What is foundation settlement?
Foundations are heavy. When they’re placed on top of soil, they press down on it, causing the soil to compact. This compaction causes the foundation to settle a little bit into the soil. This is perfectly normal and happens to all foundations.
The amount of settling a foundation experiences depends upon a variety of factors, including the type of soil and the type of foundation. Foundations built on top of bedrock don’t settle much at all, while foundations built on top of some types of soil, will settle more.
A striking example of settling is Mexico City’s Palace of Fine Arts, which has settled 13 feet over the last 100 years. However, this is an extreme example and is not considered normal settling. Most normal foundation settlement is measured in fractions of an inch, not feet.
Types of foundation settlement
All structures will experience some foundation settlement over the years. As long as the settlement is minimal and evenly distributed (i.e. uniform), there’s probably nothing to worry about. Differential foundation settlement is another matter. Let’s take a look at what that means now…
Differential foundation settlement
Differential foundation settlement happens when different sections of a structure are not experiencing the same rate of settling. Signs of this include – but are not limited to – doors and windows that don’t open and close as easily as they once did, sloped floors, and wall cracks. This happens because differential settlement distorts the building’s frame. Everything is now out of plumb. When a structure’s differential settlement is severe, it means the soil is failing to support the foundation. When this happens, the foundation needs to be underpinned.
Uniform foundation settlement
Uniform foundation settlement happens when all sections of a structure are experiencing the same rate of settling. Uniform settlement usually isn’t a problem. In fact, a certain amount of uniform settling is perfectly normal.
The image below illustrates the difference between differential and uniform settlement.
What causes foundation settlement?
As we mentioned above, a certain amount of uniform foundation settlement is normal. Over time, depending on the local climate and soil conditions, all structures will settle.
The problem is differential settlement. So, let’s talk about what causes that.
Differential settlement can be caused by a variety of things including, but not limited to,
- Soil that wasn’t properly prepared prior to the start of construction. If the building site is not properly compacted with the correct material, the soil underneath the foundation could experience issues from expansion, contraction, consolidation or erosion.
- Expansive soil. Expansive soils are those that easily absorb and release water. As they do, they expand and shrink considerably as the seasons alternate between wet and dry. If the expansive soil under a foundation isn’t expanding and shrinking uniformly, the foundation isn’t moving as one unit. This puts a lot of stress on a foundation. Stress that can lead to structural damage.
- Weather changes. This might happen when a structure is built during a dry season on top of clay soil, which is expansive. When the wet season arrives, the soil expands. Then, the dry season returns and the soil shrinks as it dries out again. Soils with a lot of clay in them shrink and expand a lot as they take on and lose moisture. This repeated expansion and shrinkage puts a lot of stress on a foundation.
- Extensive digging next to the foundation. If you remove too much soil around any foundation, it loses its support. Imagine digging a hole on the beach next to someone sitting in a chair. Eventually, the ground the person is sitting on will cave into the hole. Likewise, if one side of a house loses its support, it could result in differential settlement. So, basement construction, pool excavation, or really any type of excavation, next to a foundation could result in differential settlement.
- Seismic activity. It goes without saying that earthquakes can severely damage foundations.
- Floods. Even slowly moving water can pack a real punch, and when it pushes up against the side of a structure it can cause it to separate from its foundation. Flood water that goes deep into the soil under a foundation can shift pilings that aren’t penetrating the load-bearing strata. This can cause enormous damage to a foundation.
How much foundation settlement is too much?
The industry standard is 1 inch of differential settlement in 20 feet. Anything greater than this can be considered too much.
Foundation settlement repair
There are a variety of ways to repair foundation settlement including steel push piers, helical piers, drilled concrete piers, and slab piers. We’ll briefly go over each here. For more information see our service pages.
Heavy duty steel resistance piers (also called push piers) are driven deep down into the load-bearing strata via hydraulic pressure and the weight of the building. Hydraulic jacks, placed on the top of the piers, then lift the building back up.
Helical piers are shaped like giant screws and, like push piers, are driven deep down into the load-bearing strata. The torque necessary to drive them deep down into the soil determines their load-bearing capacity. Hydraulic jacks, placed on the top of the piers, then lift the building back up.
Drilled concrete piers
Drilled concrete piers – going deep down into the bedrock – are often used to stabilize homes situated on hillsides that are experiencing foundation problems due to soil creep and lateral movement.
Slab piers are either steel push piers or helical piers that have been installed via small holes that have been cut into the floor or slab. Anchors are inserted through the holes until they hit load-bearing strata. Steel brackets fasten the anchors to the foundation.
Signs of foundation settlement
Wondering how to check foundation settlement? Here are some common signs to look for…
- Windows and doors that stick are a common sign of a foundation with differential settling.
- Uneven floors. Look for floors that are uneven in various ways including sunken floors, sloped floors, and bowed floors.
- Ceilings and floors that have separated from the wall. These don’t need to be large, extremely noticeable separations. Even slight separations could indicate a serious structural problem.
- Floor cracks. Look for floor cracks that run in a straight line across the floor from wall to wall. Floor cracks caused by dropping something on the floor are usually limited to one or two tiles.
- Cracked or bowed walls. The wall could either be on the exterior or interior of your home.
- Tears in wallpaper. This could be a sign that the wall behind the wallpaper is cracked.
- Wall rotation. This happens when the soil under a foundation has a lot of water in it. The building’s weight pressing down on the foundation will cause the foundation’s outside edge to sink deeper into the saturated soil. Meanwhile, the inside edge of the foundation – which sits atop drier soil – pulls up. This results in the wall rotating.
- Diagonal cracks going from the corners of doors and windows to the ceiling. If these are very thin, hairline cracks it might be from normal settling. However, larger cracks could be a sign of foundation damage.
- Moldings that have separated from the wall or ceiling. This happens because the floor, ceiling, and walls are no longer moving as one unit.
- Stair step cracks in brick and masonry. This is another sign of foundation trouble. Sometimes these cracks will be in the mortar only.
- Chimneys or porches that are separating from the rest of the house. There’s a chance this might be related to a problem with the foundation under the chimney or porch. However, it should be looked at by a professional to make sure it isn’t being caused by a problem with your home’s foundation.