Use of epoxy in developing miniature ball penetrometers

Abstract

Ball penetrometers used in geotechnical centrifuge facilities for characterising the soil strength profile are typically around 12 mm in diameter. The minimum size is limited by the traditional design approach, with ball and shaft machined from steel or aluminium alloys, and axial strain gauges placed on the shaft to measure the penetration resistance. This paper describes a new fabrication method that uses epoxy to cast and cure the ball penetrometer and shaft as one piece. This allows the strain gauges to be set inside the penetrometer shaft, reducing the size of the ball penetrometers. The manufacture of three miniature ball penetrometers of 4 mm, 5 mm and 6 mm diameter is described. Example results using the penetrometers in a drum centrifuge highlight the versatility and consistency of the new devices. A comparison with the more established T-bar in characterising the undrained shear strength profile of soft clays is also provided.