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(13) In the case of the Tytan Dam, the late Sir Robert Rawlinson K.C.B., directed numerous small drains to be formed in the mass of the concrete behind the inner lining of masonry intended to be water-tight, in order that should any water find its way through the said lining it would flow freely away, and not accumulate in any fissure or cavity, causing an upward hydrostatic pressure, tending to reduce the stability of the structure. I consider that this would be a wise precaution.
In the case of Tytan it was not however adopted, because those in charge of construction thought that the concrete would be sufficiently porous, to prevent any such prejudicial accumulation.
As a matter of fact, the Tytan dam leaks considerably through the mass of the concrete. I maintain that concrete should be impervious, the interstices between the stones being completely filled with mortar. Percolation through concrete tends to its disintegration.
Mr. Mansergh informs me, that in the case of the Birmingham Dams, he is not draining the whole mass of the concrete, in the manner recommended by the late Sir Robert Rawlinson, but he is taking every precaution to render the whole mass absolutely homogeneous and impervious.
The only drains which he introduces, are to remove the land springs, in the excavations for the foundations, and he has also provided a single drain, at a low level, below the whole length of the Dam, to remove any water which might by chance percolate through the water phase, below the foundations, and this drain communicates with the out-let culvert.
(14) The surface of the drainage area should be as little disturbed as possible, the natural growth of herbage
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