Part 7 (1/2)
An ideal, or even a fairly satisfactory soil for a binder may not exist in the vicinity of a proposed improvement, and consequently an inferior binder is frequently the only ravel contain a considerable percentage of gypsum which serves as a binder and is particularly effective when used in coravel
In many places a soil of the type used for adobe and called ”caliche”
ravel
=Top-Soil or Natural Mixtures=--Deposits consisting of a natural redients happen to exist in about the correct relative proportions for sand-clay road surfaces are found in many localities 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 areconstruction Very satisfactory road surfaces are sometimes constructed with mixtures that appear to be far from ideal in composition, but experience and frequent trials are needed to determine the best way in which to handle these mixtures
=Sand-Clay Surfaces on Sandy Roads=--Sand-clay surfacesclay andit with the sand to secure the desired composition, or a layer of a natural sand-clay ypsum may be placed on top of the sand
The most widely used method is to mix clay or other binder with the sand Since there is no need to provide for ditches to carry storraded off nearly flat across the road and no ditches are provided The clay is dumped on the road in a layer about 8 inches thick and is then h sand with the clay to produce a mixture co is acco to use a heavy plow at first and to follow this with a heavy disc harrow The h acco is most readily done when the materials are saturated ater and in practice it is custoh in the final stages water may be hauled and sprinkled on the road to facilitate final co has been corader and is kept s will be required, during the first year especially, and to some extent each year in order to keep the surface s can be successfully accomplished only when the road is wet
[Illustration: Fig 14--Cross Sections for Sand-Clay Roads]
In regions where several months of continued hot, dry weather is to be expected each year, the sand-clay h unless it is of considerable thickness and generally the surface layer is ions where the annual rainfall is fairly well distributed This is especially necessary when the binder is of inferior quality It is not uncommon in such cases to make the sand-clay surface as resses it may appear that patches here and there are deficient in either clay or sand and the mixture in these places is corrected by the addition of a little sand or clay as may be required
If the top-soil is used it is deposited on the sand in the required quantity and is remixed in place to insure uniforive a satisfactory mixture, the proper resses The surface is finally s
=Sand-Clay on Clay or Loae will be required as discussed in Chapter IV The surface may be constructed of a natural sand-clay mixture or of a sand mixed with the natural soil If the for so as to insure good drainage and the natural mixture is then deposited and the surface co section
If the surface is for soil, the sands may be deposited in a layer about six inches thick which will gradually 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 surface will lack uniformity of composition and it see and discing as previously described
=Characteristics=--Sand-clay road surfaces do not have sufficient durability for heavily traveled highways, but will be satisfactory for a moderate amount of traffic These surfaces 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 continued eather and to beco continued dry weather At their best, they are dustless, somewhat resilient and of low tractive resistance
GRAVEL ROAD SURFACES
[Illustration: Fig 15--Cross Sections for Gravel Highways]
=Natural Gravel=--Gravel is the na of a mixture of more or less rounded stones, sand and earthy material, which is found in natural deposits These deposits exist in al especially nulaciated areas, but by no means confined to the in size from those of a cubic yard or ing in size fro down to fine sand predoer pieces are usually more or less rounded and the finer particles ular Many varieties of rocks are to be found ain and possessing a considerable degree of hardness generally predominate Intermixed with the pieces of rock there is likely to be clay or other soil, the quantity varying greatly in different deposits and even in various places in the same deposits
Often there are found deposits of ravel, which are really clayey sand or sand containing a few pebbles, but which are of value to the road builder for the sand clay type of surfacing The tereneral and unless specifically defined, gives little indication of the exact nature of to which it is applied
TABLE 7
SHOWING CEMENTING PROPERTIES OF SEVERAL SAMPLES OF GRAVEL
-----------------+---------------------------- | Ce Value Per Cent Clay by +---------------+------------ Weight | As Received | Washed -----------------+---------------+------------ 44 | 276 | 43 64 | 105 | 285 51 | 241 | 70 145 | 500 | 279 85 | 500 | 112 101 | 300 | 267 148 | 500 | 107 75 | 184 | 198 165 | 500 | 428 20 | 185 | 239 15 | 500 | 500 45 | 212 | 204 25 | 116 | 363 -----------------+---------------+------------
The value of any gravel for road surfacing depends upon the degree to which it possesses the properties of an ideal gravel for road surfacing Ideal gravel is seldom encountered, but a consideration of its characteristics serves to establish a measure by which to estimate the probable value of any deposit
=The Ideal Road Gravel=--The ideal road gravel is a mixture of pebbles, sand and earthyfroravel is coer pebbles are filled with the finer hly resistant to wear so that the road surface ravel will have the quality of durability The gravel possesses good ceether in the road surface The ce property may be due to the rock powder in the deposit or to earthy material mixed with the rock particles, or to both Table 7 shows the results of a nuravels and indicates that the ceravel does not always depend upon the clay content
=Perravel are less likely to crush under loads than smaller pebbles of the saher varieties such as trap, there is very little likelihood of even the s If the pebbles are of rock of ht be crushed under the heavier loads It is the usual practice to perravel to be used for the foundation course in which the pebbles are as large as will pass a 3-1/2-inch circular screen opening, and for the wearing course, as large as will pass a 2-1/2-inch circular screen opening If larger pebbles are allowed in the wearing course, the surface is certain to becole course as is a very common practice, then the maximum size should not exceed that which will pass a 2-1/2-inch circular screen opening