Effect of Initial Consolidation Stress Path on Shear Behavior of Calcareous Sand

Calcareous sand is widely used as backfill material for land reclamation, but due to its special mechanical characteristics such as easily broken particles, calcareous sand foundation is facing more complex engineering problems. In this study, drained and undrained shear tests were carried out on calcareous sand samples from the South China Sea using K0 consolidation and isotropic consolidation conditions. It was found that the particle breakage of calcareous sand has obvious dependence on the initial consolidation stress path and drainage condition, thus showing different shear strength behavior. The particle breakage under drained shear is greater than that under undrained shear, and the particle breakage under isotropic consolidated shear is greater than that under K0 consolidated shear. The larger the particle breakage, the smaller the dilatancy and internal friction angle of calcareous sand. The dilatancy and internal friction angle of K0 consolidated specimen are larger than those of isotropic consolidated specimen. It is suggested that the actual stress path of calcareous sand should be considered in the engineering.

Experimental investigation and constitutive modeling of the behaviour of highly plastic Lower Rhine Clay under monotonic and cyclic loading

A new experimental series on the highly plastic (I_P = 34 %) Lower Rhine Clay (LRC) is presented. The study comprises tests on normally as well as over consolidated samples under monotonic and cyclic loading. The loading velocity has been varied in order to evaluate the strain rate dependency of the LRC behaviour testifying i.a. the well-known reduction of undrained shear strength with decreasing displacement rate. Isotropic consolidation followed by a cyclic loading with constant deviatoric stress amplitude leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the tests. The inherent anisotropy has been additionally evaluated using samples cut out in either the vertical or the horizontal direction. Furthermore, the behaviour of LRC is compared with the behaviour of low plastic Kaolin silt (I_P = 12:2 %). A new visco-hypoplastic-type constitutive model with a historiotropic yield surface has been used to simulate some of the experiments with cyclic loading. Even the eight-shaped stress loops at cyclic mobility are reproduced well with this model. The data of this paper can be also used by other researchers for the examination, calibration, improvement or development of constitutive models dedicated to fine-grained soils under monotonic and cyclic loading.

Acoustic Emission Behavior of Tropical Residual Soil

emission technique has extensively been used for a wide variety of engineering materials, but less attention has yet been paid on the application of this non-destructive technique to investigate the fundamental behaviors of geomaterial. In the present study, acoustic emission method was adopted in conjunction with the conventional stress-strain-time measurement to investigate the mechanical behaviors of a selected tropical residual soil. A systematic acoustic emission instrumentation setup, which was devised in a monotonic triaxial shear apparatus, was evidenced to provide reasonable experimental results. From the isotropic consolidation results, it was realized that the Kaiser’s effect was observable and the pre-stressed level as induced by compaction could be determined through acoustic emission. In undrained shearing, the acoustic emission response was found to be corresponding with the axial strain measurement. Initial soil yielding, which was mobilized at small strain range, was also able to be determined. The acoustic emission response of the studied tropical residual soil also showed good similarity with the reported soils constituting considerable fines content.

The suction,yield and strength characteristics of unsaturated intact loess based on true tri-axial tests

To simulate the failure of loess under undrained condition in the actual engineering,a series of isotropic consolidation and shear tests with different intermediate principal stress ratio b under constant water content were performed on intact loess with various initial suctions using the true tri-axial apparatus for unsaturated soil. The relationship between the saturations and initial suctions,the characteristics of yield,suction and strength of unsaturated intact loess were studied. The results show that the initial suctions and the suctions after the isotropic consolidation decrease with the increase of saturations. The suctions increase with the increase of the intermediate principal stress ratio b at the true triaxial shear failure. The net mean yield stress increase with the increase of the initial suction. The yield suction is a constant,but not always equal to the maximum suction that the soil specimen experienced in the history. The strength of soil increase with the increase of the net confining pressure,initial suction and the intermediate principal stress ratio b.

Effects of Initial Shear Stress and Vibration Frequency on the Dynamic Pore-Water Pressure of Saturated Sands

The cyclic triaxial system is used to investigate the effects of confining pressure, initial shear stress, vibration frequency, and dynamic stress on the pore-water pressure characteristics of saturated sand in the Wenchuan area. Results show that the initial shear stress has a remarkable effect on the development of the dynamic pore-water pressure of saturated sand. The greater the initial shear stress, the slower the development curve of the pore-water pressure of saturated sand and the higher the number of cycles required to reach the same pore-water pressure. The larger the initial shear stress, the smaller the dynamic pore-water pressure when the sample is destroyed. Moreover, the maximum pore-water pressure ratio decreases linearly with the increase of the consolidation ratio. The normalised relationship curve between the dynamic pore-water pressure and failure time of vibration is consistent with the development law of the power function. The power function model parameters are affected by the initial shear stress and confining pressure. At the time of isotropic consolidation, the accumulation law of pore-water pressure presents a growth pattern of “fast-stable-intensified.” A modified pore-water pressure model considering vibration frequency is proposed on the basis of the Seed pore-water pressure model, and the model parameters are linear with the vibration frequency. When the vibration frequency remains unchanged, the parameter does not change with the confining pressure and dynamic stress. This modified model can predict the change rule of pore-water pressure with the frequency under isotropic consolidation.

The elastic undrained modulus Eu50 for stiff consolidated clays related to the concept of stress history and normalized soil properties

The paper presents the results of a triaxial test conducted on stiff, consolidated clays. The standard TXCIU procedure (isotropic consolidation and undrained shearing) was applied in the laboratory soil tests. The undrained elastic modulus Eu50 was determined from each test. The Eu50 values were determined for soil samples cut out from different depths and tested under different confining pressures. There was a significant scatter of values with depth, and no relationships between Eu50 modules or other geotechnical parameters (e.g., cu) were observed. This work presents the concept of normalization of Eu50 modulus values using modified normalization SHANSEP (Stress History And Normalized Soil Engineering Properties). This method was first proposed for estimating the value of the undrained shear strength cu normalizing the parameter relative to the in situ effective vertical stress σ′vo and loading history (overconsolidation stress σ′p and overconsolidation ratio OCR) of the soil. The study demonstrated that the concept of normalization of soil properties can also be used for testing elastic modulus Eu50 of consolidated natural clays and normalized values of geotechnical parameters taking into account the state of stress and load history can be correlated with the value of the overburden pressure.