SUB-SLAB DEPRESSURIZATION SYSTEMS: EFFECTIVENESS AND SOIL PERMEABILITY. Thákurova 7, Praha 6, Czech Republic Praha 8, Czech Republic

Transcription

1 Radon in the Living Environment, 038 SUB-SLAB DEPRESSURIZATION SYSTEMS: EFFECTIVENESS AND SOIL PERMEABILITY Martin Jiránek 1, Matěj Neznal 2, Martin Neznal 2 1 Czech Technical University, Faculty of Civil Engineering, Thákurova 7, Praha 6, Czech Republic 2 RADON v.o.s. corp., Novákových 6, Praha 8, Czech Republic Sub-slab depressurization (SSD) systems in the form of radon sumps are being considered as the most effective and the cheapest radon remedial measures for existing buildings. Detailed measurements were made in 17 family houses that had been remediated using this system. Measurements results indicate that the performance of SSD systems depends substantially on the soil permeability and on the presence or absence of a drainage layer beneath the floors. In case of unpermeable soils without the drainage layer the effectiveness of SSD systems decreases, even if they are ventilated by continuously operating fans. On the other hand, an intermittent operation of fans is usually sufficient, when SSD systems are designed for houses built on permeable soils. INTRODUCTION A sub-slab ventilation as an efficient method to reduce indoor radon concentration was described by Cliff et a. (1992), Welsh (1996), Bonnefous et al. (1996) and others. On the other hand, the SSD systems cannot be used to solve radon problems in all houses. Their performance requires some special conditions. In the Czech Republic, majority of old houses with increased radon concentration has uninsulated concrete slabs, but there exists a lot of houses with timber floors placed directly on the soil. Partial basements are also very common. These conditions determine the applicability of various SSD systems. Drainage pipes are preferred for timber floors, while pits for concrete slabs. A combination of both methods is also frequent. Several years ago, a special method of drilling steel tubes beneath existing floors without their puncturing were developed (Jiránek et al., 1998). It is convenient for houses with floors above adjacent terrain or for houses with partial cellars, where pipes can be drilled into the sub-floor region from the terrain or from the cellar. In points, where pipes should be inserted, holes are drilled through external foundations or cellar walls. Perforated steel tubes are then drilled into the sub-floor region through these holes. The maximum tube length, which can be drilled in by this method, is 6 m. The drilled tubes serve as a casting, the soil is removed by a twist drill. Since the steel tubes may get rusty, perforated plastic (drainage) pipes are placed inside. Several drainage pipes or pits are connected to a central ventilator, which is controlled in dependence on actual indoor radon concentration. IN SITU VERIFICATION OF SSD SYSTEMS PERFORMANCE Performance of SSD systems has been studied at 17 single-family houses with the high indoor radon concentration that were built on subsoils characterized by different classes of soil permeability. To 367

2 038 Radon in the Living Environment, obtain all necessary data for efficiency analysis, the stress was given to a detailed investigation of each house. Investigation consisted of a detailed building survey, measurements of indoor radon concentration and soil gas radon concentration in underfloor layers before and after remediation, radon risk classification (Neznal et al., 1996) and geological description of foundation soils and permeability measurements of soil and underfloor layers. The need for fan operation in dependence on the soil and underfloor layers permeabilities has been investigated by ventilation experiments at two houses. After switching on the fan, the time dependence of the radon concentration decrease in the subfloor layers has been registered. In the same way, the increase of subfloor radon concentration has been documented after switching off the fan. Radon concentration indoors has been simultaneously measured, too. RESULTS The most important measurements results are summarized in Table 1. Basic characteristics of subsoil, permeability of sub-floor layers, equivalent equilibrium radon concentration indoors before and after remediation and resulting reduction factor are presented for each house. Permeabilities of subsoil and of sub-floor layers as well as soil-gas radon concentrations were determined by direct in-situ measurements. Values of soil-gas radon concentration are represented by the third quartile (i.e. the 75th percentile) of each data set. Indoor equivalent equilibrium radon concentrations before remediation were measured by passive track detectors with a one year exposure period. Equivalent radon concentrations after remediation were determined using semiconductor detectors with a one week exposure time. Data presented in Table 1 are average values of at least two measurements in different rooms. The reduction factor is the ratio between indoor equivalent equilibrium radon concentrations before and after remediation. As can be seen, the reduction factors ranged from 3.4 to 44. The highest effectiveness of SSD systems has been found in houses built on thick layers of highly permeable fills. In a majority of these houses, the equivalent indoor radon concentration decreased 10 times or more. Lower effectiveness has been observed in houses beneath which less permeable soils were indicated. A comparison of effectiveness of SSD systems in houses built on highly permeable sub-floor layers and in houses on sub-floor layers with heterogeneous permeability are given in Figure 1. Results of one ventilation experiment are presented in Figure 2. The experiment has shown that 30 minutes after switching on the fan, the radon concentration in the subfloor layers decreased in case of permeable soils at least 10 times, while in case of unpermeable soils the decrease was only about 30 %. In unpermeable soils, the steady - state has been reached after approximately 10 hours. A slow increase of radon concentration has been observed in both types of soils after switching off the fan. Initial values were reached after approximately 24 hours. CONCLUSION Performed investigation of 17 existing houses remediated by SSD systems shows that the applicability of SSD systems and the frequency of operating periods is affected by the soil permeability. The response of the radon concentration on the fan operation is much quicker in permeable soils than in unpermeable ones. It can be also concluded that the necessity of fan 368

3 Radon in the Living Environment, 038 operation is usually less than several hours per day in permeable soils, while the continuously running fans are not exceptional in soils with low permeability. This difference is of course expressed in savings of operation costs, prolonged life of fans, reduced maintenance and reduced negative effects to subsoil (drying, freezing etc). Effectiveness of SSD systems has been studied also by Bonnefous et al. (1992), Gadgil et al. (1994) and Crips (1994). Their results concerning the dependence of the effectiveness on the soil permeability are in good agreement with our findings. REFERENCES [1] Bonnefous YC, Gadgil AJ, Fisk WJ, Prill RJ, Nematollahl AR. Field study and numerical simulation of subslab ventilation systems. Environ. Sci. Technol. 1992; 26: [2] Bonnefous YC, Richon P, Tarlay V, Arnautou JC, Sabroux JC, Goutelard F. Subslab ventilation system: Installation and follow-up in a high-radon house in Brittany, France. Environment International 1996; 22, Suppl. 1: S1069-S1072. [3] Cliff KD, Green BMR, Lomas PR. Domestic radon remedies. Radiat. Prot. Dosim. 1992; 45: [4] Cripps A. Flow rates and pressure distributions produced by radon sumps. Radiat. Prot. Dosim. 1994; 56: [5] Gadgil AJ, Bonnefous YC, Fisk WJ. Relative effectiveness of sub-slab pressurization and depressurization systems for indoor radon mitigation: Studies with an experimentally verified numerical model. Indoor air 1994; 4: [6] Jiránek M, Neznal M, Neznal M. Czech experience with sub-slab depressurization systems. In: Barnet I, Neznal M, editors. Radon Investigations in the Czech Republic VII. Czech Geological Survey, Radon corp., Prague, 1998, pp [7] Neznal M, Neznal M, Šmarda J. Assessment of radon potential of soils - a five-year experience. Environment International 1996; 22, Suppl. 1: S819-S828. [8] Welsh P. Trials of radon remedies in a UK test house: An introduction. Environment International 1996; 22, Suppl. 1: S1059-S

4 038 Radon in the Living Environment, Table 1: Results of measurements in 17 houses remediated by SSD systems House Subsoil Sub-floor layers Equivalent equilibrium Reduct. No. Perm. Rn conc. Risk Permeability indoor Rn conc. (Bq.m -3 ) factor (kbq.m -3 ) before rem. after rem. 1 low 180 high medium - high high 100 high low - high high 125 high high high 70 high high low 30 medium low - high high 66 high high low 33 medium high high 63 high high high 55 high high high 56 high low medium 97 high high medium 75 high medium - high high 83 high medium - high medium 94 high high medium 30 medium high low 450 high medium - high high 156 high high

5 Radon in the Living Environment, 038 Figure 1: Reduction factors of indoor equivalent equilibrium radon concentrations in houses built on highly permeable and on heterogeneous sub-floor layers 371

6 038 Radon in the Living Environment, Figure 2: Radon concentrations in the sub-floor layer and indoors in dependence on fan operation 372