Pressure (mat) slabs
The slab-on-grade is designed to resist the uplift pressure of the water when the slab is below the groundwater line. The slab can be increased in thickness to provide sufficient weight to counteract the buoyancy of the water or it can be anchored into position with rock anchors or caissons.
A 50- to 76-mm (2- to 3-in.) mud mat is usually provided on grade to receive the waterproofing membrane. It also serves as a relatively dry working slab. The mud mat is preferable to a tamped gravel substrate, because gravel compaction is never uniform, particularly at rising walls and penetrations. Moreover, penetrations and drains should be anchored with concrete to ensure they are held in position during the installation of the pressure slab. Special attention must be paid to the selection of rebar chairs if the tamped gravel alternate is selected.
Conclusion
This author offers design professionals the following recommendations when faced with a project.
- Consult with the excavation contractor to ascertain the type of SOE system it intends to use. Determine if the components will require special protection from SOE components that encroach on the foundation wall.
- Post-applied waterproofing membranes are less costly than pre-applied ones. Inquire if the SOE can be moved back from the foundation sufficiently far enough (about 760 mm [30 in.]) to enable its application.
- Thicken the pressure slab to absorb piping and shallow pits to simplify waterproofing details.
- Avoid systems using a wearing slab on gravel over pressure slabs to contain pipes and conduits.
- Ensure backfill is deposited in shallow layers and properly compacted.
- Sequencing the pressure slab to be cast prior to the foundation walls will simplify the waterproofing application.
Justin Henshell, CSI, FAIA, FASTM, is a principal at Henshell & Buccellato Consulting Architects, specializing in moisture-related issues in the building envelope, since 1974. His focus includes roofing, masonry walls, waterproofing, and condensation. Henshell has authored more than 40 technical articles and papers, presenting them in the United States, Canada, and Europe. He wrote The Manual of Below-grade Waterproofing (Routledge, 2016). Henshell can be contacted via e-mail at justin.henshell@verizon.net.
