三板拼接式小型U形混凝土衬砌渠道冻胀破坏力学模型

Mechanical model of small U-shaped concrete lining canal with three arc-plates under frost heaving

研究土体冻胀对渠道混凝土衬砌结构的影响,对于保障季节性冻土地区渠道输水效率具有重要意义。以三板拼接式小型U形渠道混凝土衬砌结构为对象,研究其在冻胀作用下的受力状态。依据叠加原理将冻胀视为重力、切向冻结力、法向冻结力及法向冻胀力的共同作用,利用结构整体及局部的平衡关系,根据渠道的结构型式、破坏特征及原型观测结果,建立了衬砌结构的冻胀破坏力学模型,给出了便于设计应用的计算公式。对野外观测中的某渠道各部位内力计算结果表明,三板拼接式小型U形渠道混凝土衬砌结构以压弯为主,与实际破坏特征相吻合。刚性接缝处承受较大的轴力和剪力,是结构受力的薄弱位置。为减轻冻胀破坏程度,进一步建议衬砌板直线段倾角α在[22,26]度间取值,坡板弧段圆心角θ略大于U形渠底圆心半角,法向冻结力的合力作用点为从渠顶沿直线段向下2/3处。研究结果可以为季冻区三拼式小型U形混凝土衬砌渠道的设计提供参考依据。

Exploring the frost heaving effects of structures in cold regions is quite necessary, especially in Northern China, since frost heave will be a major cause of damage to some geotechnical structures there. The present paper takes the small U-shaped canal of splicing concrete lining with three arc-plates as a case study sample, which is a common structure type at the construction of canal in Yellow River irrigation area of Ningxia, and investigates the effects of frost heave on stress state of canal＇s concrete lining. At first, take 6 assumptions to simplify the complicated factors of frost heave. According to the superposition principle, we consider the frost heave action on the concrete linings as the combined result of gravity, tangential freezing stress, normal frost heaving stress and freezing stress, and give the equilibrium relationship between them both integrally and locally; then we analyze the force for each part of concrete lining subjected, and establish the mechanical model under frost heave according to the structure and failure characteristics of concrete linings and the site monitoring experiment data in Yellow River irrigation area of Ningxia during freeze-thaw period from 2013 to 2015. Thus, the simple computational formulas of internal force for linings＇ each part including the slabs of lining, the arc plates and the U-shaped bottom of lining are obtained. Using the above formulas, in one U-shaped canal with three arc-plates concrete lining with known size in the prototype observation region, the frost heave and the internal force are calculated. The calculating results show that： Firstly, the small U-shaped canal with three-plates bears compression and relatively uniform bending moment together, which is in accordance with actual engineering; Secondly, the most complicated stress situation of the concrete lining exists in the areas from the junction of slab and arc plate to the rigid seams with cement mortar of fine aggregate concrete, where the section easily cracks or is destroyed if the frost heaving and freezing stress are big enough; Thirdly, if the camber of the slab of lining is too small, it may lead to the fragility of concrete linings under frost heaving situation; Fourthly, the position of normal frost freezing stress decides the bending moment of canal＇s slope. At last, this paper further analyzes the effects of camber of slab and central angle of arc plate on normal heaving stress, discusses the position of normal frost freezing stress, and gives the following suggestions that the camber of the slab of lining ranges from 20 to 26 degrees, and meanwhile, normal frost freezing force actions at the position of 2/3 of slope from the top. Especially, the internal forces of rigid seam are researched by using the central angle of arc plate, which indicates the position of rigid seam. Results show that, due to great axial and shearing force, the rigid seam is weakness position of concrete lining, and thus, the central angle of arc plate should be greater than the half of the central angle of curved bottom slightly when it is designed in order to obtain lesser frost heaving force and damages. All these works are beneficial for the design of U-shaped canal with three-ace plates in frozen soil area.