A METHOD TO PREDICT THE MECHANICAL STRENGTH OF AGRICULTURAL SOILS

During a 3-year period the physical and mechanical properties of 37 typical, differently textured and structured agricultural soils in Bavaria were determined in order to predict their mechanical compressibility and trafficability. The soil physical properties (bulk density, pore size distribution, saturated hydraulic conductivity, air permeability and penetration resistance) and the soil mechanical properties (pre-consolidation load and the shear strength parameters angle of internal friction and cohesion), were determined on undisturbed, differently pre-dried soil samples (60 and 300 hPa water tension). In order to quantify the changes in soil physical properties affected by loading, all soil physical parameters were measured before and after loading by the confined compression test (load range 10-800 kPa). It was found that in homogeneous, non-structured soils, such as sands and silts with low clay content (< 15%, w/w), the shear parameters are mainly texture-dependent. However, in structured soils (clay content > 15% w/w), stability increased with increasing degree of aggregation (coherent < prismatic < blocky < subangular blocky), due to higher values of the angle of internal friction between the aggregates and higher values for the cohesion within the dense, stable aggregates. The mechanical soil strength, which is determined as the value of the pre-consolidation load, could be predicted by multiple regression analysis with a high degree of significance, when the shear parameters angle of internal friction and cohesion (load range 0-400 kPa) were included as independent variables.