MICROSCOPE

A good microscope with micrometer and micrometer stage is useful. Low powers and long focal distances are best. A good hand lens is also useful. The appearance of the particles of sand as a whole and in the portions on the various sieves will frequently lead to the discovery of conditions that might not otherwise be suspected. The micrometer gives a control of the sizes and is especially useful in examining very fine materials. It should be remembered that the width of grain and not its length, as seen in the microscope approximates the diameter of a sphere of equal volume. The width is usually about three-quarters of the length and the thickness or third dimension will average one-half the length.

STATING THE RESULTS

The effective size is that size such that 10 per cent of the sand grains per weight are finer than it. The size of sand grain is always taken as the diameter of a sphere of equal volume. The results obtained in the first Lawrence experimental filters indicated that the finest 10 per cent per weight of particles had as much influence upon the action of the materials in filtration as the coarser 90 per cent. This is explained by the fact that in a mixed material containing particles of various sizes the water is forced to go around the larger particles and through the finest portions which occupy the intervening spaces and so it is this finest portion which mainly determines frictional resistance and filtering capacity.

Another important matter to be determined is the degree of uniformity in the sand grains. If the particles are mainly of the same size the properties of the sand will not be the same as if there were a great range in their diameters. The degree of mixing is indicated by the uniformity coefficient which designates the ratio between the effective size and that size such that 60 per cent of the sand is finer than it.

To facilitate ascertaining these values the 10 per cent and 60 per cent lines on special analysis sheets are printed heavier than others. The points of intersection are noted and the ratio computed.

BASIS FOR EFFECTIVE SIZE STATEMENT

The filtering materials for which data were available at Lawrence when the method was adopted comprised sands with a wide range in uniformity coefficients. Data for frictional resistance, capillarity, etc., for these sands were available. Comparative studies showed that if 8 per cent was taken as the limit for computing the effective size instead of 10 per cent, an unduly 'great weight was given to the effect of the finer particles upon the physical properties of the material; while if 12 per cent were taken, the effect of the finer particles was inadequately represented. The Lawrence data, although not numerous, were clear upon this point, and the 10 per cent limit was selected because it brought all the materials for which data were then available, whatever their uniformity coefficients, into their correct relative positions with respect to frictional resistance and capillarity.

There is no reason to suppose that 10 per cent is a precise mathematical limit always true of all materials. Nevertheless, practical experience extending over 35 years, and to many thousand samples of sand, and the construction of hundreds of filters, and the collection of large amounts of additional data on physical properties, has failed to show any way in which the original expression could be improved in a substantial manner.

EFFECTIVE SIZE AS DEFINED BY SLICHTER

This is mentioned simply to guard against confusion. C. S. Slichter developed a method of studying sand that was entirely different from the analysis now described.* This was published seven years after the Lawrence paper, which contained the definition now used. Unfortunately Slichter also used the term "effective size," but with a different definition. References to Slichter's work are often met. No discussion of it is here presented, but the caution is given that effective size as defined by Slichter is different, and his formulas must not be used with effective size as determined by the method here described.

SAND ANALYSIS FOR CONCRETE AND OTHER PURPOSES

The methods of analysis here described may be employed for sand to be used for concrete or for any other purpose. The terms effective size and the uniformity coefficient relate to the properties of sand for filtering purposes, and other designations of sand qualities may be better for other use.

The Committee on Soils for the American Society of Civil Engineers suggested the following terms descriptive of sand for other uses. The values can be taken from analyses and plottings made as described and call for no variation in procedure.

The mid-grain size is that size such that 50 per cent by weight of the sample is finer than it. This is the most generally applicable term for describing the grain size of granular material'.

The term "mid-grain size" is of first importance, but it does not give all the information that is needed regarding a granular material. It remains to be stated whether the particles have a small or a wide range in size. The range may be stated in terms to include 80 per cent of the particles. Ten per cent of the particles will be smaller than the smallest limit, and another 10 per cent will be larger than the larger limit.

The sizes of material that correspond to the various limits may be taken from a mass diagram. It is necessary to plot the figures for each analysis. Doing this has the advantage of furnishing a check upon the work; for marked irregularity in plotted line will tend to attract attention to an erroneous result and lead to its immediate correction.

The range ratio is the ratio between the limits as above described. It can be obtained directly by dividing the larger figure by the smaller. It will be useful in many comparative studies. The range ratio is a term that

* Motion of Ground Water. United States Geological Survey, 1899.

is parallel with the uniformity coefficient used in the analysis of filter sand. As it is a parallel term only one should be used. The uniformity coefficient is approximately equal to the 0.6 power of the range ratio.

A complete description of the grain size of a common sand for concrete making may thus be stated:

Mid-grain size..
Range..

Range Ratio.

0.79 mm.

.0.33 to 1.90 mm. 5.8

ANALYSIS OF REPRESENTATIVE SAND SAMPLES

The following table contains analyses of representative samples (not necessarily average) from filter plants, in sufficient number to give a fair idea of the usual ranges in slow and rapid sand filters.

STANDARD SPECIFICATIONS FOR CAST IRON PIPE AND SPECIAL

CASTINGS*

DESCRIPTION OF PIPES

SECTION 1. The pipe'shall be made with hub and spigot joints and shall accurately conform to the dimensions given in tables 1 and 2. They shall be straight and shall be true circles in section, with their inner and outer surfaces concentric, and shall be of the specified dimensions in outside diameter. They shall be at least 12 feet in length, exclusive of socket.

Pipes with thickness and weight intermediate between the classes in table 2 shall be made of the same outside diameter as the next heavier class. Pipes with thickness and weight less than shown by table 2 shall be made of the same outside diameter as the Class A pipe; and pipes with thickness and weight more than shown by table 2 shall be made of the same outside diameter as the Class D pipe.

All pipes having the same outside diameter shall have the same inside diameter at both ends. The inside diameter of the lighter pipes of each standard outside diameter shall be gradually increased for a distance of about 6 inches from each end of the pipe so as to obtain the required standard thickness and weight for each size and class of pipe.

For pipes of each size from 4- to 24-inch inclusive, there shall be two standards of outside diameter, and for pipes from 30- to 60-inch inclusive, there shall be four standards of outside diameter, as shown by table 1.

For pipes 4- to 12-inch inclusive, one class of special castings shall be furnished, made from Class D pattern. Those having spigot ends shall have outside diameters of spigot ends midway between the two standards of outside diameter as shown by table 1, and shall be tapered back for a distance of 6 inches.

For pipes from 14- to 24-inch inclusive, two classes of special castings shall be furnished; Class B special castings with Classes A and B pipes, and

These specifications were adopted by the Association on May 12, 1908. They are in general use and are often included in local specifications, without being reprinted, by the use of the phrase, "the cast iron pipe shall meet the requirements of the Standard Specifications of the American Water Works Association for such pipe."

At this time (1925) some experienced engineers believe it to be desirable to use a higher grade of metal than those specifications call for. There is also a growing belief that better pipe coatings should be required. These opinions are being considered by the Association's Committee on Standard Specifications for Cast Iron Pipe and Specials.

Changes in the art of casting and coating pipe are progressing at a rate which indicates that in a short time specifications must be standardized for centrifugal cast pipe and cement linings applied by the centrifugal process. Standardization must wait, however, for more definite and comprehensive knowledge of the capabilities and limitations of these methods than is had now.

Where it is proposed to use Universal, McWane or other proprietary forms of cast iron pipe, the standard specifications must be modified so as to substitute descriptions of their special features for the corresponding features of standard American Water Works Association Pipe.