超软土真空预压透明土模型试验及土桩形成机理

吹填超软土真空预压加固地基过程中,塑料排水板周围易形成“土桩”,“土桩”的存在降低了其周围土体的固结效率,严重时甚至会导致地基处理失效;然而,对“土桩”形成机理的认识仍相对不足.为此,基于透明土材料和粒子图像测速法（PIV）,开展超软土真空预压透明土模型试验,非嵌入式地可视化测量真空预压过程中排水板周围土体位移场.结果表明:排水板周围的土体变形与排水速率存在联系,在排水高峰期和排水平稳期,排水板周围土体产生明显的水平位移,且产生水平位移的范围随抽真空时间的增长逐渐扩大.而在排水缓慢期,排水板周围的土体几乎仅有竖向位移;土颗粒在渗流力的作用下向着排水板方向移动并聚积在排水板周边,是造成“土桩”和“软弱带”现象的主要原因;浅层土体的颗粒较细、上覆土重较低,在渗流力作用下更容易发生向着排水板方向的位移,形成的“土桩”由浅至深半径逐渐减小,“土桩”的最大半径可达11 cm;“土桩”范围内的土体密度较“软弱带”土体的密度更高,在自重和“真空荷载”作用下的压缩量更小,在土体表面形成“桩头”.

Transparent soil model test of vacuum preloading method on ultra-soft soil and formation mechanism of "soil pile"

During the vacuum preloading of hydraulic fill ultra-soft soil, "soil pile" is often formed around drainage plate, which hinders the consolidation of the surrounding soil and even leads to the failure of the foundation treatment, but the forming mechanism of "soil pile" is still unclear. To this end, a model test was carried out to implement non-intrusive measurement of the surrounding soil deformation during vacuum preloading using the transparent soil experimental technique and the PIV (particle image velocimetry) technique. The research showed that the surrounding soil deformation of drainage plate was related to drainage rate. During the quick and steady drainage period, the surrounding soil of the drainage plate had obvious horizontal displacement, and the range of the horizontal displacement expanded gradually with the increase of the vacuuming time. During the slow drainage period, the vertical displacement was obvious in the surrounding soil of the drainage plate, and there was almost no horizontal displacement. The formations of "soil pile" and "soft zone" were mainly due to the transportation and gathering of the soil particles around the drainage plate. Compared with the deep soil layer, the shallow soil layer had finer particle size and lower self-weight, which was easier to transport under the seepage force, thus leading to the decrease of the radius of the "soil pile" along the depth, and the maximum radius of the "soil pile" could reach 11 cm. The density of the "soil pile" was higher than that of the "soft zones", and the compression of the "soil pile" was smaller under self-weight and "vacuum load", resulting in the formation of the "pile head" on the foundation surface.