Undrained Shear Strength Of Clay Research Articles

An exponential strength degradation model for clay: From mathematical model to numerical application

Investigation in strain-softening behavior of clay is of great significance for offshore geotechnical engineering. Although several models could be used to characterize the degradation of undrained shear strength of clay with cumulative shear strain, it is necessary to improve accuracy of them. In this study, based on full-flow cyclic penetration test, an exponential strength degradation model with one parameter α is proposed, where α is the function of the fully remolded resistance factor and the ratio of initial extraction to initial penetration resistance. The reliability of the proposed model is verified by comparing indices (strength and sensitivity) calculated by it and by other existing strain-softening models. In addition, a large deformation finite element (LDFE) analysis based on the proposed model is conducted for pipe-clay interaction, and efficiency of the model is verified by comparing numerical results with centrifuge test results. The influence of α on the normalized penetration resistance were discussed, and demonstrate the necessity of accurately evaluating degradation of undrained shear strength of clay.

Utjecaj mineralogije gline na nedreniranu posmičnu čvrstoću pomoću pokusa s padajućim šiljkom

Consistency limits are the basic parameters used as standard inputs for any soil investigation program to evaluate the geotechnical properties of fine-grained soils. The primary objective of this study is to investigate the variations in the undrained shear strength of clays with different plasticity properties based on the fine content, clay mineralogy, and water absorption capacity. To achieve this objective, 33 Fall cone tests were conducted on low-, high-, and very high-plastic clay mixtures. Various blends of different plasticity were ensured by preparing mixtures of Na-montmorillonite (NaM)/Ca-montmorillonite (CaM), Na-montmorillonite (NaM)/Kaolinite (K), and Na-montmorillonite (NaM)/Sepiolite (S). According to the experimental results, the highest liquid limit (LL) was obtained in 100 % NaM clay at 255.07 %, whereas the lowest LL was obtained in 100 % S clay at 33.55 %. Similarly, the highest undrained shear strength was obtained at 160 kPa at 30 % water content in the 100 % NaM clay, whereas the lowest undrained shear strength was obtained at 10 kPa in the 100 % S clay at the same water content. Consequently, based on an experimental framework, empirical equations were obtained which could be used to establish the relationships among cone penetration-water content, liquid limit-clay content, and undrained shear strength-cone penetration.

CatBoost–Bayesian Hybrid Model Adaptively Coupled with Modified Theoretical Equations for Estimating the Undrained Shear Strength of Clay

The undrained shear strength of clay is an important index for the calculation of the bearing capacity of the foundation soil, the calculation of the soil pressure of the foundation pit, and the analysis of the slope stability. Therefore, the purpose of this paper is to conduct a comprehensive study of the combined use of machine learning with clay theoretical equations to estimate it. Under the Bayesian framework, the CatBoost algorithm (CatBoost–Bayesian) based on Bayesian optimization algorithm was developed to obtain the feature importance level of soil parameters affecting the undrained shear strength of clay, so as to adaptively couple the theoretical equation of undrained shear strength of K0 consolidated clay, which was derived from the modified Cambridge model. Then, the theoretical equation of undrained shear strength of the isotropically consolidated clay was established from the critical state of the clay parameters. Finally, it was illustrated and verified using the experimental samples of Finnish clay. The results indicate that the theoretical equation established by the overconsolidation ratio and effective overburden pressure parameters can well estimate the undrained shear strength of isotropically consolidated clays, and the parameter uncertainty can be considered explicitly and rigorously.

Estimating the Bearing Capacity of a Square Footing on a Geogrid-Stabilised Granular Layer over Clay Using the Finite Element Method

The bearing capacity of a square footing on an unstabilised and geogrid-stabilised granular layer over clay is estimated using finite element techniques in ABAQUS (a commercial finite element software). The model incorporates a range of soil parameters and layer thicknesses. The two-layered soil system in the model assumes a strong upper granular layer and a weak lower clay layer. The parametric results of the finite element analyses are used to develop design curves with dimensionless parameters in terms of linear gradient and the ratio of the undrained shear strength of clay to the effective vertical stress of the granular layer. The design curves are compared with the published results in the literature and found to be promising. The developed design curves are applied to estimate the minimum thickness of the granular layer and are compared with other existing methods. This application illustrates the simplicity of the proposed technique and reveals that the proposed approach yields a reasonable prediction for the bearing capacity when compared with other established methods.

The behavior of ball penetrometer in soft-over-stiff soil deposits

Proper understanding of the ball penetration behavior under centrifuge condition is important for accurate soil property determination. In this study, large deformation finite element (LDFE) analyses are conducted to investigate the scale effect of ball penetrometer on its behavior in clay-over-sand deposit in centrifuge tests. The finite element model of this study is validated against previous theoretical solutions and available centrifuge test data with good agreement. Parametric studies are then carried out to explore the effects of potential factors, including the layer thickness ratio, the shaft-ball area ratio, the undrained shear strength of clay, and the relative density of sand. The results indicate that the soil failure mechanisms can be divided into three stages: i) shallow embedment stage, ii) cavity evolution stage, and iii) full-flow stage. The penetration resistance profiles in centrifuge tests can be better quantified, which can provide more accurate shear strength interpretation for the top clay.

Undrained basal stability of braced circular excavations in anisotropic and non-homogeneous clays

This paper introduces the plastic stability solutions of braced circular excavations in anisotropic and non–homogeneous clays. Using the framework of Finite Element Limit Analysis (FELA) under axisymmetric conditions, the upper bound (UB) and lower bound (LB) solutions of the stability of excavations can be obtained. The clay is set to be anisotropic, where the Anisotropic Undrained Shear (AUS) model is used as a failure criterion of the surrounding soil. The results of this study are the proposed stability number which is the normalized parameter of the maximum unit weight and the anisotropic undrained shear strength of clay. Four dimensionless parameters are considered in the study: the anisotropic strength ratio, the depth–radius ratio, the depth–embedment ratio, and the strength gradient ratio. The impact of all considered dimensionless parameters on the results of the FELA solutions is examined. A machine learning regression approach, Multivariate Adaptive Regression Splines (MARS), is employed to develop an empirical design equation to predict the stability number of braced circular excavations in anisotropic and non–homogeneous clays. The proposed MARS equation can be a useful and reliable equation to estimate the basal stability of this excavation problem in practice.

Undrained strength of clay determined from simple shear test

The undrained shear strength of clay is an important parameter for the design of embankments, shallow foundations, and pile foundations. Among the various methods of testing undrained strength, the simple shear is essential when the shearing mode of the soil surrounding the pile is similar to that in a simple shear test. This study employs a series of undrained strength tests with a Berkeley simple shear apparatus. Three reconstituted natural clays and one artificial clay were tested with a range of coefficients of consolidation. The influence factors, including sample pre-consolidation pressure, saturation back pressure, shearing rate, height of specimen, consolidation stress, and lateral stress ratio, were investigated in undrained simple shear tests. The failure mode in simple shear and corresponding strength parameters are also examined. Based on the test data set, models for describing the undrained strength with simple shear for high plasticity clays are developed and compared to test data for normally consolidated reconstituted clay. Good agreement between models and intact Onsoy clay is also observed when allowance is made for the coefficient of consolidation and strength parameters of undisturbed clay and in situ stress state.

Effects of Strain-Softening and Strain-Rate Dependence on the Anchor Dragging Simulation of Clay through Large Deformation Finite Element Analysis

Large-deformation finite element (LDFE) analysis with the coupled Eulerian–Lagrangian (CEL) technique for large-deformation soil functions without twisting or distorting the mesh. However, the model does not consider the strain-softening and strain-rate dependence of clay-based soils. The undrained shear strength of clay is sensitive to the strain rate. In addition, the strain-softening effect of soil strength reduction accompanied by large-scale shear deformation should be considered. In this study, anchor dragging simulations were performed for large-deformation analysis considering strain-softening and strain-rate dependence. Furthermore, a shear strength equation expressing the strain-softening and strain-rate dependence of the Tresca constitutive model was developed based on VUMAT, an ABAQUS/Explicit subroutine. The equation was designed so that it could be linked to the LDFE/CEL model. The model was verified by performing comparative analysis with the Mohr–Coulomb (M–C) perfect-plasticity model. The newly constructed Tresca base strain-softening and strain-rate-dependence VUMAT algorithm in the LDFE/CEL model analysis confirmed the effects of strain-softening and strain-rate dependence. The proposed model enabled a highly realistic simulation of the actual phenomenon than the M–C model. Finally, a parametric study on strain-softening and strain-rate dependence was conducted, and the behavior of clay due to the anchor drag phenomenon was revealed.

A New Approach to Estimate Bearing Capacity of Strip Footings on Geogrid-Stabilised Granular Layer over Clay

In this study, the finite element method (FEM) is used to estimate the bearing capacity of strip footing on a non-stabilised and geogrid-stabilised granular layer over clay. The FEM model was developed using ABAQUS software that accommodates a range of angles of internal friction and thicknesses of overlying stronger granular layer and strength of underlying weaker clay layer. The results are used to develop design charts with dimensionless parameters in terms of linear-gradient (m) and the ratio of undrained shear strength of clay to effective vertical stress at the base of the granular layer ({{c}_{u}/p}_{0}). The proposed charts also contain trend lines with simple power-rule equations for different angles of internal friction of the granular layer. The charts are validated and compared with the published results in the literature and found to be closely similar. A design example is provided to illustrate the comparison of the design curves and to benchmark with other established design methods. The comparative results reveal that the proposed approach yields a comparable outcome and predicts reasonable bearing capacity. Moreover, the developed design charts and the associated equations are relatively simple and easy to use. A typical result of the analysis indicates that the use of a geogrid-stabilised granular layer provides a capacity improvement factor (CIF) of 1.1 to 1.6 depending on the soil parameters and the thickness of the granular layer.

Experimental study on the behaviour of granular column-treated soft clay under shear loading

ABSTRACT Compacted granular columns are frequently used to mitigate construction difficulties over soft soil deposits through a combination of densification, increasing load-bearing capacity and improvement in consolidation features. The behaviour of granular columns under vertical loading has been well recognised; however, this research is aimed to investigate the behaviour of granular column-reinforced soft soil subjected to shear stress. In this contribution, large-scale direct shear tests were performed, and the influence of undrained shear strength of clay, diameter of granular column and relative density of aggregates was examined. In general, the use of granular column has been shown to improve the shear resistance of the composite ground, which was found to be much significant at lower undrained shear strength of the soft clay. Increasing density of aggregates from Dr = 50% to 90% caused a large portion of the shear stress to be sustained by the accumulated friction of aggregates as the internal friction angle of the treated clay increased from 0 to 28°. In addition, test results revealed that the use of highly compacted column yields a lowered shear displacement at failure, which was indirect evidence of stiffness improvement of the composite ground.

Effect of Pore–Fluid Chemistry on the Undrained Shear Strength of Xanthan Gum Biopolymer-Treated Clays

Xanthan gum (XG) biopolymer-based soil treatment is an effective soil improvement method. In this study, we explored the effect of XG on the undrained shear strength of clays (kaolinite and montmorillonite) in chemically distinct pore fluids. Among the pore fluids tested, the undrained shear strength of kaolinite was the highest in kerosene, followed by deionized (DI) water and brine. This study hypothesized that the interparticle forces and interactions dominated the undrained shear strength of the kaolinite clay. In contrast, montmorillonite showed the highest undrained shear strength with DI water, followed by brine and kerosene, likely due to the increase in thickness of the viscous double layer that reduced the undrained strength of the montmorillonite clay. XG also affected the undrained shear strength of the clays, possibly due to the increase in viscosity of the pore fluids and modification of the clay interparticle fabrics. In DI water, XG increased the undrained shear strength of kaolinite (maximum shear strength was observed in the sample with an XG-to-soil mass ratio of 0.5%) via water absorption. Simultaneously, XG decreased the undrained shear strength of montmorillonite, likely due to XG-induced particle aggregation resulting from the changes in the montmorillonite particle surface charges. In 2-M-NaCl brine, the undrained shear strength increased with the increase in XG content, regardless of the mineral types, owing to the salt-induced double-layer compression and increase in concurrent XG-induced pore–fluid viscosity. However, the unaffected and constant undrained shear strength of both clays in nonpolar kerosene suggests that hydrating XG is a prerequisite for its application in soil treatment.

Effect of Total Stress Path and Gas Volume Change on Undrained Shear Strength of Gassy Clay

Clay with free gas bubbles can be frequently encountered in the seabed. Gassy clay is an unsaturated soil, but its mechanical behavior cannot be described using conventional unsaturated soil mechanics because it has a composite internal structure with a saturated soil matrix and gas bubbles. The gas bubbles can have either a detrimental or beneficial effect on the undrained shear strength of clay. New lower and upper bounds for the undrained shear strength of gassy clay are derived by considering the effect of total stress path and plastic hardening of the saturated soil matrix. For the upper bound, it is assumed that there is only bubble flooding, and the shear strength of an unsaturated soil sample is the same as that of the saturated soil matrix. Bubble flooding makes the saturated soil matrix partially drained and increases the undrained shear strength. The amount of bubble flooding is calculated using the modified Cam-Clay model and Boyle's law for ideal gas. The lower bound is derived based on the assumption that the entire soil fails without bubble flooding and the gas cavity size evolves due to plastic hardening of the saturated soil matrix. Compared with Wheeler's upper and lower bounds that do not consider plastic hardening of the saturated soil matrix, the new theoretical results give a better prediction of the undrained shear strength of gassy clays, especially for the upper bound. Implications for constitutive modeling of gassy clay are discussed based on the new research outcomes.

The Strength Assessment for T-bar Penetrometer Tests at Shallow Embedment in Clay considering Strain Softening

The undrained shear strength of clay is important for deep-sea foundation design. T-bar penetrometer as a key tool has been widely used in laboratory and offshore site investigation. To obtain a continuous soil strength profile, it is necessary to apply shallow corrections before the penetrometer reaching the full-flow state. Meanwhile, the actual soil strength may be underestimated if the strain softening behavior is not considered. In this study, the process of T-bar penetration is analyzed for the soil with strain softening. The buoyancy effect on the bearing capacity factor is studied, which shown that the buoyancy mainly has an effect on the test soils with extremely low strength, but have little effect in the field tests. The characteristics of soil heave in the T-bar tests are analyzed to estimate soil sensitivities. To estimate the soil strength at shallow penetration depth, a relation to correct the T-bar bearing factors is built both on the soil with strain softening and no strain softening.

Uncertainties in modelling undrained shear strength of clays using Critical State Soil Mechanics and SHANSEP

The determination of the undrained shear strength of clays relies upon the goodness of the available in-situ and laboratory tests. Often, limited soil investigation data is available, or the collected data may suffer of low quality associated with poor test execution or sampling operations. The use of reliable correlations can then play an important role in geotechnical design. In that perspective, it is vital to choose the most appropriate correlation models that are suitable with the local soil conditions and that are possibly characterized by low uncertainty. The SHANSEP empirical model describes the undrained shear strength of clays in terms of normalized properties, where the soil strength is defined by means of the overconsolidation ratio (OCR) and two material coefficients that require empirical calibration. The SHANSEP model can be further combined with analytical solutions based on Critical State Soil Mechanics (CSSM) in order to define the undrained shear strength as a function of two fundamental properties, such as the preconsolidation stress (or the OCR) and the friction angle at critical state. The paper deals with the uncertainties associated with modelling the undrained shear strength of clays using a hybrid CSSM-SHANSEP model. The performance of the model is assessed by comparing the predicted undrained shear strength to an existing multivariate database of field vane data points from Finland. For each data point, the friction angle is estimated indirectly from the plasticity index using a correlation in the literature, while the OCR is taken directly from the database. Bias and uncertainties of the CSSM-SHANSEP model associated with the multivariate database are evaluated. Finally, a sensitivity study on the model parameters is presented.

DSHANSEP: Alternative considerations of the SHANSEP approach

Since it was first proposed by Ladd and Foott in 1974, the SHANSEP approach has become a quite useful and convenient approach for the evaluation of the undrained shear strength of clay soils in-situ. SHANSEP is commonly practiced with determination of its parameters using various correlations based on consistency limits. While seemingly handy, such correlations hide some details of the behaviour of clayey soils that are useful for engineering judgement. In this article, an alternative way of looking at the SHANSEP method is presented by examining an integrated framework for strength behaviour of clays. This resulted in a new approach called DSHANSEP, where the D represents the so-called “dilatancy parameter” which measures changes in pore pressure due to changes in shear stresses during undrained loading. The resulting association with dilatancy is logical as this parameter reflects failure mechanisms of the clays which is integral aspect of the undrained shear strength of clays. The implication of this relation is explored by looking at its dependence on the overconsolidation ratio, the mobilized stress ratio and sample quality. Furthermore, a formalized system of classification for clays based on the dilatancy parameter and their overconsolidation ratio is proposed and applied for Norwegian clays along with suggested parameters. Use of DSHANSEP and the proposed classifications are in line with the safety philosophy adopted for stability of slopes involving soft clays in Norway.

Using SHANSEP for verification of unreliable piezocone data in clays

The evaluation of the undrained stability of old embankments on soft clays relies upon the accurate modelling of undrained shear strength in the subsoil. It is vital to account for the effects of consolidation to determine the increased shear strength below the embankment. The CPTU test is a fast in-situ test that provides continuous measurements with depth and can be easily performed through the embankment to evaluate the strength increase. The interpretation of undrained shear strength from CPTU requires the selection of cone factors, which are normally based on laboratory or in-situ (e.g. field vane) tests or empirical correlations. However, CPTU measurements can be negatively affected if the test is poorly executed or if the apparatus lacks proper calibration. This paper discusses how the SHANSEP empirical model for clays can be used to verify the reliability of the available CPTU measurements in a given site investigation case. Further, the paper attempts to provide insights into the selection of empirical cone factors for modelling undrained shear strength for embankment stability. The SHANSEP model describes the undrained shear strength of clays in terms of normalized properties, where the soil strength is defined by means of the overconsolidation ratio (OCR) and two material coefficients that require empirical calibration. The applicability of the method is presented for a case study from Western Finland, where CPTU testing was carried out both on the side and underneath an old railway embankment. The results of the study are exploited to suggest recommendations for engineering practice.

Measurement uncertainty of the fall cone (FC) test

The most common method to determine liquid limit, sensitivity and undrained shear strength of clay in Nordic countries is to use fall cone test (FC). However, all these properties include some uncertainty arising from different sources e.g. the geological conditions, test procedure and sample disturbance. In this study, the measurement uncertainty of the fall cone test was analysed. The parameters of interest were the penetration depth of the cone and the interpreted intact undrained shear strength. The test set consisted of high-quality block samples (d=132mm) collected from six different depths at soft clay site in Finland. The diameter of the samples made it possible to perform four parallel fall cone tests per depth, so total of 24 fall cone tests were done. The calculated COV-values for each parameter at each depth were between 0.02 and 0.08, thus indicating very low measurement uncertainty. These values also included the inherent variability of the soil. Overall, the results showed that the fall cone test is reliable method for determining the intact undrained shear strength, if the test is done with care.

Estimation of Distribution Factor for Peak Penetration Resistance Prediction of Spudcan Foundations in Loose to Medium-Dense Sand Overlying Clay

A series of 3D finite element (FE) analyses were performed to estimate the peak penetration resistance of spudcan foundations in sand over clay soil profiles. Elasto-perfectly plastic models following Mohr–Coulomb and Tresca failure criteria were used for sand and clay layers, respectively. The coupled Eulerian–Lagrangian (CEL) approach was used to simulate the large deformation in soil that occurs during the spudcan penetration. The performance of the numerical model was validated against centrifuge test results. A parametric study with a broad range of strength parameters for sand and clay was performed. The numerical results were used to assess the influence of sand thickness (Hs), the diameter of spudcan (D), friction angle of sand, and undrained shear strength of clay (su). A wide range of su was utilized to predict the resistance both of the soft and stiff clays. The calculated peak resistances are compared with a published analytical model. It is demonstrated that the model highly overestimates the peak resistance for stiff clays, most likely because it was developed specifically for soft clays and, therefore, does not account for the influence of su. One of the parameters of the model is revised to account both for su. Comparisons highlight that the modified model is able to capture the simulated peak penetration resistance for both soft and stiff clays.

Two-Dimensional Bearing Capacity of a Spudcan in Clay After Penetrating Through Top Sand

This paper presents a series of numerical investigations of the capacity of the foundations of a spudcan in clay after it penetrates through a sand layer and under planar combined loading. The model is validated against available relationships for uniaxial capacity of spudcan foundations on clay soils. The model is then used to investigate the uniaxial capacity and combined VH, VM, and HM capacity of spudcan foundations in the underlying clay of sand overlying clay soils. The results are presented in the form of failure envelopes. The effects of the embedment depth, undrained shear strength of clay, and sand plug thickness on the uniaxial and combined capacity are investigated and discussed.

The capacities of tripod bucket foundation under uniaxial and combined loading

Tripod bucket foundations are an alternative foundation for offshore wind turbines. Three-dimensional finite element analyses were carried out to determine the capacities of tripod bucket foundations in clay under uniaxial and combined loading. The undrained shear strength of clay is assumed to increase linearly with depth, and the remolded soil around the bucket and reduced foundation-soil friction caused by bucket penetration are modeled through an adhesion factor. Based on the numerical results, expressions for uniaxial capacity factors and failure loci are proposed, as functions of soil non-homogeneity, aspect ratio and bucket spacing ratio. It is found that the failure loci of tripod bucket foundations are similar to that of conventional buckets in mathematic expression, although the bucket spacing ratio that appears in uniaxial capacities is used as a key fitting parameter.