Embankment compaction equipment After these natural soils and rock are hauled to the project, they must be placed to form a firm foundation for the new highway. This is usually accomplished by compacting the material by rolling it with one or more types of equipment. Some of these are conventional equipment that have been in use for some time. In addition to the conventional steel wheel rollers and the towed sheepsfoot, tamping, and pneumatic types, there are many new compaction units operating on the embankment layers and subgrade. Location and use AGGREGATES Aggregates are one of the more important highway materials and generate over 15 percent of the onsite cost of highway construction. The current annual requirements for highway construction are about 580 million tons. This represents an increase of about 50 percent since 1955. In addition, highway maintenance takes about 170 million tons annually. The Bureau of Public Roads records show that for construction purposes highway contractors produce 54 percent of their aggregates and purchase the remainder. Aggregates are produced principally from two basic sources: (1) From parent rock by quarrying operations, and (2) From open-pit mining of natural deposits laid by glacial or sedimentary action. Some aggregates are also produced from blast furnace slag. One of the most pressing problems associated with highway aggregates is the depletion in glaciated areas where there is no continuing natural replenishment as in the case of river-borne deposits. Surveys of supplies.-Surveys conducted by the Construction and Maintenance Division of the Bureau of Public Roads in collaboration with the Physical Research Division during the first stages of the expanded highway construction program revealed that aggregate supplies were not considered adequate in Colorado, Mississippi, Oklahoma, and in parts of Kansas, Nebraska, and Virginia. Some of the States which expect shortages resulting from the depletion of glacial deposits include New Hampshire, New York, Oregon, and Pennsylvania. In such areas, processing of other sources and longer hauls will be necessary. Comprehensive surveys of aggregate sources are available in many States usually on a county basis. States which have critical areas of aggregate shortages, and where surveys have not been made, have been urged to do so as part of their highway planning program. Aerial photographs were used extensively in both Maine and North Dakota to locate possible sources. In North Dakota, preliminary explorations were conducted with electrical resistivity equipment at the sites located by photogrammetric methods. Upgrading aggregates. While the construction of highway surfaces and structures demands the continued use of high-quality aggregates, there are many opportunities by which the low-quality materials, including types that formerly would have been rejected, can be used in the base and subgrade area with highly satisfactory results. This may be done either by using admixtures to alter the behavior of natural materials or by removing deleterious particles by a process of beneficiation. For example, lime, limestone dust or chips, or sand may be used to reduce the plastic characteristics of clay-gravels. Gravels containing objectionable lightweight particles are commonly brought within specification requirements by the heavy media process. This process uses flotation to separate light and heavy stone. It will not remove high specific gravity material such as undesirable |