Transparent Soil Experimental On Pile-soil-cap Interaction Of Necking Pile Under Vertical Load

Pile is widely used in national infrastructure construction because of its high bearing capacity,small settlement and reasonable stress.However,in the process of construction and use of pile foundation,the pile is easy to produce necking defects,which greatly affect it’s bearing capacity.For single pile cap,due to the existence of necking defect and cap,the cap,necking and deformation of soil around the pile are very complex under vertical load,which constitutes a complex pile-soil-cap interaction system.Therefore,Based on the general project of National Natural Science Foundation of China(No.51978247)and Key R&D and Promotion Special Project of Henan Province(Science and Technology Targeting)(No.202102310242),using transparent soil test technology and MatPIV,the vertical loading test of transparent soil is carried out on 1 intact single pile with cap and 27 necking single piles with cap,to study the effect of necking(necking position,axial size and radial size)on deformation of soil around pile,load transfer and bearing capacity.The specific research work and main conclusions are as follows:(1)The preparation scheme of transparent soil is as follows: The transparent soil skeleton adopts fused quartz sand with particle size of 0.5mm-2mm,and the particle size grading of soil sample is good;The pore liquid of transparent soil is made of 1:8.8 volume ratio of n-dodecane and 90 # white oil,and its refractive index is 1.4582 at 20℃,which is the same as that of fused quartz sand.After testing,the transparency and speckle effect of transparent soil meet the test requirements,and can be used to simulate saturated natural sand.(2)Based on the load displacement curve of 28 groups of piles,the influence of axial dimension,radial dimension and necking position on bearing capacity of single pile cap under vertical load is studied systematically.Research shows: Under the same necking size,the shallower the necking position is,the greater the vertical bearing capacity loss is,the greater the impact on the bearing capacity is,and the earlier the load displacement curve deviates from the intact pile;When the axial dimension of the necking is less than 2.5% of the pile length and the radial dimension is less than 20% of the pile radius,the influence on the single pile of the cap is not obvious;When the radial dimension of the necking is greater than 80% of the pile radius,the pile is prone to instability failure;when the axial dimension is the same,the larger the radial dimension is,the greater the bearing capacity loss is;When the radial dimension is the same,the larger the axial dimension is,the greater the bearing capacity loss is.(3)Based on the displacement field of 28 groups of soil around piles,the influence of axial dimension,radial dimension and necking position on the deformation of soil around pile is studied systematically.Research shows: The influence of Shallow necking on the deformation of soil around the bearing platform is significant,which increases the deformation range of soil under the bearing cap,and the soil near the pile and around the cap produces more vertical downward deformation;The necking in the middle makes the soil at the necking position produce inclined downward concentrated deformation,When the necking size is large,the necking position and the soil around the cap act together to produce through deformation;The deep necking has a significant effect on the deformation of the soil at the pile end,and the soil at the necking position has a concentrated deformation downward,and has a through deformation with the soil at the pile end;When the necking size is large,the deformation range of the soil at the pile end increases and more horizontal deformation occurs.(4)According to the deformation law of the soil around the pile,the change reason of the bearing capacity of necking single pile with cap is studied.Research shows: The soil is under the shallow necking single pile with cap and close to the pile body has more vertical deformation,it reduces the relative displacement between the pile and the soil,which leads to the loss of pile friction,and is prone to negative friction;the concentrated deformation of the soil at the necking position of the single pile in the middle reduced diameter cap causes the friction loss;the concentrated deformation of the soil at the necking position of the middle necking single pile with cap causes the friction loss;When the size of necking in the middle part is large,the position of necking and the soil around the pile cap will produce through deformation,which will cause the loss of larger range of friction of the pile;Deep necking is easy for the soil between the pile tip and the necking position to penetrate diagonally downward,which leads to the loss of friction resistance of the pile at the lower part of the necking.When the necking is located in the shallow and middle part(the necking is larger),mobilize the surrounding soil to bear the bearing capacity of the loss by the cap.When the necking is deep,mobilize the soil to bear the bearing capacity of the loss by the pile end.