水库降水速度对大坝上下游边坡安全系数和失效概率的影响研究

The influence of water drawdown on factor of safety and failure probability of the reservoir-dam slope

为了探究水库降水速度对大坝上、下游边坡安全系数和失效概率的影响,首先,在Geo-studio中建立水库大坝模型,通过控制上游水位下降高度,在非饱和非稳定渗流条件下,在Seep/w模块模拟了大坝上、下游边坡在不同降水速度情况下各个时步浸润线的位置,进而利用Slope/w模块计算各个时步大坝上、下游边坡最小安全系数。其次,结合蒙特卡罗抽样法分析了不同降水速度下大坝上、下游边坡最大失效概率变化规律。最后,研究了上、下游坡角对大坝边坡最小安全系数的影响。结果表明:①大坝上游边坡最小安全系数随着库水位的下降开始减小,降幅完成时刻最小安全系数降到最低值,以后随着坝体内浸润线的逐渐下降,其最小安全系数又开始增加,直至稳定在某一数值左右;②降水速度对降幅完成时刻上游边坡的最小安全系数有显著影响,随降水速度的增加,其最小安全系数迅速减小,在降水速度为4 m/d时,其最小安全系数为0.96,需引起高度重视;③大坝上游边坡最大失效概率随降水速度增加而增加;④大坝下游边坡的最小安全系数和最大失效概率基本不受降水速度、降幅的影响;⑤降幅对大坝边坡的控制工况有影响,在确定控制工况时可综合考量最小安全系数和最大失效概率两个指标;⑥当下游坡角保持不变,定义上游坡角与下游坡角之比为坡角比值,上、下游边坡最小安全系数相等的坡角比值记为临界坡角比值。当坡角比值大于临界坡角比值时,上游边坡最小安全系数小于下游边坡最小安全系数。反之亦然。随着降水速度的增加,临界坡角比值逐渐减小。在进行水库大坝上、下游边坡稳定程度比较时,建议综合考量坡角比值、降幅和降水速度的影响。

The impact of the water drawdown on the factor of safety and failure probability of reservoir-dam slope is investigated in this paper.Firstly,a reservoir-dam model is established in Geo-studio and the seepage analysis is simulated within Seep/w by setting the drop height of the upstream water level.Based on the phreatic surface of the dam at each time step,the Slope/w module is used to calculate the minimum factor of safety for upstream and downstream slope.Secondly,the Monte Carlo sampling method is used to investigate the variation of the maximum failure probability of the upstream and downstream dam slopes.Finally,the influence of the upstream and downstream slope angles on the minimum factor of safety of the dam slope is studied.The proposed method is demonstrated in a reservoir-dam slope.The calculation results indicate that:①the minimum factor of safety for the upstream dam slope decreases with the water drawdown,and it reaches the lowest value at the time when the water drawdown is completed.After that,the minimum factor of safety turns to increase slightly;②the water drawdown rate poses a significant influence on the lowest factor of safety.As the water drawdown rate increases,the lowest factor of safety drops dramatically.It arrives at a value of 0.96 at water drawdown rate of 4 m/d,which should attract much attention from reservoir practitioners;③the maximum failure probability of the upstream slope of the dam increases with the increase of precipitation rate;④the minimum safety factor and maximum failure probability of the downstream slope of the dam are not affected by the precipitation rate and decreasing amplitude;⑤the control condition of the slope of the dam is affected by the decreasing amplitude,and the minimum safety factor and maximum failure probability should be considered comprehensively when determining the control condition;⑥when the downstream slope angle remains unchanged,the ratio of the upstream slope angle to the downstream slope angle is defined as the slope angle ratio,and the slope angle ratio with the same minimum safety factor is denoised as the critical slope angle ratio.When the slope angle ratio is bigger than the critical slope angle ratio,the minimum safety factor of the upstream slope is smaller than the minimum safety factor of the downstream slope.And vice versa.With the increase of precipitation rate,the ratio of critical slope angle decreases gradually.When comparing the stability degree of the upper and lower slopes of the reservoir dam,it is suggested that the influence of slope angle ratio,decreasing amplitude and precipitation rate should be taken into consideration.