Hydro-Mechanical Properties of an Unsaturated Frictional Material

Chapter 2
State of the Art
2.1 General
In the chapter the basics of unsaturated soil mechanics relevant to this study are briefly
presented. Concepts of stress state variables in saturated and unsaturated soils are discussed.
A survey on hydraulic functions in unsaturated soils, namely the soil-water characteristic curve
and the hydraulic conductivity function, is given. Techniques for measurement of suction and
water content, laboratory as well as field methods for determination of hydraulic functions and
several approaches to model these relationships are critically discussed. A review on modeling
of unsaturated soils is presented. Studies on volumetric mechanical behavior (stress-strain
behavior, collapse and preconsolidation pressure) of soil are also presented. This literature
review is focused on work performed on frictional granular materials.
2.2 Unsaturated Soils
Depending on the soil type a variety of methods for the characterization of soils and their
properties are available. According to the principles of civil engineering, soils are classified
into the following main soil groups:
• gravel, sand (non-cohesive soils)
• silt (cohesive soil)
• clay (cohesive soil)
The classification is based on the grain-size distribution, the coefficient of gradation and the
coefficient of uniformity for non-cohesive soils and the plasticity index as well as the liquid
limit for cohesive soils. Each type of soil is characterized by its mechanical and hydraulic
behavior, that is also dependent on the water content or suction in the soil. The saturated
conductivity for sand is about ks ≈ 1 · 10 −3 to 1 · 10 −5 m/s, where a large cross sectional
area of pore space for conduction of water flow is available. The typical value of saturated
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