Expansive Soils in Colorado

Carl Brahe, CHI CCI

Expansive Soil - Colorado is home to expansive soil,
commonly bentonite. The leading cause of foundation
damage in this type of soil is uneven moisture. Drying soil
can shift and crack your foundation as it shrinks. When
moisture in applied the resulting swelling can crumble your
foundation and actually break your house in two, if your
basement is not properly framed. There are other
dangers from water movement in the soil

Inspecting for moisture conditions that could cause damage to
your foundation.

Dirt should slope away from all sides of your foundation six inches
in ten feet. When this is not possible a French drain, or other
drainage method, is used to drain water away from the foundation.
You may notice cracks in walls and ceilings. Doors and windows may
stick. This may be the result of uneven moisture under your foundation. If water drains away from three sides of your house, but brings the water to the foundation on the other side, extensive damage to your foundation may result.

Clay and organic matter in soil expands when wet and shrinks when dry. Most foundations in Colorado are made to float on the expanding and contracting soil. If the moisture is even around the entire foundation the expansion and contraction will be uniform. The foundation will float as a single unit and remain undamaged.If one section expands, lifting the house, while the rest doesn’t, the foundation can break.

A simple way to test for uneven moisture levels around your foundation is to use an inexpensive plant watering gauge. These are available at hardware stores for around $10.

The gauge has two long, metal probes that are pushed into soil to measure water content. Measure the moisture content in the soil around your foundation at 10' intervals. Test the moisture levels about 1' from the foundation. Move around the entire house mapping moisture levels around the foundation.

If you find that the moisture levels are higher in some places than others, it is likely you will find some sign of foundation movement around the areas of higher moisture. This may result in doors and windows that stick, or cracks in walls, ceiling or foundation.

Most foundations in Colorado are made to float on the expanding and contracting soil. If the moisture is even around the entire foundation the expansion and contraction will be uniform. The foundation will float as a single unit and remain undamaged.

If moisture is uneven simple measure can usually be taken to correct the problem. In areas where the moisture content of the soil is higher than others direct runoff water way from the foundation. Make sure the dirt slopes away from all sides of the house and all gutters and downspouts are in working condition. Be sure sprinklers don't spray against the foundation or side of the house.

In some cases expensive retaining walls and/or French drains may be needed. If you have doubts consult a landscape engineer or architect.

Testing for expansive soils is not a usual part of a home inspection.

EXPANSIVE SOIL INDEX:

Map of expansive soils in Colorado

How Expansive Soils Cause Damage