Unsaturated soils
So far - in this course, but also in other courses in your curriculum - the concepts of soil mechanics have primarily navigated the extremes: soils in dry states or fully saturated conditions, whether drained or undrained. These scenarios, while fundamental, represent only the tip of the iceberg in our understanding of soil behavior.
Consider the natural world around you – the soil in your garden, the sand at the beach, or even the earth under a construction site. Rarely are these soils purely dry or completely saturated; they exist in a more complex and dynamic state. These are partially saturated soils, where the water content is nonzero yet insufficient to achieve full saturation. In partially saturated soils, four zones/states can be distinguished, as shown in below figures.
- The fully saturated zone lies below the water table: almost all pore spaces are filled with pore fluid, which can move freely or is bound to the soil particles by attractive forces of various origins. The fully saturated zone is bounded at the top by the water table. The pore water pressure \(p^w = 0\) kPa prevails along the water table (relative to atmospheric pressure). Below the water table, the pore water pressure increases linearly with depth (\(p^w(z) = \gamma^w z\), where \(\gamma^w\) is the unit weight of water, and \(z\) is the depth below the water table). However, a very small proportion of the pore space is filled by residual pore gas inclusions, known as residual air.
- Above the water table is the closed capillary zone, where all pore spaces are still filled with water. The pore water is bound by capillary forces in the pore channels and is therefore under tension despite full saturation (\(p^w < 0\) kPa). The closed capillary zone is limited upwards by the capillary rise, which can range from a few centimetres (sand) to several metres (clay), depending on the soil type. In both the fully saturated zone and the closed capillary zone, the weight of the soil acts as a buoyant force.
- Above the closed capillary zone is the partially saturated zone in which both pore fluids (pore water and pore gas) are mobile. The height of this zone depends largely on the soil's water tension properties. A relationship between the suction and the degree of saturation is provided by the Water Retention Curve (WRC), which is discussed in more detail in Saturation-suction relation.
- Soil particles above the area filled with capillary water may be surrounded by adhesive water which is held against gravity and, unlike free pore water, is immobile. In addition to the electrostatically and osmotically bound water, the pore angle water is also part of the adhesive water, which is bound by capillary forces at the contact points of the soil particles. The pore air is mobile.
In this lecture, we delve into the realm of partially saturated soils. Our objective is twofold: to expand your conceptual understanding of soil-water interactions beyond the binary of 'dry' or 'fully wet' and to appreciate the nuanced ways in which partial saturation influences soil properties.
A comprehensive description of partially saturated soils requires consideration of many aspects of fluid-solid interaction at the microscale. A mere look at the phenomenological relationships - usually measured at the macroscale - is not sufficient for a deep understanding of the matter. Unfortunately, this depth cannot be covered in this course. While this course will provide an overview and introduction to this important topic, it's worth noting that the complete picture of partially saturated soils involves a depth of understanding that goes beyond our current scope. Nevertheless, our aim is to ignite curiosity and provide a foundational understanding of the key principles and considerations in dealing with partially saturated soils.
In the following sections important components of the description of partially saturated soils are introduced: