In this part the effect on soil gas flow of the pressures caused by fans has been
considered. This reflects the widespread use of 'sump' systems for reducing indoor radon levels, and the wish to optimise their design. The way in which these sumps behave has been considered, along with the pressures they achieve in different hard core materials and the soil around a building.
Modelling
The first area of study used a two-dimensional linear finite difference model of the gas X flow. This is significantly flawed because neither the 2D or linear approximations are
valid. Hence the results can only be used with caution and in a qualitative way. Nevertheless they do help us to understand where the flow is taking place, and the impact of different layers of hardcore material.
In particular the fact that most of the flow into a sump is coming from the house allows an estimate of the heating cost of the sump system to be made; it is in the range £20-£50 for a typical size house. Another key understanding from this type of modelling, but not original to this work, is the fact that the width of cracks in a floor slab is not very important in soil gas flow. This is because the resistance to flow due to the soil is much greater than that due to the crack (under most circumstances). This therefore explains why it is so difficult to seal a floor to keep out radon.
The possibility of using source and sink theory to examine soil gas flow was also looked into. The lack of flexibility in the geometry meant that this work was not taken very far, but there is some qualitative value in the solutions anyway. They could also serve to give reference solutions for comparing with computational solutions.
The Darcy-Forcheimer Law addresses the non-linear nature of soil gas flow at the pressures occurring when sump systems are used. It requires a second parameter to the Darcy Law, which is known as the Forcheimer term. This work recognises the need for the more complex law, and uses it in some very simple, essentially one dimensional,
situations.
Experiments in the laboratory
These results were then used to interpret the results from laboratory and site
measurements of pressures and flow rates. The simplest were carried out on a series of samples of hard core in a box in the laboratory. When the leakage of the box itself can be understood these give good results for the permeability and Forcheimer terms of these materials, but of course only for the conditions in the box. The issues of compaction and moisture content were not within the scope of the equipment available, and because of leakage effects not all hard core materials can be measured.
The problem caused by the conditions of the tests can be seen from the comparison of the measured results to those from the various theories (mostly empirical) relating permeability to the grading curve information for a hardcore material. None of the theories gives a result which consistently matches that of the experiments, but they can give a reasonable approximation, and these are much to obtain than direct measurements.
Experiments on site
The next set of data presented concern measurements of pressure extension on real sites. Some of these were carried out by colleagues at BRE, others by a contractor. The interpretation of these gives some indication of how different hardcore materials affect gas flow in practice, through the concept known as pressure field extension.
In the tests carried out for BRE by Wimpey Environmental Ltd (now Wimtech) 78 floor slabs were measured after the concrete had set but before the walls were built. The test could be a post construction test for the suitability of the hardcore for use with a sump, but the test is not likely to be cost effective. Combined with measurements of the grading curves of the materials used these results give considerable insight into the variation in air flow between floor slabs.
The tests carried out by BRE and Cornwall County Council used tubes inserted before
the concrete floor was laid to measure the pressure field. This gives a wider spread of measurements without affecting the floor, but is more expensive. They also show the variation between floors, and in particular the difficulty of achieving really good pressure extension.
The most significant finding is that the differences between the behaviour of different hardcore materials are not as large as expected, and they probably do not justify the cost of using 'better' materials. This financial check is an area of work which needs to be taken further, but it reflects the high cost of using and transporting 'special' hard core materials, and the many complications of what appears to be a simple problem. The cost savings from better pressure extension should come from being able to use a smaller fan to remove radon. Since most new buildings don't need to use the fan anyway, and the cost saving is quite small, it is not worth spending more on the hard core material unless it is needed for some other purpose, drainage being the most likely.