Behaviour of ball penetrometer in uniform single- and double-layer clays

Abstract

The spherical ball penetrometer is increasingly being used for profiling the undrained shear strength of soils in centrifuge and offshore site investigations. This paper reports the results from largedeformation finite-element (LDFE) analysis undertaken to provide insight into ball penetrometer behaviour during undrained vertical penetration through uniform and stratified clay deposits. The LDFE analyses simulated continuous penetration of ball penetrometers from the seabed surface. The results were validated against centrifuge test data and plasticity solutions prior to undertaking a detailed parametric study, exploring a range of normalised soil properties and layer thicknesses and roughnesses of the soil/ball interface. The influence of the shaft (or area ratio) was also identified. The evolving soil failure patterns in single-and double-layer soils revealed two interesting aspects: soil backflow above the penetrometer, and trapping of the stronger material beneath the penetrometer. A framework has been proposed to account for these effects, which will allow accurate interpretation of soil undrained shear strength from the ball penetration resistance.

Strength assessment during shallow penetration of a sphere in clay

“…Merifield et al, 2009;White et al, 2010;Chatterjee et al, 2012;Zhou et al, 2013). An alternative approach to derive this multiplier directly, is to consider the work required to lift the soil that is displaced by the incrementally advancing ball.…”

“…This is critical for the design of almost all shallowly embedded offshore infrastructure (Puech et al, 2010) including subsea pipelines, steel catenary risers and mudmats. Large-deformation finite-element (LDFE) analyses on a T-bar (White et al, 2010;Tho et al, 2012), spudcan (Hossain et al, 2005) and ball penetrometer (Zhou et al, 2013) have shown that the transition depth from N b-shallow to N b-deep is dependent on the dimensionless strength ratio s u /c9D, where c9 is the effective unit weight of the soil and D is the diameter of the penetrometer. Higher strength ratios are associated with a delay in the transition to a steady N b-deep .…”

“…Higher strength ratios are associated with a delay in the transition to a steady N b-deep . Correlations for the transition depth and N b-shallow have been derived and, in the case of the T-bar and ball, a basis for correcting penetration data within the shallow zone has been proposed (White et al, 2010;Zhou et al, 2013). However, from the perspective of a spherical penetrometer, which is the focus of this paper, the range of strength ratios previously examined (s u /c9D52?95-44?25 (Zhou et al, 2013)) is narrower than the range that is of practical interest for offshore problems.…”

“…Correlations for the transition depth and N b-shallow have been derived and, in the case of the T-bar and ball, a basis for correcting penetration data within the shallow zone has been proposed (White et al, 2010;Zhou et al, 2013). However, from the perspective of a spherical penetrometer, which is the focus of this paper, the range of strength ratios previously examined (s u /c9D52?95-44?25 (Zhou et al, 2013)) is narrower than the range that is of practical interest for offshore problems. For example, a 250 mm diameter freefall spherical penetrometer (for measuring the strength of the seabed (Morton & O'Loughlin, 2012)) penetrating very soft soil is associated with low values of s u /c9D approaching 0?1 at one diameter embedment, whereas an 80 mm piezoball penetrating the seabed with a crust strength of <20 kPa is associated with high values of s u /c9D, approaching 40.…”

“…The experimental data are combined with reinterpreted LDFE results (Zhou et al, 2013) and form the basis of a new correlation that describes the evolution of N b with depth.…”