Shear Strength And Influencing Factors Of Topsoil In Ephemeral Gullies Under Pinus Massoniana Forest

As an important grain and oil production area in China,the red soil hilly area in the south plays an important role in national ecological security and food security.However,due to the existence of a large area of pure Pinus massoniana forest,the problem of soil erosion under the forest restricts the ecological civilization construction and economic development in the region.Ephemeral gully contributes a lot to soil erosion as the main form of soil erosion.Soil shear strength is an important mechanical index to characterize soil erosion resistance.Therefore,the study on the shear strength of shallow soil surface of Pinus massoniana forest and its influencing factors important theoretical and practical implications.The ephemeral gully of Pinus massoniana forest in the red soil hilly region of southern China was used as the research object in this paper.The field shearing strength and three influencing factors(water content,particle size gradation and bulk density)were studied by field sampling and laboratory test.The spatial distribution characteristics were used to quantitatively study the specific effects of three influencing factors on the soil shear strength.The results of the study were conducted to investigate the spatial distribution characteristics of the shear strength in the Pinus massoniana forest.The research results can provide a scientific basis for the establishment of ephemeral gully erosion prediction model and control of ephemeral gully erosion in Pinus massoniana forest.The main findings are as follows:(1)Spatial distribution characteristics of shear strength at ephemeral gully surface soil.In the three shallow trenches,the internal friction angle of the surface soil varies from 7.30° to 24.02°,and the cohesion force ranges from 2 kpa to 37.77 kpa.In the direction of the long groove,with the increase of the top distance of the slope,the shear strength and internal friction angle of the soil showed a tendency to decrease at the bottom of the ditch,the slope and the edge of the groove(p<0.05),and the decrease at the bottom of the ditch.The small amplitude is significantly larger than the slope and groove edge(p<0.01);the overall change trend of cohesion along the groove length is not obvious.In the direction of the cross slope,the soil shear strength,internal friction angle and cohesion are significantly smaller at the bottom of the ditch than at the ditch slope and the edge of the ditch(p<0.01),while the position of the ditch slope is slightly larger than the ditch edge.The overall performance is:Ditch slope>groove edge>groove bottom.(2)Spatial distribution characteristics of soil water content,soil particle distribution and unit weight at ephemeral gully surface soil.Along the length of the ditch,with the increase of the top distance of the slope,the water content increased significantly at the bottom of the ditch(p<0.05),and increased slightly at the edge of the ditch,but the trend was significant(p<0.05).There was no obvious trend in the ditch slope.With the increase of the top distance,the fine particles at the bottom of the groove(particle size<2mm)increased significantly(p<0.05),while the coarse particles(particle size>5mm,5-2mm)decreased significantly(p<0.05),groove slope and ditch.The particle change of the edge is exactly opposite to the bottom of the ditch,and the variation of the content of each particle at the bottom of the ditch is larger,while the variation range of the ditch slope and the ditch edge is weaker;the fine particles at the bottom of the ditch are more,while the ditch slope and the ditch edge are more More coarse particles.With the increase of the top distance of the slope,the bulk density decreased significantly at the bottom of the ditch(p<0.05),while the edge of the ditch decreased slightly but the trend was significant(p<0.05),and the change in the position of the ditch slope was not obvious.S???c(3)Analysis of factors affecting the shear strength of ephemeral gully soil.When the Unit weight is 1.5 g/cm3,the shear strength of soil with different particle size shows a similar phase trend with the increase of water content:S reaches the peak when the water content is 20.87%,when the water content is<20.87%,S As the water content increases,the intrinsic performance shows a significant increase in c,while ? decreases gradually.When the water content increases in the range of 20.87%?26.32%,S violently decays,and the intrinsic performance is c sharply decaying and ? gradually decreasing;When the water content is>26.32%,S has a tendency to slowly decrease with the increase of water content,and the intrinsic performance is that c decreases slowly and gradually becomes stable,while ? gradually decreases.When the bulk density is 1.5 g/cm3,the soil shear strength of each moisture content shows a linear decrease with the decrease of the particle size of<2mm particle size.The intrinsic performance shows that ? decreases linearly,while c has no obvious change trend.With the Unit weight increasing gradually from 1.3 g/cm3 to 1.8 g/cm3,S showed a rapid increase and then increased rapidly.The intrinsic performance was?,which increased slowly and then increased rapidly,and c continued to increase.