The development of full-flow penetrometers, including the T-bar, ball and plate, for in situ measurement of undrained intact and remoulded shear strength has required research regarding the conditions necessary to induce ‘full flow’ around the penetrometer. A primary parameter governing this condition is the ratio of the probe to drill string projected area, termed ‘probe area ratio’. Initial investigations assumed area ratios of 10:1 are adequate to minimise the influence of the drill string on the formation of full flow of soil around the probe. The objective of this paper is to quantitatively assess the influence of projected area ratio on penetration resistance. An in situ research programme with four T-bar lengths representing area ratios of 2:1, 5:1, 10:1, and 15:1 was performed in a lightly over-consolidated clay for evaluation of the effects of area ratio on the interpreted undrained intact and remoulded strengths. The results are also compared with full displacement piezocone and 10:1 ball penetration tests. Results indicate that area ratios of 5:1 and greater appear to minimise the effects of the drill string on measured penetration and extraction resistances. A reduction in the length of the T-bar, resulting from the use of area ratios of less than 10:1, is attractive practical option, as it allows for field deployment with seabed units, in smaller diameter drill casings and reduced bending loads on the load cell.


The use of full-flow penetrometers, such as the ball and T-bar (Figure 1), has increased as their ability to measure accurately the undrained intact and remoulded shear strength of soft clays has been demonstrated. Standard size full-flow penetrometers have an area ratio, AR (projected area of the probe to drill string), of 10:1 and a projected area of 100cm2. The larger projected area (compared to a standard 10cm2 piezocone) measures the response of a larger volume of soil, increasing accuracy in soft clay where the penetration resistance may have otherwise been undetectable relative to electrical noise and sensor resolution.

The flow of soil around the penetrometer (rather than full displacement as for the piezocone) results in nearly equal overburden stress above and below the penetrometer body, reducing the overburden correction to 10% of the correction required for the piezocone (CPTu). The correction for the imbalance of overburden stress on measured penetration was initially assumed to be insignificant, but more recent practice has been to account for this influence on measured resistance. This study assesses the effectiveness of corrections for overburden stress and the area ratio required for full-flow penetration through testing T-bars with four different area ratios, a 10:1 ball penetrometer and a 10cm2 CPTu.

Testing Programme

T-bars with area ratios equal to 2:1, 5:1, 10:1 and 15:1 were manufactured to investigate the effects of area ratio on penetration resistance (Figure 1, Table 1). A moment compensated load cell was used with the full-flow penetrometers to prevent erroneous measurements during eccentric loading (particularly likely with the T-bar penetrometers). T-bar and ball surfaces were sandblasted to provide a roughened surface.

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