The instrument is of use to an inexperienced person in inspect ing the curvature of the crown of a broken-stone road.
The above instrument may easily be modified so as to be of service in inspecting the longitudinal slope of the side ditches, for which purpose neither the strips projecting below the straight edge nor the template are required. Without these, the instru ment is simply a level, and it only remains to graduate it. This may be done as follows: Proceeding as in the second paragraph of § 316, drive two nails into the floor until they are exactly at the same level; and then adjacent to one of these nails, drive another until its head is exactly 1 inch above the adjoining one. Set the straight edge E F upon this and the opposite nail, and mark the position of the plumb-line. The lower edge of E F is now on a grade of 1 inch in 8 feet 4 inches, or 1 inch in 100 inches, or 1 foot in 100 feet. By obvious modifications of the above process, the upper face of either the piece A B or C D can be graduated to correspond to any gracie:.
In this form, the instrument is of material help in determining whether the bottom of a ditch has a uniform grade. The instru ment is not capable of mathematical precision, and hence should not be employed to run long lines of levels when accuracy is re quired ; but it is valuable in checking the grade between points determined by an engineer's leveling instrument. To obtain the highest accuracy, the plumb-line should be fine and smooth.
stone depends upon the nature of the soil, the drainage, the traffic, and the binding material; and the initial thickness depends upon the amount of wear permitted before new material is added. If repairs are continuous, the initial thickness may be the mini mum; but if repairs are made periodically, the 'initial thickness must be equal to the minimum thickness plus the amount allowed for wear. After a road has been worn down 3 inches, it is usually so uneven as to require re-surfacing; and therefore it is uneconom ical if the road in this stage is much or any thicker than the mini mum required to prevent its breaking through.
There has been much discussion and there is a great difference of opinion as to the proper depth of a broken-stone road. The depth considered necessary by the most extreme advocates of thick roads has decreased with the introduction of more improved methods of construction—particularly the use of binder and a steam roller,—and as the advantage of thorough under-drainage has been better appreciated. Early in the century a depth of IS to 24 inches was frequently considered necessary for heavy traffic. but later it was reduced to 12 or 15 inches, while now 6 inches, or less, is usually considered sufficient.
The concentrated load of a wheel is transmitted through the broken stone to the earth in lines diverging downward, and the wheel may be assumed as resting upon the apex of a cone whose base is upon the earth subgrade. The sides of this cone probably make an angle of about 30° with the vertical.* It is not wise to attempt to find a mathematical relation between the load on the wheel and the resulting pressure on the earth, since neither the angle of the cone nor the distribution of the pressure on the base of the cone are known. It is reasonably certain, however, that the supporting power of a crushed stone road varies as the square of the depth. This is an important relation to bear in mind when it road is to be strengthened.