Soil Flow Mechanisms Research Articles

Boundary effect on the soil strength estimation from T-bar penetration tests

The T-bar penetration tests have gained widespread use in assessing the undrained shear strength of marine clays. However, when dealing with model tests or sampled soil with a small diameter of the tube, the reliability of T-bar test results is influenced by both the boundary effect and the soil flow mechanism around the T-bar. In this paper, a comprehensive investigation into the boundary effect on T-bar penetration resistance was conducted by using the large deformation finite element (LDFE) method in conjunction with laboratory tests. The study reveals that the boundary conditions significantly impact the T-bar resistance when the lateral boundary dimension is smaller than 12D. Specifically, the T-bar penetration resistance is underestimated for smooth boundaries and overestimated for rough boundaries. For soils with higher strength, a larger boundary size is required. The bottom boundary also affects the T-bar resistance until the full-flow mechanism is established. However, once the full-flow mechanism is achieved, the boundary effect on T-bar penetration resistance diminishes, even for small lateral boundary sizes (2D). Considering the fact that it is challenging to attain the full-flow mechanism, especially for higher strength soils, a simple algorithm is proposed to estimate soil strength based on a shallow penetration depth (2D). This algorithm is validated through laboratory tests. To address the difficulty in achieving the full-flow mechanism, by applying the overburden pressure during the T-bar test, a novel T-bar test method is suggested. This innovative approach promises to resolve the issue effectively.

Pullout behaviour of strip plate anchors in clay overlying sand

Plate anchors are being increasingly employed to establish mooring systems for offshore floating facilities owing to their notable efficiency and cost-effectiveness. The majority of prior studies have concentrated on plate anchors, examining the vertical to horizontal orientations, particularly when embedded in single layer clay or sand. However, it is common to encounter multiple layers of soil with varying characteristics in practical engineering, leading to inaccurate estimation of pullout capacity when based solely on the properties of a single soil layer. This paper reports pullout behaviours of strip plate anchors in clay overlying sand by adopting two dimensional finite element analysis. To simulate the hardening and subsequent softening behaviour of medium to dense sand, a modified Mohr-Coulomb model is considered. The current Finite Element (FE) results are in good agreement when compared with both previous FE simulations and physical model test data. Then, the soil flow mechanisms (e.g. the accumulated plastic shear strain, the friction angle) surrounding the anchor during pullout out are revealed. Based on a series of FE results, a calculation framework considering the effect of thickness of clay layer is designed to predict the pullout behaviour of plate anchors in clay overlying sand.

Penetration resistance of stiffened caissons in uniform and soft-over-stiff clays

The prediction of the penetration resistance of stiffened caisson involves uncertainties due to the complex soil failure mechanisms, which makes an obstacle for its application. In this paper, stiffened caisson installations in single-layer uniform and double-layer soft-over-stiff clays are investigated. Large deformation finite-element (LDFE) analyses are employed to simulate the caisson installation process. Soil flow mechanisms, especially inside the caisson along the inner stiffeners, and the caisson installation resistance are studied. Along the inner stiffeners, both soil/soil sliding and soil/skirt sliding mechanisms are observed based on the soil strength and caisson geometry, especially the stiffener geometry and spacing. During the caisson installation in soft-over-stiff clays, the soil flow mechanisms observed in the single layer uniform clay can be replicated in the top soft clay layer before the caisson penetrates into the bottom stiff clay. Caisson full installation resistance can be predicted based on the combined soil-flow mechanisms revealed in single- and double-layer uniform clays.

A large deformation finite element investigation of SEPLA: Failure mechanism and bearing capacities when loaded differently

The Suction Embedded Plate Anchor (SEPLA) is an innovative deep-sea anchoring measure with precise positioning, easy installation, and high anchoring capacity. Most current research on SEPLA revolves around vertical pullout mechanism and thus its vertical bearing capacity. There are relatively few studies concerning its bearing behavior and failure mechanism when SEPLA is subjected to horizontal and moment loading. The latter is practically relevant when SEPLA is exposed to extreme environmental conditions or keyed under preloading. A more in-depth understanding of its failure mechanism under horizontal and moment loading is thereby equally important. In this work, the Coupled Eulerian-Lagrange (CEL) method is used to conduct large-deformation finite element analyses of SEPLA under vertical, horizontal, and moment loading. Emphasis is placed on ascertaining the dissimilarity of soil flow and failure mechanisms when SEPLA is loaded in different directions. Parametric investigation is done to explore the effect of anchor shape and thickness, embedment depth, and soil-anchor interfacial roughness. Semi-theoretical closed form solutions are developed to estimate the uniaxial bearing capacity factor of plate anchors in three directions. These research outcomes are likely helpful in deepening the understanding of SEPLA failure mechanism when loaded in different directions and obtaining more rational estimates of anchor bearing capacity.

Bending moment response of batter piles in clay under spudcan-pile interaction

The bending moment response of straight pile foundations, as the dominant factor of spudcan-pile interaction, has received focused attention in the close joint operation between jacket platforms and jack-up drilling rigs. As the foundation for the same widely used in jacket platforms, the research on the batter pile is still a knowledge gap in this field. This paper presents a numerical study of the bending moment response of batter piles at different inclination angles and azimuthal angles during spudcan installation. Using a coupled Eulerian-Lagrangian finite element method, the effects of flexural rigidity, stiffness ratio, spudcan-pile clearance distance, and pile length on the pile head bending moment were sequentially evaluated. It was found that the pile bending moment response is related to the change in soil flow mechanism caused by spudcan penetration and becomes more significant with the increase of inclination angle; under the same inclination angle, the bending moment response is the most dangerous when the azimuthal angle is 0°. Based on the parameter study, a simple method is proposed to estimate the pile head bending moment during spudcan installation. The outcome may facilitate incorporating pile spatial inclination into spudcan-pile interaction research and contribute to related safety design.

Effects of strain rate and strain softening on the installation of helical pile in soft clay

Helical piles are alternative foundation for offshore wind turbine as a green foundation. When a helical pile is screwed into soft clay, the soils exhibit strain-rate dependency as well as strain softening effect when sheared and remolded. In this study, large deformation finite-element (LDFE) model is established to investigate the penetration behavior of helical pile, where the soil flow mechanism and penetration resistance are studied considering strain rate effect and strain softening effect in soil. The numerical model is validated by comparing with available centrifuge results. Parametric studies are then conducted covering a wide range of strain rate and strain softening parameters including the strain-rate parameter, the sensitivity and ductility of the soil, and the original shear strength. It is found that the penetration behavior of helical pile is significantly influenced by the strain rate and strain softening of soil. The difference of the torque profiles between the ideal Tresca soil and the rate-dependent, strain-softening soil is 0–30%. Based on the numerical modelling results, the theoretical model to predict torque requirements for the installation of helical piles is extended by accounting for the strain rate effect and strain softening effect.

Soil flow mechanisms around cone penetrometer in layered clays

The cone penetrometer test is widely used for in situ site investigations and for establishing direct penetrometer-to-foundation or -anchor design correlations. This paper focuses on soil flow mechanisms during the continuous penetration of a cone penetrometer in layered clays. A series of centrifuge tests was conducted with the cone penetrating through soft–stiff, stiff–soft, soft–stiff–soft and stiff–soft–stiff clay profiles. Particle image velocimetry allowed accurate resolution of the soil flow mechanism around the cone where a half-cone model was penetrated into layered clays against a transparent window. The observed soil movement was compared both with previous observations for piles/cones and with movement from the shallow strain path method (SSPM). A comparison with SSPM results showed that the SSPM can provide reasonable evaluations on maximum lateral and vertical displacements even though the upheave movement can be overestimated. The effect of soil layering on the failure mechanisms was studied extensively by exploring soil flow mechanisms and soil displacement paths at various distances from the advancing cone centreline and soil layer interface. The reported characteristics of cone penetration in layered soils provided in-depth understanding of cone penetration responses that will lead to the development of mechanism-based theoretical models for cone penetration in layered fine-grained soils.

Numerical investigation of caisson with pad-eye stiffener installation into nonhomogeneous clay

Caisson foundations are widely used in offshore engineering. During installation, the soil flow mechanism of stiffened caissons with local thickening at the pad-eye is still not well understood. In this study, large deformation finite element (LDFE) model is established to investigate the penetration behavior of caisson with pad-eye stiffener, where the soil flow mechanism is studied considering soil strain softening effect. The numerical model is validated by comparing with available testing data. Parametric study is then conducted to examine potential influencing factors, including the penetration depth, the geometry of stiffener, the clay shear strength, and strain softening parameters. It is found that the soil failure mechanisms and the corresponding penetration resistances of locally stiffened caisson at the pad-eye are significantly different from that of unstiffened caisson or stiffened caisson with interval rings. Two parameters, i.e. the critical rotational soil flow depth (Hr) and the limiting cavity depth (Hc) are introduced to quantitatively describe the behavior of the observed soil flow mechanisms. Based on the modeling results, a simplified flow mechanism is proposed, and approximating expressions are derived to predict the penetration resistance of the caisson for engineering design application.

Numerical Study on the Behavior of Square Stiffened Caissons Penetrating into Normally Consolidated Clay

Significant difference between predicted and measured installation resistance of stiffened suction caissons was identified due to the existing uncertainty regarding the mobilized soil flow mechanisms. This paper describes an extensive investigation of square stiffened caisson penetration in nonhomogeneous clays undertaken through large deformation FE (LDFE) analysis to identify the soil flow mechanisms around and between lateral ring stiffeners. A detailed parametric study has been carried out, exploring a range of nondimensional parameters related to stiffened caisson geometry, caisson roughness, and soil strength. The LDFE results were compared with centrifuge test data in terms of soil flow mechanisms, with good agreement obtained. Two interesting features of soil flow inside the caisson were observed including soil backflow into the gaps between the embedded stiffeners and soil heaving at the surface. It shows that the cavity depth can reach ∼5 m. Finally, simple expressions were proposed for estimating the critical depths of soil backflow and cavity formation.

A new soil reaction model for large-diameter monopiles in clay

Considering the limits of the traditional p-y method and the omission of the non-negligible additional lateral resistance components for large-diameter monopiles, the effects of rotational soil flow and pile tip lateral components on the soil-pile lateral behavior are discussed in detail. A new unified soil reaction model is proposed to obtain a more accurate prediction of the lateral behavior of monopiles supporting offshore wind turbines (OWTs), which consists of the lateral p-y spring, the base moment (Mbase-θ) spring and the base shear (T-u) spring. This model is suitable for cases of slender, semi-rigid and rigid piles, and can be modified to include the effects of different soil flow mechanisms, large diameter and base lateral resistances. From the analysis results, the model proposed in this study provides a better prediction and is more appropriate for the design of OWT foundation. The traditional p-y method underestimates the lateral resistance and stiffness of the soil-pile system, is overly conservative and is not economical for the design of OWT foundations.

Experimental studies on a circular open caisson

The resistance offered by the soil to the cutting edge of a caisson and soil flow around the cutting edge will vary continuously during sinking. In this study, a series of 1g model tests have been carried out to investigate the load–penetration response and the soil flow mechanism in sand around the cutting edge of a circular open caisson. Full- and half-open caisson tests have been conducted considering different tapered angles of the cutting edge. The load–penetration response is investigated using the full-open caisson tests with different (a) tapered angles of the cutting edge, (b) types of penetration and (c) depths of sinking. The soil flow mechanism is investigated using the half-open caisson tests with different (a) tapered angles of the cutting edge, (b) magnitudes of penetration and (c) depths of sinking. The soil flow mechanism is evaluated using image-based deformation measurement techniques. The results of the study presented in the form of load–penetration response and soil flow mechanism are useful in addressing the interaction between the cutting edge of the caisson and soil during the sinking operation.

Soil flow mechanisms of full-flow penetrometers in layered clays through particle image velocimetry analysis in centrifuge test

This paper reports the soil flow mechanisms observed in centrifuge tests around full-flow (T-bar and ball) penetrometers in layered clays. The layered clay samples consisted of soft–stiff, stiff–soft, soft–stiff–soft, and stiff–soft–stiff soil profiles. Particle image velocimetry (PIV), also known as digital image correlation (DIC), allowed accurate resolution of the flow mechanism around the faces of the T-bar and half-ball penetrated adjacent to a transparent window. For the T-bar, overall, a full symmetrical rotational flow around the T-bar dominated the behavior. A novel “trapped cavity mechanism” was revealed in stiff clay layers, with the evolution of the trapped cavity being tracked. No soil plug was trapped at the base of the advancing T-bar regardless of penetration from stiff to soft layer or the reverse. For the ball, two key features of the soil flow mechanism were identified, including (i) a combination of vertical flow, cavity expansion type flow, and rotational flow for a fully embedded ball and (ii) a stiff soil plug trapped at the base of the ball advancing in a stiff–soft clay deposit. For both penetrometers, a squeezing mechanism mobilized as they approached a soft–stiff layer interface.

Undrained bearing capacity of irregular T-bar by the lower bound method in clay

The lower bound method is employed to investigate the ultimate bearing capacity of the irregular T-bar in uniform soil. The stress field is constructed to illustrate the evolution of the soil flow mechanisms and considering the frictional influence. Simultaneously finite element analyses and two special conditions are also employed to verify the reasonability of the lower bound model. The general expressions of the bearing capacity factors for two types of irregular T-bars are presented, which have reference value for similar ocean structures. The main results are shown as follows: ①The friction coefficients significantly influence the bearing capacity factor with increasing aspect ratio. ② The rigid zones are presented at the front and rear of the elliptical probe and progressively decrease with increasing ξ (probe aspect ratios) and reducing ω (friction factor), until disappearing at a larger aspect ratio following a smooth interface. ③ For ξ > 1, the degree of streamlining of the elliptical probe easily results in an attached flow mechanism to the probe surface and a decreasing rigid zone with increasing ξ in the high strength soil. ④ Compared with the CPT test, the advantage of the diamond T-bar is without the need for overburden stress correction in the full flow mechanism. ⑤ The large aspect ratio of the irregular T-bar tries to determine the interface friction factor based on the bearing factor equation.

A unified lateral soil reaction model for monopiles in soft clay considering various length-to-diameter (L/D) ratios

Large-diameter monopiles are the most commonly used foundation to support offshore wind turbines. Early designs usually adopted pile diameters (D) between 4 and 6 m, which is recently extended to 8 m and will target 10 m in the future. It is increasingly evident that the existing design method (i.e., API's p-y model) can significantly under-predict the lateral stiffness and capacity of large-diameter monopiles in soft clay, due to ignoring the soil resistances from base shear and base moment which become more pronounces as L/D reduces. In this study, a two-spring approach is proposed, aiming to predict the lateral behaviour of monopiles with varied L/D ratios in a unified manner. In light of the soil flow mechanisms around monopiles, the pure lateral soil resistance above the rotation point (RP) is quantified using a p-y model, while the resistances below the RP including the base shear and base moment are integrated into a moment-rotation spring (characterized by a MR-θR model) at the RP. It can naturally recover to a p-y model while analyzing flexible piles, where θR= 0 at RP. Formulations of the ‘p-y + MR-θR’ model (including diameter-related p-y and MR-θR models, and the depth of the RP) are proposed based on the results of a series of well-calibrated 3D numerical models. The proposed model has satisfactorily reproduced a number of field and centrifuge test results on laterally loaded monopiles with a wide range of L/D ratios (including flexible, semi-rigid and rigid piles), using a unified set of parameters. Compared to the standard p-y model, the adoption of the proposed ‘p-y + MR-θR’ model is shown to substantially reduce design conservatism.

Effect of Trapped Cavity Mechanism on Interpretation of T-Bar Penetrometer Data in Uniform Clay

AbstractThis paper describes large deformation finite element (LDFE) analysis of the penetration of the T-bar penetrometer in uniform clay, identifying soil flow mechanisms around the T-bar, the ex...

Strain softening parameters estimation of soft clay by T-bar penetrometers

The T-bar penetrometer is a key tool to estimate the strain softening parameters by repeating penetration and extraction. In this study, the cyclic penetration process of T-bar is simulated using the large deformation finite element (LDFE) method, considering the strain softening behavior of clay. The criteria to estimate the critical depth during initial penetration are discussed. A new equation of the critical depth is proposed to consider the effect of soil strain softening. The characteristics of the T-bar resistance curves are discussed, and they can also be used to judge whether the soil reaches the full-flow mechanism. The soil flow mechanisms for shallow cyclic T-bar penetration are discussed. It is found that a cavity is formed during T-bar extraction and it leads to the following resistance gradually decreases. This means that, for shallow cyclic tests, the measured cyclic penetration or extraction resistance cannot be used to estimate the clay sensitivity. Moreover, a simple method is proposed to estimate the strain softening parameters of soil. The proposed method is verified by comparing the estimated values with the numerical simulation results and the field test data.

Model tests and finite element analysis for vertically loaded anchors subjected to cyclic loads in soft clays

More attention has been focused on the pull-out behaviour of plate anchors under monotonic loads. Relatively less attention has been focused on the cyclic behaviours of vertically loaded anchors (VLAs) subjected to combined average and cyclic loads in soft clays. Therefore, these anchors are investigated by conducting reduced-scale model tests under 1 g conditions, and the cyclic deformation process of VLAs in soft clays are simulated through a 3D finite element method using an advanced elastoplastic bounding surface constitutive model of soft clays proposed by the authors. The failure displacement of VLAs for the shallow-embedded and deep-embedded conditions was determined by performing monotonic loading tests. The change rules of average and cyclic displacement and the cyclic bearing capacity of anchors with the number of cycles are revealed by performing cyclic loading tests. The influence of the average and cyclic load levels and the embedded depth on the cyclic bearing capacity is clarified. The displacement time, load-displacement histories and cyclic bearing capacity of VLAs can be predicted relatively well through 3D finite element analysis. The significant plastic strain accumulation and nonlinear cyclic hysteresis responses of VLAs in soft clays and the soil flow mechanisms surrounding the plate anchors at failure can be captured.

Effects of strain softening on the penetration resistance of offshore bucket foundation in nonhomogeneous clay

The concrete-made bucket foundations were employed as emerging solution to support the wind turbine or breakwater, where the relatively large thicknesses of skirts and the strain softening effect related to the soil sensitivity and ductility were necessary to be explicitly accounted for. However, few of current study concerned this topic. This paper therefore reported an extensive parametric investigation of penetrating behaviour for bucket foundation in nonhomogeneous clay to allow a better estimation of penetration resistance of bucket foundation. The three-dimensional (3D) large deformation finite element (LDFE) analysis model incorporating the effect of strain softening on soil strength was set up and validated by comparing to the published centrifuge data. Then the evolution of soil flow mechanism and the distribution of soil strength softening were examined with the soil plug generated inside skirt assessed. Based on the numerical results, a new analytical model was proposed to predict the penetration resistance of bucket foundation in clay accounting for a partially-remoulded condition.

Mechanisms of soil flow during submarine slide-pipe impact

This paper investigates submarine slide-pipe interaction through centrifuge tests and computational fluid dynamics (CFD), with particular focus on the soil flow mechanisms that occur around a suspended pipe at non-Newtonian Reynolds numbers (Renon-Newtonian) between 0.5 and 40. The centrifuge tests involve horizontal translation of a model pipe in samples of kaolin at different densities and undrained strengths whilst measuring pore pressures at the model pipe front and rear. These measurements provide insights into the pressure evolution at these pipe locations with increasing Renon-Newtonian. In the CFD simulations, the submarine slide is idealised as a single phase non-Newtonian, power law fluid that flows around a two-dimensional pipe (fully engulfed). CFD modelling demonstrates that the drag coefficient depends on both Renon-Newtonian and the power law index controlling the slide material viscous behaviour. Examination of the observed CFD flow features reveals that slide flow can separate from the pipe surface when Renon-Newtonian > ∼10. It is shown that the Renon-Newtonian at which soil flow separation occurs approximately coincides with increases in the: 1) differences in front and rear pore pressures measured in the experiments; and 2) the measured net horizontal drag pressure acting on the model pipe.

Experimental evaluation of ultimate bearing capacity of the cutting edge of an open caisson

Open caissons are sunk into the ground by their own weight. A cutting edge of the caisson having a tapered inner face on loading – that is, raising of the steining – results in bearing failure by displacing the soil which is in contact with the cutting edge. The bearing capacity of the cutting edge and the soil flow mechanism depend on the configuration of the cutting edge, sinking depth and soil type. This paper presents the results of a series of 1g model tests, which investigate the effect of varying tapered angles of the cutting edge on the penetration resistance of the open caisson. The vertical failure load and corresponding vertical bearing capacity factor, N′γ, and the soil flow mechanism around the cutting edge are investigated. The soil flow mechanism and the influence of surcharge formed at the top level of the cutting edge due to advancement of the caisson in the ground are examined using the image-based deformation measurement technique. The results highlight that the cutting angle of the cutting edge and sinking depth play important roles in the load–penetration response and soil flow mechanism.