Research On The Principle And Application Of Folding Blasting In The Same Direction For Ultra-high Chimneys In Complex Environments

This paper focuses on the principle and key technical issues of the demolition blasting of ultra-high chimneys in complex environments.For the blasting demolition of ultra-high chimneys,one-way directional falling blasting,reverse folding blasting,segmental blasting and other technical methods are adopted domestic and abroad.However,under the influence of the surrounding complicated environment and the structural factors of the ultra-high chimney,conventional blasting techniques cannot meet the requirements.There have been several successful cases of folding blasting of ultra-high chimneys over 200m in complex environments,but they all adopted reverse folding design.The delay intervals of the upper and lower cuts were different.There were failure cases,occasionally,due to improper delay time between cuts or chimney recoil force caused by cuts.There is no precedent of folding blasting in the same direction and setting the upper cut at a height of 110m or more.With the development of blasting technology,the folding blasting of the super-high chimney in a complex environment would become a key problem to be solved in recent years.The determination of the delay time between the upper and lower cuts is a difficult point in the design of folding blasting of the ultra-high chimney and a key factor in the successful blasting demolition.It is of great significance to study the technology of folding blasting in the same direction for the design and operation of blasting demolition of ultra-high chimneys in complex environments,which could provide technical guidance and reference for similar demolition projects.This study starts from the analysis of the motion mechanics of folding blasting in the same direction of the super-high chimney.It can be seen from the formula of the blasting pile length that the factors influencing the length of the blasting pile are the location of cuts,and the rotation angle of the bottom of the chimney,the velocity of the barrel center,and the time for the free fall of the upper chimney to the ground when the upper cut is closed.After the position of the upper cut is determined,the landing time of the upper part of the chimney is mainly related to the angle of movement of the upper and lower parts.Therefore,when the position of the cut is determined,the key factor that affects the length of the muck pile of the same-direction folding blasting is the blasting time difference between the upper and lower parts.If the time difference is too long,the limit state would be that the chimney is detonated twice,and the blast pile is the larger value in the upper and lower chimney heights.If the time difference is too short,the limit situation would be that the upper and lower parts of the chimney are simultaneously detonated,and the length of the muck pile is the height of the entire chimney.This paper improves the study of falling mechanism of chimneys by studying the motion laws of chimney during the closing process of the upper cut and after it is closed,and the mechanical conditions of the destruction of the support at the cut,instead of the commonly-used empirical formulas or estimation.This paper focuses on the establishment of the finite element model for chimney blasting demolition,and with the help of the Ansys/Ls-Dyna numerical simulation software,uses the dynamic analysis module in Ansys numerical simulation software to deal with the selection and definition of the unit type,the material model selection,the model establishment and the contact,constraints,loads,and the set of initial conditions.Based on the objective conditions and the subjective understanding of the researchers to build and solve models,the processes of teetering,falling,disintegration and crushing of the chimney under certain conditions are obtained,which can demonstrate the feasibility of the collapse scheme,predict the shape of muck pile,check the harmful effects,like slump vibration,and finally optimize the blasting design.In the numerical simulation of blasting demolition of chimneys,the collapse of the structure is achieved by gravity after the blasting cut is formed.When the structure forms a certain overturning moment,the support part fails and the concrete unit is directly deleted.Due to the failure of the concrete unit at the support part,the structure will sit and recoil in an instant.The hinge structure and the shearing structure working together,the potential energy is converted into the kinetic energy.Under the condition of large eccentric compression,the concrete unit fails and the instantaneous impact velocity of the structure is proportional to the square root of the structural unit size.During the collapse,the cumulative effect of the size factor of the structural unit will cause each part to collide with each other,which would affect the shape of the muck pile.By the numerical simulation of the folding blasting of the super-high chimney,the collapse processes of different upper and lower time intervals are analyzed,and the reasonable delay interval of the upper and lower blasting cuts is found to be 3s.Through the principle analysis and the numerical simulation of the same-direction folding of the ultra-high chimney,the reasonable upper and lower delay time is determined.In the case of the 210m reinforced concrete chimney of the Xijiao Thermal Power Plant in Tangshan,the upper cut is set at+110m,the lower cut is set at+0.3m,and the time difference between the upper and lower cuts is 3s.The demolition set a successful example for the demolition of super high chimney by the same-direction folding blasting.By comparing the numerical simulation results to those of the actual blasting,the expected results are fully achieved.It provides a theoretical basis and an engineering reference for the application of folding blasting in the same direction in demolition ultra-high chimneys within similar complex environments.