Modification of the radon concentration field in the soil by placing a house into the soil profile
|Location||International Geological Congress,oslo 2008|
|Holding Date||11 October 2008|
The risk of radon penetration from soils into buildings is in the Czech Republic expressed by the radon index of the building site. Its determination is based on the assessment of the soil permeability and soil gas radon concentration. Both parameters are measured directly on the particular building site at a depth of 0,8 m below the ground level. Results of the radon index assessment are used for the design of protective measures against radon from the soil. It is usually assumed that radon concentration under the house corresponds to the concentration measured at a depth of 0,8 m below the uncovered ground level. However, radon concentrations measured in the sub-floor layer under completed houses revealed that this assumption is mostly not true.
The effect of placing a house on the radon concentration field in the soil air under the house and in its vicinity was studied by numerical modelling with the help of the computer program Radon2D. This program solves the well-known equation describing two dimensional steady state radon transport in a porous medium caused by diffusion and convection. The program was used for the calculation of radon concentration fields in several soil profiles consisting of 6 layers of different parameters and under houses placed into these profiles.
Results of numerical simulation showed that under the floor of the houses resting on the ground level soil gas radon concentration can be up to 3,4 times higher compared to the concentration measured at the depth 0,8 m. Even higher increase was predicted for houses with the floor embedded 2 m below the ground level. In this case the sub-floor concentrations increased up to 9,3 times. In general, the smallest differences were observed for houses with floors raised 0,6 m above the ground level. Concentrations calculated in the sub-floor layer of such houses were in maximum 2,6 times higher compared to the values at the depth 0,8 m. In particular cases the sub-floor radon concentrations are influenced mainly by arrangement of soil layers of different permeability and radon production rate and by house parameters, such as airtightness of floors, underpressure within the house or presence of highly permeable drainage layers made of coarse gravel under the house.
It can be concluded that soil gas radon concentrations under the houses can be significantly different from concentrations measured on the building site and used for the assessment of radon risk categories. The highest differences were predicted for soil profiles with highly permeable upper layers.