Engineering and Construction

As previously mentioned, the geotechnical engineer often provides the design and construction specifications that are used to prevent both water vapor and capillary rise through floor slabs. An example of moisture barrier recommendations below the slab-on-grade foundation is as follows (WFCA, 1984):

Over the subgrade, place 4  to 8 in. (10  to 20 cm) of washed and graded gravel. Place a leveling bed of 1 to 2 in. (2.5  to 5 cm) of sand over the gravel to prevent moisture barrier puncture. Place a moisture barrier over the sand leveling bed and seal the joints to prevent moisture penetration. Place a 2 in. (5 cm) sand layer over the moisture barrier.

The gravel layer should consist of  an open graded gravel. This means that the gravel should not contain any fines and that all of the soil particles are retained on the No. 4 (4.75-mm) sieve.

This will provide for large void spaces between the gravel particles that will help prevent capillary rise of water through the gravel (Day, 1992c). In addition to a gravel layer, the installation of a moisture barrier (such as visqueen) will further reduce the moisture migration through concrete (Brewer, 1965).

In some areas, there may be local building requirements for the construction of moisture barriers.

For example, Fig. 12.20 indicates the County of San Diego requirements for below slab moisture barriers when constructing on clays. Note in this figure that 4 in. (10 cm) of open-graded gravel or rock is required below the concrete slab-on-grade foundation. A 6-mil. visqueen moisture barrier is required below the open-graded gravel. These recommendations are similar to the example listed earlier, except that the sand layers have been omitted. The purpose of the sand layer is to prevent puncture of the visqueen moisture barrier. When using specifications similar to Fig. 12.20, it is best to use rounded gravel or add a geotextile so that the visqueen is not punctured during placement and densification of the gravel.



Inadequate surface drainage can be an important factor in moisture migration through concrete slab-on-grade foundations. For example, water ponding adjacent to a foundation can weep in through the perimeter footing and/or seep underneath the perimeter footing and up through the slab. The infiltration of ponding water can also raise the groundwater table, which could subject the foundation to hydrostatic water pressures. Figure 12.21 shows an example of poor surface drainage and ponding of water at a condominium complex. In Fig. 12.21, the condominium is visible along the right side of the photograph. In order to prevent such ponding of water at the site, it is important to have proper drainage gradients around the structure. In addition, the surface runoff must be transferred to suitable disposal systems, such as storm drain lines.


Using a below slab moisture barrier system as described previously and providing positive surface drainage around the foundation will not be enough to resist a naturally occurring high ground-water table or artesian groundwater condition. In these cases, a more extensive waterproofing system will need to be installed. One common approach is to provide a below foundation waterproofing system and a sump equipped with a pump to catch and dispose of any water that seeps through the waterproofing system. A sump is defined as a small pit excavated in the ground or through the basement floor to serve as a collection basin for groundwater. A sump pump is used to periodically drain the pit when it fills with water. When the water level reaches a certain level in the collector box, the submerged pump will be activated, and the collection box will be emptied of water.