Analysis Of The Stability Of Parallel Boreholes Constructed By HDD

With the rapid development of social economy, and the continuous expansion of city size, the insufficientunderground pipeline construction problems seriously affected the normal operation of the city. Trenchless technology due to its economy, convenience, small occupation area, and theadvantages that does not affect the transportation has gradually replaced the traditional excavation method. HDD as a universal application of non excavation method, the hole wall stability problem is a important factor for the project’s success. This paper based on the predecessors study of single hole HDD borehole wall stability, Analysis of parallel boreholes stability constructed by HDD,the main work content includes:(1) Use theanalytical method of rock mechanics, combined withthe Cauchy integral method and Schwarz alternating method, to solove the anisotropyparallelhorizontal directional drill that under internal pressure by 4 steps, and get two analytic functions.(2) Summaring the factors that infuluences the stability of horizontal directional borehole wall, including: geological condition, drilling fluid, drilling technology. Use Mohr Coulomb criterion to analysis the borehole wall plastic radius size,and calculate the plastic zone of the same diameter with different drilling fluid pressure, the samedrilling fluid pressure with same diameter, different spacing, differentstatic earth pressure,respectively.(3) Through finite element software MIDAS/GTS simulate four conditions under parallel boreholes stability,compare the simulation results and the theoretical results, draws the following conclusions:(1)Under the condition of the same diameter and different drilling fluid pressure, the plastic zone of the hole wall decreases with the increase of the drilling fluid pressure,if the drilling fluid pressure is too high, the borehole wall soild is tend to move outside;(2)Underthe condition ofthe same drilling fluid pressure and differentdiameter, with the increase of the bore diameter, the plastic zone also increases, however, the radius of the plastic zone and the inner diameter ratio tend to be stable when the diameter increases to a certain extent;(3)Under the condition of the different hole spacing, with the increase of the distance between the holes, the plastic zone increases gradually. When the distance between the holes is increased to a certain extent, the radius of the plastic radius increases rapidly;(4)Theclay soil underthecondition of differentstatic earth pressure,the hole wall plastic radius remain basically unchanged.