Application Study On Soil-Nailing Technique In Guanmen High Slope Of Xichang-Panzhihua Expressway

Xichang-Panzhihua Expressway crosses a mountainous area with complex geology. Some high cut slopes have been constructed here. The stratum is composed of deep completely decomposed granite (CDG), highly weathered granite and the Quaternary colluviums and eluviums. The cut slopes will be unstable. So the soil nailing technique is proposed to reinforce some of these high slopes. However, as a permanent structure used for stabilizing a high slope, the use of soil nailed wall has broken the limit of height and time listed in current design codes and specifications. That breakthrough has a potential risk.In this thesis, the high slope of Guanmen at K64+759～900 of Xichang-Panzhihua expressway is mainly studied, which includes field survey and exploration, physical and mechanical test of soil, stress and deformation monitoring, theoretic analysis and finite element analysis using PLAXIS V8.2.Based on the monitoring data of axial force of nails, earth pressure on the facing and so on, and the simulation using two-dimensional finite element computer program, PLAXIS, a systematic analysis which includes deformation and stress behaviors in construction stages is made. The strength reduction method is used to analyse the extension of plastic zone, formation and evolution of slip surface. The position of slip surface and the safety factors of slope stability are ascertained. The slope is in a disadvantageous condition when the earthquake of VII degree occurs or the slope is exposed to a long-term heavy rain. The static simulation and strength reduction to failure analysis of these disadvantageous conditions are made. The stability of the slope is systematically estimated according to monitoring and calculating results above. The main products and conclusion are as follows:By comparing the two results of monitoring data and numeral simulation, it is indicated that it's feasible to use the two-dimensional numeral simulation in the case of soil nailed Guanmen high slope. The numeral simulation using PLAXIS can give out exact and visual results of the deformation and stress behaviors, it also can make a quantificational stability analysis of slope under complex conditions.The results of monitoring data and numeral simulation indicate that the design of Guanmen high slope can meet the demands of specifications. By far, the slope is stable. Under the precondition of good construction quality and no unpredictable or irresistible fatal natural or man-made disaster, the long-term stability is in conformity with the specifications. Yet, we cannot entirely exclude the possibility of local disease of slope. The soil nailing method is fit for reinforcing high slopes that are located in the stratum of CDG and highly weathered granite, such as Guanmen high slope. Furthermore, the method is efficient and economical.The monitoring and numeral simulation of the L2 section of Guanmen high slope shows that the average axial force (tention) in nails located at the lower layer in the middle grade of the slope is the largest. And the earth pressure on the facing shows the fusiform characteristic in the stratums of CDG and the colluviums and eluviums: small at the upper and lower and big in the midst. To adopt Coulomb's earth pressure theory is not fit the situation here.The numeral analysis indicates that the failure surface of slope with soil nailed wall is much deeper and bigger than the one without retaining structures. The failure surface assumes a big cambered surface. The main slip surface passes through the soil nailed wall from the bottom with the borders extend a bit far, while the subsidiary failure surface passes through 2-3 layers of nails at the lower wall, with its forepart to be located in toe of slope. Obviously, the strengthening and the reinforcement function of soil nailing structure can force the failure surface to move deeper, thus to enhance the slope stability.The soil-nailing wall should be constructed in successive phases from top downward. The excavation should be limited in depth and possibly limited in length depending on the type of ground being stabilized. A much large mass cut can cause inreversible displacements that are disadvantageous to the slope stability. It will greatly threaten the construction safety, and also bring disadvantage for the long-term stability of slope.