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PEAK PERFORMANCE

SITE INVESTIGATION

Falling North Sea gas reserves mean a major new UK gas pipeline must be built. But Comprehensive SI is no mean feat in the rough terrain of Wales.

The UK has been self sufficient in gas thanks largely to abundant supplies from North Sea fields. But offshore gas production is decreasing at the same time as demand for gas is rising and analysis indicates the UK will become increasingly dependent on imported gas.

To meet the demand, two new terminals to import liquefied natural gas (LNG) are being built at Milford Haven in South Wales.

Connecting new supplies to the national gas transmission system to ensure that it is safely and economically delivered to homes and industry is the responsibility of National Grid. The company is therefore planning to construct a new 1.22m diameter high pressure gas transmission pipeline from Felindre, near Swansea, to a point near Tirley (between Newent and Tewkesbury in Gloucestershire), in addition to the Milford Haven to Aberdulais section, currently under construction.

When completed in 2007, the new pipeline will have the capacity to carry 20% of the UK's natural gas supply. As such, the whole project is of vital strategic importance for the future of the UK economy.

Murphy Pipelines won a contract from National Grid to build a 107km long section of the pipeline from Brecon to Tirley and Stats is contracted to provide specialist technical services to the project. This initially involved the design, procurement, management and supervision of the proposed route's ground and groundwater investigation, post-investigation analysis of the collected data and the submission of appropriate recommendations with respect to design, construction and the final choice of route.

The project has been active for nearly a year and has a value to Stats of almost £1M. The next phase of investigation and analysis is likely to take the project well into 2007 and it is anticipated that work will be ongoing until the 'gas on' date of October 2007.

The main ground investigation provided considerable challenges due to the often remote locations and the rugged terrain in the foothills of the Wales' Brecon Beacons national park. Due to the environmentally sensitive location, National Grid was particularly concerned that this phase of work was carried out to the highest standards with minimal impact on the land, both during and after drilling.

Stats therefore elected to use crawler mounted Pioneer drilling rigs provided by Geotechnical Engineering. These rigs are clean, modern, highly mobile and ideally suited to the soil profile.

Ground conditions along the route generally comprised a veneer of glacial till overlying weathered rock, with competent rock, usually Devonian sandstone and mudstone, at shallow depth. River alluvium and more extensive glacial deposits were encountered along the river valleys.

Cable percussion techniques would have refused on rock head while rotary coring often results in poor recovery of the superficial deposits. The Pioneer rig is capable of dynamic soil sampling in the superficial deposits to obtain near undisturbed continuous samples up to 112mm diameter retained in a plastic liner. On encountering rockhead, the rig can readily switch to rotary coring mode using a water flush, recovering high quality cores at up to 116mm diameter.

A total of 221 boreholes have been drilled to date, with a further phase of investigation about to begin. However, even this represents a relatively low frequency per km and in some of the remoter areas even the Pioneer rigs could not gain access in the time available as they would have been tracking in for several days.

To overcome this problem, Stats' in-house geophysics team developed the concept of 'virtual trial pits' to investigate the soil characteristics and rock head profile beneath localities inaccessible or inappropriate for intrusive surveys.

The virtual trial pits involved using the seismic refraction technique.

This is based on the propagation of seismic waves through the subsurface and their refraction at interfaces across which there is a sufficient increased contrast in acoustic velocity.

Analysis of the time taken for the seismic energy to travel from the shot point to an array of distances from the source can provide information on the geometry, depth and elastic properties of subsurface materials, which can be related to geology.

Site workers used this technique at 17 locations, deploying two orthogonal spreads of geophones, each 48m long. After processing, the resulting data allowed imaging to a depth of up to 5m into bedrock.

Check drilling at two locations confi rmed the ability of the virtual trial pits to identify rock head to an accuracy of +/-50mm.

Geophysical techniques such as electromagnetics and magnetics have also been used to define the extent of features such as areas of landfill in addition to underground tunnels and shafts associated with the former Hereford to Gloucester Canal at Oxenhall.

'Although we have used innovative and unusual approaches to gathering ground investigation data, we have tried to not lose sight of the use to which the information will be put, ' says Stats project manager, Jon Bassett, commenting on the main site investigation works.

'The site based team worked closely with the designers, construction crews and the quantity surveyors to develop new ways to make the borehole and laboratory test data timely, relevant and meaningful.

He explains that one outcome of this was that the interpretative report had very little text. 'Most of the analysis of the engineering properties of the soils, rocks and groundwater data and our assessment of construction issues such as trench stability, trafficability etc were presented as a series of 'traffic light' colour coded bars along the base of A3 size long sections, ' he says.

'The non-specialist members of the project team can easily identify potentially problematic areas and an intuitive cross referencing system provides simple access to greater detail within the factual report. We have been pleased to discover that these books of long sections have become widely used throughout the project team and that the quantity surveyors in particular, have enthusiastically welcomed this approach.'

The pipeline route will also cross an aquifer, six rivers and more than 30 ditches, and Stats has provided an assessment of the likely affects on surface groundwater resources, including od risk assessments wherever the route crosses the odplain. The aquifer was of particular concern and it is critical that pipeline construction does not affect water quality or yield.

Following a detailed SI, Stats commissioned a British Geological Survey (BGS) team to construct a numerical groundwater ow model. They used a state of the art, object oriented ZOOMQ3D package, jointly developed by the BGS, Environment Agency and Birmingham University.

After running numerous 'what if-' scenarios the modelling has predicted that dewatering during construction will have a small but acceptable impact on the aquifer and that any turbidity will be lost during the 100 day travel time. The impact of the completed pipeline on the groundwater ow and groundwater heads has also been shown to be minimal. The conclusions from the modelling work are due to be tested during a full scale well point dewatering test early in 2007.

A mining and quarrying hazard assessment has been undertaken to provide a basis for evaluating the presence and likely zones of in uence from historic, current and planned mining, together with an assessment of potentially economic mineral reserves that could be mined in the future.

This assessment has involved both desk studies and ground investigation. The work has aided the creation of a generic model of geological and related potential mineral resources versus historical, contemporary and possible future mineral extraction patterns. From this, a pipeline route specic matrix of potential mineral resources versus generic, and the more detailed, regionally customised methods of extraction have been developed for all of the possible types of mineral resources present.

All forms of hazard were assessed including the traditional hazards such as ground subsidence, void migration and shaft collapse, as well as less common hazards such as gas emissions, aggressive mine waters and biological hazards from diseased animals and waste.

The resulting data set has been assessed using a qualitative risk assessment approach with four risk categories, very low to high, assigned to each actual or potential occurrence. For each moderate to high risk rating, mitigation measures have been proposed. These range from complete excavation and recompaction of loose backll to a shallow former stone quarry, to in-trench clay barriers where the pipeline passes close to an animal burial site.

Stats did a landslide hazard assessment to consider the pipeline corridor for areas of potential slope instability. The initial screening process used an advanced geographical information system (GIS) based approach to integrate LIDAR digital elevation data with BGS digital geology maps and air photographs.

Maximum safe slope angles were allocated to each geological unit and where the LIDAR data indicated that these angles had been exceeded, further investigation was done. The GIS output comprised slope angle summary maps with colour coded zoning of slope angle.

Supervising Stats director, Shon Williams, says: 'The approach we used, while common in the US, is practically unique in the UK. The high-tech GIS based analysis saved weeks of conventional walkover survey work, but when it came down to it, the only way to properly investigate the high risk areas identi ed by the GIS was to get our boots on.'

Stats is planning further investigation works which will incorporate more innovative use of technology. This will include down hole optical and acoustic scanning to provide high resolution, true colour images of the borehole wall that will be used for detecting bedding, fractures and discontinuities. This data can be automatically analysed to provide computer generated rose diagrams and stereonet depictions of dip and dip azimuth, in conjunction with conventional core logging and more detailed slope stability analysis.

Stats director Paul Stearns says: 'This has been a fascinating project.

The integrated team approach favoured by Murphy allows specialist consultants to work closely alongside the commercial, design and construction teams so that the information we provide is accurate, timely, and relevant.

'Our biggest challenge has been to nd the resources to manage the project and the unsung heroes have been the eld engineers who have worked unbelievably hard over many months to produce top quality ground investigation data, without which the specialist expert reports could not have been produced.'

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