Unsaturated Soils, Capillary Suction and Expansive Clays: Why Soil Strength Changes with Water
- MTS DNC ENERGY CONSULTANTS LIMITED
- 7 hours ago
- 4 min read
Soil is not just “dirt”. Its strength and behaviour change dramatically depending on how much water it contains and how that water is held. Three important concepts every engineer and builder should understand are:
Capillary suction in unsaturated soil
Pore water pressure in saturated soil
Swelling of expansive clay minerals
These phenomena explain why some soils suddenly lose strength, why certain ground collapses when wetted, and why floors, pavements and foundations can be lifted or cracked by swelling clay.
1. Capillary Suction – The Hidden Strength of Unsaturated Soil
In an unsaturated soil (soil that is not completely full of water), part of the strength comes from the pull or suction of capillary water.
Think of a fine drinking straw. When you put the straw in water, the water rises up the straw by itself. The narrower the straw, the higher the water climbs. This is capillary action.
Exactly the same thing happens in soil. The tiny spaces between soil particles act like millions of microscopic straws. Water is pulled into these narrow gaps and held under tension (suction). This suction pulls the soil particles toward each other, increasing the friction between them and making the soil stronger.
Key point: As long as the soil stays unsaturated, this capillary suction adds useful strength.
When the soil becomes fully saturated (all the voids fill with water), the capillary menisci disappear and the suction is lost. The soil therefore loses that extra strength.
Practical example – Collapsible loess soils
Some wind-blown soils called loess have never been saturated since they were deposited. They look strong and stable when dry. However, if they become saturated for the first time (for example by irrigation, a leaking pipe, or a rise in groundwater), they can suddenly collapse under their own weight. Buildings and roads built on these soils have suffered severe settlement after wetting.
2. Pore Water Pressure – How Water Shares the Load
When a soil is saturated, any load applied to it is carried partly by the soil grains (grain-to-grain contact) and partly by the water in the pores.
Imagine two people carrying a heavy table. If one person starts supporting more of the weight, the other person has to work less. In soil, if the water pressure in the pores increases, the grains have to carry less of the load. The contact forces between grains decrease, friction drops, and the soil becomes weaker.
This is why measuring and controlling pore water pressure is so important during construction. High pore pressures can reduce the shear strength of soil and lead to:
Slope failures
Foundation instability
Softening of fills and embankments
Engineers often install drainage systems or allow time for excess pore pressures to dissipate so that the soil can regain strength.
3. Expansive Clays – The Accordion Effect
Certain clay minerals (especially montmorillonite / smectite) behave like tiny accordions. When they come into contact with water, water molecules enter between the mineral layers and the clay particles expand dramatically.
This expansion can generate very large pressures – enough to:
Lift pavements and floors
Crack and push in basement walls
Heave foundations
Damage buildings and infrastructure
The damage caused by expansive clays is often underestimated. In many countries the annual cost of repairs due to expansive soil damage is greater than the combined cost of landslides and earthquakes, running into billions of dollars every year.
Why it surprises people
Dry expansive clay looks firm and stable. Once it gets wet (seasonal rainfall, garden watering, poor drainage, leaking services), it can swell several percent in volume and generate pressures of hundreds of kilopascals – more than enough to damage ordinary structures.
Summary Table
Condition / Phenomenon | What happens | Practical consequence |
Unsaturated soil | Capillary suction pulls particles together | Extra strength |
Soil becomes saturated | Capillary suction disappears | Loss of strength; possible collapse (loess) |
Saturated soil under load | Pore water pressure rises | Reduced friction → weaker soil |
Expansive clay minerals | Absorb water and swell like an accordion | Large heave pressures → structural damage |
Why These Concepts Matter in Design and Construction
Always check whether a soil is potentially collapsible or expansive.
Control surface water and groundwater so that dry soils do not become saturated unexpectedly.
Monitor pore water pressures during and after construction where high water tables or rapid loading are involved.
For expansive clays, use appropriate foundation solutions (deeper foundations, moisture barriers, lime/cement stabilisation, or void systems under slabs).
Understanding the simple physics of water in soil – suction, pore pressure and mineral swelling – helps engineers avoid expensive surprises and design safer, more durable structures.
Need Professional Help? Contact the Experts!
For Building Services Design – whether it’s HVAC, plumbing, or civil engineering – reach out to Nexus M&E Design for expert solutions tailored to your project needs.
If you require a technical assessment, BER rating, or assistance with SEAI grants, get in touch with the professionals at MTS DNC Energy Consultants for comprehensive guidance and support.
Disclaimer:
The content provided in this post is for informational purposes only and should not be construed as professional engineering, architectural, or surveying advice. While every effort is made to ensure the accuracy and reliability of the information presented, it is not a substitute for a thorough, site-specific inspection or the expertise of a qualified professional. For detailed guidance on foundation issues, structural integrity, or repairs, always consult with a licensed engineer, architect, or surveyor. The authors and publishers are not responsible for any damages or losses resulting from the use or reliance on this information.
