The Thirlmere water scheme of the Manchester Corporation : with a few remarks on the Longdendale Works, and water-supply generally / [James Mansergh].

  • Mansergh, James
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: The Thirlmere water scheme of the Manchester Corporation : with a few remarks on the Longdendale Works, and water-supply generally / [James Mansergh]. Source: Wellcome Collection.

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    a single sheet covering 24 square miles, and you will at once understand how the work of the engineer is facilitated by these magnificent maps. It is to be regretted that this survey is not more rapidly prose- cuted and published throughout the whole country. So far the reservoir site has been chosen with reference only to the outward and visible surface formation. Next must be ascertained the geological fitness of the site, for upon this depends the amount of the original outlay in construction and the utility of the reservoir when finished. It would be useless to make a large embankment across a valley if there were faults and fissures in the strata that would allow the water to escape. There have been instances of reservoirs being abandoned be- cause they would not hold water, especially in the “ coal measures.” Then, again, the exact spot upon which the embankment is to be founded must be carefully explored, by sinking trial holes or bdrings all the way across the valley. For you must understand that the bank is not simply a great mound of earth planted upon the surface soil; but it must be rooted as it were into the solid ground. For this purpose a trench is cut right across from side to side through the alluvium which more or less is found in such situations, and is carried down until it reaches some watertight stratum, which in all probability forms the true base of the valley and is con- tinuous throughout the whole area to be submerged in the intended reservoir. In a “ millstone grit ” district like the Longdendale, the rocks are very much disturbed and dislocated, and in some of these embankments the trenches have had to be excavated to very great depths before a watertight layer of shale has been reached. At Woodhead the cutting was between 50 and 60 feet below the bed of the brook, and as the same level had practically to be main- tained right into the sides of the valley, the total extreme depth became as much as 160 feet. Into this trench, when excavated, was filled an immense wTall of blue-lias concrete, tied down into the watertight shale all across the valley, so as to complete the impervious basin forming the reservoir. In valleys which have been overlaid thickly by glacial drift, such