2. Project overview The project is a bridge on XX River in Junlian County. The bridge site is located in XX County, Junlian County. The net span between the two banks of the river embankment is 60 meters. It is designed as a reinforced concrete two span simply supported bridge (31.2mX2). The total length of the bridge is 76.5 meters, and the bridge deck is 18 meters wide (12 meters for carriageway and 2X3 meters for sidewalk). 2.1 Main technical indicators (1). Design load: Level 20 vehicles, Level 100 trailers, and - 3.5KN/m2 crowds (2). Clear width of bridge deck: 2X3.75m motorway+2X2.25m non motorway+2X3m sidewalk=18m (3). Design flood elevation: 438.8m (elevation system of the county water conservancy and power bureau) (4). Design flood frequency: P = 5% (20-year return period) (5) Seismic cracking degree: 6 ° 2.2. Topography and geomorphology: Junlian County is located at the southwest edge of Sichuan Basin in the region, adjacent to the Yunnan Guizhou Plateau in the south, and the terrain in the south is high in the north and low in the north. The mountains are mostly distributed in the north, southeast and west, with an elevation of 433~807 meters, deep masonry and cutting, and a large relative elevation difference. Generally, it is a middle and low mountain area. The Silk Factory bank in the bridge site is relatively high, and the old city bank is flat. 2.5 Bridge structure (1). The superstructure adopts a non-standard span of 31.2m skew prestressed hollow slab with a skew angle of 5.5 degrees. The prestressed hollow slab is 1.4m high and 1.5m wide, and the base plate and web adopt variable section and width. The prestressed hollow slab is provided with 6 strands of prestressed steel strand, including 4X Φ 15.24 steel strand, 2 strands of 2X Φ 15.24 steel strand and flat anchorage. The bridge is 18m wide and consists of 12 prestressed hollow slabs. (2) Substructure a. Abutment at Silk Mill Bank: gravity abutment is adopted. b. Mid span pier: three column bent pile foundation is adopted, with a pile diameter of 1.5m. The pile passes through the karst cave and the broken layer, and is placed under the karst cave and the broken layer with a rock socketed depth of more than twice the pile diameter. The diameter of the pile body is still 1.5m, and the cap beam is 1.6m high and 1.7m wide. c. Adjacent to the abutment in the old city: the abutment adopts pile foundation, which passes through karst caves and cavities and is placed in the complete bedrock with a rock socketed depth of twice the pile diameter. Masonry retaining wall shall be set to resist earth pressure, and the foundation shall be placed on pebble soil layer. (3) . Bridge deck pavement: 10 cm thick 40 # waterproof concrete, plus 10 × 10 cm Φ 12 reinforcement mesh. (4) The sidewalk is about 35cm higher than the travel lane. The water pipe, cable, optical fiber and communication pipe are reserved under the sidewalk slab to ensure that the urban domestic water, communication and power can cross the river. (5) The sidewalk railing is made of carbon composite impermeable steel.