Study On Bearing Mechanism Of Necking Pile Under Horizontal Load

In recent years,with the further development and construction of ocean engineering in China,pile foundations have been widely used in ocean engineering due to their unique advantages.However,due to the unique hydrogeological conditions of offshore engineering,pile foundations are prone to shrinkage defects,which has a significant impact on the bearing performance of pile foundations.At the same time,buildings in ocean engineering are often subjected to horizontal loads from seawater and wind,so it is very important to study the horizontal bearing performance of necking piles.However,due to the complexity of the pile body structure of the necking pile,its bearing mechanism is extremely complex.Therefore,based on the general project of the National Natural Science Foundation of China(No.51978247),this study uses transparent soil and particle image velocimetry(PIV)technology to horizontally load 1 complete pile and 7 single piles with different necking,and uses Abaqus software to conduct numerical simulation calculations on 8 piles.The effects of different necking lengths,locations,and diameters on the horizontal bearing capacity of a single pile,the variation of soil around the pile,the distribution of pile bending moments,and the soil pressure at the pile side were systematically studied.The main research contents and conclusions of this study are as follows:(1)Through graded loading on 1 complete pile and 7 necking piles,and using a dial indicator to record the pile top displacement in real time,a load-pile top displacement curve was drawn based on the test results,and the influence of different necking lengths,positions,and diameters on the horizontal extreme bearing capacity of a single pile was obtained.The research shows that the existence of necking seriously affects the horizontal bearing capacity of a single pile.With the increase of the necking length,the ultimate horizontal bearing capacity of a single pile continuously decreases.As the distance from the necking position to the pile bottom increases,the impact of the necking on the single pile becomes greater,and the horizontal ultimate bearing capacity of the single pile becomes smaller.With the decrease of the necking diameter,the horizontal ultimate bearing capacity of a single pile becomes smaller.When the necking diameter is 20% of the complete pile diameter,the length of the necking pile is 10%,at this time,the horizontal ultimate bearing capacity loss of the necking pile is the largest,with a loss rate of 76%.(2)Using Abaqus software,numerical simulation calculations were conducted on 8single piles,and the distribution curves of pile bending moment and soil pressure along the pile shaft were obtained.The results show that although there is a slight error between the simulation results and the experimental results,they are generally consistent,so the simulation results can be used to simulate the actual situation.The maximum bending moment of the pile shaft of the necking pile occurs at the reduced diameter position,and the maximum bending moment of the pile shaft of the necking pile is greater than that of the complete pile.At the same time,the longer the necking length,the greater the maximum bending moment of the pile body.The smaller the necking reduction,the greater the maximum bending moment of the pile body.The farther the necking is from the pile bottom,the greater the maximum bending moment of the pile body.The distribution of soil pressure on the pile side of necking piles is greatly different from that of a complete pile.At the pile top and near the pile end,the soil pressure on the pile side of the necking pile will be greater than that of the complete pile,but in other areas,the soil pressure on the pile side of the complete pile will be greater than that of the necking pile.The longer the necking length,the greater the soil pressure on the pile side;The longer the necking diameter of the pile,the greater the soil pressure on the pile side;The deeper the necking is located at the pile shaft,the smaller the soil pressure at the pile side.And the zero point position of earth pressure of the necking pile is deeper than that of the complete pile.The soil pressure value of each single pile near the pile bottom will be converted from a positive value to a negative value,and the soil pressure at the pile side will be converted from passive soil pressure to active soil pressure.(3)Based on the test results,the speckle images obtained from the test were processed by Mat PIV program,and the displacement vector map and contour cloud map of the soil around the pile were obtained.By analyzing the variation of soil around the pile and combining the results of numerical simulation,the reason for the loss of bearing capacity of the necking pile is analyzed.The results show that under horizontal load,the soil on the tensile side of the top of a single pile moves obliquely upward due to the compression of the pile body,and the surface of the soil exhibits a phenomenon of uplift;Due to the displacement of the pile,there is a gap between the soil on the tension side and the soil,and the soil moves obliquely downward under the action of gravity,resulting in subsidence on the surface of the soil.The existence of necking will increase the deformation range and displacement of the soil around a single pile,and the more severe the necking,the greater the soil deformation.For single piles with different necking lengths and diameters,the more severe the necking,the poorer the strength at the necking,the poorer the flexural performance,the larger the range of soil deformation,and ultimately the poorer the bearing capacity.For necking piles with different necking positions,the shallower the necking position,the greater the soil pressure on the pile side,indicating that the greater the deformation of the pile body,the poorer the bending resistance of the pile body,resulting in greater deformation of the pile body,and the poorer the bearing capacity.