| The soft soil layer in coastal area is relatively deep,and the bearing stratum is buried deeply.Large post-construction settlements generally occur for highways that are reinforced with shallow foundation treatment methods.Large settlements will se-riously affect the function of the expressway,and it is difficult to repair.The adoption of pile foundations that penetrate through the deep soft soil layer into the bearing lay-er will greatly increase the project cost,and it will generate uncoordinated defor-mation between the roadbed and the surrounding soil.The prefabricated rigid pile-net composite foundation utilizes the advantage characteristics of the ground soil that the soil strength increases with depth.With the advantages of easy control of construction quality,good reinforcement effect,timeliness and economy of engineering and other advantages,it has been widely used in the reinforcement of the soft ground of ex-pressways.However,the research on its working mechanism and design calculation method is far behind the engineering practice.Field tests,laboratory model tests,nu-merical simulations and theoretical methods were carried out to study the flexible beam properties of reinforced cushions in composite foundations,the coordination mechanism of“Multi-layer Reinforced Cushion-Pile-Soil”system,the settlements calculation methods and consolidation characteristics in this paper.The main research work and innovative achievements are as follows:(1)The flexible beam properties of multi-layer reinforced cushions have im-portant influence on the load distribution and deformation characteristics of composite foundation.Firstly,the forming mechanism of flexible beam properties of the cushion was analyzed.Combing the material properties of the geogrids and gravel,relevant analysis and calculation methods are proposed based on the Duncan-Chang model and the characteristics of the flexible beam.(2)Targeted on-site and indoor tests were conducted on prefabricated rigid pile composite foundations in typical geological conditions in the coastal area.The load transfer characteristics,work characteristics and load sharing ratio of lateral friction resistance and end-resistance of single piles and composite foundations,the ratio of pile-soil sharing ratio and negative frictional resistance were studied in detail.Com-bined with a large number of other engineering data,the regional laws of the bearing and deformation characteristics are summarized.(3)Based on large deflection thin plate theory,two kinds of reinforced cushion deflection control differential equations were deduced.The strain continuous and in-terfacial frictional effects between geogrids and gravel were considered respectively.Combining its actual three-dimensional boundary conditions and pile-soil interactions,the deflections were deduced through the principle of function and Galerkin’s method.Two kinds of calculation models for load transfer of“Multi-layer Reinforced Cush-ion-Pile-Soil”were proposed and analyzed.The methods were validated by indoor full scale tests and the corresponding application scopes were discussed.The pro-posed methods can provide reference for engineering practice.(4)The total number and optimal location of the laying geogrids are important issues for reinforcement design.However,traditional analysis methods cannot analyze the influence of the number of reinforcement layers and positions.The flexible beam properties of multi-layer reinforced cushions were analyzed through the laminating theory.The stiffness matrix of reinforced cushion was deduced.Combined with static balance and stress-strain relationship,the tension force distribution of multi-layered reinforced cushion calculation model was proposed.The calculation model was con-nected with the load transfer calculation model proposed in this paper to analyze the influences of the number of layers,location and other factors on the load transfer characteristics of the“Multi-layer Reinforced Cushion-Pile-Soil”system and tension force distribution.A method for calculating the optimal number of layers and posi-tions was proposed.(5)The stress diffusion method and equivalent entity method are recommended by Technical code for composite foundation(GBT 50783-2012)to calculate settle-ments.However,the stress diffusion angle and equivalent side friction resistance are lack of reasonable calculation methods.Combining field tests with a large number of numerical simulations,calculation functions for stress diffusion angle and equivalent lateral friction resistance were established through inverse algorithm method and least squares method.These functions take the effect of pile spacing,pile length,soil prop-erties,cushion thickness and other factors into account.The methods were validated by indoor tests.The results have wide practicability.(6)Traditional consolidation theory of composite ground with impervious piles is based on the conventional equal vertical strain assumption and ignores the influence of the initial consolidation stress on the underlying layer.Considering the load trans-fer characteristics of prefabricated rigid pile composite foundation,the equal strain calculation model was modified.Taking the influence of load transfer efficiency of pile and soil,additional stress distribution pattern and the speed of filling into consid-eration,the differential equations of consolidation were established and the Fourier series solutions were obtained.Numerical simulation was used to verify its scienti-ficity and factor analysis were carried out.The method improves the consolidation theory of composite ground with impervious piles and it can provide useful reference for the project.(7)The applicable scope of prefabricated rigid pile-net composite foundation was discussed.The results show that it is suitable for reinforcing soft ground which equivalent sub-bed coefficient is greater than 3 MN/m~3. |
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