The bearing capacity of spudcan foundations under combined loading in spatially variable soils

doi:10.1016/j.enggeo.2017.03.022

Engineering Geology

Volume 227, 21 September 2017, Pages 139-148

https://doi.org/10.1016/j.enggeo.2017.03.022 Get rights and content

Highlights

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How spatial variability affects the bearing capacity under combined loadings is studied.
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3D random fields are combined with a finite element analysis to model spatially varying soils.
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The failure envelopes with various probability of occurrence are constructed for spudcan in random soils.
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There are 50% probability the failure envelopes in random soils are smaller than that in uniform soil.

Abstract

Predicting the bearing capacity of a spudcan foundation under combined vertical (V), horizontal (H) and moment (M) loads is a challenging problem encountered by geotechnical engineers. In previous studies the combined VHM capacity was defined for a uniform soil profile, ignoring any variability in soil stratification and properties. In offshore conditions, however, both the soil profile and soil properties vary spatially. Therefore, it is of interest to account for the spatial variability of soil in the analysis of the bearing capacity of a spudcan. It is shown in this paper how the spatial variability of a clay affects the bearing capacity of a deeply buried spudcan foundation under combined loadings. Three-dimensional random fields are generated to model the spatial variability of undrained shear strength of clay and combined with a non-linear finite element analysis to investigate and define the VHM failure envelope of a spudcan foundation. Because of the random nature of soils VHM failure envelopes of different probability of occurrence are proposed. Results from this study provide guidance to the practical assessment of spudcan foundations in spatially varied soil conditions that can be encountered offshore.

Introduction

In the offshore industry, mobile jack-up drilling rigs are a key contributor to explore oil and gas reservoirs in water depths up to around 150 m. The drilling rigs are typically supported by individual, conical spudcan foundations that are around 20 m in diameter. During installation and preloading of jack-up platforms, spudcan foundations can penetrate deeply into the seabed (up to 3 spudcan diameters) (Menzies and Roper, 2008). Industrial guidelines, such as SNAME (2008) and ISO (2012), provide formulas to assess the vertical load-penetration curves for jack-ups. Based on bearing capacity theory, these have been calibrated from model experiments, finite-element and limit analysis (e.g., Taiebat and Carter, 2000, Templeton, 2009, Hossain and Randolph, 2010, Zhang et al., 2011). These studies have assumed homogeneous soils or uniform soils of strength linearly increasing with depth. However, under more realistic conditions found offshore, the strength of seabed soils often varied spatially as a result of depositional and post-depositional process (Baecher and Christian, 2003).

The influence of spatial variability of soil properties (e.g., undrained shear strength) on the bearing capacity of a strip footing and slope stability has been studied extensively (e.g., Fenton and Griffiths, 2008, Popescu et al., 2005, Jiang et al. 2014, Li et al., 2017). It is found that the spatial variability can dramatically reduce the bearing capacity of a foundation (e.g., Popescu et al., 2005; Li et al., 2015, Li et al., 2016b). The majority of the studies employed two-dimensional random fields and plane-strain conditions (i.e. an infinite foundation with soil properties not varying into the plane). This simplifying assumption could be influential on the bearing capacity of conical-shaped spudcan foundation. Therefore, in this paper, three-dimensional (3D) random fields representing the spatial variation of soil properties in all directions are investigated.

Once a jack-up is installed and operational, the spudcan foundations need to be able to resist the combined vertical (V), horizontal (H) and moment (M) loads due to the environmental loadings (e.g., wind, waves and currents) on the superstructure. However, there currently is no guidance in the effect of the uncertainty in the spatially varied seabed on the combined VHM bearing capacity. The application being addressed in this paper is illustrated in Fig. 1. Combined loading problems have traditionally been addressed by the classical bearing capacity approach (Hansen, 1970), which considers the correction with the load eccentricity and inclination for the vertical bearing capacity. However, it is more accurate to demonstrate the ultimate bearing capacity of the foundation under combined loads written directly in VHM loading envelopes. The load combinations inside the envelope are assumed as safe, but the load combinations outside the envelope are deemed to result in failure. The jack-up industry uses this approach when assessing if a jack-up is safe to operate at a particular site (SNAME, 2008; ISO, 2012), with the VHM failure envelopes defined through numerical and experimental studies in uniform soils (Martin and Houlsby, 2000, Templeton, 2009, Zhang et al., 2013).

With no advice currently available, this paper provides guidance on how the spatial variability of clay soil affects the bearing capacity of a deeply embedded spudcan foundation under combined loads. Randomness of undrained shear strength in three dimensions is accounted for and VHM failure envelopes with different probability of occurrence are proposed. The results can be used in assessing the probability of exceeding an ultimate limit state of failure of a spudcan foundation within the methodology used to assess the safety of jack-ups structure operating in large storms.

Section snippets

Spatially varying soil

The spatial variability of a soil is often described by a trend and a residual variable. A trend is estimated by fitting well-defined mathematical functions to spatial data points. The residuals around the trend are spatially correlated to one another in space. This correlation is generally a function of the distance, described by an autocorrelation function and a parameter called scale of fluctuation (see details in Baecher and Christian, 2003).

In this study, the undrained shear strength (s_u)

Random finite element method

Random finite element method (RFEM) was performed using the nonlinear finite-element software ABAQUS to investigate the bearing capacity of a spudcan foundation buried in spatially varying soils (Li et al. 2016a). The spudcan was wished in place which meant the installation process was not simulated. The foundation was modelled as a rigid body with loads and displacements applied to a reference point (see Fig. 3). The reference point was chosen at the center of the section of lowest maximum

Deterministic analysis

A deterministic case with the spudcan foundation embedded in soil with the undrained shear strength linearly increasing with depth is first simulated. Fig. 6 demonstrates the failure envelopes for the spudcan in VH (M = 0), VM (H = 0) and HM (V = 0) planes. A comparison of the failure envelope obtained from the circular plate footing (thickness-to-diameter aspect ratio (T/D) of 0.05 in Zhang et al. (2012)) and a spudcan embedded in 3.5D (Zhang et al., 2011) are also presented in Fig. 6. The envelopes

Summary and conclusions

This study investigated the failure mechanism and failure envelopes of a spudcan foundation embedded in spatially variable soils under combined loading. The spatial variability of the soil was modelled by three-dimensional random fields. The failure envelopes were investigated by applying combined vertical (V), horizontal (H) and moment (M) loads. By expressing the VHM failure envelopes within a probability framework, failure envelopes with different probability of occurrence were defined. An

Acknowledgments

The authors would like to acknowledge the support of Natural Science Foundation of China (Grant nos: 51379053, 51422905 and 51679060) and Shenzhen Science and Technology Commission (Grant No. CXZZ20151117174345411). The authors would like to thank Dr. Youhu Zhang from NGI for his valuable discussion. This study represents part of the activities of the Centre for Offshore Foundation Systems (COFS) in the University of Western Australia and the Australian Research Council Centre of Excellence for

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View full text

The bearing capacity of spudcan foundations under combined loading in spatially variable soils

Highlights

Abstract

Introduction

Section snippets

Spatially varying soil

Random finite element method

Deterministic analysis

Summary and conclusions

Acknowledgments

Comput. Geotech.

Struct. Saf.

Eng. Geol.

Eng. Geol.

Eng. Geol.

Probab Eng Mech

Mar. Struct.

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Georisk

Reliability and Statistics in Geotechnical Engineering

Standardized sign conventions and notation for generally loaded foundations

Géotechnique

The behaviour of drag-in plate anchors in soft cohesive soils

Risk Assessment in Geotechnical Engineering

Effect of strength non-homogeneity on the shape of failure envelopes for combined loading of strip and circular foundations on clay

Géotechnique

Bearing capacity of rough rigid strip footing on cohesive soil: probabilistic study

J. Geotech. Geoenviron. Eng.

Revised and extended formula for bearing capacity

Earth Pressure

Deep-penetrating spudcan foundations on layered clays: centrifuge tests

Géotechnique