Steynton SPS

The village of Steynton is located approximately two miles to the North/North-East of Milford Haven in South Wales. The existing Steynton Wet Well/Pumping Station is adjacent to the A4076, the main road linking Milford Haven and Haverfordwest. Dwr Cymru/Welsh Water and associated approved contractors undertook a 34 week project to upgrade the Pumping Station which contained a high level unscreened overflow, and replace with a new screened CSO, Wet Well and Valve chamber. This would be accompanied by a new 250m³ Asset International Weholite Pipe Storage Tank (2.5m diameter pipes). Associated manholes, outfall, outfall headwall, concrete access road and hard standing area would complete the arrangement.

McCarthy's Role

Initially McCarthy's were approached by the client to develop a number of methods and solutions to the many difficulties/issues which were present on this site. These included:

• How to install the two number 2.5m diameter Weholite storage tanks 22m long running parallel to each other in variable ground at depths ranging from 5.5m to 7.5m
• How to efficiently deal with the estimated 2,000m³ of material which was to be excavated

Due to the underlying rock strata present at 2.5m to 4.5m depth, traditional sheet piling as shoring was viewed as an unsuitable method, and would have been extremely costly for the size of excavation which was required.

The final solution utilised a variety of techniques. McCarthy's expertise in the use of blockstone was utilised to construct a blockstone retaining wall. This was built up from the Sandstone level and run the extents of the proposed excavation, effectively retaining the more course granular material above the Sandstone.

Above the Blockstone level the ground was suitably battered to provide a safe angle of repose. Below the blockstone the Sandstone was cut/broken out vertically to give the remaining required depth to formation.

In order to efficiently deal with the excavated material, the WRAP protocol was utilised by the Principal Contractor who empowered McCarthy's to carry out the process. This involved the cartaway of the excavated material to a Dwr Cymru/Welsh Water exempt site within 5 miles of the site. At this location McCarthy's utilised a variety of crushing/screening plant to process the material ultimately producing three suitable grades of product.

The granular single size material was utilised for the backfilling of the Asset tank. The Granular graded material was used as the primary backfill material above the Asset Tank and the undersize material was used for the sub soil layer.

In essence using this approach achieved the status of zero waste being produced from the site. Processed material left over from the process has since been utilised for compound construction on several other Dwr Cymru/Welsh Water jobs within the locality.

Success Story

Testament to the success of the scheme as a whole, the site gained commendation from Dwr Cymru/Welsh Water, gaining the monthly team award and narrowly missing out on the overall yearly award coming second. In addition to this the success of the site and processes has been communicated to the wider press through the publication of an article published in 'UK Water Projects 2009'.

Article published in UK Water Projects 2009; 'Steynton Planning for Success'