Calculation Method for Uplift Capacity of Suction Caisson in Sand Considering Different Drainage Conditions

Round 1

Reviewer 1 Report

The paper is written and presented well with proper explanation. There is some minor grammatical mistakes in writing which needs to be corrected. 

Following minor correction is required:

1. Line 128, "The centrifuge tests are divided into drained uplift with very slow pullout rate".....author needs to give the value of slow pullout rate.

2. Line 200....author need to define h1 used in the Eqn 2.

3. Line 236.....The differential pressure expressions of Eq. (5) and Eq. (6) are different.....Is Eqn. 5 representing differential pressure equation?

Author Response

General comments: The paper is written and presented well with proper explanation. There are some minor grammatical mistakes in writing which needs to be corrected. 

R: Thank you very much for your opportunity to our works.

Following minor correction is required:

Q1: Line 128, "The centrifuge tests are divided into drained uplift with very slow pullout rate".....author needs to give the value of slow pullout rate

R1: Thank you very much for your comment. The centrifuge tests are divided into drained uplift (the air hole of caisson lid is opened) with the pullout velocity of 0.1 mm/s and partially drained uplift (the air hole of caisson lid is closed) with the pullout velocity of 0.1 mm/s and 0.5 mm/s.

Q2: Line 200....author need to define h1 used in the Eqn 2.

R2: Thank you very much for your comment. Eq. 2 represents the seepage length is a function of aspect ratio (L/D) [1-2]. Some researchers have proposed a specific expression for the seepage length, but There are differences in their results. Eq.2 is used to illustrate that the differential pressure is related to aspect ratio, and the specific expression of seepage length is not used in the following analysis. Therefore, Eq. 2 is given as , and “h” represents the functional relationship between normalized seepage length and aspect ratio .

[1] Mana, Divya S. K., Susan Gourvenec, and Mark F. Randolph. 2014. Numerical Modelling of Seepage beneath Skirted Foun-dations Subjected to Vertical Uplift. Computers and Geotechnics 55: 150–157.

[2] Senders, M., 2008. Suction Caissons in Sand as Tripod Foundations for Offshore Wind Turbines. The University of Western Australia. Ph.D. thesis.

Reviewer 2 Report

I have reviewed the manuscript titled Calculation method for uplift capacity of suction caisson in sand ..... ‘.
The manuscript is well written with good literature review covering the past studies. In my opinion, the research would be of interest to the readers of the Journal of Sustainability.
I have the following queries and sugesstions regarding the study.

1- It woud be better add a sensivity analysis for finite element analysis on the models.Mesh size and load ( or displacement) steps effects should be considered.

2- Explain  the soil dialation tendency effects on the uplift capacity of suction caisson foundations.

Author Response

General comments: The manuscript is well written with good literature review covering the past studies. In my opinion, the research would be of interest to the readers of the Journal of Sustainability.

R: Thank you very much for your opportunity to our works.

Q1: It would be better add a sensitivity analysis for finite element analysis on the models. Mesh size and load (or displacement) steps effects should be considered

R1: Thank you very much for your comment. The mesh width near the skirt reaches 0.05 m, which is fine enough verified by the mesh sensitivity study. This part of the content is also supplemented in the paper.

Q2. Explain the soil dilation tendency effects on the uplift capacity of suction caisson foundations.

R2: Thank you very much for your comment. The skirt wall friction is calculated by tand ( is the interface normal stress of soil-skirt). Under the constraint of surrounding soil, the soil dilatancy caused by interface shear stress generates an increase in the normal effective stress [1], causing the enlargement of skirt wall friction. Under fully drained condition, the uplift capacity of caisson is mainly borne by skirt wall friction. Therefore, the soil dilation effects enlarge the uplift capacity of caisson.

[1] White DJ, Bolton MD., 2004. Displacement and strain paths during plane-strain model pile installation in sand. Geotechnique, 54(6):375–97.

Reviewer 3 Report

Comments:
In this manuscript, the classic SANISAND model is adopted to study the uplift responses of suction
caisson. A refined calculation method of uplift capacity is proposed based on simulation results and
validated by centrifuge tests. The manuscript conducts interesting investigations through rigorous
approach. The manuscript is presented in a very excellent form. Valuable results and conclusions
are obtained. The following comments are expected to be addressed:
(a) Line 97: Please add the adopted values of the important parameter, i.e., u used in the present
master-slave contacts. Also, please explain the process regarding the determination of this
parameter.
(b) Line 103: “The radius and the depth of soil domain are 2.5D and 5L respectively, which can
eliminate the boundary” Please supplement references to prove that this model size can
eliminate boundary effects.
(c) 2.2. Model verification section: In this section, the present verification results based on model
tests show that the calculation error exists at the initial stage. Moreover, the parameter of
SANISAND model is derived from literature. Therefore, it is recommended to simulate the
element test in the literature to increase the reliability of model verification, or explain the
underlying reason for the calculation error in initial stage.
(d) Please further explain the influence of friction coefficient in Table 4.
(e) Line 462: Please add the potential practical significance and limitations for the simplified
calculation method of uplift capacity proposed in this study.

Comments for author File: Comments.pdf

Author Response

General comments: In this manuscript, the classic SANISAND model is adopted to study the uplift responses of suction caisson.  A refined calculation method of uplift capacity is proposed based on simulation results and validated by centrifuge tests. The manuscript conducts interesting investigations through rigorous approach. The manuscript is presented in a very excellent form. Valuable results and conclusions are obtained.

R: Thank you very much for your opportunity to our works.

Q1: Line 97: Please add the adopted values of the important parameter, i.e., used in the present master-slave contacts. Also, please explain the process regarding the determination of this parameter.

R1: Thank you very much for your comment. The main contact strategies of master-slave are described as follows: The ultimate shear stress is calculated by the Coulomb criterion and can be expressed by , where m is the friction coefficient of soil-structure and  is the horizontal soil stress on caisson. The elastic sliding distance of skirt-soil interface was set to 0.5mm.

Q2: Line 103: “The radius and the depth of soil domain are 2.5D and 5L respectively, which can eliminate the boundary” Please supplement references to prove that this model size can eliminate boundary effects.

R2: Thank you very much for your comment. The main reference are as follows:

[1] Shen, K., Zhang, Y., Klinkvort, R. T., Sturm, H., Jostad, H. P., Sivasithamparam, N., Guo, Z., 2017. Numerical simulation of suction bucket under vertical tension loading. Proceedings of the 8th international conference on offshore site investigation and geotechnics, London, UK, vol. 1, pp. 488–497.

[2] Shen, K., Zhang, Y., Wang, K., Wang, B., Zhao, X., 2021. Effect of partial drainage on the pullout behaviour of a suction bucket foundation. Europ. J. Environ. Civil Eng. 1–29.

Q3: 2.2. Model verification section: In this section, the present verification results based on model tests show that the calculation error exists at the initial stage. Moreover, the parameter of SANISAND model is derived from literature. Therefore, it is recommended to simulate the element test in the literature to increase the reliability of model verification, or explain the underlying reason for the calculation error in initial stage.

R3: Thank you very much for your comment. Based on the laboratory tests of Tran (2005), Shen et al. [2] calibrated the parameters of SANISAND model, and the calibration results against the drained triaxial test are shown in Fig. 1 (in the attachment). The SANISAND model can properly reflect the peak strength and residual strength of Dogger Bank sand under drained condition, and the shear dilatancy characteristics are basically consistent with the test results. The accuracy of calibration parameters is verified by [2].

Considering that the caisson is modelled as wish-in-place in this paper, the disturbance of 1g suction installation is not accounted for. As a result, the change in permeability and soil stress are neglected in the simulation, leading to inaccurate calculation results in the initial stage.

Q4: Please further explain the influence of friction coefficient in Table 4.

R4: Thank you very much for your comment. Under undrained condition, the uplift capacity can reach 10 times or more of the fully drained resistance, so the differential pressure has a pivotal effect on anti-pullout of caisson. The influence of friction coefficient is reflected in skirt wall friction, whose contribution to uplift capacity of caisson is smaller compared to differential pressure. Therefore, the influence of friction coefficient on uplift capacity is insignificant in Table. 4.

Q5:  Line 462: Please add the potential practical significance and limitations for the simplified calculation method of uplift capacity proposed in this study.

R5: Thank you very much for your comment. Under undrained condition, the soil stress is greatly affected by seepage field, bring difficulty to estimate the soil effective stress outside and inside the caisson accurately. The contribution of differential pressure to uplift caisson is much greater than skirt wall friction under undrained condition. In order to simplify the calculation, it is assumed that the increase in outer skirt wall friction due to seepage force is the same as the decrease in inner that [3-4].

[3] Vicent, S., Kim, S.R., Bong, T., 2020. Effect of loading rate on the pullout capacity of offshore bucket foundations in sand. Ocean Eng. 210, 107427.

[4] Vicent, S., Hong, S., Bong, T., Kim, S.R., 2021. Effects of embedment depth on the pull-out capacity of bucket foundations in sand. Ocean Eng. 237, 109643

Author Response File: Author Response.pdf