Effect Of Forest Stand Type And Root Architecture On Hillslope Stability

Mountainous areas are widely distributed,coupled with the high frequency and large distribution of mountain disasters in China,making it one of the countries with the most serious landslide disasters in the world.Since the 21st century,with the space of human production and construction activities increased,the impact of global climate change,landslide disasters are occurring more and more frequently.Rainfall is one of the most ordinary inducements of shallow landslides.It is not clear that the mechanism of soil fixation and slope protection by vegetation under rainfall conditions,even though vegetation measures as an important ecological means to prevent and control shallow landslides is widly adopted nowaday.In expecialy,the dynamic response and the interaction of mechanical and hydrological effects under rainfall infiltration process are still a challenge in this field.In this study the mixed forest,evergreen broadleaf forest,phyllostachys pubescens forest,shrub forest and 6 kinds of plant root architecture in Chongqing Jinyun Mountain are selected as the study object.Meanwhile,field investigation and monitoring,soil mechanics test and numerical simulation are conducted indoor to analyze the influence of dynamic slope stability among different forest stands;the main interested facters contains different root architectures,and the critical slope gradient and rainfall threshold related to facter of safety are caculated.The results are as follows:(1)Due to the deep vertical distribution and largest amount of roots the root strengthening effect of phyllostachys pubescens forest was the best among the four forest stands.The soil moisture and suction stress in typical forest stands shows seasonal variation affected by rainfall and temperature.Among the four types typical forest stands,phyllostachys pubescens forest has the strongest rainfall interception ability,followed by shrub forest,evergreen broadleaf forest and mixed forest.In the same rainfall condition,the phyllostachys pubescens forest and shrub forest have lower soil moisture variation rate than that of the evergreen broadleaf forest and mixed forest.(2)Comparing the factor of safety and their influencing factors among the four typical forest stand,the suction stress is the major factor affecting slope stability in shrub forest,evergreen broadlaf forest and mixed forest,while the plant root is the major factor in phyllostachys pubescens forest.With the increase of soil depth,the effects of suction stress and plant roots on the safety factor of hillslope generally decrease,and the effects of shear strength parameter on the safety factor of hillslope generally increase.The hillslope safety factor fluctuation indicates that steep slopes covered by mixed forest and evergreen broadleaf forest may have a higher failure potential than others in that study area.Shrub forest hillslope safety factor fluctuates the most among the four typical forest stands.The safety factor variation of the steep slopes covered by phyllostachys pubescens forest is stable and without the possibility of failure.In conclusion,the Phyllostachys pubescens forest may be considered as the suitable types for protecting steep hillslope in some forest management implementation.(3)The plant roots can significantly improve the mechanical and hydrological properties of root-soil composite.In mechanics,the existence of plant roots can increase the shear stress peak value and energy consumption in the shear process,the bigger the normal stress,the greater the increasement of shear stress peak value and energy consumption;plant roots improve the cohesion and internal friction angle by 35.19-81.79% and 12.92-42.36%,respectively.In hydrology,plant roots improve the steady infiltration rate by 1-4.23 times,the relationship between root length density,root volume density,root fractal dimension and the steady infiltration rate of soil was positively correlated.(4)According to the infinite slope stability analysis,the presence of plant roots significantly increased the minimum of the hillslope critical slope by 10-20°.The plants with V-type roots and R-type roots can be plant on steep slopes,while the plants with H-type roots are only suitable for planting on gentle slopes.At their respective minimum critical slope,the existence of the plant root reduced the rainfall duration of slope failure.When rainfall intensity greater than 14.25 mm/h,all the 6 root-soil composites hillslope may failure within 5 h.Due to the gentle slope and low permeability of the bare slope,the slope failure may not appear within 30 h.