Research On The Soil Macropores In Vadose Zone Of Well Vegetated Slope

Since the middle of last century, the frequencies of well vegetated slope failure events following extreme intense rainfall in mountain valley walls are getting higher and higher. The problems of the threat to the broadest masses of the people and the damage to ecological environment have caused wide scholars’concern over the recent years. Many research progresses have been made breakthroughs in formation mechanism, disaster process and its effects, but on the whole the achievements are unbalanced, such as the study about the process of rainfall transfers into groundwater influence by macropores network which is the basic link of the relationship between rainfall and landslides and the characterization accuracy of permeable media are relatively weakly. Hence, it is necessary to carry out researches about soil macropores in vadose zone of well vegetated slope and the contribution to landslide preparation. In this dissertation, we take two different vegetated slopes (five experimental sites) for research objects, witnessed and laboratorial experiments, samples detection and numerical simulation methods are adopted respectively for following problems about essential characteristics and influence factors, quantitative and3-D reconstruction of macropores, the process of macropores flow, contribution of macropores to landslide preparation. Specific works and main results are as follows:1、The results about the physical, chemical, hydro-physical property study of soil in vadose zone show that the soil organic content especially in Touzhai experimental sites are relatively higher, which lead to high porosity-low volume weight-high water content and high swell-shrink characteristic. The physical parameters of unsaturated soils have obviously differences to agricultural and bare slope soil. The weathering crust of Emeishan basalt has come through the severely process of silicon relative loss and aluminum relative increase, and weathering eluviations. The weathering process conforms to pedogenesis law in hypergene condition.2、The interaction among such three factors as vegetation-fauna-organic content provide favorable conditions for macropores formation and evolution. The results of methylene blue and rhodamine-B dyeing infiltration tests under different simulated rainfall intensity demonstrated that macropore flow possess fiercely heterogeneity, and that macropore flow in different unsaturated soils of vegetated slopes are quite different. Partly dyeing features were illustrated that the types of soil macropores are root channels, interspace between root-soil and gravel-root, inter-aggregate porosity, faunal tunnels, freezing-thawing and shrinkage cracks. Among all factors, vegetation roots hold dominant influence position.3、Competed tomography technique was applied to large-scale soil cores. CT values were used for the separation of macropores and soil matrix. The results from macropores quantitative study indicate that development characteristics of unsaturated soil macropores in well vegetated slope are larger scale than farmland soils. Two independent parameters of the round rate and complexity have little discreteness with soil depth, which could be regarded as representation of macropores spatial and plane variability. Based on fractal theory, soil macropores have self-similarity characteristics in statistical sense and there was a positive correlation between macroporosity and their box-counting dimension which is decreased with soil depth. When dimension is constant, cumulative macroporosity is increased in power function along with the pore diameter. On account of VolView software,3-D reconstruction of the soil cores and macropores were perfectly achieved which demonstrated that the method is workable and image quality is ideal.4、The results about macropores flow process based on lattice Boltzmann method showed that the numerical analysis method could clearly and quantitatively describe the expansion process of wetting front, when macropores effect is remarkable, which weakened during the lattice point velocity gradually increase and then remain stable. There is high velocity in the region of well connectivity and large porosity, and there is a decrease in velocity from pore-center to pore-wall. The simulation process is consistent with actual situation. Therefore, the method has well applicability in macro-pores flow study.5、Unsaturated soil in well vegetated slope have sufficient features of strong absorption-severe storage-high permeability because of macropores wide existence, which can bring about the effects that slope soil nearly completely absorb rainwater without overland runoff under condition of rainfall and can optimize underground water supply environment. Additionally, there are characters of no any obvious delay and sensibility about the response of streamflow to rainfall, and most of streamflow is from groundwater discharge, which can indicate that the runoff alternation of slope groundwater is ferocious. Furthermore, the macropores can provide material basis and humidity condition for chemical water-rock interaction, which can keep saturation index of exchange phases in a low level and ensure chemical water-rock interaction can continue on a timescale. In conclusion, unsaturated soil macropores in vegetated slope can appreciable impact water infiltration under the condition of rainfall, so that they can promote groundwater recharge and circulation, as well as chemical water-rock interaction, and eventually expedite the process of slope instability.The innovations in this dissertation are as follows:1、There are clear differences between well vegetated slope and bare slope about the macropore distribution characteristics and the response of groundwater to rainfall. Many existed researches about field and indoor permeability tests and mathematic models have been achieved without consideration on vegetation, even occasionally remove deliberately. We take two different vegetated slopes for research objects in this dissertation. The results would play a very important role in theory and practice.2、The research about macropore distribution has focused on agricultural soils, and few studies adopting by CT scan method and taking well vegetated slope failure events as research background. We take four25cm width,25cm length,50cm height vegetated slope soil cores for CT scan samples. The study objects and research scale has a certain break through.3、The another innovation of this dissertation is combination of lattice Boltzmann method and CT slice. The study achievements about macropore flow process would lay the foundation for water penetration in3D macropore network.