不同剪切方式下崩岗红土层抗剪特征随水分变化规律

Shear strengths of collapsing hill in red soil as affected by soil moisture under different experimental methods

崩壁土体抗剪强度随水分变化的规律是研究崩岗发生机理的关键,但不同剪切方式得出的抗剪特性可能存在差异。该研究采用直接剪切和三轴剪切试验方法,在100、200、300和400 k Pa 4个围压下,测量10%~30%之间5个不同体积含水率下崩岗红土的抗剪应力和内摩擦角,确定典型崩岗土体抗剪特征随水分变化规律。结果表明:直接剪切试验中当土壤体积含水率在10%~15%之间时,黏聚力的最高值达80 k Pa,随着土壤含水率增加,黏聚力和内摩擦角逐渐降低到最小。三轴剪切试验得出的结果与直接剪切试验相似,但黏聚力总体接近或大于直接剪切试验结果,而内摩擦角小于直接剪切试验结果,这与2种试验的土样制备方法、试验原理等密切相关。研究可为花岗岩崩岗区崩岗土体抗剪强度测定方法合理选择提供依据。

Gully erosion(especially from a permanent gully) is a widespread phenomenon in many agricultural landscapes and it is a serious land management issue in many parts of the world. Gully erosion occurs randomly in sediment from the gully bank deposited into a slump. Thus,simulation of the gully bank retreat condition is important. In order to understand the erosion mechanism of permanent gully, it's key to know that the variation of gully wall's shear strength. However, different experimental methods get different results. Therefore, characteristics of shear strength vary with soil water content were studied in permanent gully's red soil layer, by using direct shear and tri-axial shear method. Soil samples were collected from the Longmen catchment of Anxi County, Fujian Province, in southeast China. The samples were pre-treatment according to the experimental standers, then the samples were tested for different confining pressure(100, 200, 300, 400 k Pa) and various volumetric soil moisture(from 10% to 30%). Results showed that the highest cohesive force of soil reaches 80 k Pa when soil moisture content changed from 10% to 15% under the direct shear test. The increase in volumetric soil moisture content(from 15% to 25%), cohesive force and internal friction angle gradually reduced to a minimum. The cohesive force were less than 5 k Pa in most of the soil samples when soil moisture was over 25 %, and the internal friction angle was also reduced from 38° to 25°. In tri-axial shear test, with the increase in soil moisture content, cohesive force and internal friction angle decreased to a minimum in tri-axial shear test. The cohesive friction angle decreased more than 65% in most of the samples, and the internal friction angle showed the same trend. There are similar results of tri-axial shear and direct shear test. However, the cohesive forces of tri-axial shear test were greater than the direct shear test. However, internal friction angles of tri-axial shear test are lower than direct shear test. There were two reasons causing the variation between those two methods. First, discrete soil samples preparation. All the soil samples, which used in direct shear test, were undisturbed samples. However, there were remolded samples were used in tri-axial shear test. Although the remolded samples can get a uniform test piece, it breaks the 'natural' connection between soils particles, compare to undisturbed samples. Second, different failure modes of soil samples in direct shear test and tri-axial shear test. In the direct shear test, shear planes is inherent and not drained in the test, breakage of test piece estimated based on the theory. However, it usually disagrees with the actual conditions and difficult to control the drained. In tri-axial shear test, it can be complete not drained, and the shear planes are not fixed. This means the shear breakage of the test piece can be close to the natural situation in tri-axial shear test. This research provides a quantified evaluation of different test method in shear strength. Therefore, it is wise to choose a better method according to the demand for further research.