674
MODERN GASWORKS PRACTICE
up the brickwork in short lifts and then to fill in with puddle between the wall and the ground. The puddle may consist of pure clay, but some engineers at one time preferred to introduce a small proportion of sand or rnoiild in order to prevent cracking when the material dries off. As each layer of puddle is fillecL in it must be well moistened and thoroughly “ punned.” It will be noticed from the accom-panying figure that the puddle is spread entirely over the “ dumpling.”
An elementary consideration of the static pressure of water is sufficient to show that the bursting pressure at any section due to the contained water varies in direct
Fig. 417.—Part section of Masonry Tank showing “ Dumpling.”
ratio to the depth of the section below the surface. Thus for determining the pressure due to water at any point the following simple formula may be used :—
P — w d
where P = pressure in Ib. per square foot w = weight of a cubic foot of water d = depth below surface in feet.
In a circular tank the radial thrust ■will, theref ore, gradually increase from the surface of the water to the bottom of the tank ; in other words, the magnitude of the pressure is shown by the line AB (Fig. 418), and the thickness of the tank wall may theoretically be graduated in accordance with this line. Theoretically, no thickness at all is required at the surface of the water, but for reasons too obvious to need explanation such a condition is impossible in practice.
From a consideration of the simple formula previously given, it might be presumed that the calculation of the stresses in a masonry tank, and hence its design, would be a comparatively easy operation. Such, liowever, is by no means the case, for many factors of an uncertain nature (such, as the cohesive resistan.ee of the brick-work) are introduced, with. the result that a purely theoietical method of design is extremely difficult of application. There is no doubt that in the design of a masonry tank some sound theoretical treatment would prove of immense valne, but at present