Behavior of pore water pressure and stability evaluation against seepage failure in earth slopes

为了研究水库快速排水过程中土坝残余孔压的变化规律及其对土坝边坡稳定性的影响，采用离心机恒加速度下降上游水位的方法，对几种模型填土进行了离心机模型试验，在模型体内安装了小尺寸的孔压传感器进行孔压测量。试验中设置了几种不同的条件，研究了不同条件对路堤累积孔压累积的影响，即改变同类型均质路堤的压降速率、在上游面上设置薄的防渗土层作为倾斜心墙带、在路堤中部设置L排水渠以促进孔压消散。一些破坏试验中，滑动破坏在上游斜坡上重现，也是…更多的是讨论由于下降过程中形成的剩余孔压的增加而引起的斜坡的不稳定。本研究的结论如下：(1)在库水下降率较大的情况下，均质坝内部向上游面排水受到限制，上游边坡产生的残余孔压对边坡稳定性有较大影响。(2)在防渗层较厚的坝体中，由于上游面排水量较少，库水位与坝体浸润面之间的各种状态都会出现不连续的水头差。(3)在路堤内部设置L式排水沟后，虽然路堤内部的残余孔压迅速降低，但在不设排水设施的情况下，填土表层的残余孔压仍然较高，且随着水位的降低，出现较深的过坎滑动破坏。通过一系列的离心机试验，研究了渗透土地基上的河堤地基上的渗透扬压力的行为，讨论了扬压力对堤防管涌、承载力丧失等渗流破坏的影响。利用有限元饱和和非饱和渗流分析方法进行了分析研究，从孔压引起的滑移破坏和水力梯度集中引起的渗流破坏(水力破裂)两个角度，提出了考虑滑动面上孔压分布和填土内水力坡度分布的土坡稳定性评价方法。较少

In order to study behavior of the residual pore-water pressure and its influence on slope stability in earth dams during a rapid draw-down of the reservoir water, centrifuge model tests were conducted on several model fills by realizing draw-down of the upstream water level under a constant centrifuge acceleration, in which small size transducers were installed in matrix for pore-water pressure measurement. Several different conditions to be supposed were set in the experiment to investigate influential factors on accumulation of the built-up pore-water pressure in embankments : that is, the case where the rate of draw-down is changed for the same type homogeneous embankments, the case where a thin impervious soil layer is set on the upstream face as an inclined core zone, and the case where a L-type drain is set to promote dissipation of pore-water pressure in the middle part of an embankment. Some failure tests, in which sliding failure is reproduced in the upstream slope, were also … More conducted to discuss instability of slopes caused by the increase of the residual pore-water pressure built up during draw-down. Concluding remarks drawn from the present study are summarized as follows : (1) In the case where the rate of draw-down of the reservoir water is high, drainage water from the interior to the up stream face is restricted even in a homogeneous dam, and the residual pore-water pressure develops in the upstream slope to have a large influence in slope stability. (2) In a dam with a thick impervious core zone, a discontinuous head difference appears in every state between the reservoir water level and the phreatic surface in the dam, due to little flow of drainage water to the upstream face. (3) Although the residual pore-water pressure reduces rapidly in the interior of an embankment by setting a L-type drain, it still shows high value in the surface part of the fill to the extent of the case without drainage facilities, and comparatively deep sliding failure passing through the crest appeared as the water level decreases.Another series of centrifuge tests were done on the behavior of seepage uplift pore-water pressure acting on the base of a river dyke resting on a permeable soil foundation, and discussion was made on the influence of the uplift pressure on such seepage failures of embankment as piping and loss of bearing capacity. Analytical studies were also performed by use of the FEM saturated and unsaturated seepage analysis, and a method of stability evaluation of earth slopes was proposed, by taking into account of distribution of pore-water pressure along a sliding plane and that of hydraulic gradient in the fill, from both view points of sliding failure due to pore-water pressure and seepage failure (hydraulic fracture) due to concentration of hydraulic gradient. Less