GB2160246A

GB2160246A - Protection of surfaces

Info

Publication number: GB2160246A
Application number: GB08414999A
Authority: GB
Inventors: Adrian Ralph O'hea
Original assignee: O HEA ADRIAN RALPH
Current assignee: O HEA ADRIAN RALPH
Priority date: 1984-06-13
Filing date: 1984-06-13
Publication date: 1985-12-18
Also published as: GB8414999D0

Abstract

The surface of an erodible material (6) such as an earth dyke is protected against a fast-moving fluid (5) such as a breaking wave by a layer (3) of resistant material such as concrete or concrete blocks. Holes (1) penetrate the concrete, equalising the fluid pressures under the concrete (8) and in the region (7) above each hole. Each hole comes out at the top of a hump 2 in the upper surface of the concrete, where because of the accelerated flow the pressure is less than the average pressure on the top of the concrete. In this way the concrete is sucked firmly on to the ground beneath. If the holes are at the top of humps whatever the direction of the cross section through the concrete, then the suction effect works whatever the direction of flow. <IMAGE>

Description

SPECIFICATION Protection of surfaces This invention relates to the protection of the surface of one material by a protective layer of a more resistant material against erosion or other attack by a fast-flowing fluid the direction of which varies or is not known.

Examples are the protection of earth dykes against wave action, of hydraulic stilling basins against turbulent flow, and of space vehicles reentering the atmosphere. The invention is intended to reduce the forces tending to remove the protective layer and so reduce its cost.

The vulnerable surface is protected by a more resistant material which may take the form of a continuous layer or of tied, interlocking or juxtaposed blocks or tiles. The protective layer is perforated at intervals and this invention relates in particular to the shape of the outer surface of the protective layer in the neighbourhood of the holes.

According to the invention the top of each hole 1, as shown in the accompanying Fig. 1, coincides with the top of a cone or otherwiseshaped hump 2 in the outer surface of the protective layer 3 so that the flow is accelerated as it approaches the top of the hole and its streamlines 4 are bent downwards as it passes over the hole (up is here defined as towards the fluid 5, down towards the material 6 being protected) so causing a local reduction in fluid pressure outside the top of the hole at 7 whatever the direction of the flow. If there is no great quantity of fluid flowing through the holes, the fluid in the pores of the material being protected, and in any gap 8 between that material and the protective layer, is at the same static head as the fluid 7 immediately outside the hole.

Since at this location the static head is less than elsewhere on the top surface of the protective layer, it follows that the average head on the bottom of the layer is less than the average head above it, and that this difference increases with the speed of flow.

The protective layer is thus held down by suction in addition to whatever other method is used to anchor it to the material it is protecting. Further and optional features of the invention appear in the following description and are set forth in the accompanying

claims.

An example of the invention as it might be used in protecting an earth dyke against waves is described below and illustrated in the accompanying Fig. 2. The protective layer is formed of interlocking precast concrete blocks 9 at least some of which have one or more holes 1 penetrating through to their underside. The top of each hole is at the top of a hump 2. The total plan area of the holes is substantially greater than the total area of the gaps 10 between the blocks, otherwise flow of water through the gaps will tend to cancel the head difference induced by the holes.

The humps may be substantially circular or square in plan as in Fig. 2, so that the water pressure is reduced at their top whatever the direction of flow, or linear like a ridge and with the hole as a slot along its top so that the water pressure is reduced only when there is a component of the flow perpendicular to the ridge. If the holes are slots along ridges then the joints 11 between the blocks may be used for this purpose as shown in Fig. 3. If all the joints are so used, or some of them in at least two different directions, then there will always be a component of flow perpendicular to at least some of them.

If it is known that the flow is likely to be more violent in one particular direction, then the tops of the holes may be tilted to one side or appear to the lee of the top of the humps.

However, if the position of the tops of the holes is too asymmetrical with respect to the humps, then flow in one direction may increase the pressure in the holes instead of decreasing it as intended. For this reason, if joints are used as the holes as in Fig. 3, the method of interlocking the blocks must be such that adjacent blocks are kept substantially level with each other.

If the particles of the soil being protected are smaller than the holes, the blocks may be laid on a gravel, sand or fabric filter, or a filter may be built into each hole.

As in other forms of permeable revetment, the high pressures under a breaking wave 1 2 in Fig. 4 are transmitted through the holes 1 3 under the wave and spread out under the revetment to adjacent areas 1 4 where they tend to lift the revetment up and so destroy it.

These adjacent areas lie under fast flow 1 5 from the wave, and the present invention uses this fast flow to reduce the uplift in the way described. To limit this propagation of pressure outwards from under a breaking wave, cutoffs 1 6 may be built into the bottom of the blocks and pressed through any filter layer 1 7 into the underlying soil 1 8.

CLAIMS 1. A layer of resistant material designed to protect the surface of another material against attack by a fast-flowing fluid and penetrated by openings set in the tops of humps or ridges in the upper surface of the layer and in flowing over which the fluid reduces the pressures on the underside of the layer.

2. A layer according to claim 1 in which particles of the material to be protected are prevented from passing through the holes by means of a filter.

3. A layer according to claims 1 or 2 but regardless of the location of the openings in which the propagation of fluid pressures along

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Protection of surfaces This invention relates to the protection of the surface of one material by a protective layer of a more resistant material against erosion or other attack by a fast-flowing fluid the direction of which varies or is not known. Examples are the protection of earth dykes against wave action, of hydraulic stilling basins against turbulent flow, and of space vehicles reentering the atmosphere. The invention is intended to reduce the forces tending to remove the protective layer and so reduce its cost. The vulnerable surface is protected by a more resistant material which may take the form of a continuous layer or of tied, interlocking or juxtaposed blocks or tiles. The protective layer is perforated at intervals and this invention relates in particular to the shape of the outer surface of the protective layer in the neighbourhood of the holes. According to the invention the top of each hole 1, as shown in the accompanying Fig. 1, coincides with the top of a cone or otherwiseshaped hump 2 in the outer surface of the protective layer 3 so that the flow is accelerated as it approaches the top of the hole and its streamlines 4 are bent downwards as it passes over the hole (up is here defined as towards the fluid 5, down towards the material 6 being protected) so causing a local reduction in fluid pressure outside the top of the hole at 7 whatever the direction of the flow. If there is no great quantity of fluid flowing through the holes, the fluid in the pores of the material being protected, and in any gap 8 between that material and the protective layer, is at the same static head as the fluid 7 immediately outside the hole. Since at this location the static head is less than elsewhere on the top surface of the protective layer, it follows that the average head on the bottom of the layer is less than the average head above it, and that this difference increases with the speed of flow. The protective layer is thus held down by suction in addition to whatever other method is used to anchor it to the material it is protecting. Further and optional features of the invention appear in the following description and are set forth in the accompanying claims. An example of the invention as it might be used in protecting an earth dyke against waves is described below and illustrated in the accompanying Fig. 2. The protective layer is formed of interlocking precast concrete blocks 9 at least some of which have one or more holes 1 penetrating through to their underside. The top of each hole is at the top of a hump 2. The total plan area of the holes is substantially greater than the total area of the gaps 10 between the blocks, otherwise flow of water through the gaps will tend to cancel the head difference induced by the holes. The humps may be substantially circular or square in plan as in Fig. 2, so that the water pressure is reduced at their top whatever the direction of flow, or linear like a ridge and with the hole as a slot along its top so that the water pressure is reduced only when there is a component of the flow perpendicular to the ridge. If the holes are slots along ridges then the joints 11 between the blocks may be used for this purpose as shown in Fig. 3. If all the joints are so used, or some of them in at least two different directions, then there will always be a component of flow perpendicular to at least some of them. If it is known that the flow is likely to be more violent in one particular direction, then the tops of the holes may be tilted to one side or appear to the lee of the top of the humps. However, if the position of the tops of the holes is too asymmetrical with respect to the humps, then flow in one direction may increase the pressure in the holes instead of decreasing it as intended. For this reason, if joints are used as the holes as in Fig. 3, the method of interlocking the blocks must be such that adjacent blocks are kept substantially level with each other. If the particles of the soil being protected are smaller than the holes, the blocks may be laid on a gravel, sand or fabric filter, or a filter may be built into each hole. As in other forms of permeable revetment, the high pressures under a breaking wave 1 2 in Fig. 4 are transmitted through the holes 1 3 under the wave and spread out under the revetment to adjacent areas 1 4 where they tend to lift the revetment up and so destroy it. These adjacent areas lie under fast flow 1 5 from the wave, and the present invention uses this fast flow to reduce the uplift in the way described. To limit this propagation of pressure outwards from under a breaking wave, cutoffs 1 6 may be built into the bottom of the blocks and pressed through any filter layer 1 7 into the underlying soil 1 8. CLAIMS

1. A layer of resistant material designed to protect the surface of another material against attack by a fast-flowing fluid and penetrated by openings set in the tops of humps or ridges in the upper surface of the layer and in flowing over which the fluid reduces the pressures on the underside of the layer.

3. A layer according to claims 1 or 2 but regardless of the location of the openings in which the propagation of fluid pressures along the underside of the layer is reduced by substantially impermeable cutoffs fixed at intervals to the bottom of the layer and penetrating the underlying material.

4. A layer according to claim 1 incorporating any of the features described in the accompanying text or figures.