Study On Construction Effect And Control Technology Of Horizontal Directional Drilling

With the acceleration of urbanization in China,it is necessary to build a large number of municipal underground pipelines to meet the needs of urban development.Without affecting the surface transportation,buildings（structures）,trenchless excavation technology is extensively employed because of its unique technical advantages.In trenchless technology,compared to pipe jacking and TBM（Tunnel Boring Machine）construction,Horizontal Directional Drilling（HDD）is more suitable for medium and small cross-section pipelines.It is widely used in pipeline laying because of the advantages of fast construction speed,low cost,and controllable direction.However,HDD is also faced with a series of problems such as mud pressure damaging to adjacent buildings（structures）,hole collapse,and drilling trajectory deviation.Therefore,research on such problems has theoretical and engineering significance.Based on the principle of elastic-plastic mechanism,the construction disturbance mechanism of HDD is investigated,and the distribution range of the plastic zone around the HDD channel is obtained.Used the ABAQUS numerical simulation software,combined with the exemplifying project of the Liuwan Water Plant of Zhengzhou City to pass through the main Middle Canal of South-to-North Water Transfer Project,verified the numerical analysis model and parameters by the field measurement data.On this basis,the HDD construction effect mechanism and corresponding control technology are summarized to provide a certain reference for design and construction.The main results are as follows:（1）According to the investigation of the example project and the analysis of the disturbance mechanism of the HDD construction,the main disturbance stage is considered to be"hole-expanding phase",and the hole-expanding is not a simple"squeeze soil"function,but mainly cuts the soil with process of squeezing simultaneously.（2）Through the study of the circumferential stress state of HDD,explore the in-situ stress coefficient（the ratio of horizontal stress to vertical stress）λ≠1,and analyze the influence of various factors on the plastic stress distribution around the hole.The research shows that the maximum plastic radius₈（6）decreases with the increase of the soil cohesion₈)and the friction angle₈);increases with the increase of the borehole radius R₀;as the in-situ stress coefficientλincreases from 0 to 0.5 shows an increasing trend and then gradually decreases;as the drilling depth z increases,it shows a parabolic increasing trend.The recommended value of the plastic development coefficient n of different soils is given.The distribution shape of the plastic zone mainly shows“ellipse”with different parameter values,and technical measures to improve the stability of the hole wall are proposed.（3）The whole process is monitored in the Liuwan Water Plant of Zhengzhou City to pass through the main Middle Canal of South-to-North Water Transfer Project,the settlement features of the top roads,surrounding soils of the canal during the construction are analyzed and summarized.It is proposed to use the differential settlement rate of lining 0.5‰as the main canal safety control alarming threshold.（4）Combined with the project monitoring data of the example project,the ABAQUS finite element software was used to simulate,compare and analyze the distribution feature and plastic distribution characteristics of the directional borehole stress.HDD mainly produces large plastic strains in the vertical direction around the construction hole,so collapse mainly occurs in the vertical direction.（5）Using proven numerical analysis models and parameters to simulate the subsidence feature and concrete surface strain feature of the South-to-North Water Transfer Main Canal caused by different directional borehole depths.And with differential settlement,absolute settlement limit,and concrete strain limit as control values,the recommended drilling depth to ensure the safety of the main canal is obtained.It is recommended that the buried depth of the directional borehole should be no less than 5m from the bottom of the main canal,that is,it should not be less than 5times the diameter of the designed final hole.（6）The mud pressure of HDD was studied.For the drilling depth greater than 10times the diameter of the tunnel,the existing formula of the ultimate mud pressure was revised.When nearby,it is suggested that the mud pressure should be less than 0.3MPa.