Rising sea levels and changing weather patterns mean that construction and maintenance of coastal defences is becoming essential on some coasts in Europe. ABE takes a look at the complexities of supplying rock to meet the needs of these demanding projects
Dymchurch on the UK's south coast is an extreme example of the need for sea defences but it is not unique in Europe. The land around Dymchurch lies 1.7m below sea level and a breach of the crumbling sea wall would inundate an area with a 5km radius around the village.
Main contractor Van Oord is working to place 3 to 12tonne blocks of granite - 340,000tonnes in total - to form a 2.1km long, 7.7m high rock revetment in front of a new concrete wave wall to protect the area. But rock with sufficient strength and structural integrity to complete the work is not available locally and the granite is being brought in by barge from Larvik in Norway.
European market
Like the need for coastal defences, the project is not unusual in Europe in the fact that Van Oord has had to look further a field than the local area to source rock for the project. In fact around 10million tonnes of rock are used for coastal defence projects in Europe each year and the market to supply material for these schemes is worth an estimated €10billion.
To underline the importance of this market, CIRIA has just published a new Rock Manual, which draws on expertise from the UK, France and the Netherlands. Although the manual is aimed at designers of coastal defence projects, the book also emphasises the need to carefully specify rock for such projects.
"European standards dictate the need for a minimum specific gravity of 2.3tonnes/m3 for coastal defence work but rock with a specific gravity of 2.9tonne/m3 is the ideal solution," said a technical specialist from
Nonetheless, for many projects where the location is exposed to strong wave action, importing high density rock by barge is the only solution. Scandinavia, France, Germany, Spain, the UK and Ireland are the major suppliers of rock for armourstone in Europe, not just because of the presence of rock that meets the high specifications, but also because quarries to extract this rock are located in coastal areas. The potential for sea transport of up to 10,000tonnes rock by a single barge means that journeys of around 500km from quarry to a project site are not unusual.
"In addition to being located in the right place for export and meeting the critical density requirements, the rock must also have very low absorption as well as high resistance to degradation, freeze/thaw, magnesium sulphate attack and abrasion," Cemex's specialist told ABE.
Imperial College senior lecturer of Geomaterials John-Paul Latham added, "The rock source must be capable of producing large blocks of rock and be durable. Typically unweathered igneous, nonfoliated metamorphic and low porosity sedimentary rocks are best suited to coastal defence applications." But checking the characteristics of the rock meet the specification is not the end of the story - it still needs to be proved that the rock can be extracted in the right sized blocks for the project without causing secondary cracking.
Blasting
Extracting rock for armourstone is not simple and often results in a high level of wastage. "Blasting for a specific size of rock needed for coastal defence may only give a 10% yield," said blasting specialist Ritchies technical manager Ian Christie. "So for a scheme that needs 10,000tonnes of rock, up to 100,000tonnes of rock may need to be extracted.".
Blasts for armourstone need to be carefully designed to ensure the size of blocks are achieved without the need for further processing and with minimal fragmentation.
"When blasting for conventional aggregates, quarry operators are looking for maximum fragmentation and a uniform size to make it easier to feed the crusher. But with armourstone, the aim is the opposite but the actual blast design will be down to the rock itself." According to Christie, a preliminary assessment is essential for any quarry looking to get into the armoustone sector. "Many quarries mistake production of oversize material during a normal blast as an indication that the site is suitable for amourstone production. Some store oversize rock in the hope of securing an armourstone contract in the future but this is risky," he said. "Storing the amount of rock needed in a way that prevents any damage can take up a lot of space and then breaking this rock for conventional use, if a suitable contract doesn't materialise, can be a costly task."
According to Latham, the blast needs to carefully designed to meet the end use. "The aim is to get the maximum amount of large blocks in the blast pile," he explained. "The blast is designed to put a minimum of energy into the rock mass but enough to loosen the existing in-situ blocks that are geologically formed by the intersecting joints and planes, and to push out the toe of the bench.
Handling
Once extracted, the rock needs to be handled with care to ensure its integrity is maintained. The blasting work needs to be carefully coordinated with the need for the rock on the project site to avoid unnecessary re-handling of blocks, which can weigh anything from a few tonnes up to 20tonnes each.
Although the main strength consideration for armourstone is for its final destination, the strength to resist damage during transportation is also important.
"Uniaxial compressive strength or point load testing is usually a good indicator of resistance to fragmentation and crushing," said Latham, who also contributed to CIRIA's new Rock Manual. "Rock with a uniaxial compressive strength greater than 60MPa should be able to resist excessive corner or edge breakage during barge transport to site." Due to the size and quantity of rock needed by coastal defence projects, the material is usually delivered by tipping barges that drop the material close to the construction site. From there it is up to the contractor to manoeuvre the rocks to their final position
