The title of this blog directs to residential building structures. However the principles presented can apply to just about any type of construction that may be placed on the ground.
Specific reference is directed to both pages 4 and 5, notes 2 and 3 for Tables R401.2(a) and R401.2(b) of the Panhandle Residential Foundation Manual:
“Note 2. The effective plasticity index (PI) of the subgrade fill is 15 to 20 (R401.2(a)) or 20 to 25 (R401.2(b)) when computed by methods presented by the WRI…. The supporting foundation soils are assumed to have a minimum allowable soil bearing capacity of 1500 psf.”
Note 3. All subgrade/fill material shall be compacted to a minimum 95% of maximum density and shall be within 2% of optimum moisture content as determined by ASTM D-698, Standard Proctor, in lifts not exceeding 12″ in depth.”
The aforementioned tables and notes set forth the criteria necessary for compliance should the Panhandle Residential Foundation Manual be utilized for a particular residential building site preparation.
Initial site preparation remains the most crucial aspect of the foundation construction process. The actual foundation structure depends heavily on good site work.
The criteria states that all subgrade and embankment fill be placed at proper compaction at optimum moisture content. Before any of this work is done, though, consideration of historical circumstances of the site is required.
The typical homesite occurs as part of a development. The developer tailors homesite lots for future home construction. Most developments entail a degree of site work on each homesite lot. For example, the developer often cuts streets to grades lower than the actual lot building site for the sake of drainage. The excess embankment soil removed from the street is often placed on the homesite lot. This effort usually augments good off homesite drainage.
Sometimes the developer processes this spread out embankment fill, but no existing code requirement demands this work. It remains the responsibility of the home builder to assure full depth compaction and processing has been completed.
The treatment history of each homesite should be properly investigated to assure that no non-engineered (non-compacted) embankment soil has been placed on the site in the past. If non-compacted soils of more than one foot of depth exist, then the entire building site should be reprocessed down at least to the virgin soils. Even if the site had been properly prepared prior to construction, the surface soils, at least, should be reprocessed if sufficient time has past since the initial work. Surface soils typically dry and deteriorate with time.
These issues are of particular importance with non-level sites.
The Panhandle Residential Foundation Manual design Tables R401.2(a) and R401.2(b) provide specific footing/slab criteria for the more lean clays (R401.2(a)), and for the more expansive clay (R401.2(b)). These notes provide a methodology for determining the particular soil type for a site by using the Web Soil Survey website. For the most part, the use of these stated guidelines are sufficient. A subsequent blog shall discuss the use and the limitations of the Web Soil Survey.
It must be kept in mind, however, that the Web Soil Survey presumes on site virgin soils. In most development projects, the movements of the soils about a site as previously described here do not affect changes in the nature of the soils to any notable degree, so the Web Soil Survey will be useful in these circumstances.
It is possible,though, significant earthwork could be accomplished to the extent that the nature of the on-site soils can be changed. The Web Soil Survey may not apply under these conditions. Awareness of these potential changes is essential. Any foreign soil placed on a virgin site should be compatable from a foundation performance standpoint.
Significantly dis-similar soils should not be layered on a particular homesite. Sandy, non plastic soils should not be placed directly on a clayey soil site. Conversely, clayey soils should not be placed on a sandy site.
For example, a site’s suitability for construction shall be degraded if embankment soils from an area of fatty, more highly expansive soils are placed on a naturally more sandy clayey or lean clay site. On the other hand, if a sandy, more pervious soil is placed on a more impervious clay soil site, then the sand layers near the surface will readily absorb drainage, only to have that excess moisture trapped over the deeper, more impervious clay strata. This tends to create perched moisture zones on top of the virgin clay layers; which shall add to the expansive potential of the underlying clays.
On a future blog, the implications of poor and improperly prepared site conditions on the foundations systems shall be discussed.
The bottom line in all home building site preparation efforts is that all parties be aware of the actual site conditions and be attuned for the proposed foundation design criteria. Numerous resources are available to help assure the most optimal treatment of any site is accomplished. Future blogs shall discuss various options to properly investigate soil issues on sites.