Computers and Geotechnics 45 (2012) 127–139

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Computers and Geotechnics

journal homepage: www.elsevier .com/locate /compgeo

Numerical modelling of a hybrid skirted foundation under combined loading

Britta Bienen a,⇑, Christophe Gaudin a, Mark Jason Cassidy a, Ludger Rausch a, Okky Ahmad Purwana b,Henry Krisdani b

a Centre for Offshore Foundation Systems, University of Western Australia, Perth, WA 6009, Australiab Keppel Offshore & Marine Technology Centre Pte Ltd (KOMtech), 31 Shipyard Road, Singapore 628130, Singapore

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 December 2011Received in revised form 9 May 2012Accepted 10 May 2012

Keywords:Skirted foundationOffshore foundationCombined loadingCapacity

0266-352X/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compgeo.2012.05.009

⇑ Corresponding author. Tel.: +61 8 6488 4246; faxE-mail addresses: britta.bienen@uwa.edu.au (B.

uwa.edu.au (C. Gaudin), mark.cassidy@uwa.edu.auuwa.edu.au (L. Rausch), OkkyAhmad.Purwana@kepHenry.Krisdani@keppelom.com (H. Krisdani).

Offshore foundations are subject to combinations of vertical, horizontal and moment (VHM) loading. Dif-ferent foundation solutions are appropriate, depending amongst others, on the expected load combina-tions. This paper investigates in detail the soil failure mechanisms and resulting foundation capacityunder undrained VHM loading of a hybrid foundation concept. Comprising of a skirted mat combinedwith an internal caisson compartment, this foundation design is found to provide significantly increasedhorizontal capacity that is often critical in offshore applications, with key influences for increasedmoment capacity identified as well. The numerical study investigated both variations in the geometryof the internal caisson compartment and the soil shear strength profile. The paper concludes by proposingexpressions that allow slightly conservative prediction of the combined capacity of a hybrid foundation.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Shallow foundations for offshore structures typically includeskirts to provide the additional horizontal and moment resistancerequired by offshore environmental loading. Depending on thelength of the skirt L and the resulting aspect ratio L/D (withD the diameter or breath of the foundation), these shallowfoundations are called skirted mat (L/D 6 0.5) or bucket founda-tions (L/D 6 1). In comparison to a surface foundation, the skirttransfers the load to deeper, typically stronger, soil, thus mobilisinghigher bearing capacity (e.g. [2]). For some applications, includingsubsea systems, horizontal and moment foundation capacity isoften more critical than vertical capacity due to the structuralresponse of the offshore system in combination with the environ-mental loading. The seemingly simplest solution, i.e. the extensionof the skirt length, can meet horizontal capacity requirements in astraightforward manner but at the same time may result in anoverdesign of the overall foundation capacity. Considerations inaddition to the increase in steel weight relate to the structuralintegrity of the foundation. A long, large diameter skirt requires asignificant amount of stiffeners to prevent skirt buckling duringinstallation. On the other hand, any additional stiffeners mayadversely impact ease of foundation installation.

ll rights reserved.

: +61 8 6488 1044.Bienen), christophe.gaudin@(M.J. Cassidy), ludger@civil.pelfels.com (O.A. Purwana),

Aiming for an optimum design, a hybrid foundation concept isinvestigated here. The hybrid foundation aims to provide addi-tional horizontal and moment capacity by optimising the amountof steel required and minimising structural design complexityassociated with long and large diameter skirted foundation. It con-sists of a skirted mat with (an) internal caisson compartment(s)(which can also be viewed as extended internal skirts). The generalconcept is shown in Fig. 1. This design provides the large top areaof a skirted mat, while increased capacity, in particular under hor-izontal loading, is expected due to the longer internal skirts.

The geometry of the hybrid foundation is likely to result in differ-ent failure mechanisms compared to a foundation with only exter-nal skirts, such that existing solutions [2,17,5–7,13,11,3,18,14,10,1]to predict the capacity under vertical, horizontal and momentloading cannot be relied on. This study therefore investigates thefailure mechanisms and resulting combined vertical (V), horizontal(H) and moment (M) capacity of the hybrid foundation underundrained loading conditions immediately following installation(i.e. no consolidation effects are taken into account) through finiteelement analysis. In the numerical study, the installation processis not modelled. Installation-related excess pore pressures andremoulding at the skirt–soil interface are thus ignored. In practice,these effects may reduce the mobilised capacity, though excess porepressures at the skirts are expected to dissipate rapidly. Six hybridfoundation geometries were considered through variation of thegeometry of the internal caisson compartment (diameter and skirtlength). The effect of variation in soil shear strength profile was alsoinvestigated. The results are compared to available solutions forskirted and bucket foundations (characterised by different aspect

Control unit

Pump unit

Connecting frame

Ballast tank

Vent hatch

Reaction base

Suction compartment

Peripheral skirt

Fig. 1. Hybrid foundation concept (schematic).

dm

Dm

Dc

dc

Fig. 2. Hybrid foundation model adopted in finite element analysis.

128 B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139

ratios) as well as preliminary centrifuge experimental data of simi-lar hybrid foundations.

2. Finite element model

Mobilisation of the undrained capacity of a foundation, once in-stalled, only requires relatively small displacement excursions.Therefore, small strain finite element analyses were employed hereto establish the undrained capacity of the hybrid foundation undercombined VHM loading, using the commercial software Abaqus.The installation process was not simulated, i.e. the foundationwas wished in place.

2.1. Footing model

This study sought to quantify the contribution of the internalskirts that protrude below the skirt tip level of the skirted mat tothe VHM capacity of the hybrid foundation. For this purpose, a cir-cular skirted mat with a central circular caisson compartment wasmodelled as shown in Fig. 2. Note that the elevated caisson lid re-quired to preload the mat area through the application of suction isnot modelled here. This is because once the caisson compartmentis sealed, due to its incompressibility the water inside the caissoncompartment is expected to transfer any undrained loading to thesoil. Six geometrical variations were considered, varying the inter-nal caisson compartment diameter and skirt length, with an over-view of the dimensions and relevant aspect ratios provided inTable 1, which also sets out the naming of the different models.The first number refers to the caisson diameter, the second tothe internal skirt length. All hybrid footings (HFs) have a 16 mdiameter mat with a skirt length of 2 m. For instance, HF9.2–6 re-fers to a hybrid footing which has an internal compartment of9.2 m in diameter and 6 m skirt length. The hybrid footing wasmodelled as a rigid body, with displacements and load relating toa single reference point (RP). The choice of RP location is discussedin a separate subsection. In order to obtain the net foundationcapacity, without buoyancy effects, the skirts were assigned thesame effective unit weight as the soil.

Gravity was applied to the entire numerical model. In order toobtain the net capacity, the embedded parts of the footing, i.e.the skirts, were assigned the same effective unit weight as the soil.The remainder of the footing was modelled as weightless.

2.2. Soil properties

The soil was modelled as a linearly elastic, perfectly plasticmaterial, governed by the Tresca failure criterion. The yield stressis determined by the undrained shear strength su. Both uniformand non-uniform (linearly increasing with depth) undrained soilshear strength profiles were considered. They can be expressed as:

su ¼ sum þ kz ð1Þwhere sum is the undrained shear strength at the mudline and k theshear strength gradient with depth z. Three different undrainedshear strength profiles were investigated:1. Uniform soil strength with su = 3 kPa.2. A lightly over-consolidated (LOC) soil with sum = 3 kPa and

k = 1.05 kPa/m. These values were specifically chosen to reflectthe target shear strength profile of complementary centrifuge

Table 1Hybrid foundation geometries investigated.

Geometry name Dm dm Dc dc dc/Dm dc/Dc Dc/Dm Ac/Am

Skirted mat (SM) 16 2 – – – – – –Hybrid foundation HF 7–4 16 2 7 4 0.25 0.57 0.4375 0.19HF 7–6 16 2 7 6 0.375 0.86 0.4375 0.19HF 7–8 16 2 7 8 0.5 1.14 0.4375 0.19HF 9.2–4 16 2 9.2 4 0.25 0.43 0.5750 0.33HF 9.2–6 16 2 9.2 6 0.375 0.65 0.5750 0.33HF 9.2–8 16 2 9.2 8 0.5 0.87 0.5750 0.33

Table 2Undrained soil shear strength profiles su = sum + kz investigated.

Shear strength profile sum (kPa) k (kPa/m)

Uniform 3 0Lightly over-consolidated (LOC) 3 1.05Normally consolidated (NC) 0.1 1.0

B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139 129

experiments [9] that were planned to further validate thenumerical analysis results. The shear strength gradient of about1 kPa/m is relevant to most deep sea areas [20], where subseainstallations may be considered.

3. An essentially normally consolidated (NC) profile withsum = 0.1 kPa and k = 1.0 kPa/m. The small non-zero value forsum was required for numerical reasons. In comparison to theLOC soil, the NC shear strength profile will highlight the effectof undrained shear strength at the mudline, which is expectedto have the greatest influence on the failure mechanism underhorizontal load.

The soil shear strength profiles, as well as the nomenclatureused for the remainder of the paper, are summarised in Table 2.The results are presented normalised by the undrained shearstrength as detailed below.

An effective unit weight (c0) of 5.7 kN/m3 was assigned to thesoil, though the initial stresses do not affect the soil resistancedue to the constitutive model used in the numerical analysis beingelastic perfectly plastic. The rigidity index E/su = 500 was assumedto be constant, the Poisson’s ratio was 0.49. Both values are typicalof the kaolin clay used in the centrifuge experiments. The coeffi-cient of earth pressure at rest, K0, was assumed here as 1, thoughin a Tresca material the choice of K0 does not affect the results.

2.3. Meshes

The problem under consideration requires a three-dimensionalmodel, though symmetry about the vertical plane (due to in-planecombined loading) allows discretisation of a semi-cylindrical mod-el (Fig. 5). The soil domain extended sufficiently far from the foun-dation (7Dm in width and 6.25Dm in height, with Dm being the matdiameter as shown in Fig. 2) to avoid boundary effects. The sideswere restricted from horizontal movement, while the base wasfixed in all three translational degrees-of-freedom.

The foundation–soil interface was assumed rough, with themaximum allowable shear stress equal to the soil’s undrainedshear strength. Tensile stress was allowed to develop at the fullybonded interface (with tensile resistance during undrained loadingin the field being mobilised by transient suction).

The initial geostatic stresses were specified in the input, withgravity applied to the model in a geostatic step preceding loadapplication.

The model was validated against published solutions for foun-dations with an external skirt. Due to the stress concentrations,mesh refinement is required around the skirt tips. Following vali-

dation, the same meshing principles were adopted when includingthe internal skirts in the model. First order hexahedral elementswere used in the analyses. Abaqus uses a selectively reduced inte-gration technique for these elements, replacing the actual volumechanges at the Gauss points by the average volume change of theelement. This helps to prevent mesh locking and thus providesaccurate solutions for (near) incompressible materials.

2.4. Reference point and normalisation of results

The foundation is modelled as a rigid body, such that the VHMcapacity determined for one point (reference point) can be calcu-lated for another point using statics. The undrained shear strengthat the reference point is denoted su0.

For a surface foundation or an infinitely thin embedded plate,the results can be uniquely presented in terms of the local soil het-erogeneity kD/su0. While this characteristic parameter can also becalculated for embedded or skirted foundations, this informationalone does not fully describe the problem as the footing aspect ra-tio in combination with the shear strength profile determines thefailure mechanism. For the hybrid foundation with external andinternal skirts of different lengths, the overall geometry as wellas the relative dimensions of the internal skirted compartment tothe foundation diameter and external skirts and the soil undrainedshear strength profile affect the failure mechanism and thus thefoundation bearing capacity.

Typically, the reference point of a skirted mat or bucket founda-tion is taken at the foundation centre at skirt tip level as this is thelevel the load is transferred to the soil. Due to the variation ingeometry of the hybrid foundation considered here, the locationof the reference point is not obvious. The soil failure mechanismsare expected to be influenced by both the internal (long) and exter-nal (short) skirts.

Where the results of the hybrid foundation capacity analysesare compared to those of a skirted mat or bucket foundation, there-fore, the respective reference point location is used. Note that therelevant undrained shear strength in this case corresponds to thatat the respective skirt tip level (i.e. su,mat skirt tip or su,internal skirt tip).

For the assessment of the overall system performance, i.e. thesoil–structure interaction due to the loads imposed on the foun-dation-superstructure system, the foundation capacity at theconnection point to the superstructure is required to be known.As this may be different in different applications, the mudlinelevel is selected here. The location of this reference point (RP)is shown in Fig. 3. This is the only RP location used when dis-cussing the VHM capacity of the hybrid foundation. The corre-sponding reference strength used for the interpretation, su0, isequal to sum.

2.5. Sign convention

The sign convention for positive loads and displacements em-ployed here is shown in Fig. 3. It follows the standardised conven-tion proposed by Butterfield et al. [4] for combined loadingproblems.

Fig. 3. Sign convention and reference point location.

130 B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139

The ultimate uniaxial capacities are denoted Vult (H = M = 0),Hult (V = M = 0) and Mult (V = H = 0) for vertical, horizontal and mo-ment directions, respectively.

Due to cross-coupling of the horizontal and rotational degrees-of-freedom, the maximum horizontal or moment capacities (Hmax

and Mmax, respectively) do not coincide with the uniaxial capaci-ties. The bearing capacity factors are derived from the ultimateuniaxial capacities with respect to the undrained shear strengthat the RP as follows: NcV = Vult/Amatsu0, NcH = Hult/Amatsu0 andNcM = Mult/AmatDmsu0, where Amat and Dm are the plan area andthe diameter of the mat of the hybrid foundation, respectively.

2.6. Load paths

The three-dimensional VHM capacity surface was mapped outin two-dimensional sections corresponding to the VH (M = 0), VM(H = 0) and HM (V = 0) planes. All analyses of this study were per-formed with displacement-controlled probe tests. In these tests,vertical, horizontal and rotational displacements are applied at aconstant ratio until failure is reached, defined as no further in-crease in load with increasing displacement.

2.7. Validation

The accuracy of the numerical solutions was validated throughcomparison to known solutions for foundations with an externalskirt only. As the reference point for a skirted mat is typically cho-sen at skirt tip level, this location is chosen for comparison.

The analysis was validated by comparing the bearing capacityfactor obtained for a circular skirted mat of aspect ratiodm/Dm = 0.125 under uniaxial vertical loading, resting on clay ofuniform shear strength. A value of NcV of 7.36 was obtained, whichlies between the lower (NcV = 7.0) and upper (NcV = 7.5) boundsestablished by Martin [15] and is in agreement with Mana et al.

Fig. 4. The hybrid foundation and rele

[14] (NcV = 7.5), with a difference of only 1.9%. On soil with a shearstrength profile of su = 3 + 1.05z, the resulting NcV of 10.00 fallsabout midway between the upper and lower bounds of 9.00 and11.25 of Martin [15], respectively.

No solutions are available for ultimate horizontal load or mo-ment for circular skirted footings of appropriate aspect ratios.However, comparison of both the kinematic mechanisms and themagnitude of bearing capacity for a plane strain model of similardiscretisation provides confidence in the three-dimensionalnumerical model. The results obtained in this study (NcH = 1.65,NcM = 0.85) differed by only 3.6% in ultimate horizontal loadingcompared to the solution of Bransby and Randolph [2](NcH = 1.71) and 4.5% in ultimate moment loading (NcM = 0.81,[12] for uniform soil strength (and DNcV = 1.3%, DNcH = 1.3%,DNcM = 1.7% for non-uniform soil strength compared to [2].Further, the size and shape of the failure mechanism of thetwo-dimensional and three-dimensional analyses showed goodagreement. This exercise therefore provides confidence in thevalidity and accuracy of the numerical results.

3. Results and discussion

3.1. Uniaxial capacities

As the hybrid foundation represents a composite of a skirtedmat and a bucket (caisson), the results of this study are presentedin a manner that allows comparison to the following, in order toevaluate the performance of the hybrid foundation concept:

� A skirted mat (SM) of d/D = 0.125, corresponding to the hybridfoundation without the addition of internal skirts. This compar-ison highlights the influence of the internal caisson compart-ment. Therefore, the hybrid foundation capacities should bepresented in terms of the aspect ratio of the skirted matdm/Dm. As this would result in the data plotting along a verticalline of dm/Dm = 0.125, however, representation in terms of theaspect ratio of the protruding internal caisson skirts, dc, to theoverall foundation diameter, Dm, is considered more meaning-ful. The bearing capacity factors are normalised by the mat area(Am) and the undrained shear strength at the mat skirt tips(su,mat skirt tip).

� A bucket foundation of the same overall dimensions as thehybrid foundation geometries considered (i.e. d = 4, 6, 8 m andD = 16 m, resulting in aspect ratios of dc/Dm = 0.25, 0.375, 0.5).These foundation geometries may be arrived at by simplyextending the mat skirt to mobilise higher capacity, as dis-cussed in the introduction, though this imposes challenges inthe structural design, installation, and optimisation of founda-tion capacity. The bearing capacity factors are normalised bythe overall (mat) area (Am) and the undrained shear strength

vant foundations for comparison.

Fig. 5. Finite element model of HF 9.2–6.

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

NcV

= V u

lt/(A

ms u

,mat

ski

rt ti

p)

dc/Dm

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SMExp., SMExp., HF 7-4Exp., HF 7-8Exp., HF 9.2-4Exp., HF 9.2-8

Uniform su

LOC soil

SM d/D = 0.125

NC soil

Fig. 6. Vertical bearing capacity factor of the hybrid foundation (normalised by su atmat skirt tips).

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

NcV

= V u

lt/(A m

s u)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

Uniform su

aspect ratio of overall bucket foundationaspect ratio of skirted mat

Skirted stripGourvenec & Barnett (2011)

Mana et al. (2010)

UB (Martin,2001)

LB (Martin, 2001)

Fig. 7. Vertical bearing capacity factor of the hybrid foundation on uniform soil incomparison with published solutions for simple skirted foundations.

B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139 131

at the internal (caisson) skirt tips (su,internal skirt tip) as this allowsdirect comparison to published solutions for foundations with-out the internal skirts.

The comparison strategy is presented in Fig. 4.Further, the numerical results are also compared with experi-

mental data obtained with hybrid foundation models similar tothose investigated in the numerical study here. The experimentswere performed on lightly over-consolidated (LOC) kaolin clay inthe drum geotechnical centrifuge at UWA [21] at an accelerationof 200 times the Earth’s gravity (200 g). Details of the test set-upand procedure can be found in Gaudin et al. [9].

For the first comparison, with the skirted mat, the bearing capac-ity factors obtained for the different hybrid foundation geometriesand soil strength profiles are normalised by Am and su,mat skirt tip.These bearing capacity factors are plotted against the aspect ratioof the protruding internal caisson skirts, dc, to the overall founda-tion diameter, Dm, in Figs. 6 (NcV), 11 (NcH) and 15 (NcM). Usingthe same normalisation, Figs. 7 and 8a (NcV), 12 and 13a (NcH),and 16 and 17a (NcM) compare the results for the hybrid foundationto published solution for skirted mats.

Figs. 7 and 8b (NcV), 12 and 13b (NcH) and 16 and 17b (NcM)present the bearing capacity factors established by normalisingthe capacities by Am and su,internal skirt tip, as detailed for the secondcomparison, with the bucket foundation.

The published solutions used for comparison include upper andlower bound solutions of a solid circular foundation, embedded byd/D, for the relevant strength heterogeneity [15] as well as numer-ical analysis results of a circular skirted foundation [14] on uniformand non-uniform soil with a high degree of strength heterogeneity(kD/sum = 20). Solutions for skirted strip foundations [3,10], with Bdenoting the foundation breadth) are supplied in addition to re-sults from this study where no published data of the circularskirted geometry by other authors is available.

The uniaxial capacities (as bearing capacity factors NcV, NcH,NcM) of the different hybrid foundation geometries consideredare summarised in Table 3 for uniform soil strength. For compari-son, the relevant values for the skirted mat (bucket) foundationshave been included in the Table. Table 4 lists the respective valuesfor the LOC soil strength profile.

3.1.1. Uniaxial vertical capacity Vult

Fig. 6 illustrates the influences of hybrid foundation geometryand soil strength profile on the vertical bearing capacity factor.Fig. 7 establishes context with published solutions for simpleskirted foundations in uniform soil. The hybrid foundation resultsare compared with ultimate uniaxial vertical capacities expectedof skirted mats in Fig. 8a, while Fig. 8b shows the same data but

(a)

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

NcV

= V u

lt/(A

ms u

,mat

ski

rt ti

p)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

LOC soil

Skirted stripGourvenec & Barnett (2011)

kD/sum = 20(Mana et al., 2010)

UB (Martin,2001)

LB (Martin, 2001)

(b)

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

NcV

= V u

lt/(A

ms u

,inte

rnal

ski

rt ti

p)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

LOC soil

aspect ratio of overall bucket foundation

Skirted stripGourvenec & Barnett (2011)

kD/sum = 20(Mana et al., 2010)

UB (Martin,2001)

LB (Martin, 2001)

Fig. 8. Uniaxial vertical capacity of the hybrid foundation on non-uniform soilcompared with (a) skirted mats and (b) bucket foundations.

Table 3Bearing capacity factors under ultimate uniaxial loading for uniform soil.

Foundation NcV NcH NcM

This study: HF 7–4 7.38 2.56 0.90This study: HF 9.2–4 7.38 2.72 0.90This study: HF 7–6 7.41 3.49 0.90This study: HF 9.2–6 7.50 3.79 0.90This study: HF 7–8 7.55 4.27 0.93This study: HF 9.2–8 7.73 4.53 0.99This study: SM d/D = 0.125 7.37 2.13 0.91This study: SM d/D = 0.25 8.54 3.28 1.15This study: SM d/D = 0.375 9.55 4.56 1.57This study: SM d/D = 0.5 10.46 5.91 2.19

Bucket d/D = 4/16 = 0.25 8.55a N/A N/ABucket d/D = 6/16 = 0.375 9.55a N/A N/ABucket d/D = 8/16 = 0.5 10.40a N/A N/A

a The values for the skirted mat/bucket foundations were taken from Mana et al.[14] and interpolated where required.

Table 4Bearing capacity factors under ultimate uniaxial loading for LOC soil.

Foundation NcV NcH NcM

HF 7–4 17.63 (10.37) 4.56 (2.68) 2.16 (1.27)HF 9.2–4 18.09 (10.64) 5.02 (2.95) 2.15 (1.26)HF 7–6 19.23 (11.31) 7.04 (4.14) 2.18 (1.28)HF 9.2–6 20.69 (12.17) 8.03 (4.72) 2.24 (1.32)HF 7–8 21.58 (12.70) 8.92 (5.25) 2.53 (1.49)HF 9.2–8 24.14 (14.20) 10.00 (5.88) 2.79 (1.64)

SM d/D = 0.125 17.01 (10.00) 3.28 (1.93) 1.93 (1.13)SM d/D = 0.25 27.63 (11.51) 7.45 (3.10) 3.51 (1.46)SM d/D = 0.375 33.26 (10.73) 10.41 (3.36) 4.45 (1.44)SM d/D = 0.5 41.93 (11.03) 15.50 (4.08) 6.99 (1.84)

The values provided refer to the selected RP location (at the mudline), whereas thevalues in brackets refer to the mat skirt tip level.

132 B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139

referring to the internal skirt tips for comparison with relevantbucket foundations.

It is evident from Fig. 6 that the addition of internal skirts re-sults in an increase of the vertical bearing capacity factor in soilswith strength increasing with depth (LOC and NC soils). However,the shear strength heterogeneity determines whether the capacity

increase is more pronounced with increasing skirt length (as in theNC profile, Fig. 6) or with increasing caisson diameter (as in theLOC profile, Fig. 6).

Importantly, the results of complementary centrifuge experi-ments confirm the capacity established through numerical resultsin this study (Fig. 6), both qualitatively and quantitatively, whichprovides additional confidence. Though the undrained soil shearstrength in the experiments was slightly softer (su = 1.8 + 0.92z)than the target profile adopted in the numerical study(su = 3 + 1.05z), the influence on the failure mechanism was evi-dently insignificant as the normalised results, the bearing capacityfactors, are similar.

The following observations comparing the bearing capacity fac-tors obtained for the hybrid foundation geometries with publishedsolutions for simple skirted foundations are made:

� Under uniaxial vertical loading on soil of uniform strength, thehybrid foundation essentially behaves like a skirted mat sincethe capacity is not significantly increased by the internal skirts(Figs. 6 and 7). The maximum difference in NcV (for HF 9.2–8)compared to the skirted mat is 5%. This is due to the volumeof soil sheared. The failure mechanism under undrained verticalloading on soil with uniform strength for the hybrid founda-tions HF 9.2–8 and HF9.2–4 is shown in Fig. 9a and b respec-tively. It is compared with the failure mechanism of a skirtedmat with an aspect ratio of d/D = 0.125 (i.e. the hybrid founda-tion without internal skirts), presented in Fig. 9c. Unless theinternal skirts extend beyond the rigid cone that forms under-neath the skirted mat, the failure mechanism remains the sameas that mobilised by the skirted mat alone, and so does the bear-ing capacity factor. This is the case for the hybrid foundation HF9.2–4. As the skirt intercepts the initial rigid cone (HF 9.2–8,Fig. 9a), the failure mechanism extends slightly in both the hor-izontal and vertical directions, resulting in an increase in thebearing capacity factor. This increase is 5% for the hybrid foun-dation HF 9.2–8, but would be higher for longer internal skirts.

� In non-uniform soil, however, the addition of internal skirts sig-nificantly increases the vertical capacity of the hybrid founda-tion compared to the skirted mat (by up to 42%, Fig. 8a, Table4), with results from this study for hybrid foundation geome-tries with larger internal caisson compartments (HF 9.2–6, HF7–8 and HF 9.2–8) being higher than the relevant upper boundsolution of Martin [15] for skirted mats and bucket foundations.The reason for the significant increase in capacity is revealedwhen examining the respective failure mechanism. The failuremechanism of a hybrid foundation (HF 9.2–8) is shown inFig. 10a, that of a skirted mat with an aspect ratio ofd/D = 0.125 without internal skirts in Fig. 10b. The internalskirts of the hybrid foundation change both the shape,increasing the volume of soil sheared, and extent of the failure

(a)

0.05D

0.005D

0.01D

(b)

0.05D

0.005D

0.01D

(c)

0.05D

0.015D

0.135D

HF9.2-8 HF9.2-8

HF9.2-4

Skirted mat

Fig. 9. Failure mechanism (displacement contours at intervals of 0.5% of Dm) under uniaxial vertical load on soil with uniform strength of (a) a hybrid foundation with a largecaisson compartment (HF 9.2–8) compared with (b) a hybrid foundation with a shorter caisson compartment (HF 9.2–4) and (c) a skirted foundation alone.

B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139 133

mechanism (Fig. 10a), forcing it deeper, compared to that of askirted mat (Fig. 10b). Since the undrained soil shear strengthincreases with depth, the hybrid foundation at failure shearsstronger soil, resulting in significantly increased foundationcapacity. As the majority of the vertical capacity is mobilisedthrough end bearing rather than skin friction, however, thehybrid foundation does not mobilise the same resistance as abucket foundation of the same overall size (Fig. 8b, with thehybrid foundation results falling below the lower bound solu-tion for a circular bucket foundation of [15].

The target application areas of the hybrid foundation concepttypically have soil with increasing shear strength with depth (withthe shear strength gradient typically around 1 kPa/m). Based onthe evidence from this study, the vertical capacity of the hybridfoundation is significantly higher than that of a mat alone. Theinfluence of undrained shear strength at the mudline is shown inFig. 6. The chosen normalisation affects the appearance of the re-sults. When normalised by the undrained shear strength at the

mat skirt tip, the resulting bearing capacity factor in an NCstrength profile lies above that in a LOC strength profile with high-er mudline strength. In other words, given the relatively short matskirts and the modest shear strength gradient, the ultimate verticalcapacity is normalised by a significantly lower shear strength inthe case of the NC profile, resulting in a high bearing capacity fac-tor. This influence diminishes at depth, such that the values calcu-lated at the level of the internal skirts are slightly lower in the NCprofile.

3.1.2. Uniaxial horizontal capacity Hult

The horizontal bearing capacity factors for the different hybridfoundation geometries are shown in Fig. 11, including lines indi-cating the trends in uniform, LOC and NC soils. Figs. 12 and 13compare the results for the hybrid foundation with skirted mator bucket foundations as well as solutions for skirted strip founda-tions from the literature. In Fig. 13a the hybrid foundation capaci-ties are normalised by the undrained shear strength at the matskirt tip level, whereas in Fig. 13b the reference shear strength is

(a)

(b)

0.015D

0.06D

0.005D

0.01D

0.06D

HF9.2-8

Skirted mat

Fig. 10. Failure mechanism (displacement contours at intervals of 0.5% of Dm) under uniaxial vertical load on LOC soil of (a) a hybrid foundation (HF 9.2–8) compared with (b)a skirted foundation alone.

0

1

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8 1

NcH

= H

ult/(

Am

s u,m

at s

kirt

tip)

dc/Dm

HF 7-4

HF 7-6

HF 7-8

HF 9.2-4

HF 9.2-6

HF 9.2-8

SM

Exp., SM

Exp., HF 7-4

Exp., HF 7-8

Exp., HF 9.2-4

Exp., HF 9.2-8Uniform su

LOC soil

NC soil

Fig. 11. Horizontal bearing capacity factor of the hybrid foundation (normalised bysu at mat skirt tips).

0

1

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8 1

NcH

= H

ult/(

Am

s u)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

Uniform su

Skirted stripGourvenec & Barnett (2011)

aspect ratio of overall bucket foundationaspect ratio of skirted mat

Fig. 12. Horizontal bearing capacity factor of the hybrid foundation on uniform soilin comparison with published solutions for simple skirted foundations.

(a)

0

1

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8 1

NcH

= H

ult/(

Am

s u,m

at s

kirt

tip)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

LOC soil

Skirted strip

Gourvenec & Barnett (2011)

Bransby & Yun (2009)

(b)

0

1

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8 1

NcH

= H

ult/(

Am

s u,in

tern

al s

kirt

tip)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

LOC soil

Skirted strip

aspect ratio of overall bucket foundation

Gourvenec & Barnett (2011)

Bransby & Yun (2009)

Fig. 13. Uniaxial horizontal capacity of the hybrid foundation on LOC soil comparedwith (a) skirted mats and (b) bucket foundations.

134 B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139

(a)

(b)

0.005D

0.01D

0.005D

HF9.2-8

Skirted mat

Fig. 14. Failure mechanism (displacement contours at intervals of 0.5% of Dm) under uniaxial horizontal load on LOC soil of (a) a hybrid foundation (HF 9.2–8) compared with(b) a skirted foundation.

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1

NcM

= M

ult/(

Am

Dm

s u,m

at s

kirt

tip)

dc/Dm

HF 7-8HF 7-4HF 7-6HF 9.2-8HF 9.2-6HF 9.2-4SM

LOC soil

Uniform su

NC soil

Fig. 15. Moment bearing capacity factor of the hybrid foundation (normalised by su

at mat skirt tips).

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1

NcM

= M

ult/(

Am

Dm

s u)

d/D, dc/Dm, d/B

HF 7-4HF 7-8HF 7-6HF 9.2-8HF 9.2-4HF 9.2-6SM/bucket this study

Uniform su

Skirted stripGourvenec & Barnett (2011)

aspect ratio of overall bucket foundationaspect ratio of skirted mat

Fig. 16. Moment bearing capacity factor of the hybrid foundation on uniform soil incomparison with published solutions for simple skirted foundations.

(a)

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1

NcM

= M

ult/(

Am

Dm

s u,m

at s

kirt

tip)

d/D, dc/Dm, d/B

HF 7-4HF 7-6HF 7-8HF 9.2-4HF 9.2-6HF 9.2-8SM/bucket this study

LOC soil

Skirted strip

Gourvenec & Barnett (2011)

Bransby & Yun (2009)

(b)

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1

NcM

= M

ult/(

Am

Dm

s u,in

tern

al s

kirt

tip)

d/D, dc/Dm, d/B

HF 7-8HF 7-6HF 9.2-8HF 9.2-6HF 9.2-4HF 7-4SM/bucket this study

LOC soil

Skirted strip

aspect ratio of overall bucket foundation

Gourvenec & Barnett (2011)

Bransby & Yun (2009)

Fig. 17. Uniaxial moment capacity of the hybrid foundation on LOC soil comparedwith (a) skirted mats and (b) bucket foundations.

B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139 135

taken at the internal skirt tip level. The published solutions plottedfor skirted strip foundations are for a shear strength gradientk = 1.2 kPa [3], and for uniform soil and kD/sum relevant to thenon-uniform profile considered here [10].

While an increase in diameter of the internal compartment re-sults in increased horizontal capacity, the influence of the length of

the internal skirts is far greater (within the range investigatedhere), as Fig. 11 illustrates. Inclusion of the internal skirts of HF9.2–8 results in 113% additional capacity compared to the skirtedmat alone, in uniform soil. The soil strength profile influences therate of capacity increase (both sum and k), though the sensitivityis reasonable and the geometry of the internal hybrid foundationcompartment is the more dominant influence.

136 B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139

Comments are as follows:

� Considerable increases in capacity (of up to 113% in uniformand 205% in LOC soil) are observed, compared to the skirtedmat alone (Figs. 11, 12, 13a, Tables 3 and 4), though in uniformsoil, a bucket foundation with the same overall dimensions asthe hybrid footing offers even higher capacity (albeit at the costof increased steel volume). The absolute capacity of the hybridfoundation is higher than that of the corresponding bucketfoundation, but Fig. 13a and b indicates that in LOC soil the ref-erence level chosen to normalise the results affects the compar-ison. The influence of shape factor becomes obvious whencomparing the results for the circular foundation geometriesto strip bucket foundations of the same external aspect ratio(Fig. 12 for uniform soil, Fig. 13 a and b for LOC soil) Any changeof the internal geometry results in differences in the availablehorizontal capacity, even in uniform soil, due to changes inthe failure mechanism (as illustrated in Fig. 14) and thus thevolume (and strength where non-uniform) of the soil that issheared. A wider internal compartment results in a larger baseshear component as well as larger active and passive compo-nents. The skirt length determines the strength of the soil thatis sheared at the foundation base. It also influences the size ofthe active and passive components of the mechanism. There-fore, in the more realistic case of shear strength increasing withdepth, for the same volume of steel the hybrid foundation offersthe advantage of increased capacity through mobilisation ofstronger soil at larger depths.

� The experimental results in the horizontal direction are signifi-cantly softer than the numerical results in LOC soil (apart fromExp. HF 9.2–8, Fig. 11). This is likely due to difficulties in apply-ing a purely horizontal load in the drum centrifuge. While in thenumerical study evaluation of Hult, with M = 0, was possible, theexperimental set-up did not allow application of a path that

(a)

(c)

HF9.2-8

0.005D0.045D

0.005D

Skirted mat

Fig. 18. Failure mechanism (displacement contours at intervals of 0.5% of Dm) under uniauniform soil, compared with (c) a skirted foundation on uniform soil, (d) on LOC soil.

results in zero moment. The horizontal load measured in thetests, therefore, is reduced by the moment load applied on thefoundation.

In the case of the hybrid foundations with the longest internalskirts considered here, HF 7–8 and HF 9.2–8, at failure differentialsoil displacements are observed within the internal skirted com-partment (see Fig. 14a), the soil does not move as a rigid body. Thisis similar to a skirted mat with low aspect ratio (and/or soil with ahigh degree of strength heterogeneity) (Fig. 12 as well as [3]), suchthat consideration of a solid embedded foundation would result inover-prediction of the available capacity.

3.1.3. Uniaxial moment capacity Mult

The results obtained for the hybrid foundation in terms of bear-ing capacity factor under uniaxial moment loading, are summa-rised in Fig. 15. Fig. 16 shows the results for the hybridfoundation on uniform soil compared with simple skirted stripand circular foundations, while Fig. 17 compares the results onLOC soil with solutions for skirted strip and circular mats inFig. 17a and bucket foundations in Fig. 17b. Tables 3 (for uniformsoil strength) and 4 (LOC soil) summarise the bearing capacity fac-tors. Comments are as follows:

� The general trends (qualitatively and quantitatively) of thehybrid foundation results under uniaxial moment loading aresimilar to those under uniaxial vertical loading.

� Similar to uniaxial vertical loading on uniformly strong soil, themoment capacity remains unchanged by insertion of relativelyshort internal skirts as the failure mechanism is unchanged.Addition of longer skirts results in an increase in capacity ofup to 10% for the geometries considered here (Fig. 15, Table 3).In uniform soil, the hybrid foundations offer similar momentcapacity as a skirted strip of d/D (or B for breadth) of about

(b)

(d)

0.005D 0.045D

HF9.2-8

0.005D

Skirted mat

xial moment load of (a) a hybrid foundation (HF 9.2–8) on uniform soil, (b) on non-

(a)

0

2

4

6

8

10

12

V/(Asu0)

H/(A

s u0)

HF 7-4 HF 9.2-4

HF 7-6 HF 9.2-6

HF 7/8 HF 9.2-8

Uniform su

LOC soil

(b)

0

0.5

1

1.5

2

2.5

3

V/(Asu0)

M/(A

Ds u

0)

HF 7/4 HF 9.2-4

HF 7-6 HF 9.2-6

HF 7-8 HF 9.2-8

Uniform su

LOC soil

(c)

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30

0 5 10 15 20 25 30

-15 -10 -5 0 5 10 15

M/(A

Ds u

0)

H/(Asu0)

HF 7-4 HF 7-6

HF 7-8 HF 9.2-4

HF 9.2-6 HF 9.2-8

Uniform su

LOC soil

Fig. 19. Normalised capacity envelopes in (a) the VH (M = 0) plane, (b) the VM(H = 0) plane and (c) the HM (V = 0) plane.

B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139 137

0.25 (Fig. 16), though results should not be extrapolated beyondthis study.

� Increasing shear strength with depth results in significantincreases in moment capacity. For the LOC strength profile con-sidered in this study, the increase in moment capacity of hybridfoundation HF 9.2–8 compared to a skirted mat alone is 45%(Figs. 15, 17a). In non-uniform soil, bucket foundations of thesame overall aspect ratio were found to only result in slightlyhigher moment capacity than the circular hybrid foundations.The moment capacity of the circular geometries was generallyfound to be higher than that of comparable skirted strip founda-tions (Fig. 17a and b).

Analysis of the failure mechanisms provides insight into theresulting capacity. Fig. 18 illustrates the differences and similari-ties. In uniform soil, the soil displacements at failure form an al-most circular mechanism which governs the ultimate momentcapacity of a skirted mat (Fig. 18c). The internal skirts of the hybridfoundation, even for HF 9.2–8, do not extend this mechanism con-siderably, resulting in the modest increase in capacity. In LOC soil,however, the mechanism of the skirted mat is compressed to ashallow spline (Fig. 18d), shearing the softer soil closer to the mud-line. In the case of the hybrid foundation (Fig. 18a and b), themechanism passes through the internal and external skirt tips,such that the hybrid foundation geometry (in particular the inter-nal skirt length) significantly influences the moment capacity.Internal skirts that are longer than those considered in this studyare therefore expected to extend the mechanism and thus increasecapacity. Unlike the upper bound mechanisms considered by Bran-sby and Yun [3], the failure mechanism does not continue in itsshape to the soil surface. Instead, it is truncated at the mat skirts,from where the incremental plastic shear strains follow theskirt–soil interface to the mudline. This is also evident in the finiteelement results of Bransby and Yun [3] for skirted strip founda-tions. Even for a rough foundation–soil interface, such as consid-ered here, less work is required in shearing along this interfacecompared to the mechanism continuing to extend through the soil,as the length of the shear plane is shortest.

3.2. Combined loading – VHM capacity

The envelopes that describe the available capacity (at the refer-ence point location at the mudline) under VHM combined loadingfor the different hybrid foundations are shown in Fig. 19 as sec-tions in the VH plane (M = 0), the VM plane (H = 0) and the HMplane (V = 0). The capacity increases discussed above due tochanges in the hybrid foundation geometry as well as the influenceof the undrained shear strength profile are evident.

The interaction of the vertical and moment degrees-of-freedom(Fig. 19b) is slightly stronger than that in the VH plane (Fig. 19a),i.e. the moment capacity reduces more quickly with increasing ver-tical load than the horizontal capacity does. However, the soilstrength profile influences the shape of the envelope slightly morein the VH plane, with LOC strength leading to slightly less interac-tion between these two degrees-of-freedom (i.e. a slightly moreconvex envelope shape, Fig. 19a). In the VM plane, the soil strengthprofile does not lead to any systematic differences in the envelopeshape (Fig. 19b). Overall though, the differences both in the VH andVM planes are small for the different hybrid foundation geometriesas illustrated through normalisation by the respective ultimateuniaxial loads in Fig. 20. Therefore, no significant changes are ex-pected for a normally consolidated soil profile.

The envelope can be conservatively approximated (as shown inFig. 20a and b) with the expressions provided in Eqs. (2) and (3),respectively. The degree of interaction influences the exponentassociated to the horizontal and moment load components.

jHjHult

� �1:4

þ VVult

� �2

� 1 ¼ 0 ð2Þ

jMjMult

� �1:5

þ VVult

� �2

� 1 ¼ 0 ð3Þ

These expressions were selected for their simplicity. A closermatch with the envelopes may be achieved through the introduc-tion of additional shaping factors.

The envelope shape in the HM plane is complex and changeswith the geometry of the hybrid foundation as well as the soilshear strength profile, as Fig. 19c illustrates. The asymmetry ofthe envelope for any one hybrid foundation geometry can be re-duced, but not eliminated, by choosing a different RP location. Inother words, the foundation behaves differently if the horizontal

(a)

0

0.2

0.4

0.6

0.8

1

1.2

V/Vult

H/H

ult

HF 7-4 HF 9.2-4

HF 7-6 HF 9.2-6

HF 7/8 HF 9.2-8

Eqn 1

Uniform su

LOC soil

(b)

0

0.2

0.4

0.6

0.8

1

1.2

V/Vult

M/M

ult

HF 7/4 HF 9.2-4

HF 7-6 HF 9.2-6

HF 7-8 HF 9.2-8

Eqn 2

Uniform su

LOC soil

(c)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2

0 0.2 0.4 0.6 0.8 1 1.2

-1.5 -1 -0.5 0 0.5 1 1.5

M/M

ult

H/Hult

HF 7-4 HF 7-6HF 7-8 HF 9.2-4HF 9.2-6 HF 9.2-8Eqn. 4

Uniform su

LOC soil

Fig. 20. Capacity envelopes in (a) the VH (M = 0) plane, (b) the VM (H = 0) plane and(c) the HM (V = 0) plane in terms of ultimate uniaxial capacity.

138 B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139

and moment loading act in the same direction or in opposition dueto differences in the soil displacements at failure. For lower aspectratios (dc/Dm), the maximum HM capacity occurs when horizontalload and moment act in the same direction. For higher aspect ra-tios, however, the maximum capacity is associated with opposinghorizontal load and moment. These observations apply to the cho-sen RP location. As the governing mechanism is determined by thecomposition of the external mat and the internal skirts, the HMcapacity envelopes of the different hybrid foundation geometriescannot be unified through the choice of any RP location.

Eq. (4), which is obtained through combination of Eqs. (2) and(3), describes the coupled VHM capacity of the hybrid foundationsinvestigated here. Similar expressions have been proposed for cir-cular surface footings on homogeneous soil [22], spudcan footings[16,23] and anchors [19,8].

jHjHult

� �1:4

þ jMjMult

� �1:5

þ VVult

� �2

� 1 ¼ 0 ð4Þ

The interaction of horizontal and moment capacity Eq. (4) de-scribes for V = 0 is shown in Fig. 20c in comparison with the enve-lopes obtained from the finite element analyses, indicating agenerally conservative fit. The approximating expression proposedby Gourvenec and Barnett [10] for skirted strip footings, which as aside note is not defined for negative values of H or M due to the ab-sence of the absolute sign, is indistinguishable from the simpler fitproposed here. Note that cross-coupling of the horizontal and mo-ment degrees-of-freedom is not considered. This could be intro-duced. An additional parameter that varies the eccentricity in theHM plane for the different hybrid foundation geometries wouldbe required.

3.3. Comment on hybrid foundation geometry

This numerical study considered a circular foundation geome-try. This shape may be applicable to multi-footing systems or foun-dations with a low torsional load component. Applications withhigh horizontal and torsional load components require a differentdesign of the hybrid foundation, with a rectangular mat and possi-bly a larger number of internal skirts. Further, while the verticaland horizontal response is not expected to change significantly,inclusion of several internal skirted compartments (Fig. 1) may al-ter the behaviour under moment loading to a push–pullmechanism.

4. Conclusions

The foundations of a number of offshore installations requirehigh horizontal and moment load capacity, which acts in combina-tion with vertical loading. However, skirted mats, which are oftenused for subsea applications, for instance, do not provide sufficienthorizontal and moment capacity such that innovative solutions arerequired. This study investigated the capacity under undrainedvertical, horizontal and moment (VHM) loading of a skirted foun-dation with additional internal skirts which exceed the length ofthe external skirts. This geometry results in considerable changesin the soil failure mechanisms compared to a simple skirted mat,which importantly leads to large increases in particular in horizon-tal capacity (with key influences for increased moment capacityidentified as well). This is significant as it indicates the potentialof this foundation concept to be an economical foundation alterna-tive, especially compared with a large bucket foundation. For thesame steel volume, the smaller diameter internal skirts of the hy-brid foundation reach stronger soil at larger depths, which resultsin increased capacity.

The influence of variations in the geometry of the internal skirts(length and diameter) as well as the soil strength profile wereinvestigated and discussed. Further, the interaction of the loadingdirections was analysed and conservative expressions for the esti-mation of combined capacity of the hybrid foundation system wereproposed.

Acknowledgements

This project forms part of ongoing collaborations betweenKeppel Offshore & Marine Technology Centre and the Centre forOffshore Foundation Systems. Support through the AustralianResearch Council’s (ARC) Linkage Program LP0989433 as well asKeppel’s contribution and permission to report these results aregratefully acknowledged. The work described here forms part ofthe activities of the Centre for Offshore Foundation Systems(COFS), the ARC Centre of Excellence in Geotechnical Science and

B. Bienen et al. / Computers and Geotechnics 45 (2012) 127–139 139

Engineering and The Lloyd’s Register Educational Trust Chair andCentre of Excellence in Offshore Foundations. The first and thirdauthors acknowledge the support of the ARC through theirAustralian Postdoctoral (DP110101603) and Future Fellowships(FT0990301), respectively.

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