The parkable piezoprobe for determining c_v and strength – modelling and interpretation methods

Abstract

The parkable piezoprobe (PPP) is a site investigation tool for measuring the coefficient of consolidation, in-situ offshore, at shallow embedment depths. The device applies a similar bearing pressure to subsea infrastructure so reaches a comparable self-weight penetration when 'parked' at the seabed, representative of an unburied pipeline. Instrumentation on the device allows the dissipation of penetration-induced excess pore pressure to be recorded at various locations on the surface. From these dissipation responses the coefficient of consolidation can be inferred, which is a key parameter in the design of many offshore structures founded in surficial soil, such as pipelines or shallow foundations. The intent is that this device is deployed from a seabed frame while other activities such as penetrometer testing or sampling take place in parallel. This paper presents robust interpretation methods for the parkable piezoprobe by using a combination of centrifuge experiments and large deformation finite element analyses. The centrifuge tests demonstrate that the penetration response of the parkable piezoprobe is adequately captured by existing bearing capacity models, allowing the optimum device weight to be identified. A comprehensive interpretation method is then developed for the dissipation stage. This yields accurate estimates of the coefficient of consolidation, even for cases where there is no prior knowledge of the soil parameters or the depth to which the device embeds under its own self-weight.

Shallow penetrometer tests: theoretical and experimental modelling of penetration and dissipation stages

“…Figure 13 compares the inferred coefficients of consolidation with miniature piezocone (c h ) and Rowe cell (c v ) data measured independently in the same kaolin clay over the past decade or more at The University of Western Australia (Richardson 2007;Chow et al 2014;Cocjin et al 2014). Data at shallow embedment are conspicuously absent, but the shallow penetrometer data presented here are consistent with the measurements generated using the parkable piezoprobe, another device intended to measure surficial soil properties (Chatterjee et al 2014;Schneider et al 2019) and the back-extrapolated trends from the piezocone and Rowe cell data.…”

“…A small surcharge, q 0 , of ϳ1 kPa (quasi-uniform conditions of s u and c v over depth range of interest, as q 0 /␥ D = 2) was required to achieve convergence at the free surface of the soil and to create a shear strength intercept at the mudline. The appropriateness of the adopted mesh density and penetration rate (V pen ϳ 100) was confirmed through sensitivity analyses (see also Schneider et al 2019).…”

Measuring strength and consolidation properties in lacustrine clay using piezocone and self-boring pressuremeter tests

“…Mahmoodzadeh et al Schneider, M.A., Whittle, R.W. and Springman, S.M. Measuring strength and consolidation properties in lacustrine clay using piezocone and self-boring pressuremeter tests 21 isotropic pore water flow and permeability anisotropy seems to be one of the most prominent drivers of the observed discrepancy between field and laboratory measurements, the equation proposed by Mahmoodzadeh et al (2014) shall be extended by the findings from Schneider et al (2019) to give:…”

“…7)whereas vʹ, α and f(n k ) represent the Poisson ratio, a weighting factor controlling the elastoplastic soil behaviour around the probe and a function incorporating the effects of anisotropic flow, respectively. Based on an extensive LDFE-study analysing the dissipation response of the Parkable Piezoprobe (shallow penetration of a hemispherical probe),Schneider et al (2019) found the following relationship: f(n k ) = (2n k +1)/3, which is essentially equivalent to the geometric mean of all three permeability components Wang et al (2015). found a similar expression for the piezoball.…”

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

“…These were previously calibrated using standard geotechnical element tests performed on natural Pisa and reconstituted kaolin clay samples and the various parameters adopted are presented in Table 1. For the Pisa clay, the identification of the parameters [36], which has been adopted extensively in past numerical studies( [37], [38], [39], [40], [41]). Considering the effective stress levels important for the boundary value problem simulation presented later in this study (i.e.…”