In the vicinity of the Tabeaud Dam, recently built under the personal supervision of the author, the construction of which will be described later, there is to be found a species of potash mica, which in decomposing yields a yellow clay (being ochre-colored from the presence of iron), mixed with particles of undecomposed mica. This material is subject to expansion, and by reason of its lack of grit and its unctuous character it was rejected or used very sparingly. Analysis of this material gave, Silica, 54.1 to 59.5%; potash, 1.5 to 2.3%; soda, 2.7 to 3.7%.

Soil analysis may be either mechanical or chemical. For purposes of earthwork, we are most interested in the former, having to deal with the physical properties of matter. Chemical analysis, however, will often afford information of great value regarding certain materials entering into the construction of earth dams. The most important physical properties are:

There are two distinct methods of mechanical analysis: (1) Granulating with sieves, having round holes. (2) Elutriating with water, the process being known as silt analysis.

It would require a large volume to present the subject of soil analysis in any way commensurate with its importance. Experiments bearing upon the subjects of imbibition, permeability, capillarity, absorption and evaporation, of different earth materials, are equally interesting and important.[1]

The permeability of soils will be discussed incidentally in connection with certain infiltration experiments to be given later.

Puddle.

Puddle without qualification may be defined as clayey and gravelly earth thoroughly wetted and mixed, having a consistency of stiff mud or mortar. Puddle in which the predominating ingredient of the mixture is pure clay, is called clay puddle. Gravel puddle contains a much higher percentage of grit and gravel than the last-named and yet is supposed to have enough clayey material to bind the matrix together and to fill all the voids in the gravel.

The term earthen concrete may also be applied to this class of material, especially when only a small quantity of water is used in the mixture. These different kinds of puddling materials may be found in natural deposits ready for use, only requiring the addition of the proper amount of water. It is usually necessary, however, to mix, artificially, or combine the different ingredients in order to obtain the right proportions. Some engineers think grinding in a pug-mill absolutely essential to obtain satisfactory results.

Puddle is handled very much as cement concrete, which is so well understood that detailed description is hardly necessary. Instead of tampers, sharp cutting implements are usually employed in putting puddle into place. Trampling with hoofed animals is frequently resorted to, both for the purpose of mixing and compacting.