SETTING THE TELFORD. The distinguishing feature of a telford road is its paved foundation. After the road-bed has been brought to the proper form and been rolled, rough stones are set upon the surface to form a pavement 5 to 8 inches thick, the thickness depending upon that to be given to the finished road (§ 318), the general practice being to make the paved foundation about two thirds of the total thickness of the road. The practice of Telford was to grade the road-bed flat, and then construct his pavement deeper in the middle than at the sides, using for a road way 16 feet wide, stones about 8 inches deep at the middle and 5 inches at the sides. This practice is still followed by some engi neers, but it is now more common and usually considered preferable to make the surface of the road-bed parallel to the finished surface and the pavement of uniform thickness. Fig. 45, page 198, shows a telford road with a level subgrade; and Fig. 46, page 198, a tel ford road with the subgrade parallel to the finished surface.
The size of the stones for the telford pavement is of no great importance, at least there is a great difference in the practice of the best road builders. The width of these stones varies from 3 to 10 inches, 3 to 6 being most common; and the length varies from 6 to 20 inches, 8 to 12 being most common. It is desirable to have the width on any particular job somewhat nearly uniform, and the stones in any course should be still more nearly equal. The stones
are set upon their widest edge with their greatest length across the road, the joints being broken as much as possible. Each stone should stand independently of its neighbor, i. e., one stone should not lean against another. The irregularities of the upper surface are then broken off with a hammer, and the interstices between the stones are filled with spalls lightly driven into place with a ham mer or a crow-bar. This knocking off of the projecting points and the driving of spalls into the interstices should not be done so near the face of the pavement as to dislocate the stones last set. It is frequently specified that no wedging shall be done within 10 or 15 feet of the front edge of the pavement. After the projecting points have been knocked off and the interstices have been filled with stone chips or ordinary crushed stone, the pavement is usually rolled. It is usually specified that the roller shall not go nearer to the front of the pavement than 25 to 30 feet.
The cardinal requisite of a telford foundation is the interlocking of the stone closely and compactly together by barring, wedging, and rolling until the entire structure is brought in action to resist disturbance as a single mass.