Building on clay or reactive soil

Clay soil has a bad reputation due to the fact that it moves a lot. Preparation of the area where you will build a shed is key to preventing any problems with it down the road. Clay soil swells in moist conditions and shrinks when dry; so, you need to take care of the problem beforehand to avoid any structural damage. If you want your shed to remain stable and secure, you should start with proper ground work.

How reactive clay soil affects shed slabs and footings

Reactive clay soils have unique behaviours that may bring about serious engineering problems. Clay soils behave in such a way that the volume of the soil changes according to the amount of moisture in it. In situations where there is an absorption of moisture in the clay soil, the soil will swell. On the other hand, when the soil becomes dry and loses moisture, the soil will shrink. The process of expansion and contraction of soil is referred to as soil movement.

This leads to cracked concrete, irregular floors, sticking doors, and cracks developing around the door frame area. In cases where the soil tends to be highly reactive, the forces may be strong enough to even shear a slab, especially if it is not designed to handle such conditions. The level of reactivity largely depends on the amount of clay present in the soil: the higher the clay content, the greater the force exerted on whatever lies above.

How to test soil reactivity and site classification before building

Before making any decisions or starting construction, it's crucial to conduct a geotechnical soil test. During this process, an experienced engineer collects soil samples from your site to assess its reactivity. This is typically determined by calculating the plasticity index, which indicates the extent to which clay in the soil expands and contracts with varying moisture levels.

The results of this test provide your site with a classification according to the Australian Standard AS 2870. This code is essential as it dictates the requirements for residential slabs and footings. The classification system categorises the site based on the degree of ground movement expected.

• Class S: slightly reactive clay or sand, with only minor ground movement.

• Class M: moderately reactive clay or silt, with moderate movement.

• Class H1 and H2: highly reactive clay, with high and very high movement.

• Class E: extremely reactive sites, with extreme ground movement.

The type of soil where you plan to build heavily influences how you should design the slab and footings. If you bypass testing, you're essentially making educated guesses, which often lead to disastrous outcomes, especially when dealing with clay. Incorrect assumptions are a major reason why slabs fail on this type of soil. Additionally, skipping this crucial step can invalidate both the engineering certification and the warranty on your construction, so it's a risk not worth taking.

Site preparation steps for a shed on clay soil

Clearing and stripping the topsoil

Topsoil is organic-rich and retains water, so it may move or deteriorate with time. In order to avoid this problem, it is necessary to strip away all plants, roots, grass, and other loose materials from the ground. After that, dig down until you reach a consistent subsoil layer across the entire construction site.

Cut and fill to level the building pad

Where you have a site that has slopes on it, then cut and fill becomes necessary. This involves moving soil from the elevated area to the lower one in order to make the surface level. The benefit of this technique is that you do not require other materials to get a levelled surface. 

You do, however, have to ensure that the fill is compacted in layers. Failure to do so will make the fill subside. Slopes bring their own challenges, and our guide on how to build a shed on sloped ground runs through them.

Compacting the sub-base

In order to build an effective foundation for your shed, it is necessary to construct a solid and flat sub-base prior to placing the slab. To do so, it is recommended to roll or plate compact the surface layer by layer. This approach will help avoid any areas that can eventually shift. The compacted sub-base will evenly distribute the load from the shed and the slab, which will prevent the slab from shifting in case there is movement on the ground. It is especially important if the subsoil is reactive clay.

Designing the slab and footings for reactive clay soil

The slab and footings play a crucial role in countering soil movement. On reactive soil, simply pouring a flat slab won't suffice. The design must align with the site's classification, as determined by your soil test. The more reactive the ground, the greater the demands on the slab.

A stiffened raft slab often provides a solution. This involves thick concrete beams arranged in a grid pattern within the slab, ensuring it moves uniformly rather than bending and cracking. In areas with highly reactive soil, these beams extend deeper, and the reinforcement becomes more substantial. Additionally, footings are dug deeper, reaching beyond the zone where moisture variations have the most impact, anchoring them in more stable soil.

Here's where the expertise of engineering comes in. A slab tailored to your AS 2870 classification maintains its integrity and supports the shed's structure over time. Every Fair Dinkum shed is engineered to suit its specific site, ensuring the foundation is properly aligned with the ground conditions rather than relying on guesswork.

Managing drainage and moisture around the shed

Maintaining consistent moisture levels around the slab is crucial on clay soil. This type of soil tends to shift when moisture levels change, so preventing water from pooling near the footings and ensuring even moisture distribution is key. 

To achieve this, grade the ground around the shed so that it slopes away, guiding rainwater away from the slab's edge. Install drainage systems in areas where water tends to gather, and make sure gutters and downpipes direct water well away from the footings. Poor drainage can quickly lead to the problematic moisture conditions that cause reactive clay to expand and move.

It's also important to monitor what's placed near the slab. Garden beds and irrigation systems can introduce unwanted water into the ground, and a leaking pipe near the edge can saturate one side of the pad while leaving the other side dry. This uneven moisture distribution is a common cause of foundation movement and heave.

Common mistakes when building a shed on reactive clay soil

There are several common errors that occur when constructing clay blocks, all of which have something to do with moisture or an improperly constructed slab.

• Planting gardens and placing irrigation near the slab: continuous watering on one side while the soil gets dried on the other causes unequal pressure, which results in cracking of the slab.

• Removing mature trees just before construction: mature trees absorb large quantities of water from the clay soil, and their removal allows it to become moist and heave for many years to come.

• Grading the ground towards the shed: water flowing back towards the slab creates a puddle that feeds the swelling process.

• Improperly designing the slab: casting a slab designed for non-reactive soil on a reactive one almost guarantees cracking, and repairs cost much more than initial proper construction.

Getting your shed foundation right on clay soil

Constructing on reactive clay requires understanding the soil's behaviour and planning accordingly. Start by testing the soil to gauge its properties. Properly prepare the pad, ensure the slab aligns with the site's classification, and control water drainage effectively. These steps prevent issues, ensuring your shed remains stable and square for years.

Success hinges on solid groundwork, so it's wise to collaborate with a builder who can tailor the foundation to your specific site conditions. If you're considering erecting a shed on clay soil, the Fair Dinkum team can handle the site preparation and provide a quote based on the actual conditions of your land.