82 research outputs found
Examination of the estimation of relative permeability for unsaturated soils
No full textThe unsaturated permeability function is an important soil property function used in the numerical modeling of saturated-unsaturated soil systems. The permeability function is generally predicted by integrating along the soil-water characteristic curve (SWCC) starting at saturated soil conditions. The integration is based on a particular integral formula. The Fredlund-Xing-Huang permeability function is a flexible integration techniques used for calculating the unsaturated permeability function. The original permeability theory published by Fredlund et al., (1994) specified that the air-entry value, ψaev (AEV), be used as the lower limit of the integration when calculating the permeability function. However, since there was no analytical procedure available for the calculation of the air-entry value on the SWCC, it became common practice to start the integration procedure from a value near zero. The assumption was made that the error associated with starting the integration from an arbitrary low value was minimal. While this might be the case in some situations, the error can be quite substantial in other situations. This paper undertakes a study of the effect of the lower limit of integration on the calculation of the permeability function. Comparisons are made between starting the integration from various values below the AEV and starting the integration from the calculated air-entry value, ψaev. A mathematical algorithm is also proposed for the calculation of the AEV for integration purposes. The results show that the relative coefficient of permeability can be significantly under-estimated when the lower limit of integration is smaller than the AEV. The recommendation is that the AEV always be used as the lower limit of integration in the Fredlund-Xing-Huang permeability equation.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
A bearing capacity approach to the design of low-volume traffic roads
No full textPavement design methods based on the elastic layer theory idealize the pavement structure as consisting of linear elastic layers and utilize the theory of elasticity to predict limiting stresses and strains. The assumption of elastic behavior may be valid for relatively stiff pavement materials. In unpaved roads, consisting of unbound granular bases overlying cohesive subgrades, the assumption of elastic behavior is unlikely to be valid. The behavior of such pavements under traffic stresses is markedly nonlinear. Pavement design methods based on the ultimate strength approach assume shear failure of the pavement structure at sufficiently high traffic stresses. Pavement material behavior is assumed to be plastic rather than elastic. The assumption of plastic response is more realistic for unpaved roads in which traffic stresses exceed the elastic range of the pavement materials. The determination of the ultimate wheel load that a pavement structure can sustain is the most important component of a design process based on bearing capacity theory. Existing solutions are restricted to a narrow range of material properties and are also deficient in the manner in which they determine ultimate wheel loads. General and accurate solutions for the determination of the bearing capacity of pavement structures are required. The incorporation of climatic factors in the pavement design process is another important component of design based on bearing capacity theory. Existing methods assume full saturation of the subgrade. Experience has shown that in many regions of the world full saturation rarely occurs and the assumption of full saturation leads to overdesign. There is a need to incorporate the influence of matric suction in the determination of ultimate wheel loads. A limit equilibrium solution, which can handle any combination of pavement material properties, is proposed for the determination of bearing capacity in a 2-layer pavement system. To enable the incorporation of climatic factors in the determination of ultimate wheel loads, limit equilibrium solutions are proposed for the determination of the effects of positive pore-water pressures and matric suction on bearing capacity. The solution developed for the influence of matric suction on bearing capacity is verified in the laboratory using model footing tests in homogeneous soils equilibrated under constant levels of matric suction. A simple method of testing compacted soils in the direct shear apparatus as well as a method of analyzing the test results in terms of the stress state variables is proposed. The method of testing and analysis is shown to give results which are comparable to the results of the modified direct shear test. The method is considered to be a simple and viable alternative for the characterization of shear strength of compacted unsaturated soils. Finally, a method based on bearing capacity theory is proposed for designing unpaved roads whose structure consists of a base layer overlying a subgrade. The method can handle any combination of shear strength parameters as well as constant levels of matric suction in the pavement layers
Verification of the Fredlund (2019) Unsaturated Shear Strength Function
No full textThere has been a proliferation of equations proposed to describe the unsaturated shear strength envelope going back to the 1970s. However, there have been limited studies to verify the suitability of one unsaturated shear strength equation over another. Most proposed shear strength equations have attempted to relate the shear strength of an unsaturated soil to some aspect(s) of the soil–water characteristic curve (SWCC). Estimation procedures have generally focused on using that of air-entry value (AEV) as defined by the drying (or desorption) branch of the degree of saturation SWCC (S-SWCC). This paper studies the suitability of using two “anchor points” (or reference points) along the drying S-SWCC to estimate the unsaturated soil shear strength function. The anchor points referred to are the air-entry value (AEV) of the soil and the “residual suction point” of the soil defined in terms of the S-SWCC. Shear strength conditions associated with both so-called anchor points are used as “boundary conditions” that should be satisfied when estimating the shear strength function for unsaturated soils. Past research laboratory measurements published in the research literature are used as part of the verification process for this study
Soil evaporative fluxes for geotechnical engineering problems
No full textPredicting the flow of water between the soil surface and the atmosphere is a critical issue which geotechnical engineers must resolve for many practical problems. The analysis of saturated-unsaturated groundwater flow problems requires the specification of the flux of water at the upper soil boundary. The flow of moisture at the ground surface is also important for many problems in soil mechanics. These include the analysis of volume change in expansive soils and the stability of slopes.
The transfer of water across the soil-atmosphere surface occurs as infiltration and evaporation. The mechanics of infiltration into the soil surface is well understood and has been widely addressed in the literature. Alternately, the
process of evaporation from the soil surface is poorly understood. Extreme difficulties frequently arise while predicting evaporation from unsaturated soil surfaces.
Empirical methods of estimating evaporation from unsaturated soil surfaces can be found in the literature: however, the suitability and accuracy of these methods can be questioned.
A theoretical approach for evaluating the rate of evaporation from an unsaturated soil surface is developed. The theory is based on the principles of Darcy's Law and Fick's Law to describe the flow of liquid water and water vapour in the saturated-unsaturated soil below the surface. Dalton's Law and a modified form of Penman's Method for evaporation are utilized to predict evaporation from the soil surface.
Drying tests were conducted using three distinct soil types of sand, silt and clay. The soil surfaces were found to evaporate at the same rate as free water surfaces when saturated. The rate of evaporation begins to decline once the soil surfaces become unsaturated and total suction exceed approximately 3000 kPa. The rate of evaporation is proportional to total suction and continues to decline as suction increases. This principle appears independent of soil type and universal for the three texturally distinct soils selected for testing. The rate of evaporation may be predicted on the basis of the water content of the soil and its moisture retention curve established using routine test procedures.
The proposed theory was used to simulate the results of a 42 day evaporation test for a column of fine, uniform, clean sand. Good agreement was generally found between the computed and measured values of evaporation rate, soil water content and soil temperature. Additional analyses were conducted using various values of the saturated coefficient of permeability and the pore-size distribution index. The computed evaporative fluxes were found to be very sensitive to the permeability of the soil. Varying the coefficient of molecular diffusion for water vapour was also found to influence the rate of evaporation.
The modified Penman expression was applied to an example evaporation problem for Saskatoon, Saskatchewan during a 10 day period in July. The evaporative fluxes were computed with the watertable positioned at several depths below the surface of the sand. Evaporative rates were found to vary widely between the full potential rate of 7.7 mm/day and 0.4 mm/day depending on the position of the water table. In general, the results showed that the rate of evaporation from a soil surface depends strongly on the groundwater conditions
Preferential flow in vertically oriented, unsaturated soil layers
No full textPreferential flow paths develop where particular areas of a geologic profile become more conductive than the surrounding material. Research has been conducted in recent years on preferential flow and the flow of water through unsaturated soils. Unfortunately, the research has been completed by different disciplines of science and engineering, resulting in a wide range of terminology and research objectives.
A field program conducted during the excavation of a large waste rock pile at Golden Sunlight Mine in Montana defined the structure of the pile to consist of steeply-dipping, fine and coarse layers. The fine layers located near the top of the pile were wet and oxidized while the coarse layers were dry and unoxidized. The results of the field program indicated the development of preferential pathways through the fine-grained layers.
A column study was developed to investigate the potential development of preferential flow in vertically layered, unsaturated systems. To achieve this objective, a column was constructed that enabled the amount of lateral flow between two adjacent materials to be quantified and related to the applied surface flux and the hydraulic properties of the individual materials under steady-state conditions.
The results of the column study and subsequent numerical modelling program showed that water prefers to flow where water exists. In unsaturated systems, a fine-grained soil has smaller interparticle voids and is able to maintain fluid-filled pores at suctions greater than that of a coarse-grained material. Once suctions exceed the air entry value of the material the largest voids begin to drain, air enters the system and the hydraulic conductivity decreases. The decrease in hydraulic conductivity with increased suctions for each material is dependent on the distribution of pore sizes. In an unsaturated system it is this mechanism of decreasing hydraulic conductivity with increasing suction that can result in a fine-grained material becoming more conductive than a coarse-grained material.
When a surface flux is applied to a vertically layered, unsaturated system under steady state conditions, the preferential flow path is determined by the relationship between the applied surface flux rate and the saturated hydraulic conductivity of the fine layer. If the applied flux rate is greater than the saturated hydraulic conductivity of the fine material, the equilibrium suction that forms within the column results in the coarse layer becoming the preferential flow path. Reducing the surface flux to a rate less than the saturated hydraulic conductivity of the fine material results in an equilibrium suction where the fine layer becomes the path of preferential flow. It is critical that the interaction between the hydraulic properties of materials within a system be quantified in order to predict the behaviour of the system.
Following the analysis of the fine and coarse sand column, another column was constructed using fine and coarse waste rock. The results from the second column experiment showed that when the applied surface flux was reduced to a rate of 5.56 x 10-8 m/s (i.e., 1753 mm/year), the fine waste rock layer became the path of preferential flow
The study of undrained and drained behavior of unsaturated soils
No full textCompacted soils, expansive soils and collapsible soils are typical unsaturated soils that are commonly considered as problematic soils. The
behavior of unsaturated soils during undrained loading and consolidation processes have not been extensively studied from an experimental standpoint in comparison to saturated soils.
The primary objective of this research program was to study the pore pressure development and the consolidation behavior of unsaturated soils resulting from the application of external loads and changes in pore-water pressure. The study dealt with one-dimensional loading (Ko-conditions) and consisted of theoretical and experimental programs. The theoretical program was started with a brief literature review on unsaturated soil behavior. The relevant theories were developed by first introducing the physics involved during undrained and drained loadings. Boyle's gas law and the constitutive equations for unsaturated soils were used in the derivation of pore-pressure parameters for undrained loading. Darcy's law and the constitutive equations for unsaturated soils were used in deriving the governing flow equation for the water phase. The stress state variables for an unsaturated soil were used in the formulation of the constitutive equations.
An experimental program was established and the program involved the development of appropriate equipment for testing, the selection of a suitable soil and testing procedure; and eventually the carrying-out of the undrained and drained tests. A Ko-cylinder that allowed undrained and consolidation tests to be performed on an unsaturated soil specimen was designed and built. Simultaneous measurements of pore-air and pore-water pressures were made throughout the soil specimen during tests. Details on the design and construction of the equipment are outlined in the thesis. Criteria used for selecting the soil and testing procedure are described.
There were four types of tests performed on the soil throughout the experimental program. The undrained or constant water content loadings were conducted for obtaining the pore pressure parameters for an unsaturated soil. The drained tests consisted of consolidation and increasing matric suction tests. The drained tests were conducted in order to study the pore pressure and volume change behavior of an unsaturated soil during a transient process. The above tests were
performed alternately in an experimental series. There were five experimental series carried-out in the program.
The experimental data were analyzed and interpreted in the light of the formulated theory. Theoretical simulations for matching the experimental data were also performed. Comparisons between the experimental data and theoretical simulations are presented and discussed in the thesis.
The experimental pore pressure parameters obtained from the undrained and constant water content loadings agree reasonably well with the formulated theory. The pore-air pressure dissipation was found to be essentially instantaneous when the air phase is continuous. The pore-water pressure dissipation during the consolidation test was found to be faster than the pore-water pressure decrease during the increasing matric suction test. The differing rates of dissipation were attributed to the differing coefficients of water volume change for both tests. The water volume changes during the consolidation test were considerably smaller than the water volume changes during the increasing matric suction tests for the same increment of pressure change
Swell properties of desiccated Regina clay
No full textDamage to structures founded on the lacustrine areas is common in western Canada. Adverse volume change exist in both fully saturated and partially saturated soils. However, their behaviour is still not completely understood. The purpose of this study is to investigate the volume change behaviour and swell potential of remolded and desiccated Regina clay.
Three series of Constant-Volume and Free-Swell consolidation tests were performed to observe the effects of back pressure on the compression and rebound curves and the difference in the rebound behaviour in the two types of tests. Also determined was the relationship between initial water content and swelling pressure, and degree of saturation and χ value. Pore pressure measurements were also made during the tests.
It is found that back pressure does not significantly alter the rebound slopes of remolded, desiccated samples for degrees of saturation in excess of 90 percent; however, there is an upward shift of the compression-rebound curves. The rate of volume change is found to decrease with applied back pressure and this is believed related to the presence of air in the sample.
The presence of air in the soil and measuring system decreases pore pressure responses considerably. The application of back pressure increases the accuracy in measuring pore pressures and gives an experimental settlement-log time curve which checks more closely with its theoretical curve.
The measured swelling pressures appear consistent with those measured by Noble on Regina clay. χ values, computed from the swelling pressure and soil suction, show a very rapid decrease with a decrease in degree of saturation.
Comparison of results from Free-Swell and Constant-Volume tests show a higher swelling pressure and rate of volume increase for the Free-Swell test. The slopes of the rebound curves are essentially the same for both types of tests.
For further confirming the effects of back pressure on volume change characteristics, future research should be carried out on samples with lower degrees of saturation. Samples prepared by static compaction would be satisfactory for such a study. It is also recommended that stress paths be taken into consideration in volume change investigations for partially saturated solls
The effects of capillary hysteresis on the measurement of matric suction using thermal conductivity sensors
No full textMatric suction has proven to be a key parameter in the study and application of soil mechanics for unsaturated soils. Field measurements of suction are necessary in many engineering analyses, such as the prediction of total heave, the analysis of the slope stability due to changes in soil suction, and the monitoring of moisture flux through a soil cover or barrier structure used to impede contaminant transport. There are a number of methods to measure soil suction in the field. The thermal conductivity sensor proves to be one of the most promising means of in situ suction measurement.
The thermal conductivity sensor measures matric suction by measuring the rate of dissipation of thermal energy in the ceramic sensor tip. The thermal diffusivity. of the ceramic is dependent upon the water content of the ceramic. The water content is a function of the matric suction in the surrounding soils. This function is referred to as capillary function or soil-water characteristic curve and exhibits hysteresis. In other words, the same value of matric suction may correspond to different ceramic water contents, thus different sensor outputs, depending upon the drying and wetting history. The objective of this study is to investigate the properties of capillary hysteresis of the sensor ceramic and its effects on the measurement of matric suction.
Two groups of laboratory tests involving drying and wetting processes were carried out; one group measured the relationship between water content and matric suction of the sensor ceramics, the other group measured the relationship between sensor output and matric suction for a newly developed sensor. The result shows that, although the hysteresis loop is relatively narrow compared with those of coarse-grained materials found in the literature, the effects of capillary hysteresis on suction measurement using the thermal conductivity sensor are not negligible. If the capillary hysteresis is not taken into account, the maximum possible relative error of suction measurement caused by the capillary hysteresis is from 24% to 50% for the sensors used in the tests. The problems associated with the conventional method of calibration are also discussed in the thesis.
To make the suction measurement more accurate, the sensor output versus suction relationship of each of the possible wetting and drying processes should be measured in calibrating the sensor. However this calibration is impractical. Therefore, it is desirable to predict the hysteresis curves from limited measured data using a mathematical procedure.
There are a number of models found in the literature to simulate the capillary hysteresis of a porous material. Some of these models were examined using the experimental data of the sensor ceramic. It was found that the models in the literature either require a large amount of measured data to make the prediction, or fail to reproduce the measured curves of hysteresis. Therefore, an analytical approximation was developed which used a curve fitting method to fit the measured main drying curve and to predict the main wetting curve and the primary scanning curves.
Based on the above experimental and modeling studies, suggestions were made on the calibration of the sensor
Simulation of swelling pressure measurements on expansive soils
No full textNumerous methods have been proposed to predict the swelling pressure and the amount of swell of an expansive soil. These methods generally involve the use of a one-dimensional consolidation apparatus (i.e., oedometer). A large amount of test results and experience involving these methods have been reported. In contrast, little attempt has been made to formulate a theoretical framework to simulate these testing procedures and to visualize the different stress paths used in the various methods. The primary objective of this research program is to formulate a theoretical framework which can embrace all swelling testing procedures. The formulations are to accommodate various boundary conditions and to simulate the stress paths that have been followed using various testing procedures. The research program commenced with a literature review which provided a summary of the research which have been conducted on laboratory swelling pressure measurement and the theoretical simulation of swell testing methods. A theoretical model for describing the pore-water pressure and volume change behavior during various swelling oedometer tests is formulated. The theory is based on the equilibrium equation, the constitutive equations for unsaturated soils and the continuity equation for the pore fluids. A computer program, SWELL, based on the theoretical model is developed using finite element method. The presented theory is used to describe the behavior observed during the experimental program. Several types of laboratory tests (i.e., falling head permeability test, Free Swell oedometer test, pressure plate test, shrinkage test and constant suction consolidation test) were performed to identify the appropriate soil properties and variables which control the swelling behavior of an unsaturated soil during the swelling oedometer tests. Several empirical equations were proposed to describe the soil properties. The proposed theory was used to simulate the results from the Free Swell oedometer tests, the Constant Volume oedometer tests, constant water content (i.e., undrainage loading) oedometer test and the Loaded Swell oedometer tests. In general, good agreement was found among the computed and measure values of volume change, vertical total stress and pore-water pressure. Additional analyses were conducted using various values for the saturated coefficient of permeability and the imposed boundary conditions. The computed rate of swelling were found to be quite sensitive to the coefficient of permeability of the soil and the length of the drainage path. The final stress state and surface evaporation were also found to influence the rate of swelling, the percent swell, and the swelling pressure
Laboratory study of evaporative fluxes in homogeneous and layered soils
No full textMany problems faced by geotechnical engineers require the prediction of the water flux boundary condition at the soil surface. Soil covers for the decommissioning of landfills and tailings piles are one of the primary engineering applications in this category.
The purpose of a cover may be to minimize infiltration, reduce radon fluxes, or prevent the generation of acidic leachates. Two cover concepts which are currently being evaluated for various applications are capillary barriers and moisture retaining covers. Both of these concepts involve multi-layered soil covers in which the soils and the layering combinations are chosen to achieve the design requirements.
The processes of evaporation and moisture redistribution in homogeneous and layered soils were studied through the use of column evaporation tests. Six soil columns were tested: three homogeneous profiles, and three layered configurations. The soils used for the column evaporation tests were an aeolian sand (Beaver Creek sand), a natural silt, and a processed silt. The column evaporation tests were conducted with a constant head boundary condition (representing a shallow water table) at the base of the column for 31 days, after which the lower boundary condition was changed to a zero flux condition. The column evaporation tests were continued for approximately 30 days with the zero flux boundary condition at the base of the column. Measurements taken during the column evaporation tests included gravimetric water contents, temperatures, suctions (using tensiometers) and evaporation rates.
A sensitivity analysis was conducted to determine the influence of soil hydraulic properties (saturated hydraulic conductivity, air entryvalue, andpore size distribution index) on the maximum evaporation rate sustainable from soil profiles corresponding to those used in the column evaporation tests. The sensitivity analyses indicated that the saturated hydraulic conductivity and the air entry value have a greater effect on the maximum evaporation rate than the pore size distribution index for the soils and layering combinations analyzed.
The column evaporation tests were modelled using the computer program SWIM (Soil Water Infiltration and Movement). SWIM is based on the Richards equation for flow in unsaturated porous media and accounts only for liquid phase flow. The SWIM model was limited by the curve fitting functions used to determine moisture retention curves and hydraulic conductivity functions. The computed moisture contents from SWIM showed reasonable agreement for the natural silt and processed silt. The results for the Beaver Creek sand showed poor agreement due to the steep moisture retention curve for this material.
This thesis indicates that selective layering of multi-layered soil covers has potential in designing capillary barriers or moisture retaining covers where the covers will be subjected to predominantly evaporative conditions
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