The document describes three failure modes for suction caissons (structures used to anchor offshore foundations) under axial pullout loads in sand: Failure Mode I involves only the pore water lifting, with soil remaining in place. Pullout capacity decreases as internal and external skirt wall friction reduces. Failure Mode II involves the core soil plug moving upward with the caisson, generating soil flow to fill the base. Pullout capacity includes caisson weight, external friction, and suction. Failure Mode III involves the core soil plug acting as a pile, but soil flow is insufficient to fill the base, causing cavities. Pullout capacity includes caisson and soil weights, external friction, and suction.

1. Criterion for Failure Modes of Suction Caisson
in Sand under Axial Pullout Load
Jinfu Xiao
jxiao3@ncsu.edu
Department of Civil, Construction and Environmental
Engineering, North Carolina State University
Doctoral Preliminary Oral Presentation

2. Failure Modes

3. In Failure Mode I the soil remains in position during the pullout process of suction caisson, only
the pore water is lifted to fill the induced space beneath the caisson lid as illustrated in Figure (a).
The pullout capacity is composed of the caisson’s weight, internal skirt wall friction, external skirt
wall friction, suction beneath the lid and the reverse end bearing resistance of the annular caisson’s
tip. The internal and external skirt wall friction takes the main contribution to the pullout capacity,
so generally the pullout capacity decreases during the pullout process as illustrated in Figure (b).
Failure Mode I

4. In Failure Mode II the core soil-plug moves upward with caisson during the pullout process of
suction caisson, the soil flow around the caisson is generated to fill the induced space at the
caisson’s base as illustrated in Figure (a).
The total pullout capacity is composed of the total weight of caisson and core soil, the external
skirt wall friction due to soil flow and the suction at caisson’s base as illustrated in Figure (b).
Failure Mode II

5. In Failure Mode III the core soil-plug inside the caisson moves upward with caisson during the
pullout process, the caisson and core soil-plug act together as a pile, the velocity of soil flow
generated around the caisson is smaller than the pullout rate of suction caisson, so the soil flow is
not enough to fill the induced space at the caisson’s base and cavity occurs at the caisson’s base
due to the soil tension failure, as illustrated in Figure (a).
The pullout capacity is composed of the total weight of caisson and core soil, the external skirt wall
friction and the suction at caisson’s base as illustrated in Figure (b).
Failure Mode III

6. Criterion

7. Criterion for Failure Modes
The failure modes are complex influenced by multiple factors, such as pullout rate,
hydraulic conductivity, length of drainage path, shear strength properties of soil.
With other parameters being constant, the pullout rate increases during the
transition from Failure Mode I to Failure Mode III.

8. Terminology in the Criterion

9. Terminology in the Criterion
For more details please refer to the author’s publications.

10. Failure Mode I, II & III
Failure Mode I, II & III
Failure Mode II & IIIFailure Mode I
Failure Mode II Failure Mode III
if Vpullout > Vmax
if Vpullout > Vliq
if Vpullout < Vmax
if Vpullout < Vliq
if Vpullout < Umax if Vpullout > Umax
Step 1a
Step 1b
Step 1
Step 2
Procedures of judging failure modes
(Note: Vmax > Vliq is always and unconditionally true)

11. Case Study

12. In the case study the criterion proposed by in this paper is verified by the experimental data
from five model tests (test 9, test 10, test 11, test 16 and test 23) performed by Houlsby et al
(2005). In the model tests, the dimension of the caisson model is 280 mm in diameter and
180 mm in skirt length (Kelly et al. 2006). The Redhill sand and HPF5 sand were used as
test soil samples.

13. Failure modes for the model tests determined by criterion

14. Failure modes for the model tests determined by criterion

15. Failure modes for the model tests determined by criterion

16. Failure modes for the model tests determined by criterion

17. Failure modes for the model tests determined by criterion

18. Summary
The criterion for failure modes of suction caisson in
sand under axial pullout load has been verified by the
experimental data (Houlsby et al 2005).
The failure modes determined by the criterion show
good agreement with the pullout capacity curve
pattern from experimental data.

19. Thank You!