THE EFFECTS OF RADIAL SOIL CONFINEMENT AND PROBE DIAMETER ON PENETROMETER RESISTANCE

The reliability of penetrometer resistance (P) measurements on forest sites with the presence of soil structural discontinuity (such as crac ks, channels, rocks or tree roots) near the penetrometer probe tip was tested indirectly in this study, The effects of varying radial soil c onfinement on P for penetrometer probes of different diameters, d (ove r 2 mm) was used as a measure of the sensitivity of a penetrometer to the presence of structural discontinuity. Measurements of P were made on two homogenized forest soils (at constant water contents) compacted in tubes of various diameters (D) to bulk densities of 0.8 and 1.0 Mg m(-3). The combinations of D and d were used to obtain radial soil co nfinement (D/d) in the range of 4-35. Note that smaller values of radi al confinement here indicate greater restriction to radial soil expans ion during probe penetration. Although the maximum values of P (i.e. P ') ranged from 0.5 to 5 MPa, no significant effects of radial soil con finement on P' was evident. In a separate experiment, additional measu rements of P and P' at a higher bulk density of 1.2 Mg m(-3) showed su bstantial increase in P' with increased restriction to radial soil exp ansion for both soils. These results imply that for lightly compacted soils, reliable measurements of P and P' can be made even if structura l discontinuity occurs near a penetrometer test site. Results also sho wed no dependence of P' on probe diameter within the diameter range 2- 8 mm.