Imagine soil that’s made up of incredibly tiny pieces, much, much smaller than what you’d find in sand. That’s essentially clay soil. These microscopic bits are what give clay its unique characteristics, forming over a very long time as rocks break down into these super-fine particles.

Because the particles are so small and pack together tightly, clay soil has a few distinctive traits. It’s like a sponge, holding onto a lot of water. This is great during dry spells because plants have a reserve, but if it rains too much, the water can just sit there, making the soil soggy because it struggles to drain. 

Those tiny clay particles are also excellent at grabbing onto and holding the food (nutrients) that plants need to grow, so clay soil is often naturally fertile. When it’s wet, clay soil is very pliable, meaning you can easily shape it—think of pottery clay! 

However, it can get compacted easily when wet, especially if you walk on it, which makes it harder for plant roots to grow and for air to get in. Many types of clay also expand when they get wet and then shrink and crack when they dry out, a phenomenon that can cause problems for things like building foundations.

Because it holds so much water, clay soil also takes a while to heat up in the spring sunshine. And finally, it’s notoriously tough to work with: sticky and heavy when wet, and incredibly hard and clumpy when dry.

Where can I find clay soil?

Essentially, if you’re in a lowland, relatively flat area in England, (especially in the south or east), there’s a good chance you’ll encounter clay soil. Geological maps of the UK, often available from the British Geological Survey, are excellent resources if you want to know the specific soil type in your exact location.

How do I know I’ve found clay soil?

If you are digging in an area, clay soil can be relatively easy to spot after a few checks. Here are three key indicators:

1. It’s tough to dig: This is often the first thing people notice. Your spade or shovel will feel like it’s hitting something incredibly dense and heavy. It takes a lot more effort to push the tool into the ground, and to lift each scoop of soil.
2. It’s sticky when it’s wet: If the soil is at all damp, it will be incredibly sticky and clingy. It’ll stick to your boots, to your shovel, and form thick, muddy clumps that are hard to shake off. It might feel almost like putty or wet cement.
3. Smooth and slippery (or soapy) when rubbed: Take a small amount of the damp soil and rub it between your fingers. It will feel very smooth, almost greasy or soapy, rather than gritty (like sand) or silky (like silt). If you can rub it enough to make it look shiny, that’s a strong indicator of high clay content.

Does clay soil impact the stability of a retaining wall?

Yes, if there are no mitigations at play, and construction isn’t conducted carefully, clay soil can significantly impact the stability of a retaining wall negatively. This can result in:

• The wall falling
• Warps in the walls
• Bends in the wall
• The wall leaning over
• The wall leaking

Regardless of the kind of block you use to construct your retaining wall, these issues can still happen. Even interlocking concrete blocks, known for their extreme robustness and durability, can be significantly affected by the stress from clay soil if no proper mitigations are made. 

Here’s how clay soil affects the stability of a retaining wall:

1. Hydrostatic Pressure (Water Buildup):

• The issue: 

Clay soil is notorious for its poor drainage. When it rains, especially heavily, clay soil behind the wall acts like a sponge and holds water. This water doesn’t drain away easily. As water accumulates, it creates immense hydrostatic pressure – the force exerted by standing water – against the back of the retaining wall.

• The effect: 

This pressure can be enormous, far greater than the force of dry soil. It constantly pushes against the wall. While interlocking blocks have some give, this constant, powerful outward force can cause the wall to bulge, lean, crack, or even completely fail. The joints between the blocks, even if designed to interlock, are vulnerable to this pushing force if not properly managed.

2. The shrinking and swelling of clay soil

• The problem:

Generally, clay soil is “expansive.” This means they swell significantly when they absorb water and shrink when they dry out. This isn’t just a minor movement; it can be substantial, depending on the amount of water and clay soil, as well as the residual temperature. 

• The effect:

Swelling: When the clay behind the wall gets wet and swells, it exerts tremendous additional pressure on the wall, similar to hydrostatic pressure, pushing it forward. In most cases, hydrostatic pressure and the swelling of clay can happen at the same time. This force can be enough to overcome the interlocking mechanism and cause the blocks to separate, shift, or be pushed out of alignment.

Shrinking: When the clay dries out and shrinks, it can create a void or gap behind the wall. While this might seem less problematic initially, it means the support behind the wall is temporarily lost. If the ground then gets wet again quickly, the sudden re-expansion can create a “shock” or differential pressure that stresses the wall. 

Can you prevent clay soil from affecting a retaining wall?