40 BULLETIN 463, U; S. DEPARTMENT OP AGRICULTURE. 



determined for each sample prior to the time of making the slaking 

 test, so that the percentage may be corrected by adding sand or clay. 

 This test may be employed also to compare the slaking qualities of 

 the same clay when mixed with different percentages of sand, and 

 when so employed may afford a valuable guide in fixing the proper 

 proportions for an artificial sand-clay mixture. In making the test 

 several balls should be made and tested from each kind of clay, so 

 as to afford check results. Also, in all the tests care must be exercised 

 to keep the different samples from becoming confused after the balls 

 are placed in the water. To do this, identifying marks may be placed 

 upon the balls, or the balls from each sample may be placed in a 

 different part of the containing vessel. 



(3) The amount which clay shrinks on drying out also may be a 

 measure of its fitness for use in a sand-clay surface. The shrinkage 

 may be tested by observing the behavior of the clay balls while they 

 are being dried out, when those which shrink most will develop most 

 cracks. Other things being equal, the sand-clay mixture showing the 

 smallest shrinkage is best suited for use in a road surface. 



(4) Valuable comparative information regarding different avail- 

 able clays or sand-clay mixtures may be gained sometimes by simply 

 observing exposed surfaces, where the materials outcrop, under 

 different weather conditions. The best claj^s will be capable of stand- 

 ing on relatively steep slopes and will develop fewer surface cracks 

 upon drying out after a wet season. They also will be little affected 

 by frost and will appear dense and firm in dry weather. 



(5) To test the suitability of sand for use in a sand-clay surface, 

 place a sample of the sand in a vessel containing water and agitate 

 the water until the sand is thoroughly in suspension. Then, after 

 the sand has been allowed a few moments to settle, pour off the water 

 slowly. If of good quality, the sand will not be carried out with 

 the water but will remain in the vessel until practically all of the 

 water has been drained off. Sand containing a large percentage of 

 mica or other light mineral matter will not meet this test and is not 

 generally suitable for use. 



(6) The methods described above for testing sand-clay materials 

 are based on the assumption that the selection must be restricted to 

 materials locally available and are intended simply to aid in com- 

 paring the relative merits of such materials. But these methods 

 can, of course, be adapted to more exact laboratory practice if so 

 desired. 1 



1 A rather detailed plan for laboratory procedure, which has been followed to some 

 extent in connection with the sand-clay roads of Georgia, is described by John C. Koch 

 in the " Transactions of the American Society of Civil Engineers," Vol. LXXVII, Decem- 

 ber, 1914, p. 1454. 



