Study On The Influence Of Post-impact Combined Hole Forming Method On Hole Wall Stability

In China,bored pile is one of the main application forms of pile foundation in the field of foundation engineering.Considering the diversity and complexity of engineering foundation,the hole forming method of bored pile also presents the development trend of " Multiple-parallel".However,no matter how advanced the hole forming technology is,a single hole forming method can not break through the technical upper limit of its "Targeted-advantage",and the advantages and disadvantages of the hole forming method largely depend on the engineering object it faces.In view of this problem,the combined hole forming method has inherent advantages.The switching and combination of hole forming methods will maximize the technical matching degree between different soil layers and different hole forming methods,realize the complementary advantages,improve the tunneling efficiency and reduce the construction cost.Tunneling efficiency is an important index to evaluate the advantages and disadvantages of hole forming methods,but it must be based on the stability of hole wall,and there is little research on the stability of hole wall of combined hole wall.In this paper,the combination of post-impact hole forming method is selected as the research object,and there are two reasons: The impact drilling is the most easy to form explosive external force disturbance in all the drilling methods,so the post-impact combination is the most unfavorable combination for the stability of the hole wall in the pairwise drilling combination;Compared with other pore forming methods,the impact pore forming method is more suitable for hard and complex strata,which is consistent with the distribution law of natural foundation from top to bottom.so the post-impact combination is the most engineering universal combination method in the pairwise drilling combination.In this paper,the post-impact hole forming method process is divided into pre hole forming process and post hole forming process,the main research contents and conclusions are as follows:(1)Research on the pre drilling process and the "Disturbance" of mud to soil mechanical properties.There are two reasons for the disturbance in the process of pre drilling: one is the result disturbance,that is,only the stress redistribution of the hole wall caused by the unloading effect after excavation is taken into account without considering the influence of drilling technology;The other is the process disturbance,that is,considering the influence of pore forming technology on the performance of soil.As slurry wall protection has become a common process of bored pile drilling,laboratory tests are carried out simultaneously to study the influence of slurry permeability on soil mechanical properties.The results show that: 1)the earth pressure decreases due to the construction disturbance,and the more the shallow soil is close to the orifice,the greater the decreasing range is;2)At the initial stage of infiltration,the water loss rate of the mud is large,the water content of the soil around the hole increases,the solid particles in the mud adhere to the hole wall to form mud skin,and the water loss rate of the mud gradually decreases.The mud infiltration reduces the cohesion of the soil near the mud hole by 15.91% and the internal friction angle by 30.36%.(2)Calculation of hammer impact during post hole forming process.In the process of post-impact,the hammer movement is affected by mud resistance,and the impact process is very complex.In this paper,the analytical solution of hammer falling velocity time history is obtained by theoretical calculation,which can provide a quick calculation channel for hammer bottom touching kinetic energy at any lifting height.The results are in good agreement with the simulation results of ABAQUS.The results show that: 1)when the hammer head falls,the acceleration first decreases,then the mud resistance increases with the speed and finally balances with the gravity,and the hammer head changes into uniform falling motion.At this time,the hammer head touches the bottom,and the maximum impact energy can be achieved in the shortest time,which is the most ideal hole forming process;2)It is the most economical way to improve the drilling efficiency by determining the appropriate lifting height to maximize the impact energy.(3)The distribution and transmission of impact during post hole forming process.When the hammer touches the bottom,part of the impact kinetic energy is used to break the rock and squeeze the soil,and the residual energy propagates along the hole wall in the form of vibration wave,and finally affects the stability of the hole wall as the disturbance input of the soil on the hole wall.In this paper,ABAQUS finite element software is used to simulate the propagation and attenuation of stress wave at the moment of impact,and combined with the scale test of rock breaking,the response of soil on the hole wall to vibration is monitored.The results show that: 1)the rock breaking energy in impact kinetic energy is related to the rock breaking absorption capacity,and the absorption capacity of intact rock is less than that of broken rock;2)The vibration response of the soil near the vibration source is the strongest,and the acceleration,displacement and velocity response of the particle are relatively large.The vibration propagates to the top of the hole and gradually attenuates.The first 1 / 3 section attenuates the fastest,and the closer to the top of the hole,the slower the attenuation.(4)The stability evaluation of the wall soil in the process of combined pore forming.Based on the instability mechanism of pore wall soil in the process of post impact combined pore forming,the evaluation criterion of pore wall stability considering the disturbance of pre pore forming performance and the effect of post impact external force is established.The results show that: 1)the evaluation results of the modified criteria are more secure than the original criteria;2)The results show that the bottom of the hole and the end of the casing are most likely to be damaged and unstable in the process of post impact combined drilling.