Study On The Long-term Stability Of Spray Sowing Protective System For Vegetation Rehabilitation On Slopes

Comparing to other eco-protection technologies, spray sowing technology has some prominent advantages, such as mechanized construction, fast construction speed, restoring the vegetation in short time and so on. With the further and rapid development of spray sowing, many new spraying machinery, material, and construction technique with independent intellectual property were put forward. But as time passed, some problems of spray sowing appeared, such as destruction of the structure, sliding of foreign soil and degradation of vegetation. People begin to doubt the long-term stability of spray sowing system and wonder whether the protecting effect of spray sowing is reliable and sustainable. The main reason for these problems is that the study on the long-term stability of spray sowing is inadequate and there is no corresponding mature design theory and construction method to ensure the long-term stability. Therefore, funded by the key project ’The vegetation rehabilitation technology for the slopes along the railway" of China railway ministry, this dissertation studied and analyzed the long-term stability of spray sowing technology systematically by the methods of theoretical analysis, laboratory tests, field tests, field monitoring,etc. The main contents and conclusions of this dissertation are summarized as follows.(1) The classification of slopes for vegetation rehabilitationIt’s obvious that the difficulty of vegetation rehabilitation is not the same on various slopes in different area of China. The dissertation analyses the difference from three aspects:1. the surrounding environment of the slopes;2. the conditions of the slopes;3. the available foreign soil in local area. According to different difficulty of vegetation rehabilitation, the author proposed the FVR (Feasibility index of Vegetation Rehabilitation) evaluation system to classify the different difficulty levels of slopes for vegetation rehabilitation and evaluate the suitability of spray sowing method. The FVR system also provides the scientific basis and references for planning and design of spray sowing technology.(2) The function of bolt-net structure and its designFrom the theoretical aspect, this dissertation analyzed the function of bolt-net structure and its reinforcement mechanism in the spray sowing protective system. The analysis indicates that the bolt-net structure reinforces the foreign soil mainly by three ways:First, the structure establishes the framework of foreign soil. Secondly, the structure reinforces the foreign soil like geotechnical fabric. Third, it transfers the system load to the rock. Of which the nets use the frictional effect and occlude method to reinforce the foreign soil. Based on the mechanical analysis, the dissertation gives suggestions on the selection of net on different gradient slopes. The author also summarized the factors that should be taken into consideration during the design and specification of the structure, and presented the planning procedure and design flow chart of bolt-net structure.(3) The study on durability of bolt-net structureThe deficiency of durability of bolt-net structure may lead to poor protective effect or failure of spray sowing method. Therefore, this dissertation analyzed the similarities and differences of durability between the bolt-net structure and the normal reinforced concrete structures. The paper also introduced the corrosion mechanism of metal materials and summarized the factors that affect the corrosion speed in the foreign soil. Combined with the features of spray sowing method, the paper makes a deduction of the time limit when the loaded bolt-net structure will fail theoretically. Based on the Fick second law, the author used the software ANSYS to simulate the corroding process of mortar under different boundary conditions. Besides, the author offered a proposal on how to improve the durability of bolt-net structure in practical projects from many aspects, such as selecting material, construction method and projects management and so on.(4)The study on the stability parameters of foreign soilIf the foreign soil that holding the vegetation slides, the spray sowing protective system will fail immediately. The reasons that lead to the failure mainly lies in two aspects:the rain fall and the deficient strength of foreign soil. According to this situation, the paper analyses the relationship between the erosion and the rainfall intensity theoretically, establishing a formula to judge the occurrence condition of erosion due to the rainfall and verified the formula by the artificial rainfall experiment. The changes of foreign soil strength during the different growth period of plants are also investigated by laboratory tests. In addition, based on the results listed above, the paper discusses the stability of foreign soil on different gradient slopes using the limit equilibrium analysis method and proposes the suitable slope gradient range for the spray sowing protective system.(5)The water equilibrium of spray sowing protective systemPlant can not survive without water. Once the water content of spray sowing protective system is insufficient and can not meet the requirement of plants, they will wither or die. So, the proper water content, which not only can meet the requirement of plants but also can maintain in a state of dynamic balance, is a key precondition for a successful spray sowing protective system. In the light of the above considerations, In the laboratory tests, the author study the water parameter changing rule of foreign soil combined with different content of organic matter and water retaining agent during the different growth period of plants. Secondly, the proportioning of foreign soil in the laboratory tests was used in field tests to study the water content changing rule of foreign soil with different gradients and covering condition. Third, based on the results above, the author proposed a water equilibrium equation. It can predict the water content of foreign soil with meteorological data and judge whether the water content is enough for the survival of plants.(6) Succession rule of plants community and its stability evaluationThe spray sowing protective effect only can be demonstrated by the plants eventually. Many spray sowing projects failed because the plants on the slopes can not form a stable community structure. Therefore, based on years of observation on many successful spray sowing projects, the paper concludes the plants succession rule of a stable community structure. It will experience three phases: the growth period of pioneer plants, the invasion period of native plants and the stable community period. Furthermore, to evaluate the stability of the plants community, the paper chooses seven indexes, such as survival time, coverage degree, species similarity, etc, and establishes an evaluation system.In the end of the dissertation, the author takes four typical spray sowing projects as examples to illustrate the application of the research results. Among these projects, the shortest time of successful vegetation rehabilitation is more than nine years, and the longest time is over twelve years. These projects prove that the long-term stability of spray sowing protective system can be achieved as long as the proper measures are taken.