Use of piezocone tests to predict consolidation yield stress and overconsolidation ratio of lagoonal deposit soil

Existing empirical methods for interpreting the consolidation yield stress and the overconsolidation ratio （OCR） in clays from piezocone tests are briefly reviewed. It can be seen that no universal correlation exists for all worldwide sites. However, for a given clay deposit, there does exist a most appropriate method to reflect the consolidation yield stress and the OCR based on piezocone test data. Three empirical methods are compared based on the piezocone test data collected on Lixia River lagoonal deposit soil sites in the north of Jiangsu province. The objective of this study is to evaluate the validity of the existing relationships linking the consolidation yield stress to piezocone test data and identify the appropriate method for Lixia River lagoonal deposit soil. It is shown that the correlation based on the net tip resistance has much higher accuracy for estimating the consolidation yield stress of lagoonal deposit soil than other methods.

Investigation of the overconsolidation and structural behavior of Shanghai clays by element testing and constitutive modeling

The mechanical properties and constitutive modeling of Shanghai clays are very important for numerical analysis on geotechnical engineering in Shanghai,where continuous layers of soft clays run 30-40 m deep.The clays are divided into 5 major layers.A series of laboratory tests are carried out to investigate their mechanical properties.The top and bottom layers are overconsolidated hard clays,and the middle layers are normally consolidated or lightly overconsolidated sensitive marine clays.A constitutive model,which can describe the overconsolidation and structure of soils using only 8 parameters,is modified to simulate the test results.A rational procedure to determine the values of the material parameters and initial conditions is also proposed.The model is able to effectively reproduce both one-dimensional(1D)consolidation and drained/undrained triaxial test results of Shanghai clays,with one set of parameters for each layer.From element testing and constitutive modeling,two findings are obtained.First,the decay rates of overconsolidation are smaller in overconsolidated layers than in normally consolidated layers.Second,the natural microstructure of layer 4 is relatively stable,that is,a large degree of structure is still maintained in the specimen even after 1D consolidation and drained triaxial tests.The modified model and obtained parameter values can be used for numerical analysis of geotechnical projects in Shanghai.

Elastoplastic Model for Soils Considering Structure and Overconsolidation

An isotropic hardening elastoplastic model for soil is presented, which takes into consideration the influence of structure and overconsolidation on strength and deformation of clays. Based on the superloading concept and subloading concept, the inner structural variable ω and overconsolidation variable ρ are introduced to describe the structure and overconsolidation of soil. The present model requires three additional parameters which can be obtained by conventional triaxial test, and the other parameters are same as those of modified Cam-clay(MCC) model. The performance of the proposed model is verified by undrained and drained triaxial tests.

Methods for evaluating overconsolidation ratio from piezocone sounding results

Methods for estimating the overconsolidation ratio(OCR)of soil deposits from piezocone sounding results have been investigated.Three existing methods have been briefly reviewed and a new method has been proposed.The proposed method can be applied to both normally consolidated/overconsolidated and underconsolidated deposits.Furthermore,existing methods have been modified so that they can be applied to underconsolidated deposits.Then,all the methods have been applied to 12 case histories collected from 6 different countries.The estimated values of OCR have been compared with the measured data.It has been shown that if the parameters(soil properties and empirical parameters)can be determined appropriately,then all the methods can achieve a reasonable prediction.It has also been shown that the proposed method exhibits a relatively better performance and results in less scattered data than the other methods.

Cyclic stress-controlled tests on offshore clay

One of the main concerns in cyclic behavior of soft clay is gradual degradation with the progression of loading cycle.A series of cyclic constant-volume direct simple shear(CDSS) loading tests was performed on Malaysia offshore clay to study its undrained degradation.The testing program consists of stresscontrolled tests with cyclic shear ratio ranging from 0.34 to 0.83 at different overconsolidation ratios(OCRs).For a given cyclic stress ratio in stress-controlled tests,the accumulated cyclic strain and pore water pressure increase with elevated number of cycles.In heavily overconsolidated clay specimens,the negative cyclic pore water pressure is generated followed by positive cyclic pore water pressure as cyclic tests progress.The post-cyclic strength of offshore clay specimens is reduced by undrained cyclic stresscontrolled loading.

Revising the unified hardening model by using a smoothed Hvorslev envelope

The original unified hardening(UH) model, in which a straight Hvorslev envelope was employed to determine the potential peak stress ratio of overconsolidated soils, is revised using a smoothed Hvorslev envelope(Hermite-Hvorslev envelope). The strength at the intersection between the straight Hvorslev envelope and the critical state surface(i.e. Mohr-Coulomb envelope) can be undefined due to the discontinuous change in the slope of the two linear strength envelopes mentioned above. A smoothed Hvorslev envelope is derived through Hermite interpolation to ensure a smooth change between the proposed Hvorslev envelope and the zero-tension surface as well as a smoothed transition between the proposed Hvorslev envelope and the critical state surface. The Hermite-Hvorslev envelope is then integrated into the original UH model, and then the UH models with four different functions of the Hvorslev envelope are compared with each other. The UH model revised by the Hermite-Hvorslev envelope can well predict the mechanical behaviors of normally consolidated and overconsolidated soils in drained and undrained conditions with the same parameters in the modified Cam-Clay model.

Using Piezocone Dissipation Test to Estimate the Undrained Shear Strength (<i>s_u</i>), <i>N_kt</i>and <i>N_Δu</i>Factors in Cohesive Soils

The current practice of geotechnical engineering commonly uses a combination of theoretical and empirical correlations to estimate the soil undrained shear strength in clays from the piezocone test. In order to complement the use of such correlations, the application of a method to estimate the soil undrained shear strength, using measures of the excess pore pressure in dissipation tests of piezocone is presented. In cohesive soils, excess pore pressure and undrained shear strength are dependent on the same variables (stress state, stress history, soil stiffness), which allows them to be related by the theoretical cavity expansion-critical state framework. This paper mentions the mathematical formulation that supports the theoretical framework used, its relationship with the Nkt and NΔu factors and their estimation in a case studied. The results obtained are consistent within the dispersion found in the international literature and encourage the use of the method in engineering practice.

An elastic-viscous-plastic model for overconsolidated clays

The influences of time on clays are discussed first,and the concept of the instant normal compression line is proposed by analyzing the existing theories and experimental results.Based on the creep law,the relationship between the aging time and the overconsolidation parameter is built.With the reloading equation of the UH model(unified hardening model for overconsolidated clays) used to calculate the instant compression deformation,a one-dimensional stress-strain-time relationship is proposed.Furthermore,the evolution of this relationship is analyzed,and the characteristic rate that is a function of the overconsolidation parameter is defined.Then a three-dimensional elastic-viscous-plastic constitutive model is suggested by incorporating equivalent time into the current yield function of the UH model.The new model can describe not only creep,rate effect and other viscous phenomena,but also shear dilatancy,strain softening and other behaviors of overconsolidated clays.Besides,compared with the modified Cam-clay model it requires only one additional parameter(the coefficient of secondary compression) to consider the creep law.Finally,because the proposed model can be changed into the UH model under instantaneous loading,the elastic-plastic and elastic-viscous-plastic frameworks are unified.

UH model considering temperature effects

The influences of temperature on the mechanical behavior of saturated clays are discussed first. Based on the concept of true strength and the revised calculation method of the potential failure stress ratio, the equation of the critical state stress ratio for saturated clays under different temperatures is deduced. Temperature is introduced as a variable into the UH model （3-dimensional elastoplastic model for overconsolidated clays adopting unified hardening parameter） proposed by Yao et al. and then the UH model considering temperature effects is proposed. By means of the transformed stress method proposed by Yao et al., the proposed model can be applied conveniently to 3-dimensional stress states. The strain-hardening, softening and dilatancy behavior of overconsolidated clays at a given temperature can be described using the proposed model, and the volume change behavior caused by heating can also be predicted. Compared with the modified Cam-clay model, the proposed model requires only one additional parameter to consider the behavior of the decrease of preconsolidation pressure with an increase of temperature. At room temperature, the proposed model can be changed into the original UH model and the modified Cam-clay model for overconsolidated clays and normally consolidated clays, respectively. The considered temperature range here is from the melting point to the boiling point of the pore water （e.g. the experimental temperatures （20℃-95℃） mentioned in this paper are within this range）. Comparison with existing test results shows that the model can reasonably describe the basic mechanical behavior of overconsolidated clays under various temperature paths.

Comparison Study of Constitutive Models for Overconsolidated Clays

Widely distributed in natural deposits,the overconsolidated(OC)clays have attracted extensive experimental investigations on their mechanical behaviors,especially in the 1960s and 1970s.Based on these results,numerous constitutive models have also been established.These models generally fall into two categories:one based on the classical plasticity theory and the other the bounding surface(BS)plasticity theory,with the latter being more popular and successful.The BS concept and the subloading surface(SS)concept are the two major BS plasticity theories.The features of these two concepts and the representative models based on them are introduced,respectively.The unified hardening(UH)model for OC clays is also based on the BS plasticity theory but distinguishes itself from other models by the integration of the reference yield surface,unified hardening parameter,potential failure stress ratio,arid transformed stress tensor.Modification is made to the Hvorslev envelop employed in the UH model to improve its capability of describing the behaviors of clays with extremely high overconsolidation ratio in this paper.The comparison among the BS model,SS model,and UH model is performed.Evidence shows that all these three models can characterize the fundamental behaviors of OC clays,such as the stress dilatancy,strain softening and attainment of the critical state.The UH model with the revised Hvorslev envelop has the fewest parameters which are identical to those of the modified Cam-Clay model.

Thermo-elasto-viscoplastic mechanical behavior of manmade rock and its numerical modeling

In this study,a thermo-elasto-viscoplastic model for soft rock is proposed to describe its fundamental mechanical behavior of soft rock such as the influences of overconsolidation,intermediate principal stress,temperature,time dependency and inherent structure with a unified set of parameters.In the model,the concepts of subloading and superloading yielding surfaces are introduced to describe the influence of overconsolidation and structure on the deformation and strength of soft rock.The influence of the intermediate principal stress is also properly considered by adopting a transformed stress tensor t_(ij).To consider the temperature effect,a thermo-induced equivalent stress is introduced to consider its influence on the yielding surfaces,the evolution of the overconsolidation,and the structure when subjected to shearing loading under different conditions.By comparing the calculated results with test results of a manmade rock under different loading and temperature conditions,the availability and accuracy of the proposed model are carefully investigated,and the performance of the proposed model is verified in detail.

Constitutive model for overconsolidated clays

Based on the relationships between the Hvorslev envelope,the current yield sur-face and the reference yield surface,a new constitutive model for overconsolidated clays is proposed. It adopts the unified hardening parameter,to which the potential failure stress ratio and the characteristic state stress ratio are introduced. The model can describe many characteristics of overconsolidated clays,including stress-strain relationships,strain hardening and softening,stress dilatancy,and stress path dependency. Compared with the Cam-clay model,the model only re-quires one additional soil parameter which is the slope of the Hvorslev envelope. Comparisons with data from triaxial drained compression tests for Fujinomori clay show that the proposed model can rationally describe overconsolidated properties. In addition,the model is also used to predict the stress-strain relationship in the isotropic consolidation condition and the stress paths in the undrained triaxial compression tests.

SSUH model: A small-strain extension of the unified hardening model

A small strain unified hardening(SSUH) model is proposed in the present study to tackle the small strain behavior of clay. The model is an extension of the unified hardening(UH) model for overconsolidated(OC) clays accounting for the small strain stiffness. The new features of the SSUH model over the UH model include:(a) a new elastic hysteretic stress-strain relationship to evaluate the stiffness degradation at small strains and to generate the hysteresis loop under cyclic loading condition;(b) a revised unified hardening parameter to enhance the plastic stiffness at small strains; and(c) a new overconsolidation parameter, which is crucial to make the UH model working with the elastic hysteretic stress-strain relationship effectively. With these enhancements, the SSUH model can describe a high initial stiffness and the highly nonlinear stress-strain relationship at small strains, in addition to the shear dilatancy and strain hardening/softening behaviors of OC clays at large strains. In comparison with the Modified Cam-clay(MCC) model, the proposed model needs two more small strain related parameters, which can be easily obtained from laboratory tests. Finally, some drained triaxial compression tests at large strains, drained triaxial compression/extension tests at small strains, an undrained compression test at small strains and a drained cyclic constant radial stress test are employed to validate the new model.

Modeling of strength and deformation of overconsolidated clays based on bounding surface plasticity

An elastoplastic constitutive model for overconsolidated clays is established in the framework of the critical state theory and bounding surface plasticity theory. The bounding surface is defined as the maximum yield surface in the loading history. A yielding ratio, i.e., an internal variant, is defined as the size ratio of the current yield surface to the corresponding bounding surface. The yielding ratio instead of the overconsolidation ratio(OCR) is used to evaluate the strength and stress-strain behaviors of overconsolidated clays in the shearing process. The bounding stress ratio incorporating the effect of the yielding ratio is used to characterize the potential failure strength of the overconsolidated clays. The dilation stress ratio taking into account the effect of the yielding ratio is applied to describe the dilatancy behaviors of the overconsolidated clays. Comparisons between model predictions and test data show that the proposed model could well capture the strength and stress-strain behaviors of normally consolidated and overconsolidated clays.