b BULLETIN 220, U. S. DEPARTMENT OF AGRICULTURE. 



of the road was kept even and smooth by the addition of fresh mate- 

 rial where necessary. This material was placed on the road in thin 

 layers in damp weather in order that the new material might more 

 readily bond and incorporate with the old. 



TELFORD METHOD OF ROAD BUILDING. 



This form of construction takes its n'ame from the celebrated 

 engineer, Thomas Telford (1757-1834), who, besides doing many 

 other notable things, constructed 920 miles of roads in the High- 

 lands of Scotland and also a large mileage in the mountainous sec- 

 tions of Wales and in the north of England. To-day the chief 

 characteristic of the telford road is a base of fairly regular stones, 

 about 3 by 5 by 7 inches in their smallest dimensions, placed by 

 hand on the wider of the long narrow faces and with the greatest 

 dimension perpendicular to the axis of the road. The blocks are 

 then "keyed in" by filling the interstices with stone spalls, chips, 

 or small, gravel. Any projecting points are broken off. On this 

 base is placed a wearing surface of broken stone from 4 to 7 inches 

 deep. Originally the telford road was constructed with a flat 

 subgrade, and a slight crown was obtained by using larger stones 

 in the center. In present construction, however, the subgrade is 

 given the same crown as the finished road, and stones of uniform 

 size are used throughout. Formerly, too, the wearing surface 

 was placed and consolidated in the same manner as the wearing 

 surface of the old macadam road. 



The telford base is used very generally in the construction of 

 important roads in Austria-Hungary, Germany, Russia, Switzer- 

 land, and the Scandinavian countries. Some of the roads of England 

 and Scotland which were formerly macadam are proving too weak 

 for the present heavy traffic and are being relaid with a telford base. 



Plate IV, figure 1, illustrates a telford road as constructed during 

 the first period, which began about 1820. 



Section A shows the earth foundation, from 16 to 20 feet wide, 

 and flat. 



Section B represents the telford base, composed of stones about 7 

 inches in depth. No stone more than 3 inches wide was placed at 

 the top. 



Section C shows the top course, about 7 inches thick at the crown. 

 It was composed of hand-broken stone, in pieces not heavier than 

 6 ounces, which would pass through a circular ring not larger than 

 2\ inches in diameter. 



Section D shows the finished road, bonded with 1 inch of gravel. 

 The crown was made 6 inches for the road, which was surfaced to 

 a width of 18 feet. 



