Indoor Experiment Study And Mechanism Analysis Of Long-short-pile Composite Foundation

Based on the National Natural Science Foundation of China(Project No.51178185)"The characterization and predication method of composite pile foundation settlement under rigid foundation",a more comprehensive long short piles composite foundation test model was proposed in this study through different ways including analyzing the shortage of existing test model,investigating new experimental equipment,adjusting the test procedure,optimizing the experimental method and adjusting the test procedure.The bearing capacity and the settlement of ten sets of long short composite pile foundation test model under natural foundation were evaluated in this study,and the effect of pile length,pile diameter and pile distance were also considered here.Additionally,four sets of simulation numerical data from FLAC.3D were analyzed and compared to experimentaldata.The brief objectives and summary of this study are detailed as below:1.A comprehensive long short piles composite foundation test model was proposed in this study by optimizing the chamber size,loading system,data acquisition systems and settlement observation system.The accuracy and reliability of the collected data were significantly improved.This novel system can acquire synchronous external load,pile soil stress-strain and full-time foundation settlement data,and control the loading stage precisely.Also,the pile soil stress-strain data were recorded automatically and the settlement data at different depth of soil were measured accurately.2.Ten sets of nine long short piles(five long and four short,precast small scale concrete piles)composite foundation test model with different pile length,pile diameter and pile distance were investigated in this study.Soil stress and settlement strain were collected and analyzed at each loading stage,and these data was measured at multiple locations including the top of the pile,the middle the reinforced area of the long short composite pile,the interface of long pile and short pile,the middle and bottom of reinforced single pile and the based soil.3.The results shows that the bearing capacity of the pile was improved as the length of the long pile increase,diameter anddistance of the long pile decrease,while the soil properties and short pile’s length were kept constant.Moreover,the length of the long pile has more significant effects on the pile’s bearing capacity while the pile of soil replacement rate is fixed.4.The stress in long short composite foundation increases when the applied load increased.The maximum stress increase occurs at the middle reinforced section,where the magnitude of the load is also maximum.The peak of the stress distribution happens in the middle of the column,followed by the top of the column and the bottom bear the least stress.The stress of soil between the piles increases as the load increases.And the stress of the soil diminished as the depth increased.If the length of the column increases,the proportion of the stress in soil and the proportion of load increases too,and vice versa.Same result if the radius of the column decreases.However,if the distance among the columns becomes more separate,the ratio of the soil stress and the load increases.The stress on every section of the pile responds differently to the load.While the middle reinforced section bears the most pressure,and the bottom takes the least,which shows the range of the carrying capacity coefficient in the pile.5.The stress in the soil base declined dramatically as the depth of the soil increases,and the stress approximately reached zero at 600 mm depth.The soil stress decrease rate is higher for the soil under the central of the pile rather than that in the soil under the edge of pile.6.The indoor experimental data are consistent with the numerical stimulated data from FLAC.3D software,within the bearing capacity of composite foundation.7.It can be concluded from the result that settlement was decreased as the length of long pile was increased,settlement was increased while increase diameter of the pile.whereas settlement was increased while increase the pile distance.Additionally,soil replacement ratio for pile is one of the most vital factors which controls the settlement,change of length of long pile also influence the settlement of the foundation.Besides,ignorance variation of ES because of loading changes maybe one of the potential reasons that caused the calculated settlement value for reinforced areas is smaller than experimental data.