SoilVision Systems Ltd

Saskatoon, Canada

SoilVision Systems Ltd

Saskatoon, Canada

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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.


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.


Lu H.H.,SoilVision Systems Ltd. | Xu L.M.,SoilVision Systems Ltd. | Fredlund M.D.,SoilVision Systems Ltd. | Fredlund D.G.,1721 8th St. E.
Geotechnical Special Publication | Year: 2014

Limit equilibrium analysis of slopes has been commonplace in the geotechnical industry for many years. The 2D approach is conservative in that 3D geometric influences are not accounted for in the 2D analysis. The conservatism associated with a 2D analysis has been viewed as a «buffer» or added factor of safety but such conservatism can be problematic. Recent software tools allow for an improved analysis of 3D slopes through limit equilibrium analysis techniques. The industry as a whole must also examine the use of 3D analysis in light of how design expectations are managed. The shear strength reduction (SSR) technique calculates the factor of safety based on an FEM analysis of stresses. However, the SSR technique remains a new analysis in the geotechnical industry. The purpose of this paper is to compare 3D finite element stability analysis with 3D limit equilibrium analysis through the examination of benchmark examples. The classic differences between 2D finite element stress analysis and 2D limit equilibrium stress analysis will be examined. Continuity when going from 2D to 3D analysis is examined. © 2014 American Society of Civil Engineers.


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.


PubMed | SoilVision Systems Ltd. and University of Zagreb
Type: | Journal: Waste management (New York, N.Y.) | Year: 2016

The variability of untreated municipal solid waste (MSW) shear strength parameters, namely cohesion and shear friction angle, with respect to waste stability problems, is of primary concern due to the strong heterogeneity of MSW. A large number of municipal solid waste (MSW) shear strength parameters (friction angle and cohesion) were collected from published literature and analyzed. The basic statistical analysis has shown that the central tendency of both shear strength parameters fits reasonably well within the ranges of recommended values proposed by different authors. In addition, it was established that the correlation between shear friction angle and cohesion is not strong but it still remained significant. Through use of a distribution fitting method it was found that the shear friction angle could be adjusted to a normal probability density function while cohesion follows the log-normal density function. The continuous normal-lognormal bivariate density function was therefore selected as an adequate model to ascertain rational boundary values (confidence interval) for MSW shear strength parameters. It was concluded that a curve with a 70% confidence level generates a confidence interval within the reasonable limits. With respect to the decomposition stage of the waste material, three different ranges of appropriate shear strength parameters were indicated. Defined parameters were then used as input parameters for an Alternative Point Estimated Method (APEM) stability analysis on a real case scenario of the Jakusevec landfill. The Jakusevec landfill is the disposal site of the capital of Croatia - Zagreb. The analysis shows that in the case of a dry landfill the most significant factor influencing the safety factor was the shear friction angle of old, decomposed waste material, while in the case of a landfill with significant leachate level the most significant factor influencing the safety factor was the cohesion of old, decomposed waste material. The analysis also showed that a satisfactory level of performance with a small probability of failure was produced for the standard practice design of waste landfills as well as an analysis scenario immediately after the landfill closure.


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.


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.


Fredlund M.,SoilVision Systems Ltd | Lu H.,SoilVision Systems Ltd | Feng T.,Royal Dutch Shell
Geotechnical Special Publication | Year: 2011

The strength of levees can be affected during fluctuations in the water table. It is also possible for the climate to have an influence on the position of the water table in an earth levee. Traditional methods have resulted in approximate methods for dealing with the transient fluctuations of the water table in a levee. These approximations are generally accepted in engineering practice but the question can be rightfully raised as to how these approximations compare to a rigorous transient combined seepage and slope stability analysis. Software technology has significantly changed in recent years and is now at the point where it is much easier to perform transient seepage analyses. There are new questions that can be asked. Does an effective stress analysis diverge significantly from the 3-stage Duncan (1990) analysis? If so, under what conditions? This paper compares the Duncan (1990) three-stage methodology for analyzing rapid drawdown scenarios to a combined transient seepage and slope stability analysis. Traditional limit equilibrium methods will be utilized in the slope stability analysis and the accommodation of saturated and unsaturated pore-water pressures will be considered. Analyses of a number of typical cross-sections will be considered in order to determine the potential influence of geometry. The intent of the paper is to illustrate scenarios under which the Duncan (loc. cit.) methodology produces similar results to the results of a more rigorous analysis. © 2011 ASCE.


Fredlund M.,SoilVision Systems Ltd | Gitirana Jr G.,Federal University of Goais
Geotechnical and Geological Engineering | Year: 2011

The numerical modeling of unsaturated soil processes is becoming more prevalent worldwide. Although numerical modeling is becoming increasingly accepted in geotechnical engineering practice, care must be exercised and improper modeling techniques and procedures must be avoided. Many issues such as nodal resolution and imperfect convergence can result in inaccurate solutions. Unfortunately, analyses of highly nonlinear unsaturated soil flow and slope stability models can significantly increase the modeling time required. As a result, there is a trend to reduce the number of model runs. Results are often presented to client as single model runs or simplistic sensitivity analysis. This paper presents methodologies for applying probabilistic methods to unsaturated soils seepage and slope stability analysis models. The focus is on the application of the alternative point estimate method to practical problems in such a way as to minimize the number of model runs. The demonstration of a successful application to a waste rock pile is presented. © 2011 Springer Science+Business Media B.V.


Fredlund M.,SoilVision Systems Ltd | Zhang J.,SoilVision Systems Ltd
Proceedings of the International Conference on Cold Regions Engineering | Year: 2012

The cold climate environment can cause damage to engineering structures such as roadway/railways and pipelines. There has been an increased use of finite element numerical models to study and understand the processes involved. The primary difficulty in the application of such numerical models is the lack of proper coupling between the related processes of water, thermal, and airflow. This paper presents classic benchmarks solved with a finite element numerical modeling code which demonstrates the coupling of thermal, hydraulic, and air (THA) processes. The use of this technology potentially opens the use of numerical models to a wider range of applications in artic regions. © 2012 American Society of Civil Engineering.

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