Analysis of ground deformation development and settlement prediction by air-boosted vacuum preloading

Vacuum preloading has been widely used to improve soft soils in coastal areas of China.An increasing amount of evidence from field operations has shown that conventional vacuum preloading is prone to clogging in prefabricated vertical drains(PVDs)and demands a large volume of sand fills.In recent years,air-boosted vacuum preloading has been developed to overcome these limitations;however,this method still requires more data to verify its performance.In this study,a field test for air-boosted vacuum preloading was conducted,and a large-strain two-dimensional(2D)finite element(FE)model was developed and validated against the field test data.Then,a series of FE parametric analyses was performed to assess key factors,i.e.the air injection pressure,the injection spacing,and the characteristics of cyclic injection,which affect the performance of the air-boosted vacuum preloading.The results showed that the ground settlement and lateral displacement of the soils increased due to an increase in the injection pressure,a decrease in the injection spacing,or increases in the number and duration of the injection cycles.Based on the parametric analyses,an empirical formula for ground settlement prediction was proposed and compared with a case history reported in the literature,showing good agreement.

Influencing factors and control measures of excavation on adjacent bridge foundation based on analytic hierarchy process and finite element method

Many uncertain factors in the excavation process may lead to excessive lateral displacement or overlimited internal force of the piles,as well as inordinate settlement of soil surrounding the existing bridge foundation.Safety control is pivotal to ensuring the safety of adjacent structures.In this paper,an innovative method is proposed that combines an analytic hierarchy process(AHP)with a finite element method(FEM)to reveal the potential impact risk of uncertain factors on the surrounding environment.The AHP was adopted to determine key influencing factors based on the weight of each influencing factor.The FEM was used to quantify the impact of the key influencing factors on the surrounding environment.In terms of the AHP,the index system of uncertain factors was established based on an engineering investigation.A matrix comparing the lower index layer to the upper index layer,and the weight of each influencing factor,were calculated.It was found that the excavation depth and the distance between the foundation pit and the bridge foundation were fundamental factors.For the FEM,the FE baseline model was calibrated based on the case of no bridge surrounding the foundation pit.The consistency between the monitoring data and the numerical simulation data for a ground settlement was analyzed.FE simulations were then conducted to quantitatively analyze the degree of influence of the key influencing factors on the bridge foundation.Furthermore,the lateral displacement of the bridge pile foundation,the internal force of the piles,and the settlement of the soil surrounding the pile foundation were emphatically analyzed.The most hazardous construction condition was also determined.Finally,two safety control measures for increasing the numbers of support levels and the rooted depths of the enclosure structure were suggested.A novel method for combining AHP with FEM can be used to determine the key influencing aspects among many uncertain factors during a construction,which can provide some beneficial references for engineering design and construction.