What is Slab Settlement?

Simply stated, slab settlement is the movement a concrete slab experiences when the soil below can no longer support the weight of it.
You may be asking yourself, "Hey, the soil used to support my (driveway, patio, garage, slab floor) is just fine. So what changed?"!
Most likely…the soil changed. And while there are many reasons the soil may change underneath your concrete clan, let's focus on the three most common reasons:

1. Changes in the soil's moisture content

2. Wash-out of soil under the slab

3. Poorly compacted soil

Changes in the Soil's Moisture Content

During replacement, concrete is poured "into the forms on the tio if the soil." So let's talk about soil and its relationship with moisture.

Two very common soil types are sandy soil and clay soil.

Sand

Moisture does not have a big effect on sandy soil. When sand gets wet, it doesn't expand in size, and when it is dry it doesn't shrink. But sand does wash out...(more on this later!)

Clay

When clay is wet, it holds onto the water and expands in size. When it is dry, it shrinks.

Look at this sample of moist clay soil. When this soil has been allowed to dry out, even the naked eye can see how much the soil shrinks. Imagine how dramatic this is when several feet of clay soil exists below your concrete slab- whether it be your driveway, patio, or beneath the concrete floor in your home! 

Soil can dry out for several reasons, including drought conditions, tree roots drawing valuable moiature from the soil, and leaking HVAC systems under your floor slabs. In any case, as the soil beneath your concrete slab shrinks in size, it creates an empty space for the slab to settle into. The result? Cracked an unlevel concrete. 

Wash-out of Soil Under the Concrete

As water moves beneath your concrete slab, it can wash away the soil that's supporting the weight of the concrete. This can be caused by plumbing leaks, erosion, large rains, and a multitude of other reasons.

We mentioned earlier that sandy soils are not typically affected by changes in moisture content. However, if there is a path for water to flow underneath a concrete slab, sandy soil, as well as other soil types, can flow right along with that water from beneath the concrete. Regardless of the reason, over time with nothing to support the concrete, the slab will begin to sink or cave in. 

Poorly Compacted Fill Soil

During the construction of a home, driveway, patio, or sidewalk, soil is commonly moved around or spread out to get to the desired grade level. Oftentimes, the concrete slab is poured right on top of these fill soils that have been moved around. If the fill was poorly compacted, the fill soil compresses and settles, and a void is created under the concrete. again, with nothing to support it, the concrete cracks, breaks, and settles into the void. 

Options for repair:

1. PolyLEVEL: click to learn more

2. Mudjacking: click to learn more

Click here to see the comparision these 2 options. 

An excerpt from our book: Slab Repair Science