真空预压的最终负压分布模式

为了揭示真空度衰减特性和确定最终竖向有效应力,研究了底部半透水边界下真空预压的最终负压分布模式。首先通过边界齐次化理论建立了真空预压和堆载预压固结方程之间存在的等效关系,获得最终负压沿深度分布公式。其次根据固结完成后的稳定渗流方程,研究了单层地基、多层地基和砂井地基的最终负压分布特性。研究表明,在半透水边界条件下最终负压呈现随深度衰减现象。对于单层地基或竖向排水体通水量恒定的砂井地基,最终负压沿深度呈单一线性衰减;对于多层地基或竖向排水体通水量分段恒定的砂井地基,最终负压沿深度分段线性衰减;对于渗透系数连续变化地基或竖向排水体通水量连续变化砂井地基,最终负压沿深度曲线衰减。最终负压衰减速率随着边界透水系数增大而增大。渗透系数越大的地层,最终负压衰减越慢;上层土体与最底层土体的渗透系数之比越大,底部边界处的最终负压越大,反之越小。算例表明,采用半透水边界地基模型能较合理地模拟真空预压最终负压和最终竖向有效应力沿深度减小特性。

Distribution patterns of final negative pressure in vacuum preloading

The distribution patterns of the final negative pressure are investigated in vacuum preloading under impeded bottom boundary in order to demonstrate the degenerated properties of vacuum degree and to determine the final vertical effective stress. Firstly, the equivalent relation of consolidation equations between vacuum preloading and surcharge preloading is established according to the boundary homogenizing theory. The distribution patterns of the final negative pressure with depth are obtained. Secondly, according to the steady seepage equation of post-consolidation, the distribution properties of the final negative pressure are studied for single-layer ground, multi-layer ground and sand-drained ground. The study shows that the final negative pressure decreases along depth under impeded boundary condition. For the single-layer ground or sand-drained ground with a constant water flux of vertical drain, the final negative pressure decreases single-linearly along depth. For the multi-layers ground or sand-drained ground with piecewise constant water flux of vertical drain, the final negative pressure decreases piecewise-linearly along depth. For the ground with continuous permeability coefficient or sand-drained ground with continuous water flux of vertical drain, the final negative pressure decreases nonlinearly along depth. The rate of final negative pressure increases with the permeability coefficient of boundary. The final negative pressure decreases slowly in high permeable layer. The larger the ratio of permeability coefficient between the upper layer and the bottom layer is, the larger the final negative pressure at bottom boundary is, and vice versa. The case study shows that the ground model with impeded boundary can reasonably simulate the properties that the final negative pressure and the final vertical effective stress decrease with depth in vacuum preloading.