Design Analysis for Bridge No. 10 on Gardez to Khost Road, Afghanistan

10 Design Analysis provides documentation of the basis for the design of the project. The proposed bridge is a two-span structure, similar in design and detailing to Bridge #09, with a reinforced concrete superstructure and substructure. The bridge includes a concrete scour mattress for protection against scour. The hydraulic analysis for the proposed bridge was conducted using HEC-RAS version 4.1.0, encoded using the topographic survey. The main river model consists of fourteen cross sections encoded at an interval between 10 and 60 meters, with a Manning's n value of 0.045 selected to represent the rocky, largely unvegetated condition within the channel and the overbank areas. Along the tributary, eleven cross sections were encoded at an interval between 10 and 30 meters, with a Manning's n value of 0.045 selected to represent the rocky, unvegetated conditions in the river channel. The proposed design is a two-span cast-in-place slab bridge with one pier, each span being 16.8 meters long from centerline abutment bearing to centerline pier. The total conveyance width is 31.1 meters, with the width of the bridge being approximately 11 meters. The finished grade of the river bottom under the bridge will be approximately elevation 1816.31 meters upstream of the bridge and at approximately 1815.51 meters downstream of the bridge. A transition channel connecting the bridge opening and the existing tributary channel is recommended. The transition channel would tie in the bridge opening to the existing channel at a point approximately 50 meters upstream, lowering the effective slope of the creek. The channel would have a Manning's n value of 0.06 to represent some areas of vegetation and agriculture on each overbank area upstream of Bridge #10. The hydraulic capacity of the bridge and scour potential were evaluated using the estimated peak 50-year discharge on the tributary as stipulated by the project scope. Two hydraulic scenarios were assessed: 1) analysis of coincident peak flows on the main stem and on the tributary, producing the highest flow depths at the bridge, and 2) low flows in the main river and the 50-year discharge in the tributary producing the highest velocities calculated at the bridge. The proposed bridge design is intended to increase the hydraulic capacity of the crossing and provide adequate protection against scour. The design analysis provides documentation of the basis for the design, including engineering calculations and technical specifications. The proposed bridge design is a two-span cast-in-place slab bridge with one pier, with a reinforced concrete superstructure and substructure, and a concrete scour mattress for protection against scour. The geotechnical investigation and analysis were conducted to determine the suitability of the site for the proposed bridge design. The investigation included a topographical survey, geotechnical investigation, and geotechnical analysis. The results of the investigation indicate that the site is suitable for the proposed bridge design, with a stable foundation and adequate drainage.