Study of the Bearing Capacity at the Variable Cross-Section of A Riser- Surface Casing Composite Pile

Reducing the cost of offshore platform construction is an urgent issue for marginal oilfield development.The offshore oil well structure includes a riser and a surface casing.The riser,surface casing and oil well cement can be considered special variable cross-section piles.Replacing or partially replacing the steel pipe pile foundation with a variable cross-section pile to provide the required bearing capacity for an offshore oil platform can reduce the cost of foundation construction and improve the economic efficiency of production.In this paper,the finite element analysis method is used to investigate the variable cross-section bearing mode of composite piles composed of a riser and a surface casing in saturated clay under a vertical load.The calculation formula of the bearing capacity at the variable section is derived based on the theory of spherical cavity expansion,the influencing factors of the bearing capacity coefficient N_(c) are revealed,and the calculation method of N_(c) is proposed.By comparing the calculation results with the results of the centrifuge test,the accuracy and applicability of the calculation method are verified.The results show that the riser composite pile has a rigid core in the soil under the variable cross-section,which increases the bearing capacity at the variable cross-section.

Field measurement analysis of the influence of simultaneous construction of river channel and bridge on existing double shield tunnels

This study presents the construction of a river channel and a bridge adjacent to existing metro tunnels in Changzhou.The influence of simultaneous construction on these existing tunnels was investigated via 3D numerical modelling to predict tunnel deformation before construction.Then,a targeted protection scheme was developed according to the obtained numerical results.The full construction period field monitoring scheme can monitor tunnel responses during construction.Subsequently,the safety of the tunnel structures was evaluated according to the monitoring results,and the evolution of tunnel deformations was analysed.The analytical results can help to clarify the influence characteristics of different construction stages,verify the effect of the proposed protection scheme,and determine the disturbance mechanism of short-distance pile construction.According to the results,the tunnel deformation mainly occurred during pile construction and river channel excavation,and the tunnel vertical displacement and convergence were mainly affected by the construction.The anti-floating scheme of the partition excavation and casting effectively controlled the tunnel heave with an alarm value of approximately 6 mm.The penetration of the short-distance casing resulted in a tunnel deformation.The main construction influence area of the casing pile was within 6D(D is the pile diameter).