Saturated Soft Clay Research Articles

Influence of drainage conditions on piezocone penetration mechanism in offshore clays

ABSTRACT CPTu (piezocone penetration test) is widely used in engineering practice to determine various parameters of clays under partially drained conditions. However, most existing research is based on undrained or fully drained conditions for clays, leading to underestimation or overestimation of soil strength. By applying the Eulerian-Lagrangian large deformation finite element method to analyse the water-soil interaction, the CPTu driving mechanism in offshore saturated soft clays under different drainage conditions is revealed. An advanced hypoplastic constitutive model for clays is used to simulate the nonlinear behaviour of kaolin under different drainage conditions. The generation, accumulation, and dissipation of excess pore water pressure under different drainage conditions are analysed, as well as the influence of excess pore water pressure on cone tip resistance and the effective stress of the soil during the CPTu penetration process.

An elastoplastic model with egg-shaped yield surface for coastal soft clay

A novel elastoplastic constitutive model with an egg-shaped yield surface (ESE model) is proposed herein to elucidate the strength-deformation characteristics of marine soft clay. One of the key improvements offered by this model is its ability to address the singularity associated with plastic strain increments at yield surface corners—a limitation commonly encountered in traditional elastoplastic models. Additionally, the proposed model demonstrates flexibility, as it can transform into various forms under different conditions. Subsequently, to evaluate the efficacy of this egg-shaped elastoplastic model, numerical simulations are performed within the finite element software ABAQUS using an implicit integration algorithm through the user material subroutine interface (UMAT). The simulations encompass standard triaxial consolidation tests involving saturated soft clay sourced from the Itsukaichi marine clay as well as two types of saturated kaolin clays. The outcomes derived from these simulations by the ESE model are compared with both empirical data and numerical simulations originating from the widely employed Modified Cam-Clay (MCC) model. The simulations of the ESE model for three conventional triaxial tests on clay demonstrate superior concordance with experimental measurements in comparison to the MCC model. This underscores the efficacy of the proposed ESE model in predicting the strength and deformation characteristics manifested by coastal soft clays under undrained conditions. Finally, the predictions for deformation in an adjacent tunnel according to the ESE model closely align with experimental measurements and prior to the simulations conducted using the MCC model and advanced models such as the bounding surface model and the unified model, thereby affirming its practical utility in engineering applications.

Numerical study on undrained multi-directional bearing capacity of hybrid subsea foundation

The traditional foundation design does not consider the bearing capacity under multi-directional coupling load. However, the foundation is usually subjected to vertical (V), horizontal (H) and moment (M) loading simultaneously on the seabed. The present study examines the undrained multi-directional bearing capacity of hybrid subsea foundation in saturated soft clay using finite element analysis. The contribution of the mudmat and the suction bucket to the bearing capacity of the hybrid subsea foundation was evaluated. The results show that the suction bucket can effectively make up for the lack of horizontal anti-slip and anti-overturning ability of the mudmat. It has a significant contribution to the horizontal and moment bearing capacity. In addition, the failure envelope of bearing capacity in horizontal-moment (H-M), vertical-horizontal (V-H), and vertical-moment (V-M) load space is obtained by using prescribed displacement ratio loading method. Based on the simulation results, the expression of approximate envelope of foundation bearing capacity under combined load of horizontal and moment is proposed. The study also found that a certain value of vertical load can increase the horizontal and moment bearing capacity of the hybrid subsea foundation, while excessive vertical load will lead to a decrease in bearing capacity.

Evaluation of bearing capacity envelope of mat foundations in soft clay seabed under wave loads using kriging-based response surface method

This study examines the influence of cyclic wave loads on the bearing capacity envelope of mat foundations for bottom-supported platforms in saturated soft clay seabed. These platforms offer advantages in terms of quick installation and removal but may experience reduced bearing capacity due to cyclic wave loads. An evaluation method for the failure envelope of mat foundation using Kriging-based response surface methodology under wave action is proposed. To achieve this, we incorporated an elastoplastic constitutive model for cyclic loading into the ABAQUS finite element method platform, utilizing a self-compiled user-defined material (UMAT) code to describe clay behavior. The evolution of uniaxial bearing capacity and vertical-horizontal-moment (VHM) bearing capacity failure envelope are investigated. The performances of Kriging-based response surface methodology and original response surface methodology are compared, and the evaluation of the VHM failure envelope using the Kriging-based response surface methodology is presented. Results indicate that cyclic wave loads can cause significant weakening of the initial bearing capacity failure envelopes of mat foundations. However, it is worth noting that cyclic wave loads only affect the magnitude of the envelope surface and not its shape. Furthermore, the bearing capacity failure envelope of mat foundations decreases with increasing cyclic stress ratio, frequency, and time factor of cyclic wave loads. The evaluation method for the failure envelope of mat foundation bearing capacity based on Kriging-based response surface methodology under the action of wave loads outperforms the original response surface methodology and can serve as a valuable reference for engineering design.

One-dimensional rheological consolidation study on two-sided semi permeable boundary and fractional viscoelastic saturated soil under cyclic loading

Considering the viscoelastic of soft clay, the fractional kelvin model is used to describe the constitutive relationship of soft clay. At the same time, considering that both sides of the soft clay layer have certain permeability, the two-sided semi-permeable boundary is used as its boundary condition. Combined with the cyclic load of different waveforms, the one-dimensional consolidation settlement of saturated soft clay is analyzed. Firstly, the consolidation equation is obtained through Laplace transformation, and the semi-analytic result is obtained by Laplace inverse transformation. The accuracy of the solve method and the model’s applicability are checked by the degradation of boundary conditions and the model. Finally, one-dimensional consolidation settlement under cyclic load with different waveforms is compared.

Implicit integration of an anisotropic egg-shaped elastoplastic model for saturated soft clay

PurposeThe purpose of this paper is to suggest an implicit integration method for updating the constitutive relationships in the newly proposed anisotropic egg-shaped elastoplastic (AESE) model and to apply it in ABAQUS.Design/methodology/approachThe implicit integration algorithm based on the Newton–Raphson method and the closest point projection scheme containing an elastic predictor and plastic corrector are implemented in the AESE model. Then, the integration code for this model is incorporated into the commercial finite element software ABAQUS through the user material subroutine (UMAT) interface to simulate undrained monotonic triaxial tests for various saturated soft clays under different consolidation conditions.FindingsThe comparison between the simulated results from ABAQUS and the experimental results demonstrates the satisfactory performance of this implicit integration algorithm in terms of effectiveness and robustness and the ability of the proposed model to predict the characteristics of soft clay.Research limitations/implicationsThe rotational hardening rule in the AESE model together with the implicit integration algorithm cannot be considered.Originality/valueThe singularity problem existing in most elastoplastic models is eliminated by the closed, smooth and flexible anisotropic egg-shaped yield surface form in the AESE model. In addition, this notion leads to an efficient implicit integration algorithm for updating the highly nonlinear constitutive equations for unsaturated soft clay.

An anisotropic egg-shaped bounding surface model for saturated soft clay

An anisotropic egg-shaped bounding surface plasticity model (AESBS model) is proposed to describe cyclic and monotonic behaviors of saturated clays. The model is based on an innovative egg-shaped bounding surface, which not only eliminates the singularity present in most plasticity models and facilitates numerical calculation but also can be degenerated into various configurations. Moreover, to improve its representation when considering the initial consolidated state, an anisotropic tensor is incorporated into the bounding surface. Furthermore, a novel generalized isotropic hardening rule that combines isotropic and kinematic hardening is applied. It regards reversal stress points as homological mapping centers, hence leading to discrete and movable mapping centers. Notably, the elastic zone concept is employed to produce a more realistic performance of nonlinearity during cyclic loading. These modifications formulate the novel model that can be utilized to enhance the ability in predicting the behavior of saturated clays. To improve the convergence of the model, an implicit integration scheme is used to solved the constitutive relationship through some nonlinear algebraic equations. The comparison of simulation results and published experimental data from one-way undrained cyclic and monotonic triaxial tests for saturated clays demonstrates the efficiency of this model under different consolidated conditions.

A Comparative Centrifuge Test Study on the Influence of Overlying Seawater on Seismic Response Spectrum

In this study, dynamic centrifugal model tests for two homogeneous saturated soft clay sites are designed to analyze the influence of overlying sea water on ground motion. We obtain the response spectrum value and amplification coefficient from the acceleration sensor records. Through comparative analysis, we conclude that with the increase in input peak ground motion, the acceleration response spectrum value at the same depth gradually increases. When the input peak value is small, there is no significant difference between the response spectra with and without water; when the input peak value is large, there is no significant difference in the high-frequency part of the response spectrum value. In the middle- and low-frequency part, the response spectrum value with water is obviously smaller than that without water. The amplitude of the response spectrum of the water-free model gradually moves in the long-period direction, but the water model tends to the short-period direction. In the long-period part, the amplification coefficient of the water model is obviously smaller than that of the water free model.

Excess pore water pressure behavior of saturated soft clay under cyclic confining pressure with different frequencies

The excess pore water pressure is main factor affecting the dynamic response of saturated soft clay under traffic loads. Recognize that, cyclic triaxial tests with and without cyclic confining pressure were carried out to study the development of excess pore water pressure of saturated soft clay. The impacts of factors, such as cyclic stress ratio (CSR), loading frequency, and cyclic confining pressure, were analyzed. Excess pore water pressure increases with increasing cyclic stress ratios, while decreases with increasing loading frequencies. Nevertheless, with increasing cyclic confining pressures, the maximum excess pore water pressure increases, while the minimum excess pore water pressure remains unchanged. Besides that, the greater CSR and cyclic confining pressure deliver an increment of both maximum and minimum excess pore water pressure rate. However, the excess pore water pressure rate - time curves coincide under different loading frequencies. Meanwhile, both the normalized maximum and minimum excess pore water pressure rates are linearly proportional with time under different test conditions in logarithmic coordinates. Based on that, both maximum and minimum excess pore water pressure prediction models, relating to the above factors, are proposed, and the predicted results accord well with the measured data.

Effect of stress rotation angles on dynamic responses of soft clay under multi-directional loading

Offshore wind turbines are subjected to significant combination environmental loads from wind and wave action. Under such conditions, the directions of these environmental loads vary over the structure's service life, and the stress directions of subsoil rotates during the wind turbine operating process. Tests were conducted on saturated soft clay using multi-directional cyclic simple apparatus to investigate the influence of stress rotation angle. All test results implied that both the stress rotation angles and cyclic stress ratios influence the dynamic responses of the reconstituted clay greatly and regularly: shear strain and pore pressure decrease as θ values vary from 0° to 90° and increase as θ changes between 120° and 180°. Shear strain accumulative ratio for θ = 0°, 30°, 150°, and 180° is more incredible than that of 60°, 120°, and 90°. Increasing the cyclic shear stress ratio will enhance the effect of θ on the cyclic strain and excess pore water pressure.

Viscoelastic-viscoplastic damage analysis of transversely isotropic soft soils

Soft soils show noticeable creep, damage and transversely isotropic properties. An incremental three-dimensional Nishihara model is proposed to present the stress-strain relationship of soft soils in this paper. The Drucker-Prager yield criterion of the transversely isotropic media, the transversely isotropic flexibility matrix, and the damage law to Nishihara's constitutive model are applied to construct the transversely isotropic viscoelastic-viscoplastic damage constitutive equation of the soil. Afterwards, the constitutive model is implemented by applying a stress return mapping algorithm. We verified the proposed model by comparing the results calculated by ABAQUS with triaxial compression experimental data from Hong Kong marine deposits (HKMD) and Shanghai soft clay. Finally, based on the proposed model, we present several numerical examples to investigate the influences of damage parameters on the time-dependent behaviors of saturated soft clay.

Superimposed Stress Calculation of Soil Underlying Anchor Beam considering Anisotropy and Strength Nonhomogeneity

Prestress anchor beams are an important reinforcement method for underground engineering and rock slopes, and they are crucial for maintaining the stability of civil engineering structures. In this study, the computational formula of superimposed stress of soil underlying an anchor beam was proposed in this paper, based on the assumption that the anchor beam is completely rigid and the prestress of the anchor cable is acting on soil in the form of a strip uniform load. By considering the anisotropy and strength nonhomogeneity of the saturated soft clay, the failure envelopes of the soil and the maximum effective depth underlying the anchor beam were obtained by the D-P yield criterion suitable for soft clay analysis. Furthermore, the experimental comparisons with the previous findings were used to validate the proposed approach. The proposed method was valid to assess the maximum effective depth. Considering the influence of the factors on the failure envelopes of soil, a parametric study was carried out to optimize the design of the prestressed anchor cable. The findings of this study provide a theoretical basis for designing prestressed anchor cables; these can also be used to predict the response of prestressed anchor cables as well as to optimize the design of the cables, such as the anchorage length of cable and grouting radius.

SOIL SUBSTANCE ANALYSIS USING PVD WITH VACUUM CONSOLIDATION METHOD (VCM)

The construction of toll roads on soft clay has the main problems of the relatively low bearing capacity of the subgrade and the relatively large and long duration of subgrade compression. The infrastructure built on the subgrade could be damaged before it reaches its planned age if the subgrade not repaired first. The selection of the soft clay soil handling method considers the cost and time at the time of construction in the field. For this reason, alternative methods needed that are cost and time efficient and have minimal risk to environmental impacts. The vacuum consolidation method (VCM) is an alternative to improve water saturated soft clay. Soil improvement using the vacuum consolidation method is intended to accelerate the settlement and increase the bearing capacity of the soft native soil by vacuum pumping the soil to reduce the moisture content and air content of the soil grains to accelerate long-term settlement and differential settlement. This study aimed to analyze the soil settlement using PVD with the vacuum consolidation method (VCM). The analysis conducted showed that to achieve 90% consolidation without PVD, it would take 13,922 years and using PVD, it would take 18 months, and using PVD and the vacuum consolidation method (VCM), it would take 4,8 months. The use of PVD with the vacuum consolidation method (VCM) can speed up the consolidation process compared to using PVD or only using PVD. The total decrease that occurred was 2,043 m.

Research on Nanoeffect of Penetration and Consolidation under Nonlinear Characteristics of Saturated Soft Clay.

In order to systematically study the nanopermeability properties of saturated soft clay under different consolidation pressures and different osmotic pressures, this paper analyzes various loading and unloading conditions that affect the permeability of soil based on the material description equation with displacement as the control variable of large deformation consolidation theory. By summarizing the empirical relationship between permeability coefficient and consolidation pressure and permeability coefficient and void ratio, the infiltration law and seepage failure characteristics of soft clay are revealed. For the soil studied in this paper, a = 0.65, b = 0.001, and q = 3.55 are acceptable. The effect of the initial permeability coefficient on large deformation consolidation is studied, and the necessity and plausibility of considering the nonlinearity of the compression and permeability coefficient when calculating the soft-land base large deformation consolidation is also studied.

Behaviour of Model Stone Column Subjected to Cyclic Loading

Behaviour of saturated soft clay deposits subjected to cyclic stresses is of considerable importance in the design of railway infrastructure, since these deposits are characterised by very low bearing capacity and excessive settlement. Stone column is a well-established method that used to improve both bearing capacity and reduce settlement of such soil. However, much of the work to date has dealt with the behaviour of stone columns under static loading. Little is known about their response to cyclic loading. A laboratory based study was undertaken to investigate the response of both column and the surrounding soft soil to cyclic loading using a series of static and cyclic loading triaxial tests under undrained condition. Testing was performed on normally consolidated specimens of soft clay (undrained shear strength of ≈ 12 kPa, diameter 100 mm, and 200 mm height) reinforced with 28 mm diameter stone columns. Effects of loading frequencies at range of cyclic stress ratios were examined. It was found that the cumulative strain developed was dependent on the cyclic stress ratio, but independent of frequency from 0.5 to 3 Hz.Overall, stone columns have shown the potential to strength subgrade when exposed to cyclic loading, and hence reduce both capital costs (reduce the ballast needed for subbase), and operational cost through extended maintenance period.

Constitutive modeling for cyclic responses of saturated soft clay under principal stress rotation induced by wave loads

A bounding surface model is proposed to reproduce the cyclic responses of saturated soft clay under principal stress rotation induced by wave loads. While extensively reported in experimental studies, this particular type of clay response has not been adequately addressed by constitutive modeling. By incorporating the mapping rules of relocatable projection center into the bounding surface, the plastic effects under pure principal stress rotation can be established. To represent the non-coaxiality during pure principal stress rotation, the presented model employs a non-coaxial flow rule sharing the same directions with the non-coaxial stress rate. However, it has been modified to related to the coaxial strain rate and the current stress ratio. Besides, the anisotropic elasticity is introduced for simulating the effects on plastic accumulation behavior during principal stress rotation. The developed model is validated through pure principal stress rotation tests of Shanghai clay and Wenzhou clay. The comparisons show that the proposed model can reasonably reproduce cyclic responses of saturated soft clay under principal stress rotation induced by wave loads.

Cyclic Loading Test for the Small-Strain Shear Modulus of Saturated Soft Clay and Its Failure Mechanism

Small-strain shear modulus, G max , is a key evaluation index to study the dynamic characteristics of soil in geotechnical engineering. It is widely adopted to evaluate the stiffness of soft soil in soil dynamic engineering. In this paper, the cyclic triaxial tests and resonance column tests were carried out to explore the variation of G max of soft clay with respect to various confining stresses, cyclic shear stress ratios, pore pressures, and effective stress paths. Test results indicated that the effective stress decreased gradually with the increase of the cycle shear stress ratio. The failure points were mainly concentrated in a rectangular area, defined by the normalized effective stress from 0.56 to 0.64 and the normalized shear modulus from 0.72 to 0.78. Additionally, a short pause caused a small increase of 1-2% in G max as well as pore pressure. This study demonstrates that G max can be effectively used to characterize the failure of saturated soft clay in a more intuitive and convenient way, compared to the commonly used strain failure standards.

Performance of elevated energy pile groups with different pile spacing in clay subjected to cyclic non-symmetrical thermal loading

There is an increasing interest and use of energy foundation worldwide. Full-scale tests, physical model tests at one Earth’s gravity and at elevated gravity in a geotechnical centrifuge and numerical simulations have been widely reported in the literature. Almost all studies have focused on single energy piles (EPs) and pile groups subjected to symmetrical thermal loads, although it is not unusual to have energy pile groups stressed by non-symmetrical thermal loads. In this study, a series of non-symmetrical thermal loading centrifuge model tests were conducted in saturated soft clay (OCR = 1.7), which is much vulnerable to temperature changes. The tests aim to investigate the effects of pile spacing on the serviceability of floating energy pile groups. Fifteen cycles of non-symmetrical thermal loading are applied to three EPs in 2 x 2 elevated energy pile groups with 3D and 5D (D denotes pile diameter) pile spacing. The thermally induced irreversible settlement and tilting in the 3D-pile group are 200% and 300% larger than those in the 5D one, respectively. Contrary to the 5D-pile group where the serviceability criteria (EN 1997-1:2004) are satisfied, the settlement and tilting are 30% and 200% larger than the criteria in the 3D one, respectively.

Experimental and numerical study for the optimization of bottom of foundation shapes on soft soils

Shallow foundations resting on a deep layer of soft saturated clay are considered a geotechnical challenge. The design engineer is faced with challenges in selecting safe and economic solutions for the foundations of these light weight structures by turning to the shell-shaped foundations to deal with such foundation problems and avoid the use of the costly deep foundations or other soil improvement schemes. This paper introduces different bottom of foundations shapes resting on extended soft clay layer in an effort to minimize the soil stresses and reduce the settlements. Elliptical, trapezoidal and inverted folded footings have been used as an alternative to the conventional flat bottom shallow footings. The purpose of the current study is to reach the optimal shape of the foundations, which achieves reduced stresses, reduced settlements and consequently cost. The geotechnical behavior of different bottom of foundation shapes resting on saturated soft clay is investigated experimentally and numerically. The experimental investigation started with studying a model for the conventional square flat footing shape resting over saturated soft clay. Elliptical, trapezoidal and inverted folded bottom of foundation models are tested and compared with the reference conventional square flat ones. In addition to the experimental study, a numerical analysis is performed using the finite element software ADINA. Verification of the numerical model is conducted first against the experimental results for all the studied cases. The verified numerical model is then extended to cover full-scale footing sizes of conventional and other studied footing cases and shapes. The optimum bottom of foundation shape is the one that gives the lowest soil stresses and consequently the lowest settlements.

Relationship between monotonic and cyclic behavior of saturated soft clay in undrained triaxial compression tests

Cyclic loading–induced deformation of soil is a common problem in engineering practice. In current practice, however, monotonic triaxial tests are more commonly used due to the availability of apparatus and ease of operation. Thus, it would be very useful and practical if monotonic triaxial tests could be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress–strain surface in a three-dimensional p′–q–εa space (p′, effective mean principal stress; q, deviatoric stress; εa, axial strain). It was also found that it is possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure is proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against additional test data from the previous literature.