Experimental Study Of The Influence Of Vegetation On Soil Matrix Suction And Mechanical Effect

In recent years,the concept of slope protection with vegetation has attracted a high attention at home and abroad,and its core is the interaction of soil-vegetation-atmosphere.Vegetation plays an important role in strengthening the stability of shallow surface slope,maintaining water and soil,and improving the ecological environment.Therefore,the interaction mechanism of soil-vegetation-atmosphere is studied to furtherly enrich the hydraulics mechanism of plant in soil stability,which is conducive to realize the long-term goal of sustainable development.At present,the evaluation mechanism of plant protection in slope protection mainly includes mechanical mechanism and hydraulic mechanism of plant roots.The mechanical mechanism is reflected in that plant root can improve soil structure and enhance soil shear strength.The hydraulic mechanism is expressed as plant leaf reduces the effect of rain splashing,and the rooting and netting effects of root system increase the ability of soil to resist erosion.At the same time,plant transpiration reduces soil moisture and produces matrix suction through plant root absorbing soil water,and matrix suction is one of the two independent controlled variables of the deformation and stress state in unsaturated soil.Therefore,it is very important to understand the matrix suction produced by vegetation transpiration,because unsaturated soil properties will be changed due to matrix suction changes,such as permeability.This paper focuses on the soil-vegetation-atmosphere interaction in which sandy soil,silty soil and clay soil as carriers,and Cynodon dactylon and Indigofera amblyantha as test plants.Homogeneous soil and uniform plant growth conditions are controlled,in setting up all kinds of boundary conditions,the effect mechanism and factor of vegetation on homogeneous soil matrix suction are studied.The influence of matrix suction on soil shear strength and permeability is analyzed by means of different matrix suction.The numerical model of plant characteristic parameter and soil matrix suction is derived by which soil matrix suction change is quantitatively evaluated and the hydraulic mechanism of plant protection in slope stability is furtherly expounded.The matrix suction produced by vegetation transpiration can produce the additional shear strength of 3-4 kPa in clay soil,and plant through root absorb water can influence matrix suction distribution of the relatively deep soil.The matrix suction produced by vegetation transpiration is about 2-2.5 times higher than that of no water evapotranspiration.Plant transpiration is mainly controlled by light thermal energy,and thermal light can enhance plant transpiration rate,while cold light only affects plant photosynthesis.There is a certain correlation between soil matrix suction and soil clay content and silty content,and the matrix suction of silty clay is sensitive to the change of soil moisture content.The water holding capacity of clay soil and silty soil are strong,which can maintain a large of matrix suction in the corresponding time,which can significantly reduce soil permeability coefficient.With the increase of soil depth,the matrix suction in vegetation soil of Indigofera amblyantha has an increasing trend followed by a reduction,then,the double plant root domain soil can is influenced effectively in the horizontal direction.The matrix suction of in vegetation soil of Cynodon dactylon decrease with soil depth increasing,however,outside the root domain,matrix suction decrease sharply along the horizontal distance increase.Penetration test for 2 h,water osmotic quantity in vegetation soil of Cynodon dactylon is about 1/3 of the bare soil,and Indigofera amblyantha vegetation soil is about 1/2.The permeability coefficient of soil gradually decline due to soil matrix suction increase.With the advance of soil matrix suction,soil shear strength gradually becomes larger.A significant increase in soil apparent cohesion is observed when soil matrix suction change,but not in apparent internal friction angle.Leaf area index and root area index can effectively characterize the plant potential transpiration rate,and plant root type has an important influence on the matrix suction distribution in soil.