The armoured unit, called the Xbloc, is a simple, robust and reliable breakwater armoured unit with considerable structural integrity as an individual element and considerable hydraulic stability in an armoured layer.

Moreover, says DMC, casting and positioning the Xbloc unit is straightforward and is very cost effective.

The Xbloc has been subjected to extensive research consisting of 2-D and 3-D hydraulic model tests; destructive tests with 4m 3prototype units; analyses of the structural integrity of the blocks using Finite Element calculations; logistic studies of the fabrication, storage and placement of the units; and a comparison of the Xbloc with existing armour units - such as the well-known Accropode and Core-loc - on actual projects.

DMC says it believes that the average saving yielded by using Xbloc units (less concrete needed) can be as much as 15% compared to other single-layer armour units, the difference being due to the high stability coefficient of the Xbloc unit and the low packing-density required, both of which result in a lower concrete demand.

As there are no specific requirements on the orientation of the individual units to achieve good interlocking, placement of the units is straightforward reducing both construction time and overall cost.

Xbloc armouring has been tested extensively against wave loading in 2-D and 3-D models at Delft Hydraulics in the Netherlands, and at a number of other hydraulic laboratories.

DMC says the results from the tests demonstrate that the Xbloc possesses excellent hydraulic stability and that the stability coefficient of the Xbloc is the same for breaking and non-breaking waves. According to DMC, the design wave height can be exceeded by 20 per cent without damage to the armoured layer.

Another key feature of the Xbloc is automatic interlocking Xbloc units naturally find a stable position on a slope and DMC says an armoured layer formed using Xblocs is also 'self-repairing' in as much as that after damage automatic interlocking reinforces the structure.

Xblocs are also simple to position, notes DMC, because no specific orientation of the individual units is required, and the highly porous armoured layer formed by Xblocs minimises wave overtopping.

The structural integrity of the Xbloc was evaluated using prototype drop tests and Finite Element calculations. Drop tests were carried out using units of 4m 3subjected to overturning and free fall tests in various positions.

All of the studies indicate that the Xbloc is a structurally strong unit able to cope with the loads that can be expected during its lifetime.

Another important advantage of the Xbloc is that steel moulds can be used repetitively to produce large numbers of units.

DMC says various mould designs have been developed, consisting of two or three sections. Casting is easy because of large fill openings. At the construction yard, the Xbloc units can be handled with the aid of a forklift, a clamp or a sling, and the units can be stored in various ways, for example side by side in a half-brick bond.

The storage pattern itself can be arranged in one or two layers, depending upon the capacity of the subsoil. During placing of the units on the breakwater there are no restrictions on Xbloc orientation, only on the packingdensity achieved on the slope.

The volume of the units varies between 0.75m 3and 20m 3, depending upon the extreme sea-state.

Overall, claims DMC, the use of the Xbloc armour unit has a number of significant advantages, including single layer application - random placement of only one armour layer; outstanding structural stability of the units; excellent hydraulic stability of the armour layer; the use of a simple mould that can be built easily by a local contractor; high rate of production due to the fact that they are easy to cast as the mould has large fill openings; high placement rate, with no requirements on orientation of the individual units to achieve good interlocking; lower concrete use compared to other single layer units; large reserve stability of the armour slope; and a limited storage area is required because of the unit's dense storage configuration.