Transparent Soil Technique-based Model Tests And Numerical Simulation Studies On Reinforced Foundation

Reinforced foundation means the foundation with geosynthetics such as geogrid,geocell or geotextile laid layer by layer in the filler as the bearing layer.It can improve the bearing capacity and reduce the settlement of the foundation soil.This foundation treatment method has the advantages of short construction period,low project cost and remarkable effect,so it has been widely used in the fields of municipal and transportation infrastructure construction.At present,the deformation and bearing characteristics of reinforced foundation are mainly studied,however the displacement field evolution,reinforcement mechanism,failure mode and reinforcement fracture condition of it are still less investigated.In this paper,a new type of transparent soil was prepared,and 1g model test and centrifugal model test using the new transparent soil were conducted.Then Finite difference numerical simulation was carried out to study the deformation and bearing characteristics of reinforced foundation,the evolution law of internal displacement field,foundation failure mode,tension distribution and reinforcement fracture sequence.Finally,based on the model test and numerical simulation test results,a calculation method considering BP neural network was proposed.The main work and research results of this paper are listed as follows:（1）A new type of transparent sand was prepared using glass sand and a mixture of3#and 15#white oil,and its transparency was tested.Then a series of geotechnical tests were carried out on the new transparent soil with two particle size ranges 0.1-1mm and1-2mm,and its physical and mechanical properties were analyzed.The results show that the new transparent sand prepared in this paper has high transparency.Its clear transparent depth is 14cm,and the ultimate transparent depth is 20cm.It is better than that of other types of transparent sand.The strength,deformation and permeability characteristics of the new transparent sand are similar to those of natural sand,and the new transparent sand has no toxicity and stable properties.It can be widely used in the visual model test of geotechnical engineering.（2）The 1g model loading test was conducted with the new type of transparent sand.The effects of reinforcement length,number of reinforced layers,layer spacing and reinforcement stiffness on the deformation and bearing characteristics of reinforced foundation were studied.The deformation evolution process of reinforced foundation was analyzed,and its failure mode was discussed.The test results show that the deformation evolution of reinforced foundation and the fracture process of reinforcement are closely related to the reinforcement soil modulus ratio R_r:when the stiffness of reinforcement is large and the reinforcement spacing is small,the overall stiffness of reinforced area is large,and the downward displacement trend of reinforced area as a whole with the foundation is obvious.During the loading process,the displacement occurs first on both sides below the reinforced area and expands from bottom to top to the reinforced area.The reinforcement was pulled and the reinforcement fractured from bottom to top.When the layer spacing is too large or the stiffness of the reinforcement is small,the overall stiffness of the reinforced area is small,and the displacement occurs first in the upper part of the reinforced area then gradually expands downward.As a result,the reinforcement fractures from top to bottom.The failure surface of reinforced foundation is similar to that of unreinforced foundation,and they are both Prandtl failure mode.The failure surface of reinforced foundation is larger than that of unreinforced foundation,and increases with the increase of reinforcement depth and reinforcement length.（3）Centrifugal model tests of reinforced foundation were conducted using transparent soil with two ratios of soil particle size to geogrid opening size（represented by symbol“R_d”）1.18（i.e.particle size of 1-2mm）and 0.33（i.e.particle size of 0.1-1mm）and reinforcements with different stiffness.The purpose of the centrifugal tests is to study the effects of reinforcement spacing,reinforcement layers,reinforcement stiffness and soil particle size on the deformation and failure process of reinforced foundation under real stress condition.The results show that the ultimate bearing capacity of reinforced foundation increases with the decrease of reinforcement spacing,and with the increase of number of reinforcement layers and reinforcement strength.When R_d=1.18,the soil has greater compaction damage to the top reinforcement than the bottom one,and the reinforcement fractures layer by layer from top to bottom.When R_d=0.33,the soil has weaker compaction damage to the top reinforcement,and the reinforcement fractures from bottom to top.The failure mode of reinforced foundation under real stress is the same as that of 1g model test and they are both Prandtl failure mode.With the increase of reinforcement soil modulus ratio R_r,the failure surface depth of reinforced foundation increases,but the width changes little.（4）A series of reinforcement compaction damage tests were carried out using the reinforcement used in the centrifugal model test and transparent soil with two ranges of particle size.The influence of foundation settlement on the compaction damage degree of reinforcement with different buried depths was studied under the condition of R_d=0.33 and R_d=1.18.The results show that the degree of compaction damage on reinforcement under the condition of a large R_d is significantly greater than that under the condition of a small R_d;The degree of compaction damage on reinforcement increases with the decreasing of reinforcement buried depth and the increasing of R_dand foundation settlement;The normalized compaction damage reduction coefficient formula considering the influencing factors such as R_d,foundation settlement and reinforcement buried depth was obtained.The calculated values of the formula are in good agreement with the measured ones.（5）The cable structure unit in geotechnical numerical calculation software FLAC^2D was redeveloped using fish programming language,and its function of automatic disconnection was realized.The normalized compaction damage reduction coefficient formula was also programmed into the program text.The numerical model of reinforced foundation was established and then verified by the centrifugal model test results.The influence of reinforcement layout parameters,reinforcement properties and soil properties on the load settlement characteristics of reinforced foundation,the distribution and evolution law of soil displacement field and the fracture order of reinforcement are further discussed using the verified numerical model.The results indicate that the two-dimensional numerical model of reinforced foundation considering reinforcement fracture and damage can well simulate the reinforcement deformation and fracture process in the model test.The ultimate bearing capacity of reinforced foundation increases with the increase of reinforcement layer number and reinforcement strength,first increases and then decreases with the increase of the buried depth of the first layer of reinforcements and the reinforcement spacing,and first increases,then slows down and finally tends to be constant with the increase of the length and stiffness of reinforcement.The displacement field distribution of reinforced foundation is significantly affected by the buried depth,spacing,number of layers and stiffness of the first layer of reinforcement.With the increase of the buried depth of the first layer of reinforcement,the horizontal displacement of the soil in the non-reinforced area at the left and right corners below the reinforced area gradually decreases,while the horizontal displacement of the soil in the upper part of the reinforced area gradually increases;With the increase of reinforcement spacing,the horizontal and vertical displacement of soil in the reinforced area increase significantly,and the vertical settlement distribution in the reinforced area is more uneven;The distribution depth of foundation horizontal displacement increases with the increase of reinforcement layer number and decreases significantly with the increase of reinforcement stiffness.The fracture order of reinforcement is closely related to the ratio of reinforcement to soil modulus.When the ratio of reinforcement to soil modulus is small,the reinforcement fractures from the upper part,while when the ratio of reinforcement to soil modulus is large,the reinforcement fractures from the lower part.The damage of soil to reinforcement mainly reduces the strength of reinforcement,and then affects the fracture order of reinforcement.（6）Based on the test and numerical simulation results in this paper and the test results in the literature,a calculation method of ultimate bearing capacity of reinforced foundation based on BP neural network is proposed,and compared with the existing typical calculation methods of ultimate bearing capacity of reinforced foundation.The results show that the calculation method based on BP neural network proposed in this paper considers the influencing factors from load to failure of reinforced foundation more comprehensively,can calculate the ultimate bearing capacity of reinforced foundation more accurately,and has stronger applicability.