Effects Of Herbaceous Plants On Slope Stability Under Drying Wetting Cycles

The biotechnical slope protection is a new method to prevent slope hazards, which by planting plants, so that the slope can not only satisfied the requirements of slope stability, but also meet the requirements of landscaping. It becomes the main way to prevent and control slope disaster and prevent soil erosion. However, roots can increase the permeability coefficient of soil, so that the higher pour water pressure will decrease the shear strength under rainfall conditions. On one hand, reinforcement effect of roots can increase the strength of soil, on the other hand, preferential flow of roots can decrease the strength of soil, which one plays an important role is still remain uncertain. In order to solve this problem, the influence of roots to permeability coefficient of soil, the influence of strength of soil and slope stability to comprehensively consider the effect of two aspects were studied in this thesis:In order to research the influence of roots to permeability coefficient of soil by field test three different cover layer models were established. The monitoring was started when these covers had experienced drying-weting cycles of one and half year. By comparing the data of the three covers at the same depth, it demonstrated that topsoil is affected by the weather strongly. Under the action of high rainfall and evaporation, the speed of water movement in soil covered with vertiver grass is the fastest, soil covered with Bermuda grass is the slowest. Soil covered with Vetiver grass had the fastest speed of infiltrate and evaporation at 30 cm depth, the cover layer with Bermuda grass coverage was hardly influenced by the weather. Only the soil covered with Vetiver grass can be influenced by the weather. By using the transient method to calculate the permeability coefficient of three cover layers, it shows that the permeability coefficient of soil covered with Vetiver grass is the biggest, soil covered with Bermuda grass is the smallest.In order to research the influence of roots to shear strength of soil by triaxial shear test, the removal of device by self-design was used to take the undisturbed rooted soil to do triaxial shear test. We found that the difference of curves of principal stress difference with trial strain is increasing as the trial strain increasing, and finally reached steady. They all present ed strain hardening phenomenon. Both rooted soil, including fissure soil and remolded soil can conform to the Mohr-Coulomb strength criterion. The existence of the roots can enhance soil cohesive force effectively, and the increasing rate is between 24.36% and 118.49%. The reinforcement effect of Vetiver grass roots is better than that of Bermuda grass roots.Considering the results of field test and triaxial test, the slope stability by numerical simulation is evaluated. In considering both the seepage effect and the reinforcement effect of roots to study the different slope of stability of vetiver grass coverage slope, bermuda grass coverage slope and without vegetation including crack slope under drying-weting cycles and without cracked slope. The results showed that planting Vetiver grass slope safety factor is far higher than the other three types of slopes, the difference of the other three types is not obviously. Under heavy rains, planting Vetiver grass slope has the fastest speed of rise of pour water pressure. The difference of the other three kinds of slopes is small. Although planting Vetiver grass slope is most sensitive to rain, the safety factor is still the highest after 100 h of heavy rain. After long-term stability analysis, we got that the most safety of four kind of slopes is planting Vetiver grass slope, planting Bermuda grass slope and slope without plant and cracked are both fairly. The most dangerous is no planted but with crack.