right-of-way of the road. It has been found that soils of a clayey nature in which there is a considerable percentage of sand usu

s of sand and clay, has led to a fairly comprehensive understanding of the principles of constr

and that at least 45 per cent of clay is required to fill the voids and bind the sand grains together, because the clay spreads the sand grains apart during the mixing, thus having the effect of increasing the voids. As a matter of experiment, it is

of their behavior when combined. Extensive and exhaustive experiments have been conducted with sand-clay mixtures i

ose soils that are properly and technically called clay are decidedly sticky when wet and are the best materials for sand-clay construction. Of the clays, those that produce a tough sticky mud are best. This can be tested by mixing a small quantity into a stiff mud and molding it into a ball and immersing in water. If the ball retains its shape for some little time, it is likely to prove a very satisfactory binder, but, if it becomes plastic and loses its shape

t exist in the vicinity of a proposed improvement, and consequen

centage of gypsum which serves as a binder and is particularly

dobe and called "caliche" may be found and t

ities. These mixtures are commonly referred to as top-soil. If the deposits are somewhat deficient either in sand or clay, they can be utilized if the proper corrections in the proportions are made during construction. Ver

ds either by adding clay and mixing it with the sand to secure the desired composition, or a la

1/3 clay and 2/3 sand. The mixing is accomplished in various ways, the most common being to use a heavy plow at first and to follow this with a heavy disc harrow. The mixing is a tedious and disagreeable process, but its thorough accomplishment is indispensable. The mixing is most readily done when the materials are saturated with water and in practice it is customary to depend upon rain for the water, although in the final st

Sections for

of considerable thickness and generally the surface layer is made much thicker than for regions where the annual rainfall is fairly well distributed. Thi

e deficient in either clay or sand and the mixture in these places is

sure uniformity. If either sand or clay is needed to give a satisfactory mixture, the proper material is a

constructed of a natural sand-clay mixture or of a sand mixed with the natural soil. If the former, the surface of the existing road is prepared

adually mix with the soil as the road is used. A second application of sand may follow in a year or two if it is needed. Such a road surfa

s have maximum serviceability when moist, not wet, and consequently are not as durable in dry climates as in humid areas. They are likely to become sticky and unstable in continu

Road

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nsist of pieces of rock varying in size from those of a cubic yard or more in volume to the finest stone dust, but with pieces ranging in size from that which will pass a 3-inch ring down to fine sand predominating. The larger pieces are usually more or less rounded and the finer particles may be rounded or may be angular. Many varie

taining a few pebbles, but which are of value to the road builder for the sand clay type of surfacing. The term gravel is

bl

roperties of Severa

Cent

t Cement

eived

27

10

24

500

50

300

500

18

500

18

50

21

11

s of an ideal gravel for road surfacing. Ideal gravel is seldom encountered, but a consideration of it

he pebbles are of a variety of rock that is highly resistant to wear so that the road surface made from the gravel will have the quality of durability. The gravel possesses good cementing properties, insuring that the pieces will hold together in the road surface. The cementing pr

ck of medium toughness, the smaller pebbles might be crushed under the heavier loads. It is the usual practice to permit gravel to be used for the foundation course in which the pebbles are as large as will pass a 3?-inch circular screen opening, and for the wearing course, as large as will pass a 2?-inch

very large mileage of excellent gravel roads and

sist of a mixture of gravel, sand and clay wit

n; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate to be retained on a one-inch screen;

of a mixture of gravel, sand and clay with th

ot more than seventy-five per cent of the total coarse aggregate (material over one-fourth inch in size) to be retained on a one-half-inch screen; at least six

ral state, screened gravel shall be used and the necessary sand and clay binder added as directed by the engineer. Gravel and sand

pass all the material through a two-inch screen for the bottom course, and through a one-inch screen for the top course. When the work consists of only one course, the material shall be of the sizes a

e rubber tired traffic predominates, this action is much less severe than where steel tired vehicles predominate, but the tendency exists on all roads. In additio

consequently, little impact on the surface. When somewhat worn, the impact becomes a factor of some importance and the poundi

od wearing properties, that quality will be imparted to the road surface. If mostly from rocks of little durability, the same characteristic will be imparted to the road surface. A very good g

l that can be utilized, or at any rate a supply that need not be shipped a long distance. In the nature of things, such deposits are likely to be deficient in some of t

duce a reasonably uniform product. If deficient in clay, it often proves feasible to add a small part of the clay over-burden, thereby insuring enough binder. Sometimes adjoining deposits will consist one of relatively fine material, the other of

and become a source of annoyance to the users of the road. Oversize stone can be removed while loading the gravel or while spreading it, if care is exercised and not too large a proportion is oversize. It is

den to give enough additional binder or where the overburden is of a material unsuitable for binder. Such m

other characteristics. The serviceability of a gravel road will depend largely on how nearly the gravel ap

carrying heavy truck traffic, but if gross loads of three or four tons are the heaviest anticipated, the gravel will be reasonably stable. On such roads, a layer of well compacted gravel ten inches thick will support the loads if a well drained earth foun

arket traffic, and not a great volume of that, the

or special study on account of the variations in the gravels and

ing

f this could be done a year before the gravel is placed so that no settlement of the earth foundation would occur after the gravel surface is completed. Bu

trench method and the surface or feather edge method respectively. The method to adopt for

nd depth for receiving the gravel is excavated in th

se material is generally moved out laterally to build up earth berms or "shoulders" alongside the gravel. Into

avel has been deposited in the trench for a distance of a thousand feet or more, the spreading is accomplished by dragging the surface repeatedly with the blade grader, the work being continued until all waviness disappears. The gravel

ighing about 8 tons and must be done while the gravel is wet. Generally a sprinkling wagon is used to wet down the gravel, but advantage is always taken of rains to facilitate the work. The gravel must be spread in layer

ns. Then it will begin to pack, but will become badly rutted and uneven during the process. During this period the surface must be kept smoo

placed, to the side ditches, will relieve this condition by affording an outlet for the surplus water. Nevertheless some difficulty may be expected if the trench method is used and wet weather prevails. If it is possible to close the road against traffic until the road is dry the method is applicable. M

ducted in the same general manner as for the trench method. The gravel does not compact as quickly as in the trench method and a considerable loss of material is likely to result from the effect of automobile traffic while the gravel is loose. But it has the advantage of being free from diffi

er and sprinkling and rolling until the clay squeezes up through the surface layer. It may also be accomplished by spreading dry clay on the upper course before it is harrowed and then harrowing to mix it with the gravel. Both

se disturbed by traffic. It is important during this period to restore the shape once a week or at least twice a month. The light blade grader is usually employed for the purpose so long as t

6.-Roa