| With the domestic railway passenger dedicated line is built,there will be a corresponding increase in tunnel excavation section,and the rich water loess tunnel with large cross section,because the soil of low strength,large deformation,small bearing capacity,is affected very strongly by the water,once being immersed in water,its strength will decrease,the initial support shows deformation is very large,and surrounding rock is difficult to remain stable for a long time.How in the construction of large section tunnel in rich water loess to find out a set of scientific,safe,quick construction method become the problem to be solved.This paper according to Zheng Xi passenger railway Zhang Mao tunnel excavation section,rich water loess in saturated,aqueous clay,settlement deformation of large,easy to collapse,complicated engineering geology,time limit for pressing and other characteristics,in order to achieve the safety,quality,fast construction,conduct a study about the engineering characteristics,the water rich loess research rich construction method water,large section loess tunnel large section loess tunnel construction technology,key technology,foundation treatment of rich water loess tunnel large section loess tunnel construction monitoring technology contents,conclusions and innovation points are as follows:(1)Zhang Mao tunnel is mainly of Quaternary Pleistocene(Q2)in old loess,clay loess,moisture content as high as 20%,saturation of 92%,with weak to middle expansion,belongs to Ⅳb surrounding rock of tunnel excavation,and large span,large section,the construction is more difficult than to build general loess tunnel of the same type methods,so should take reasonable and effective measures of construction and engineering.The excavation of the tunnel in rich water loess strata,because of loess disintegration,collapsibility and weak swelling,under the action of the rich water,soil excavation after rapid disintegration,strength to plummet,the capacity of self stability is greatly reduced;after the tunnel is excavated soil easily along joint swelling or cut loose,spalling occurred and the top collapse;selection of excavation method is reasonable to ensure that such geological conditions of large section tunnel security safety.(2)Through the comparative analysis of CD method,CRD method,double side heading method and three step and seven step excavation method,obtained three seven step excavation deformation values greater than CRD method and double side heading method,but the three step and seven step excavation method is a fast progress,and is conducive to the tunnel closed early,three step the seven step excavation method for construction project.(3)A three-dimensional numerical simulation of the three step seven step excavation method show that,the initial support stress concentration occurred mainly in the arch and vault;compressive stress occurs mainly in the roof and side wall,the wall;initial supporting steel frame of each control point,the lateral stress maximum value is-127.07MPa;the early supporting vault settlement is small,the final vault settlement value of about 50.2mm,vault settlement convergence distance about 2D;the initial support horizontal displacement value is small,the arch foot horizontal displacement of the final value of 6.3mm,the 10.5mm side wall,side wall at the bottom is 10.1mm;the arch foot horizontal convergence distance relative displacement about 1.5D,side wall and the bottom of the middle horizontal displacement convergence faster,convergence distance about 1D;(4)Choose typical cross section of the tunnel excavation support of surrounding rock and supporting structure stress and deformation in the process of field testing and research.From the field monitoring data,the initial support deformation is small,and the initial supporting closed quickly tends to be stable,and the stress and pressure of surrounding rock is also smaller,illustrates the large span tunnel three step and seven step excavation method is suitable for the water rich old loess stratum.(5)On the basis of studying the numerical analysis and field test,study and put forward the zhang MAO tunnel construction basic technical method of surrounding rock deformation control.The three step and seven step excavation method optimization design steps that:3M～5m for the top steps.4m～6m for the middle and low steps,in top steps’ length is 0.3 times more than its width,on the order of not less than width,step interleaving:2～3M;the step length is less than 0.8～1.0m,in the lower step,footage of not more than 1.5m,the invert excavation early supporting of a length not more than 2～3M;the remaining core soil,excavation face first injection 3～5cm concrete,set up advanced small pipe technology to effectively control the tunnel face stability;by setting high arch foot,feet lock bolt technology to control the overall settlement;construction of inverted arch shall be promptly closed,closed distance is 30～35m,closing time should be controlled within 15 days;(6)After the completion of tunnel foundation treatment,foundation coefficient of vertical increased,bearing capacity increased,and the physical properties of the tunnel arch foot got clear improvement;static cone penetration test and standard penetration value has been improved significantly;at the same time,control of the arch settlement after construction effectively,maintain tunnel section size.(7)After 21 Hz vibration frequency,the growth rate of dynamic displacement of filling surface in inverted arch becomes slow,and the maximum vertical dynamic displacement is about 0.5 mm.The maximum dynamic velocity of the filling surface in inverted arch is 1.61mm/s.The vibration stress has nothing to do with the vibration time and the vibration frequency,and under the pressure of 30kPa vibration stress,the maximum measured dynamic stress at the bottom of the inverted arch is 5.1kPa which is 1/6 of the filling surface in inverted arch.The depth is about 5.9m far from the filling surface in inverted arch where the vibration velocity reduces to 0.2 mm/s.And the maximum dynamic velocity of surrounding loess under the inverted arch is 1.03mm/s.(8)The size of the vibration strain has little change with the increase of vibration frequency in the backfill concrete of inverted arch,which can be considered it has nothing to do with the vibration frequency.Under the action of 30kPa vibration stress and 5～27Hz vibration frequency,the super hydrostatic pressure amplitude at the bottom of the tunnel is small which is about 1/50 of the vibration stress in the surrounding loess.(9)After the vibration excitation test,the blow number for each 10 cm is about 40 under the action of vibration exciter for the tunnel bottom soil,which shows that the rich water loess here is in the solid state and there is no softening phenomenon.There are no obvious changes in the gradation curves before and after the tests,especially the clay content didn’t decrease and there are no refinement and mud phenomena in the rich water loess of tunnel bottom.The shear strength indicators c and(p of loess in basement have little changes,so the excitation has nothing to do with the shear strength in the rich water loess at the bottom of tunnel. |
