Before we talk about unsaturated soil, we need to begin with the soil mechanics principles most civil engineering students learn first. These principles are essential. They help us understand how stress is carried through the ground and how water influences soil behaviour.
Begin with the principles you already know
Introductory soil mechanics often begins with saturated soil, where the voids between soil particles are assumed to be completely filled with water. Under this condition, three quantities are particularly important.
These principles are not wrong when we move into unsaturated soil mechanics. They remain crucial. The difficulty is that the assumptions behind the familiar saturated model do not describe every condition found in the ground.
Why do we need to go beyond saturated soil mechanics?
Much of the soil supporting slopes, highways, embankments, shallow foundations, landfill covers and other infrastructure lies above the groundwater table. Its pores may contain both water and air. Rainfall, evaporation, vegetation and seasonal weather can continually change that balance.
If we treat this soil as though it is either fully saturated or completely dry, we may overlook an important part of its behaviour. The result can be an incomplete assessment of strength, settlement, shrinkage, swelling, water movement or seasonal ground movement.
This is also a sustainability issue. When engineers understand how moisture changes affect soil, they are better placed to maintain existing earthworks, target investigations, avoid unnecessary replacement and design interventions that respond to the real mechanism of a problem.
So, what is unsaturated soil?
An unsaturated soil is a soil in which the spaces between the solid particles contain both water and air. It is not necessarily dry. It simply is not completely filled with water.
A useful way to understand its uniqueness is to see it as a four-phase material.
The contractile skin is what distinguishes the four-phase explanation from a simple mixture of solids, water and air. It helps explain how water can be held within the pores even when the soil is above the groundwater table.
What makes unsaturated soil behave differently?
The pressure in the pore air and the pressure in the pore water may not be the same. Their difference is called matric suction.
In simple terms, matric suction helps us describe how strongly water is being held within the soil. As soil wets or dries, suction can change. This can influence:
- soil strength and stiffness;
- shrinkage, swelling and settlement;
- how quickly water moves through the soil;
- cracking and seasonal ground movement; and
- the performance of slopes, embankments, roads, foundations and environmental barriers.
This does not mean that higher suction always guarantees a stronger or safer soil. Soil type, density, stress history, structure, wetting and drying path, and the engineering situation all matter. Suction is one part of the evidence, not a complete answer by itself.
Where does this knowledge become useful?
Unsaturated soil mechanics becomes useful whenever environmental change can alter the moisture condition of the ground. Examples include a compacted slope exposed to prolonged rainfall, an expansive clay foundation during a dry season, a pavement subgrade affected by seasonal wetting, or a landfill cover that must limit water entry.
The practical question is not simply, “Is this soil unsaturated?” The more useful questions are:
- How can water enter, leave or redistribute within this soil?
- Which soil properties may change as the moisture condition changes?
- What engineering performance could be affected?
- What evidence is needed before a decision is made?
Answering these questions can help engineers develop better research questions, plan more focused investigations, interpret monitoring data, diagnose field problems and design more appropriate solutions.
Continue learning with the free pathway
I have developed a free four-part learning pathway, Unsaturated Soils in Practice, to help you move from recognising an unsaturated soil problem to using evidence in practical engineering decisions.
The pathway uses clear explanations, realistic situations, short knowledge checks and applied assessments. It is designed to help students, researchers and practising engineers build understanding step by step and connect the theory to technical practice.
You will learn how to recognise relevant field conditions, interpret soil-water information, identify the evidence needed for a decision and plan a defensible investigation. Optional personalised technical feedback is also available for learners who want their applied work reviewed.
Explore the free learning pathwayFrequently asked questions
Is saturated soil the same as waterlogged soil?
Not exactly. Saturated soil has its voids filled with water. Waterlogged describes a field condition where excess water is present, often near the surface. A waterlogged soil may be saturated, but the terms describe different ideas.
Is all soil above the groundwater table unsaturated?
Not always. Capillary action can create saturated zones above the groundwater table, and local drainage conditions can also produce perched water. The soil profile and pore-water conditions must be investigated.
Why should I care about unsaturated soil?
Because many foundations, slopes, embankments, roads and environmental barriers operate partly above the groundwater table. Moisture changes in this zone can affect movement, strength, water flow and long-term performance.
Is matric suction simply negative pore-water pressure?
Not quite. Matric suction is the difference between pore-air pressure and pore-water pressure. When pore-air pressure is atmospheric, it is often represented using a negative pore-water pressure, but the full definition remains the pressure difference.
Does higher suction always mean stronger soil?
No. Increased suction can contribute to strength in some soils and conditions, but the relationship depends on soil type, density, stress state, structure and wetting history. It should not be used as a universal rule.
Does unsaturated soil testing always take a long time?
Some specialised tests can take longer because moisture and suction must be controlled or allowed to equalise. However, not every project requires every test. The testing programme should be selected to answer the engineering question.
Can I still use conventional soil mechanics?
Yes. Conventional principles remain essential. Unsaturated soil mechanics extends your understanding when air and water coexist in the pores and environmental changes influence behaviour. The appropriate level of analysis depends on the problem and the evidence required.
Final thought
Unsaturated soil mechanics is not a replacement for the soil mechanics you already know. It is the next layer of understanding. It helps us recognise where familiar assumptions become limited and gives us better questions to ask about real ground conditions.
When we understand how soil, air and water interact, we can make decisions that are safer, more economical and more sustainable.
This article is for education and professional development. It is not a substitute for project-specific investigation, analysis or advice from a suitably qualified engineer.
About the author
Damilola Akinniyi
Damilola is a geotechnical engineer, educator and researcher who makes engineering and research easier to understand and apply. His work brings together soil mechanics, practical learning, mentorship and career development.
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