Determination of Pressure Distribution and Radon-Laden Soil-Gas flow conditions induced by an active Sub-Slab Depressurization System

A test-house was realised and equipped with an Active Sub-Slab Depressurization (ASD) system with two suction branches. As the active element, a centrifugal fan of power 136 W was used. The characteristic point of operation of the fan was determined as 370 m³/h air flow rate, with -120 Pa pressure drop in each suction branch with respect to the atmospheric pressure. The radon activity concentration in the sub-slab region (in gravel) was increased by means of a flow-through radon-gas source of 3.7 MBq activity. Pressure distribution characteristics in the gravel-bed under the concrete slab, induced by the ASD system, was determined. About 40 Pa of pressure drop on the average was established under the concrete slab (in gravel) of the test-house while the fan was in operation. Similarly, the corresponding velocity distribution of the radon-bearing soil-gas was also determined, the soil-gas velocity was found roughly as 0.14 m/s, and the corresponding Reynolds Number (Re) was calculated as 210, indicating that the flow was turbulent and therefore non-Darcian. When the ASD system was not in operation, in other words when the fan was off, the Reynolds Number was calcualted as less than 10 (Re > 1) indicating that the flow was laminar (4) and therefore Darcian The corresponding soil-gas velocity was experimentally determined approximately as ~0.7x10-3 m/s.