The residential foundation design configurations provided by Tables 401.2(a) and 401.2(b) of the Panhandle Residential Foundation Manual are based primarily on the guidelines offered by the Welded Reinforced Wire Institute’s Concrete Foundation Design Method.
This methodology considers the expansive qualities of the on-site soils as depicted by the “effective plasticity index” of the soils on the proposed building site. This soils information considers the soil classification taken from the surface down to a depth of fifteen feet.
However, the method does not include load factors of soil condition such as moisture content, soil stiffness, or in-place density preparation. These specific factors are considered in the notes of the Manual regarding on-site preparation.
As stated in the Manual, the design criteria presumes uniform structural loadings distributed evenly across the foundation system of 200psf, 275psf, and 350psf for single story, two story, and three story structures, respectively.
For the most part, adherence to the stated Manual foundation design configurations as well as the required site preparation and soil condition criteria should provide for a foundation of adequate performance for the typical foundation structural configuration.
Never-the-less, foundation performance issues can arise under some altered circumstances. Several of the issues brought to mind involves historic examinations of foundation performance problems.
Most of the observed foundation movement problems have to do with perimeter drainage. The Panhandle Residential Foundation Manual stipulates a degree of positive drainage away from the foundation’s perimeter. The idea is to minimize accumulated moisture in the perimeter soils about the structure. However, landscape irrigation or lack of irrigation that is improperly managed creates more problems than does site drainage due to natural precipitation.
Continuous excessive wetting of soils about the perimeter will cause moisture to penetrate perimeter foundation soils well beyond the soil’s initial condition. In expansive soils, this means swelling, or uplift about the perimeter of a lightly loaded residential structure.
The converse is true if the perimeter soils are wetted, then allowed to dry at depth. The perimeter foundation system then shrinks, or settles. The ideal circumstance is to manage the perimeter soils moisture contents at a constant state at all times. Water when needed, but not excessively.
The Manual dictates that water stops be utilized between the construction joints of two pour foundation configurations. This is necessary because perimeter water can easily enter beneath the interior floor slab through the joint between the footing and the slab without water stops.
As previously noted, the Manual foundation design presumes uniform load distributions across the structure. The actual loading conditions across the structure may vary widely from the uniform state. In multi story structures, it is possible that some upper floor level support may be concentrated at certain portions of the foundation system. Conversely, other areas may have lightly loaded sections. Should the aforementioned soil moisture swelling/shrinkage circumstances arise in heavily loaded or lightly loaded foundation sections, then foundation movement problems may be enhanced.
The heavily loaded portions should be treated with local foundation configurations that spread out the loading to become more uniform. The lightly loaded areas should have design configurations that help minimize variations in the uniform loadings.
Thickness of the floor slab can have a factor. The use of five inch or thicker sections seem to aid in the more widespread distribution of loadings, which contributes to a more uniformed load distribution.
Interior basement structures require additional consideration regarding structural loadings. It is common to see the perimeter foundation system subject to uplift due to soil swelling about the non-moving basement structure. The basement behaves as an “anchor” about which the ground level portions of the house can flex. Oftentimes, significant cracking is noted about the perimeter of the interior basement because of exterior perimeter uplift. In homes with interior basements, proper management of the surrounding surface level foundation system against uplift or shrinkage is important.
Adherence to the Panhandle Residential Foundation Manual in the environment of expansive soils is crucial. Usually, though, it is essential that the loadings and site configuration of all foundations systems receive careful cognizance and consideration.