Settlement Analysis And Reliability Research Of Prefabricated Vertical Drains Preloading Soft Soil Foundation

Prefabricated Vertical Drains(PVD)combined with preloading as a common foundation treatment method was widely used in the foundation reinforcement of soft soil area.Due to the uncertainty of physical and mechanical properties of soft soil itself and the unevenness of spatial distribution,there would be great differences between the geological exploration results and the actual engineering situation,which would have a negative impact on the treatment effect,settlement analysis and reliability of soft foundation.Therefore,in this paper,based on the soft foundation treatment project of the artificial island with dredger fill in the west anchorage of Lingdingyang Bridge in the Shen-Zhong Link,the reinforcement mechanism of PVD preloading soft foundation was analyzed based on the effective stress principle of Terzaghi,and the settlement deformation characteristics of soft soil during the island construction period was discussed combined with field monitoring and numerical simulation.According to the analytical solution of radial consolidation settlement of soft soil foundation and the Monte-Carlo theory,the reliability analysis method of PVD preloading soft foundation settlement considering the variation degree of soft soil settlement parameters was proposed.The main conclusions of this paper were as follows:(1)The numerical simulation method of PVD preloading soft soil foundation based on the principle of equivalent consolidation degree could not only better simulate the consolidation settlement,but also avoid the problem that too many PVDs complicate the model.Through the analysis of consolidation settlement curve,it was found that drainage system and compression system were indispensable in PVD surcharge preloading.The analysis of influencing factors on the treatment effect of PVD preloading method showed that changing the spacing and flow rate of PVD would affect the consolidation settlement rate of soft soil foundation.The smaller the PVD spacing was,the greater the consolidation settlement rate was,but its disturbance to the soil layer would also increase,so the appropriate drainage spacing should be selected according to the actual project.The greater the water flux of PVD was,the greater the consolidation settlement rate was.In the absence of other factors,PVD with larger water flux should be selected.The influence of PVD spacing and water flow rate on final consolidation settlement was not obvious.(2)Based on the analytical solution of radial consolidation settlement and Monte Carlo method,the reliability analysis method of PVD preloading foundation settlement was proposed,and the corresponding calculation program was compiled.The influence analysis of parameter variation coefficient showed that for complex sites,the settlement control standard after foundation treatment should be calibrated according to a certain exceeding probability.When the coefficient of variation of soil was large,there was a greater risk in using a single settlement control standard.According to the relationship between exceeding probability and settlement ratio established in this paper,the settlement control standard satisfying a certain exceeding probability could be obtained.(3)Taking the west anchorage artificial island project of deep-middle channel as an example,combined with the monitoring results of stratum settlement,the rationality of the theoretical calculation method was verified by comparison.The reliability analysis of foundation settlement showed that when the coefficient of variation of soil compressibility was 0.18 and 0.21,the coefficient of variation of post-construction settlement was 0.43,and the reliability probability of using 100 mm settlement control index directly was 51%.According to the relationship between the exceedance probability and the settlement ratio,in order to achieve a reliable probability of 95%,the settlement standard after calibration was 59 mm.Field monitoring showed that the foundation settlement tends to be stable after 250 days of preloading.Based on the reliability control method,the uncertainty caused by the variability of soil parameters could be considered,which could provide reference for the safety assessment of engineering projects.