Theoretical And Model Test Study On The Development Of Negative Pore Pressure In Saterated Clay Foundation In Unloading Conditions

It is an important research topic in marine geotechnical engineering,as suction is widely existed in offshore foundations.When a gravity structure,such as a caisson or a jackup spudcan,is pulled out from the saturated soft clay,negative excess pore pres-sure will be developed,which may become the main part of the suction force and greatly increase the pullout force.When any foundation in saturated clay foundation is used to provide stable working environment for the upper structure,suction force between the base and foundation is beneficial to resist the external load.However,a structure,such as spudcan,need to be pulled out and moved to other areas,it will become a hindrance and restricting factor.Therefore,the study of negative pore water pressure in saturated clay foundation under unloading conditions is a very important subject.Theoretical and experimental research has been conducted,and the main research and results are as follows.(1)The development of excess pore pressure is studied based on triaxial extension tests under isotropic consolidation condition.The stress path,the stress-strain charac-teristics and the development of negative pore pressure are studied,and the influence factors are concluded.(2)Combining the Modified Cambridge Model with the Henkel's approach,the generation mechanism of negative pore pressure is revealed and the method to predict the magnitude of the negative pore pressure is established.Meanwhile,the development rule of pore pressure under the isotropic consolidation conditions is obtained and ex-plained why there will be a change of excess pore pressure from negative to positive.(3)A flat caisson model is used to research the influence of uplift bearing capacity,such as the consolidation stress,unloading rate.Test results are consistent with the tri-axial test.Unloading rate effect on the bearing capacity is normalized by coefficient of consolidation c_v and model dimension D.Then,the empirical relationship between bearing capacity coefficient N_c and normalized unloading rate V is established.At last,the drainage condition,the partially drained condition and the undrained condition is delimited by the normalised velocity.(4)The theory of predicting excess pore pressure used by Modified Cambridge Model is applied to the finite element analysis of skirt caisson model experiment.Re-search shows that calculation result of negative pore pressure is less than the meas-ured.Combining the rate effect of model test with triaxial test in kaolin clay,the reasons of this phenomenon is revealed.Therefore,the modified Cambridge model is appropri-ate for predicting pore pressure at lower unloading rate and the model test results are needed for the faster uplift problem.Meanwhile,the behavior of a concrete suction caisson with wedge-shaped wall tip installed in different consolidated soils was studied.According to a wedge shape of wall tip and thick wall of concrete suction caisson,the calculation method of the required suction pressure and the allowed suction pressure were proposed and validated by model tests.(5)In general,drilling ship is assembled at the dock.It may produce adverse effect on the slope and the surrounding structures.Combined with research results in this pa-per,the influence of long piled wharf is reduced by lowering the uplift rate.