Design Analysis for Sagai Crossing Bridge Construction Project

The existing concrete roadway across the wadi overtops during periods of spring ice melting and summer rainy seasons, rendering the 315m crossing impassable. Additionally, the strong flows and scour-susceptible soils at the site have severely deteriorated the concrete road. The proposed bridge crossing is designed to be adjacent to the existing concrete roadway, with a reinforced concrete superstructure and substructure. Approach roadway work will be required to transition from the existing roadway to the bridge. Riprap and guide banks will be used to protect the substructure and approaches, as required for scour protection. The guide banks were designed to protect the north abutment of the proposed bridge, which encroaches 100 meters into the wadi. The purpose of constructing a guide bank is to reduce the scour at the abutment by lessening the turbulence at the abutment and to move the scour away from the abutments. A scour analysis was performed to determine the scour depth at the proposed bridge. Without the guide banks, the scour depth was determined to be 6.92 meters on the north abutment and 5.47 meters on the south abutment. With the guide banks, the scour depth at the abutments was reduced to be 3 meters at the abutments and 2 meters at the piers. Considering an additional factor of safety, the bottom of abutment footings shall be 4 meters below grade and the bottom of pier footings shall be 2.5 meters below grade. A limited hydrologic analysis was performed for the proposed bridge construction. The bridge is designed for a 100-year return interval. Due to the lack of available data regarding rainfall depth, rainfall distribution, soil and vegetation characteristics, and topography in the project area, a detailed hydrologic model could not be performed to determine the design discharge. Instead, an indirect approach based on discharge per unit area was adopted. The design discharge was calculated to be 1,200 cubic meters per second, which is the design flow for the bridge.