Abstract: Sub-slab depressurisation systems have proven to effectively mitigate radon entry. A poor understanding of the fluid physics underlying the technique has been shown to lower the success rate substantially. This article describes a study of pressure fields in a sub-slab gravel bed induced by a soil depressurisation system consisting of perforated pipes run under the slab at a depth of 75 cm. The advantage of the approach is that pipes can be laid from outside the building to be protected. The study was conducted on a large-scale experimental facility where the variations in morphology and scope of pressure fields with different pipe combinations could be monitored and characterised. The findings showed that pressure was uniform across the entire area in the gravel bed, whereas the sensors buried in natural soil showed pressure to depend on distance from the source. Pressure transfer to the sub-slab plane was also observed to vary depending on the active pipe. Air-flow resistance studies in the layers of soil lying between the pipes and the gravel delivered different results for each pipe. That finding would appear to be related to the presence of preferential pathways in some parts of the soil. Total pressure when several pipes were activated was observed to be practically the same as the sum of the pressures transferred by each when working separately. The correlation between extraction fan power and pressure generated was also analysed. These and other factors are discussed and analysed from a perspective of the understanding of such highly effective techniques.

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