Research On The Consolidation Characteristics Of Vacuum Combined With Surcharge Preloading

Vacuum combined with surcharge preloading method is a new soft clay reinforcement technology,it is useful in many kinds of reinforcement project and popular in the practical engineering due to the combination of characteristics ofthe vacuum preloading and the surcharge preloading.However,the performance of vacuum combined with surcharge preloading method in practical application is not perfect yet.For example,the consolidation calculation theory is not systematic,the smearing effect in engineering design can only be determined by experience,and the actual consolidation mechanism is unclear,etc.The above factors limit the further development of the vacuum combined with surcharge preloading method in engineering applications.This paper mainly describes the behavior of Guangxi Fangchenggang Steel Base’s material site stabilized with prefabricated vertical drains installed in soft clay foundations under vacuum combined with surcharge preloading,based on the field measurements and the development of consolidation theory of vertical drain.A parameter cp was introduced to quantify the influence of smear effect according to relative comparsion of well resistance and smear effect in different vertical drain theory,and the variation range of φ was determined to be 0.01～0.57.The vacuum degree,surface settlement,pore water pressure and horizontal displacement of soil were analyzed in detail to evaluate the reinforcement project effect.The quantified theory of smear effect of vertical drains improves the accuracy of predictions in comparison with the field measurements.Main content of each chapter is as follows:(1)Comparing the reinforcement mechanisms of vacuum preloading method,surcharge preloading method and vacuum combined with surcharge preloading method from the aspects of construction technology,effective stress increment composition,and stress path,it can be concluded that:the effective stress increase of vacuum combined with surcharge preloading method is caused by the vacuum degree and the surcharge load.According to the effective stress principle,when the two work together,a larger pore water pressure gradient can be generated,and a better reinforcement effect can be obtained;(2)Starting from the classic Hansbo,Xie Kanghe vertical drain consolidation theory,the relative influences of well resistance and smear effect on the change of consolidation degree were compared,a new parameter φ was introduced to quantify the smear effect,and the relative magnitudes of the well resistance and smear were compared to determine the value range of the parameter φ,and the Hansbo,Xie Kanghe vertical drain theory is quantified to consist of well resistance and parameter φ;(3)Axisymmetric and plane strain finite element models were established,and a combination scheme considering different well resistance and smearing effects was designed to verify the quantitative theory;the vertical displacement cloud diagrams of the axisymmetric model and the plane strain model were compared,and the pore pressure change at 3 m in each model was used to represent the variation of the model consolidation degree,and compared the consistency between the model consolidation degree and the quantified theoretical consolidation degree;(4)According to the field measurements data in area,analyzed the vacuum degree,surface settlement,pore water pressure,and groundwater level variation laws,and evaluate the effect of the vacuum combined with surcharge preloading method;(5)Based on the existing monitoring data of the site,the relative sensitivity Str was introduced as the main parameter affecting the φ value to improve the quantitative theory.Taking the settlement data of field measurements as examples,the quantitative theory is verified,and the feasibility of determining the φ value according to the Str value is proved.