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Zhang L.L.,Shanghai JiaoTong University | Fredlund D.G.,Golder Associates | Fredlund M.D.,SoilVision Systems Ltd | Ward Wilson G.,University of Alberta
Canadian Geotechnical Journal | Year: 2013

The linear form of the extended Mohr–Coulomb shear strength equation uses a φb parameter to quantify the rate of increase in shear strength relative to matric suction. When the φb value is unknown, a φb 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, aφb 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 φb 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 φb 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, φb can generally be assumed to be equal to the effective angle of internal friction, φ, in applications where geotechnical structures have matric suctions around 100 kPa. © 2014 Published by NRC Research Press. Source


Fredlund D.G.,Golder Associates | Houston S.L.,Arizona State University | Nguyen Q.,Golder Associates | Fredlund M.D.,SoilVision Systems Ltd
Geotechnical and Geological Engineering | Year: 2010

A continuum mechanics approach is used for the formulation of unsaturated hydraulic conductivity functions and the water storage functions for fractured or cracked clay soils in this parametric study. Suggested procedures are based on available research literature related to the behavior of cracked unsaturated porous media. The soil-water characteristic curve, hydraulic conductivity and water storage functions take on the character of bi-modal unsaturated soil property functions. The bimodal character arises out of the independent behavior of the cracks and the intact clay soil. Matric suction changes beneath a slab-on-grade foundation placed on a cracked clay soil profile are modeled for varied surface flux conditions using the proposed unsaturated hydraulic conductivity and water storage functions. The impact of various levels of surface cracking on soil suction distributions is discussed. Suction distribution patterns are dependent on the initial soil surface suction. In particular, the results are dependent upon whether the initial matric suction is less than or greater than the air entry of the cracked clay. There is an extremely wide range of possible conditions that could be modeled but the parametric study results presented in this paper are limited to a series of selected crack widths and densities for an exfiltration case and an infiltration case. © 2010 Springer Science+Business Media B.V. Source


Huang M.,CAS Institute of Soil and Water Conservation | Fredlund D.G.,Golder Associates | Fredlund M.D.,SoilVision Systems Ltd
Geotechnical and Geological Engineering | Year: 2010

There are significant advantages in using indirect pedo-transfer functions, (PTFs) for the estimation of unsaturated soil properties. The pedo-transfer functions can be used for the estimation of the soil-water characteristic curve (SWCC) which in turn is used for the estimation of other unsaturated soil properties. The accuracy of the indirect pedo-transfer function method for the estimation of the SWCC depends on the PTF and the equation used to best-fit the particle-size distribution (PSD) data. The objectives of this study are to: (1) evaluate the performance of the Fredlund et al. (Can Geotech J 37:817-827, 2000) equation for best-fitting the particle-size distribution, (PSD) data, and, (2) compare the predictions made by two of the commonly used PTFs; namely, Arya and Paris (Soil Sci Soc Am J 45:1023-1030, 1981) and Fredlund et al. (Can Geotech J 39:1103-1117, 2002), for estimating the SWCC from the PSD. The authors used 258 measured PSDs and SWCC datasets from the Loess Plateau, China, for this study. The dataset consisted of 187 silt-loam soils, 41 loam soils, 11 silt-clay-loam soils, 10 sand-loam soils, 6 silt-clay soils, and 3 loam-sand soils. The SWCC and PSD datasets were measured using a Pressure Plate apparatus and the pipette method, respectively. The comparison between the estimated and measured particle-size distribution curves showed that the Fredlund et al. (Can Geotech J 37:817-827, 2000) equation closely prepresented the PSD for all soils in the Loess Plateau, with a lower root mean square error (RMSE) of 0.869%. The comparison between the estimated and measured water contents at the same suction showed that the Fredlund et al. (Can Geotech J 39:1103-1117, 2002) PTF performed somewhat better than the Arya and Paris (Soil Sci Soc Am J 45:1023-1030, 1981) function. The Fredlund et al. method had RMSE value of 0.039 cm3 cm-3 as opposed to 0.046 cm3 cm-3 for the Arya and Paris (Soil Sci Soc Am J 45:1023-1030, 1981) method. The Fredlund et al. (Can Geotech J 39:1103-1117, 2002) PTF produced the closest predictions for sand-loam, loam-sand, and loam soils, with a lower RMSE for gravimetric water content ranging from 0.006 to 0.036 cm3 cm-3. There were consistent over-estimations observed for silt-loam, silt-clay-loam, and slit-clay soils with RMSE values for gravimetric water content ranging from 0.037 to 0.043 cm3 cm-3. The measured and estimated air-entry values were closest when using the Fredlund et al. (Can Geotech J 39:1103-1117, 2002) PTF. The measured and estimated maximum slopes on the SWCC were closest when using the Arya and Paris (Soil Sci Soc Am J 45:1023-1030, 1981) PTF. © Springer Science+Business Media B.V. 2009. Source


Zhang L.L.,Shanghai JiaoTong University | Fredlund M.D.,SoilVision Systems Ltd | Fredlund D.G.,Golder Associates | Lu H.,SoilVision Systems Ltd | Wilson G.W.,University of Alberta
Engineering Geology | Year: 2015

It is commonly understood that a 2-D slope stability analysis will provide a lower factor of safety than a 3-D slope stability analysis. The difference in the calculated factors of safety between a 2-D and a 3-D analysis is generally less than 15% for simple slope geometries. Most past comparative studies between 2-D and 3-D stability analyses have ignored the effect of negative pore-water pressures (i.e., matric suctions) in the soil zone above the groundwater table. In this paper, a comparison is made between 2-D and 3-D slope stability analyses on soil slopes where a portion of the soil profile has matric suctions. The factors of safety on simple geometry slopes and complex geometry slopes (i.e., slopes which have two intersecting slope surfaces), are investigated for a range of shear strength parameters and groundwater conditions. For simple slopes with a low slope angle, the difference in factor of safety between a 2-D and a 3-D slope stability analysis, (i.e., δFs/Fs 2-D), generally ranges from 9% to 16% when ϕb is equal to 15°. The value of δFs/Fs 2-D for a steep, simple slope is generally larger than for a low angle, simple slope. When ϕb is 15°, the values of δFs/Fs 2-D for the simple, steep slope generally range from 12 to 18%. The difference between a 2-D and a 3-D stability analysis was most pronounced for concave geometries where a portion of the soil profile contained unsaturated soils. The values of δFs/Fs 2-D for corner angle concave slopes with angles ranging between 180 to 270° can be as large as 20 to 59% when ϕb is equal to 15°. Two case histories, (i.e., the highwall stability failure at the Poplar River coal mine and the Kettleman Hills landfill slope failure), were used to illustrate the effect of the unsaturated zone on changes in the factors of safety. © 2015 Elsevier B.V. Source


Fredlund M.D.,SoilVision Systems Ltd | Thode R.,SoilVision Systems Ltd | Nelson R.G.,PDE Solutions Inc.
Unsaturated Soils - Proceedings of the 5th International Conference on Unsaturated Soils | Year: 2011

The quantification of the performance of earth covers has been largely determined through the use of test plots as well as numerical modeling. Such numerical models are run with the use of complex climatological data and unsaturated soil property functions. The input information is then used by the finite element solvers to determine vertical flow rates and, ultimately, long-term percolation rates through the earth cover. Calculations are generally complicated by the fact that the infiltration into a dry soil is one of the more complex types of numerical modeling scenarios. The challenge occurs largely because of the non-linearities present in the unsaturated soil properties. These non-linearities becomes part of the numerical model which must run for 1, 10, or 50 years. This paper examines the numerical difficulties associated with these calculations and examines the impact of small numerical issues over long time periods. In particular, the role of mesh density as it is related to the accuracy of water balance calculations is examined. © 2011 Taylor & Francis Group, London. Source

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