| The goal pursued by all fields involving drilling technology has always been to improve drilling efficiency.Drilling efficiency is related to the progress of engineering construction and the cost of production.The average drilling speed within a certain drilling operation time is widely recognized as an indicator for evaluating drilling efficiency.It is inadequate to evaluate the drilling efficiency based on the drilling speed.The economic and time cost of drilling depth per pore and the pore-forming quality should also be considered.Due to the limitation of test cost and duration,relevant research projects are insufficient and in-depth exploration is difficult to be carried out.Some papers focus on the methods to improve the rock-breaking efficiency of bits.With the proposal of “cost reduction and efficiency improvement” and the progress of relevant technologies,various equipment and auxiliaries used for drilling have been improved to a certain extent.However,there are few research projects on the drilling efficiency of the hole-forming equipment for micro-pile foundation construction in the plateau mountain area.Taking the drilling efficiency of the hole-forming equipment for the micro-pile foundation construction in the plateau mountain area as the research object,how to improve the rock-breaking effect of drilling tools and optimize the technology are discussed.Focusing on the single cutting pick,the effect of operating angle and parameters on the force,temperature,vibration and wear of the cutting pick was identified based on theoretical research,finite element analysis and experimental verification,and the rock-breaking mechanism of the drilling tool was determined.On this basis,the layout of cutting picks was explored,the position parameters of each pick were obtained,the full-size bit was manufactured,and the experimental investigation of the full-size bit was implemented.Furthermore,the drilling parameters were optimized and verified in engineering practice.The main results are as follows:(1)Drillability of typical rocks(sandstone,granite,limestone and shale)Current evaluation methods of rock drillability were systematically analyzed.Protodyakonov coefficient based on the rock’s uniaxial compressive strength was adopted to evaluate the drillability of typical rocks.A uniaxial compressive strength test for typical rocks was carried out,and the corresponding Protodyakonov coefficient was determined.(2)Vertical rock-breaking mechanism of single-cutting pickThe mechanism of single-cutting pick rock-breaking was studied theoretically.A finite element model of the vertical rock-breaking single-cutting pick was established by the mean of the finite element method.The effect mechanism of operating parameters on the force-bearing capacity of the cutting pick was explored.The results indicated that the rotational speed was negatively correlated with the axial force,and the feed rate was positively correlated with the axial force.The process of cutting rock with a bit was analyzed.During the operation of the cutting pick on the rock,the mechanism of heat generated by friction and the theoretical calculation model of the cutting pick temperature was established.According to the basic theories such as frictional heating,bit rock-breaking mechanism and finite element method,the thermal-solid coupling finite element model of vertical rock-breaking with the single pick was established.The temperature field distribution of the cutting pick and the effect mechanism of the vertical rock-breaking single-cutting pick were analyzed in the process of rock cutting.The results indicated that if the feed rate was higher and the temperature was higher,the increase in temperature was negatively related to the rotational speed.Different feed rates have different effects on temperature.When the feed rate is 0.501 mm/s,the temperature rises when the rotation speed increases.Once other feed rates were adopted,the temperature decreased with the increase in rotational speed.The heat transfer of bit drilling on the rock was analyzed by the mean of the differential equation.Effect factors of bit temperature change were concluded,and the change rule of bit temperature was identified,with a view to providing a reference for the in-depth research on the failure mechanism and engineering application of the bit.In order to properly regulate drilling parameters and solve the unstable vibration of the drilling tool,research on the theory and generation mechanism of drilling tool vibration signal during rock breaking was implemented by the mean of wavelet analysis.The results indicated that the effect of rotational speed and feed rate on longitudinal vibration was less than that on transverse vibration.The smaller the stiffness of the rock,the more significant the vibration caused by impact.(3)Effect mechanism of working angle of cutting picks on mechanical properties and wearA numerical simulation model of the interaction between the pick and rock was established.The effect of cutting mode,structural parameters and operating angle parameters of the pick on its mechanical characteristics were analyzed.The results indicated that the cutting angle can significantly affect the rock-breaking performance of the pick.The axial force decreased with the increase of the cutting angle.The pick structure and operating principle were explored.The contact model of the interaction between the pick and rock was established by the mean of the discrete element method.The effect of the rock property parameters,pick working angle parameters and structural parameters on the wear degree was analyzed,thus laying a theoretical foundation for the research on the wear characteristics of the pick.The results indicated that the wear of the pick was mainly caused by the interference of the back-knife surface of the pick with the incompletely cut rock,which made the wear of the pick more serious than that of the front-knife surface.The wearing length caused by the interaction between the pick and rock decreased with the increase of cutting angle.The wearing length of the pick body decreased with the increase of the cutting angle.The greater the cutting angle,the worse the caving performance of the rock.Furthermore,the rotational speed and feed rate were positively correlated with the wear rate.Finally,the above results were verified through experiments.The wear mechanism of the pick was analyzed from three aspects: the wear positions of the pick,the main wear forms and the main effect factors of the wear.(4)Optimization of cutting pick layoutA mathematical model of the pick optimal layout covering design variables,objective functions and constraints was established.The position parameters of all picks were determined,and several bits were manufactured.Experimental research on the full-size bit was performed,including a feed force test,a non-contact temperature test based on the FILR-T600 infrared thermal imager and a vibration test based on LMS test Lab.Combined with the thermal-solid coupling finite element analysis results of vertical rock-breaking with the single pick,the temperature rise rule of drilling 40 MPa sandstone was revealed.The results can be used to determine the duration of a single drilling session in a 40 MPa uniaxial compressive strength sandstone at an engineering scale.Combined with the wear simulation study,the wear characteristics of the bit when drilling 40 MPa sandstone were revealed,which provides a basis for the life prediction of the cutter.At the same time,the above research methods can be applied to other drilling objects,and provide theoretical support for the formulation of engineering construction plan.(5)Multi-objective optimization of drilling parameters and engineering practiceNSGA-II multi-objective genetic algorithm was adopted to optimize drilling parameters.The results indicated that when drilling rate increased,specific energy increased and bit life decreased.Taking a newly built 110 k V substation near Miaotan Village,Tuzhu Town,Xuanhan County,Sichuan Province as the test site,field investigation and drilling exposure were implemented.It was found that the stratum of the site within the survey depth was composed of the Quaternary soil layer and Jurassic bedrock,and the bedrock was composed of mudstone and sandstone.The actual drilling test was performed based on the optimization method,and the reliability of the optimization method was verified.The test results indicated that the optimized drilling parameters can significantly improve drilling efficiency. |
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