| During the process of construction and excavation on dam foundation of reservoir Piaoshuiyan, it was revealed that the foot of landslide is located in the designed dam abutment of concrete faced rockfall dam. As a matter of fact, there was no obvious sliding sign of landslide before excavation in this area. The less development of surface crack is always neglected due to the smooth terrain and the flat layer. During the proceeding of stepped excavation on slope from rebuilt road to riverbed, the deformation appeared clearly in the front of it. It is possible to decrease the stability of dam, such as reservoir capacity decreasing, dam body instability and so on, if reasonable prevention measures were not taken enough in the future.Many treatments had been done in this paper for ensuring the stability of the dam body and the normal operation of reservoir. Based on regional geology and engineering-geologic condition, and considering the deformation characteristics, deformation monitoring data of landslide and the physical and mechanical properties of rock and soil mass, this paper take many analysis methods, such as the process mechanism analysis, the theoretical calculation method, UDEC, Geostudio and FLAC3 D numerical simulation method. In this way, the formation mechanism, deformation and failure mechanism of the leading edge of slide, overall and partial stability can be investigated and discussed under the condition of excavation, dam backfill, rainfall and reservoir level fluctuating. At the same time, the variation characteristics of stress and strain were discussed as well on above-mentioned conditions. Finally we acquired some achievements and recognitions as follow:(1) The landslide belongs to large landslides, and the maximum thickness of it is about 30 m, volume of it about 250×104 m3. According to the relationship among the landslide deformation characteristics, engineering structures and slope excavation, the landslide is divided into I1 deformation failure sub-region, I2 sub-region and II stable region downstream. The local collapse occurred in the front of zone I1, the volume of which is 35×104 m3, and water seeped out from the toe board of dam when heavy rain occur. There is no obvious deformation in zone I2 before and after excavation and the slope is relative stable compared to zone I1. Zone II is stable and has no obvious deformation. According to the on-site geological investigations and the excavation in the leading edge of slope did it reveal that “bedded-like” rock was found well-developed. By using drilling, borehole TV means and physical mechanics test experiments, many soft interlayer, compressive zone and hollow phenomena are existed evidently. The crack of trailing edge and upstream boundary developed and landslide toe was exposed during the excavation. After heavy rains the water seeped out from the leading edge of slope. We make a conclusion that the sliding zone was hidden before riverbed excavation.(2) The landslide used to be low-angle stratum and soft-hard interbedding rock mass of Siluric which trends to outside slope. And the interlayer staggered zones or weak intercalation developed between the layers. The analysis on formation history of landslide shows four evolution stages that the valley downcutting and slope creeping, pressure cracking face extending to bottom-up, bedding slope sliding and riverbed reengineering. The numerical simulation of UDEC reproduces the deformation and failure process of landslide though the underlying weak structural plane and weak interlayer under the weight. According to deformation characteristics including pressure crack trough at the crown of landslide, characteristics of dipping structure surface, tensile crack and "echelon" crack of slope after excavation, and the numerical simulation results, the deformation and failure mode of landslide was comprehensively judged to be “sliding- pressure cracking”.(3) Monitoring data show that deformation of I1 area emerged after excavation. And it gradually enlarged with the depth increasing of excavation, thus local slope collapsed after heavy rain. Landslide area is located in the turning parts syncline core to the flanks, where dipping structural plane developed at the crown of slope and downstream, which provides tectonic conditions for local deformation of I1 area in front of landslide under excavation. With on-site investigation and monitoring for stepped excavation process of I1 area, combined with FLAC3 D numerical simulation, it shows that a part of landslide area though slide and the crack occurred at the back of zone I1 under the common effect of the front freeing, the argillation and softening of remaining weak intercalation, the weak structural plane. At last, the "creep-cracking” deformation took place in zone I1 of slope under the action of rainfall or water flow in construction. The " creep-cracking” has the feature of traction characteristics, which is more sensitive to the effect of rainfall.(4) By using the limit equilibrium method, the stability of landslide was computed on static condition. As a result, the landslide is basically stable as a whole under various conditions. Zone II is more stable than other zones and the stability of I1 area is lower than the other. Combination software SEEP/W with SLOPE/W to study the seepage field and the stability of landslide, the water level of I1 area near slope surface is slightly higher than the internal of slope body during rainfall. And internal saturation line "lag" behind the change of it at the toe of slope under the condition of water level fluctuation. The stability of landslide is affected a bit small and stable basically on the working condition of excavation, dam backfill, rainfall and impound. The stability of zone I1, state of which is between understable and not to some extent, is greatly influenced on diverse conditions. Studies have shown that rainfall and reservoir water fluctuation do influence the stability of landslide I1 area.(5) Coupling the seepage field with stress field of slope body by using finite element software SEEP/W and SIGMA/W(Geostudio), the characteristics in different working condition can be simulated. We found that shear stress, compared with the natural condition, mainly concentrated in the lower part of zone I1 on various conditions. Further to say, it is the deformation in horizontal direction that focus obviously on the toe board at the toe of slope and the bridleway of 462 m. |
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