The OMO River Bridge/2010

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1 Sometimes the simplest bridge designs come in the most complex and innovative engineering and construction packages. Such is the case with the OMO River Bridge located in the Southern Nations and Nationalities of Ethiopia near the town of OMORATE. Midroc Foundation Specialist Plc, of the several projects currently being executed, is engaged in the construction of the second phase of the bridge after successfully completing the first phase of the project which comprised of the foundation, substructure and retaining structures back in February, the new bridge with innovative construction techniques that were the only viable solution to the project s numerous construction challenges. To this end, the company in the first phase of the project, apart from the conventional construction methods employed, has successfully put the following innovative construction techniques into practice Construction of secant pile retaining walls on the east and west pier locations of the bridge. Secant pile walls are an innovative way to build retaining walls. They are formed by a series of interlocking drilled piles and are used primarily where there is a high water table, unstable ground conditions, restricted working space and a need for control of ground movements and groundwater ingress, as was the case with the OMO River Bridge. The company, engineered by a highly experienced inhouse design department, provided the full design of the secant pile wall constructed. View of steel section of the Bridge The bridge is being constructed in the heart of an area of historic, ecological, agricultural and archeological significance. From an engineering standpoint, this project was not just a structural challenge but it required innovative geotechnical, hydraulic and environmental solutions as well. The company embraced the challenge of building 1 P a g e Secant pile drilling using BG 14

2 The secant pile wall was made of diameter 620 mm piles, and boring was made with an overlap of 8 cm. Every second pile (secondary pile) was reinforced by a reinforcement mesh to stand primarily the load from earth pressure and water pressure. The work started with construction of a guide wall, so that the center distance between the piles corresponds exactly to the required overlap. The secant pile wall was bored to a total depth of 8m. The secant pile wall was excavated to a depth of 4.5 meters, upon completion of piling of 49 structural piles to a depth of 28 meters, and bracing with H-Beams was done prior to trimming of structural pile heads and concreting of the pile cap. Secant pile wall after excavation Inaccessibility of the west side of the river was among the many challenges of the project, which in this case the company had to devise a way to deliver all the construction machineries and materials to the inaccessible side. The company in this regard employed the use of a 55 ton capacity barge, specially designed and manufactured for the project, to convey all the required construction machineries and materials to the west side. The use of the barge emanates from the construction expertise we have gathered through the years that enabled us to assess and provide a practical and economical solution to the challenge. The work starts with a construction of a guide wall, so that the center distance between the piles correspondes exactly to the required overlap Assembly of barge pontoons at the construction site 2 P a g e

3 During the first phase of the project, the barge successfully transported machineries ranging from 10 to 50 ton in weight and all the required construction materials to the west side. In addition to that, land reclamation work both on the west and east pier sides of the river was performed to facilitate the construction work and keep water side-works to a minimum. Pile drilling machine (BG 14) being transported to the west side using a barge These innovative construction techniques employed for the project come in taking a sound concept, adapting it for the new conditions and applying it in a previously unproven way. The company by employing these techniques has successfully dealt with the construction challenges previously thought to be a construction impasse. Land reclamation work being executed The second phase of the project, currently being executed, consists of erection of a CALLENDER- HAMILTON Steel Bridge, construction of approach structures and approach roads. The C- H steel bridge erection is almost on the verge of completion with incremental launching erection method for a bridge span of almost 128m weighing 600 tons being implemented for the first time in the country. The company has successfully applied the incremental launching erection technique, which eliminates the need for temporary erection towers and piece- by- piece in place erection of structural steel required by conventional methods. Erection of launching nose members in a specially excavated launching pit behind the east abutment 3 P a g e

4 Steel assembly of the C-H Bridge began in December The launch of each of the bridge s sixteen 8m spans began after completion of erection of a steel launching nose in a specially excavated pit behind the bridge s east abutment. The temporary launching nose, weighing more than 90 tons, was attached to the front of the first span, and the structural steel was gradually pushed on a system of guided roller bearings set on launching beams placed in the launching pit and pier caps of the bridge. till the front and last panels of the bridge fully rest on the west and east pier caps respectively. being launched from the east side pit being launched from the east side pit approaching the west side pier Part of the launching nose hanging over the east pier cap during launching The entire steel bridge deck system, including all diaphragms and lateral bracings, was launched from the east side pit adding one panel at a time being resting on west pier location. 4 P a g e

5 The company s immense construction expertise, commitment and willingness to take a risk have boosted its endeavor to meet the stringent technical requirements set for the construction of the bridge. The client and the community have recognized the first phase of the project as a tremendous success and it is very much likely to be the same for the ongoing second phase of the project which is being expedited to completion. The construction techniques, applied for this bridge, are blazing paths for new projects to follow and the company in this regard will keep on striving to introduce innovative construction techniques. 5 P a g e