The Sinking Mechanism and Active Control Method for Large-Scale Caissons

In the construction of large-scale caissons, it is difficult to obtain accurate values of the end resistance. There are also inclination, sand boiling, and other issues. In this study, a simplified calculation model for the broken-line end-bearing capacity was proposed on the basis of the passive extrusion failure theory of foundations. Simplified calculation formulas for the bearing capacity under different soil support conditions were established, considering the impacts of excavation or burial depth on the failure slip surface and bearing capacity of the foundation. In addition, the impacts of different shapes of caisson components on the end resistance were analyzed. On the basis of this analysis, a bearing capacity correction coefficient was introduced that considers three-dimensional spatial effects; the calculation results of the end resistance method deviate from the on-site measured earth pressure values by no more than 10%. The dominant influences of the inclining moment and resisting moment on the caisson’s attitude varied progressively as the caisson continued to sink. In the construction of large-scale caissons, end resistance emerged as the primary factor governing the caisson’s orientation. Accordingly, a “stepped progressive” sinking control method was developed and implemented during the caisson sinking operations for Pier No. 5 of the Changzhou-Taixing Yangtze River Bridge. By actively controlling the width of the supporting soil at the end of the caisson and the burial depth, the verticality of the caisson throughout the construction process remained within 1/150. The verticality of the final sinking of the caisson exceeded 1/2000, and the torsional angle of the final sinking of the caisson was only 0.07°. This achieved the active control of the end resistance of the large caisson, the process of sinking, the attitude during the sinking, and the risk of sand boiling.