WO1997006309A1

WO1997006309A1 - Coastal defences

Info

Publication number: WO1997006309A1
Application number: PCT/GB1996/001925
Authority: WO
Inventors: Allan Harry Beckett
Original assignee: Beckett Allan H
Priority date: 1995-08-09
Filing date: 1996-08-08
Publication date: 1997-02-20
Also published as: GB9516291D0; AU6665696A

Abstract

The present invention provides a ballasted armour unit (1) for use in construction for preventing coastal erosion. The armour unit (4) is formed from a material such as concrete or a plastics material, having a significant proportion of denser inclusions (7, 9) embedded in it. The inclusions are preferably steel bars, such as scrap rail sections, or smaller scrap metal particles may be used. In a preferred embodiment, the armour unit (1) is T-shaped. In this way, a number of the units can be interlocked to form a layer of a sea defence having no substantially straight into-unit joint extending from one side of the sea defence to the other.

Description

COASTAL DEFENCES

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to constructions for preventing coastal erosion, or breakwaters, and more particularly to an armour unit for use in such a construction.

SUMMARY OF THE PRIOR ART

Coastal erosion, that is erosion of shores by the action of waves, tides and storms, is a widespread phenomenon. Several methods have been used to tackle the problem.

In some earlier known methods, a barrier formed from large rocks, each rock typically weighing several tons, is placed over exposed areas of coast. The rocks can be placed on a bed of gravel or stones, or a bed comprising a layer of gravel with an overlying layer of larger stones (approximately 5-10 cm in diameter) . Whilst this arrangement may provide better resistance to erosion than the previously exposed sand underneath, it is often difficult to find suitable quantities of rocks of adequate size near the site to be protected, and transportation can be difficult and expensive.

Furthermore, the rocks may be displaced by larger waves or storms.

In a development of these methods, large cubes or cuboids of concrete have been used in place of the rocks.

These can be more readily positioned than rocks, and larger size barriers can more easily be constructed, for example from individual blocks weighing 5 tons or more. However, heavy storms can still lead to significant displacement or damage to the blocks.

Drawbacks with the above methods have been identified, and it has been proposed to use shaped concrete armour units, which allow waves to penetrate the barrier, and to be dispersed more gradually, rather than impacting suddenly against a continuous wall. One of the earliest and most commonly used examples of such a shaped armour unit is the "Tetrapod" . As shown in Fig. 1, this comprises four truncated conical sections emanating from a common centre, and facing the four vertices of a tetragonal pyramid. Such shapes permit a degree of interlocking between adjacent armour units, which assists in resisting movement of the armour units in heavy wave conditions.

Recent developments in the art have shifted emphasis away from continuous solid barriers, such as those employing cubical units, and have concentrated on refining the shape of the units, to produce a better defined water flow through and around the units. Ever more intricate shapes are being developed as a result of research into fluid flow patterns.

Such armour units however still suffer from the drawback that, in heavy storm conditions the units are liable to be displaced and damaged. The relatively high surface area to volume ratio of the complex shapes can increase their susceptibility to be lifted by the water. Typically, the units weigh about 5 tons. However, in places susceptible to particularly heavy waves, it has been necessary to increase the resistance to lifting of the units, and this has been done by increasing the size of the units. For some areas, individual units weighing up to about 25 tons may need to be used, and these can present severe handling and transport problems. Furthermore, the complexity of such units requires complex moulds and manufacturing techniques, which adds to the cost.

SUMMARY OF THE INVENTION

The inventor has found that increasing the density of an armour unit rather than simply making it larger has a more pronounced effect on its stability; the overall area exposed to the water decreases in addition to an increase in the effective weight for a given mass of armour unit, so the maximum velocity of water that could be tolerated before the unit is displaced is significantly raised. This could lead to a reduction in the rate of damage, and to a substantial increase in the useful lifetime of a barrier constructed from such units. In the present invention, the increase in density is achieved by embedding in the armour unit a significant proportion of inclusions denser than the material of the armour unit.

Accordingly, in one aspect, the present invention provides an armour unit comprising a mass of concrete having a significant proportion of inclusions denser than concrete embedded in the concrete.

Because of the large volumes of the units, and the large areas to be covered, the cost of the units is a very important factor, and for practical purposes concrete is the only material which has been used.

However, with the ballasted unit of the present invention the applicant has found it is possible to move away from the now conventional use of concrete for the material of the armour unit and other previously impractical materials, for instance plastics, may be used.

In another aspect, therefore, the present invention provides an armour unit comprising a mass of a plastics material having a significant proportion of inclusions denser than the plastics material embedded in the plastics material.

In the context of the present application, a significant amount of dense material means sufficient material to bring the overall density of the armour unit to a value at least about 8-10% higher than the density of concrete.

Advantageously, the inclusions have a density of at least about 4 or 5 tonnes/m³, more preferably at least about 7 tonnes/m³. The inclusions may take the form of ballast and/or reinforcing materials, conveniently they comprise a metal reinforcement, preferably steel.

Considering units employing concrete, it is, of course, known that steel wire can, in general, be used to reinforce concrete. The density of steel is about four times that of concrete. However, conventional reinforcement generally does not affect the overall density of the concrete significantly, the steel generally only accounting for about 2 or 3% of the total weight. Additionally, the cost per unit weight of steel reinforcement, presently about 10-20 times that of concrete, has meant that steel has never been considered for use a ballast in such a concrete structure.

The density of concrete is about 100 lbs/ft³ (1.8 t/m³) , which means that a unit weighing 5 tons on land has a net weight of only about 2.3 tons in water.

At a proportion of at least about 10% by weight, which is in excess of that which one would normally use if steel were being used as reinforcement, steel ballast can contribute to a reduction in the ratio of surface area to effective weight in water of concrete armour units which can surprisingly increase the lifetime of the armour unit .

When the proportion of steel is at least 30% of the total mass of the concrete armour units, the improvement in stability and strength of the armour units is more significant. Thus a proportion of at least 30% by weight of steel is preferred.

Preferably, the proportion of steel is at least about 40% or about 50%, which gives optimum results in terms of cost of manufacture, and strength and resistance to movement of the armour units in use.

At higher proportions of metal, problems ensuring that all the metal is adequately covered with concrete, to inhibit corrosion, may occur. It is undesirable to make the units too dense, or an excessive mass of material would be required to cover a given area of coastline. More than about 80% metal would not normally be used with concrete armour units, except in exceptional circumstances.

Preferably the inclusions are covered by a minimum thickness of about 3 cm (1 inch) of concrete over substantially the entire surface of the armour unit. This may inhibit corrosion of the inclusions. More preferably, the outer layer of concrete is at least about 7 cm (3 inches) .

Turning now to the plastics units, it is possible to reduce the thickness of material covering the inclusions to less than 3 cm (1 inch) . This can lead to a further decrease in the overall area exposed to the water for a given mass of armour unit. To help inhibit corrosion of the inclusions, the outer layer of plastics is preferably at least 1 cm (0.5 inch) . However, thicknesses of 5cm (2 inches) or more are also possible and may inhibit corrosion more surely.

Typically, the density of the plastics material is less than that of concrete and a higher weight percentage of steel than used in the concrete units is therefore preferred. In this respect, a proportion of at least about 50% by weight is preferable.

More preferably, the proportion of steel is at least about 60% or about 70%, or about 80%, at which values the improvement in stability and strength of the armour units, over known concrete units, is more significant. In fact, since it is possible to use a relatively thin covering of the plastics material, proportions of steel of 90% by weight or more are practical.

Whilst it is possible for the plastics material to be formed of a single type of plastics, eg. polypropylene or the like, the invention is not limited to this, and the plastics material may be a blend of different plastics. The inclusions may be dispersed throughout the plastics material, but more preferably the inclusions are concentrated in a core of the armour unit, the plastics material being coated around this core. Even more preferably, the core is coated with a plastics material which itself has fine inclusions dispersed throughout it. An example of a suitable plastics material is a proprietory material of EcoBuild International Limited, formed by blending Mixed Waste Plastics (MWP) and Automobile Shreader Residue (ASR) using a process developed by EcoBuild which is the subject of UK Patent Application No. 9602086.2.

In another aspect, the invention provides an armour unit comprising a core containing metal, preferably ferrous metal, and an outer layer comprising concrete or a plastics material and being substantially free of said metal.

When the density of the armour unit is increased, smaller units can be used to achieve the same effect as conventional units. Furthermore, simple shapes or substantially solid units, which are relatively easy to manufacture, can be effective. It has been found that by altering the shape of the armour units, scrap metal which is much cheaper than conventional steel reinforcement can conveniently be employed as ballast, without requiring intricate shaping. The term scrap metal is meant to include metal which has previously been produced or used for another purpose. The scrap metal may be in the form of elongate bodies, for example scrap rails. It is not substantially shaped or processed before incorporation in the armour unit. For example, the metal may only have been cut to a desired length prior to incorporation in the armour units. Of course some form of cleaning, for example sand blasting, may be carried out on the scrap metal.

With a barrier formed from cubical blocks, discontinuation gaps or joins must remain between adjacent rows or columns of blocks, so the rows or columns may become separated. With complex shapes such as the "Tetrapod" , some degree of interlocking is possible, but this is generally fairly haphazard, and due to the voids, relative movement of adjacent armour units may occur. Moreover, such shapes are intended to allow passage of water through the barrier.

In a further aspect, the present application provides an armour unit for a sea defence a plurality of which can be arranged in interlocking fashion to form a layer having no substantially straight inter-unit discontinuity line extending from any side of the sea defence to any other side. The invention also provides a sea defence comprising a plurality of armour units and having no substantially straight inter-unit joint extending from any side of the defence to any other side. The invention further provides a method of constructing such a sea defence by arranging armour units.

This may significantly reduce the amount of movement and hence the amount of damage to the armour units.

Preferably, the armour unit is substantially T- shaped. This allows a plurality of units to be arranged to form interlinked rows and columns of a barrier, to be manufactured easily, and allows straight bars or similar to be embedded in the units without any shaping of the bars, other than cutting of the bars to desired lengths. In a preferred embodiment of the invention a substantially T-shaped armour unit is provided for a sea defence comprising concrete and having at least about 10% by weight of a denser ballast, preferably in the form of metal bars, embedded therein.

In another preferred embodiment of the invention a substantially T-shaped armour unit is provided for a sea defence comprising a plastics material and having at least about 50% by weight of a denser ballast preferably in the form of metal bars, embedded therein. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, with reference to the accompanying schematic drawings, in which: Fig. 1 illustrates a prior art armour unit; Fig. 2 is an oblique view of an armour unit according to the invention;

Fig. 3 is a front view of the armour unit of Fig, 2; Fig. 4 is a section on A-A of Fig. 3; and Fig. 5 shows the results of a comparative test. DETAILED DESCRIPTION

Referring to Figs. 2 and 3, a concrete armour unit 1 is T-shaped, having a top leg 3 of width (t) of approximately 1.8 m (6 ft) and a height (h) of approximately 0.6 m (2 ft) . A descending leg 5 has a length (1) of about 0.6 m (2 ft) giving a total height (h+1) of approximately 1.2 m (4 ft) . The leg 5 has a width (b) of approximately 0.6m (2 ft), and is centrally positioned, leaving an overhang (o) of the top leg of approximately 0.6 m (2 ft) on either side.

As can best be seen in the section shown in Fig. 4, the depth (d) of the unit is 0.6 m (2 ft) . It can be seen that the height (h) of the top leg 3, the overhang (o) on either side of the leg 5, the width (b) and length (1) of the leg 5 and the overall depth (d) are all equal allowing the units to be tessellated in a variety of ways, as will be discussed below. As indicated by the phantom lines in Figs. 2-4, a plurality of steel bars 7, 9, here in the form of segments of used rail, having an approximately l-shaped section, are embedded in the concrete block. In this example, nine rails 7 just under 1.8 m (6 ft) long are embedded in the transverse portion 3 of the T in a regular 3 3 array, and nine rails 9, each just under 1.2 m (4 ft) long are embedded in the leg 5, and central portion of the top 3 of the T in a similar regular 3 x 3 array. The rails are interleaved in the central portion of the top 3 of the T-shaped unit, as can be appreciated from Fig. 3.

To construct such an armour unit in concrete, the rails are cut to the desired length, about 15 cm (6 inches) less than the length of the corresponding concrete portion. That is, the rails 7 are cut to about 1.65 m, and the rails 9 are cut to about 1.05 m. The rails may be sand blasted to remove excess corrosion. Notches are formed and the rails arranged to form a T- shaped frame. The rails are then tack welded together at the points where they cross to enable them to be handled as a single unit. The rail framework is placed into a T- shaped mould (or shuttering) , made of wood or other suitable material, and is supported above the bottom of the mould by precast concrete distance cubes about 7.5 cm (3 inches) high. Concrete is then poured into the mould and allowed to set. The concrete may be vibrated throughout the process to ensure that all voids between the rails are filled. With this arrangement, the rails are covered by a minimum depth of about 7.5 cm (3 inches) of concrete, which inhibits corrosion of the rails. In this arrangement, with rails having a linear density of approximately 75 lbs per yard (38 kg per metre) , the rails account for approximately 50% of the total mass of the unit, which is approximately 500 lbs (2.5 tonnes) . The embedded rails give the unit considerable strength. This reduces the likelihood of the unit being ruptured, even in the event that extremely rough seas displace the unit.

For lifting purposes, concrete lined holes (not shown) may be cast in one or more locations in the unit, for example adjacent the bottom of leg 5 and at either side of the top 3.

Where a plastics material is used instead of the concrete, the rails can be tack welded together to form a T-shaped frame as with the concrete units, and this frame can then be repeatedly dipped into a liquid bath of the plastics material until the desired thickness of coating is achieved. Alternatively, the plastics unit can be formed in a mould, similarly to the concrete units, pre¬ formed plastics blocks being used to support the T-shaped frame in the mould when the plastics material is poured in.

Advantageously, because a relatively thin layer of plastics material is sufficient to protect against corrosion, smaller more portable units can be used than with the concrete units, without a loss in effectiveness against wave action. In one embodiment of the plastics unit it is proposed to use pairs of rail sections tack welded side by side to form both the top leg and the descending leg of a T-shaped block, giving the following dimensions: a top leg width (t) of approximately 0.5m (1.5 ft) , and a top leg height (h) , descending leg length (1) and a descending leg width (b) each of approximately 0.15m (0.5 ft) .

With this size of plastics unit, having a plastics covering over the rails of about 1cm, the rails are likely to account for approximately 90% of the total mass of the unit. To further increase the density of the unit, if this is desired, any air gaps between the rails can be filled with, for example, scrap steel or even cement.

In an alternative concrete embodiment, smaller scrap metal (e.g. ferrous metal) particles can be used as inclusions, for example in the form of scrap steel punchings, plate, off-cuts, reject nuts or bolts. The metal particles are mixed with sand and cement to form a coarse aggregate. This is poured into a mould lined with a layer of cement and sand plaster about 7 cm (3 inches) thick, which forms an outer layer to inhibit corrosion of the metal when the cement has set. In this embodiment, the inclusions do not provide a reinforcing effect in the same way as the earlier embodiment. However, this embodiment can readily be used with a variety of scrap metal parts, and still provides the benefits of increased density. A similar arrangement is also possible with the plastics unit. The units 1 can be deployed as a series of spaced apart units. However a single layer of units can be interengaged to leave no plane of discontinuity passing from one side of the barrier to the other (as seen in Fig.5), thereby reducing the tendency of the barrier to separate when subjected to violent waves. The units can also be interlocked in a three dimensional fashion to form a taller substantially solid barrier, if desired.

As can be seen from Fig. 5, which shows the results of a breaking wave test in a hand-operated wave flume, the ballasted units of the present invention (on the right) have been found to perform significantly better than unballasted units of a similar size and shape (on the left) . The T-shaped units used in this test were constructed in concrete, the ballasted units being ballasted with steel inclusions to increase their density by 30% over the concrete only units. All the units were laid on shingle with a slope of 1:2

It will be appreciated that the dimensions can be varied. Although it is not generally desirable to make the units much smaller than that described, unless they are to be used in places where the anticipated amplitude of waves is small, the units can readily be scaled up as desired. The dimensions may be adjusted so that, on interlocking, gaps or voids are left between adjacent units. Furthermore, units may be contoured or profiled as desired to modify their resistance to the passage of water. The depth of the units, as shown in the section in Fig. 2, may be significantly different from the width of the upper units, as shown in the plan view in Fig. 1, for example if the units are intended to form a single layer, and only to interlock in a plane.

Claims

1. An armour unit for use in a construction for preventing coastal erosion, the unit comprising a mass of a first material, and inclusions denser than the first material embedded in the first material, wherein the average density of the armour unit is at least 8% higher than the density of concrete.

2. An armour unit according to claim 1, wherein the first material is a plastics material.

3. An armour unit according to claim 2, wherein the inclusions are present at a proportion of at least 50% by weight.

4. An armour unit according to claim 2, wherein the inclusions are present at a proportion of at least 60% by weight.

5. An armour unit according to claim 2, wherein the inclusions are present at a proportion of at least 70% by weight.

6. An armour unit according to claim 2, wherein the inclusions are present at a proportion of at least 80% by weight.

7. An armour unit according to claim 2, wherein the inclusions are present at a proportion of at least 90% by weight.

8. An armour unit according to any one of claims 2 to 7, wherein the inclusions are covered by a minimum thickness of 1 cm (0.5 inch) of plastics material over substantially the entire surface of the armour unit.

9. An armour unit according to claim 1, wherein the first material is concrete.

10. An armour unit according to claim 9, wherein the inclusions are present at a proportion of at least 10% by weight.

11. An armour unit according to claim 9, wherein the inclusions are present at a proportion of at least 30% by weight.

12. An armour unit according to claim 9, wherein the inclusions are present at a proportion of at least 40% by weight.

13. An armour unit according to claim 9, wherein the inclusions are present at a proportion of at least 50% by weight.

14. An armour unit according to any one of claims 9 to 13, wherein the inclusions are covered by a minimum thickness of 3cm (1 inch) of concrete over substantially the entire surface of the armour unit.

15. An armour unit according to any one of claims 9 to 13, wherein the inclusions are covered by a minimum thickness of 7cm (3 inch) of concrete over substantially the entire surface of the armour unit.

16. An armour unit according to any one of the preceding claims, wherein the inclusions have a density of at least 4 or 5 tonnes/m³, and preferably at least 7 tonnes/m³.

17. An armour unit according to any one of the preceding claims, wherein the inclusions comprise a metal reinforcement, preferably steel.

18. An armour unit according to any one of the preceding claims, wherein the inclusions comprise scrap metal.

19. An armour unit according to any one of the preceding claims, wherein the unit is shaped such that a plurality of the units can be arranged in interlocking fashion to form a layer of a sea defence having no substantially straight inter-unit discontinuity line extending from any side of the sea defence to any other side.

20. An armour unit according to claim 19, wherein the unit is substantially T-shaped.

21. A construction for preventing coastal erosion comprising a plurality of armour units according to claim 18 or claim 19, the construction having no substantially straight inter-unit joint lines extending from any side of the construction to any other side.

22. An armour unit for a sea defence, a plurality of which can be arranged in interlocking fashion to form a layer of the sea defence having no substantially straight inter-unit discontinuity line extending from any side of the sea defence to any other side.

23. A sea defence comprising a plurality of armour units and having no substantially straight inter-unit joint extending from any side of the defence to any other side.