Research On Mechanism Of Confined Groundwater In The Gently Inclined Landslide

Gently inclined landslide is a kind of landslide, sliding surface obliquity of which is less than20degrees. Influence of confined groundwater is very important to sliding damage of gently inclined landslide.Translational landslide is a special case of gently inclined landslide. Methods of engineering geology analysis, theoretical analysis, numerical simulation and physical model test are used to study the influence mechanism of confined groundwater in gently inclined landslide. The hydrogeology condition and the action range of confined groundwater are analyzed. Effect of permeability parameters variation on landslide stability is analyzed. The start-up criterion of translational landslide is modified. The temporal and spatial variation of pore water pressure induced by confined groundwater is analyzed. The hysteresis of confined groundwater effect which acting on gently inclined landslide is analyzed. Finally, the main results and new ideas in this study are as follows:(1) Based on engineering geology analysis of Xiashan landslide, formation mechanism and its effect on this landslide are analyzed. Monitoring data shows that the acting of the action range of confined groundwater is less than the length of sliding surface. Combining with geological environmental condition of Xiashan landslide, the typical groundwater seepage model of translational landslide is established, and the result of slope stability calculation becomes more reasonable considering action range of confined water. Based on one-dimensional steady seepage theory, calculation formula of action range caused by confined groundwater is deduced, and then parameter sensitivity of this formula is analyzed. The calculation result shows that action range of confined groundwater follows established law within a certain range of each parameter:①Action range of confined groundwater decreases linearly with the increase of permeability coefficient.②Action range of confined groundwater increases linearly with seepage discharge.③Action range of confined groundwater takes on parabola shape with thickness of permeable layer, and it is accelerated decreasing trend with increase of permeable layer thickness. Calculation result also shows that the stability coefficients of slope decreases linearly with the increase of action range of confined groundwater.(2) The existing research about start-up criterion of translational landslide neglects two aspects:one is the normal component force of hydrostatic pressure at posterior border of slope, which acts on slip surface, the other is the action range of confined water. Therefore the calculation results of start-up criterion of translational landslide are inaccurate. Overall force analysis are exerted on translational landslide, meanwhile the conception of action range of confined water is introduced, then the modified start-up criterion is deduced. The modified start-up criterion is applied to analyze the results of physical simulation, and proved to be accurate. Mechanism analysis is applied on the differences of critical hydraulic between test results and calculation results obtained from the modified start-up criterion of translational landslide.(3) The present simulation of confined groundwater effect in slope stability analysis is too simplified to guarantee an appropriate analysis mode. Considering that some existing research has given confusing interpretation about the groundwater effect, this thesis proposes a concept of using pore water pressure near the slip surface to indicate this effect, and then analyzing the temporal and spatial variation laws of pore water pressure induced by confined groundwater and the consequent influence on slope stability caused by this variation. According to the analysis of slope stability results under different permeability conditions, the concept of seepage coefficient ratio is proposed, and the spatial distribution of pore water pressure in dry season with various seepage coefficient ratios is analyzed, and then the variation laws of slope stability corresponding to different seepage coefficient ratios are obtained. Under two conditions including the permeable layer blockage condition and non-block condition at front edge in wet season, the temporal and spatial variation laws of pore water pressure and effect on the slop stability factor caused by the variation with different rainfall duration are analyzed. Through analyzing engineering practice, the effect on slope stability caused by the variation of confined groundwater is verified, and the variation of confined groundwater is induced by the rainfall in wet season.(4) Via engineering examples analysis, the hysteresis of gently inclined landslide failure caused by confined groundwater effect is analyzed. A physical slope model including confined groundwater is constructed to measure volume water content, slope body deformation, slope surface deformation and slope thrust in different position of this model. According to the test results which are processed using normalization method, the hysteresis of slope deformation relative to confined groundwater effect is analyzed. The mechanism of slope failure hysteresis caused by confined groundwater is discussed from some aspects, such as stresses and strain redistribution law, slope creeping law, long-term strength of slip soil and impact of water content on the strength of slip soil.