Improvements of the combined penetrometer|moisture probe (CPMP)
Yosuke YAMAKAWA, Ken'ichirou KOSUGI, Weili LIANG, Takahisa MIZUYAMA
Abstract
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 0.55 mm 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 21 mm and coiling widths
between 2 and 5 mm, were tested by field penetration experiments. Results showed
that the moisture probe 20 mm in diameter and 3 or 4 mm in coiling width was the
best for reducing the water content underestimation.