You need to fix the underlying problem.
The most common culprit is water. It can accumulate in the soil around the foundation, which expands the soil and puts pressure on walls and foundation footings, causing cracks to appear.
Check to make sure all gutters and downspout drains are in good working order, and that the soil around your foundation is properly graded — it should slope at least 6 inches for every 10 horizontal feet.
Most foundations are required to have a perimeter drain system, such as a French drain, that channels sub-surface water away from the foundation.
It’s possible for this drain to become blocked, causing water to accumulate in the soil and putting pressure on your foundation walls. If you suspect a blocked drain, you will need to hire a professional to unclog it.
If Your Foundation is Uneven
A foundation that has tipped, bowed, or severely cracked requires substantial reinforcement to prevent further deterioration.
Repair the walls from the inside with wood or steel braces, carbon-fiber mesh, or wall anchors spaced 6 feet or so apart along the entire wall.
For about $500 to $700 each, wood and steel braces install against the wall and attach to the floor and overhead joists, blocking further movement. However, they intrude into the basement area about 6 inches, making it difficult to finish the walls.
A newer option, which costs less than half as much and winds up almost invisible, involves spreading epoxy in vertical strips and then pressing on carbon-fiber mesh to lock the wall in place.
Wall anchors are similar to large bolts. They consist of metal plates in your yard (installed by excavating) and metal plates on the inside of your foundation walls. The plates are connected by steel rods buried horizontally. The connectors are gradually tightened to stabilize and help straighten the wall.
Wall anchors are placed every 6-8 feet, and cost $400-$600 each. If a foundation wall bows severely (more than 3 inches) or if you want to make it straight again, you probably won’t be able to fix the problem from the inside.
You will probably need to excavate part or all of the foundation and rebuild it — a $30,000 to $40,000 job.
If Part of Your Foundation is Washed Out
If a broken water pipe, a plugged gutter, or a drainage problem in your yard sent enough water cascading alongside a perimeter foundation to undermine an area, a contractor might be able to shore up the area with more concrete or shim the sill plate to make the area level again. Or you might need to tear out a section of the foundation, re-pour, and tie the new section into the old with rebar and epoxy.
Simple fixes with concrete and lumber might cost as little as $500 or as much as several thousand dollars. Just be sure that the underlying cause is fixed first, or the repair won’t last.