Load-controlled cyclic T-bar tests: a new method to assess effects of cyclic loading and consolidation

Abstract

Full-flow T-bar and ball penetrometer tests are often used to measure intact and remoulded soil 46 strengths, with the latter determined after several large amplitude displacement cycles. In 47 offshore design the remoulded soil strength is often the governing design parameter during 48 installation of subsea infrastructure, whilst a 'cyclic strength' applies for the less severe 49 operational cyclic loading. This paper utilises T-bar penetrometer tests to measure both 50 remoulded and cyclic strengths, where the latter is determined via a new test protocol involving 51 cycles between load rather than displacement limits. The tests use kaolin clay and a 52 reconstituted carbonate silt and involve three cyclic phases with intervening consolidation 53 periods. The results demonstrate the important and beneficial role of consolidation, with the 54 loss in strength due to remoulding sometimes surpassed by the strength recovery from 55 consolidation. The most significant gains in strength, to 2.5 times the initial value, were 56 measured in the load-controlled cyclic tests. These data demonstrate a novel way to characterise 57 undrained cyclic strength, taking advantage of consolidation to reduce conservatism.

The changing strength of carbonate silt: parallel penetrometer and foundation tests with cyclic loading and reconsolidation periods

“…The steady seepage increases the profile of vertical effective stress with depth. Other studies using the same soil, reconstituted in the same way but without the seepage problem, show a strength of gradient of 2 kPa/m (Chow et al 2019;Zhou et al 2019b;O'Loughlin et al 2019), which is half of the gradient recorded in this study. Therefore, all the present results have been interpreted considering a doubling of the geostatic effective stress profile.…”

“…Figure 3a plots the undrained shear strength with normalised depth, z/DTbar, where su = qT-bar/NT-bar was interpreted assuming a constant T-bar capacity factor of NT-bar = 10.5. This value was selected based on theoretical solutions (Einav and Randolph 2005;Martin and Randolph 2006), and it has been adopted for other studies using the same soil (Chow et al 2019;Chang et al 2019;Zhou et al 2019b;O'Loughlin et al 2019). Negative values of undrained shear strength correspond to a downwards soil resistance on the T-bar penetrometer, which is created when the penetrometer moves upwards.…”

Modelling the behaviour of sensitive clays experiencing large deformations using non-local regularisation techniques

“…d T −bar /D of 2.5) plotted against the penetration cycle number (N ), following the convention suggested by Randolph et al [17] and Zhou and Randolph [14] that the first pass is referred to as cycle number N = 0.25. The results extracted from five cyclic undrained T-bar penetration tests in UWA kaolin clay by Ragni et al [35] and O'Loughlin et al [55] are also presented in the figure with error bars representing the range and the symbols representing the mean value for the five tests. The contour plots of the local incremental equivalent plastic strain variable, εs , are presented in Figure 15 for the points A-H marked in Figure 14.…”

“…(c) UWA [54,55], the subsequent cycle generates a practically constant resistance with no reduction in resistance during the subsequent cycle. This is because the sensitivity state variable has already reached a minimum and re-consolidation did nothing to change that.…”

“…This necessitates the development of new techniques for strain-softening parameter calibration if non-locally regularised models are to be of practical use. The model developed was unable to capture the second packet of strength degradation as well as the magnitude observed in T-bar experiments by Hodder et al[54] and O'Loughlin et al[55]. This is because the sensitivity state variable does not recover during the dissipation process, which results in a monotonic response after the soil is fully remoulded for the first time.…”