Haili ZHU ,Yanting LI ,Lu SONG

1.Department of Geological Engineering,Qinghai University,Xining 810016,China;

2.Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining 810008,China

Responsible editor:Tingting XU Responsible proofreader:Xiaoyan WU

Alarge number of domestic and foreign researches have shown that the presence of plants plays an important role in preventing water and soil erosion in slope and shallow landslides,reducing the cracking degree of slope soil,reducing the slope surface soil erosion and increasing slope stability[1].Plant roots spread across soil system,and they contact with soil particles,forming into natural root-soil composite[2].The strength size of root-soil composite plays an important role in slope stability[3-11],and it is also an important in dicator for assessing the contribution of root system to slope stability.Lin et al.[4]simulated the slope planted with Phyllostachys bambusoides in Taoyuan,Taiwan using three-dimensional finite element method(3D).The results showed that when the gradient ranged from 50° to 70°,Phyllostachys bambusoides showed insignificant effect on slope stability; when the gradient was less than 25° or higher than 40°,the contribution of Phyllostachys bambusoides to slope stability was greater.Xiao et al.[6]investigated the effects of Robinia pseudoacacia on slope stress and strain distribution within gradient range of 30°-40° using the finite element software ADINA.The results showed that Robinia pseudoacacia roots could improve the stress and strain distribution in shallow slope,so shallow slope tends to be more stable;the increased gradient would increase the stress in slope toe; tensile stress would be generated gradually in slope shoulder,and it became greater and greater; the concentrated region of shear strain was enlarged.Thomas et al.[11]investigated the effects of three kinds of root types (horizontal root,scattered root and main taproot) on stabilities of slopes with gradients of 0°,5°,35°and 45°by the Monte Carlo method.They found that the taproot type had greater contribution to stability of slope with gradient of 45°,and the shear strength of root-soil composite was 15.08 kPa.At present,there are many studies showing that vegetation enhances shear strength of slope soil and improves slope stability.However,there are rare quantitative researches on effect of gradient on stability of slope containing plant roots.Therefore,an indoor triaxial shear test was conducted to measure the shear strengths of root-free slope soil and root-soil composite,and the effect of gradient on slope stability was analyzed using limit equilibrium method,thereby providing a theoretical basis for the gradient design of ecological protection slope.

Materials and Methods

Materials

The soil samples were collected from the surveyed area located in Xiaozhai Village,Yunjiakou Town,Chendong District,Xining City.The collected soil was silty clay soil,and its physical properties were shown in Table1.The roots of Caragana kor

shinskii and Atriplex canescens were also collected from the survey area.The roots of the two kinds of shrub are characterized by strong cold and drought resistance and developed root system.Caragana korshinskii and Atriplex canescens are two dominant shrub species for reducing wind and stabilizing sand and maintaining soil and water in Qinghai-Tibet Plateau.

Methods

The shear strengths of the sampled soil and root-soil composites were measured using a triaxial apparatus(TCK-1,Nanjing Soil Instrument Co.,Ltd.).The measured samples had diameter of 61.8 mm,height of 125 mm,moisture content of 12.8% and wet density of 1.58 g/cm3.For the preparation of soil samples containing roots,the roots were added vertically and horizontally.In each root-soil composite,total four shrub taproots with average length of 120 mm and diameter of 1.66 mm were added vertically; total three layers of shrub lateral roots with average diameter of 0.48 mm and uniform length of 50 mm were divided evenly and horizontally in the sample,and there was 0.235 g of roots in each layer.In order to simulate the actual situation,the used root materials were all fresh.There were four replicates for each treatment,and the measurement was conducted under confining pressure of 10,20,30 and 40 kPa,respectively.For the samples in a same treatment,Mohr’s stress circles were drawn on a τ-σ stress plan.In addition,the envelope curves of stress circles at failure under different confining pressures were drawn.Thus the cohesions(c)and internal friction angles(φ)reflecting shear strengths of soil and root-soil composite samples were obtained.

Correlation between stability coefficient and gradient of slope

Definition of slope stability coefficientIn the limit equilibrium method,the stability coefficient(K)is defined as follows:

When Mohr-Coulomb condition is taken into account,the calculation formula of stability coefficient(K)is transformed as follows:

In the Equation (2),c represents the actual cohesion of soil;φrepresents the actual internal friction angle of soil; ccrepresents the cohesion in the critical state;φcrepresents the internal friction angle of soil in the critical state;σrepresents the actual normal stress component.

Example calculation of stability coefficient of slope containing roots

It is assumed that on the surface of a shallow slope,there is a soil system with thickness of h,length of l and slope angle ofα.The soil volume weight was assumed asγ.The weight(Q)of the soil system can be calculated according to the following formula:

Q=γhl.

The root-containing slopes were all soil slopes planted with Caragana korshinskii or Atriplex canescens.Considering the action depth of roots in surface soil system of certain slope,the thickness of soil system (h)is assumed as 1 m.Thus,the Equation (2)can be transformed as follows:

The Equation (3) is applicable to both root-free and root-containing slopes.Due to the reinforcement effect of root system on slope,the cohesion c is increased,and the internal friction anglesφis also increased,thus the stability coefficient difference ΔK is increased.It is assumed that cr represents the cohesion of soil system on the surface of root-containing slope,c0represents the cohesion of soil system on the surface of root-free slope,φrrepresents the internal friction angle of soil system on the surface of root-containing slope,φ0represents the internal friction angle of soil system on the sur-face of root-free slope,γrrepresents the volume weight of soil system on the surface of root-containing system,γ0represents the volume weight of soil system on the surface of root-free slope,Krrepresents the stability coefficient of soil system on the surface of root-containing slope and K0represents the stability coefficient of soil system on the surface of root-free slope,thus the ΔK can be calculated as follows:

Table1 Physical properties of soil in surveyed area

Table2 Calculation results of slope stability parameters

Table3 Stability coefficient increments of slopes with varying gradients

Results and Analysis

By triaxial compression test and routine physical tests,the shear strength indexes and volumes of sampled root-free soil and root-soil composites were obtained (Table2).Soil shear strength is produced by the soil cohesion and internal friction angle.Many studies have shown that the increased shear strength of reinforced soil is entirely due to produced cohesion,instead of variation in internal friction angle[14-15].The results of this study showed that the internal friction angles of root-soil composites were almost the same as that of pure soil;compared with that in pure soil,the increase in cohesion of root-soil composite was more obvious,and the cohesion of root-soil composite in the Atriplex canescens-planted slope was greater than that in the Caragana korshinskii-planted slope.

According to the Equation (4),the stability coefficient increments of slopes with gradients ranging from 10°to 90° were calculated (Table3).The stability coefficient increments of the two slopes planted with Caragana kor

shinskii and Atriplex canescens were all increased with the increase in slope gradient,and the stability coefficient increment of Atriplex canescens-planted slope was larger than that of Caragana korshinskii-planted slope,indicating greater reinforcement effect of

Atriplex canescens on shallow slope.Based on the calculation results,the relationship between ΔK andαwas fitted(Fig.2).

Fig.2 showed that when the gradient was less than 25°,the stability coefficient of root-containing slope was two times higher than that of root-free slope.The root-free slope and rootcontaining slope were all stable,and their surface soil systems all could reach a stable state without protection of plant roots.Vegetation was of great significance in greening environment and preventing rainwater erosion.When the gradient ranged from 25° to 50°,the stability coefficient of Atriplex canescens-planted slope was higher than that of pure slope by 0.80-1.38,and of Caragana korshinskii-planted slope was higher than that of pure slope by 0.56 -1.03.The root-free slope was secondary stable,and it needed the protection of vegetation.When vegetation in surface soil system grows well,its intricate root system plays an important role in reinforcing entire soil system.Under the turgor pressure of roots,axial pressure is generated between roots and surface soil particles,closely binding plant roots and soil particles.In addition,the contact area between roots and soil is enlarged,so that the frictional resistance between the roots and the surface soil system is increased.This explains why the stability coefficient of surface soil system of root-containing slope was higher than that of root-free slope.When the gradient exceeded 55°,the stability coefficients of the slopes planted with two kinds of shrub were higher than that of the pure slope by 0.55 in average.The slope stability varied insignificantly with the increase in gradient,which is consistent with the study result of Lin et al[4].In short,when slope gradient was less than 55°,plant root system played an important role in improving soil strength.

Conclusions

Based on the preparation of remodeling samples,the effect of plant roots on slope stability was investigated by triaxial compression test.The results showed that internal friction angles of root-soil composites of Atriplex canescens-planted slope and Caragana korshinskii-planted slope were almost the same as that of pure slope; compared with that of pure slope,the cohesions of the two kinds of root-soil composites were increased significantly,and the cohesion of rootsoil composite of Atriplex canescensplanted slope was higher than that of Caragana korshinskii-planted slope.

The calculation and analysis showed that the stability coefficient increments of the two kinds of root-soil composites were all decreased with the increase in slope gradient,and the stability coefficient increment of Atriplex canescens-planted slope was larger than that of Caragana korshinskii-planted slope.

When the slope gradient exceeded 55°,the reinforcement effect of roots of the two kinds of shrub on shallow slope basically reached a limit,suggesting that plant roots play a role in improving slope stability when the slope gradient was less than 55°.

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