Pembroke CCGT Power Station

Friday 30 April 2010

Challenge
RWE npower needed to build a new 2000 megawatt combined cycle gas turbine (CCGT) power station at Pembroke, Wales on a site previously occupied by a former oil-fired power station. The new CCGT plant would be constructed on the former power station site and the main buildings would be located on the low lying ground to the south west of the former oil-fired power station.

Published Geology
Based on information given at the time of tender, it was understood that to the western end of the site, a 4 to 14 metre deep cutting was excavated in the 1960s to provide fill material for the old power station. The proposed CCGT Power Station would be constructed at the northern end of this “bowl cutting”, with associated works expected to pass through the site of the former power station.

The site was situated on the northern arm of an east-west trending syncline, which plunges gently to the east. The south and south east of the site is underlain by the Carboniferous Limestone situated in the core of the syncline. The central and northern areas of the site are shown to be underlain by older Lower Limestone Shales of Carboniferous age, (comprising dark grey mudstones and siltstones, with sub-ordinate limestone beds).

Underlying the Carboniferous bedrock, Old Red Sandstone (Skrinkle Sandstones) of Devonian age is shown to outcrop at the most northerly part of the site. The dip of the beds were indicated to be between 30 and 60 degrees to the south. No superficial deposits were shown on the geological maps; however Alluvium was anticipated to be encountered to the east of the site.

Based on the previous history of the site and information given by Halcrow Made Ground was anticipated to a maximum depth of up to 10 metres below existing ground level. This fill was derived from material excavated from the “ bowl cutting” situated to the west. Outside the old power station site, a shallower thickness of less than 3 metres of Made Ground was expected.

Encountered Geology
The strata encountered varied from that which was anticipated. The difficulty was the variation in weathering and lithology changes within the Carboniferous Shales and the apparent lack of alluvial deposits. In the northern part of the site competent bedrock was encountered within a trial pit excavation, less than two metres below existing ground level. Whereas approximately one hundred metres to the south west a cable percussion borehole was drilled to a depth of over 26 metres. The strata varied from firm to stiff clays to moderately strong and strong siltstone. The variation in strength was not depth related and therefore not easily anticipated across the site. The geology was further complicated by the presence of strong and very strong limestone bedrock overlain by and interbedded with completely weathered bedrock comprising clay with large limestone cobbles and boulders.

The disparity in the strata resulted in difficulties progressing both the cable percussion boring and rotary drilling. It also proved a major challenge with regards to the sample quality and recovery. The in situ pressuremeter and dilatometer testing was also greatly affected. At the time of the investigation the existence of the weathering and lithological variations were not wholly anticipated. Based on supplementary information supplied by Halcrow during the works, the deep variable weathering was generally the result of leaching caused by glacial melt waters.

It was understood that several turbines with dynamic loading would be incorporated into the new power plant, yet the original design model had been built on the premise of encountering competent bedrock rather than deeply weathered strata. Utilising this site would therefore require greater certainty with regard to the geotechnical characteristics of the superficial and solid geology.

Results
The final fieldwork was significantly different to that originally scoped and additional drilling was undertaken. However the whole investigation was successfully completed on time and to the satisfaction of the client. Following the fieldwork, geotechnical and geoenvironmental laboratory testing and factual report, Soil Mechanics was able to jointly conclude with RWE npower some revealing findings, not least the discovery of strata not previously encountered in that area. Both parties agreed that the investigation had exposed some vital geotechnical information. These finding helped to effectively manage the risk and value of the project including:-

• The cost/risk implications in the early phased project
• Helping RWE npower define total cost of project.
• RWE npower's selection of design options

Solution
In order to support RWE npower's need to fully understand the complex ground conditions and to achieve a quick turnaround, it was evident from the start that several aspects of Soil Mechanics' business would need to pull together to achieve this demanding and high profile ground investigation.

In consultation with Halcrow, the drilling and in situ testing methodologies were continually adjusted on a day to day basis in order to achieve the optimum amount of quality information for the client.  " Soil Mechanics were set a tough challenge by RWE npower and met it full on. From the engineer's perspective, it was evident from the start that many parts of the business would need to pull together to achieve the client's demanding programme. Pembroke in November was never going to be easy, but staff on site, in Bridgend and in the labs demonstrated a team commitment to match and on occasions exceed the client's expectations." - Halcrow

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