Stainless UK have supplied over 120 stainless steel Grip-Bar anchors to contractor BAM Richies for use as soil nails, rock anchors and special foundation assemblies as part of the geotechnical works required for the construction of a new lifeboat station at St Davids on the Pembrokeshire coast of Wales in the UK.
For over 140 years a lifeboat has been stationed at St Davids on the rugged Pembrokeshire coast. The old lifeboat station was over 100 years old and not able to house the new GBP 2.5 million Tamar class lifeboat. A new GBP 9.5 million lifeboat station and slipway has been constructed next to the old boathouse which is able to house the new lifeboat and also provide modern facilities for the 2 full time staff and 28 crew volunteers.

To stabilise the cliff face around the new station, 20mm diameter stainless steel Grip Bar anchors in 2m and 3m lengths were used as soil nails. Square plates were supplied with each anchor to secure the rock netting. 30mm Grip Bar anchors in 8m lengths were used as rock anchors in the most unstable areas of the cliffs.

Stainless UK also manufactured in their Sheffield factory the 8.5m long, 36mm diameter Grip-Bar anchors that are being used to tie back reinforced concrete facings to the cliffs which will act as column supports for the new lifeboat station. The Grade 316 stainless steel anchors bar are set into the rock face at 25° and then tensioned.

Access to the anchor heads has been maintained by casting a recessed area into the concrete facing blocks. Custom made stainless steel covers have been supplied to protect the anchor bar heads and provide for future access if required. “We are particularly proud to be associated with a project such as this that will provide so important a service to the community” commented Tim Wells, Managing Director of Stainless UK.

Manufacturing process

The high strength product is made from  cold drawn bar feedstock which benefits from superior strength, whilst retaining the elevated ductility associated with stainless steel. Using a cold rolled thread enhances the strength of the bar and provides a robust, self-cleaning and user friendly thread which is easy to use on construction sites. The bars are manufactured in 6m fully threaded lengths or by special order, with a bespoke thread on the ends of the bar. The threaded products utilise the thread rolling principle, which benefits from not removing any material from the bar whilst generating the coarse thread. It is in principle a cold forging process, and hence the grain flow caused by this process improves the strength of the thread relative to that of a cut thread of the same size.

Material selection

The selection of the correct grade of stainless steel for rock anchor applications must take account of the following features:

• The environment
• Structural requirements
• Maintenance
• Life of structure
• Surface finish

Factors which may influence material selection are temperature, pollutants, humidity and presence of chloride ions. In general it is reasonable to use grade 304 for most non-marine applications, however highways and marine structures generally call for grade 316 because of the high chloride concentrations. Duplex grade 1.4462 can be used in high chloride environments for increased resistance to stress corrosion, pitting and crevice corrosion.

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