Numerical Simulation Study Of Ventilation During The Construction Period Of Shaft Excavation By Downward Boring And Raise Boring Methods

The shaft is an important part of the water diversion project,and its construction method is mainly divided into the downward boring method and the raise boring method.The ventilation during the shaft construction directly affects the safety and health of the construction personnel as well as the construction efficiency.Based on the actual project,this study investigates the ventilation problems in the shaft excavation by the boring method and the raise boring method through theoretical analysis,numerical simulation and mathematical statistics.In-depth analysis of the ventilation challenges under different construction methods and investigate the effect of different factors on the ventilation of the shaft during the construction period.The distribution characteristics of the flow field inside the shaft(tunnel)and the emission law of harmful gases under different construction methods are studied.The main research results obtained are as follows:(1)The theoretical derivation of the CO distribution function in the shaft during the excavation of the shaft by the downward boring method was carried out based on the dilution ventilation model.Meanwhile,the natural wind pressure in the tunnel after the guide shaft penetration during the construction period of the raise boring method is determined by combining the theory of excess differential static pressure and thermal potential difference.The different numerical calculation models are summarized and compared,and the applicability and reliability of the selected calculation models are analyzed by comparing the calculation results with the experimental results.The grid convergence index method is used to study the applicability of the grid,and determine the appropriate grid division method to ensure the accuracy of the simulation results.(2)The flow field distribution law and the transport law of harmful gas concentration field in the shaft during the construction period of the downward boring method with press-in ventilation were elucidated,and the sensitivity of ventilation time to each influencing factor was analyzed.The mechanism of the influence by different factors on the shaft ventilation is also revealed.Based on the distribution law of the flow field in the shaft under different working conditions,the distribution function of harmful gases in the shaft with space and time is derived for the press-in ventilation of the shaft.The formula for estimating the ventilation time when excavating shafts by the downward boring method was constructed based on the shaft diameter and the ventilation volume,and the accuracy of the formula was verified by actual works.And the relevant optimization suggestions are made for the ventilation system when excavating the shaft by the boring method.(3)The flow field distribution law of each section in the tunnel and the concentration transport law of harmful gases are clarified when the shaft is excavated by the raise boring method and pressed-in ventilation is carried out in the lower flat tunnel.The effect of the lateral and longitudinal cross structure of the tunnel on the wind flow was analyzed,and the effect of the air velocity of the duct in the lower flat tunnel on the overall ventilation of the tunnel was also revealed.The mechanism of the effect of natural wind pressure on tunnel ventilation after the guide shaft is penetrated is explored,and the effect of mechanical ventilation on natural wind flow is discussed.When the natural wind flow helps to ventilate the tunnel,proper press-in ventilation in the lower flat tunnel helps to improve the ventilation efficiency.Finally,the optimization of the ventilation system is proposed,which can effectively shorten the discharge path and ventilation time.(4)The calculation method of natural wind flow during the construction period of complex water diversion tunnels is clarified by combining the project examples of complex water diversion tunnels.The flow field distribution law and harmful gas diffusion law in this engineering example are also studied.The effects of different ventilation optimization measures during the construction period of the raise boring method were compared to provide a reference basis for similar projects.