The effect of interlocking is apparent when comparing a rock revetment with a modern single layer unit for average boundary conditions while taking into account the lower specific density of concrete compared to most natural rock commonly used in breakwater construction. Assuming that natural rock would be placed at identical slope steepness, the individual rock weight would require to be 3 times as high compared to Xbloc units. Rock is generally to be placed as double layer, thus the volume of armour material which needs to be quarried, stored, handled, transported and installed can be enormous for a larger breakwater exposed to significant wave action. Due to the interlocking effect the weight and thus the volume of single layer armour units is considerably less compared to an armour consisting entirely of rock. In addition units are normally fabricated near or at project site, so that transport issues are less critical.