7-5 Expansion buckle and breaking up of pavement, also longitudinal cracking on clay silt soil, near Seely, Imperial County. given in column 1 of the table below. The old solid tire was worn to thickness of 1 inch. The new solid tire is 21 inches thick, while the pneumatic tire was 42x9 inches with a 142pound air inflation. The impacts under these cars are shown in columns marked (2) and (3). The exact data of the effect of pneumatic tires on the life of a truck itself could not be obtained. The Motor Transit Company operating 80 trucks on the highways radiating from Los Angeles, including some unpaved mountain roads, gives the following general information: They were having turned in for repair about one broken axle a day, the trucks at that time being equipped with solid rubber tires. They have now all been outfitted with pneumatic tires and the breaking of the axles has greatly decreased. They do not consider it economical to operate solid tired vehicles at speeds over 15 miles an hour, while the same machines equipped with pneumatic tires may be operated 25 miles per hour. It is said that a stage equipped with solid rubber tires attempting to run 25 miles an hour from Los Angeles to San Bernardino would break down. They estimate the life of a truck operating with pneumatic tires as twice that of one operating with solid rubber tires and they find it is economical from the operator's standpoint to use pneumatic tires. Pneumatic tires do not permit of excessive overloading of trucks which, from the standpoint of the taxpayer, is desirable. It is generally recognized in laws that have been studied that trucks with pneumatic tires should be permitted to travel at higher speeds than those having solid rubber tires. Future Policy The future policy of the California Highway Commission should be quality rather than quantity in the construction of the road system. Undoubtedly there is severe pressure for the building of many miles of roads which are relatively unimportant. Such a program leads to reduced quality of work and design in other portions of the State where the road is essential to a heavy traffic. Public opinion in California today decidedly demands a better type of road. As the expense of rebuilding the broken down pavements develops, this will become intensified. The reader is referred to the report of the Committee on the Width and Thickness of Slab for recommendations on the design for our future pavements. No pavement should be put down on a trunk highway in California less than 6 inches thick. On bad soil it should be 8 inches thick and reinforced. The minimum width of trunk lines should be at least 18 feet. California should be in the lead in the quality of its road work rather than at the other end of the procession. REPORT OF COMMITTEE ON SUB-BASE Robert Morton, Highway Engineer San Diego County, Chairman; Lawrence Moye, County Surveyor Tulare County, and J. B. Lippincott, Consulting Engineer. December 10, 1920. The character and preparation of the subgrade or foundation for a road are more important factors than the type of pavement to be laid thereon and the details of its construction, including thickness. There are notable examples of oil and water bound macadam and other structurally weak pavements which have been in service for from 5 to 10 years under heavy traffic which are in excellent condition, being maintained at small cost, because they are built on well drained sandy soils. There are other examples of higher priced and stronger pavements, both of concrete, brick and asphaltic base types, which have failed because of defective foundation. Their renewal constitutes a serious engineering and financial problem. Discussion of the type of pavement to be selected for our various highway projects should not divert our minds from the more important consideration of an unyielding, immovable foundation. In the Southwest there is a heavy clay soil, usually formed from the breaking down of shale rocks, that is locally known as "adobe." It somewhat resembles the "gumbo" in the middle west. The term applies to the structure of the material rather than to its chemical composition. It has the quality of expansion and contraction with wetting and drying. This characteristic, coupled with our California wet and dry seasons, make it an unsatisfactory foundation upon which to lay any pavement. The inspection of the improved state highways in Southern California shows that 45% of the failures have occurred on clay and adobe soils. Out of 117 miles of pavement laid on clay and adobe, 82 miles have failed, while out of 469 miles laid on sand, gravel and loam, 101 miles have failed. Other pavement failures due to foundation troubles are attributed to lack of consolidation of earth fills. The study of the Committee on Thickness, Width and Reinforcement of Slabs demonstrates that it is beyond the limit of reasonable cost to build a concrete pavement on a subbase that shrinks away from it, leaving it partially suspended on the sides. A non-rigid slab of macadam or asphaltic concrete forms into ruts and waves on an unstable base. It has been too often the custom to regard a light concrete slab placed directly upon any natural soil as having sufficient strength. The same thickness should not be used under all conditions. The frequent examples of the breaking up of our concrete pavements, and the distortion of our asphaltic base types, where laid upon heavy clays, shows the fallacy of this assumption. Table No. 20, page 101, and photographs L-X and L-13-7, page 123, and L-8-2, page 109, relate to this subject. In Washington State between Camp Lewis and Everett the concrete pavements have been laid on morainal gravels which are well drained. This is an ideal foundation. They have given good service. The width and thickness of the slab (usually 20 feet wide and 6 to 7 inches thick) is, however, more substantial than our California types. In Multnomah County, Oregon, adjacent to Portland, there is much clay. During the past excellent basaltic macadam roads have been maintained in this section. Recently many miles of both asphaltic and concrete slabs have been placed over these old macadam roads. Care has been observed with drainage. Under such conditions both types of pavements have given good service. When they have, however, built pavements on new clay grades with poor drainage trouble has developed. Macadam rolled into the clay by the traffic accompanied by good drainage is an improvement, as shown by the inspection. Speed is usually demanded in the construction of highways. Time, weather and traffic are the best agencies to stabilize fills. The highways of Europe were not the product of a season's work. The foundations, when exposed by excavations for trenches, often showed rock material three feet deep. The foundations of the roadways of France are the product of a gradual upbuilding process continuing for generations. The same care should be exercised in making fills as in building an earthen dam. It should be made so dense that water will not percolate through it and so that it will settle a minimum amount. Specifications for fills are frequently prepared with care, but often they are not followed closely. An average of $18,097.00 per mile has been paid for the construction of the roads of Southern California. Maintenance, improvements and renewals have amounted to $615.00 per mile per annum. If the life of the pavement is dependent on the foundation, it follows that on such types of soil in which danger is to be expected, substantial outlays are justified in the preparation of the subgrade. Soil surveys should be made as a part of the work of location. Much discretion should be left in the hands of the division engineer. Time should be taken to permit earth fills to be surfaced with gravel or rock and placed under traffic for one to four years, depending on their height and the nature of the material used. Levels should be taken to determine when it has ceased settling. Frequently satisfactory materials for fills or for topping off the subgrade may be obtained in short distances. Adobe should be discarded as much as possible in making fills and selected materials of a satisfactory nature used where possible. The discarding of adobe for subgrade is one of the surest means of solving the problem of shrinkage and expansion. On state highway work, the contractor is required to haul earth a thousand feet for the grading price per cubic yard. If the maximum distance allowed in this provision were enforced, many fills now made of adobe could be made of material taken from a good borrow, sometimes located perhaps off the right of way. Other specifications provide for the payment of le per cubic yard per 100 feet to haul earth onehalf mile beyond the free haul. This would mean an addition to the grading price of 26c per cubic yard for overhaul. As the grading prices often range from 75c to $1.50 per cubic yard, it would appear to be possible to obtain suitable materials in many cases for the making of subgrades or low fills within reasonable additional costs. An examination of a pavement during the summer on many sections laid over heavy soil will disclose a vertical opening from 1/4-inch to 1-inch in width between the soil and the edge of the pavement, except where shoulders are used. Beneath the pavement it is often possible to insert a yardstick (see photographs 8-10, page 128, and 13-8, page 107). The air has dried out the moisture and shrunken the supporting soil of the subgrade. If the pavement has not already failed, it will soon do so. When such a pavement is concrete, cracks develop usually 3 to 5 feet from the edge. Thereafter less weight is required to rupture the fragments. With an asphaltic concrete type of surface on similar soils the settlement is not so apparent. The material has elasticity, and follows the subgrade without cracking. Deformation occurs, however, and it is only a short time until it becomes so great that this type also fails. The San Diego County Highway Commission is making a study of soils on the various routes to determine what their future action will be under various conditions of moisture. They are adopting a system of subgrade "reinforcement" or adulteration with disintegrated granite or broken stone wherever unstable conditions are encountered. No matter what subgrade treatment is adopted, steel reinforcement will be used in the pavement over the heavy soils. It is known to sidewalk builders that a cushion of sand or cinders on adobe soil will prevent cracks. In Tulare County in 1920 bids for 12-inch of rock and 2 inches of sand rolled into the clay subgrade were at the rate of 36c to 36.3c per sq. yd., or $2,800.00 to $3,400.00 per mile. Photograph L-13-10, page 111, was taken on the Ventura County highway on the road between Ventura City and Ojai. In one block of this soil there was a shrinkage of one and one-half inches in twenty in one direction, and three inches in thirty-one in the other. Cracks two inches wide at the surface ran out at depths of twenty inches. |