重力坝计算与实测坝踵应力差异中扬压力作用研究

坝踵应力是评价重力坝安全的重要指标,按无拉应力准则设计的重力坝采用有限单元法分析时往往出现较大的拉应力,但实际工程中未见观测到拉应力的报道,有的甚至出现较大压应力,同时实测应力变幅往往远小于计算值。本文统计了国内外24座重力坝观测结果,分析了坝踵应力蓄水前后变化特点,以一典型重力坝断面为例,采用材料力学法和有限元法对坝踵应力进行计算,对比分析了理论坝踵和实测坝踵应力变化规律,重点研究了扬压力对两个坝踵点应力贡献作用,揭示了计算和观测坝踵应力差异的原因之一。结果表明：理论上的坝踵应力是坝踵基岩一侧计入扬压力作用的有效应力,实测坝踵应力则是距基础一定距离、坝体混凝土内部的总应力,两者存在一个与扬压力接近的应力差;按无拉应力准则设计的重力坝,坝内测点不会出现拉应力,最小压应力为0.5～1.0倍的上游水头;实测坝踵的有效应力取决于渗流场和孔隙水压力系数(B系数),该点渗透压力变幅小于上游水头且存在滞后,致使应力变幅明显小于理论坝踵应力变幅;扬压力在理论坝踵和实测坝踵的作用差异是实测与计算坝踵应力差异显著的重要原因之一。

Study on the influence of uplift pressure on difference between calculated and measured heel stresses for gravity dam

Dam heel stress is an important indicator for evaluating the safety of gravity dams.When the finite element method is used for analysis, gravity dams designed according to the no-tensile stress criterion tend to have large tensile stress.However, there are no reports of observed tensile stress in practical projects.The gravity dam of some projects even has great compressive stress.Besides, the measured stress variation is often much smaller than the calculated value.In this paper, the observation results of 24 gravity dams at home and abroad are counted, and the variation characteristics of the dam heel stress before and after water storage are analyzed.Taking a typical gravity dam as an example, the dam heel stress is calculated by means of material mechanics and finite element method, and the variation law of theoretical dam heel stress and measured dam heel stress is compared and analyzed.This paper focuses on the contribution of uplift pressure to the stress of two dam heel measuring points, and reveals one of the reasons for the discrepancy between calculated and observed dam heel stress.The results show that the theoretical heel stress is the effective stress considering uplift pressure at the bedrock side of dam heel, while the measured dam heel stress is the total stress inside the concrete of the dam body at a certain distance from the foundation.There is a stress difference close to the uplift pressure between the two dam heel stresses.For gravity dams designed according to the criterion of no tensile stress, the measured point in the dam is generally the compressive stress, and the minimum compressive stress is 0.5-1.0 times the upstream head.The measured effective stress of the dam heel depends on the seepage field and the pore water pressure coefficient (B coefficient).The amplitude of seepage pressure at this point is smaller than the upstream head and there is a lag.Therefore, the measured dam heel stress amplitude is significantly smaller than the theoretical dam heel stress amplitude.The difference between the effect of uplift pressure on theoretical and measured dam heel is one of the important reasons for the significant difference between measured and calculated dam heel stress.