What Causes a Foundation to Sink, Settle or Sag?

WHAT DOES YOUR HOME SIT ON?

At first glance you may say, "the ground!" And you would be right. When determining what may be causing your home to sink, settle, or sag, however we need to look deeper. What is below the "ground" that you can see as you walk out onto your lawn? Soil layers. Your home is resting upon many different layers of soil each with different thicknesses and abilities to hold the weight of your home. They were formed or deposited there thousand and thousands of years ago when the earth was formed. Some layers were carried and deposited by water, some by wind, and some by glaciers. Some layers may have even been put under your home by your homebuilder, who commonly moves soil around to create flat, buildable lots of land.

Typically, soil layers get stronger with depth. In most locations, you'll find a shallow layer near the very surface that is organic, making it easy for plants or other vegetation to grow. Below the top layer, you may find layors of sandy, silty, or clayey soils depending upon where you live and how the soil got there. Deep below these layers is a soil layer of bedrock, which is rock or very stable, dense soil.

MOSITURE AND SOIL

Different soil types are affected by moisture in different ways. Two very common soil types are sand and clay.

SAND- Moisture does not have a big effect on sandy soil. When it rains, the water usually passes through the sand very quickly. When the sand gets wet it doesn't expand in size, and when it is dry it doesn't shrink.

CLAY- the problem soil! When clay is wet, it holds onto the water and expands in size. When it is dry,  it shrinks. 

WHAT IS SETTLEMENT?

Simply stated, settlement is the movement your home experiences when the soil below can no longer support the weight of your home. 

Well, the soil used to support my house just fine. So what changed?!

Most likely, the soil changed. There are three common changes in soil that cause settlement:

1. Drying and shrinking of soil (clay soil is the problem)

2. Wetting and softening of the soil (clay strikes again!)

3. Poorly compacted fill soil

1. DRYING AND SHRINKING OF SOIL

Drought. After many months or years of drought, clay soil dries out. And, as we know, when clay dries, it shrinks. As the amount of soil around your house shrinks in size, it creates an empty space for your home to setlle into.

Maturing trees. Did you know that the root system of a tree is often two times the size of the tree canopy? This means, a tree with branches that extend over your home, most likely has roots that extend under your home, drawing valuable mositure from the soil. When the soild dries out, you know the drill!

2. Wetting and softening of soil

Heavy rain & flood conditions. As clay soil gets wet, it holds on to the water and becomes very soft. This soft soil can be weak, causing the home to shift or 'sink' down into it. In some ways, it is similar to when you step in mud and your foot 'squishes' into the soil. 

Poor drainage. If water is allowed to stand or pond next to your home, the soil will absorb the water and again, weaken the soil. 

3. Poorly compacted fill soil.

A common practice when building a new home involves removing soil from hilltops and placing it in valleys to create flat, buildable lots. If this fill soil is not compacted well, it will begin to compress under the weight of the new home, causing settlement. 

DID YOU KNOW... Settlement is often associated with older homes, but a recent article from Consumer Reports shows that an alarming 15% of new homes have structural problems. These problems generally have two causes: poor construction or homes built on poorly compacted fill soils. 

SIDE NOTE: Other less common causes of settlement include the drying and shrinking of soils associated with leaking HVAC systems under the slab floor. This tens to be more common in slab-on-grade houses and buildings. Plumbing leaks and broken water lines under the slab can also be a problem, causing a wetting and softening of the soil. 

* An excerpt from the book Foundation Repair Science that is provided to our clients who have an appointment booked to speak with a System Design Specialist about their foundation issue.