Study On Shear-resistance And Its Influence Factors In Typical Forest In Jinyun Mountains

This study was supported by the "Eleventh Five-year" National Technology Support Project "Experiment& Demonstration of the Construction Technology of Water-Conservation-Forest in the northern Water Source Area, Chongqing (2006BAD03A1802)". In this thesis, Experiment, Correlation Analysis and Principal Component Analysis were employed to study the shear strength characteristics and affecting factors of the undisturbed soil and disturbed soil from typical Mixed Coniferous forest, Evergreen-Broadleaf forest, Bamboo, Shrub and Farm land, in Jinyun Mountain Area, Chongqing.Analysis on the structure and physical and chemical properties of soil indicated that:the content of sand in soil from the five typical land types is relatively low, while the physical clay content in the soil from Bamboo Land is the lowest, about 31.33%. The content of Aggregates in the soil from forest land is more than Shrub Land and Farmland. The Aaggregate Degree and various types of Porosity decrease, while the Soil Bulk Density increases from the surface layer(A layer) to the bottom layer(C layer). Organic Matter Content, pH, Cation Content, Available Nitrogen, Phosphorus, Potassium, Total Nitrogen, Total Phosphorus and Potassium of the soil form typical forest lands array in different order. All of these indicators, except pH Value, Cation Exchange Capacity and Potassium, are decreasing with the depth of soil.The analysis of shear strength of soil from the five land types showed that:Cohesion Value(c) of undisturbed soil gradually increases with the depth of the soil, while the average c of soil from Mixed Coniferous forest is the biggest, about 42.09KPa. The Internal Friction Angle(φ) of the soil from all the five land types doesn't change with the depth of soil, andφof soil from Farmland is the biggest, about 27.31°. The c andφof disturbed soil were tested in three conditions, direct shear test, undrained test and consolidated undrained test. In the direct shear test, there is no significant change in c andφfrom A layer to C layer, and the c of soil from Shrub Land andφof soil from Bamboo Land are the largest, about 16.51kPa 25.6°respectively. In the undrained test, there is also no significant regularity in c andφfrom A layer to C layer. In the consolidated undrained test, the trends of the effective soil cohesive force and the total value of the cohesive force are the same, while trends of effectiveφand the total value ofφare different.The shear strength of undisturbed soil is closely related with soil properties indices, the correlation coefficient between them is 0.824. In the direct shear test, undrained test and consolidated undrained test, the correlation coefficient between shear strength of disturbed soil and soil properties indices is very high, between 0.90 and 0.928. Principal Component Analysis showed that:1), the average Comprehensive Index (CI)of the factors which affect the shear characteristics of soil from different land types is 30.34, the order of CI is Shrub Land (40.74), Evergreen-broadleaf forest (30.63), Mixed Forest (28.78), Bamboo Land (28.71) and Farmland(25.46);2)On the disturbed soil analysis:①. In the direct shear test, the average CI is 29.08, the order of CI is Shrub Land (51.99), Farmland(27.57), Mixed Forest (27.46), Evergreen-broadleaf forest (24.39), Bamboo Land(19.72);②. In the undrained test, the average CI is 23.69, the order of CI is Mixed Coniferous (25.27), Shrub forest (24.23), Evergreen-broadleaf forest (23.88),Farmland (21.92), Bamboo Land(21.49);③. In the consolidated undrained test, the average CI under total stress is 77.57, the order of CI is Shrub forest (109.91), Evergreen-broadleaf forest (74.82), Mixed Forest (72.20), Bamboo Land (69.63), Farmland (69.37), while the CI under effective stress is 24.12, the order of CI is Shrub (52.78), Mixed Forest (29.05), Farmland (18.67), Bamboo Land (14.45), Evergreen-broadleaf forest (12.84).