MarCom WG 12 - Subgroup D: Analysis of Rubble Mound Breakwaters - Construction Deviations and Reliability of Construction (1992-1994)

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Notes on research in UK on Rock Durability

For a number of years research has been proceeding at Queen Mary College, University of London, and Hydraulics Research, UK with armour durability and breakwater performance.

When a rock armour layer is exposed to the marine environment the material is subject to impact, abrasion and physico-chemical degradation processes causing, to a degree dependent on the material quality and the environment, fracture of blocks, rounding altering their shape and mass, and consequent changes to the profile and the porosity of the mass.

These changes will affect hydraulic performance and stability of the structure and must therefore be taken into account in design and construction.

In order to do so, it is necessary to predict for a given rock quality and environment how the breakwater performance will change from year to year during its designed service life. Methods and controls of construction together with future inspection and maintenance can then be planned in accordance with the severity of the predicted changes.

The general practice in present breakwater design is subjective, in selecting and specifying a durable"" rock, attempting to control construction to specified material and geometric tolerances and assuming that the future deterioration in service will remain within tolerable limits or that maintenance can be carried out to preserve the performance and structural integrity.

For a conventional rubble mound, designed for very small movements or damage, and built with good quality materials and control, there have been many structures where this approach has been sUccessful. In considering more novel constructions, such as berm or beach breakwaters, which may be designed for more movement, the importance of predicting rock durability and resulting rates of weight loss and rounding becomes increasingly great.

The current research has defined tests for fracture toughness and abrasion resistance of rock which are being calibrated against performance in prototype. A consistent set of results would suggest that these tests can in future form a quantifiable contribution to the design process at the stage when different rock sources are being considered. This work on rock durability testing is proceeding at present.

One method of prototype calibration being investigated is by photographic image analysis. The state of rounding of individual rocks of known abrasion resistance in a given environment is compared with a laboratory simulation of rounding.

Another method is to monitor the surface geometry of the armour layer and to assess changes in armour profile and layer porosity that are the consequence of more rounded and degraded rock as determined from laboratory models of armour layers.

The detailed measurements, needed to describe fully an armour layer, are of a frequency and precision which are unlikely to be available to construction and monitoring in the field as a matter of general site practice. Nevertheless, the present research contains considerable promise of enabling predictions to be made of armour behaviour.

Indeed, if the planned future research in this area is undertaken, simplification of the existing methods could allow them to be used on a routine basis. The quantitative prediction of armour degradation and layer geometry changes may enable hydraulic modelling to be performed for a structure at different ages of deterioration and add to the reliability of the design."

  • Written by: MarCom Working Group 12
  • Published: February 08, 1989
  • Language: English