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RESIDENTIAL BASEMENT PROBLEMS - UNDERPINNING

Cracked and shifted walls lead to expensive repairs of settled and shifted foundations. This damage may arise from poor underpinning. It is particularly aggravating when a common wall between two houses has shifted. Underpinning is the process used to extend existing building foundations to a lower level. Many older homes were constructed with only partial basements. Owners have been enlarging these basements, leading to a requirement for underpinning.

Why Underpin?

It is not only the soil directly below a footing that supports the structure. The soil below and adjacent to the footing provides important support for the footing. If the adjacent soil is removed, as it must be to deepen a basement, the soil directly below the footing will often collapse into the newly created opening and the foundation will settle. Therefore, foundations must be deepened by underpinning when basements are lowered.

How To Underpin

Most houses are constructed on strip footings which run the length of the foundation wall. To underpin the wall, a series of short alternating sections are excavated to the desired depth below the footing.

Concrete is poured into forms at the excavations. After the concrete has hardened, non-shrink, dry pack grout is used to fill the gap between the bottom of the old footing and the top of the new concrete so that these portions of the wall are once again supported, now by new concrete to the soil at a lower level. Next, soil is removed from the unexcavated sections and new concrete poured until the entire wall has been excavated and underpinned. This alternating excavation technique ensures that at least half of the wall is always supported and that no long spans are unsupported at any time.

Problems

Minor settlements are to be expected as the weight of the building is redistributed to the new foundations. This may lead to some cracking of rigid interior surfaces such as plaster and drywall. Large settlements and foundation wall collapse result when inexperienced contractors or home owners excavate long sections of soil below an existing footing. Problems are often sudden and usually occur during construction.

One of the first signs that a collapse may occur is sloughing of soil from below the exposed footing. The long unsupported length of wall may collapse or the soil below the remaining sections of wall may become overloaded leading to settlement. All portions of the structure above the shifted foundation wall will settle, often significantly.

Water Leakage

Increased moisture infiltration following underpinning is caused by several factors. Since the excavation is normally carried out from the interior, there is no exterior damp-proofing or exterior foundation drain at the level of the new footings. Interior foundation drains are often specified for underpinned basements. They are unlikely to function effectively where clayey soils exist. Leaks may occur at the joint between the old footings and the underpin. We have observed holes the size of a fist at this joint.

Underpinning usually requires the services of an engineer and must be in accordance with Part 4 (structural design) of the Building Code. Walters Consulting Corporation offers the experience and service to effectively investigate the cause of structural damage.

 
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Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com