Machine for Construction of an Artificial Perched Watertable, Part 2: Improved Sand Gun

Abstract By constructing an artificial perched watertable at about a 1 m depth from the soil surface in an area where the annual precipitation occurs only in the summer season, it was intended 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 dry spring season. For this purpose, a machine to construct the artificial perched watertable was developed. This paper deals with the development of part of the machine, a sand gun which deposits sand in the underground horizontal soil cavity (perched watertable) formed by high-pressure air. The results showed that the optimum configuration of the sand gun was that sand should be initially charged in the injector. Then, high air pressure would act on top of the sand pillar, and the sand in the injector would be injected explosively from the nozzle. Thus, the sand would not be injected continuously but injected in batch mode. In this system, if the underground nozzle was clogged by soil, the obstructing soil could be blown off by the high-pressure air. The charge tank pressure required for sand injection for both Chinese and Japanese sands was different with the soil water content and showed a maximum of 0·4 MPa at a soil water content of 10% dry basis (d.b.). There was no difference in the transportation distance for the Chinese and the Japanese sands. The pressure in the charge tank required for sand transportation showed a maximum of 0·8 MPa at a soil water content of 10% d.b. The air pressure in the charge tank required for sand transportation was always larger than the air pressure in the charge tank required for sand injection. Hence, when sand is deposited into the underground soil cavity, the charge tank pressure should be the value of the air pressure required for sand transportation. Even if the injector had more than one nozzle, the nozzles which had the least resistance, worked first and then other nozzles worked one by one and the soil cavity was perfectly filled by sand.