When excavating to increase ceiling height or create a crawl space, it’s not enough to simply dig and pour a new floor. Excavation can weaken the foundations of a detached house, leaving the structure unstable. To prevent cracks and structural damage, foundations must be anchored at a deeper level and reinforced, a process known as underpinning. Rather than demolishing and rebuilding, it is often preferable to repair the foundations from underneath.
One method of underpinning: “alternate sections”
However, foundations cannot be underpinned all at once, as this would risk destabilizing the house. The most common technique is the “alternate sections” method, also called the “hit and miss methodology.” This involves excavating small sections—typically one metre at a time—and reinforcing them with concrete, ensuring that the house remains supported on both sides of the opening. The process is repeated every two to three metres, depending on soil conditions, with a quarter to a third of the foundations strengthened in each pass. In three or four rounds, the entire foundation is securely underpinned, maintaining stability throughout the project.
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