Study On The Reasonable Obliquity Of Integral U-shaped Canal In Seasonal Frozen Area

In seasonal frozen area,under the action of negative temperature gradient,the moisture migrates and changes phase,and the volume expands of soil,resulting in the cracking,lifting and deformation of the canal lining under the action of uneven freezing,which seriously affects the normal operation of the project,and wastes irrigation water.It has practical significance for the design and selection of canal and the development of water-saving in irrigation areas though studying the frost heave of mechanism and characteristics of canal destruction and proposing effective anti-frost heave measures with calculation and numerical simulation.Aiming at the insufficient problem of the thermo-hydro-mechanical coupling process and water-frost heaving damage of integral U-shaped canal in the seasonal freezing area.The integral U-shaped mechanical model of frost heave was constructed,and the thermo-hydromechanical coupling model was verified by soil column freezing test based on the coupling model of frost heave and frost heave theory.The finite element software COMSOL Multiphysics was used to simulate the freezing process of the prototype canal.Affecting factors of frost heave damage were comprehensively considered,the obliquity sections and reasonable construction suggestions were put forward,and the following main conclusions were obtained:(1)The mechanical model of frost heaving damage of integral U-shaped canal has been established.Under the condition of ultimate damage of lining,the relationship between ultimate tangential freezing shear stress and obliquity of lining was obtained by the calculation of structure,internal force and crack resistance of concrete slab.Solving with empirical formula of simultaneous tangential freezing force.The obliquity of ultimate anti-frost heave was calculated according to the soil quality of canal foundation,concrete lining strength and thickness.The calculation process of anti-frost heave obliquity is simplified,the calculation is convenient.(2)The thermo-hydro-mechanical coupling model has been established.The freezing temperature,temperature field and water field distribution of soil were obtained by freezing test of sub-clay in Hetao irrigation area of Inner Mongolia.The simulation results of waterheat-force coupling model are consistent with the experimental results.The Nash coefficient was used to evaluate the simulation quality of the model,the model is highly reliable with slight error.(3)Taking the integral U-shaped canal of D80 in Hetao irrigation area as an example,the variation in a freezing-thawing period was simulated.The maximum freezing depths of the shady slope,sunny slope and canal bottom are 1.2m,0.8m and 0.5m respectively.Soil water migrates upward,and mainly concentrates at the lining arc plate of the negative slope under the action of negative temperature,and the maximum ice content is 0.33.The lining lifts upwards as a whole with deformation and stress increase continuously,and the deformation of the shady slope with a tendency to dump,which is greater than that of the sun slope.The maximum stress is located the junction between the straight section and the arc section of the lining of the shady slope,which is prone to frost heave damage.(4)The effects of soil quality,lining properties and groundwater depth on 0°,10°,20° and 30 ° obliquities have been simulated.The silt foundation soil of canal lining has greater stress and frost heave than sub-clay.The higher the grader and more thickness of concrete lining,the stronger the more frost heaving resistance of the canal.The groundwater depth is shallow,the maximum stress at the junction of the straight section and the arc section,the groundwater depth is shallower,the maximum stress at the bottom of the canal and the junction of the straight section and the arc section,the groundwater is deep and the maximum stress at the bottom of the canal.The cracking stress standard,water delivery capacity and cross sectional area of the concrete were taken into comprehensive consideration,the reasonable obliquities of C20 concrete lining U-shaped canal are 13～23° for sub-clay soil and 17～28° for silty soil in Hetou irrigation area.Considering the complexity of natural conditions,combined with the characteristics and location of canal lining’s frost heaving damage and soil ice content distribution,soil replacement,thermal insulation board and cushion were proposed to improve the anti-frost heaving effect of canal engineering.