Research On The Mechanism Of Groundwater In Reactivating Of Old Landslides On Reservoir Bank

Most of landslides in Three Gorges Reservoir Area are ancient or old landslides. The old landslides are mostly gentle slope landform which can provide natural place for the residents living and engineering construction. The reservoir water has rise, it can seriously scour the reservoir old landslides and cause hydrodynamic pressure and hydrostatic pressure. The reservoir old landslides would reactivate at the cycle effect of reservoir water and threat residents's life and property security and normal operation of reservoir. So the research on the mechanism of groundwater in reactivating of old landslides on reservoir bank is of great significance.This paper takes Wanzhou middle school old landslide as the major study objects, which is one of the typical old landslides in There Gorges area, and analysed the characteristics of saturated-unsaturated seepage field and stability evolution under the effect of reservoir water and revealed the mechanism of groundwater in reactivating of old landslides on reservoir bank. The main achievements are as follows:(1) The old landslide named according to the relationship between the occurrence time of the landslide and the erosion epoch of the river, it is referred to the landslides which were occurred in the floodplain period and at the stable state now. The engineering geological characteristics of old landslides are as follows:For topography and geomorphology the slopes are not flat, there are annular depressions on the slopes, reservoir bank are convex, river terrace were destroyed continuous, trees were toppled, attitude of rocks generate larger variety. For material structure the origin is multiplicity, the material composition is complex, the structure in unorganized, the material structure and composition are heterogeneity and discontinuously obviously. The soil structure of slope can be divided into clay soil type and gravelly type. The rock structure of slope can be divided into block fracture structure, cataclastic structure and granular structure, and the block are called block fracture rock,cataclastic rock and granular rock. The old landslides have characteristics of polystage activity and there are multiple stages sliding zone in it. The process of formation and evolution can be described as:Mountain unloading→loosening→cracking→rotating→sludging→collapsing and sliding→overlapping on terrace→compound accumulation bodies. The process of reactivation can be described as:compound accumulation bodies→sludging→collapsing and sliding→overlapping on terrace→new compound accumulation bodies.(2) There are two basic modes of action in landslide:unconfined aquifermode and confined aquifermode. Groundwater have many effect on landslide:softening effect, the uplift pressure effect, hydrodynamic pressure effect, water cushion effect, excess pore water pressure effect, bank rebuilding effect.(3)The groundwater effects on landslide under unconfined aquifermode influenced by permeability of landslide obviously.With the reservoir water level rising, the stability coefficient of landslide increased with decrease of landslide permeability coefficient in the same periods. The key external force is hydrodynamic pressure which point inside the landslide when the stability coefficient of landslide is relatively small (less than10-5m/s), and the stability coefficient of landslide increased with time to certain landslide. The key external force is hydrostatic pressure when the stability coefficient of landslide is larger (more than10-5m/s), the results showed the uplift pressure effect, and the stability coefficient of landslide decreased with time to certain landslide.With the reservoir water level declining, the stability coefficient of landslide increased with increase of landslide permeability coefficient in the same periods. The key external force is hydrodynamic pressure which point outside the landslide when the stability coefficient of landslide is relatively small (less than 10-5m/s), and the stability coefficient of landslide decreased with time to certain landslide. The key external force is hydrodynamic pressure at the initial stage when the stability coefficient of landslide is larger (more than10-5m/s), and the stability coefficient of landslide increased with decrease of hydrostatic pressure and increase of matric suction at later stage, the stability coefficient of landslide decreased at the initial stage and increased at later stage.(4)The groundwater effects on landslide under confined aquifermode influenced by the permeability of landslide bed obviously.With the reservoir water level rising, the stability coefficient of landslide increased with decrease of landslide bed permeability coefficient in the same periods. The key external force is hydrodynamic pressure which point inside the landslide when the stability coefficient of landslide bed is relatively small (less than10-5m/s), and the stability coefficient of landslide increased with time to certain landslide bed. The key external force is uplift force when the stability coefficient of landslide is larger (more than10-5m/s), the results showed the water cushion effect, and the stability coefficient of landslide decreased with time.With the reservoir water level declining, the stability coefficient of landslide increased with increase of landslide bed permeability coefficient in the same periods. The key external force is hydrodynamic pressure which point outside the landslide when the stability coefficient of landslide bed is relatively small (less than10-5m/s), and the stability coefficient of landslide decreased with time to certain landslide bed. The key external force is hydrodynamic pressure at the initial stage when the stability coefficient of landslide is larger (more than10-5m/s), and the stability coefficient of landslide increased with decrease of uplift force and increase of matric suction at later stage, the stability coefficient of landslide decreased at the initial stage and increased at later stage.(5)The reactivation style and scale of Wanzhou middle school old landslide are controlled by the material composition, topography and geomorphology, slip surface morphology of itself. It's impossible to slip whole integrity basically. The high content of cohesive soil is beneficial to forming sliding surface, and it can case local or shallow slip. The old landslide would mostly reactivate in the declining process of reservoir water level according to the evolution rule of landslide stability. The key external force is hydrodynamic pressure which point outside the landslide in reactivating of old landslide which caused by combined action of hydrodynamic pressure, hydrostatic pressure and uplift force. The whole body of Wanzhou middle school old landslide is stable. The secondary landslide body is not stable, and it can reactivate easily under the effect of external factors.(6)The confined water head of the groundwater in under sand and gravel is the main factor affecting the stability of this kind of old landslide, and the permeability is one of the decisive factors of confined water head. The larger the permeability of the under sand and gravel is, the faster the confined water head changed, and the uplift force changed in the same, even appear two minimum points. The larger the permeability of the under sand and gravel is, the faster the confined water head increased, and appears the first minimum point. The larger the permeability of the under sand and gravel is, the longer the lower stability the old landslide in. The results show that the larger the permeability of the under sand and gravel is, the smaller stability coefficient is, and the old landslide reactivated easier.The characteristics and innovative points of this dissertation are as follows:(1) Make a systematic summarize of characteristics of old landslide on topography and geomorphology, material structure and process of formation and evolution. The geological model of old landslide is inductived comprehensively. (2) The characteristics of saturated-unsaturated seepage field and stability evolution of reservoir landslide under the effect of unconfined aquifer mode and confined aquifer mode are systematically studied. The mechanism of groundwater in stability evolution of reservoir landslide is revealed. (3) Takes Wanzhou middle school old landslide as the major study objects, and analyses the characteristics of saturated-unsaturated seepage field and stability evolution under the effect of reservoir water and revealed the mechanism of groundwater in reactivating of old landslides on reservoir bank. It is pointed out that The whole body of Wanzhou middle school old landslide is stable, the secondary landslide body can reactivate easily under the effect of external factors, the larger the permeability of the under sand and gravel is, the smaller stability coefficient is, and the old landslide reactivated easier.