GB2265639A - Restricting the build-up of ground-emanating gases under or in a structure - Google Patents

Abstract

A static ground-supported structure (10) is provided with a system for restricting the build-up of ground-emanating gases such as radon under or within the structure. The system includes a pump (12) situated, for example, in a loft void (11) of the structure and taking on a supply of air relatively free of said gases. The pump (12) supplies air at a pressure above ambient into a pipe (18) that takes it down to air-diffusion means (28) located in the ground (5) beneath the structure. The pressurised air diffuses into the ground and reduces the upward flow of gases. A sensor system (41 to 44) sensing the build up of ground-emanating gases, may also be provided to control operation of the pump. The pump (2) of the system may additionally serve to provide ventilation air into the structure. <IMAGE>

Description

METHOD AND SYSTEM FOR RESTRICTING THE BUILD-UP OF GROUND-EMANATING GASES UNDER OR IN A STRUCTURE The present invention relates to a method and system for restricting the build-up of ground-emanating gases under or in a static ground-supporting structure such as a house, bungalow, office or warehouse.

The problems that reduced ventilation causes in current well insulated houses or buildings, due to increased condensation are well known and will not, therefore, be discussed. A problem which has only recently come to public prominence is the accumulation of high levels of radon gas, and other problematic gases, including methane, hydrocarbons and other toxic and combustible gases that in certain localities emanate from the ground and can collect beneath and enter buildings, to cause potential health risk to the inhabitants, as well as potential physical risk in the case of combustible gases.

In the following, the primary focus is on radon gas and the prevention of a build-up of this gas; however, it is to be understood that the solution offered by the present invention is also applicable to restricting build-up of other ground-emanating gases.

The present methods of radon gas reduction are based on:- - increasing ventilation levels, to dilute any gas present, - providing a radon resistant membrane between the floor and the living area, to restrict the entry of radon gas, - creating a sump below, or around the house, and mechanically vacating that sump and the surrounding area with a fan working in suction, to collect and then discharge the collected gas away into the atmosphere, - increasing the ventilation level by inputting an air flow into the house which in well sealed houses raises the internal air pressure slightly, restricting the entry of radon gas from the soil below the house, as well as correcting poor ventilation levels.

All of the above methods have certain disadvantages.

Thus, increasing ventilation levels to a degree sufficient to be fully effective would be very expensive in terms of the required heating energy costs in winter.

The use of a membrane is also very expensive, as its installation must be very carefully done at the time a building is constructed; furthermore, a membrane can serve to trap pockets of gas which could be very dangerous where the gas is methane. Negative pressure sumps are again expensive to install and also may produce condensation problems on the underside of suspended floors; additionally, where such sumps are used for radon protection, there may be increased radiation levels at the sump outlet and the equipment as a whole may present disposal problems due to low-level radioactive contamination.

According to one aspect of the present invention, there is provided a static ground-supported structure provided with a system for restricting the build-up of the groundemanating gases under or within the structure, the system comprising: - pump means having an inlet open to a supply of ambient air relatively free of said gases, and an outlet for supplying air at a pressure greater than the ambient pressure, - air-diffusion means for diffusing air into the ground beneath or adjacent the structure, and - conduit means communicating the outlet of the pump means to the air-diffusion means; the arrangement being such that in operation of said system, the air pumped into the ground through said airdiffusion means, reduces the upward flow of said gases beneath said structure.

In the context of radon gas restriction in houses, the system of the invention thus supplies an air flow of relatively unpolluted air, which is then piped into the ground below the floor of the house, the subsequent increase in air pressure at the outlet or outlets, and the area there around restricts the movement of radon gas in the ground below the house, the gas preferring to take an easier route to the ground surface. The increased pressure created also slows the progress of the radon gas through the ground; as radon gas has a short half life of about four days, the slowing down of the gas to the surface further reduces the final level of radon gas capable of entering the house.

The input of air into the ground under the house also has the effect of diluting the radon gas, the flow of "clean air" below the house producing a "bubble" of radon free air, below the floor slab of the house.

The pump inlet is preferably located in the loft void of the house as the levels of radon gas are most often at their lowest in this area of the house. The supply of air can, of course, be drawn from another area or source (for example, through an outside wall or from above the roof) so long as the point is born in mind that the air should be as "clean" of radon gas as practically possible.

By providing an airflow distribution unit in communication with the pump outlet, the pump of the system can also be used to ventilate the house (or other structure). The distribution unit divides the airflow from the pump into a flow that is pumped into the ground and a flow that is used to ventilate the house. This latter flow is fed into the house, thereby slightly increasing the internal air pressure and causing an outward flow that assists in reducing levels of unwanted gases.

Advantageously, the distribution unit is controlled by a temperature sensor arranged to sense the temperature of the air flow through the pump (the sensor can be located upstream or downstream of the pump). The nature of the control is such that when the intake air is relatively cold only a small proportion (for example, 10%) is used for ventilation purposes whereas if the intake air temperature is relatively high (as could be the case for air drawn in from the loft void of a house during sunny weather), then the proportion of air used for ventilation is increased (for example, up to 50%).

Generally as regards the air-diffusion means of the system, this may comprise, for example, at least one hole in the ground filled with aggregate and may be distributed between a plurality of locations beneath the structure. In this latter case the conduit means serves each location and preferably comprises air distribution piping laid within or below a ground floor of the structure. It is also possible to provide a plurality of pump means each with its own associated air-diffusion means and conduit means.

Generally the said conduit means will pass down through a ground floor of the structure and in this case sealing means are advantageously provided to seal the passage of the conduit means through the floor. Furthermore, oneway valve means are preferably located in the conduit means approximately at ground floor level or below to prevent said gases entering the above-ground portion of the conduit means when the pump means is not operating.

The state of the valve means, and/or the flow of air therethrough, may be sensed as a check on the operation of the system.

Advantageously, embodiments of the invention are provided with sensor means for monitoring the build-up of gases beneath a structure, and control means connected to the sensor means and the pump means . whereby to control operation of the latter in dependence on the output of the sensor means. The sensor may include pipes laid within or beneath a ground floor of the structure for enabling samples of gas to be taken from under the structure.Preferably, the said control means is operative to cause the pump means to operate both in a first mode in which it provides a sufficient air flow into the air diffusion means for the ambient pressure beneath the structure to be sufficient to cause gas to flow up the pipes of the sensor means, and in a second mode in which the pump means provides a substantially greater volume of air to the air diffusion means than in its first mode for the purpose of reducing the upf low of said ground-emanating gases, the control means changing operation of the pump means between its first and second modes in dependence on the output of said sensor means.

According to another aspect of the present invention, there is provided a method of restricting the build-up of ground-emanating gases under or within a static groundsupported structure, said method comprising the steps of providing a supply of air relatively free of said gases at a pressure greater than ambient pressure, and diffusing this supply of air into the ground under said structure whereby to reduce the upward flow of said gases beneath the structure.

The need to install a system of the invention in a structure may be uncertain at the time of initial construction of the structure. However, the cost of retro-fitting such a system will generally be considerably higher than installing it at the beginning.

It is therefore proposed that the below-ground elements of the system and associated pipework be installed at the outset and the pump means and other associated elements only be added if a problem is subsequently found.

Thus, according to a further aspect of the present invention, there is provided a static ground-supported structure provided with air-diffusion means for diffusing air into the ground beneath or adjacent the structure; conduit means built into and/or below the ground floor of the structure and communicating with the air-diffusion means, the conduit means being connectable to pump means for the purpose of pumping a supply of air into the ground through said air diffusion means to reduce the upward flow of ground-emanating gases beneath said structure; and a sensor system for sensing for the presence of said ground-emanating gases beneath the structure whereby to determine whether the provision of said pump means is called for.The sensor system advantageously comprises sensor pipes laid within or beneath said ground floor for enabling samples of gas to be taken from under the structure According to a still further aspect of the present invention, there is provided a static ground-supported structure in areas where there is a risk of groundemanating gases building up under or within the structure, the method including the steps of:: - providing air-diffusion means for diffusing air into the ground beneath or adjacent the structure; - constructing the ground floor of the structure with conduit means incorporated within and/or below the ground floor and communicating with said air-diffusion means, the conduit means being connectable to pump means for enabling a supply of air to be pumped into the ground through the diffusion means to reduce the upward flow of ground emanating gases beneath said structure, and - incorporating a sensor system for sensing for the presence of ground emanating gases beneath the structure whereby to enable a determination to be made regarding the need to provide said pump means. Again, the sensor system advantageously comprises sensor pipes laid within or beneath the ground floor for enabling samples of gas to be taken from under the structure.

Should operation of the sensor system detect a predetermined unacceptable level of ground-emanating gases, the pumps means can be installed and operated.

Embodiments of the invention will now be described by way of non-limiting example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side elevation, in cross-section, of a house showing a first embodiment of the invention in which a pump unit is located in the loft void of the house; Figure 2 is an enlarged view of the pump unit of Figure 1; Figure 3 is a diagram similar to Figure 1 but showing second and third embodiments of the invention with pump units respectively mounted on the outside of the house and on an external wall, but within the house itself; and Figure 4 is a diagram of a floor slab showing embedded air distribution and sensor pipes.

Figure 1 illustrates a two storey house 10 built on ground 5 and having both a floor slab 6 and a suspended floor 7 The nature of the ground is such that in the absence of any preventative measures, radon gas will emanate from the ground below the house and percolate into the house (as well as potentially collecting beneath the house).

In the first embodiment of the invention, shown in Figure 1 an air pump unit 12 is mounted in a loft void 11 of the house 10, the pump unit being shown in more detail in Figure 2. The pump unit 12 comprises a fan 13 (for example, electrically powered) provided on its upstream side with a filter 14 Air is drawn into the pump unit 12 through an inlet aperture 15 and is blown out through an outlet aperture 16 A distribution unit 17 is connected to the outlet aperture 16 of the pump unit 12 and serves to distribute the air flow from the pump unit between first and second pipes 18 and 19.

The pipe 18 passes down through the house, through an air-tight seal 26 in the ground floor, and into an airdiffusion arrangement 28 located in the ground 5 beneath the house. This arrangement 28 comprises a hole, of substantial surface area, filled with aggregate to enable the relatively free flow of air through it and into contact with the ground. Air diffuses from the arrangement 28 into the surrounding ground creating an over-pressure that restricts the upward flow of ground emanating gases, effectively forcing them to find another upward route, taking them to the ground surface beyond the bounds of the house 10.

It will be appreciated that the arrangement 28 may comprise several aggregate-filled holes (or other airdiffusing structures) so distributed beneath the house as to produce the required restriction on the upward flow of ground-emanating gases beneath the house. In this case, the pipe 18 may divide into several pipes as required to feed these air-diffusing structures, these pipes being arranged, for example, to fan out within or below the ground floor of the house from the main down pipe 18.

The pipe 18 is provided near or below ground level with a one-way valve 27, (for example, a mechanical or water valve) to prevent the upward flow of ground-emanating gases into the pipe 18 when the pump unit is inactive.

The other pipe 19 from the distribution unit 17 feeds a ventilation outlet 25 (for example, a ceiling diffuser) opening into the living space of the house 10. This air supply produces a ventilation input point into the house which corrects any lack of air change the house may be suffering due to high insulation levels in the house.

The air input from this point also slightly raises the internal air pressure in the house helping to reduce radon gas ingress. Of course, increased ventilation is likely to increase the cost of heating the house; in order to minimize said additional cost, the amount of ventilation air supplied through the ventilation outlet 25 is adjusted according to the temperature of this air.

To this end, the distribution unit is provided with a valve 20 controlled by a unit 21 to which is connected a temperature sensor 22 arranged to sense the temperature of air in the loft void. It will be appreciated that the air in the loft can be subject to substantial solar heating in sunny weather whilst at other times it may be no warmer than outside ambient air. The control effected by the unit 21 is such that the minimum air flow through the ventilation outlet 25 is about 108 of the available volume produced by the pump unit 12. At low external air temperatures the distributor unit allows this minimum air flow level into the house, the rest of the air flow being fed to the pipe 18.

As the external air temperature rises, the percentage of airflow allowed into the house through the ceiling diffuser 25 is increased by the distribution unit up to a maximum of about 50% or possibly more, the rest of the airflow again being fed down the pipe 18.

The distribution unit 17 may be provided with a valve arrangement to enable air flow to a selected one of the pipes 18,19 to be completely cut off. In this case, with the pipe 18 closed off, the installation serves simply as a ventilation system; with the pipe 19 closed off, the installation only pumps air into the ground.

It is, of course, possible to implement the invention without the distribution unit 17 and ventilation pipe 18 and outlet 25, the pump 12 being dedicated to the supply of air into the ground.

The control of the pump 12 (and where present, the distribution unit 17) can be made more sophisticated in relation to the supply of air to the air-diffusion arrangement 28. Thus, a sensor system 41, 42, 43 can be provided to monitor the build up of ground-emanating gases beneath the house, the sensor system being connected to the control unit 21 so that the latter can regulate the supply of air to the air diffusion arrangement 28 in dependence on the sensed levels of ground-emanating gas.

More particularly, the sensor system may comprise one or more relatively narrow pipes 41, 42 for collecting samples of gas from beneath the ground floor of the house and supplying them to an analysis unit 43. The analysis unit 43 monitors the level of the or each groundemanating gas or gases of interest and provides a corresponding output via wire 44 to the control unit 21.

As the level of a ground-emanating gas of interest increases towards an unsafe level, the control unit 21 increases the supply of air to the air-diffusion arrangement 28.

In order to facilitate the provision of gas samples through the pipes 41, 42 to the analysis unit 43, the control unit 21 may cause the continuous or intermittent supply of air to the arrangement 28 sufficient to increase the gas pressure beneath the house to a level that ensures a flow of gas along the pipes to the analysis unit 43. A pressure differential of 200 Pascal is sufficient for this purpose. Until an unsafe level of gas is detected, the loading of the pump 12 in respect of supplying air to the arrangement 28 is therefore kept low, this loading only being increased as and when necessary to maintain the level of ground-emanating gas beneath the building at an acceptable value.

The control unit 21 may also be fed via wire 46 with the output of a sensor unit 45 associated with the valve 27.

The sensor unit 45 can be arranged to detect closure of the valve (indicating that air has ceased to pass down to the air-diffusion arrangement) and if such closure is inconsistent with the settings of the pump 12 and distribution unit 17 demanded by the control unit 21, then the latter is preferably arranged to generate an alarm signal. The sensor unit 45 may alternatively or additionally be arranged to sense the pressure differential across the valve 27 again for the purpose of detecting zero flow conditions; such a sensor would also permit detection of reverse gas flow in the event of failure of the one-way valve 27.

For a typical house and with typical ground conditions, air pressures of around 200 to 400 Pascal over atmosphere will generally be adequate to produce suitable airdiffusion rates. Typical flow rates will be around 190 cubic metres/hour for full operation, reducing to 8 cubic metres/hour for sensing-only operation. It will be appreciated that the foregoing figures are only rough guides and local conditions will greatly affect the determination of appropriate values of pressure and flow rates.

Figure 3 illustrates possible variations of the Figure 1 embodiment. Thus, as is shown in Figure 3, it is possible to dispose the pump unit and the associated distribution unit in locations other than the loft void in Figure 3, a pump unit 12A is shown mounted on the outside of the house 10, in this case protected by a casing 30. The unit 12A feeds a distribution unit 17A that divides the airflow from the pump between a pipe 18A and a pipe 19A. Pipe 18A feeds an air-diffusion arrangement 28 buried in the ground in a manner similar to that described above with reference to Figure 1. Pipe 19A feeds one or more ventilation outlets 25A, here taking the form of wall mounted louvred outlets.

Figure 3 also shows a further possible location for the pump unit; in this case, a pump unit 12B is mounted inside the house 10 but on an external wall such that it can draw in air from outside through an aperture 31 in the external wall. A distribution unit 17B divides the air flow from the unit 12B between a pipe 18B and a pipe 19B Pipe 18B feeds the ground-buried air-diffusion arrangement 28 (this connection is shown dashed so as to avoid confusion with the connection made by pipe 18A, it being appreciated that normally only one pump unit, distribution unit and piping would be required in one house. The pipe 19B feeds one or more ventilation outlets 25B.

Although not shown in Figure 3, each of the distribution units 17A, 17B has an associated control unit and temperature sensor as per the Figure 1 installation.

Furthermore, sensor arrangements similar to those represented by elements 41 to 46 of Figure 1 may also be provided.

Figure 3 further shows a preferred form of air-diffusion arrangement 28 in which the aggregate filled hole is sealed with a layer 29 of flexible sealing material (for example, of an expanded closed-cell foam material). This sealing layer 29 forms a gas-tight seal over the aggregate preventing air from escaping through the top of the hole and sealing the entry of the pipe 18A into the hole. Indeed, in cases where a solid floor slab 6 is laid directly on top of the layer 29, the latter effectively performs the function of the seal 26 which may therefore be omitted.

Various modifications can, of course, be made to the afore-described embodiments of the invention. For example, it is not essential for the air-diffusion arrangement to be located directly under the structure 10 (although this is preferred) provided that the zone of influence of the air-diffusion arrangement extends beneath the structure. The air flow characteristics and distribution and size of the air-diffusion arrangement needed to provide adequate protection depends on many factors (such as the type of ground area to be protected, flow rate of gases, target safety levels) and optimisation of performance will generally be empirically based.

For larger structures such as offices and factories, embodiments of the invention are envisaged in which a plurality of pump units are provided each connected to a respective air-diffusion arrangement via respective conduit means. Typically each air-diffusion arrangement would comprise several air diffusion sites and the associated conduit means would take the form of air pipes embedded within or below the ground floor of the structure. Where sensor means are provided for analysing the gases beneath the structured this sensing arrangement is preferably zoned in correspondence with the area covered by each pump unit and its associated airdiffusion arrangement. In this manner, each pump unit can be controlled independently of the other pump unit in accordance with the build up of ground-emanating gas in the area associated with the air-diffusion arrangement of the pump unit.The sensing arrangement itself may comprise a respective analysis unit for each zone or a central analysis unit to which sensing pipes from each zone are connected, the analysis unit being arranged to selectively sample gas from each zone as required.

Figure 4 shows a typical layout of air distribution pipes 50 and sampling pipes 51 within a floor slab 52. For reasons of clarity, although the pipes 50 and 51 are embedded in the floor slab 52, these pipes have been shown in full lines. The pipes 50 are intended to supply air to an underfloor air-diffusion arrangement 28 (not shown in Figure 4) through downwardly-opening pipe portions 53; the pipe portions 53 can be provided with auto dampers. The pipes 50 would, in use, connect back to one (or possibly more) pump units. The sensor pipes 51 provide for the taking of gas samples through downwardly opening inlets 54, the pipes 51 being taken back to an appropriate analysis unit (or units).

Although the susceptibility of a particular site to the ground emanation of gases may be known prior to construction of a building, it may well not be known whether this represents a major problem in practice for any new building. However, the cost of retrospectively fitting protection could well be very high. Accordingly, new buildings being built on sites thought to be susceptibly to the ground emanation of gases are preferable installed with air distribution piping and sensor piping (such as shown in Figure 4) at the time of their construction but the installation of the pump units and associated control means and one-way valves can be deferred until such a time as gas samples taken through the sensor pipes indicate that there is a genuine problem with ground emanating gases calling for the provision of counter-measures.

As already indicated, the invention is not limited to preventing the build up of radon gas but is also applicable to preventing the build-up of other ground emanating gases. Where the risk presented by such gases can be reduced by reacting them with other substances, then embodiments of the invention can advantageously be provided with means for introducing a reactant fluid (liquid or gas) into the ground to react with the ground emanating gases to reduce the risk presented by them.

For example, a reactant fluid could be introduced into the air flow pumped into the air-diffusion arrangement.

The form and nature of the air-diffusion arrangement 28 will generally depend on the nature of the site concerned. For highly porous soils the air-diffusion arrangement need be little more than an enclosed area in contact with the ground. In contrast, where the ground at the site is heavily compacted, it may well be necessary to take appropriate measures to relieve the compaction to facilitate diffusion of air into the ground. One such method would be to force a noncompressible fluid (liquid) into the ground in order to create fissures providing sufficient ground contact surface area to give the desired rate of diffusion of air into the ground.

Claims (25)

1. A static ground-supported structure provided with a system for restricting the build-up of the groundemanating gases under or within the structure, the system comprising: - pump means having an inlet open to a supply of ambient air relatively free of said gases, and an outlet for supplying air at a pressure greater than the ambient pressure, - air-diffusion means for diffusing air into the ground beneath or adjacent the structure, and - conduit means communicating the outlet of the pump means to the air-diffusion means; the arrangement being such that in operation of said system, the air pumped into the ground through said airdiffusion means, reduces the upward flow of said gases beneath said structure.

2. A structure according to Claim 1, wherein said airdiffusion means comprises at least one hole in the ground filled with aggregate.

3. A structure according to Claim 1 or Claim 2, wherein said air-diffusion means is distributed between a plurality of locations beneath said structure with said conduit means serving each location.

4. A structure according to Claim 3, wherein said conduit means comprises air distribution piping laid within or below a ground floor of the structure.

5. A structure according to any one of the preceding claims, comprising a plurality of pump means each with its own associated air-diffusion means and conduit means.

6. A structure according to Claim 1 or claim 2, wherein said air diffusion means is located beneath a central zone of the structure.

7. A structure according to any one of the preceding claims, wherein the said conduit means passes down through a ground floor of said structure, sealing means being provided to seal the passage of the conduit means through the floor.

8. A structure according to any one of the preceding claims, wherein said conduit means includes an aboveground portion extending above a ground floor of the structure, one-way valve means being located in said conduit means approximately at ground floor level or below to prevent said gases entering said above-ground portion of said conduit means when said pump means is not operating.

9. A structure according to claim 8, including sensor means for generating an output signal upon closure of said one-way valve means indicating that air has ceased to flow therethrough from the pump means.

10. A structure according to claim 8, including sensor means for generating an output signal upon detection of a zero pressure difference across said valve means.

11. A structure according to any one of the preceding claims, including sensor means for monitoring the buildup of said gases, and control means connected to said sensor means and said pump means whereby to control operation of the latter in dependence on the output of the sensor means.

12. A structure according to claim 11, wherein said sensor means includes pipes laid within or beneath a ground floor of the structure for enabling samples of gas to be taken from under the structure.

13. A structure according to claim 12, wherein said control means is operative to cause the pump means to operate both in a first mode in which it provides a sufficient air flow into the air diffusion means for the pressure beneath the structure to be sufficient to cause gas to flow up the pipes of the sensor means, and in a second mode in which the pump means provides a substantially greater volume of air to the air diffusion means than in its first mode for the purpose of reducing the upf low of said ground-emanating gases, the control means changing operation of the pump means between its first and second modes in dependence on the output of said sensor means.

14. A structure according to any one of the preceding claims, wherein means are provided for introducing into the flow of air pumped into the air-diffusion means, a flow of a reactant fluid intended to react with said ground-emanating gases.

15. A structure according to any one of the preceding claims, further comprising ventilation means opening into an interior space of said structure, ventilation duct means communicating with the ventilation means, and airflow distribution means communicating with the outlet of said pump means and with said conduit means and duct means, said distribution means serving to divide the air flow from the pump means between said conduit means and said duct means whereby the pump means provides air at above ambient pressure not only for air-diffusion means but also to said ventilation means.

16. A structure according to claim 15, including a temperature sensor for sensing the temperate of the air flow through the pump means, and control means associated with said distribution means and responsive to the temperature sensed by said sensing means such as to vary the proportion of air being supplied to the ventilation duct means, the proportion being greater for higher sensed air temperatures.

17. A structure according to any one of the preceding claims, wherein the inlet of said pump means is situated in a loft void of said structure.

18. A structure according to claim 17, wherein the pump means itself is situated in said loft void.

19. A static ground supported structure provided with a system for restricting the build-up of ground emanating gases below or within the structure, said system being substantially as hereinbefore described with reference to Figure 1, Figure 2 or Figure 3 of the accompanying drawings.

20. A method of restricting the build-up of groundemanating gases under or within a static ground-supported structure, said method comprising the steps of providing a supply of air relatively free of said gases at a pressure greater than ambient pressure, and diffusing this supply of air into the ground under said structure whereby to reduce the upward flow of said gases beneath the structure.

21. A static ground-supported structure provided with air-diffusion means for diffusing air into the ground beneath or adjacent the structure; conduit means built into and/or below the ground floor of the structure and communicating with the air-diffusion means, the conduit means being connectable to pump means for the purpose of pumping a supply of air into the ground through said air diffusion means to reduce the upward flow of groundemanating gases beneath said structure; and a sensor system for sensing for the presence of said groundemanating gases beneath the structure whereby to determine whether the provision of said pump means is called for.

22. A structure according to claim 21 wherein said sensor system comprises sensor pipes laid within or beneath said ground floor for enabling samples of gas to be taken from under the structure.

23 A method of constructing a static ground-supported structure in areas where there is a risk of groundemanating gases building up under or within the structure, the method including the steps of: - providing air-diffusion means for diffusing air into the ground beneath or adjacent the structure;; - constructing the ground floor of the structure with conduit means incorporated within and/or below the ground floor and communicating with said air-diffusion means, the conduit means being connectable to pump means for enabling a supply of air to be pumped into the ground through the diffusion means to reduce the upward flow of ground emanating gases beneath said structure, and - incorporating a sensor system for sensing for the presence of ground emanating gases beneath the structure whereby to enable a determination to be made regarding the need to provide said pump means.

24. A method according to claim 23, wherein said sensor system comprises sensor pipes laid within or beneath said ground floor for enabling samples of gas to be taken from under the structure.

25. A method of operating a ground-supported structure constructed in accordance with any one of claims 21 to 24, comprising operating said sensor system and upon the detection of a predetermined unacceptable level of said ground-emanating gases, installing and operating said pump means.