Engineering and Construction

Introduction
Water can penetrate basement walls due to hydrostatic pressure, capillary action, and water vapor. If a groundwater table exists behind a basement wall, then the wall will be subjected to hydrostatic pressure, which can force large quantities of water through wall cracks or joints. Figures 11.21 and 11.22 show two photographs of a basement wall subjected to hydrostatic water pressure that forced large quantities of water through the wall. A subdrain is usually placed behind the basement wall to prevent the buildup of hydrostatic pressure. For the drains to be effective, the backfill material should consist of granular (i.e., permeable) soil.

Another way for moisture to penetrate basement walls is by capillary action in the soil or the wall itself. By capillary action, water can travel from a lower to higher elevation in the soil or wall.

Capillary rise in walls is related to the porosity of the wall and the fine cracks in both the masonry and, especially, the mortar. To prevent moisture migration through basement walls, an internal or surface waterproofing agent is used. Chemicals can be added to cement mixes to act as internal water-proofs. More common are the exterior applied waterproof membranes. Oliver (1988) lists and describes various types of surface-applied waterproof membranes.

A third way that moisture can penetrate through basement walls is by water vapor. Similar to concrete floor slabs, water vapor can penetrate a basement wall whenever there is a difference in vapor pressure between the two areas.