The penetration of clays by fine probes and root tips

Abstract

Penetration tests were conducted in a saturated clay using fine probes and pea radicles. The point resistance encountered by fine probes, Qp, was related inversely to the rate of penetration of the probe. The resistance increased exponentially with decreasing voids ratio, ew,; at any given value of ew more resistance was offered by clay that had been remoulded than by finely aggregated clay. When radicles grown through a bed of soil crumbs encountered cores of moderately consolidated saturated clay, they merely indented the clay before turning and growing horizontally. When their tips were inserted in channels that had been formed in the cores, they were unable to penetrate directly through moderately consolidated remoulded clay (ew < 1.10), but grew along horizontal fissures that developed in the clay. No fissures developed in the aggregated clay, and the radicles were unable to elongate at ew < 0.81. The results of the penetration tests are interpreted with the aid of a mathematical model taking into account the strength and compressibility of the clay. In addition, multiple exposure photography was used to show the paths of motion followed by clay particles near the tip of the probe and the radicle. Qualitative information on friction between the root tip and the clay was obtained by matching the local deformation of the clay around the root tip with the deformations produced by probes of known roughness. The root tips tended to compress the clay 'cylindrically', whereas the probe tips compressed the clay 'spherically'. There appeared to be little friction between the root tip and the clay.