Experimental Research And Numerical Study On Bearing Property Of Double Layer Foundation With Upper Dredger Fill

In the reclamation project of Tianjin Bin Hai New Area,the silt-dredged fillings are treated by vacuum preloading to form a typical "soft upper and lower hard" soft clay foundation with the normal soft soil.This double-layer foundation has complex engineering characteristics.It is different from both the natural ground and the double-layer ground formed by backfilling.At present,the research on the double-layer soft soil ground in reclamation land projects is scarce which leads to theoretical research lags far behind the engineering practice.Therefore,it is necessary to study the load-bearing characteristics of such ground.In this paper,the upper soft clay and the normal soft soil in the lower layer are taken as a whole.The double-layer specimens are prepared to simulate the double-layer grounds.The simulation of double-layer soft clay ground under different thickness of upper dredger fill is conducted by using indoor model test and finite element simulation.The research results and main conclusions are as follows:(1)The loading settlement curve of the double-layer soft clay foundation with upper dredger fill has three stages: initial compression section,shear section and failure section.The load-settlement curve shows that the compaction section curve of the double-layer soft soil ground is not obvious.The sedimentation value calculated by the layer-wise summation method is much smaller than the actual sedimentation one.Greater error may appear if the sedimentation value of the double-layer soft soil ground is calculated by the layer-wise summation method.The ratio between the measured value and the theoretical calculated value of the stratified summation method is defined as the settlement modification coefficient under the corresponding load.The settlement modification coefficient of double-layer soft clay foundation decreases exponentially with the increase of load.When the ratio of the width of the load plate to the thickness of the upper soil is less than 1,the shear curve shape changed from S to inverted S shape.When the ratio of the width of the load plate to the thickness of the upper soil is greater than 1,its sedimentation value increases step by step and the maximum value can increase 36% under the same load conditions with the increase in the thickness of the upper soil.The maximum sedimentation value can be increased by 160% when the ratio of the width of the load plate to the thickness of the upper soil is less than 1:1.(2)The results show that the phenomenon of stress concentration will appear when the sample is consisted of soft clay both at upper and lower layer.The ratio of the thickness of the upper soil to the width of the load plate and the stiffness difference between upper soil and lower soil are important factors of influence on the degree of stress concentration through the finite element simulation.The results show that the thinner the upper soil is,the more obvious the stress concentration is;the ratio of width to thickness is more significant,and when the ratio of width to thickness is 2:1,the stress concentration can be increased by 50%.(3)The stress field and the development process of the plastic zone of the double-layer soft clay ground under the overburden filling are analyzed through finite element simulation.Simulation results reveal that both the stress field and the development process of the plastic zone and the ultimate failure mode of the double-layer soft clay ground will be affected observably by the thickness of upper soil.The double-layer soft clay ground will appear the whole shearing failure when the ratio of the width of the load plate to the thickness of the upper soil is 2:1.It will appear the local shear failure when the ratio is 1:1,while it will appear punching failure when the ratio is 3:4.When the ratio of the width of the load plate to the thickness of the upper soil is less than 1,the plastic zone mainly concentrates under the load plate and the edge of the load plate.When the ratio of the width of the load plate to the thickness of the upper soil is larger than 1,the plastic zone mainly concentrates under edge of the load plate.