Bearing capacity factors of T-bar from surficial to stable penetration into deep-sea sediments

https://doi.org/10.1016/j.soildyn.2022.107671Get rights and content

Highlights

  • A CFD modeling of T-bar penetrometer penetrating deep-sea surficial sediments described by non-Newtonian fluids with shear thinning characteristics under ambient water is developed.

  • Differences in surficial and deep penetration of T-bar penetrometer into these sediments are discussed quantitatively, and corresponding mechanism and variation patterns are revealed.

  • A method to evaluate surficial and deep bearing capacity factors is established considering the contact relation between T-bar and sediment and variation in strengths of deep-sea sediments.

Abstract

In-situ full-flow penetrometers (i.e., the ball and T-bar) have shown great advantages and promising applications in testing and evaluating the undrained shear strength of low-strength deep-sea surficial sediments with fluidization characteristics. However, compared with a ball with the same projected area, a T-bar with the advantages of smaller volume and length along the penetration direction still suffers from the lack of better understanding of the surficial penetration mechanism and a method to estimate the bearing capacity factor under the effect of ambient water. In this study, a computational fluid dynamics method, Eulerian–Eulerian two-phase flow modeling, is used to analyze a T-bar with the no slip and free slip wall boundaries penetrating deep-sea surficial sediments defined by the dynamic viscosity of a non-Newtonian fluid with shear thinning behavior under ambient water. The stress characteristics and corresponding mechanism of the entire process (i.e., surficial and deep penetration) of a T-bar penetrating deep-sea sediments from the mudline to stable penetration are revealed from the evolution of the open and trapped water cavities above the T-bar. Finally, a methodology to evaluate the undrained shear strength of deep-sea sediments tested using a T-bar with different interface contact relations is proposed, and the corresponding equations are established, providing a basis for marine engineering geology survey, engineering construction, and hazard assessment.

Introduction

With the continuous utilization of deep water oil and gas, mineral and other resources, as well as the development of ultra-deep water resources [1], it is essential to obtain reliable mechanical parameters of deep-sea surficial sediments to serve the design, construction, and safe operation of engineering structures [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. Deep-sea surficial sediments within 2 m below the mudline are characterized by a high natural water content, large void ratio, and low undrained shear strength [[12], [13], [14], [15], [16], [17], [18]]. Consequently, it is difficult and expensive to obtain high-quality undisturbed samples of these surficial sediments, and the sampling and subsequent laboratory tests have also the following problems: (i) The transportation process might destroy the in-situ structure of these samples; (ii) It is difficult to restore the original environment of the sample in the laboratory tests, providing inaccurate strength parameters. Therefore, in the context of engineering construction requiring a large number of strength parameters (e.g., undrained shear strength) of deep-sea surficial sediments, e.g., submarine pipeline design and layout [5,[19], [20], [21]], the determination of undrained shear strength of surficial sediments increasingly depends on in-situ testing techniques.

The in-situ testing techniques used in marine engineering geological surveys mainly include the vane shear test (VST) and cone penetration test (CPT). Because the strength analysis principle of VST is relatively rough and cannot continuously provide the variation trend of sediment strength along with the depth, the applicability of VST is limited. CPT and/or piezocone penetration test (CPTu) are widely used in marine engineering geology [22,23]. However, the coupling action of deep-water high-pressure environment and low-strength deep-sea surficial sediments easily leads to excess pore water pressure concentration in the cone probe during the penetration of CPTu, thus making it difficult to ensure the accuracy of measurement data. Compared with CPTu and VST, a full-flow penetrometer with a larger probe projection area and higher testing accuracy can efficiently, accurately, and continuously provide the variation trend of deep-sea surficial sediment strength along with the penetration depth [[24], [25], [26], [27], [28]], so that the full-flow penetration test has a broader development and application prospect in the determination of strength parameter of deep-sea surficial sediments in ultra-deep water areas.

By changing the shape and size of cone probe (the projected area of the probe of a full-flow penetrometer is generally 10 times that of a cone probe, and the projected area of a standard in-situ full-flow penetrometer is 100 cm2, while that of a cone is only 10 cm2), Randolph and Houlsby [29], Stewart [30], and Randolph et al. [31] increased the contact area between a probe and the tested sediment, so that the tested sediment can reach a full flow state on the probe surface, thus avoiding the abovementioned stress concentration of cone probe and leading to a full-flow penetration test technique. The undrained shear strength of marine sediments can be more accurately determined from the penetration resistance on the probe [32], as shown in Eq. (1). Fig. 1 shows the most widely used probe shapes of a full-flow penetrometer including the T-bar and ball; however, they are mainly used to resolute the undrained shear strength of marine sediments during deep penetration where the sediment reaches the full failure mode. For surficial penetration, the tested surficial sediment flowing through the probe surface cannot achieve full reflux [[33], [34], [35], [36], [37], [38]]. Clearly, the deep bearing capacity factor cannot be used to analyze the undrained shear strength of deep-sea surficial sediments. Additionally, the problem becomes more complex when further exploring the effect of overlying seawater (i.e., ambient water) above the mudline [39], especially for deep-sea surficial sediments with fluidization characteristics that are rarely considered [40,41].su=qNwhere su is the undrained shear strength of the deep-sea sediment; q is the penetration resistance on the probe of the full-flow penetrometer; and N is the dimensionless bearing capacity factor.

Based on centrifugal experiments and computational fluid dynamics (CFD) simulations, Guo et al. [40,41] elucidated the mechanism of ball penetration from the mudline overlying ambient water into the deep-sea surficial sediment described by the soil mechanics and non-Newtonian fluid mechanics models, as shown in Fig. 2, quantitatively analyzed the important factors affecting the surficial penetration of ball, and deduced equations for determining the surficial and stable bearing capacity factors of ball penetration. In surficial penetration involving the interaction between two-phase materials (i.e., water and sediment) and probe, the state of the probe contacting two-phase materials changes continuously with increasing dimensionless penetration depth, resulting in an evolving and complex sediment–probe contact relationship. In parallel, the buoyancy on the probe also varies unevenly with the dimensionless penetration depth, which further makes the accurate evaluation of the penetration resistance and bearing capacity factor very difficult, and this complex situation is currently not considered in scientific research and practical applications. In the case of the same probe projection area, a T-bar has the advantages of a smaller volume (the volume of a ball is 755 cm3, the volume of a T-bar is 314 cm3, and the volume of a ball is approximately 2.4 times that of a T-bar) and a smaller length along the penetration direction (the length of a ball is 113 mm, the length of a T-bar is 40 mm, and the length of a ball is approximately 2.8 times that of a T-bar) compared with the ball, which can reduce the effect of buoyancy and sediment density on the surficial penetration resistance and bearing capacity factor. Therefore, there is an urgent need for an in-depth study on how to use a T-bar to evaluate the strength parameters of deep-sea surficial sediments with fluidization characteristics [13,14,42].

The remainder of this study is organized as follows. In Section 2, the undrained shear strength model of deep-sea surficial sediments is defined using the viscosity of a non-Newtonian fluid with shear thinning behavior, and a CFD two-phase flow modeling is developed to analyze the T-bar penetrating deep-sea surficial sediments under ambient water condition. In Section 3, the stress characteristics and corresponding mechanism of the entire process of T-bar penetrating deep-sea surficial sediments are proposed. In Section 4, the characteristics and mechanical mechanism of T-bar deep penetration and its difference from surficial penetration are discussed. In Section 5, a method to evaluate the strength of deep-sea surficial sediments tested using a T-bar with no slip and free slip walls is established. The conclusions of this study are presented in Section 6.

Section snippets

Undrained shear strength model of deep-sea surficial sediments

Deep-sea surficial sediments within 2 m below the mudline are generally low-strength soft clay or fluid mud [12,[15], [16], [17], [18],[42], [43], [44]]. Fig. 3 presents the undrained shear strength of deep-sea surficial sediments 1 m below the mudline in the northern continental slope of the South China Sea, and the data were obtained from an in-situ test, sampling laboratory test, and literature compiled by Nian et al. [16]. These deep-sea surficial sediments have dual properties of fluid and

Results and analysis

The vertical force T acting on the T-bar penetrating the surficial sediment from the mudline can be obtained using the CFD method established above. In fact, only this vertical force T was determined for the in-situ T-bar during the field test. The penetration resistance on T-bar can be calculated using Eq. (8). Here, the buoyancy on T-bar can be obtained using Eq. (9). Furthermore, the surficial bearing capacity factor NT-surface of T-bar can be obtained using Eq. (1). Taking the deep-sea

Mechanism and difference of deep bearing capacity factor of T-bar

According to the above method, a CFD model with the T-bar completely submerged was established, i.e., the mudline in Fig. 6 (a) is raised upward by a distance of 30D, and the other parameters are consistent with those described in Section 2. The bearing capacity factor is calculated in the same manner, as shown in Eqs. (8), (9), (1). Fig. 11 presents the deep bearing capacity factor NT-deep as functions of undrained shear strength su of deep-sea sediment under different T-bar boundary

Conclusions

We developed a CFD modeling to numerically investigate the T-bar full-flow penetrometer with the no slip and free slip wall boundaries penetrating deep-sea surficial sediments with different undrained shear strengths under ambient water. The following conclusions can be drawn:

  • (1)

    With the increase in the dimensionless penetration depth h/D, the surficial bearing capacity factor NT-surface gradually increases and then reaches a stable value (i.e., the stable surficial bearing capacity factor N

Author contribution statement

Xingsen Guo: Methodology, Writing - Original Draft. Zhenwen Liu: Writing - Review & Editing. Jiewen Zheng: Writing - Review & Editing. Qianyu Luo: Writing - Review & Editing. Xiaolei Liu: Conceptualization, Writing - Review & Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Funding for the research has been supported by the National Natural Science Foundation of China (Nos. 42022052 and 41877221) and the Shandong Provincial Natural Science Foundation (No. ZR2020YQ29). This support is gratefully acknowledged.

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