This document presents a novel method for estimating the axial pullout capacity of suction caissons in sand under flow failure mode. Current methods assume the soil is in a solid state during pullout, but this can overlook the occurrence of sand flow. The proposed method uses analytical and numerical modeling of Bingham plastic flow in the soil to better account for the effects of shear strain rate on pullout capacity. Results show improved agreement with experimental data compared to existing predictions that ignore sand flow. This new approach provides a more accurate estimation of axial pullout capacity for suction caissons failing in the sand flow mode.

1. A Novel Estimation of Axial Pullout Capacity of
Suction Caisson in Sand Flow Failure Mode
Jinfu Xiao
jxiao3@ncsu.edu
Department of Civil, Construction and Environmental
Engineering, North Carolina State University
Doctoral Preliminary Oral Presentation

2. 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)
orE G    
Soil in solid state
Soil-caisson
interface
 tann   

3. In the sand flow zone, the constitutive model of sand flow is
Where 𝜏 is shear stress, G is shear modulus, 𝛾 is shear strain, 𝛾 is the shear strain rate,
𝜏0 is the yield stress, 𝜇 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
Core
Soil
Soil
Flow
Soil
Flow
Soil at Rest
0
0 0
for
for
G  

   

 
 

4. 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

5. Methodology I
Analytical solution for Bingham plastic flow in straight annular domain

6. Methodology II
Numerical simulation in multi-physics program COMSOL
Axisymmetric model

7. Pressure and Velocity Profile from Numerical Analysis

8. Numerical Results vs. Analytical Solution
     
       
       
2 2 2
0 1
2 2
0 2 2 2 1 2
2 2
0 2
1
ln
2
1
ln 1 1
2
1
ln 1 1 1
2
o
z
r
C r B r r r r
U r r B r r r r r
r r B r r r r
  

  
       
 

      


     

1
0
z
rz rz
z
z
rz
dUB
B
dU dz dr
dU
B
dr
 

  
       

 
Analytical solution
Numerical data

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

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

11. 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.

12. Thank You!