Messages and Online Forum Q&A

Re: Foundation Leveling

From: info@builderswebsource.com
Category: Foundations and Basements
Remote Name: 63.198.181.43
Date: 11 May 2001
Time: 08:19 PM

Comments

The best solution to foundation problems relating to clay soils involves proper engineering upfront, before a structure is built. Our online book, Building a Dream, has an excellent chapter on this very subject which you can read online for a small fee.

However, like millions of people, you own an existing structure that is exhibiting the destructive effects of clay soil and you're looking for the best remedy.

First, some background for those who aren't familiar with this issue.

Clay soil are highly expansive. Depending on moisture content, the soil volume can expand or contract up to several percent. As the moisture content changes throughout the years and the seasons, the clay shrinks and expands. If a home is built on top of this clay soil without regard to settlement issues, sooner or later the homeowner will be faced with some type of retrofit to mitigate the problem.

The trouble with "after-the-fact" remedies is that most of them are not optimum and may not last indefinitely. Generally, if a clay problem is known to exist, such is widely known in the Houston area, a foundation should be built initially with a pier and grade beam foundation. In this case, soil samples are taken to determine how deep the clay goes. Generally, the destructive clay soils go at least 3 feet down, and often to 10 or more feet. The key to foundation stabilization is to get past this poor structural layer to more stable soil. Once soil samples are taken, a friction test is made in the lab to determine the soil friction. This ultimately sets the PSF of soil area required to support a structure when using piers.

Once the weight of the structure is known (including, for example all dead loads, such as tile roofs, etc.), then this can be transferred to the perimeter foundation for load analysis. Depending on the size and spacing of the piers, the structural engineer will determine a suitable depth (past the clay soil) that the piers must penetrate in order to provide ample skin friction to support the structure. Often, interior foundation walls are also used which support load-bearing internal walls. This type of grid foundation provides a superior method of foundation stabilization. Differential settlement can be expected to be less that 0.25" over the life of the structure if designed properly. Depending on the conditions, piers may go 10-20 feet deep, ranging in diameter between 12" - 18" typically for a residential application.

Strictly speaking, a reamed pier (bell bottom) isn't required so long as the skin friction design is sufficiently conservative. Often, bell-type piers are used when relying not on skin friction, but actual support at the bottom of the pier itself. The bell allows the load to be transferred to a wider base. However, the bell won't hurt and can help to further lock-in the position of the pier.

Often, in order to avoid the soil from pushing up against the grade beam or structural slab, styrofoam or other compressible material is used between the foundation and soil to prevent future strain on the foundation.

Now, in retrofit applications, installing the proper piers, particularly in the middle of the foundation, is very expensive and disruptive to the structure. While it can be done, this method is never as good as a foundation designed this way from the outset. Future problems, such as internal settlement can happen, particularly if load-bearing partitions are not supported properly.

Another technique uses computer-controlled moisture stabilization to keep the soil at a constant moisture level. This works well along the perimeter of the house, but is more complex to administer on the interior portions of the structure. This can have its own issues, particularly if there are large trees nearby that can suck hundreds of gallons of water per day. However, the reason you see your cracks "healing" is simply due to the fact that soil has increased its moisture content after an extended dry period.

Generally, it is true that if you are going to remedy your foundation, you want to do it after the soil's moisture content is close to normal. That way, you're not optimizing the repair for an extreme condition (such as too dry or too wet).

We found a good article on this subject, specifically focusing on the Houston area. Please see: http://www.houseandhomeonline.com/renovations1199.html

Regardless of which way you go (pier stabilization or moisture stabilization), you should be aware that neither process is as good as building it right from the outset. Depending on the age of your structure, you may have some legal recourse with the original builder.

The most frustrating part of this is that the Houston area is KNOWN for having severe clay problems, yet structures are erected without proper design considerations. This problem is very easy to "engineer around" if it's planned for up front. But after the fact, it's expensive and inconvenient. We empathize with your situation...and can particularly relate to it: our first home was built in the 1930's and had the same problems...and eventually we decided to rebuild it with a proper foundation. We no longer have any foundation problems.

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