the portion of the report dealing with the traffic census.

Despite these excess weights, it is believed that most of the failures in the roads have not been caused by overloading alone, but in combination with conditions of poor sub-base and speed. The Highway Commission maintains that legislation is adequate to regulate these violations but delinquency lies in their enforcement. They recommend a state motor police under the Motor Vehicle Department. The State Highway Commission has power to revoke licenses.* The Motor Vehicle Department has police powers. On many of our valley roads it is possible to approach the City of Los Angeles without surmounting any steep grades. Roads originating in the San Fernando Valley take a water grade into the city. Roads leading from the San Bernardino and San Gabriel Valleys and from the Coastal Plain south and west of Los Angeles and from the harbor are free from hills. As the pavements are smooth, the truck owners find that the economy in their operation lies in carrying maximum loads. The limit of the loading is often based on the ability of the truck to start with the load. The drivers say if they can start they can keep going to their destination.

Particularly where sub-base conditions are poor as with an adobe or clay soil and the slab is imperfectly supported as indicated in Photograph No. 13-8, page 107, where the soil has shrunk away from the concrete at the edge of the slabs, loads of the character described above quickly develop not only longitudinal cracks, near the sides or in the center of the slabs, but also break triangular blocks of concrete at the corners where the unavoidable transverse cracks or expansion joints occur. There rapidly follows a breaking up into smaller fragments and possibly because the sub-base is made with a crown the jar resulting from the traffic sets up a slight lateral movement of the slab causing the widening of the crack. Unless it is promptly filled with some impervious material such as asphalt, the winter rains get through these cracks in the slab, soften the ground thereunder and in the case of adobe soil produce a swelling which aggravates the situation. There results a broken down pavement, the remedy of which is expensive and unsatisfactory. The reinforcing steel put in the slab prevents this enlargement of these cracks and holds the fragments together rather than preventing the original break. The report of the Committee on Sub-base describes this condition.

The growth in the number and tonnage of trucks in California during the last seven years has been in excess of conditions that could have been reasonably anticipated at the time the original design was made for the pavement for the state highways, as shown by Diagram No. 5, page 58. Statistics are not available on the increase in carrying capacity of the trucks during this period but it is obvious that the growth in this regard is equally great.

Out of the 500 trucks that were weighed on the public scales of Los Angeles in August and September, 1920, 114 were one ton; 61 were one and one-half ton; 93 were two ton; 128 were two and one-half ton; 27 were three ton; 51 were three and one-half ton; 53 were four ton and 110 were five ton. Out of 637 trucks that have been weighed in and outside the City of Los Angeles, 15% were five-ton trucks. Table No. 21 indicates that the mean and maximum gross loads of these 637 trucks were as follows:

It is impossible to make any design for bridges or pavements that will carry unlimited loads. No railroad system would tolerate unlimited loads. The present law forbids it. There is no fairness in permitting a small number of truck owners, for the sake of slight additional profit to themselves, to overload the expensive structures to their destruction in disregard of the rights of the great mass of the taxpayers of the community. We should put some reasonable limit on the load such as 10,000 pounds on any one wheel and substantially improve and strengthen the roads we build in the future. It is demonstrated and admitted that the California pavements that have been built in the past, fifteen feet wide and four inches thick, are now inadequate.

Photograph No. L-8-2, page 109, shows the effect of traffic of this nature on the concrete pavement on the south side of the San Fernando Valley between Universal City and Encino. The original pavement in this section was 15 feet wide, 4 inches thick, not reinforced. The concrete is in large part broken up in blocks approximating from 6 inches to a foot square. This pavement was built in 1913. It is 6.55 miles in length. Its original cost was $12,920.00 per mile. There has been expended on it for resurfacing with asphaltic concrete, widening shoulders, reconstruction and other forms of maintenance during the last six and a half years, approximately an average of $13,000.00 per mile and there are substantial portions of this road that are now in an unsatisfactory condition. The photograph shows the repair gang at work. The concrete is picked out in these small blocks rather indefinitely and patched either with precast blocks or by new sections of slab. The maintenance gang has a discouraging task continually before them because of the extent of this breakdown. This section of road is in worse condition than the average and is referred to because more data is now (October, 1920) avail. able in reference to it than other sections. This Ventura Highway, from Universal City through the San Fernando Valley to Ventura County, has been out of commission for nearly a year's time, largely on account of repair work, but partly on account of the laying of a new pavement on the Conejo grade. The public has thus been deprived of the benefit of its investment therein during this period. This condition of the great overloading of the highway by the larger sizes of trucks is serious and should be remedied by the enforcement of the laws or else we must contemplate a continuance of these conditions with the roads we have so far built, at least on the clay soils, which apparently will soon lead to the destruction of our present highway system.

Sub-Base

A report on Sub-base Conditions is presented on page 121 of this report.

Within practical limits of cost no pavement can be designed to stand present day traffic conditions unless it is completely supported on its sub-base. Only a painstaking attention to construction details will result in a satisfactory foundation for the slab. In California we have a great deal of a soil that is described as adobe and nearly all pavements of all classes which have been laid thereon, where traffic conditions have been severe, have broken down, as is shown in Table No. 20, page 101. Adobe is a term applied to any fine grain soil whether clay or silt. The typical structure shows shrinkage cracks at close intervals when dry. It increases in volume on being wet. Materials having adobe structure are "reversible colloids" which means that they have the property of alternately when dry absorbing moisture and becoming gelatinous in character, thereby swelling and when drying of shrinking with their varying moisture content. Photograph No. L-13-10, page 111, shows shrinkage cracks in adobe soils. Under the direct action of the sun they will contract about 10% of their volume laterally and an undetermined amount vertically. Adobe cracks are frequently found in which a yardstick can be inserted from 24 to 30 inches. A block of adobe excavated on the road between Ventura and Ojai shows cracks 2 inches wide at the ground surface, 34 of an inch wide 14 inches below the surface and readily detected at depths of 20 inches. Samples

of this adobe soil have been selected from various portions of Southern California and sent to the Civil Engineering Laboratories at Berkeley for the purpose of testing the extent to which they will expand and contract and the force with which they expand.

We, therefore, have in large districts of California, plastic soils that change their forms. between wet and dry seasons upon which a rigid pavement slab is laid. No pavement within reasonable financial limits can be built to withstand heavy loads, if it is not fully supported. When taken in connection with the loading of the pavement, as described above, failures follow.

Photograph No. L-13-1, page 111, shows a concrete slab that was built on adobe by experienced road engineers and contractors in Santa Barbara County near the Santa Barbara Mission. A four-foot lath can readily be run under the sides of this pavement at numerous points. Photograph No. 12-6, page 106, shows the character of the results occurring when a slab is loaded under such conditions.

Search has been made by your engineers to find satisfactory pavement conditions on soils of this character both in Northern and Southern California. As a rule pavements that have been so laid five years or more where the traffic is heavy, have been broken down to such an extent that they are either in bad condition at present or they have been resurfaced at an expense as great as the original cost of the pavement by putting a wearing surface of asphaltic concrete from 12 to 2 inches over them or in some instances by casting another slab of concrete thereon.

The California Highway Commission has demonstrated with a three-thousand-foot slab 6 inches thick and 15 feet wide reinforced and laid on the Ventura Highway west of Calabasas on adobe soil that it is possible to build a pavement under such conditions that will stand up under heavy traffic while standard types of 4-inch unreinforced slabs adjoining have completely failed. This is described in the report of the Committee on Sub-base (page 125). Between Goleta and Gaviota on the Coast Highway some reinforcing has been placed in the 4-inch concrete slab where laid on adobe soils with results that are beneficial.

It is surprising that in view of this evidence of success and failure, the Commission continued constructing this standard type of pavement 4 inches thick without reinforcing on so many miles of similar soils until quite recently in California.

In order to prevent the change in volume of the adobe soil in the sub-base it is essential to maintain a uniform moisture consistency. Apparently the adobe will shrink either laterally or vertically to a depth or width of three feet from the exposed surface. This drying out is said to be due to the existence of capillary tubes in the soil. Experiments in different classes of soils in irrigation work indicate that these dense clay soils will evaporate moisture from their surface, which is drawn from greater depth than in the case of sandy soil. It was found by tests conducted by the Los Angeles Aqueduct Bureau in Owens Valley that underground water supplies will evaporate from the ground surface when the water plane is eight feet or less from the surface. This, however, is when they are planted with native grasses. The rate of evaporation is more rapid when the water levels are nearer to the surface. It is possible that the thorough breaking up of these capillary tubes in the adobe to substantial depths will tend to diminish the changing in their moisture contents as indicated. by the following:

A road that was built under the direction of Mr. Lloyd Aldrich when County Engineer of Stanislaus County in 1917, between Newman and Patterson and for a distance of several miles north of Patterson, was visited by your engineers. This pavement is concrete, 16 feet wide and 4 inches thick, slightly reinforced at the expansion joints and corners. Portions of it are covered with 1 inch of Topeka asphalt concrete. The surface of the road is maintained about 18 inches above the general ground level. The Department of Agriculture has classi fied this soil as "Yolo adobe" and "Yolo sandy clay." The road is now three years old and