贺兰山云杉林根土复合体提高边坡稳定性分析

为了研究贺兰山青海云杉林(Picea crassifolia)边坡根土复合体对边坡稳定性的影响,该文在直剪试验、三轴试验等获得的土壤参数基础上,建立了基于有限元理论的贺兰山青海云杉林边坡稳定性计算数值模型,并在5种坡度条件下(18.43°、21.80°、26.57°、33.69°、45.00°)计算了无林边坡和有林边坡的安全系数、最大塑性应变、最大位移、破坏时间及它们的增长率随坡度的变化规律,并计算了土壤强度参数摩擦角、黏聚力、剪胀角与安全系数的灰色关联度矩阵和平均关联度。结果表明:1)无林边坡与有林边坡的安全系数随坡度的变化规律是一致的,都以幂函数递减。有林边坡相对于无林边坡安全系数的增长率以指数函数增加,且坡度越陡根土复合体对边坡稳定性的提高作用越强;2)根土复合体的存在可以延长边坡的破坏时间,有林边坡的破坏时间均高于无林边坡,根土复合体提高边坡破坏时间的增长率随坡度的增大呈抛物线增加,且坡度越陡提高作用越明显。边坡破坏时有无塑性贯通区会明显改变最大塑性应变、最大位移、破坏时间的值;3)摩擦角、黏聚力、剪胀角3个土壤强度参数中,黏聚力是影响边坡稳定的主导因素,摩擦角、剪胀角的影响次之,此规律不受边坡坡度的影响,这是根土复合体可以显著影响边坡稳定性的根本原因之一。该文的研究结果对于解释森林根土复合体加固边坡作用的本质、推进边坡稳定性计算的数值模拟、防治贺兰山浅层滑坡及水土流失灾害都具有重要的意义。

Analysis on improvement of slope stability in root-soil composite of Picea crassifolia forest in Helan Mountain

Abstract: Helan Mountain is an area prone to geological hazard and an ecological shelter in Yinchuan Plain. Plants have been widely used for controlling shallow landslides and soil and water loss in this area. The exact study site is located at the Tu''erkeng forest protection point in Suyukou national forest park. To reveal mechanisms of slope stability reinforced by root-soil composite, we built a finite element numerical model to simulate the impact of root-soil composite on slope stability. Picea crassifolia was selected as research object, which is one of the most important trees for soil and water conservation in Helan Moutain. In this paper, direct shear tests and triaxial tests of root-soil composite and pure soil were conducted for getting soil strength parameters, and based on the soil parameters, the finite element numerical model for the stability calculation of Picea crassifolia forest slope was established. All the soils, including the root-soil composite and the plain soil, were undisturbed samples and were obtained from the same time and field. In the numerical model, the soil was defined as ideal elastoplastic material and following the Mohr-Coulomb yield criterion. The soil and rock were divided by triangular elements of plane strain (CPE6MP and CPE3). Using the model, factor of safety, maximum plastic strain, maximum displacement, failure time, and their growth rate with the slope grade (18.43°, 21.80°, 26.57°, 33.69° and 45.00°) were studied. In this paper we also calculated the grey relational coefficient between the factor of safety and soil strength parameters including friction angle, cohesion, and dilation angle. It was found that: 1) The relationships between factor of safety and slope grade are similar in the slopes with or without trees, and they all decrease in power function. The growth rate of factor of safety increases with the increasing of the slope grade and shows the exponential change. The steeper the slope, the stronger root-soil composite improves the slope stability. 2) The existence of the root-soil composite can prolong the failure time of the slope, and the failure time of the slope with trees is longer than that without trees. The growth rate of the slope failure time increases with the increasing of the slope grade, showing a parabolic curve, and the steeper the slope, the more obvious the effect. With or without plastic-perforation zone, the maximum plastic strain, maximum displacement and failure time of the slope will have an obvious difference. 3) In the 3 soil strength parameters, the cohesion has the greatest influence on the slope stability. The next is the friction angle and the dilation angle. Moreover, this order is not affected by the slope grade. This is also one of the fundamental reasons why vegetation can affect slope stability obviously. The successful implement of this research can play a key role in understanding the nature of root-soil composite reinforcement, promoting numerical simulation study on slope stability and preventing shallow landslides and soil and water losses in Helan Mountain.