Soil-gas entry into an experimental basement: model measurement comparisons and seasonal effects

Abstract

Previous field studies of soil-gas entry into existing houses have found large discrepancies between modeled and measured values of soil-gas entry rate and pressure coupling between house substructures and the soil that could have been explained by inherently poor understanding of complex field sites.This paper reports the results of detailed investigation of soil-gas entry into an extensively instrumented and controlled experimental basement designed for the verification of existing numerical models of soilgas and radon entry into houses.The experimental design provides greatly reduced uncertainty regarding structure and soil characteristics.Estimates of soil-gas entry based on radon mass-balance in the structure are compared with predictions of a 3-dimensionaI.steady-state numerical model.This study corroborates the earlier findings of significant model underprediction of soil-gas entry and far-field pressure coupling.Observed soil-gas entry at 21 Pa structure depressurization was 8 times greater than predicted by the model and far-field pressure coupling was as much as 6 times greater than predicted.This suggests that field conditions at the site, and possibly at real houses, are inconsistent with model assumptions, or that typical methods of assessing bulk soil permeability are misleading.A number of hypotheses are raised to explain the discrepancy that will be tested in the future at the experimental facility.The effect of seasonal changes in soil conditions on soil-gas entry is also examined.Despite large seasonal changes in near-surface soil moisture content and air permeability, there is no observable effect on soil-gas entry because critical soil conditions near the soil-gas entry location in the structure floor remain relatively constant.