Modified state-surface approach to the study of unsaturated soil behavior. Part III: Modeling of coupled hydromechanical effect

2012 ◽  
Vol 49 (1) ◽  
pp. 98-120 ◽  
Author(s):  
Xiong Zhang ◽  
Robert L. Lytton
Keyword(s):  
Unsaturated Soils ◽  
Coupling Effect ◽  
Degree Of Saturation ◽  
Compression Tests ◽  
Soil Behavior ◽  
Volume Changes ◽  
Surface Approach ◽  
Soil Skeleton ◽  
Hydromechanical Behavior ◽  
Undrained Conditions

A modified state-surface approach (MSSA) was proposed in the authors’ previous study to investigate volume change of the soil skeleton for unsaturated soils. This paper discussed the coupling effect between volume changes of soil skeleton, water phase, and air phase for unsaturated soils based on the proposed MSSA and experimental results presented by other researchers. The MSSA was further extended to study the coupled hydromechanical behavior for unsaturated soils. Besides void ratio constitutive surface, conventional unique water content and degree of saturation constitutive surfaces were also divided into elastic and plastic regions by loading–collapse (LC) yield curves and simultaneously used to describe the coupled hydromechanical behavior for unsaturated soils. A general theoretic formulation was derived for the simultaneous use of the MSSA under isotropic conditions. Based on the derived formulation, existing elastoplastic models were reviewed, and compatibility and consistency in modeling the mechanical and hydraulic behavior of unsaturated soils were discussed. Afterwards, the results of a number of suction-controlled laboratory isotropic compression tests at different suctions were used to demonstrate the ability of the proposed approach to reproduce the observed soil behavior quantitatively and soil behavior under undrained conditions qualitatively.

Modified state-surface approach to the study of unsaturated soil behavior. Part I: Basic concept

2009 ◽  
Vol 46 (5) ◽  
pp. 536-552 ◽  
Author(s):  
Xiong Zhang ◽  
Robert L. Lytton
Keyword(s):  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Stress Path ◽  
Complex Stress ◽  
Soil Behavior ◽  
Elastic Surface ◽  
Surface Approach ◽  
Basic Model ◽  
Plastic Hardening ◽  
Complex Stress Path

The traditional state-surface approach to the study of unsaturated soil behavior is becoming much less popular these days, as it uses unique constitutive surfaces to represent unsaturated soil behavior. This approach is essentially a nonlinear elastic formation and cannot be used to explain complex stress-path dependency for unsaturated soils. In this paper, a modified state-surface approach (MSSA) is proposed to represent unsaturated soil behavior under isotropic stress conditions in which a conventional void-ratio state surface is considered to be made up of an elastic surface and a plastic hardening surface. The plastic hardening surface remains stationary at all times, whereas the elastic surface remains unchanged when the soil experiences elastic deformation and moves downward when there is plastic hardening occurrence. Using the MSSA, the loading–collapse (LC) and the suction increase (SI) yield curves in the Barcelona basic model (BBM) are derived. The prediction of three typical cases of soils under isotropic conditions and experimental results using the proposed approach confirmed its feasibility, simplicity, and potential for the study of unsaturated soil behavior.

scholarly journals Compression mechanisms of unsaturated clay under high stresses

2015 ◽  
Vol 52 (12) ◽  
pp. 2099-2112 ◽  
Author(s):  
Woongju Mun ◽  
John S. McCartney
Keyword(s):  
Unsaturated Soils ◽  
Water Pressure ◽  
High Stress ◽  
Control Mode ◽  
Gradual Transition ◽  
Compression Tests ◽  
Suction Control ◽  
Unsaturated Clay ◽  
Undrained Conditions ◽  
Drained And Undrained Conditions

This paper investigates the compression behavior of unsaturated clay under mean stresses up to 160 MPa and different drainage conditions. A new isotropic pressure cell was developed that incorporates matric suction control using the axis-translation technique, and a high-pressure syringe pump operated in displacement-control mode was used to control the total stress and track specimen volume changes. In addition to presenting results from characterization tests on the cell, results from a series of isotropic compression tests performed on compacted clay specimens under drained and undrained conditions are presented. These results permit evaluation of the hardening mechanisms and transition points in the compression curve with increasing effective stress. As expected, specimens tested under undrained conditions were much stiffer than those tested under drained conditions. In the drained tests, the rate of compression was sufficient to permit steady-state dissipation of excess pore-water pressure except under the highest stress ranges. Suction-induced hardening was observed when comparing saturated and unsaturated specimens tested in the drained compression tests. In both the drained and undrained compression tests, the range of applied stresses was sufficient to cause collapse or dissolution of the air voids (pressurized saturation) and convergence of the virgin compression lines for unsaturated specimens with that measured for saturated specimens. A gradual transition to full-void closure was observed at high stresses when the compression curves were plotted on a natural scale, but the shapes of the compression curves at high stresses were not consistent with conventional soil mechanics models when plotted on a semilogarithmic scale. The results from this study provide insight into how constitutive models for unsaturated soils can be extended to high stress conditions for drained and undrained conditions.

scholarly journals A Unified Elastoplastic Model of Unsaturated Soils Considering Capillary Hysteresis

2013 ◽  
Vol 2013 ◽  
pp. 1-15
Author(s):  
Tiantian Ma ◽  
Changfu Wei ◽  
Pan Chen ◽  
Huihui Tian ◽  
De'an Sun
Keyword(s):  
Soil Water ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Mechanical Response ◽  
Internal State ◽  
Degree Of Saturation ◽  
State Variables ◽  
Soil Behavior ◽  
Capillary Hysteresis ◽  
Modified Cam Clay

Unlike its saturated counterparts, the mechanical behavior of an unsaturated soil depends not only upon its stress history but also upon its hydraulic history. In this paper, a soil-water characteristic relationship which is capable of describing the effect of capillary hysteresis is introduced to characterize the influence of hydraulic history on the skeletal deformation. The capillary hysteresis is viewed as a phenomenon associated with the internal structural rearrangements in unsaturated soils, which can be characterized by using a set of internal state variables. It is shown that both capillary hysteresis and plastic deformation can be consistently addressed in a unified theoretical framework. Within this context, a constitutive model of unsaturated soils is developed by generalizing the modified Cam-Clay model. A hardening function is introduced, in which both the matric suction and the degree of saturation are explicitly included as hardening variables, so that the effect of hydraulic history on the mechanical response can be properly addressed. The proposed model is capable of capturing the main features of the unsaturated soil behavior. The new model has a hierarchical structure, and, depending upon application, it can describe the stress-strain relation and the soil-water characteristics in a coupled or uncoupled manner.

Hydromechanical behaviour of overconsolidated unsaturated soil in undrained conditions

2019 ◽  
Vol 56 (11) ◽  
pp. 1609-1621 ◽  
Author(s):  
Shengshen Wu ◽  
Annan Zhou ◽  
Jie Li ◽  
Jayantha Kodikara ◽  
Wen-Chieh Cheng
Keyword(s):  
Effective Stress ◽  
Unsaturated Soils ◽  
Axial Strain ◽  
Triaxial Compression ◽  
Volumetric Strain ◽  
Degree Of Saturation ◽  
Triaxial Tests ◽  
Conventional Triaxial Compression ◽  
Hydromechanical Behaviour ◽  
Undrained Conditions

Hydromechanical behaviour of an unsaturated silt with various suctions and different overconsolidated ratios (OCRs) was investigated through a series of undrained triaxial tests (constant water contents, CW). All the samples were prepared from the slurry state. Different OCRs (= 1, 2, 4, and 8 in net stress) were achieved by unloading the samples to 400, 200, 100, and 50 kPa from an initial confining net pressure of 400 kPa. Then the samples were dried to various suctions (0, 100, 200, 300, and 400 kPa). Unsaturated samples with different OCRs were then sheared at CW conditions following the conventional triaxial compression (CTC) paths. Full hydromechanical responses including the changes in deviator stress, stress ratio, volumetric strain, suction, and degree of saturation with axial strain were monitored and are presented in this paper. Some key findings include (i) the critical state for unsaturated soils with different OCRs can be well defined by Bishop’s effective stress; (ii) the peak strength in Bishop’s effective stress increases with increase of OCR, but decreases with increase of suction in the undrained condition; and (iii) the volume change of unsaturated soils in undrained conditions is related to OCRs and the volume of pore air.

Capillary-based effective stress formulation for predicting shear strength of unsaturated soils

2015 ◽  
Vol 52 (12) ◽  
pp. 2067-2076 ◽  
Author(s):  
Jean-Marie Konrad ◽  
Marc Lebeau
Keyword(s):  
Shear Strength ◽  
Effective Stress ◽  
Unsaturated Soils ◽  
Water Retention ◽  
Adsorbed Water ◽  
Soil Strength ◽  
Liquid Films ◽  
Degree Of Saturation ◽  
Stress Parameter ◽  
Effective Stress Parameter

A number of investigations have shown that the shear strength of unsaturated soils can be defined in terms of effective stress. The difficulty in this approach lies in quantifying the effective stress parameter, or Bishop’s parameter. Although often set equal to the degree of saturation, it has recently been suggested that the effective stress parameter should be related to an effective degree of saturation, which defines the fraction of water that contributes to soil strength. A problematic element in this approach resides in differentiating the water that contributes to soil strength from that which does not contribute to soil strength. To address this difficulty, the paper uses theoretical considerations and experimental observations to partition the water retention function into capillary and adsorptive components. Given that the thin liquid films of adsorbed water should not contribute to effective stress, the effective stress parameter is solely related to the capillary component of water retention. In sample calculations, this alternative effective stress parameter provided very good agreement with experimental data of shear strength for a variety of soil types.

scholarly journals Modelling free gas overpressure in peat layers

2020 ◽  
Vol 195 ◽  
pp. 02027
Author(s):  
Stefano Muraro ◽  
Cristina Jommi
Keyword(s):  
Unsaturated Soils ◽  
Barometric Pressure ◽  
Pressure Oscillations ◽  
Global Response ◽  
Free Gas ◽  
Soil Skeleton ◽  
Changes Over Time ◽  
Fully Coupled ◽  
Over Time ◽  
Numerical Approaches

The paper assesses fully coupled hydro-mechanical numerical approaches developed for unsaturated soils to model the effect of free gas overpressure on the response of peat layers. A simple linear model is used for the soil skeleton, however, the global response is non-linear due to changes over time of the compressibility of the solid skeleton over the compressibility of the fluid, and solubility of gas in water. The overpressure generated in foundation peat layers by barometric pressure oscillations is modelled, and the results are compared to literature data. The development of pore overpressure upon unloading is analysed as a function of the soil skeleton compressibility, and the consequences on the average stress acting on the soil skeleton are discussed.

A flexible wall permeameter for measurements of water and air coefficients of permeability of residual soils

2003 ◽  
Vol 40 (3) ◽  
pp. 559-574 ◽  
Author(s):  
Agus Setianto Samingan ◽  
Eng-Choon Leong ◽  
Harianto Rahardjo
Keyword(s):  
Flow Rate ◽  
Unsaturated Soils ◽  
Temperature Fluctuations ◽  
Air Permeability ◽  
Matric Suction ◽  
Residual Soils ◽  
Volume Changes ◽  
Flexible Wall ◽  
Drying And Wetting Cycles ◽  
Suction Flow

This paper describes the development of a flexible wall permeameter apparatus for measuring the water and air coefficients of permeability of residual soils. Water and air coefficients of permeability were obtained for both the drying and wetting cycles for residual soils using the flexible wall permeameter. Volume changes of the soil specimens during unsaturated consolidation and during the water and air permeability measurements were also monitored. Factors that influence the measurements, such as hydraulic and pneumatic gradients and temperature fluctuations are described and discussed in this paper. The developed flexible wall permeameter was found to be capable of measuring water and air coefficients of permeability as low as 10–12 m/s.Key words: flexible wall permeameter, permeability, residual soils, unsaturated soils, matric suction, flow rate.

scholarly journals Impact of the moisture content in medium sands on CPTU test results

2016 ◽  
Vol 48 (3) ◽  
pp. 221-231 ◽  
Author(s):  
Łukasz Zawadzki ◽  
Marek Bajda
Keyword(s):  
Moisture Content ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Degree Of Saturation ◽  
Test Results ◽  
Time Dynamic ◽  
Tip Resistance ◽  
The Impact ◽  
Resistance Value

Abstract Soils occurring in the soil “active zone” are in contact with the surface and are directly influenced by external factors (mainly climatic changes) that cause variation in their parameters over time. Dynamic and uncontrolled changes of soil properties e.g. due to rainfall and evapotranspiration processes may affect field test results leading to the misinterpretation of the obtained data. This paper presents investigations on the influence of moisture content changes in sandy soils on CPTU results. For this purpose, a field ground model has been constructed and five CPTU tests with a different moisture content of soil were carried out. During the investigations, the tip resistance (qc), friction on sleeve (fs), and pore water pressure (u2) were measured. Moreover, a TDR probe was applied to determine the distribution of the moisture content in the studied soil columns. Differences between CPT results obtained in saturated and unsaturated soils have been shown. Furthermore, a simple equation to correct the tip resistance value due to the impact of the degree of saturation has been proposed.

Modified State Surface Approach to Study Unsaturated Soil Hysteresis Behavior

2020 ◽  
Vol 2674 (10) ◽  
pp. 484-498
Author(s):  
Beshoy Riad ◽  
Xiong Zhang
Keyword(s):  
Mechanical Behavior ◽  
Unsaturated Soils ◽  
Construction Material ◽  
Constitutive Models ◽  
Experimental Results ◽  
Special Focus ◽  
Surface Approach ◽  
Hysteresis Behavior ◽  
Mechanical Hysteresis ◽  
Hydraulic Hysteresis

Unsaturated soils are often used as a construction material in transportation infrastructures. In this situation, unsaturated soils are subjected to cyclic mechanical loading from traffic loads or wetting-drying cycles in seasonal climatic conditions. While mechanical hysteresis is a common feature of soils in general, hydraulic hysteresis is associated with unsaturated soils. Although several constitutive models for unsaturated soils have been proposed, the mechanical and hydraulic hysteresis behavior of unsaturated soils has been little studied. A modified state surface approach (MSSA) was first proposed for investigating the mechanical behavior of unsaturated soils. It was then extended to study the coupled hydro-mechanical behavior of unsaturated soils with a special focus on the consistency between different soil phases. However, hydraulic and mechanical hysteresis were neglected in MSSA formulations. In this paper, based on evidence from experimental results, the MSSA is extended further to study the coupled hydro-mechanical hysteresis behavior of unsaturated soils. The extended MSSA can reproduce several forms of mechanical and hydraulic behavior observed in experimental results that cannot be represented by existing constitutive models. To demonstrate the capabilities of the extended MSSA, typical behaviors are simulated and compared, qualitatively, with the characteristic trends of the behavior of unsaturated soils. Experimental results from the literature are then used to evaluate the model to predict, quantitatively, the observed behaviors. The agreement between measured and predicted results is considered satisfactory and confirms the possibility of the proposed approach to reproduce the hysteresis behavior of unsaturated soils.

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