Horizontal Bearing Capacity And Failure Mode Of The Large Cylinder Structure In Soft Clay Composite Foundation

The large cylinder structure has been widely used in the offshore engineering as a retaining structure of mega sea-crossing infrastructure artificial island due to the advantages of simple construction and low environmental impacts.When the artificial island constructing on the soft clay,the soft clay is easy to occur large strain under loading which results in the obviously nonlinear change of its undrained shear strength with the increasing of strain.Therefore the failure mechanism of large cylinder structure in soft clay is complex.Nowadays,the horizontal bearing capacity and failure mode of the large cylinder structure in soft clay are still unclear.Therefore it is important to investigate the large cylinder structure in soft clay which can shed light on the design of large cylinders as a retaining structure for artificial island.This study include 4 parts: the investigation of large strain properties on soft clay by unit tests,the failure mechanism of large cylinder structure in soft clay composite foundation by model tests,the numerical simulation method of large cylinder structure in soft clay composite foundation and the stability calculation method of large cylinder structure in soft clay composite foundation.Firstly,a new triaxial apparatus is designed to investigate the undrained shear strength of soft clay with large strain.The homogeneousness of the soft clay sample under three-dimensional large strain is guaranteed.The tests of soft clay with large strain are conducted under the undrained and unconsolidated condition.The results demonstrate that the soft clay develops strain softening under large strain condition.The residual shear strength of soft clay is 80% of the peak shear strength.The large strain properties of soft clay provide soil parameters to the numerical simulation method of large cylinder structure in the soft clay.Thereafter,an experimental apparatus for measuring the horizontal bearing capacity of large cylinder structure in soft clay and sand compaction pile composite foundation is developed.The PIV technique is used to measure the soil movement around the structure.The failure mode of the structure is determined based on the soil movement.The results demonstrate that as a retaining structure of artificial island in soft clay,sliding failure is the main failure mode which is different with the overturning failure in the sand.The slip failure plane in the soil is determined based on the soil displacement surrounding the large cylinder structure.The tests result of large cylinder structure is used to validate the numerical simulation method of large cylinder structure in this study.The effect of replacement area ratio and embedment depth of sand compaction pile on the large cylinder structure is also investigated by the experimental method.The results demonstrate that when the replacement area ratio of the sand compaction pile increases to 30%,the ultimate load of large cylinder structure is 2.42 times of that in the soft clay.When the embedment depth of the sand compaction pile euqals to 1.8 times of the pile diameter,the ultimate load of large cylinder structure is 2.25 times larger than that in the soft clay.The increasing of the internal friction angle and the extending of slip failure plane length in the composite foundation are important to raise the bearing capacity of large cylinder structure.Meanwhile,a numerical simulation method is developed to investigate the constructing process of the large cylinder structure in soft clay composite foundation.This method is validated by simulating the test result of horizontal bearing capacity and the failure mechanism in the soft clay.Then a case study of the large cylinder structure in Hong Kong-Zhuhai-Macao Bridge Project is conducted by this numerical method.The results demonstrate that the horizontal bearing capacity of large cylinder structure rises with the increasing of embedment depth of large cylinder structure.The numerical simulation results of large cylinder structure in the composite foundation is used to revise the calculation method of the stability of the large cylinder structure in soft clay composite foundation.Finally,based on the test result and numerical result of the horizontal bearing capacity of large cylinder structure in the composite foundation,an empirical equation of the improved factor is proposed which can consider the effect of replacement area ratio and embedment depth of sand compaction pile.Therefore,the design method to calculate the sliding stability of the large cylinder structure in sand compaction pile composite foundation is revised.