PENETROMETER RESISTANCE EQUATION - ITS DERIVATION AND THE EFFECT OF SOIL ADHESION

The widely quoted equation of Farrell and Greacen (1966;)1 for penetrometer resistance does not include a term for soil-metal adhesion. A modified form of this equation which includes the effect of adhesion is Qp = σn(1 + μ cot α) + ca cot α, where Qp is the penetrometer resistance, σn is the stress normal to the cone surface, α is the probe semi-angle, μ is the coefficient of soil-metal friction, ca is the tangential adhesion, and ca cot α is the adhesion component of penetrometer resistance. The value of ca can be determined experimentally either by measuring the stress required to pull a metal plate over a soil surface under several different externally applied loads, or by measuring the normal stress required to pull the metal and soil surfaces apart. The value of ca is generally smaller in sandy soils than in clay soils, and is dependent on the soil matric potential. The adhesion correction (ca cot α) is greatest for penetrometers with small cone semi-angles, being over six times greater for a probe with α = 5 °, than for a probe with α = 30 °. Neither this equation nor the conventional one is valid if a soil-body forms in front of the probe tip, as can occur for penetrometer cones with a large semi-angle (e.g. α > 45° - φ/2, where φ = angle of internal friction). © 1992 Silsoe Research Institute.