60 巻 (2007-2008) 4 号 p. 34-39
The combined penetrometer-moisture probe (CPMP) developed in our previous study is a useful tool for measuring vertical profiles of soil penetration resistance and water content simultaneously, providing essential information for detecting vulnerable points for shallow landslide on steep landscapes. However, we found that the moisture probe of CPMP is occasionally broken when it hits against hard pebbles in soil profile. Moreover, soil water content measured with CPMP tends to be lower than actual water content probably because of inadequate contact between soil and the moisture probe. In this study, the structure of the moisture probe was improved so that it is hard enough to measure water content in a soil profile containing many pebbles. The improved probe consists of two parallel stainless wires, each 0.55mm in diameter, coiled around acrylic column along grooves on the surface of the column. In order to confirm the cause for the underestimation of water content, we conducted detailed observations of soil surface in contact with the moisture probe during laboratory penetration experiments. As a result, many cracks were found on the soil surface affected by penetrations of the sensor rod, which was detected to be a main reason for the measurement error. Then, we evaluated most appropriate structure of the moisture probe for minimizing the error. Nine probes, which have diameters between 19 and 21mm and coiling widths between 2 and 5mm, were tested by field penetration experiments. Results showed that the moisture probe 20mm in diameter and 3 or 4mm in coiling width was the best for reducing the water content underestimation.