EARTH, SAND-CLAY, AND GRAVEL ROADS. 41 



CONSTRUCTION METHODS. 



The proper method to employ in constructing a sand-clay road 

 surface depends on the conditions to be met. In general, there are 

 four distinct sets of conditions, all of which may occur in the same 

 locality, or even on different sections of the same road. These are : 



Case 1, where the original roadbed is sandy and it is desired to 

 construct the surface by admixing clay ; 



Case 2, where the original roadbed is composed of clay and it is 

 desired to construct the surface by admixing sand ; 



Case 3, w T here it is desired to construct the surface of topsoil or 

 other natural sand-clay mixture without admixing any of the roadbed 

 material ; 



Case 4, where it is desired to supply the clay and sand separately 

 and mix the two materials together in place to form the surface. 



Before discussing the different cases separately, the general ends 

 which all of the construction methods should aim to attain will be 

 summarized briefly. 



(1) The questions of location, design, drainage, grading, etc., are, 

 if anything, more important in the case of sand-clay roads than in the 

 case of earth roads. These questions have all been discussed in 

 another part of this bulletin. 



(2) The amount of clay contained in a finished sand-clay road sur- 

 face should be only slightly more than sufficient to fill the voids in 

 the sand. Ordinarily about one part of clay to two parts of sand 

 gives satisfactory results, though the proper proportion for any par- 

 ticular case can be determined best by experiment. 



(3) For average country-road traffic and a stable roadbed, the 

 depth of a sand-clay surface, should be about 8 inches after it is com- 

 pacted. If clean sand and pure clay were used to make the mixture, 

 the respective depths of the sand layer and the clay layer required 

 would be approximately 8 inches and 4 inches, measured loose. It is 

 customary to decrease the surface thickness from center to sides, with 

 a feather edge extending out over the shoulders, but this practice 

 makes it necessary for most of the traffic to use the center of the 

 road, and heavy vehicles using the edge of the surface are liable to 

 break through the surfacing material. 



(4) In constructing a sand-clay surface, the two constituent mate- 

 rials should be thoroughly and intimately mixed together in all cases. 

 In making artificial mixtures, therefore, much plowing and harrow- 

 ing are required. 



(5) No matter what method is followed in constructing a sand- 

 clay surface, traffic usually must be depended upon to puddle and 

 compact the surfacing material, and the road never should be con- 



