SUBSOIL HEATING This invention relates to subsoil heating. Outdoor areas are obviously affected by weather and ambient temperature conditions. Outdoor areas on which sports are intended to be played and in cold and wintery conditions, can have surface conditions making it dangerous to players. When the sports area is a football pitch, it is highly disruptive for games to be cancelled from several viewpoints. For football clubs there is a loss of revenue, and for administrators there is the need for alternative dates to be found for a match to be replayed. Decisions to call off a football match are frequently made on the day of the match, and it means that large numbers of supporters are inconvenienced, having travelled, sometimes large distances, only to find that the game is not being played. With professional football clubs, there is the need for players to be able to train without risk, and whilst training sessions can be transferred indoors to a suitable sports hall or gymnasium, the training session cannot be as effective as when taking place outdoors. Numerous attempts have been made to provide heat to a surface area of such as for example a football pitch, from simply covering the area with an insulating material such as straw when bad weather conditions are forecast, to providing subsoil heating means. It is known to lay electric cables below the surface able to be that can serve as heating elements. However, the heating effect is very localised and a considerable variation of heating effect is generated causing a variety of surface conditions to be created. Also known is to lay pipes below the surface through which hot water or steam can be passed, but this has not only the considerable cost of laying multiple pipes in position, but also the cost of hot water or steam generation. The object of the invention is to provide a subsoil heating means, able to provide a substantially uniform heating effect over a large surface area, without those disadvantages mentioned above. According to the present invention, a subsoil heating means comprises a pad formed by a core of insulating material overlaid by a layer of a flexible semi-conductive heatable material with
connections to a source of electrical power, the core and the heatable material being encased in an impervious material. The outer case of impervious material may be any suitable PVC, VISQUEEN (RTM) or polyethylene welded at its edges and its ends to form a waterproof casing, and the core may be of an appropriate foamed plastics material, ormineral wool, or a combination. The layer of flexible semi-conductive heatable material is most desirably such as to provide a substantially constant heating effect over its full width and length. Such a material may be that known as INDITHERM (RTM) and discussed in European Patent Application 1127356. Each pad may be of a considerable length and width, to provide a heating effect over a wide surface area. However, given that the same heating requirement might not be required overthe full playing area of such as a football pitch or a training area, it is preferred to provide a number of individual pads laid side-by-side and end-to-end, to extend overthe full surface area, and to have all said pads either individually connected to a source of electrical power, or grouped into blocks with pads of each block connected to a source of electrical power. All of the pads, when heated individually or when in groups, are preferably connected to a transformer or to a series of transformers located in close proximity to the area to be heated, that if required can be positioned below ground level. The power supply to a pad or to a group of pads is governed by its position in relation to the source of power, and to compensate for a voltage drop in cables connecting a remote pad or group of pads, a higher voltage supply is provided to remote pads or groups. In a practical application of the invention, a designated area, such as a sports ground, football pitch or training area can be stripped of its surface turf, and dug out to a required and relatively shallow depth. A number of pads can be laid in end-to-end and side-by-side relationship to cover the whole of the required area, and a number of pads in side-by-side configuration connected as a group to a control panel via a transformer, all of the groups in end-to-end configuration being electrically connected to the same transformer and control panel. Once laid, the pads can be covered with earth that can be compacted, and the turf relaid, to provide the required surface. The control panel may be such that when switched on, voltage at one level is
provided to a number of pads or groups of pads closer to the control panel, and voltage at a higher level supplied to pads or groups of pads furtherfrom the control panel to compensate for a voltage drop in the electrical cables, and to ensure that the heating effect of the heatable sheet in each pad is substantially the same. At times when a pitch is fully frozen, all of the pads can be switched on and left on to provide sufficient heat to soften the playing surface. At times when a weather forecast is such as to cause the expectation that a pitch will become frozen, all of the pads can be switched on to prevent the pitch from freezing. To reduce the running costs and power supply requirements in advance of actual freezing of a pitch, a number of individual pads or groups of pads can be heated alternately and sequentially over predetermined periods of time, to guard against a pitch freezing but without having to heat all pads. Equally, and if a partially frozen pitch needs to be dealt with control means can be such that only those pads or groups of pads below a frozen area are switched on to provide a local heating effect. One schematic embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a view of part of an outdoor area being prepared for the subsoil heating means of the invention. Figure 2 is a schematic sectional view of a pad according to the invention; and Figure 3 is a schematic plan view of a surface area provided with pads according to the invention. Figure 1 , shows part of an outdoor area 1 dug to a required depth of the top soil removed.
Laid on the dug area are pads 2 with electrical connecting wires 3, the pads being laid in spaced parallel relationship, to leave gaps 4 to allow for water drainage. Once all the pads are laid, top soil is replaced to cover the pads, and the surface either seeded or turfed to provide a playing surface. As is illustrated in Figure 2, each pad 2 comprises a core 5 of an appropriate insulating material such as foamed plastics, mineral wool or the like, over which is laid a layer 6 of flexible
semi-conductive heatable material connected by the wire3 to an appropriate power source. The flexible semi-conductive material is preferably that known as INDITHERM (RTM) described and Claimed in European Patent Application 1127356. The core and the heatable layer are encased in impervious outer cover 7. As is shown schematically in Figure 3, the whole of an area 1 is provided with the pads 2 in parallel longitudinal rows, and the pads connected to transformers 8 along one edge of the area 1 , each transformer having a control panel. Preferably, and for safety the transformers are buried alongside the area to be heated. When the area is such as a football pitch, pads 2 can also be provided below what is known as the technical area 9, where officials of teams playing on the area gather. Each pad 2 can be connected separately to one of the transformers, and the transformers simultaneously activated to provide power to all of the pads to cause the heating of the layers 6 of flexible semi-conductive material. However, it is more convenient to form a block 10 from a number of transversely spaced pads 2, with the blocks 10 of a longitudinal row being connected to one transformer 8 and control panel. The control panel is such that each block 10 is separately powered, and is such that all of the blocks 10 of a row can be heated, or selective blocks 10 of pads of a row heated. Equally so, the control panel can cause a voltage supply at one lower level to blocks 10 of pads nearer to the transformer 8 and a different higher level of voltage to blocks 10 of pads 2 further away from the transformer 8, to compensate for voltage drop in the connecting wires 3. At times when such as a football pitch is fully frozen, all of the blocks 10 of pads 2 can be switched on and left on to provide sufficient heat to soften the playing surface. At times when a weather forecast is such as to cause the expectation that a pitch will become frozen, all of the blocks 10 of pads 2 can be switched on to prevent the pitch from freezing. To reduce the running costs and power supply requirements in advance of actual freezing of a pitch, a number of individual blocks 10 of pads 2 can be heated alternately and sequentially over predetermined periods of time, to guard against a pitch freezing but without having to heat all of the blocks 10 of pads.
Equally , and if a partially frozen pitch needs to be dealt with control means can be such that only those blocks 10 of pads or groups of blocks of pads, below a frozen area are switched on to provide a local heating effect. Once melting of a frozen surface takes place, surface water penetrates through the grass surface, and is allowed to pass the pads by virtue of their laying with spaces 4 between adjacent pads 2, with water running off the impervious outer cover 7 into the gaps 4.