CG Engineering, civil and structural engineering and planning for residential and commercial projects

Underpinning: A Common Foundation Repair Method

Jan 15, 2019 | Foundation Repair, Residential, Structural Engineering

Soft soil is the most common reason homes in the Pacific Northwest suffer foundation settlement. If the weight of a house is too great for the soft soil it rests upon, the house may begin to sink. This process can take a matter months, years or even decades, but if the foundation is not stabilized, eventually the structural integrity of the entire home will be compromised. Soft soils may have existed since the home was built, or they may have been created by improper drainage or poor compaction methods during construction. In some cases, it is a combination of many factors that lead to the settlement of a home.

In order to stabilize a sinking foundation, it must supported on stable ground or underpinned. Underpinning is a method where piles are installed below the foundation into the ground until they hit soils that are strong enough to support the structure. Some structures are underpinned from the very start, others need to be underpinned due to settlement that has occurred.

Pin Piles

Pin Piles are commonly used to stabilize the foundations of sinking homes and light weight structures, such as decks and porches. Pin Piles are 2.5-inch steel pipes that are driven into the ground with a 150 pound jack hammer until refusal is reached. A structure’s weight and the depth of the soft soil determine the size of the pin pile. The goal is to transfer the weight of the foundation into more compact soils below. Once refusal is reached, the piles are attached to the foundation with a steel bracket bolted to the concrete foundation. The home can be stabilized or lifted to re-level the house.

Helical Anchors

Helical anchors (also called screw piles or helical piles) look like giant steel screws. Similar to pipe and pin piles, helical anchors are used to transfer the weight of a building to better bearing soils beneath the foundation. The difference is that helical anchors are screwed into the earth hydraulically, rather than hammered in. They offer the benefits of quicker installation and less ground disruption and larger load capacity than traditional piles, however some soils may not be well-suited for helical anchors.

Push Piers

Sometimes contractors will propose push piers for the repair of settled foundations. In our opinion these systems work for heavier homes with concrete basement walls or heavy brick structures, however, we usually don’t recommend these piers for lightweight home foundations.

The plans for underpinning a structure should be designed a structural engineer. Often, the structural engineer will employ the help of a Geotechnical engineer to perform soil tests that help determine which type of underpinning system to use. The engineer will also carefully determine the placement of the underpinning mechanisms, taking into account excavation needs for installation and other factors. Technical expertise is used to ensure the home is supported evenly and the job can be performed safely. Once the design is completed and permitted, the engineer will work with the contractor and homeowner to ensure the repair is completed properly.

Photo credit: Helical Anchor photo: By ArgyriouSelf-photographed, CC BY-SA 4.0, Link

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