Soil Shrinkage, Compaction, and Settling

Many homeowners focus on repairing minor issues on their concrete, mostly to keep it visually appealing. However, the structural integrity of your concrete must be addressed as well. Doing so will keep the concrete functional and safe to use. 

Shrinkage, compaction, and settling in your soil can lead to impressive amounts of damage. Understanding why this happens, and how to safeguard your home against it, will keep the concrete surfaces on your property in good condition. While you can’t always avoid the damage caused by these soil changes, you can slow the deterioration and apply the right solutions. 

Why Is My Concrete Sinking or Cracking? 

Your home’s concrete driveway, patio, pool deck, or garage floor may seem strong and unbreakable. However, it still needs solid support. Without it, it can’t handle any extra weight placed on it. Homeowners are constantly walking on their concrete pavements, driving heavy cars, and placing heavy objects. Over time, a piece of the concrete breaks and settles low on the ground. Why does this happen? 

It all has to do with the soil. Soil shrinking and compaction causes settling. Once your concrete is forced to support its own weight, any weak points in the design will begin to crack. If your concrete is resilient, then it may instead begin sinking in an uneven fashion. By understanding the soil that holds up your concrete, you can better protect your floors, steps, and other concrete surfaces on your property. 

The Causes of Soil Shrinkage 

There are different kinds of soil used as a foundation for concrete structures. How well that soil holds water depends on how much clay, sand, and silt is in the soil. Soils that have 10% or more clay content are highly absorbent. They will go through a drastic change when wet, as well as when it is deprived of moisture. This phenomenon is called swell-shrink. 

You may also know it as expansive soils. When clay-rich soil gets wet, it expands. The water eases its way through the space between the soil particles and makes the soil swell and become looser. However, when the moisture dries up, the soil shrinks considerably. This is because the clay in the soil makes it stick together very easily, so in the absence of water, the particles unite and don’t separate again until air or water makes its way between the spaces again. 

When soil shrinks, it compacts together and gets denser. Its surface volume also decreases, meaning that it rests lower than it was before shrinking. When it comes to soil underneath concrete, this decrease in volume means that a gap exists between the soil and the concrete. This is detrimental to your pavement because without the proper support from the soil, the concrete will cave in when enough weight is placed on it.  

• What Makes My Soil Shrink? 

There’s very little a homeowner can do to reduce soil shrinkage. Just because the soil sits under heavy concrete doesn’t mean it can’t shift around and get wet. Soil shrinkage is a naturally occurring process. Here are some of the things that cause it: 

• The sun 
• Dry climate 
• Plants 

Exposure to the sun makes the moisture in the soil evaporate. Even under the concrete, the sun’s rays still reach the soil. The same goes for dry climates. In Missouri, the summers are hot and last a long time. This means that soil shrinkage is a common occurrence during this time. A good soil shrinkage prevention method is covering your concrete with some sort of roof. 

Plants absorb the moisture from the ground in order to survive. Depending on the kind of plants you have, the soil they’re planted in will often be drier than the soil around it. If you don’t want the soil under your concrete to shrink, you should plant trees, flowers, and bushes far from concrete areas or in pots. 

The Causes of Soil Compaction 

Soil compaction occurs when the air in the soil is displaced due to the pressure of a heavy object. In construction, soil gets compacted before it is placed down as a foundation. This is because compacted soil is a lot denser than non-compacted soil. 

When soil has a lot of air between each soil particle, it’s a lot looser. Loose soil has a tendency to shift and move around. Water is also able to filtrate through loose soil a lot easier, making erosion more likely to happen. This doesn’t make for a very solid foundation, so the soil is laid down in layers, with each layer getting compacted by heavy machinery. 

The soil underneath concrete can get compacted after many years of supporting heavy weight. The soil under driveways, garage floors, and concrete pools is especially susceptible to compaction. The average car weighs around 4,000 pounds, and although it may not seem like it, the weight affects the soil. Homeowners also have a tendency to store heavy things in their garages. 

As for concrete pools, it depends on the amount of water it contains. A lot of water can be extremely heavy. It can quickly become too much for the pool to bear if the foundation underneath is faulty. 

Another cause of soil compaction is swelling. When the top layer of clay-rich soils gets wet and expands, it puts a lot of pressure on the un-moisturized soil under it. 

• What Makes My Soil Compact? 

Much like soil shrinkage, compaction is inevitable. Our concrete is tough enough to not break when we put weight on it, but that doesn’t mean the soil under it doesn’t get compressed. 

Compaction isn’t a straightforward process. Placing a single heavy item on concrete isn’t enough to compact the soil completely, but it does contribute, especially if the concrete was thinly layered or is of low density. Here are some of the ways pressure is applied on concrete to the point of affecting the soil underneath: 

• Cars 
• Heavy items 

• Snow 
• Soil swelling 

Low-density concrete (also called lightweight concrete) compresses the soil underneath when supporting a heavy weight. Cars, snow, and heavy boxes all contribute to soil compaction. 

Of course, it’s important to try to prevent the soil from getting wet. That’s because soil swelling can cause compaction and accelerate settling. The moisture may not reach the bottom layers of the soil, but the expanding soil puts so much pressure on those layers that it compacts. Once the top layers shrink, there are gaps between the top and bottom layers. 

The Causes of Concrete Settling 

When a concrete slab becomes uneven and sinks into the ground, that’s known as settling. Concrete settling is something that should be taken very seriously. It ruins the functionality of your pavement and creates an obstacle you have to watch out for. A broken, uneven path increases the likelihood of tripping and falling, so it’s important that you try to resolve it right away. But how does settling happen in the first place? 

When soil shrinks and leaves a gap under the concrete, the slab is no longer supported. Over the years, as the concrete has to withstand the weight of humans, cars, snow, and other objects, it starts cracking and breaking off. It then sinks and settles on top of the soil, which sits lower than it was when it was first placed. 

Concrete edges and corners are the areas most susceptible to settling. This is because a lot of concrete around the home meets grass. Because the soil in grassy areas is exposed to weather and gets wet and dry easily, the soil where the grass meets the concrete goes through a more extreme swell-shrink experience. Concrete edges and corners are also the weakest part of the slab, so they very easily break off or sink once the soil underneath can’t support it anymore.  

• What Makes My Concrete Settle? 

There are many reasons why concrete would settle. The moment the soil is unable to support the concrete, it’s at risk of sinking. However, your concrete might not settle immediately after the soil shrinks. Apart from soil shrinkage, concrete can settle due to: 

• Extra weight 
• Poorly compacted soil 
• Soil erosion 

When the soil placed under concrete is poorly compacted, it’s less dense. Since it’s looser, it will shift around in certain places, creating an uneven foundation. This can lead to soil erosion, which is when soil wears away due to flowing water. 

How Can Concrete Settling Be Fixed? 

Homeowners should be able to walk on their own property without fear of tripping and hurting themselves. Settled concrete is a huge menace to the peace of mind that comes with knowing your path is smooth and even. 

It’s something that should be fixed right away. Even if the damage is minor, before it gets worse and limits the concrete’s functionality, you should start planning on how to fix it. But how do you start? 

The first step would be choosing the right local professional. Concrete settling is something you need an expert’s help with. If you try to fix it yourself, you might end up making the shrinking, compaction, or settling worse. You might even have to replace the entire slab, which is a lot more expensive than concrete lifting. 

Depending on who you call, you can either choose to go with mudjacking or polyurethane injections as a concrete lifting method. If you would like a fast, long-lasting method that uses the latest technology for concrete lifting, you should go with a contractor that uses polyurethane injections. 

• Advantages of Polyurethane Injections  

Mudjacking is an outdated method that involves lifting sunken concrete by filling the gaps with a cement slurry. Multiple large holes must be drilled for the slurry to be pumped through, and these are then covered up once the job is done. PolyRenewal™ applies the same method of drilling a small hole, but instead of a cement mixture, polyurethane foam is used to level the sunken concrete. The foam used is closed-cell polyurethane, so it’s light enough to not compact the soil underneath but firm enough to level the concrete. 

Not all contractors use polyurethane injections, despite it being the best method of concrete lifting. The reason you should call an expert that uses this method is that polyurethane injections have several advantages over mudjacking. 

The work takes less time to complete and the polyurethane foam dries faster, which means you can step on your concrete shortly after the job is done. Polyurethane injections also last longer and are 100% guaranteed to fix settling. Because the cement used for mudjacking is so heavy, it puts a lot of pressure on the soil it rests on, which is already debilitated due to shrinking and compacting. The mixture’s rough texture doesn’t allow it to fully fill the gaps in the soil, so future settling is to be expected. Mudjacking is a solution that is said to only last five to 10 years. 

Once you’ve found a local expert who used polyurethane injections as a concrete lifting method, you can contact them to see what can be done about your settled concrete. 

Soil Shrinkage, Compaction, and Settling

FAQ's

What is the Difference Between Soil Compaction and Soil Consolidation?

When talking about soil compaction, it is often confused for consolidation. Both involve a very similar action that leads to a similar result, though the contents involved are different. When trying to understand why your concrete would settle, it’s important to know how to differentiate the two. 

Concrete settling may not be something homeowners can fully avoid, but it is something they can prepare for. Clearing up any misconceptions about how it happens helps with prevention methods, as well as planning when it’s time to fix the concrete.  

• Compaction 

Compaction is the displacement of air in soil due to pressure. When soil is loose and separated, there is a lot of air between each particle. When the soil is pressed together, the particles unite and the air gets pushed out. It depends on the soil, but some soils stay densely packed together when compacted, while others come loose once more. 

Clay-rich soil, which is commonly used, compacts really well. Not a lot of pressure is needed in order for the air to be displaced. While this has many advantages, it also means that settlement is common. Alongside shrinkage, compaction is a major cause for concrete sinking. 

• Consolidation  

Consolidation is similar to compaction in the sense that it involves applying pressure to displace an element in soil. However, while compaction is the displacement of air, consolidation is the displacement of water. The water between each soil particle pushes the soil apart, but it all comes together if the water is displaced. More pressure needs to be applied when soil is being consolidated because water weighs more than air. 

Just because water is displaced doesn’t mean it evaporates. The water flows down to the next layer of soil. Consolidation spreads moisture to a larger area of effect. The only way the moisture can be stopped from spreading too far is if it reaches soil too dense for the water to go through. 

Can I Change the Soil Underneath My Concrete?

Different kinds of soil have different swell-shrink capacities. Any soil that has 10% or more clay content is likely to swell and shrink very easily. This can lead to settling, which is why many site managers don’t like to use it. That said, there are a lot of benefits to using clay soil as a foundation, so it’s still widely used. 

Other soils like loam and sand shrink less than clay soil. Hearing this, you might want to change the soil underneath your concrete to one that has less of a chance to shrink.  

• Changing Your Soil 

It’s not recommended you change the soil your concrete rests upon. There are a lot of other solutions you can choose from that don’t involve altering your home’s foundation. It’s a very invasive process that requires many weeks to months, depending on the scale of the project. It’s also a very expensive process that should only be done when a home is severely settling. This is not to say it can’t be done, but it’s not the way to go when it comes to concrete settling due to shrinkage and compaction. 

If the structural integrity of your home is in good condition, but your concrete pavement is uneven or cracked, foundation repair is not necessary. The specific area that needs to be treated can be the focus of the operation when you call a contractor for concrete lifting. It’s a much more cost-effective, time-saving method than a complete change in foundation.  

• Other Methods 

So, if repairing the entire foundation under the sunken concrete is unnecessary, what other methods are there? The answer is concrete lifting. Concrete lifting fills the gap that’s formed under your concrete. It lifts up the sunken slab until it’s level with the rest of the pavement. There are two different concrete lifting methods used by contractors, and depending on the one you choose, it is a lifelong solution to concrete settling. 

It’s important that you don’t try any DIY methods to solve concrete settling. Repairing a structure so it’s stable is a very delicate job that needs to be left to the professionals. DIY may work for simple, minor fixes, but not for repairing concrete structures, especially those as important as the pavement you walk on. 

Why is Clay Soil Used if it Has a High Shrinking and Compaction Capacity?

Clay soil is commonly used as both a foundation and a filler soil. It has the highest swell-shrink capacity of any soil, which means settling is to be expected. So why is it still used in construction if it causes settling? Well, just as there are a few downsides to using clay soils, there are upsides as well. 

• Easy Compaction  

It’s important for soil to be well compacted if it’s going to be placed down as a foundation. The more compacted soil is, the better it is as a foundation. Water does not flow through dense soil very easily, so compaction prevents erosion as well as excessive shifting in the soil. 

Clay soils are very difficult to break apart due to how well they hold together once pressed. This is very important, and site managers would rather have soil that doesn’t shift easily over soil that doesn’t hold well, even if settling is more frequent in the former. 

• Fast Results  

Because clay compacts so well, it’s very easy to lay down as a foundation. When compacting, contractors have to make sure they are thorough. Poorly compacted soil causes many problems and can destroy a structure’s integrity. However, if a soil that isn’t easily compacted is used, it takes a lot longer to lay down. 

Clay soils are easy to manipulate. It doesn’t take long to thoroughly compact, so it can be laid down a lot faster than other soils. Well-compacted soil and fast results are a lot more important than little shrinkage capacities for some site managers. This is why clay soils are still used as a foundation, despite the higher rates of settling with this kind of foundation material. 

Count on Foundation Recovery Systems for Your Concrete Lifting Needs 

If you need to get your concrete lifted, Foundation Recovery Systems is here to help. We’ve been servicing Moberly, Kansas City, Springfield, and St. Louis for decades and have become the most trusted home repair experts in Missouri. Contact us today to schedule your free inspection and repair estimate with one of our experienced professionals. 

Foundation Recovery Systems uses PolyRenewal™, the best, most reliable polyurethane concrete lifting method. We care about your property’s visual and structural arrangement, so we use PolyRenewal™ as a non-invasive, highly effective solution. 

If you think that your property might be struggling with concrete settling, don’t hesitate to give us a call.

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_{Disclaimer: “Concrete leveling” means the process by which cracked, uneven concrete is stabilized, and in many cases lifted, by means of PolyRenewal™ polyurethane foam. Foundation Recovery Systems does not guarantee that PolyRenewal™ can make your concrete perfectly level.}