What Causes Settlement or Subsidence?
There are several potential causes of subsidence or settlement of the ground underneath a property. Some causes are man-made while others are natural geological changes. Natural causes such as fault movement or sediment compaction can cause subsidence. More commonly, however, the cause is man-made. Groundwater pumping, mining, oil and gas extraction, river channelization, and other manipulation of the ground can cause soil disruption that leads to subsidence.
Soils such as saline, non-cohesive soils, gypsum, silt, and clay are not as stable as some other soils. These soils can dry out due to lowering water tables and, when the water is not replenished, it can cause the soil to compact (subsidence). In other cases, soils can breakdown and vertically compress due to the weight of the structure on the soil combined with other factors (settlement).
Several things can disrupt the soil under your property and cause either settlement or subsidence. Explore the most common causes below.
Cause: Groundwater Pumping
In the US, 80% of subsidence is directly related to groundwater pumping, according to a report from the United States Geological Survey, making groundwater pumping the leading cause of subsidence in the US. Groundwater pumping lowers the water table that naturally holds up the soil. This can dry out soils and cause subsidence in various soil types, especially in hotter weather and droughts where the water table takes longer to replenish. Most government agencies have water management policies in place to proactively manage the water table, but issues can still occur depending on weather conditions and soil type.
Cause: Soil Types
The type of soil is a major determining factor when understanding subsidence. Some soils, such as clay and silt are classed as cohesive soils. This means that when wet and dry, they have a capacity to swell and shrink. Foundations built on such surfaces can witness changes from day to day, month to month and season to season. Soils such as sand and gravel are what are known as non-cohesive soils. Shrinking and swelling doesn’t occur with this soil type. Unfortunately, they are not exempt from subsidence. They are made up of tiny particles which can erode and wash away over time. Additionally, organic soils can naturally decompose due to their makeup. Oxygen naturally shrinks particles resulting in a smaller load bearing capacity.
Cause: Unstable Foundations
In some cases, homes can experience settlement as the soils underneath the foundation have a reduction in volume. This can be caused by the weight of the building over the years. Some foundation settlement is perfectly normal over time. Your home is heavy, and soils can compact more over time under the weight of your home. The amount of settlement typically varies based on the types of soils under your home. Soils such as saline, non-cohesive soils, gypsum, silt, and clay can create more settlement. In contrast, bedrock will settle much less.
Cause: Drainage Issues
Many subsidence issues are the result of leaks from pipes or drains. Water can seep through and affect the foundations in one of two ways. Water can make the ground softer, causing the supporting soil to lose structural support. As a result, the foundation starts to sink. In other cases, water will wash away non-cohesive soil particles and remove volume from the soil, leaving foundations to sink with the loss of volume.
Cause: Tree Roots
Trees, plants, and shrubs can be detrimental to your home’s foundation. Big trees, especially, are guilty of taking moisture away from the supporting soil under your foundation if planted too close to the foundation. Plants need water to survive and roots naturally seek out moisture. When the soils are shrinkable cohesive soils, plants can take away too much water from the foundation. This causes soils to shrink and foundations to sink. As you might expect, different species absorb different amounts of water. As a general guideline to planting distances from a property, root length and tree height are around the same.
Cause: Collapsing Cavities
In more rare cases, areas of the ground can collapse. This type of localized collapse is often triggered by severe groundwater level declines. A collapse is rare because it tends to be associated with very specific rock types such as salt or gypsum. These rock types are more soluble. While 35-40% of the US has evaporite rocks underneath, most of this rock is buried very deeply and is less of a concern for most areas. Additional information can be found from the USGS.