Stable foundations are necessary for secure structures, whether they’re residential or commercial. When the soil subsides or shifts around the base of a building, there’s a good chance that the foundation is facing serious damage. Underpinning is one of the best ways to tackle issues with unsupportive or missing soil.
While it’s not the only option for stabilizing a foundation at risk, it’s one of the most affordable options. Most underpinning methods require less excavation than other foundation repair procedures. However, they only work in certain situations as well. Learn when underpinning is recommended and what options you have for repairing your foundation in this way.
How Does Underpinning a Foundation Work?
Underpinning a foundation involves drilling or excavating to extend the bottom of the foundation deeper than before. This allows the supports to extend past unstable or missing soil layers, reaching down to more stable levels of compacted soil or even bedrock in some cases.
The goal is to spread the weight of the structure onto supportive layers capable of bearing the weight. In the case of a natural disaster like a flood or an earthquake, it’s often the only option for securing the structure and preventing further damage. But it’s also used preemptively when soil shifting is identified early. There’s no need to wait until the foundation is cracking or damaged just to undergo underpinning.
When Is Underpinning Recommended?
Unlike some foundation repair methods, underpinning can work for almost every type of foundation. There are methods used for slab, crawlspace, pier, pile, basement, and block foundations. It’s simply a matter of matching the right technique to the situation.
Underpinning is often recommended for homes and businesses built over expansive soil. Soils with a high clay percentage tend to swell up when wet and then shrink dramatically when dry. This slowly leaves your foundation unsupported and at risk of cracking or sinking, damaging the rest of the structure when it does.
Other issues that cause soil settlement or erosion, such as poor drainage or slopes that direct water at the foundation, also tend to lead to the need for underpinning. It’s best to improve drainage at the same time with features like French drains and weeping tiles. If the soil wasn’t compacted or excavated properly during construction, underpinning can permanently fix the problem. Additions that increase the weight of the home can also trigger the need for extra support.
Methods of Underpinning: Pros and Cons
There are quite a few methods for securing your foundation and reducing the chance of future damage. The four primary methods of underpinning are known as helical or mini piers, beam and base installation, grout or resin injections, and concrete mass or pit installation. The right choice largely depends on the amount of damage, the depth of stable soil, and the type of basement you have. Your foundation repair team can guide you to the right type of underpinning based on your unique situation.
Helical Piers
Also known as mini-pile underpinning, this method involves driving large metal screws deep into the soil. They require a lot less excavation than any other method for stabilizing foundations and can reach as deep as necessary to connect with stable soil or rock. It’s not the most affordable solution in some situations, but it’s a permanent solution that works well for a variety of foundation types. It’s often the only choice in difficult soil conditions where pier or pile foundations need immediate support.
Beam and Base
Also known as pier and beam, this is one of the oldest forms of underpinning and remains popular today. Extensive excavation is needed to create space for concrete beams and the piers or bases that support them. This creates a large supportive structure that can anchor a badly damaged foundation and prevent any further sinking and shifting all at once. Yet it’s a slow and costly method that is often best replaced with helical piers instead.
Grout or Resin Injections
Related to the injection methods that use foam or mud to lift sections of a sunken foundation, injections can be used to support foundations where soil has eroded away. A high-pressure grout or dense resin is typically used to fill the void. It’s a quick and often affordable option due to the reduced need for excavation and the minimal access needed to inject the material. However, it’s highly limited in how much support it can provide. There’s also little to no protection against sliding or lateral forces, reducing its usefulness in many situations. It’s recommended for sandy soils where compression is more common than expansion or erosion.
Concrete Mass Method
Affordable and a good fit for most foundations, concrete mass underpinning remains one of the most widely used methods today. It generally involves pouring the same footers as the existing foundation, just deeper and on more stable soil. Since it’s poured in sections, excavation is limited and there’s relatively little disturbance around the structure. One of its only limitations is access. Some structures simply don’t have the access from the crawlspace, basement, or sides of the building to allow for this kind of underpinning method.
The Steps of Installing Underpinning and Foundation Supports
Each foundation repair is unique, but there are a few stages common to all underpinning methods.
1. Site Assessment
Test holes and excavations are usually required to assess the extent of soil loss and shifting. This can take a few days to a week, so it’s best to wait until there’s more information about the foundation’s condition before choosing an approach.
2. Excavation to Expose the Footings
Once an underpinning or other repair method is selected, it’s time to expose the footings. Most underpinning methods require excavation to dig deeper than the base of the current foundation structure. Helical piers can be driven into the soil with minimal digging, but they still must be installed in soil below the current foundation.
3. Installation of Chosen Supports
The installation of the underpinning supports can take anywhere from a few days to over a month. Methods that call for pouring concrete in sections, such as mass supports, will take the longest.
4. Load Transfer
Once the new concrete structures or steel piers are in place, it’s time to tie them together with the existing foundation walls or footers. This helps transfer the load of the home onto the new supports and ensures everything is holding up.
5. Stabilization and Soil Compaction
The last stage is returning soil to the excavated areas and compacting it properly to prevent erosion and shifting in the future. Any further stabilization needed to repair the foundation is carried out at this point.
Reach Out Today
Wondering if underpinning is the right solution to your foundation issues? Reach out to us at Pinnacle Foundation Repair to find out more. We can schedule an inspection and assessment of your basement to determine the right approach to repairs.
