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 a sufficient bearing surface. For a conservative relationship between soil type and load-bearing, refer to Table 1.
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 one-foot penetration (blow count) is recorded. Values can be roughly correlated to soil-bearing values as shown in Table 2.
The instrumentation and cost of conducting the SPT test method are usually not warranted for typical residential applications. Nonetheless, the SPT test method provides information on deeper soil strata and can offer valuable guidance for foundation design and building location particularly when subsurface conditions are suspected to be problematic. The values in Table 2 are associated with the blow count from the SPT test method.
Many engineers can provide a reasonable estimate 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. The values in Tables 1 and 2 are generally associated with a safety factor of 3 (Naval Facilities Engineering Command, 1996) and are considered appropriate for non-continuous or independent spread footings supporting columns or piers (i.e. point loads). The 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. To achieve a safety factor of 2, the designer may multiply the values in Tables 1 and 2 by 1.5.
1N – denotes the standard penetrometer blow count in blows per foot in accordance with ASTM D1586; shown in parentheses.
2compaction should be considered in these conditions, particularly when the blow count is five blows per foot or less.
3pile and grade beam foundation should be considered in these conditions, particularly when the blow count is five blows per foot or less.
The required width or area of a spread footing is determined by dividing the building load on the footing by the soil bearing capacity from Tables 1 and 2. Building design loads, including dead and live loads, should be determined using the combinations below according to allowable stress design (ASD) load combinations.
Areaindependent spread footing = (Load in lbs)/ (Soil bearing capacity in psf)
Widthcontinuous footing = (Load in plf)/ (Soil bearing capacity in psf)
Objectives of Footing Design
1. To provide a level surface for the construction of the foundation wall.
2. To provide adequate transfer and distribution of building loads to the underlying soil.
3. To provide adequate strength in addition to the foundation wall, to prevent differential settlement of the building in weak or uncertain soil conditions.
4. To place the building foundation at a sufficient depth to avoid frost heave or thaw weakening in frost-susceptible soils and to avoid organic surface soil layers, and
5. To provide adequate anchorage or mass (when needed in addition to the foundation wall) to resist potential uplift and overturning forces resulting from high winds or severe seismic events.