A Novel Estimation of Axial Pullout Capacity of

Suction Caisson in Sand Flow Failure Mode

Jinfu Xiaojxiao3@ncsu.edu

Department of Civil, Construction and Environmental Engineering, North Carolina State University

Doctoral Preliminary Oral Presentation

Pullout Capacity Components:• Caisson weight/ Caisson + Core soil weight• Internal wall friction • External wall friction• Reverse end bearing capacity

The actual pullout capacity is the combinations of the components depending on the failure mode. However, in current study, the soil in all the failure modes is assumed to be in solid state and the pullout capacity is independent to the pullout rate!

Current Estimation Methods and Failure Modes (Randolph and Gourvenec 2011)

or E G Soil in solid state

Soil-caissoninterface

tann

In the sand flow zone, the constitutive model of sand flow is

Where is shear stress, G is shear modulus, is shear strain, is the shear strain rate, is the yield stress, is viscosity of sand flow

For sand flow, the shear stress is dependent to the shear strain rate, consequently the pullout capacity is dependent to the pullout rate!

Estimation Considering Sand Flow Occurrence

I

II III I – Suction caisson

II – Core soil plug

III – Sand flow zone

IV – Sand at rest

IV

CoreSoil

SoilFlow

SoilFlow

Soil at Rest

0

0 0

for

for

G

Model test - Failure wedge in soft marine clay (Rao 1996)

Field test - Failure surface under cyclic eccentric load in clay (Dyvik et al. 1993)

Model test - Failure surface under axial load in cemented siliceous sand (Chen 2013)

The sand flow occurrence was observed in literature but mistaken for solid failure wedge of soil

I

II III I – Suction caisson

II – Core soil plug

III – Sand flow zone

IV – Sand at rest

IV

Methodology I

Analytical solution for Bingham plastic flow in straight annular domain

Methodology II

Numerical simulation in multi-physics program COMSOL Axisymmetric model

Pressure and Velocity Profile from Numerical Analysis

Numerical Results vs. Analytical Solution

2 2 20 1

2 20 2 2 2 1 2

2 20 2

1ln

2

1ln 1 1

21

ln 1 1 12

o

z

rC r B r r r r

U r r B r r r r r

r r B r r r r

1

0

zrz rz

z

zrz

dUBB

dU dz dr

dUB

dr

Analytical solution

Numerical data

Analytical Solution Vs. Experimental data (Kelly et al. 2006) & Houlsby’s prediction (Houlsby et al. 2005)

Analytical Solution Vs. Experimental data (Kelly et al. 2006) & Houlsby’s prediction (Houlsby et al. 2005)

Summary

The method proposed here makes a better estimation of axial pullout capacity of suction caisson in the sand flow failure mode*.

*This statement is ONLY for sand flow failure mode. In other failure modes, the soil can be still in solid state. In that case, the conventional method is still valid. The criterion for determination of failure modes of suction caisson in sand under axial pullout load is discussed in another section.

Thank You!