Study On The Evolution Law Of Frozen Shaft Lining Temperature Field In Deep And Complex Strata

In recent years,China has to turn for deep and complex stratum for exploiting many kinds of resources such like coal and iron result from shallow resources has been being used up.Freezing Method is the most primary shaft sinking method in deep and complex stratum,whose key technology are the shaft wall and the frozen wall.In deep and complex stratum,there is complex and violent thermal interaction between frozen wall and high strength mass concrete shaft wall,but there is a lack of systematic and in-depth research on it in China and abroad.Studying and mastering the evolution law of frozen shaft wall temperature field is very important for preventing concrete frost damage,developing the prevention techniques for wall temperature fractures and studying the mechanism of shaft wall crack because of frost heaving and thawing settlement.Aiming at cast-in-situ concrete frozen shaft wall with thickness of 1-2.5m and strength grade C60-C80 and CF60-CF90,this paper studies systematically and deeply about the shaft lining temperature field during freezing period,thawing recovery period and periodicity temperature change period,by using a variety of methods as theoretical analysis,physical modelling,numerical calculation and field measurement.Firstly,Through physical simulation and numerical simulation,a high precision cement hydration temperature expression—composite index expression with a fitting error of less than 5% is presented and the range of correlation parameters is given and the calculation method and the data range of coefficient of heat transfer at inner surface of both inner and outer lining are obtained by analyzing different working condition.This provides a key reliable basis for the study of frozen shaft wall temperature field with time-varying internal heat source and complex heat transfer boundary conditions.Secondly,the evolution rules of shaft lining temperature field during freezing and thawing recovery are studied.Based on the actual engineering,the distribution and variation of temperature field in the early stage of shaft lining are obtained,through 12 large physical simulation tests for 2 strength grades and 4 mixing ratios of large thickness and high strength concrete.A large-scale 3-dimensional numerical calculation model for the whole process of shaft sinking by freezing method including ground freezing,excavation,the lining of outer wall(or single lining)and the lining of the inner wall,is established.The temperature field evolution rules of the outer lining(or single wall),the inner lining and surrounding rock(soil)during the freezing and the thawing recovery period are analyzed and mastered through four aspect(14 factors)including external boundary condition,inner factor,internal boundary condition and construction technology and some important technical parameters such like the variation range of lining's temperature at each construction stage,maximum temperature difference,heating rate,temperature falling rate,melting range of freezing wall caused by cement hydration and thawing time are obtained.The effects of various factors on the temperature field during freezing and thawing recovery are compared and analyzed.Field measurements were carried out on 12 shaft walls,and the variation rules of the shaft wall's temperature fields were obtained in three stage,respectively,cement hydration temperature rising,rapid cooling and slow cooling.These 12 shaft wall include: 4 outer wall of thick alluvium well(the depth of topsoil is 675.6~754.96 m,which is the most in the world),1 wall seat with large diameter and thickness(both the inner diameter and the wall thickness is the biggest of frozen wall,respectively is 10.5m and 2.5m),3 inner wall of well,4 new single wall in water-rich bedrock.Finally,the evolution law of the shaft wall's temperature field in periodicity temperature change period is mastered.An analytical equation for the temperature distribution of the shaft lining during periodicity temperature change period is derived and the evolution law of the temperature field when the daily and annual air temperature in the shaft well changes with simple harmonic is analyzed.The numerical model of temperature field in cyclic thermogenesis is established and the effect law of some parameters such like wall thickness,thermal conductivity,internal boundary temperature and time to temperature field.By monitoring 5 shaft wall for many years,the variation rule of shaft wall temperature was mastered.Research results show that: Reasonable control of the temperature of the shaft wall and cast concrete can prevent the concrete of the frozen bedrock section from freezing.Anti-cracking measures should be taken because the phenomenon of large radial temperature difference and large average temperature change generally exists for high strength,thick outer shaft wall(or single wall)and thick inner shaft wall which is result in there is a high risk of temperature cracks in the wall.In daily production operation stages,the shaft wall temperature fluctuates with the air temperature in the shaft well and for the downcast shaft wall,the change of temperature can be over 20?,which is one of the causes of the increase of shaft wall vertical addition force and the main reason that the deep and thick alluvium shaft wall usually breaks in April to October every year.