Key Techniques for Construction of North Anchorage Caisson Foundation of the Fourth Nanjing Changjiang River Bridge

The caisson in dimensions of 69.0 m×58.0 m×52.8 m is used for the foundation of the north anchorage of the Fourth Nanjing Changjiang River Bridge and is set in the dense pebble stratum where the geologic conditions are complicated.The caisson is composed of totally 11 sections with its first section a steel shell concrete and the rest of the 10 sections the reinforced concrete.At the site of the caisson,the ground was first reinforced by the combined reinforcement method of building sand piles and backfilling sand soil,the steel shells were then assembled on the reinforced ground and the concrete for the first section of the caisson was cast.The 11 sections of the caisson were extended in height and were sunk down in 4 times.For the first time sinking of the caisson,the soil in the caisson was air lifted out and the water there was discharged,and for the rest of the 3 time sinking,the soil was also air lifted out,but the water was not discharged.After the caisson was sunk in place,the tremie concrete for the bottom sealing of the caisson was cast area by area according to the four divided areas.The monitoring of the construction indicates that the sinking status and the offsetting tolerance of the caisson are all controlled within the provisions in the technical specification.