How Foundation Soils Affect Your Home Foundation
Sandy soil and sandy loam soils expand and contract very little with moisture changes. They can be very reliable when supporting a foundation.
Clay soils expand and shrink in volume dramatically with moisture changes and can cause significant foundation damage
What Is Your Home Sitting On?
The simple answer is “the ground”. However, the real answer is a bit more complicated than that.
Soils are composed of different ingredients like sand, silt, loam and clay. These ingredients determine how soils behave under wet and dry conditions and when they need to support weight. Soil characteristics have a major effect on a house foundation.
Moisture And Soil
Different soil types are affected by moisture in different ways. Each of these three soils react to water differently:
- Sandy Soils – Water passes through sandy soils rather than being absorbed. This fact makes sandy soils very stable. Instead of expanding as they absorb moisture and contracting as they dry out, sandy soils maintain a fairly consistent volume and density. Because of their stability and good load-bearing qualities, sandy soils are less likely to shift and settle, so they rarely cause foundation problems. Unfortunately, sandy soils are less commonly found than other more problematic soil types.
- Clay Soils – Soils rich in clay and silt have the greatest potential to damage a foundation. Clay absorbs water easily, expanding in volume as it becomes more saturated. So-called “expansive clays” can cause foundations to crack, heave and shift.When clay soils dry out, they shrink and crack, leaving gaps around a house where water from the next storm can penetrate easily and deeply to repeat the expansion cycle. Clay-rich soils usually cause more foundation damage by expanding than by contracting.
- Sandy Loam Soils – Loamy soils are usually a very stable soil that shows little change with the increase or decrease of moisture temperature.The primary concern with foundations built on loamy soils is erosion. When soils underneath your foundation erode, they may begin to be inappropriate strata for sustaining the weight of a foundation and home structure.
Because of the constant cycle of wet and dry periods that occur as the weather changes, certain types of soil can expand and contract indefinitely, subjecting your foundation to settling or expansive stresses that often cause damage.
The “Active Zone” of Foundation Soils
Illustration of the active zone around and underneath a foundation.
Your home is resting many different layers of soil, each with different thicknesses and performance characteristics that can affect a house foundation.
These soils been formed or deposited there over thousands of years — some by water, some by wind, some by glaciers, and some by the contractor who built your home.
Typically, soil layers gain in stability and load-bearing capacity with depth. The surface layer is made up of organic materials, making it easy for plants and vegetation to grow.
As you go down, you’ll find layers of sand, silt, clay, and loam soils, depending on where you live. Deep below these layers is a layer of bedrock. Bedrock is a layer composed of either rock or very stable, densely packed soils.
The soil you should be most concerned about is known as the active zone immediately around and underneath hour house. This soil is most affected by changes in moisture and climate — and the source of most foundation problems. The active zone may vary from a few feet below the surface to more than 30′ below grade, depending on what area of the country you live in.