流变对悬挂式防渗墙应力变形的影响初探

Preliminary study on the influence of rheology on vertical stress and displacement of suspended concrete cut-off wall

覆盖层中坝下防渗墙竖向应力变形在很大程度来源于覆盖层向混凝土防渗墙传递的负摩阻力。为解决覆盖层防渗墙应力计算值异于工程中实测值的问题,在非线性计算中引入变形黏滞特性,以Maxwell元件模型为基础,融合模拟瞬时变形的Duncan非线性模型和模拟黏性变形的Newton黏性定律,并推广到三维,建立了联合模拟非线性变形和流变(黏性变形)的计算模型。基于某规划中在超深厚覆盖层上修建150 m级心墙堆石坝的实际工程,建立了河谷段覆盖层和混凝土防渗墙的平面应变简化分析模型。深厚覆盖层表面坝体填筑过程被简化为逐级施加的均布荷载。模拟了填筑过程及填筑完成后短期内河谷段悬挂式防渗墙的竖向变形和竖向应力发展过程。考虑剪切变形的黏性时,覆盖层因附加应力作用下沉降变形而向防渗墙传递的负摩阻力会出现应力松弛。覆盖层变形对防渗墙整体向下拖曳、向位于墙体中下部的覆盖层-墙体沉降差中性点处挤压的作用减小,防渗墙较不考虑黏性时存在少量向上整体位移增量,内部沉降梯度减小,竖向应力减小。考虑黏性时,覆盖层顶面荷载施加结束不再提高后,防渗墙应力还会继续下降,直到稳定。假定墙体处于弹性状态,亦暂不考虑墙体混凝土徐变,采用恒定模量计算混凝土应力,河床段防渗墙内部最大竖向应力可从不考虑黏性时的64.8 MPa减小到26.6 MPa,减小幅度超过1/2;考虑黏性对混凝土防渗墙沉降影响较小,仅较不考虑黏性时略有减小。

The vertical stress and settlement of the cutoff wall under the embankment dam, are largely de-rived from the negative friction transmitted from the overburden to the concrete cutoff wall. In order to solve the problem that the calculated stress value of cutoff wall is often different with the measured value in engineering, the viscosity characteristic is introduced into nonlinear stress-strain calculation. Based on Maxwell element model, Duncan′s nonlinear model simulating instant deformation and Newton′s viscosity law simulating viscous deformation are integrated, and then extended to three dimensions. Thus, a joint model for nonlinear deformation and viscous deformation is established. A plane strain simplified analysis model of overburden and cutoff wall in valley center section is established, according to the dimension of a 150 m level earth core rockfill dam built on ultra-wide and ultra-deep overburden in river valley. The embanking process of the dam is simplified as uniformly distributed load applied on the surface of overburden step by step. The vertical deformation and vertical stress development of the suspended cutoff wall in the valley section during and shortly after the completion of embanking are simulated. When the viscosity of shear deformation is considered, stress relaxation appears in the negative frictional resistance transferred to the cutoff wall due to the settlement deformation of the overburden layer under the action of additional stress. The effect of overburden deformation on the overall downward drag of cutoff wall and on the extrusion to the neutral point of settlement difference between overburden and cutoff wall, which locates at the middle and lower part of cutoff wall, is reduced. Compared with the one without considering the viscosity, there is a small upward displacement increment in cutoff wall. Moreover, the internal settlement gradient and the vertical stress in cutoff wall decreases, compared with the one without considering the viscosity. If the viscosity is considered, after the loading on the top surface of overburden layer stops increasing, the stress of cut-off wall will continue to decrease until it is stable. It is assumed that the wall is in an elastic state. Moreover, the creep of concrete is not considered. Then a constant value of the modulus is introduced in the computation of stress in concrete cutoff wall. The maximum vertical stress inside the cutoff wall in the river valley section can be reduced from 64.8 MPa(without consideration of the viscosity) to 26.6 MPa(with consideration of the viscosity), decreasing by more than half. However, the calculated settlement with consideration of the viscosity is only a little smaller than that without considering viscosity.