Conditions and Uses
Normal Soil Conditions
Diamond Pier foundations sold through retail outlets are designed for projects that are founded in normal sound soils. Normal soils are typical in most residential neighborhoods throughout the United States and are defined in the International Residential Code (IRC) Table R401.4.1. Presumptive Load-Bearing Values of Foundation Materials. For residential applications, the two most common prescriptive bearing soil types relied upon in the IRC table, and in most local codes, are 2000 psf sands/gravels and 1500 psf silts/clays. Diamond Pier foundations sold through retail outlets must be founded in soils with a minimum 1500 psf bearing strength. Supporting soils that do not meet the presumptive bearing strength defined in the applicable code for your area will not provide expected foundation capacity, and their bearing capacity may need to be determined by a soils investigation. Ask your local code official for soil information regarding your site. Additional soils information may also be available at the U.S. Geological Soils Survey website managed by the U.S. Department of Agriculture—see http://websoilsurvey.sc.egov.usda.gov/.
Some soils may not be appropriate for supporting Diamond Pier foundations. Some examples include soils that are weaker than 1500 psf, soils that are highly expansive, shifting or sliding soils, soils on slopes greater than 2:1 (27 degrees), contaminated soils, or soils where traditional concrete piers, accepted by local codes, are unable to provide adequate bearing to support the loads of the project or to protect the structure from the negative effects of frost heave. Where unsound soils exist, a registered design professional may be required to review the project. Soils can also be weakened when they retain standing water or are improperly drained, and in certain types of soil this can also cause heave problems. A site depression with standing water or the potential for water to pond, pool, or saturate the soil may be an indication that the soil is not sound. Downspouts that discharge at or near a foundation may also cause soil problems, and setting a Diamond Pier foundation adjacent to any water body should be considered carefully. Depending on the variables involved, soils at the edge of or within lakes, ponds, rivers, streams, or tidal zones may be considerably weaker (as much as 40% or more) than dry or well-drained soils. Soils adjacent to existing foundations may also have been improperly or loosely backfilled, which may also cause poor drainage or poor soil conditions. Be sure to inform your project designer if any of these conditions exist. Please contact PFI if you have any questions regarding your project or soil conditions, and/or the proper use of the Diamond Pier product or “Residential Diamond Pier Load Chart,” provided in Table 1.
WARNING: You must check for underground utilities and follow the instructions described under “Locate Buried Utilities” (page 8) before Diamond Pier foundations can be installed.
Residential Diamond Pier Load Chart
Table 1. Residential Diamond Pier Load Chart
IAS-Accredited Third-Party Bearing, Uplift, and Lateral Field Tests2
- This load chart is intended for simple structures supported by columns, posts, and beams loaded up to, but not exceeding, the stated capacities. It is not intended for structures with asymmetrical, rotational, overturning, or dynamic forces. Intended uses are described in section 2.0 of ICC-ES prescriptive bearing evaluation report ESR-1895. For projects that exceed the capacities or limitations defined herein, or the intended uses described in ESR-1895, contact PFI for additional information or site-specific capacity evaluation. See also the Use and Applications download at www.diamondpiers.com.
- Capacities shown are tested to a Factor of Safety of 2, and are applicable in properly drained, normal sound soils only, with minimum soil bearing capacities as indicated. Copies of the field test reports are available from PFI upon request.
- See IRC Table R401.4.1, “Presumptive Load-Bearing Values of Foundation Materials,” for a full description of applicable 1500 psf and 2000 psf soil types. For soils below 1500 psf, or soils with unknown characteristics, additional site and design analysis is required. For soils above 2000 psf, the values in this chart shall apply.
- All capacities use four pins of the specified length per foundation. Pin length includes that portion of the pin embedded within the concrete head. See “Check Your Layout” on page 9 for more information on pin/pier layout and spacing restrictions.
- For professional engineers designing for short-term transient loads, contact PFI for further information.
Use and Applications
The intended use for Diamond Pier DP-50 and DP-75 foundations sold through retail stores is to support simple residential projects constructed with columns, posts, and beams. The scope of project is defined as decks, covered decks, walkways, stairways, and accessory structures or similar projects that meet this intent. Project loads are limited to the capacities defined in the “Residential Diamond Pier Load Chart” shown in Table 1. The load chart shows that Diamond Pier foundations provide equal or better performance when compared to traditional concrete foundations claimed as equivalent.
In the residential load chart, “cylinder comparison” and “frost zone” values are given. These two values define the size of the traditional concrete pier foundation that a given Diamond Pier foundation is equivalent to in bearing capacity and frost heave resistance. For example, a DP-50 with 50" bearing pins shows a cylinder comparison of 20" and a frost zone rating of 48". This compares with a traditional 20" diameter, 48" deep poured concrete foundation. For more information, please refer to the Use and Applications document at www.diamondpiers.com.
Frost is not an unusual or unsound soil condition unless the site has a history of locally accepted conventional foundations failing due to frost heave or freeze/thaw cycling. In frost zones, a properly drained, sound soil will freeze solid and hold its foundations tight. In heaving areas, water sources, the rate of temperature drop, and certain soil grain sizes can combine to cause pressures on foundations in all directions. The most important of these three factors is the presence of water in the soil, and this makes proper drainage a must—for all types of foundations.
Most traditional concrete foundations in frost zones rely on depth and gross weight as protections against frost heave. They use significant volumes of site-poured concrete, which has the potential for many field condition variables and inconsistent mix designs, and their installation requires considerable excavation, which weakens the existing soil structure, invites water problems, and leaves substantial amounts of soil to be removed from a site.
Diamond Pier foundations resist heave pressures and are often used in areas requiring frost protection. See our new 10-year Frost Study of over 95,000 installations and a 99.7% success rate against frost heave.
Rather than reaching a specific vertical depth or gross weight, Diamond Pier foundations resist heave pressures with their wide-spreading pin pile groups. Embedded in the intact soil structure, the pins are prevented from changing angle under load by the concrete head, creating a stable foundation for both bearing and uplift forces. Because of the unique design of the Diamond Pier head, the pins are also free to move along their axes without compromising the position of the head or its lock on the pin cluster. This feature allows the Diamond Pier foundation to absorb soil strains caused by frost heave or expansive conditions without losing alignment or transferring these strains to the supported structure.
When assessing projects in extreme frost areas, be aware of sites where traditional concrete footings—48" to 60" deep—have failed to resist frost heave, requiring larger, deeper concrete piers. Project sites that require concrete footings deeper than 60" to resist frost heave exceed the definition of normal soil conditions and the limits of the “Residential Diamond Pier Load Chart.”