考虑冻土双向冻胀与衬砌板冻缩的大型渠道冻胀力学模型

由于大型渠道断面大、渠坡长,渠基冻土沿坡长方向的切向冻胀及衬砌板的冻缩变形不可忽略,该文把大型渠道衬砌板的冻胀破坏视为两者共同作用的结果,结合冻土的Winkler弹性地基假设,并考虑冻土冻胀变形的双向冻胀差异,提出一种开放系统梯形渠道衬砌板法向和切向冻胀力的计算方法及内力计算公式。基于弹性地基理论推导了衬砌板的冻缩应力表达式,并由迭加原理提出大型混凝土梯形渠道衬砌板的抗裂验算方法。以甘肃靖会灌区某梯形渠道为原型,分析了衬砌板各截面内力和冻缩应力的分布规律,进而确定了各截面最大拉应力的分布规律及危险截面位置。对综合考虑冻土双向冻胀和衬砌板冻缩及仅考虑法向冻胀的2种情形进行对比分析表明,基于前者的衬砌板最大拉应力为2.134 MPa,而基于后者计算的相应值仅为1.494 MPa,与前者相比偏小、偏不安全。因此,在大型渠道的抗冻胀设计中建议综合考虑冻土双向冻胀和衬砌板冻缩变形的影响。

Mechanical model for frost heave damage of large-sized canal considering bi-directional frost heave of frozen soil and lining plate frozen shrinkage

Abstract: In vast and cold permafrost regions, concrete lining canals are highly vulnerable to frost heave damage because of the freezing and expansion of foundation soil. Frost heave deformation of canal foundation frozen soil is orthotropic, namely bi-directional frost heave. Bi-directional frost heave of foundation soil consists of frost heave both parallel to temperature gradient direction and perpendicular to temperature gradient direction. As large-sized concrete lining trapezoidal canals have larger cross-section dimension and longer canal slope lining plate than ordinary canals, the mechanism of frost heave damage of large-sized canals is different from medium or small sized ones. Both storage effect of tangential frost heave force along canals concrete lining plates and frozen shrinkage stress of canals concrete lining plates are significant. The reasons for frost heave damage of canals concrete lining plates in cold regions consist of frost heave of canal foundation frozen soil and frozen shrinkage of canals concrete lining plates. As low rainfall and no water diversion in winter in north-western cold and arid regions in China, initial moisture content of canal foundation soil is extremely low. In specific regions with specific meteorological conditions and soil quality conditions, groundwater replenishment becomes dominant influence factor which determines frost heave intensity of each point on canal concrete lining plates. By combining Winkler elastic foundation assumption for canal foundation frozen soil and considering orthotropy of frost heave deformation of frozen soil namely bi-directional frost heave of frozen soil, calculating methods to determine distribution of normal frost heave force and tangential frost heave force on canals concrete lining plates in open-system conditions were proposed. Eventually calculation formulas of internal force of canal concrete lining plates were carried out. Analytic formulas to calculate frozen shrinkage stress of canal lining plates were deduced based on elastic foundation beam theory, and methods for crack resistance checking computations of canal lining plates of large-sized concrete lining trapezoidal canal were proposed. By taking a trapezoidal canal in Gansu Jinghui irrigation district as prototype, the distribution of internal force and frozen shrinkage stress of each section of lining plates were determined. Then the distribution of maximum tensile stress (namely, tensile stress on upper surface of canal lining plates) on canal lining plates and the position coordinate of the most dangerous section were calculated. Contrast analysis results between the situation considering both bi-directional frost heave and frozen shrinkage stress and the situation only considering normal frost heave force irrespective of frozen shrinkage stress showed that the calculated value of maximum section tensile stress according to the latter was notably less than the previously calculated value. Therefore, taking no consideration of the effects caused by bi-directional frost heave of canal foundation frozen soil and lining plate frozen shrinkage in mechanics analysis and frost heave resistance designs of large-sized concrete lining canal was unsafe. While considering both bi-directional frost heave and frozen shrinkage stress, the maximum cross-section tensile stress on shady-slope plate was 2.134 MPa. While considering normal frost heave only and irrespective frozen shrinkage stress, the maximum cross-section tensile stress on shady-slope plate was 1.494 MPa. Thus it can be seen that the calculated values would be smaller if considering normal frost heave only and irrespective frozen shrinkage stress. The research results can provide references for mechanics analysis and frost heave resistance designs of large-sized concrete lining canal.