If there’s one thing I’ve learned during my PhD journey so far, it’s that soil is incredibly stubborn.
In my lab at the University of Auckland, I spend a lot of time looking at how sand-kaolin-bentonite mixes dry out and crack. While my research focuses on those visible surface changes, there is an invisible physical battle that every geotechnical engineer has to account for: the tug-of-war between Drained and Undrained conditions.
If you’ve ever found these terms confusing, you’re not alone. The secret is to stop thinking about them as “types of soil” and start thinking about them as a race against the clock. That’s at least how I force myself to remember, and not get confused again.
The fight between the water and soil skeleton
Think of saturated soil as a team. You’ve got the Soil Skeleton (the actual grains touching each other) and the Pore Water (the water filling the gaps between them).
When we build a foundation or dump a load of fill on a site, that weight has to go somewhere. The “winner” of who carries that weight, the water or the soil is determined by one thing: How fast can the water get out of the way? It is entirely about whether the water is allowed to escape during the timeframe of the load.
1. The Undrained State (The “Trapped” Phase)
Imagine a sponge soaked in water and wrapped tightly in a plastic bag. If you step on it, the sponge doesn’t compress. Why? Because the water is trapped inside.
In the low-permeability clays I work with, the “pores” are so tiny that water can’t escape quickly. When we apply a sudden load, the water pressure spikes, and takes a role of what we call excess pore water pressure, and it instantly supports the weight.
- The Danger: Because the water is doing all the heavy lifting, the soil particles aren’t being pushed together. This means there’s zero increase in friction. The soil stays weak, and if we aren’t careful, it can lead to a sudden, catastrophic shear failure.
2. The Drained State (The “Strong” Phase)
Now, imagine taking that sponge out of the bag. As you step on it, the water squirts out.
As the water leaves, the sponge (representing the soil skeleton) finally feels your weight. This is where the magic happens. The particles in the soil skeleton are forced to interlock, increasing the Effective Stress.
* The Result: In the engineering world, more effective stress is like more friction between the soil particles. And more friction means a stronger, more stable ground. This is the goal, but in clay, reaching this state can take weeks, months, or even years (i.e. removal of water from the pores in the soil)
Why I Care: The Lab Reality
In the lab, we may have to perform tests, and they may be termed drained or undrained tests. Right now, I’m preparing for a series of direct shear tests on my soil samples, with the goal of performing a constant water content drained test. This is where the “race against time” becomes a literal setting on my testing equipment.
To make sure I’m actually measuring the strength of the soil skeleton (the drained state) and not just the trapped water pressure, I have to be incredibly careful with two things:
- Saturation Levels: I’m keeping the degree of saturation below 90%. This ensures there is enough air in the voids to keep the air phase continuous, so pressure doesn’t get “bottled up” during the test.
- The Speed of the Test: I have to shear the soil at a very low rate. If I go too fast, I risk building up pore pressure and getting an “undrained” result by mistake.
If I get the rate right, the water has time to redistribute, the air stays connected, and I get a true look at how these soil mixes hold together under stress.
The Takeaway
Geotechnical design isn’t just about knowing what the soil is made of; it’s about knowing how much time the water needs to move out so the soil can move in and do its job.
Whether you’re working with clean beach sand (where drainage is instant) or a stubborn clay mix (where it’s a marathon), I believe this is one of the core concept one needs to understand.