To prevent overrun, the operator must be careful not to push the material over the edge of the pile. If delivered by conveyor belt, the pile is formed by pushing it into horizontal layers. If delivered by truck, the pile is formed by pushing the material into inclining layers. If this equipment is not available a dozer may substitute. Each layer of the layered stockpile will have the same segregation characteristics of a large conical pile, but the multiple layers mitigate segregation at the point of loading, and eliminate the need to reblend.Ī radial or telescopic conveyor can be used to construct a layered stockpile.
The formation of a continuous layered stockpile will reduce the effects of segregation and eliminate the need to reblend the base before shipment. Careful and even loading of the truck bed will prevent further overrun segregation. Loading exclusively from either the front or back face of the stockpile will result in loads that are overly fine or overly coarse. This is best accomplished by drawing the material evenly from the edge of the pile perpendicular to the belt, thereby loading material from both the front and back face. When loading from a segregated stockpile, the loader operator will attempt to blend material from the fine and coarse portions. The resulting stockpile is segregated, with coarse particles settled at the toes, and fine particles in the center portion of the pile. Fine particles, which have settled against the surface of the conveyor belt, tend to cling to the belt and drop to the back face of the pile.
In addition, the coarser particles hit the front face of the stockpile with a greater momentum, and roll down the outer edge of the pile, creating overrun: an accumulation of particles at the pile’s bottom edge, or toe. Coarse particles at the top of the material stream have a higher velocity as the material reaches the head pulley, and are thrown a greater distance. As the product is transported by conveyor, vibration and motion of the belt causes fine particles to settle on the bottom. Segregation occurs in the formation of a traditional conical stockpile. A ¾″ maximum base product exceeds this ratio and will readily segregate. Segregation of an aggregate product will likely occur where the ratio of the size of the largest particles to the size of the smallest exceeds 2:1. Segregation of aggregate base can lead to rejection or costly penalties for the producer, and base material is especially susceptible to segregation because of a greater variation in particle size than that of other stockpiled products. Specifications for ¾″ and 1 ½″ maximum base products are provided by Caltrans Section 26. It is composed approximately 50% coarse and 50% fine aggregate, and its tight compaction is achieved by an even gradation and angular particle shape. Aggregate base is placed between the compacted subgrade and the concrete or AC surface layer to provide support and drainage.
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