Comparison of Helical Piers vs Drilled Concrete Piers for Expansive Soils
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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Understanding the Installation Process of Helical Piers
Helical piers are installed using a hydraulic torque motor. The motor rotates the pier into the ground like a giant screw. Installation depth is determined by measuring torque resistance. Helical piers can be installed in tight spaces with minimal soil disturbance. The process is quick, often completing in a single day. Helical pier installation techniques can vary based on soil conditions, requiring expert assessment for optimal results.
Advantages of Helical Piers in Expansive Soils
Helical piers offer superior resistance to uplift forces in expansive soils. These piers can be installed to greater depths than concrete piers. Helical piers provide immediate load-bearing capacity upon installation. The slender profile of helical piers minimizes soil displacement during installation. Helical piers can be easily removed or adjusted if necessary. Foundation repair using helical piers often results in less disruption to the surrounding landscape and structure.
Comparing Load Capacities of Helical and Concrete Piers
Helical piers typically have lower individual load capacities than concrete piers. However, multiple helical piers can be used to match concrete pier capacity. Helical pier capacity depends on helix size and soil conditions. Concrete pier capacity relies on shaft diameter and soil friction. Both types require proper engineering calculations for load determination. Structural engineers can determine the most suitable pier type based on specific site conditions and building loads.
Environmental Impact of Helical vs Concrete Piers
Helical piers have a smaller environmental footprint than concrete piers. No soil removal is required for helical pier installation. Concrete piers generate waste soil that needs disposal. Helical piers do not require curing time, reducing on-site emissions. The production of concrete for piers contributes to carbon emissions. Helical piers can be recycled at the end of their lifespan. Considering environmental factors before choosing a foundation system can contribute to more sustainable construction practices.
Cost Comparison Between Helical and Concrete Piers
Initial costs for helical piers are often higher than concrete piers. However, helical piers may offer long-term cost savings. Installation of helical piers is typically faster, reducing labor costs. Concrete piers may require additional equipment for deep installations, increasing costs. Helical piers offer more predictable pricing due to standardized materials. Understanding the full scope of foundation repair costs is crucial for making an informed decision between pier types.
Maintenance and Long-Term Performance Considerations
Helical piers generally require less maintenance than concrete piers. Concrete piers may develop cracks over time, requiring monitoring and repair. Helical piers are less susceptible to degradation from soil chemicals. Both pier types can be affected by corrosion in certain soil conditions. Regular inspections are recommended for both helical and concrete pier systems. Recognizing early signs of foundation problems can help prevent costly repairs regardless of pier type.
Suitability of Pier Types for Different Soil Conditions
Helical piers perform well in a wide range of soil types. Concrete piers may struggle in areas with high water tables. Sandy soils may require deeper helical pier installation. Clay soils can benefit from the uplift resistance of helical piers. Rocky soils may pose challenges for both pier types. Professional foundation inspections can determine the most suitable pier type for specific soil conditions at your site.
Limitations and Challenges of Both Pier Types
Helical piers may encounter difficulties with very hard soils or bedrock. Concrete piers can be affected by soil shrinkage in dry conditions. Both pier types require careful engineering to avoid adjacent structure damage. Helical piers may have limited availability in some regions. Concrete piers may face depth limitations due to equipment constraints. Understanding the differences in foundation repair methods is crucial for selecting the most appropriate solution for your specific situation.