A device to measure soil penetration resistance in a root box (24cm×2cm×40cm) was developed, and its validity was examined for the revised root box method. The device was constructed with the metal probe (2.3 mm diameter and 50 cm length piano wire), strain gauge, linear displacement transducer, reversible motor and X-Y recorder. The main characteristic of this device is to push the needle type probe into the soil continuously about 40 cm downward at a constant rate to measure the penetration resistance easily and quickly as a function of soil depth.
Edge effect was examined using probes with different diameters ranging from 0.95 to 2.3 mm. Edge effect was found to be negligible for any measured soil resistance value. Comparing spot resistance and continuous resistance, the skin friction was almost statistically constant (meaning it did not accumulate) under around 10 cm soil layer. The trend of the variation of both resistances as a function of soil depth was similar especially in the pattern of their peaks. Thus, the measured values were considered to be valid and therefore practical to use in determining penetration resistance as a function of soil depth, as a relative value.
Age hardening process of the soil in the root box as a function of soil depth was examined under submerged condition. Penetration resistance tended to get bigger as soil depth increased.