By Ethan Davis
Understanding the underlying soil and its ability to bear the weight of a proposed structure is vital to the success of a project. Soil bearing investigations are rarely required for residential construction except in the case of known risks, as evidenced by a history of local problems (e.g., organic deposits, landfills, expansive soils, etc.). Soil-bearing tests on stronger-than-average soils can, however, justify smaller footings or eliminate footings entirely if the foundation wall provides sufficient bearing surface.
When a soil-bearing investigation is desired to determine more accurate and economical footing requirements, the designer commonly turns to ASTM D1586, Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils (ASTM, 1999). This test relies on a 2-inch-diameter device driven into the ground with a 140-pound hammer dropped from a distance of 30 inches. The number of hammer drops or blows needed to create a 1-foot penetration (or blow count) is recorded. Values can be roughly correlated to soil-bearing values as shown in Table 4.3. The instrumentation and cost of conducting the SPT test is usually not warranted for typical residential applications. Nonetheless, the SPT test method provides information on deeper soil strata and thus can offer valuable guidance for foundation design and building location, particularly when subsurface conditions are suspected to be problematic. The values in Table 4.3 are associated with the blow count from the SPT test method. Many engineers can provide reasonable estimates of soil-bearing by using smaller penetrometers at less cost, although such devices and methods may require an independent calibration to determine presumptive soil-bearing values and may not be able to detect deep subsurface problems. Calibrations may be provided by the manufacturer or, alternatively, developed by the engineer.
The designer should exercise judgment when selecting the final design value, and be prepared to make adjustments (increases or decreases) in interpreting and applying the results to a specific design. Use of a minimum safety factor of 2 (corresponding to a higher presumptive soil-bearing value) is recommended for smaller structures with continuous spread footings, such as houses.
The required width or area of a spread footing is determined by dividing the building load on the footing by the soil-bearing capacity. Building design loads, including dead and live loads, should be determined by using allowable stress design (ASD) load combinations.
In our next blog, we will be discussing Footings.
(This information is taken from an article by Nick Gromicko and Ben Gromiko on the International Association of Certified Home Inspections website)