Unchecked, the flow of water towards a house can lead to the buildup of water pressure along basement walls, and weaken foundation soils, contributing to foundation settlement. Footings, foundation slabs, driveway pavements, and sidewalks may be undermined, and cracks often develop in brickwork, slabs, and concrete and block walls. In extreme cases, the side-way movement of loose, wet soils can cause walls and foundations to collapse.
Cracks may also develop for reasons unrelated to drainage problems: weak foundation soils, basic design or construction flaws, inferior quality construction materials, faulty workmanship, and even the natural behavior of concrete. Whatever the reason for the cracks, water can use existing cracks, unsealed joints, and other openings to enter basements or other below-grade areas. Widening of originally insignificant openings by seeping water may eventually lead to serious water intrusion problems. Some sort of drainage improvement is almost always required prior to crack repair and foundation stabilization.
Several approaches are available for stabilizing a footing or foundation slab suspected of settling. Underpinning is the term for supporting a settling footing by installing new, usually deep-seated “pier footings” below the settling footing. Helical-shaped steel piers are probably the most common way to stabilize a settling footing. Helical piers are hydraulically driven so as to anchor into competent soil or rock lying below the weak soils near the surface. Helical piers can even be driven at sloping angles to secure a wall that has tilted or been pushed by soil or water pressure. Sometimes, settlement can be stopped more economically by simply digging or boring to suitable soils below the problem footing and filling the excavation with concrete. In all cases of foundation repair, it is important to make sure that digging around an existing foundation, or transferring loads to a new foundation support, will not dangerously weaken the structure. Foundations often have to be shored and repaired in stages. An experienced geotechnical engineer is able to determine the best type of pier, how much of the old foundation requires underpinning, how deep to drive the pier, the number and spacing of piers required, and the procedure to secure the existing foundation during underpinning.
Pressure grouting is another option for preventing continued settlement of a slab (or a footing, in some cases). Pressure grouting involves drilling into the soil below a left-in-place slab or foundation and injecting grout into the soil to strengthen it. Depending on the soil conditions and the extent of the settlement or crack problem, pressure grouting can be an extremely cost-effective approach. When it is not possible to repair a slab or wall that is severely cracked, removal and replacement may be necessary. Poor consistency soils below a cracked slab can sometimes be stabilized or replaced with gravel or newly compacted sand and clay fill material. Alternatively, it more cost effective to leave the original foundation soils in place, and instead construct a grid-work of support footings (referred to as grade beams) to support the slab. The ends of the grade beams usually sit on footings that extend to competent soil that may lie a few feet to tens of feet below the surface.
The significance of cracks and the repair costs vary widely. It is very important to determine if the crack movement is on-going and likely to continue, or the result of a “one-time” event arising from past conditions. An engineering evaluation of a house experiencing crack development or other drainage or foundation related problems will determine how the various problems relate to each other. (Sometimes they don’t.) Cracks may develop from shrinking of concrete the day the concrete was poured, and while these types of cracks are not very pleasing to the eye, (and potentially allow water to enter a basement), they certainly are not as significant as cracks that suddenly develop in basement walls after several cycles of rain and draught, or that are clearly associated with ongoing settlement. Without careful evaluation of the soils, the structural details, the extent of crack development, and the indications of interior (framing) movement, it is difficult to determine whether the solution is basic brick replacement in a very limited area, major foundation stabilization, or some other repair measure.
In all cases involving foundations and water intrusion, the more information you have about foundation soil and groundwater conditions and the construction of the structure itself, the easier it is to select the best, most cost effective approach, or combination of approaches to solving the problem.
Copyright 2011 Daniel A. Freiberg, PE, PG