Sub Slab Drainage / Discharge System (SSDS)

What is Sub Slab Drainage / Discharge System (SSDS)  ?

Definition of Sub slab drainage / discharge system ( SSDS ) in Construction

Sub slab drainage ( or discharge ) system. This term SSDS refers to a water drainage system that is installed under the concrete slab on grade, to eliminate the build- up of hydrostatic pressure, due to a high water table, or simply to drain water from under the slabs. A new use for the SSDS has been introduced, that is not intended to discharge water , but to reduce the pressure under the slab. This reduction of pressure is required to create a negative pressure ensuring that any contaminants that could be in the ground, or material below the slab on grade, do not create fumes, which could be forced into the occupied structure, due to a pressure buildup. The difficulty in finding clean sites has caused engineers to further evaluate techniques to construct buildings on contaminated sites, without the need to either cap, or remove all the contaminants.

The SSDS systems, which are a series of PVC pipes installed under the slab and connected to fans on the roof of the structure, have been designed to allow the capping of contaminants under the slab on grade. The fans are reversed to pull air from the piping and stone embedment under the slabs, causing a negative pressure to be created under the slab. In this manner, even if there was a hazardous gas being produced under the slab, it would not vent through cracks, or openings in the concrete slab, into the occupied spaces, due to the negative pressures. Many systems such as this, start out as a static system, which means there are no fans installed to draw the air from beneath the slab on grade. Instead there are monitoring ports installed, to enable a testing lab to test the air below the slab on grade, on a scheduled cycle.

If contaminants are indicated, then fans are installed to mechanically draw the air from below the slab and reduce the static pressures under the slab on grade. This use of the SSDS system is relative new and its ability to continue to create the negative pressures under the slab is dependent upon the installation of the system as well as the cleanliness of the stone used beneath the slab on grade. If the stone is contaminated with too many fines, these fines will clog the pores of the stone and not allow the movement of air between the stone particles. This will eventually cause the system to fail. The use of filter fabric correctly in the correct location around the stone and piping system is also necessary to ensure that the stone remains clean and porous. The SSDS system for under-slab depressurization is a relatively new process and its development is ongoing.