Limiting resistance of a spherical penetrometer in cohesive material

The shear strength of soft sediments may be assessed by field tests, principally either vane tests or cone penetrometer tests. The continuous profile of strength obtained from the cone penetrometer is an advantage over the vane test, although the significant corrections for overburden stress, and pore pressure acting on the back face of the cone, limit its accuracy in soft soils. Alternative shapes for penetrometers have been suggested, for example a cylindrical T-bar and a spherical ball, both of which allow full flow of soil around the probe, thereby obviating the need for corrections due to the ambient stress level. Plasticity solutions have been available for some while relating the (average) shear strength of the soil to the net bearing pressure for plane strain flow around the cylindrical T-bar. This paper documents a corresponding solution for a spherical penetrometer, using upper and lower bound approaches supported by finite element analysis. Effects of strength anisotropy are discussed in the light of example results comparing the measured resistance of cone, T-bar and ball penetrometers.