Undermining & Underpinning

Is my foundation going to crumble?

Even after 18 years and counting, sometimes I must stop and do a double take on what I see. Recently I inspected a beautiful hillside home that had the most amazing view of downtown Los Angeles in the horizon, the Pacific Ocean on its left and the LA harbor the right. What a view!!  While traversing the lower crawlspace area and proceeding through my normal analytic perspective, I see this 2-to-3-inch horizontal gap running the entire length of the concrete slab edge. At first, it really didn’t dawn on me as to what I was seeing. Then the light went on in my head!! Oh my gosh, the foundation is completely undermined…. I crawl up the hill to take a closer look and to my amazement, I can stick my entire hand under the concrete slab foundation… Normally there is no gap or separation!!

So, what is undermining and what is underpinning? This article is about shedding light on the difference. Undermining at a foundation refers to the process by which the soil underneath the foundation is eroded or removed, leading to a loss of support and stability for the foundation. This can be caused by a variety of factors, such as water infiltration, improper grading, or soil settlement to name a few.

Underpinning refers to a process that is used to stabilize and reinforce a foundation that has been affected by undermining. Underpinning involves strengthening the foundation by extending its depth or breadth, either by installing new supports beneath it or by reinforcing existing supports. This can be done by various methods. Repairing a lifted and separated concrete slab foundation typically involves lifting the affected area and re-leveling it to its original position. Here are some underpinning options, processes, and procedures that can be used for this type of repair:

1.    Helical piers - Helical piers are often used because they can be installed quickly and easily, and they provide excellent support. The process involves drilling holes in the ground beneath the affected area and screwing in helical piers to the desired depth. The piers are then attached to the foundation using various methods, and the foundation is lifted and leveled.

2.    Push piers - Push piers are another option for underpinning a lifted and separated concrete slab foundation. The process involves driving steel piers into the ground beneath the affected area until they reach a stable soil layer. The piers are then attached to the foundation using brackets and hydraulic jacks, and the foundation is lifted and leveled.

3.    Slab jacking - Slab jacking is a process that involves injecting a mixture of grout, sand, and other materials beneath the affected area to lift and level the foundation. This method is less invasive than Piering and can be completed quickly, but it may not provide as much support as other methods.

4.    Excavation - In some cases, excavation may be necessary to repair a lifted and separated concrete slab foundation. This process involves removing the affected area of the slab and the soil beneath it, installing new footings and supports, and then pouring a new concrete slab.

Ultimately, the underpinning option, process, and procedure used for a lifted and separated concrete slab foundation repair will depend on the extent of the damage, the type of soil, and other factors unique to the specific situation. By stabilizing and reinforcing the foundation, underpinning can prevent further damage and ensure the long-term stability of the structure. Just know, underpinning is a complex process, and it is important to work with a professional soil expert and foundation repair contractor who can assess the situation and provide the best solution for your particular needs in the event your property has been affected by this condition.