Resistance to penetration of fine probes in compressible soil

Fine metal probes are being used to simulate the penetration of plant roots into soil. Existing soil mechanics theories on the bearing capacity of piles were used to estimate the point resistance of these probes but were found to be unsatisfactory. It is considered that this was due to the fact that sufficient emphasis is not given to soil compression by these theories. A new theory is proposed that is based on the model of an expanding sphere in a plastic-elastic medium at the point of the probe. Compression of the soil to accommodate the volume of the probe is assumed to occur in two main zones, a zone of compression with plastic failure surrounding the probe and, outside this, a zone of elastic compression. In estimating the point resistance to the penetrometer, the mean contact pressure on the conical surface of the point has been assumed to be equal to the pressure required to form a cavity large enough to take the probe. For three soils ranging from a loam (12% clay) to a clay soil (48% clay) at three bulk densities and two water contents the calculated point resistances were on the average within 10% of the values determined by experiment.