the Bridge. A remarkable aspect of this finally recommended design is the fact that all projected structures are to be constructed with a core of sand cement stone and concrete blocks as slope protection. Instead of the (almost classical) method of a soil core with a slope protection of fascine mattresses etc., which is also elaborated being one of the alternatives, it is concluded that the application of sand cement stone as construction material here is the best option. This is explained by a combination of reasons. As the country of Bangladesh is located in one of the largest deltas in the world its soil consists mainly of the fine alluvial deposits as (rather fine) sand. Therefore rock and boulders are rather scarce construction materials here. Another important aspect is the fact that Bangladesh is a developing country. Therefore "high tech" construction techniques as dredging in deep water and the placing of elements under water are locally not (yet) widely known and available. Using sand cement stone and concrete blocks as construction materials requires only simple construction techniques. Another merit is the fact that in the structures rather steep slopes can be applied which results in a (relative) saving of material and therefore results in low costs. These (and others) reasons finally resulted in the recommendation of the mentioned alternative.