Rock Armour From Quarry Waste

Protecting the coast with dimension stone waste from Larvik

By Terje Johansen, managing director, Stema Shipping (UK) Ltd

The quarries that produce the unique and highly attractive monumental and decorative natural stones known as Emerald Pearl, Black Pearl and Blue Pearl, and all other varieties of larvikite, are situated in and around the town/municipality of Larvik, in south-eastern Norway.

Although much sought-after as a building product, precise production and quality requirements mean that approximately 95% of the extracted rock (more than 3 million tonnes a year) is regarded as waste that has to be disposed of. In the past this created a huge problem and financial cost, as the only permitted way of disposing of the surplus larvikite was to create artificial mountains.

Stema Shipping, one of the largest suppliers of mineral-based building materials in Europe, in co-operation with the Larvik quarry owners, are continually investigating possible solutions to this ‘waste’ problem. One such solution is the production and delivery of rocks and stones for use in sea defences, port construction and other hydraulic engineering works throughout Europe, and, in particular, coastal protection along some of the most vulnerable sections of the UK coast.

Stema Shipping have a sole-agency agreement with the quarries for the use of the surplus stone as rock armour. Other uses for the surplus rock are also being considered but as yet are not economically viable.

The intrinsic properties of the rocks from Larvik result in a very high-quality and durable rock armour, which is why this has become the main product of the surplus materials, although this is essentially a recycled waste product, the aggregate tax still applies in the UK.

Thanks to careful removal of the primary dimension stone product by low-intensity blasting and/or narrow-bore line drilling and diamond-wire saws, the final rock armour products are devoid of cracks and well suited for use in sea defences and breakwaters.

Stema Shipping store useful rocks and stones at the quarries for use in rock armour production. These range in size up to 45 tonnes and each one is sorted and shaped to comply with the relevant CIRIA Standard. For smaller gradings (up to 1 tonne), the stones are sized using a screener and/or screen buckets, while most other gradings are produced from larger blocks that are reduced to the correct dimensions. Generally, rocks over 1 tonne are shaped and reduced to the appropriate size using a hydraulic hammer, before being individually weighed and stored in ranks of different sizes.

When loading their ships and barges, Stema Shipping pick the rocks from the different ranks according to programmed delivery cycles in compliance with the required project design specifications. Normal production and delivery can include all sizes from graded core material to individual blocks weighing up to 45 tonnes, and larger rocks can also be supplied, subject to the availability and capacity of lifting equipment. All weight sizes and classes comply with EN 13383.

Stema Shipping currently utilize two loading facilities for the shipment of the rocks. The company has five acres of land for storing rocks at the main Port of Larvik, with all loading carried out over the port’s main quay. Approximately 100,000 tonnes of rock is shipped out of this facility every year, although the port has the capacity to handle up to 500,000 tonnes if necessary.

The Port of Larvik facility is used for rocks from the eastern quarries, however Svartebukt harbour, situated some 10 miles from the main Port of Larvik, is where the main production of larvikite is carried out. Here Stema Shipping have invested in dedicated quay facilities and storage space for up to 100,000 tonnes of rock, as well as a 3km private road from the quarry areas to the harbour. The loading capacity at Svartebukt harbour is approximately 1.2 million tonnes a year.

The company has also invested in a fleet of five specially strengthened, flat-top, rock-carrying barges with cargo capacities ranging from 4,000 tonnes to 25,000 tonnes. The bigger barges are used to transport rocks from the Larvik ports to destinations in the UK, France, Holland, Denmark, Germany and the Baltic states, with the southern and eastern coastlines of UK being the company’s main market. To date, the furthest destination that rocks have been delivered to is the Azores.

The five barges range in size from 80m x 20m up to 140m x 42m and are all fitted with side-walls around the deck area to secure the cargo in place. Front shovels are used to load the barges via a ramp from the quay to the vessel. To keep the barges level with the quay, ballast is pumped in and out as necessary during the loading process.

Tug boats are used to tow the barges to their final destination, which may be a harbour but in most cases is an offshore anchorage where the cargo is transhipped on to the company’s smallest barge for direct delivery to the beaches where the rocks are to be used. A Caterpillar 385 excavator is used to tranship the rocks from the larger barges to the smaller vessel, while a Cat 988 front shovel on the small barge is used both for positioning the rocks on the deck and for the subsequent discharge operation.

When loaded, the small barge is towed to the foreshore where it is positioned to drop the rocks in the required location. Transhipped cargoes range from 1,000–2,500 tonnes, depending on the depth of water at the dropping point. The rocks are discharged over the stern of the barge on high water, allowing the receiver (mainly contractors specialized in the building of sea defences or breakwaters) to retrieve the cargo at low tide.

Some current and recent projects that Stema Shipping have supplied with armour-stone from the Larvik quarries include: the Great Yarmouth EastPort Development (ongoing: 850,000 tonnes), Sumburgh Airport extension, Shetland (2006: 200,000 tonnes), Dymchurch, Kent (2005–07, 200,000 tonnes), Lyme Regis, Dorset (2006: 40,000 tonnes) and Jersey, Channel Islands (2006: 40,000 tonnes).