Month: January 2018

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.

The Websoil Survey website is essentially the digital version of the USDA Soil Survey that had been initiated many years ago. The Survey had publications for individual counties. The soils information data has been available for some time, but the ready access provided on-line has certainly enhanced it’s value.

The City of Amarillo’s Building Safety Department has taken advantage of this on-line access by incorporating the soil description provided in the Panhandle Residential Foundation Manual. For the most part, the Websoil Survey is useful in determining the soil type for any particular site. The described soil type can be utilized for the residential foundation design configuration. However, soil type data is not provided by the Survey in certain specific locations in the Panhandle region. Typically, these areas where no soil data is available are the areas where the soil type is usually lakebed type soil which is a fat clay. These fat clays are the most expansive clays found in the region.

Therefore, in areas where the soil data are most critical for building purposes, no information is provided. Under these circumstances, the Panhandle Residential Foundation Manual directs the builder to use the worst case more substantial foundation system provided by the Manual.

The Websoil Survey does generally provide useful soil type information. In some cases, it gives soil types at varying depths. However, the data is rather generic in nature, and cannot account for recent artificial changes, development modifications, or site specific variations that are probably not noted in the Websoil Survey.

Comments provided by the previous blog, “Initial Site Building Preparations”, where awareness of the history of any site conditions should be the order of the day.

The Panhandle Residential Foundation Manual’s specific design criteria are primarily based on the “effective plasticity index” of a building site. Reference to page 4-1, (Methodology for Determining Effective PI) of the Manual is noted.

The “effective plasticity index” is determined by an analysis of the weighted plasticity index (PI) values of the layers of the soils taken down to the depth of fifteen feet below the finished soil surface. This kind of information on a particular site is of considerably more value that assumed Websoil Survey provided data.

In fact, obtaining actual on-site data may prove to be more economical than reliance on on-line information. The site may be found to contain superior soils than those presumed by the on-line data, hence lower cost foundation.

I have encountered building sites, most particularly in terrain of significant elevation variations, where soil conditions vary within the site. Under these circumstances, it would be wise to examine the soils at the various changing locations.

It must be recalled, though, that at least the “effective plasticity index” must be determined. A surface soil sample is not sufficient. Other considerations such as site drainage, soil moisture contents, soil stiffness, or water table considerations may be called for.

In the long run, a careful understanding of the building site’s soil conditions are essential, whether determined by Websoil Survey or actual on site determinations.