The Safety Study And Calculating Analysis For Large Preloading On Deep-thick Soft Soil Near River And Mountain

Aiming at the preloading engineering on oil tank areas of a refinery,a fluid-solid coupling finite element model is established and the model parameters are obtained through parameter back analysis,based on the data from the geological survey report and the in-situ monitoring and using GeoStudio geotechnical analysis software.Next,the paper has studied the stress and strain state of the foundation and the safety control index in the construction stage.Then,the paper has studied the stability of surcharge slope and embankment,and put forward some improvement suggestions to some problems existing in the current industry standard,using four stability analysis methods,including simplified Bishop effective stress method,finite element coupling analysis method,analysis method considering strength increasing and composite sliding surface method.According to the measured results,the paper has analysed and compared the settlement prediction methods,including the three-point method etc.and corresponding unloading standard.A prediction method considering the consolidation degree for post construction settlement is studied and put forward.The main contents and results are summarized as follows:(1)Based on the mechanical parameters of geological detailed report,the key parameter of the modified Cambridge mathematical model including ? is obtained by finite element back analysis taking the measured settlement,horizontal displacement and pore pressure data as the reference standard.According to the sand well consolidation theory,a fluid solid coupling finite element model is established,based on the Sigma/w module of GeoStudio and result of back analysis.Comparing with the measured results,the calculation error is less than 3% in settlement,horizontal displacement and excess pore pressure.Finally,the parameter ? is obtained.(2)According to the in-situ monitoring and alarm status,the paper has analysed stress and strain state of the foundation under actual condition and two simulation conditions and researched the reasonableness of the danger point and stop loading moment.Then the paper put forward the control index according with the project: sedimentation rate and horizontal displacement rate respectively is 20 mm/d,5 mm/d;the alarm value of pore pressure coefficient is 0.9;The ratio of maximum horizontal displacement to maximum settlement is less than 0.22.(3)According to the monitoring data and the situation of soil layer and loading,the paperhas studied four methods of stability analysis,including simplified Bishop effective stress method,finite element coupling analysis method,analysis method considering strength increasing and composite sliding surface method.Then the paper has proposed that only the analysis method considering the strength increasing in the period of loading is the most reliable,with the result being in accordance with the measured safety state.It is suggested that the method considering strength increasing should be adopted to calculate the steady state during the construction period in the petrochemical specification.For the condition of soil layer containing soft silt interlayer,the most dangerous sliding surface should be found by using the composite sliding surface method and the other methods are partial to danger.(4)By contrasting and analysing the advantages and disadvantages of the three-point method,hyperbolic method,Asaoka method and finite element method,the paper has studied and proposed method of predicting post-construction settlement considering the degree of consolidation under overloading preloading.Combined with the later creep observation,it is proved that the improved method of predicting post-construction settlement has the best prediction effect.Based on the three-point method and the improved method of predicting post-construction settlement,combined with the standard of settlement control in the petrochemical specification,the unloading standard of preloading of oil tank foundation is put forward: when the three-point method is used,the consolidation degree should be greater than89%;when the improved method of predicting post-construction settlement is used,the deformation ratio should be greater than 78%;the corresponding settlement rate is 1~2 mm/d.