This article by Ryan J. Hardesty, P.E., of Hardesty Consulting LLC discusses helical piers vs drilled concrete piers in highly expansive soil areas. We found it interesting and wanted to share.
It is well known along the front range of Colorado that extensive structural damage to lightly loaded structures can occur in areas underlain by expansive soils. In addition to the damages incurred by such structures, especially residential homes, there is a considerable amount of risk that is shared amongst the community when dealing with expansive soils.
For example, the risk of building on such soils affects the builders who construct the homes, the engineers who design the foundations, the warranty companies who provide structural warranties on the homes, the insurance companies who insure the warranty companies for such homes, and more importantly, the people who own the homes. In order to mitigate the risk among parties, proper design and construction should be attained and viable foundation alternatives sought out.
In the Rocky Mountain region, drilled concrete pier foundations have been utilized since the early 1950’s to minimize the damage caused by the structural movement of homes built on expansive soils. Drilled concrete piers are a means of transferring structural loads from an upper layer of undesirable material to a lower layer of more desirable material. (CAGE, 1999) The load-carrying capacity of a drilled concrete pier depends not only upon its end-bearing value but also to a great extent on the skin friction value between the concrete and the surrounding soil. Concrete piers are generally drilled into bedrock or other competent material in a zone unaffected by moisture change. Within this zone, the skin friction between the concrete and soil generates the withholding force along the pier shaft. Above this zone, the soil is prone to swell along the pier shaft, producing an uplifting force along the top portion of the pier. The uplifting force is to be resisted by both the withholding force along the bottom portion of the pier and the amount of dead load pressure applied to the top of the pier from the superstructure of the home. The theory that leads to the rational method of designing drilled concrete pier foundations in expansive soils assumes that the sources of soil wetting are derived from the surface and gradually penetrate into the subsoil.
One major problem with using drilled concrete piers in highly expansive soil is the extreme embedment depths that are required to anchor the pier and resist vertical movement. Penetration into a zone of soil that is not expected to undergo significant moisture variations should be deep enough to develop enough friction between the shaft and the soil to offset the tendency for the expanding soil to lift the shaft. It may not be feasible and/or possible to construct drilled concrete piers in highly expansive areas due to current drilling constraints. Drilling rigs within the Denver area are currently capable of drilling concrete pier holes to a depth of 38 to 40 feet. This depth constraint may hinder the ability of the drilled pier to resist the uplifting forces along the pier shaft. Due to the large surface areas of drilled piers, the pier shafts would need to penetrate beyond the possible 38 to 40 feet in order to resist such uplifting forces. Additionally, installing pier reinforcement and placing concrete at greater depths would also become a problem.
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