EARTH, SAND-CLAY, AND GRAVEL ROADS. 



13 



5. Table 3 of loads is based on the foregoing statements, assum- 

 ing the weight of the average horse at 1,200 pounds, the pull he 

 is capable of exerting continuously for several consecutive hours 

 on a level road at one-tenth of his weight, his maximum pull on a 

 level road, over a distance not to exceed 500 feet without a rest, at 

 twenty-five one-hundredths of his weight ; and assuming that a pro- 

 portionate part of a horse's energy is devoted to lifting his own 

 weight on grades. 



Table 3. — Effect of grades on the gross load a 1,200-pound horse can haul? 



Per cent of grade. 







Pounds. 

 760 



1... 



645 



2 



540 



3 



445 



4 



365 



5 



290 



7 



160 



10 





15 





Road through 

 deep sand. 



Contin- 

 uous 

 load. 



Maxi- 

 mum 

 load. 



Pounds 



1,905 



1,720 



1,550 



1,410 



1,275 



1,155 



950 



700 



390 



Road over 



average 

 dry earth. 



Contin- 

 uous 

 load. 



Pounds. 



1,600 



1,270 



1,010 



800 



625 



480 



250 



Maxi- 

 mum 

 load. 



Pounds, 

 4,000 

 3,390 

 2,905 

 2,515 

 2,190 

 L,920 

 1,490 

 1,030 

 535 



Road over 



firm earth 



or sand-clay. 



Contin- 

 uous 

 load. 



Pounds. 



2,285 



1,725 



1,325 



1,020 



780 



585 



295 



Maxi- 

 mum 

 load. 



Pounds. 

 5,715 

 4,610 

 3,805 

 3,200 

 2,725 

 2,340 

 1,765 

 1,180 

 590 



Average 

 gravel road. 



Contin- 

 uous 

 load. 



Pounds. 



3,000 



2,160 



1.600 



1.200 



900 



665 



330 



Maxi- 

 mum 

 load. 



Pounds. 

 7,500 

 5,760 

 4,600 

 3,770 

 3,150 

 2,665 

 1,965 

 1,285 

 630 



First-class 



gravel or 



macadam road. 



Contin- 

 uous 

 load. 



Pounds. 

 4,365 

 2,880 

 2,020 

 1,460 

 1,065 

 775 

 370 



Maxi- 

 mum 

 load. 



Pounds. 

 10,910 

 7,680 

 5,810 

 4,590 

 3,735 

 3,100 

 2,215 

 1,410 

 675 



1 This table is based upon the assumption that for the same road surface the pull varies 

 directly with the weight of the load. Tests to be made may make it necessary to modify 

 this assumption. 



Table 3 shows conclusively that for economy the maximum grade 

 for any particular road should be fixed with due regard for the type 

 of surface to be employed. For example, where the road is through 

 deep sand, a horse of the character assumed, by extraordinary exer- 

 tion, could pull his allowable continuous load of 760 pounds for level 

 grades up a short grade of about 10 per cent, while in the case of a 

 firm earth or sand-clay road the allowable continuous load for level 

 grades is 2,285 pounds, and the steepest grade up which the horse 

 could possibly pull this load is about 5 per cent. For average gravel 

 roads the corresponding comparison would show a maximum grade 

 of about 4 per cent. 



Comparisons of the kind made in the preceding paragraph are not 

 sufficient, of course, to determine the maximum allowable grades for 

 any particular road, because, in general, the judgment should be 

 largely influenced, in fixing the maximum grade, by the topography 

 of the region which the road traverses. According to the best current 

 practice, where the road is or is expected to become of sufficient im- 

 portance to warrant a highly improved surface, the maximum grade 

 usually is fixed with reference to this feature about as follows : 



Per cent. 



Coastal plain and prairie regions 2 to 3 



Average rolling country 4 to 6 



Hilly or mountainous regions 6 to 8 



