Construction of an Artificial Perched Watertable, Part 3: Sand Gun

Abstract

By constructing an artificial perched watertable at about an 1 m depth from the soil surface in an area where the annual precipitation occurs only in the summer season, it was envisaged to retain summer runoff in this watertable and prevent excess moisture loss. The water in the artificially permeable layer could be used for plants as capillary water in the spring sowing season. For this purpose, a machine to construct the artificial perched watertable was needed. This paper deals with the development of a sand gun, which is a part of the machine and deposits sand in the underground horizontal soil cavity (perched watertable) formed by high pressure air. The results showed that in order to obtain continuous sand injection from the nozzle, an ejector should be mounted at the connection of the sand pipe and the nozzle. A vacuum was produced by high-speed air injection from the ejector at the tip of the sand pipe. The sand in the sand tank was pressed into the sand pipe by atmospheric pressure and smooth sand injection was easily obtained. Based on the charge tank pressure, the opening ratio of the air valve should be within 1/10-1/5 for practical application, resulting in a primary air velocity at the nozzle of 10–30 m s −1. Based on these conditions, the maximum size of the sand particles for practical use should be less than 2 mm. When the primary air velocity of the air injection at the nozzle port was 5 m s −1, sand accumulated 0·3–0·7 m from the nozzle. At 10 m s −1, the sand reached a distance of more than 2·0 m but sand piled up at 0·5–1·0 m from the nozzle. At 20 m s −1, a relatively uniform sand distribution was obtained at 0·25–2·25 m. At 30 m s −1, there was no sand distribution 0–1 m from the nozzle, but the sand accumulated uniformly at 1·0–2·25 m. Thus, the optimum primary air velocity of the air injection at nozzle port was more than 20 m s −1.