| In the process of construction and use,single pile is easy to produce stem shrinkage defect,which greatly reduces its bearing capacity and brings security risks to the upper structure.The pile-soil interaction is the key to study the bearing capacity of single pile.The stem shrinkage defect of the pile foundation brings hidden dangers to its safe use in practice.The systematic analysis of the bearing characteristics of single pile foundation with stem shrinkage defects and the soil deformation around the piles are of great significance for studying the pile-soil interaction of single pile foundations with stem shrinkage defects.In this paper,based on the General Program of National Natural Science Foundation of China: “Probabilistic analysis of toe debris and study of influence of toe debris on vertical bearing capacity for drill bored piles based on Bayesian information fusion”(51608177);“Transparent soil experiment analysis of piles-soil interaction for necking piles and study of influence of shrinkage on reliability for pile capacity”(51978247).Transparent soil,particle image velocimetry(PIV)technology and vertical loading platform were used to carry out vertical loading tests on 28 single pile foundations with different stem shrinkage defects,study the effects of different types of stem shrinkage defects on the vertical bearing capacity of the single pile and the soil deformation around the pile.The specific work and main conclusions of this paper are:(1)Method for preparing transparent soil in this paper: Using fused silica sand solid particles with particle diameters of 0.5-1mm and 1-2mm to prepare transparent soil solid particles according to a mass ratio of 1:2.Using N-dodecane and NO.90 white oil at a volume ratio of 1: 9.5,a transparent soil pore fluid with a refractive index of 1.4590 at 20°C was prepared.The test proves that on the basis of satisfying the transparency and speckle field production of the transparent soil required for the test,and also meets the dense sandy soil.(2)Using vertical loading system,digital image acquisition system,532 nm area light,and PIV carry out vertical hierarchical loading tests on 28 types of single pile foundations.Recording the pile's top settlement and soil speckle field changes in real time under each hierarchical load in test.In the later period,based on the test data,the load-settling curve of each single pile's top were draw.Using MatPIV to calculate the change of soil speckle field under a certain load level compared with the original soil speckle field,the vector and contour map of soil displacement around the pile under a certain load level are obtained.Which opened up a new path for the study of pile-soil interaction of stem shrinkage defective single pile under vertical load.(3)Based on the load-settling curves of the top of 1 type of complete pile and 27 types of piles with stem shrinkage defects from the test.The influence of stem shrinkage defects length,stem shrinkage defects radial size and stem shrinkage defects position on the load settlement curve of single pile top under vertical load is analyzed.Based on this,studying the influence of the existence of the stem shrinkage defects on the bearing capacity of the single pile foundation at the initial bearing stage,the vertical ultimate bearing stage and the damage load stage.The following conclusions are drawn: Compare the bearing type of the complete pile as a friction pile under vertical load,the existence of the stem shrinkage defects does not change the bearing type of the single pile.When the radial size of the stem shrinkage is greater than 80% of the complete pile diameter,the stem shrinkage has no significant effect on the bearing characteristics of the single pile,and this effect is getting weaker as the shrinkage length decreases.The influence of stem shrinkage location on the bearing capacity of single pile is in the order of shallow stem shrinkage defect,middle stem shrinkage defect and deep stem shrinkage defect.(4)Based on the soil displacement vector and contour map of soil around the 28 types of single piles at the three load levels of 140 N,220N and 260 N from the test,the influence of stem shrinkage length,stem shrinkage radial size and stem shrinkage position on the deformation of soil around single pile under vertical load were comprehensively analyzed.Based on this,studying the influence of the existence of the stem shrinkage defects on the displacement of the soil around the single pile foundation at the initial bearing stage,the vertical ultimate bearing stage and the damage load stage.The following conclusions are drawn: Compared with the complete pile,the failure characteristics of the single pile lose its bearing capacity due to the overall shear failure of the soil around the pile,the existence of the stem shrinkage defect will not affect the damage form of the single pile foundation,and the 28 types of single piles will lose their bearing capacity due to the overall failure of the soil around the pile.The existence of the stem shrinkage defects will affect the soil around pile,and the effect becomes larger as the degree of defects increases(the length size increases and the radial size decreases of the stem shrinkage).The influence of stem shrinkage location on the displacement of the soil around the single pile is in the order of middle stem shrinkage defect,shallow stem shrinkage defect and deep stem shrinkage defect.(5)Combine the load-settling curves and the two-dimensional deformation map of the soil around the pile of 28 types of single piles.The following conclusions are drawn: Single pile bearing capacity decreases with the degree of the stem shrinkage defect increases.The deformation of the soil around the pile increases with the degree of shrinkage defect increases.When the stem shrinkage defect is not large(The diameter of the stem shrinkage is about 80% of the complete pile diameter,the length of the deep stem shrinkage is 2.5% of the complete pile length and its diameter is larger than 40% of the complete pile diameter),its bearing capacity will not change much because of the existence of soil. |
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