34 BULLETIN" 555, U. S. DEPARTMENT OF AGRICULTURE. 
surfacing in various sections of the country. The terms sand-clay roads, top- 
soil roads, and semi-gravel roads have come into road literature as designat- 
ing various types of construction that utilize either natural surface soil, or 
subsurface deposits, or artificial mixtures wherein the chief ingredients are 
siliceous sands and natural clays. The abundant distribution of these two soil 
ingredients either separately in deposits or already mixed in many soils and sub- 
strata, has attracted attention and experiment to an extent which now justifies 
some attempt at classification and more definite statement of specifications and 
methods upon which satisfactory results may be predicated. 
The better known and more expensive road surfacing materials have had the 
benefit of long study and detailed specification as a basis for control of con- 
tract work. But much of the large mileage of the types herein considered 
has been constructed by force account and with comparatively recent study 
of the proper bases and considerations out of which systematic specifications 
and methods could be formulated. The prospect of Federal-aid contracts in 
various States makes it very desirable that some such standardization should 
now be attempted as a guide to expenditures in those States where such types 
of construction either now prevail or where conditions indicate their desira- 
bility and efficiency in connection with possible Federal-aid projects. 
The present state of knowledge regarding these materials is derived from 
the comparative study of the road history under traffic of such roads and the 
mechanical analyses of the constituent materials. The data secured indicate 
the necessity of classification and tests based upon a more complete separation 
of the ingredients than is usually given in a laboratory analysis. Special 
definitions of the terms " sand," " silt," and " clay " are required, also a theory 
of their mutual interaction after consolidation. It is believed that the adhesive 
clay, acting as a binder in dry weather and as a stopper to capillary openings 
by swelling in wet weather, must be supplemented by the interlocking strength 
of the sand aggregate both in dry and in wet weather to support the weight 
and the wear of the traffic, while the nonadhesive silt gives aid to density and 
water resistance. The proportions and sizes of the sand content are highly 
important for the strength and wearing element. The clay, owing to its ex- 
pansion under wet conditions, should be present in minimum amount consistent 
with sufficient adhesion in dry weather, and the silt should form the inert 
filler for those voids not occupied by the clay. 
The thorough intermixing of the ingredients is a vital factor and a process 
of consolidation which will pack the loose mass from the bottom upward is of 
much consequence. 
A further conception of the action of this class of materials suggests that the 
fine aggregate passing a 10-mesh sieve may be regarded as a mortar capable of 
being used separately to form the road slab or of carrying any amount of 
gravel or coarse aggregate up to the point where a complete gravel road is 
reached with this mortar as its binder. Experience has shown that the addi- 
tion of coarse material retained on a 10-mesh sieve and less than 2 inches in 
size, provided the coarse material is itself present in graded sizes, increases 
the durability of these soil mixtures. Further, the analysis of the fine material 
passing a 10-mesh sieve in the well-known Augusta (Ga.) gravel, which pos- 
sesses excellent binding value, reveals a composition similar to some of the 
best top-soil roads in that State. There is reason to think that the binders in 
washed gravel roads might very well be compounded on the same basis as 
successful soil road mixtures. 
The successful use of these road soil mixtures, whether obtained on the 
surface, as top soils, from substrata, or from artificial mixtures of clay and 
sand, depends primarily on the composition as to size, amount, quality, and 
