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Unsaturated soil mechanics has rapidly become a part of geotechnical engineering practice as a result of solutions that have emerged to a number of key problems (or challenges). The solutions have emerged from numerous research studies focusing on issues that have a hindrance to the usage of unsaturated soil mechanics. The primary challenges to the implementation of unsaturated soil mechanics can be stated as follows: (1) The need to understand the fundamental, theoretical behavior of an unsaturated soil; (2) the formulation of suitable constitutive equations and the testing for uniqueness of proposed constitutive relationships; (3) the ability to formulate and solve one or more nonlinear partial differential equations using numerical methods; (4) the determination of indirect techniques for the estimation of unsaturated soil property functions, and (5) in situ and laboratory devices for the measurement of a wide range of soil suctions. This paper explains the nature of each of the previous challenges and...

http://keshavarz.persiangig.com/document/Fredlund2006.pdf

Unsaturated Soil Mechanics in Engineering Practice

The Climate Near the Ground

Loess soil deposits are widely distributed in arid and semi-arid regions and constitute about 10% of land area of the world. These soils typically have a loose honeycomb-type meta-stable structure that is susceptible to a large reduction in total volume or collapse upon wetting. Collapse characteristics contribute to various problems to infrastructures that are constructed on loess soils. For this reason, collapse triggering mechanism for loess soils has been of significant interest for researchers and practitioners all over the world. This paper aims at providing a state-of-the-art review on collapse mechanism with special reference to loess soil deposits. The collapse mechanism studies are summarized under three different categories, i.e. traditional approaches, microstructure approach, and soil mechanics-based approaches. The traditional and microstructure approaches for interpreting the collapse behavior are comprehensively summarized and critically reviewed based on the experimental results from the literature. The soil mechanics-based approaches proposed based on the experimental results of both compacted soils and natural loess soils are reviewed highlighting their strengths and limitations for estimating the collapse behavior. Simpler soil mechanics-based approaches with less parameters or parameters that are easy-to-determine from conventional tests are suggested for future research to better understand the collapse behavior of natural loess soils. Such studies would be more valuable for use in conventional geotechnical engineering practice applications.

https://cyberleninka.org/article/n/575040.pdf

Review of collapse triggering mechanism of collapsible soils due to wetting

In this technical note, a genetic programming GP approach is employed to predict the soil-water characteristic curve SWCC of soils. The GP model requires an input terminal set that consists of initial void ratio, initial gravimetric water content, logarithm of suction normalized with respect to atmospheric air pressure, clay content, and silt content. The output terminal set consists of the gravimetric water content corresponding to the assigned input suction. The function set includes operators such as plus, minus, product, division, and power. Results from pressure plate tests carried out on clay, silty clay, sandy loam, and loam compiled in the SoilVision software were adopted as a database for developing and validating the genetic model. For this purpose, and after data digitization, GP software GPLAB provided by MATLAB was employed for the analysis. Furthermore, GP simulations were compared with the experi- mental results as well as the models proposed by other investigators. This comparison indicated superior performance of the proposed model for predicting the SWCC.

/pdf/prediction-of-soil-water-characteristic-curve-using-genetic-3czj3d6u71.pdf

Prediction of Soil-Water Characteristic Curve Using Genetic Programming

Soil-water characteristic curve variability

The suitability of the current resilient modulus test protocol (NCHRP 1-28A) for its application to unsaturated soils was assessed. Modifications to the stress state conditions of the protocol are necessary due to the axis-translation needed during the test when measuring matrix suction. This study presents the modulus of unbound materials resulting from tests performed under unsaturated soil conditions. Two different materials were tested. The base material was tested under drained and undrained boundary conditions, while the subgrade was tested under drained boundary condition. The results allowed for the enhancement of the Universal Model for resilient modulus prediction by incorporating suction as a stress state. This model predicts the resilient response of unbound materials as a function of external stress state and matrix suction levels and therefore, it is independent of moisture variation.

Resilient modulus for unsaturated unbound materials

Conditions in arid and semi-arid climates favor the formation of the most problematic collapsible soils. The mechanisms that account for almost all naturally occurring collapsible soil deposits are debris flows, rapid alluvial depositions, and wind-blown deposits (loess). Collapsible soils are moisture sensitive in that increase in moisture content is the primary triggering mechanism for the volume reduction of these soils. One result of urbanization in arid regions is an increase in soil moisture content. Therefore, the impact of development-induced changes in surface and groundwater regimes on the engineering performance of moisture sensitive arid soils, including collapsible soils, becomes a critical issue for continued sustainable population expansion into arid regions.

Geotechnical engineering practice for collapsible soils

Environmental conditions have a significant effect on the performance of both flexible and rigid pavements. External factors such as precipitation, temperature, freeze-thaw cycles, and depth to water table play a key role in defining the bounds of the impact the environment can have on the pavement performance. As part of the new US Mechanistic-Empirical Pavement Design Guide (MEPDG) being developed under the overall project sponsored by the National Cooperative Highway Research Program (NCHRP project 1–37A), a climatic modelling tool called the Enhanced Integrated Climatic Model (EICM) was implemented to incorporate the changes in temperature and moisture of unbound materials into the design process. Currently a new independent review project (NCHRP 1–40) is reviewing this model to correct errors and to develop further enhancements to produce a final methodology ready for approval/disapproval vote by AASHTO in 2006. This paper reflects the methodology used for the MEPDG and present the models in...

Incorporation of Environmental Effects in Pavement Design

The grain-size-distribution (GSD) of a soil is intimately related to its pore size distribution and hence, the GSD holds a close relation with the soil-water characteristic curve (SWCC). In addition, the plasticity index (PI) is a measure of the water holding capacity of the soil and therefore, it plays an important role in shaping the SWCC. This paper presents two sets of statistically derived equations that describe the SWCC of non-plastic and plastic soils. Data from 154 non-plastic soils and 63 plastic soils were analyzed. Soil samples were collected as part of the National Cooperative Highway Research Program (NCHRP) 9–23 project entitled Environmental Effects in Pavement Mix and Structural Design Systems . Samples were obtained from underneath paved roads of 30 sites located throughout the United States. The soil samples were subjected to laboratory testing that included index testing and SWCC testing. SWCCs were determined using a newly developed pressure plate device capable of overburden pressure application, continuous measurements of moisture content, and volume change monitoring. In addition to the collected field data, a database of published soil index properties and SWCCs was incorporated to the analysis. Each SWCC data set was fitted with Fredlund and Xing curve, which provided an S-shaped curve with four parameters, a f , b f , c f , and h rf . Using multiple regression analysis, equations were derived for these four parameters based on predictors derived from GSD and PI. The equations presented in this paper are useful in predicting the SWCC of any given soil without carrying out actual SWCC testing and they can easily be incorporated into computer codes to solve various unsaturated soil mechanics problems such as determining moisture beneath covered areas.

Claudia E. Zapata

Papers

Soil-water characteristic curve variability

Resilient modulus for unsaturated unbound materials

Geotechnical engineering practice for collapsible soils

Incorporation of Environmental Effects in Pavement Design

Prediction of the soil-water characteristic curve based on grain-size-distribution and index properties