scholarly journals Investigating the Effect of Geocell Changes on Slope Stability in Unsaturated Soil

2020 ◽  
Vol 14 (1) ◽  
pp. 66-75
Author(s):  
Behnam Mehdipour ◽  
Hamid Hashemolhosseini ◽  
Bahram Nadi ◽  
Masoud Mirmohamadsadeghi
Keyword(s):  
Slope Stability ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Bending Strength ◽  
Load Capacity ◽  
Three Dimensional ◽  
Bending Moment ◽  
Soil Conditions ◽  
Vertical Side ◽  
Basic Model

The purpose of this research is to investigate the performance and efficiency of reinforced slope in the stability of geocell layers in unsaturated soil conditions. Slope reinforced with geocell acts like a beam in the soil due to the geocell having a height (three-dimensional). Due to its flexural properties, it has moment of inertia as well as bending strength, which reduces the displacement and increases the safety factor of the slope. Taking into consideration unsaturated conditions of soil contributes a lot to making results close to reality. One of the well-known models among elastoplastic models for modeling unsaturated soils is Barcelona Basic Model, which has been added to the FLAC2D software by codification. Changes in thickness, length and number of geocell layers are remarkably effective on slope stability. The results show that the geocell's reinforcing efficiency depends on the number of layers and depth of its placement. As the depth of the geocell's first layer increases, the lateral and vertical side elevation of the upper part of the slope increases with respect to the elevation. Load capacity increases with increasing geocell length. By increasing the length of the geocell layer, the joint strength, the mobilized tensile strength, and the bending moment are increased. At u/H = 0.2, an increase in the bending momentum of about 20% occurs with increasing geocell thickness. In u/H = 1, the increase in bending momentum is 10.4%. In addition, by increasing the thickness of the geocell, the Value of moment of the inertia increases and, as a result, the amount of geocell reinforcement bending moment increases.

Stress Prediction of Buried Pipes Subjected to Operational Loadings in Unsaturated Soils

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Chamal Randeniya ◽  
Dilan Robert ◽  
Chun-Qing Li
Keyword(s):  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Three Dimensional ◽  
Soil Condition ◽  
Temporal Variations ◽  
Soil Conditions ◽  
Fe Model ◽  
Fe Method ◽  
Buried Pipes ◽  
Stress Prediction

Abstract Pipelines are used to provide variety of services in modern community and have grown rapidly in past few decades due to growing socio-economic requirements. Most of the water mains are buried in shallow depths where the soil is partially saturated with significant spatial and temporal variations. Even though the behavior of buried pipes in such unsaturated soil condition is substantially different when compared to dry or fully saturated soil, the effect of soil saturations is overlooked in the current pipe stress prediction methods, leading to unrealistic predictions of the pipe stresses. In this study, three-dimensional (3D) finite element (FE) method was employed with advanced constitutive soil models to analyze the behavior of pipes buried in unsaturated soil condition. Having validated the FE model using reported field test data, an analytical model was proposed to predict the maximum stress in buried pipes considering soil saturation effect using a series of 3D FE analyses. Results from the FE analyses reveal that the maximum pipe stress can be significantly different when soil is in unsaturated condition when compared to dry condition. The proposed formula shows a good agreement with the field data and FE results, so that the expression can be used in the prediction of maximum pipe stress when they are buried under realistic (i.e., nondry) soil conditions.

Soil Strength Reduction Method Induced by Variation of Water Content

2012 ◽  
Vol 170-173 ◽  
pp. 847-852
Author(s):  
Peng Ming Jiang ◽  
Zhong Lei Yan ◽  
Peng Li
Keyword(s):  
Slope Stability ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Strength Reduction ◽  
Actual State ◽  
Soil Slope ◽  
Simple Method ◽  
The Common ◽  
The Stability

As the complexity of unsaturated soil theory, and it must have a long test period when we study the unsaturated soils, so the conventional design analysis software does not provide such analysis, so we can imagine that such a slope stability analysis does not accurately reflect the actual state of the slope. Based on the known soil moisture content，this paper use the soil water characteristic curve and strength theory of unsaturated soil to calculate the strength reduction parameters of soil which can calculate the stability of the soil slope when using the common calculation method. It is noticeable that this method can be extended and applied if we establish regional databases for this simple method, and these databases can improve the accuracy of the calculation of slope stability.

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.

Second Canadian Geotechnical Colloquium: Appropriate concepts and technology for unsaturated soils

1979 ◽  
Vol 16 (1) ◽  
pp. 121-139 ◽  
Author(s):  
D. G. Fredlund
Keyword(s):  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Three Dimensional ◽  
Earth Pressure ◽  
Saturated Soil ◽  
Engineering Practice ◽  
The Matrix ◽  
Matrix Suction

A practical science has not been fully developed for unsaturated soils for two main reasons. First, there has been the lack of an appropriate science with a theoretical base. Second, there has been the lack of an appropriate technology to render engineering practice financially viable.This paper presents concepts that can be used to develop an appropriate engineering practice for unsaturated soils. The nature of an unsaturated soil is first described along with the accompanying stress conditions. The basic equations related to mechanical properties are then proposed. These are applied to practical problems such as earth pressure, limiting equilibrium, and volume change.An attempt is made to demonstrate the manner in which saturated soil mechanics must be extended when a soil is unsaturated. Two variables are required to describe the stress state of an unsaturated soil (e.g., (σ – ua) and (ua – uW). There is a smooth transition from the unsaturated case to the saturated case since the pore-air pressure becomes equal to the pore-water pressure as the degree of saturation approaches 100%. Therefore, the matrix suction (i.e., (ua – uW) goes to 0 and the pore-water pressure can be substituted for the pore-air pressure (i.e., (σ – uW)).The complete volumetric deformation of an unsaturated soil requires two three-dimensional constitutive surfaces. These converge to one two-dimensional relationship for a saturated soil. The shear strength for an unsaturated soil is a three-dimensional surface that reduces to the conventional Mohr–Coulomb envelope for a saturated soil.The manner of applying the volumetric deformation equations and the shear strength equation to practical problems is demonstrated. For earth pressure and limiting equilibrium problems, the unsaturated soil can be viewed as a saturated soil with an increased cohesion. The increase in cohesion is proportional to the matrix suction of the soil. For volume change problems it is necessary to have an indication of the relationship between the various soil moduli.There is a need for further experimental studies and case histories to substantiate the proposed concepts and theories.

Modeling the unsaturated soil zone in slope stability analysis

2014 ◽  
Vol 51 (12) ◽  
pp. 1384-1398 ◽  
Author(s):  
L. L. Zhang ◽  
Delwyn G. Fredlund ◽  
Murray D. Fredlund ◽  
G. Ward Wilson
Keyword(s):  
Shear Strength ◽  
Stability Analysis ◽  
Slope Stability ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Matric Suction ◽  
Slope Stability Analysis ◽  
Engineering Practice ◽  
Rate Of Increase ◽  
Nonlinear Shear

The linear form of the extended Mohr–Coulomb shear strength equation uses a [Formula: see text] parameter to quantify the rate of increase in shear strength relative to matric suction. When the [Formula: see text] value is unknown, a [Formula: see text] equal to 15° is sometimes used in the slope stability study to assess the influence of matric suction on the stability of a slope. In many cases, however, a [Formula: see text] value of zero is used, signifying that the effect of matric suction is ignored. Experiment results have shown that the relationship between the shear strength of an unsaturated soil and matric suction is nonlinear. Several semi-empirical estimation equations have been proposed relating the unsaturated shear strength to the soil-water characteristic curve. In this paper, the results of a study using two-dimensional slope stability analysis along with an estimated nonlinear shear strength equations is presented. The effects of using an estimated nonlinear shear strength equation for the unsaturated soils are illustrated using three example problems. Several recommendations are made for engineering practice based on the results of the example problems. If the air-entry value (AEV) of a soil is smaller than 1 kPa, the effect of matric suction on the calculated factor of safety is trivial and the [Formula: see text] value can be assumed to be zero. If the AEV of a soil is between 1 and 20 kPa, the nonlinear equations of unsaturated shear strength should be adopted. For soils with an AEV value between 20 and 200 kPa, an assumed [Formula: see text] value of 15° provides a reasonable estimation of the effects of unsaturated shear strength in most cases. For soils with an AEV greater than 200 kPa, [Formula: see text] can generally be assumed to be equal to the effective angle of internal friction, [Formula: see text], in applications where geotechnical structures have matric suctions around 100 kPa.

Volume–mass unsaturated soil constitutive model for drying–wetting under isotropic loading–unloading conditions

2011 ◽  
Vol 48 (2) ◽  
pp. 280-313 ◽  
Author(s):  
Hung Q. Pham ◽  
Delwyn G. Fredlund
Keyword(s):  
Constitutive Model ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Research Literature ◽  
Degree Of Saturation ◽  
Soil Conditions ◽  
Constitutive Relationships ◽  
Proposed Model ◽  
Wide Range

A rigorous volume–mass constitutive model is proposed for the representation of drying–wetting under isotropic loading–unloading conditions for unsaturated soils. The proposed model utilizes concepts arising from soil physics and geotechnical engineering research and requires readily obtainable soils data for soil properties. The model can be used to predict void ratio and water content constitutive relationships (and therefore degree of saturation) for a wide range of unsaturated soils. Various stress paths (i.e., loading–unloading and drying–wetting) can be simulated, and hysteresis associated with the soil-water characteristic curve is taken into account. Two closed-form equations for the volume–mass constitutive relationships are presented for soils starting from slurry conditions. A number of test results (i.e., from experimental programs reported in the research literature) were used during the verification of the proposed volume–mass constitutive model. The volume–mass constitutive model captures key unsaturated soil conditions such as air-entry value, water-entry value, and residual conditions. The proposed model appears to satisfactorily predict unsaturated soil behavior for soils ranging from low compressible sands to high compressible clays.

Unified hardening (UH) model for overconsolidated unsaturated soils

2014 ◽  
Vol 51 (7) ◽  
pp. 810-821 ◽  
Author(s):  
Y.P. Yao ◽  
L. Niu ◽  
W.J. Cui
Keyword(s):  
Unsaturated Soils ◽  
Path Dependence ◽  
Three Dimensional ◽  
Constitutive Models ◽  
Stress Path ◽  
Triaxial Tests ◽  
Additional Material ◽  
Basic Model ◽  
Proposed Model ◽  
Unsaturated Clays

Naturally deposited clays are often unsaturated and overconsolidated. Within the frameworks of the Barcelona Basic model (BBM) for normally consolidated unsaturated clays and the unified hardening (UH) model for overconsolidated saturated clays, a three-dimensional constitutive model for overconsolidated unsaturated clays is proposed in this paper. This model can be reduced to the original UH model for overconsolidated saturated clays when suction becomes zero and the BBM when the overconsolidated behaviour disappears. Compared with existing constitutive models for unsaturated clays, the influence of a high overconsolidation ratio (OCR) on wetting deformation can be adequately described. Also, many other characteristics of overconsolidated unsaturated clays can be modelled, including strain-hardening, softening, shear dilatancy, and stress path–dependence behaviour. Compared with the BBM, the proposed model requires no additional material parameter. The validity of the UH model for overconsolidated unsaturated clays has been confirmed by data from two groups of wetting tests performed by the authors and previous triaxial tests in the literature.

scholarly journals Development of a pore gas pressure transducer used in unsaturated soils at high water content

2019 ◽  
Vol 92 ◽  
pp. 02003
Author(s):  
Rui Chen ◽  
Zhongkui Chen ◽  
Charles Wang Wai Ng ◽  
Jian Liu
Keyword(s):  
Slope Stability ◽  
Water Content ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Pressure Transducer ◽  
High Water ◽  
Gas Pressure ◽  
Water Infiltration ◽  
High Water Content ◽  
Pore Gas Pressure

Pore gas pressure in soil is an important parameter in many geoscience applications such as evaluating the effects of trapped pore gas pressure on water infiltration through soil mass, optimizing the design of gas extraction wells in landfills and assessing the performance of landfill covers in reducing landfill gas emission. In addition, it has been observed that pore gas pressure affects slope stability in unsaturated soils. However, the pore gas pressure build-up induced by water infiltration is generally ignored in most slope stability analysis by assuming gas pressure to be zero. Therefore, pore gas pressure measurement in soils is crucial to better understand the unsaturated soil behaviour. However, most of current measuring techniques of pore gas pressure are affected by water interruption during the measurement in unsaturated soils, especially at high water content. In this study, a novel gas pressure transducer was developed to measure the pore gas pressure in unsaturated soil within a wide range of water content. The newly developed pore gas pressure transducer mainly consists of an electrical pressure sensor package and an integrated membrane filter which can prevent water leaching through the membrane but allow gas to pass it freely. The performance of the gas pressure transducer was evaluated by a series of permeation tests. The results show that the developed gas pressure transducer has a good repeatability to monitor gas pressure and has a relatively fast response to the gas pressure change in compacted soils. This transducer is able to measure pore gas pressure range of 0~50 kPa of soils within a relatively high range of soil water content.

scholarly journals STUDY OF THE INFLUENCE OF A BORED PILE DIAMETER ON CHANGE OF COEFFICIENT OF SUBGRADE REACTION AT CALCULATION FOR HORIZONTAL LOADS

2019 ◽  
Vol 9 (4) ◽  
pp. 11-15
Author(s):  
Airat Z. GAISIN ◽  
Sergey A. KRUTYAEV ◽  
Anton O. GLAZACHEV
Keyword(s):  
Large Diameter ◽  
Three Dimensional ◽  
Bending Moment ◽  
Soil Conditions ◽  
Subgrade Reaction ◽  
Pile Head ◽  
Bored Pile ◽  
Pile Diameter ◽  
Numerical Studies ◽  
Bored Piles

The problem of designing foundations using long bored piles of large diameter is shown. Such piles are most often used in the construction of buildings and structures, on the foundations of which large loads are transferred, and such buildings are often built on sites with difficult soil conditions. When designing foundations using such piles, it becomes necessary to calculate them for horizontal load and bending moment. The article is devoted to studies of the dependence of the coefficient of subgrade reaction on the diameter of piles when calculating long bored piles of large diameter in clay soils. To determine the patt erns of changes in the coefficient of subgrade reaction from the diameter of the piles, numerical studies in a three-dimensional setting were performed. Based on the results obtained, the graphs «load - displacement» are constructed. The method of calculating the coefficient of subgrade reaction with known movements of the pile head and the applied load is shown. The regularities of changes in the deformability of the soil base with an increase in the diameter of the pile are revealed and a coefficient taking into account this dependence is proposed.

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