Basement Walls & Soil Pressure

How Much Pressure

• Sideways soil pressure increases with depth. A 9-foot basement wall experiences 50 percent more lateral pressure than an 8-foot wall, while a 10-foot wall must resist twice the pressure of an 8-foot wall. The zone of greatest inward-pushing lateral stress is about halfway up the basement wall. The pressure also varies with the type of soil. Slow-draining clay soils will exert roughly twice the lateral pressure on walls as fast-draining sandy soils. A 10-foot-high wall in sandy soil experiences lateral pressure of one ton per horizontal foot, but the same wall in clay soil will experience two tons of lateral pressure per foot.

Know Your Soil

• Before building your basement, consult an engineer to have borings done. Find out what kind of soil you have at your building site; design your basement walls accordingly. A wall’s strength depends on the quality of materials, wall thickness and presence of reinforcements. Wet, poorly drained soil may also require that you install exterior perimeter drains around your basement walls to reduce soil water content and thereby reduce the lateral forces pressing inward on your basement walls.

Trouble Signs

• Basement walls show certain common trouble signs when they start to lose their ability to resist soil pressure. Walls that are holding back water-saturated soil will show peeling paint, water staining, mineral deposits, dampness or seeping water. As the inward pressure increases, poured concrete walls will show horizontal cracks at roughly mid-height on the wall. Block walls will show stairstep cracks near the corners as well as mid-level horizontal cracking. If the top of the basement wall was not secured with bolts to the sill plate of the house, the top of the basement wall may bow inward, and vertical cracks may appear at mid-length of the wall. If you see these signs, consult a structural engineer.

Wall Repair

• If the inward bowing of your basement wall isn’t too severe, you may be able to repair it. Repair involves excavating the soil behind the wall to relieve the inward pressure and allowing the wall to flex back outward. You then must reinforce the wall by installing steel reinforcing bars, pouring concrete fill into the blocks and repointing the block joints. You may also have to install additional perimeter drainage to prevent oversaturation when the excavated soil is filled back in. If the wall is severely bowed, you may have to replace it. This work isn’t cheap. As of 2011, a foundation wall repair can exceed $100 per horizontal foot, while wall replacement can exceed $400 per horizontal foot.