Assessing the punch-through hazard of a spudcan on sand overlying clay

Abstract

A complete analytical method to describe the full load-penetration resistance profile of a mobile jackup spudcan footing penetrating a sand over clay stratigraphy is described. It is based on both large deformation finite-element analyses and geotechnical centrifuge experiments. The coupled Eulerian-Lagrangian (CEL) approach is used to accommodate the large deformations of a spudcan footing penetrating sand overlying clay. Modified Mohr-Coulomb and Tresca models describe the sand and clay behaviour, with modifications accounting for the effects of strain softening on the response of the soil. The CEL results are shown to match centrifuge tests well, allowing the numerical study to be extended parametrically, and with confidence, to cover the range of layer geometries, sand relative densities and footing shapes that are of practical interest to offshore jack-ups. The results are used to (a) assess the performance of an existing model to predict the peak resistance in the sand layer (extending its range of application to medium dense to dense sands and to conical footings of angle 0°t o 21°), and (b) develop an expression for the bearing capacity factor when the footing penetrates into the underlying clay. Using the analytical formulas proposed, retrospective simulations of centrifuge tests show that the method provides a reasonable estimate of the peak punch-through load, the behaviour in the underlying clay, as well as the punch-through distance; the latter being a basic reflection of the severity of a potential punch-through failure.

Predicting jack-up spudcan installation in sand overlying stiff clay

“…This large-deformation analysis approach provides a full spudcan penetration resistance profile and reveals the evolving soil flow patterns. The CEL approach has been employed to simulate spudcan penetration in singleand two-layer clays (Tho et al 2012) and spudcan penetration in sand overlying clay (Qiu and Henke 2011; Qiu and Grabe 2012; Hu et al 2014b;2015a). The spudcan and soils were discretised using Lagrangian and Eulerian meshes, respectively.…”

“…Poisson's ratio was 0.49 to approximate constant-volume shearing under undrained conditions. More detailed discussions of the adopted constitutive models are given in Hu et al (2015a).…”

“…The expression for Nc given in Houlsby and Martin (2003) was recommended by the ISO guideline and it is used in this comparison. It is obvious that the previous bearing capacity factor expression of Hu et al (2015a), as expressed in Eq. (1), over-estimates the penetration resistance by as much as 40% for these cases of low kD/sum gradients.…”

“…Recently, a new analytical penetration resistance-depth profile prediction method (hereinafter called the Hu et al method) was proposed (Lee et al 2013b;Hu et al 2014bHu et al , 2015a. It was calibrated using a testing and numerical database with soft clay strength of 10 -40 kPa at the sand-clay interface and strength gradient of 1.5 -2.5 kPa/m (Lee et al 2013a;Hu et al 2014aHu et al , 2015aHu et al , 2016. In the prediction of the peak penetration resistance in the upper sand layer, the stress level and dilatant response, as well as the embedment depth, are taken into account using a discretised silo approach.…”

“…Based on a large dataset of 111 cases,Hu et al (2015a) derived an equation that fits all the data as:…”