What is a Soils Test ?
As you stand on the ground you are thinking about building a home on, the dirt feels solid, unmovable and full of life. Surely a home can be built here with no problems you feel. And then someone mentions a need for a soils test. What is that?
A soils test, or by the more technical name “Geotechnical Test”, analyzes the composition of the soil below the surface for its ability to support a foundation of a building. In the Denver area, and Front Range of Colorado, a soils test is required to be done in order for a foundation to be correctly designed and you need to turn in a copy of it to the Building Department there if you want a building permit for a home there. Since the test determines how much weight the soil can take on without sinking or sometimes expanding, it is done by a separate geotechnical engineering firm and is compiled into a report that the structural engineer uses for their design
Along the Front Range of Colorado and throughout Denver, the soils contain a significant amount of clay and bentonite. Both substances have beneficial uses in cosmetics, and in gardening and in making things like bricks and clay fired dishes. But under a home or building, they are responsible for the shifting and lifting of homes and concrete floors and movement in streets and sidewalks. That simple lifting or called “heaving” creates cracks in foundations and floors and also show up as drywall cracks inside and cracks in bricks outside.
The test determines the amount of these substances and other soil constituents to determine the possible amount of movement they can create. Parcels of ground with significant amounts of these problem items can change a foundation design from a simple spread footing to one that requires expensive drilled reinforced concrete piers to hold the home in place. Basements can go from having a concrete floor to a wood floor framed over a crawl space under the basement. Walls in basements are usually hung from the floor above and left floating above a basement floor to prevent the transfer of upward lift into the house above. All these things of mitigation add significant costs to a home.
In Twin Falls, Idaho, the Building Department there has a presumption that the bearing capacity of the soil inside the city limits is 1500 psf. So for every square foot of soil, that the spread footer of the foundation sits on, it can uphold 1500 pounds of weight. Therefore a footing that is 18 inches wide at its bottom can transfer up to 2250 pounds of weight from the home for every foot of length of its foundation walls. Add up the lengths in feet of all of the home’s foundation walls and multiply that number by 2250 and you will have the maximum weight the home can be before it possibly settles or sinks. This of course is a very simplified example to give an idea of what a bearing capacity number can mean to a home design. One of the purposes of the soils test is to determine that bearing capacity number.
Other items a geotechnical test can provide is at what depth within the depth of the hole drilled for the test, is water present. It always amazed my inner child need to drop a stone into the test hole and weight to hear either that thud of a dry hole or the splash of it hitting a surface of water. A tape measure is then used to determine how close to the surface is this water table. Water that is discovered higher then the planned depth of a basement can mean possible trouble with water entering a basement in the future and added expenses in a basement design to try to keep that water out. Sometimes though, the water level is too invasive and a basement idea may need to be abandoned all together. Knowing this thru the use of this test saves a lot of headache in changing plans for a basement.
Another item discovered is a point called the “Point of Refusal”. This is simply where the rock under the ground is hard enough and close enough to the surface that the drill device that takes samples for the test comes to a point where it can not proceed. If the “Point of Refusal” is to high, then the excavation for the foundation could become more expensive, especially if blasting is needed. Or again, a decision for a basement is abandoned.
Typically the test is performed by a company drilling a 6 inch diameter hole going down nearly 25 feet into the ground. Samples of the soil are brought up at different depths in the hole and collected. Those samples are taken to a laboratory where they undergo analysis of composition as well as swell and shrink potential tests. Usually I would see in Denver two holes drilled in the area of the proposed basement could be and one hole made in the area of any garage may be.
After seeing the extent of how underground soil can effect the home above and how easily a home can develop cracks or become flooded by water, I encourage this test as it helps to ensure a better and more sound foundation for the dream you have.
Gary Miller
Principal and Architectural Designer,
Timbertree and Stonecastle Design, LLC
