28 BULLETIN 691, U. S. DEPARTMENT OF AGRICULTURE. 
form layer over a concrete or well compacted broken stone or 
gravel foundation to such depth that it will yield a uniform layer 
2 inches in thickness after thorough rolling. A seal coat of the 
same bituminous material should then be applied at the rate of 
(K4 to 0.6 gallon per square yard and covered with a sufficient 
amount of stone chips to just absorb the excess bitumen. Roads 
of this character may be maintained with oil surface treatments 
using the materials and methods given under Specifications OC-2 
or OH-1. The latter is usually to be preferred. Such mainte- 
nance treatments should be applied whenever the wear comes 
directly upon the wearing course rock. 
