CO129-204 - Acting Governor Marsh - 1882 [11-12] — Page 371

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366

12.

13.

the centre or

hearting of the concrete base there is no

-objection to the employment of the large rough blocks of Granite which may expected to be tipped out at the mouth of the tunnel works. These blocks

may

be as large

be

as

can be conveniently curried by coolies and if thrown into the middle of the concrete will very

greatly econornize coment, but due care must be taken that all such blocks are thoroughly well imbedded, surrounded and covered in a complete matrice of the smaller concreter

This use of large rough and irregular shaped blocks of granite has been adopted by the Engineer to the Harbour Works of the Port of Dublin, with great- economy and perfect efficiency.

The Concrete substructure being finished off at the top in steps. as shown in Drawing N 2 the casings and other limbering may be removed, and the spaces that have been left in the trench; between the sides of the latter and the perpendicular sides of the concrete mass, may now be filled in with layers of concrete until the level of the ground is reached. In order to economize concrete therefore it will be important that the excavation of the trench for the foundations should not be wider than is absolutely requisite, for every foot of unnecessary width will entail so many more cube feet of Concrete to fill up the void. This filling in will not require concrete of so rich. a character as for the main dam.

I should remark with regard to the concrete foundation. for the dam. that it is not désirable to expedite the work unduly . The slower the progress, the greater will be the opportunity afforded to the concrete to harden. The superposition of fresh layers of concrete over great masses that are yet imperfectly set may tend to retard the induration of the latter.