Criterion for Failure Modes of Suction Caisson

in Sand under Axial Pullout Load

Jinfu Xiaojxiao3@ncsu.edu

Department of Civil, Construction and Environmental

Engineering, North Carolina State University

Doctoral Preliminary Oral Presentation

Failure Modes

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

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

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

Criterion

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.

Terminology in the Criterion

Terminology in the Criterion

For more details please refer to the author’s publications.

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)

Case Study

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.

Failure modes for the model tests determined by criterion

Failure modes for the model tests determined by criterion

Failure modes for the model tests determined by criterion

Failure modes for the model tests determined by criterion

Failure modes for the model tests determined by criterion

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.

Thank You!