坪床排水管进水孔与过滤层材料颗粒的搭桥作用初探

在草坪坪床排水系统中,由于排水管周围填充的过滤层材料与排水管上的进水孔孔径不匹配,过滤层材料中的细小颗粒通过进水孔进入排水管发生沉积引发堵塞是导致排水系统失效的主要原因。本研究选用管径75 mm的单壁PVC排水管,在其一侧打孔,孔径分别为2.5、4.0、5.0和6.0 mm,选择当地3种不同种类的细沙、中沙和粗沙作为过滤层材料,在实验室条件下将排水管两端封堵并置于容器中保持水孔向上,用风干过滤层材料完全覆盖排水管,容器水平方向振动20和40 s,以此模拟排水系统在实际工程中在受到外力作用下过滤层材料进入排水管道的情况,探讨过滤层材料粒径分布特点与进水孔孔径之间的关系。结果表明,进入排水管的沙子数量随着孔径的增大,其沉积量会以指数函数的量级而上升。级配较好且粒径较粗的中沙的沉积量最少,最适合作为排水管周围的过滤层材料,但排水管进水孔孔径不能超过5.0 mm;级配差且粒径小的细沙掉落到管子里的沉积量最多,不能作为排水管周围的过滤层材料;粗沙可以作为过滤层材料,但进水孔的孔径不能超过4.0 mm。

Preliminary study of bridging between water holes in subsurface drainage pipes and particles of filter materials

Failure of the turf subsurface drainage system in many cases is mainly caused by improper matching between the particle size distribution of the filter material around the pipe in the trench and the perforation size of the water-entry hole in the drainage pipe. In this experiment, a single wall PVC drainage pipe with a 76 mm diameter was selected. The drainage pipe was perforated based on the position of the upper part of the pipe. The diameter sizes of the water holes were 2.5, 4.0, 5.0, and 6.0 mm. Three different types of local fine sand, medium sand, and coarse sand were selected as the filter material. The drainage pipe was placed in a container with both ends covered by plastic sheeting and holes facing up. The drainage pipe was completely buried in the air-dry filter material in the container. The container was then shaken horizontally for either 20 or 40 seconds. The sand entering the drainage pipe through the water holes was taken out and its volume was measured. The results showed that the volumes of sand entering the pipes exponentially increased with expansion of the diameter of water holes. The highest volume of sand that entered the pipe was observed for poorly graded fine sand because of its smaller particle diameter and unstable particle bridging structure. This sand should therefore never be used as a filter material, whereas the least volume of sand in the pipe was observed for the well-graded medium sand, which was thus best suited for use as filter material surrounding drainage pipes; drain perforations no greater than 5 mm in diameter was highly recommended. For the coarse sand, the perforation diameter should not be more than 4 mm. However, the well-graded medium sand formed a stable and tight bridging in the vicinity of the water holes, locking other sand particles together and preventing them from entering holes in the drainage pipes.