ABSTRACT
The last two to three decades have seen significant advances in the mechanics of unsaturated soils. It is now widely recognized that the fundamental principles in soil mechanics must cover both saturated and unsaturated soils. Nevertheless, there is still a great deal of uncertainties in the geotechnical community about how soil mechanics principles well-established for saturated soils can be extended to unsaturated soils. There is even wide skepticism about the necessity of such extension in engineering practice. This paper discusses some common pitfalls related to the fundamental principles that govern the volume change, shear strength and hydromechanical behaviour of unsaturated soils. It also attempts to address the issue of engineering relevance of unsaturated soil mechanics.
KEY QUESTIONS?
In particular, some basic questions are often raised on the fundamental principles that govern the hydromechanical behaviour of unsaturated soils and on the engineering relevance:
(1) Reconstituted soil versus compacted soil. What are the main differences in the hydromechanical behaviour of these soils? What are the implications of different pore size distributions (PSD), in constitutive modelling of unsaturated soils? Can a reconstituted soil become collapsible?
(2) Relationship between volume change, yield stress and shear strength. Can the constitutive equations for volume change, yield stress and shear strength be defined separately? Does the loading-collapse yield surface have to recover the apparent tensile strength surface? Do we need the suction-increase surface to capture possible plastic volume change when a soil is dried to a historically high suction? What are the implications of stress state variables in defining volume change and shear strength equations?
(3) Implications of using a Bishop effective stress. Can we use a Bishop-type effective stress in modelling unsaturated soil behaviour and what are the implications?
(4) Engineering relevance. What is the relevance of the unsaturated soil mechanics in engineering practice? Is a design based on the saturated soil mechanics always conservative? Considering the difficulty and uncertainty in measuring or monitoring in-situ suctions, the applicability of the unsaturated soil mechanics to engineering practice has also been questioned. These questions represent some of the most fundamental issues in unsaturated soil mechanics. There are currently no unified answers to these questions.
SOLUTIONS/ KEY ANWERS?
1) It seems possible to use the same theoretical framework to model reconstituted soils and compacted soils. The pore size distribution evolves with stress and suction paths and can be modelled by the evolution of the loading collapse yield surface. However, much experimental evidence is needed before an affirmative conclusion can be drawn. In particular, we need more experimental data on the behaviour of natural soils or soils reconstituted from slurry.
2) The volume change, yield stress and shear strength behaviour of an unsaturated soil are co-related to each other and it is not recommended to define these functions separately. Of all these functions, the volume change equation is the most fundamental one and it underpins the yield stresssuction and shear strength-suction relations.
3) The loading-collapse yield surface should recover the apparent tensile strength surface when the preconsolidation stress at zero suction is set to zero, to avoid non-uniqueness of the yield surface. The suction-increase yield surface used to capture the possible plastic volume change associated with drying is not truly needed, if the loading-collapse yield surface is properly defined.
4) The shear strength of an unsaturated soil can be defined by a single effective stress, if the friction angle of the soil does not change with suction. On the other hand, the volume change behaviour of unsaturated soils usually has to be expressed in two stress variables.
5) There are implications associated with using the Bishop effective stress. Because there is only one compression index associated with both stress and suction changes in the volume change equation, this compression index is constrained to decrease with increasing suction. Such a constraint is not always supported by experimental data. A possible solution is to adopt a saturation-dependent compression index and to form the constitutive equations in the stress-saturation space.
6) All the existing elastoplastic models for unsaturated soils have stress-path dependent elastic behaviour. It is still a challenging task to solve this theoretical problem. The mechanics of unsaturated soils has strong engineering relevance. Most geotechnical problems involve some variations of suction, water content or degree of saturation. These variations can cause significant volume change and strength variation, leading to undesirable deformation and stability problems. It is likely that we will see further developments and more engineering applications of unsaturated soil mechanics in the next few decades
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