Xiaoxia Guo

Recognition of the Stress-Strain Curve Based on the Local Deformation Measurement of Soil Specimens in the Triaxial Test

A digital image technique was developed and used to measure the deformation distribution over the entire surface of soil specimens in triaxial tests. The measured deformation process shows that the specimen apparently exhibits three states during the test, i.e., pre-failure, in-failure, and post-failure, in correspondence to the different features of its deformation. The deformation feature in each state is then analyzed by the displacement and strain contours. Additionally, the stress level, S, is calculated to determine whether the soil is in failure at a point (representative element volume (REV)) on the surface of the specimen. Next, the failure zone, namely the shear band, was considered to be enveloped by the curve defined by S = 1 on the stress-level contour map. The stress level is calculated based on the strain, Youngs modulus and Poissons ratio. According to analysis of the deformation feature and the failure process, we recognized the following properties: (i) the deformation of the specimen was approximately uniform in the pre-failure state; (ii) failure occurred from a point and developed gradually until the shear band cleaves the specimen; (iii) in the post-failure state, deformation was exclusively due to the blocks of the specimen sliding along the shear band; (iv) the deformation feature in the shear band was quite different from that outside the shear band. In conclusion, the stress-strain curve of the specimen revealed a structural response, not an elementary response, especially in the in-failure and post-failure states, in which the deformation features of different corner points were different, and the observed deformation for the entire specimen may be the combination of local deformations. Therefore, it is not appropriate to build the constitutive model for soil according to the stress-strain curves of the entire deformation process and to take the specimen as a uniform element in the entire process.

A Crack Segmentation Approach Using the Combination of Gray Thresholds and Fractal Feature

A segmentation method of combining gray-level threshold and fractal feature for crack images is proposed, and the fractal law for the perimeter and area of the target is introduced as the constraint condition for the image segmentation of crack. At first, Otsu algorithm is used for the initial segmentation of the crack image, and then the edge of crack is optimized in accordance with fractal law. At last, boundary of crack is determined, and the final result of the image segmentation is obtained. This method makes full use of the fractal geometry law and image information, to effectively solve the problems such as crack contour detection, regional connection and cross crack identification. Several typical examples are analyzed, and the results show that this method has a good segmentation effect on crack images, and it can also be used to identify the other images which have fractal feature.

A Method for Directly Measuring the Hydraulic Conductivity of Unsaturated Soil

Numerous methods, such as the Gardner outflow analysis method and the van Genuchten-Mualem prediction model, have been developed to estimate the hydraulic conductivity of unsaturated soil in multistep outflow experiments. In the Gardner outflow analysis method, the hydraulic conductivity is estimated based on the diffusivity calculated from the line fitted to the water outflow. In contrast, in the van Genuchten-Mualem prediction model, it is predicted via the curve-fitting parameters based on the measured soil water retention curve (SWRC). However, a major drawback of these two approaches is that variation of hydraulic conductivity during each step of the outflow process cannot be represented. In this paper, an improved pressure plate instrument with an evaporation compensation system is developed to measure the transient water content curve (TWCC) of the soil in multi-step outflow experiments. The SWRCs of silicon micro-powder (SMP) and Guangxi Guiping clay (GGC) are derived from this transient curve. Therefore, a direct measurement method is presented to describe the change of hydraulic conductivity during each step. Comparison with the Gardner outflow analysis method and the van Genuchten-Mualem prediction model indicates that our approach shows better agreement with test data for these two types of soils.

Rapidly Determining the Unsaturated Permeability by Transient Two-Steps Outflow Experiment

Numerous methods have been developed to measure the unsaturated permeability in multi-step outflow experiments. Silicon micro-powder (SMP) and Guangxi Guiping clay (GGC) have been conducted by transient outflow methods in this paper. The water release upon two-steps increase in matric suction. For each step of the experiment, the water outflow from the soil specimen is weighed in real time in order to determine the change of the water content in the specimen. As a gas pressure is exerted, the difference of the matric potential between the transient water content curve (TWCC) and the soil water retention curve (SWRC) at the same water content is calculated. This difference is considered to have the potential to drive the flow of pore water. Then, the unsaturated permeability can be calculated by using Darcy’s law. This two-steps measurement method agrees with the Gardner outflow analysis method (1956), which have the same ranges of variation for unsaturated permeability. Additionally, the test time is greatly reduced, and generally takes 10 days.

Current Situation of Constitutive Model for Soils Based on Thermodynamics Approach

Traditional constitutive models of soils were based on Drucker’s Stability Postulate and the theory of plastic potential. The yield condition, flow rule and hardening law are determined independently and contradict each other sometimes, which results the probability of violating thermodynanics laws. Thermodynamic-based constitutive models for soils are newly developed constitutive models. This approach has the advantage that the models are guaranteed to obey the laws of thermodynamics by use of dissipative incremental function and free energy function, while retrospective criteria need not be applied. The development history of constitutive models based on thermodynamic approach is outlined, the recent study situations are analysed, and the classification of the models is discussed which indicate a broad field of applications to modeling constitutive behavior for soils.

A New Setup to Measure Hydraulic Properties of Unsaturated Soils

The measurement of the hydraulic properties of an unsaturated soil is time consuming and complex, and the water evaporation loss may cause error to the measurement of hydraulic conductivity in multi-step outflow experiment. In this paper, an improved pressure plate instrument is introduced, which can overcome the deficiency of the impact of the water evaporation loss on the measurement of the hydraulic properties. Then, the hydraulic properties of silicon micro-powder (SMP) and Guangxi Guiping clay (GGC) were conducted under five cycles of drying and wetting by this instrument, respectively. We found that the hysteresis effect was apparent less with the number of drying and wetting cycles increases, and tended to exhibit a zero hysteresis under fifth cycle. Additionally, the transient water content curves (TWCCs) of these two soils can be obtained, and the hydraulic conductivity for these two soils can be calculated by Shao et al. (2017) method based on TWCC and SWRC, which can get more data points to present the variation of hydraulic conductivity during each step of the outflow process.

On Effective Stress and Effective Stress Equation

The concept of effective stress and the effective stress equation is fundamental for establishing the theory of strength and the relationship of stress and strain in soil mechanics. However, up till now, the physical meaning of effective stress has not been explained clearly, and the theoretical basis of the effective stress equation has not been proposed. Researchers have not yet reached a common understanding of the feasibility of the concept of effective stress and effective stress equation for unsaturated soils. Focusing on these problems, new viewpoints for explicitly elucidating the effective stress and deriving the effective stress equation are given in this paper, including that the effective stress should be defined as the soil skeleton stress due to all the external forces excluding pore fluid pressure, and that the soil skeleton should include a fraction of pore water which can bears and passes the load together with soil particles. The relationship between the effective stress and the shear strength and the deformation of unsaturated soils is preliminarily verified by experiments and quoting test data from literature.

Digital Image Measurement System for Soil Specimens in Triaxial Tests

In order to measure the deformation distribution over the entire surface of soil specimens in triaxial tests, a digital image technique is developed by use of a complementary metal-oxide-semiconductor (CMOS) camera and two mirrors to obtain the deformation field and the strain field of the whole surface of the specimen. Two sets of circular light-emitting diode (LED) lights are placed on the top and the bottom of the pressure chamber. A shading box seals the camera and the plate glass in front of the chamber to ensure an unchanged lighting environment. The process of data processing involves the error check, the expanding and the splicing of the images, and the calculation of the strain in each element. The measurement precision of the strain is approximately 10–4 ~ 10–5. And then, the application of this digital image processing technique is introduced in the study of shear band problem, membrane embedding problem, the heterogeneous soil, the end contact and end constraint, and so on. At last, based on the digital image measurement system, a series of geotechnical test instruments have been developed, including: (1) Advanced triaxial compression test apparatus; (2) Bidirectional dynamic triaxial apparatus; (3) High pressure triaxial compression apparatus; (4) Plane strain compression apparatus; (5) Unsaturated soil triaxial compression apparatus; (6) Frozen soil triaxial apparatus; (7) Asphalt concrete stress-strain apparatus; (8) Hydrate triaxial apparatus; (9) Soft material testing machine.

Permeability function for unsaturated soil

Abstract This paper proposes a permeability function to estimate the water permeability coefficient of unsaturated soil. It is derived on the basis of the equilibrium equation of the water phase. Only one parameter is required in the proposed function, which is similar to the Brooks–Corey model. However, this parameter can be more easily obtained by using permeability tests of saturated soil with different porosity values. A number of test results in the literature are employed to validate the proposed permeability function, and the model prediction exhibits good agreement with the experimental data.

Investigation of the mechanical behaviour of an unsaturated soil mixture using a digital image measurement system

Abstract A triaxial apparatus that includes a digital image measurement system (DIMS) is used to investigate the mechanical behaviour of a compacted soil mixture of tailing sand and speswhite kaolin. The experimental programme involves a series of soil–water retention curve (SWRC) tests, isotropic compression tests and suction-controlled triaxial tests. A non-contact method for measuring the sample volume is developed based on the strain field obtained from the DIMS. Particular attention is given to the volumetric behaviour of the soil, and three distinct stages are observed during the consolidation of the suction-controlled triaxial tests. Then a void ratio-dependent shear strength model is proposed, incorporating the influence of the change in void ratio on the SWRC pattern. The model is validated against the experimental data from the tests, showing that it can provide a more accurate prediction of shear strength than that of conventional predictions using a unique SWRC. Moreover, the proposed model is capable of representing several important aspects of the shear behaviour of unsaturated soils, such as the dependence of suction-induced strength on net normal stress and density effect on the shear strength.