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Geology

Geology in the North West ranges from soft deep alluvial deposits and peats on the Lancashire coast to the fertile plains of Mercia Mudstone, through the rolling hills of the Central Lancashire Coal Measures to the upland Millstone Grit moors and the drier limestone Yorkshire Dales, before reaching the steep almost Alpine terrain of much harder older igneous and metaphoric rocks of the Lake District.

All of this has been given a heavy dose of glacial activity. There is a liberal spread of glacial till (or boulder clay) and this generally provides good ground for foundations.

However the problem with glacial materials is that they can be extremely variable over a small plan area. A particular difficulty can be localised pockets of soft ground and peat. These were formed as depressions (kettleholes) in the boulder clay left by melting ice, and in the rural environment these became small ponds.

Over time these became naturally silted up with soft organic deposits, and these swampy areas were often further tilled in by farmers, making them invisible.

But perhaps most significant of all from the geotechnical engineer's point of view is the legacy of man, particularly in the central Coal Measures area which was, of course, the heart of the industrial revolution.

Man has been extremely busy underground in the Lancashire Coal Field which forms the 'heart' and urban core of the North West. There can be over 30 coal seams in some areas, such as, Wigan, and thickness of many metres have been evacuated over wide areas. However, as mining has now generally ceased, the geotechnical concentration is on the problems of investigating and stabilising shallow mine- workings, mineshafts and adits.

The centre of the British salt industry is Cheshire, and historical pumping of 'wild' brine had given rise to some spectacular subsidence, photo- graphs of which often grace piling contractors' calendars.

Numerous old quarries also exist, with good sandstone being removed for building stone and mudstones and shales for producing bricks and pipes. A particular difficulty is that many of the smaller activities are not well chartered and are difficult to find. And whatever is dug out tends to be filled back in again, often with unsavoury household waste where the problems of gas generation, gas migration and leachate need to be resolved.

Over recent decades, as the traditionally strong manufacturing base in the North West has dwindled, factories and mills have closed and released land for redevelopment. After decades and in some cases centuries of heavy industrial use, there is plenty of work for the environmental side of the geotechnical industry to investigate and remedy these problems and bring the site back into beneficial use.

Elsewhere natural difficulties can be soft compressible peats and alluvial deposits in the coastal areas where piling is usually required, and on the Pennine valley fringes landslips can be a difficulty.

Thanks to the notoriously wet weather, tree root shrinkage effects on clay soils do not seem to be much of problem in this area.

There are plenty of interesting tasks facing the geotechnical industry, but the sector is a professional and thriving and readily rises to the challenge.

Graham Cannon, Worms Eye Geotechnical, Burnley

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