Design Analysis for Bridge #09 Replacement on Gardez to Khost Road, Afghanistan

The existing bridge, a two-span concrete deck/concrete girder bridge, has partially collapsed due to scour and abutment failure. The proposed bridge replacement is a three-span beam bridge with two piers, constructed out of reinforced concrete. The new bridge will be approximately the same width as the existing bridge but will include an additional pier, providing increased capacity. Hydraulic analysis was conducted using HEC-RAS version 4.1.0, with a detailed topographic survey of the river and its vicinity conducted in January 2014. Thirteen cross sections were encoded at an interval of generally less than 20 meters between cross sections. A Manning's n value of 0.045 was selected to represent the rocky, unvegetated conditions in the river. The proposed bridge superstructure and substructure will be constructed out of reinforced concrete, with a finished grade of the river bottom under the bridge at approximately elevation 1854.2 meters. The initial recommendations for the new Bridge #09 were to install a slightly taller or higher bridge opening to provide additional capacity. However, the replacement bridge is approximately the same width and includes an additional pier, resulting in relatively high velocities and shear stresses. Hydraulic modeling results show velocities at the bridge approach of 4.2 m/s and shear stresses near 304 N/m2. Average velocities through the bridge of 3.5 m/s, ranging from 3 to 4.2 m/s across the cross section, were also observed. A geotechnical investigation was conducted by KA Construction Materials Testing and Soil Investigation Laboratory in Kabul, Afghanistan in January 2014. Five boreholes were advanced at the proposed locations of the bridge abutments and each of the piers. Gradations were obtained for samples taken at a one-meter depth interval for each of the boreholes. The results of the gradation analyses show that average d50 for all samples is approximately 23 mm. Boreholes 1 and 2 show smaller d50 values nearest the surface, while smaller values also occur in boreholes 4 and 5. The proposed bridge design includes a three-span beam bridge with two piers, constructed out of reinforced concrete. The new bridge will be approximately the same width as the existing bridge but will include an additional pier, providing increased capacity. The hydraulic analysis was conducted using HEC-RAS version 4.1.0, with a detailed topographic survey of the river and its vicinity conducted in January 2014. The geotechnical investigation was conducted by KA Construction Materials Testing and Soil Investigation Laboratory in Kabul, Afghanistan in January 2014. The design analysis provides documentation of the basis for the design of the new Bridge #09. It describes the project requirements, identifies governing codes and criteria being utilized, explains proposed design solutions, and documents other situations that affect the design. Engineering calculations are also included where appropriate. The design analysis is organized by content and technical design discipline, with sections on hydraulics, geotechnical, civil, structural, and bill of quantities.