What is Residential Slab Underpinning?

After World War II, Slab on Grade Foundation Systems became very popular. Many veterans returning from war needed reliable and affordable housing in suburban areas. Because they are relatively affordable, builders and developers chose to build large homes with shallow-bearing concrete foundations systems. They were half the time required to build foundations like basements.

These primarily residential foundations look like a "waffle" in structure, with "ribs on the bottom and a smooth flat top. The slab is stiffened by the ribs to prevent movement and the smooth top surface allows for living space floors. Slabs on grade construction methods and design have not changed significantly over the past 50-years. It is still one of the most popular and reliable foundation systems on the market. If you are interested in underpinning in south Melbourne, you should consider these systems.

What should you do if these foundations fail? What is the best repair method? What is the difference between a reliable repair system and a good repair method?

The "single unit" behavior of shallow bearing slab-on-grade foundation systems is one of their key design principles. Slab-on-grade foundations act as single structural elements that have uniform stiffness to do their job. These foundation systems are designed to resist downward or upward movement due to soil pressure from below and heavy structural loads above. These slab-on-grade foundations are strong enough to withstand soil movement and other potentially harmful subgrade materials, but flexible enough to protect supported structural elements from unsafe or excessive planar tilting.

A design that is consistent with the original design principle is required to properly repair a slab on a grade foundation system that has failed or become structurally unstable. Because shallow bearing foundation systems are single units, they must be repaired as one unit. The remedial structural design should be uniformly stable to ensure that the foundation can withstand the soil, dead, and live loads.

Partial Piering to Repair Part of the Slab on-Grade Foundation System.

Stabilizing only a portion of the system will limit its movement to a specific area and allow the rest of the foundation to move freely. As a lever moving up and down, the unrestrained slab transfers its stress and focuses it on the area between the restrained slab and the movable slab. The transfer of stress can cause further damage to the concrete foundation structure and lead to structural failure.

Full Piering of the entire slab-on-Grade System.

The foundation system, regardless of whether it is repaired or not. It is intended to function as one structural element. Engineers design this type of foundation system. Shear, deflection, and strength are evaluated for the whole foundation, not just a portion. Why should a repair only address a portion of the slab?

One unit slab-on-grade design allows all foundation systems to respond uniformly to soil movement. This reduces the soil movement's impact on the fragile house materials (brick, sheetrock, etc.). This minimizes the potential for structural or cosmetic damage to the house and slab and helps into foundation repairs Melbourne.

Underpinning is a reliable, durable, and proven method of uniformly stabilizing (full-piering) slab-on-grade foundation systems. This involves a slab-on-grade foundation (interior or exterior grade beams) that bears upon and spans underpins (drilled-and-steel reinforced concrete piers or DRCP's). These underpin are placed strategically beneath the foundation system's expanse to provide maximum support. DRCP's can be placed on stable, deep soils without seasonal moisture fluctuations, unlike the cheaper, multiple segmented concrete piles that are stacked one over another like unstable kinder block blocks. Unlike segmented piles that can push the underpin upward (float) and from one side to another (lateral), DRCPs are extremely resistant to soil friction. DRCP's are the solid bearing surface that limits the foundation system's ability to move horizontally or vertically. A 30-40 reinforced concrete underpin may be used for a residence with similar quality, age, and size. These should be drilled to a depth of 10-15 feet and placed around the perimeter and interior of the foundation system.

If your engineer recommends foundation repairs, DRCP's should be seriously considered by the owner. DRCP's are expensive but must be used to repair slab-on-grade foundation systems. While mud-jacking and soil stabilization are cheaper options, partial peering or using segmented piles may be more expensive in the long term.