The Massachusetts Highway Commission assumes * the pressure to be uniformly distributed over an area equal to the square of twice the thickness of the layer of crushed stone, which is equiva lent to assuming that the sides of the cone make an angle of 48/ degrees with the vertical and that the pressure is uniformly dis tributed over the base. According to this theory, if t = the thick ness of the stone in inches, w = the maximum weight in pounds per wheel, and p = the supporting power of the soil in pounds per square inch, then The Commission has applied this formula to roads already con structed to determine the safe bearing power of the soil, and con cludes that non-porous soils, drained of ground water, at their worst will support a load of 4 pounds per square inch, and that sand and gravel will safely support 20 pounds per square inch.t Although the method of arriving at equation (1) is not correct, the manner of deducing the supporting power of the soil in a meas ure offsets the error, and consequently the formula may be used with some confidence.
In Massachusetts the thickness for state-aid roads varies from 4 to 16 inches, the standard for crushed stone with macadam foundation on well drained sand or gravel being 6 inches, " which appears to be ample for the heaviest traffic.": In New Jersey, on state-aid roads, the depth of stone with macadam foundation varies from 4 to 12 inches, but is generally 6 itches; and the telford roads are from 8 to 12 inches thick, usu ally 8 inches. Most of the roads have a macadam foundation, the telford being used as a rule only where field stones suitable for a telford foundation are found alongside of the road.
The experience at Bridgeport, Conn., is frequently cited to prove that a comparatively thin road is sufficient. Something like 60 miles of 4-inch macadam roads built in that place gave excellent service even under a heavy traffic. The conditions were very favorable for a thin road: (1) the soil was sand or sandy loam, and had fairly good natural drainage; (2) the subgrade was thoroughly rolled with a 15-ton roller; (3) the broken stone was trap, which is hard and durable; (4) the binder was hard and durable, being either stone dust or siliceous sand, and was free from clay or loam; (5) the binder was worked in until the voids in the crushed trap were practically filled, the effect of frost being thus reduced to a minimum and the soil being prevented from working up from below; (6) the stone was thoroughly consolidated with a steam roller of adequate weight; and (7) the roads were main tained by the system of continuous repairs.
The experience at Bridgeport has been repeated at several other places; but such experiences should be regarded as the ex ception, rather than the rule, since 4-inch roads are adequate only under favorable natural conditions and with the most painstaking construction and careful maintenance. The fact that a very thin road can carry the traffic does not prove that such a road is the most economical, for the increased cost of maintenance may more than counter-balance the decreased cost of construction. The engineer should always attempt to construct economically and adapt his construction to fit the natural conditions.
In many cases the problem is whether to construct a thick road on the undrained soil or to secure thorough underdrainage and build a thin road. The latter is usually better and cheaper.