DEAIlSrAGE METHODS FOR COUNTY ROADS. 51 



ditches filled with stone and provided with suitable outlets or by the 

 use of tiling. Methods of constructing drains of either kind are de- 

 scribed in another part of this bulletin. 



Where old road crusts or old base of broken stone, gravel, Telford, 

 etc., are to be covered, the disposition of the old construction is im- 

 portant. A highway engineer can commit no greater error than to 

 abandon or destroy an old and well-compacted road crust if it can 

 be used as a foundation for a new road. If the new construction is 

 to be gravel or macadam, the lesser irregularities of the old surface 

 can be filled with gravel or fine stone after the old surface has been 

 swept thoroughly. These patches should be well watered, covered 

 with screenings, and rolled. On a base so prepared the new road 

 can be built. If a bituminous top is to be placed directly on the old 

 crust, lean bituminous concrete can be used to excellent advantage. 

 The old surface should be swept thoroughly, the depressed areas 

 painted with the same bitumen used in the mix, and the patches laid 

 and raked with care. A 3 or 4 foot straightedge with a handhole is 

 convenient in bringing the patch to a true surface uniform with the 

 old. The entire series of patches should be rolled and allowed to set 

 for at least 12 to 24 hours ahead of the topping. 



If the new surface is to be wider than the old base or crust, it is 

 more difficult to produce an even surface that will not settle at the 

 edges. In many casei the only practicable disposition of the old crust 

 is to loosen it and either waste the material or salvage it for the new 

 top. Salvage methods usually are expensive. 



It should be borne in mind that wdien the old crust is left undis- 

 turbed and simply evened up by the addition of new material, it 

 is difficult to compact the new material as thoroughly as the old, and 

 a tendency to uneven settlement of the subgrade will result. For 

 this reason the greatest care should be used in making the prelimi- 

 nary patches. Plate VII shows the results of neglecting this detail. 



In the premises the whole problem of making adequate use of old 

 base is largely one of initial engineering. The grades should be laid 

 with unusual care to take advantage of the old crusts, and where 

 these are thin room should be left for necessary additional material. 



Where practicable it is always well to have the grading and drain- 

 age structures for an improved road completed some time in advance 

 of the pavement. This enables the roadbed to settle without doing 

 any damage, and obviates the necessity for much of the tamping 

 and compacting which otherwise would be necessary. 



COST OF STJBGRADE PEEPARATION. 



The cost of preparing the subgrade of a road to receive the founda- 

 tion or road crust depends to a very considerable extent on the degree 



