Scrapers are commonly used in road construction, site formation, other earthmoving and surface mining operations. It is known that loading a scraper to capacity may not be best in terms of unit operational cost, because the load-growth curve for a scraper is nonlinear À the loading rate decreases with load time. However, there is no published research considering the effect of load time and fleet size on unit emissions, and what the corresponding optimal load time and fleet size might be. This paper shows that the optimum scraper load time for unit emissions may be the same or slightly less than that for unit cost depending on the scraper type and operation, while the optimum fleet size in terms of minimum unit emissions, is slightly higher than or the same as that in terms of minimum unit cost. Cost-load-time, emissions-load-time and hauling characteristics, particular for different equipment and operations, influence the conclusion. For practice purposes, it could be said that the optimal unit cost and unit emissions solutions essentially coincide. Case study site data, in conjunction with a scraper operational analysis, are used to demonstrate the optima. Both push-pull scrapers and push-loaded scrapers are considered.
IntroductionScrapers are useful for road construction, other earthmoving and surface mining operations. Extensive research has been conducted on accurately predicting cycle times and production for fleets of scrapers, and determining optimum (minimum unit cost; equivalently minimum cost per production) configurations and loading practices. Recent attention has turned to also looking at minimizing unit emissions (equivalently minimizing emissions per production) for the related excavatortruck operations (for example, Carmichael et al. 2012Carmichael et al. , 2014 and extended in Kaboli & Carmichael 2012, 2014). However, the relationship between unit cost and unit emissions is unknown for scrapers, and it is unclear the extent to which managing scraper operations in terms of minimum unit emissions changes the unit cost of operations. There is an identified need, and addressed in this paper, to study scraper operations in this context.Diesel-powered equipment used in earthmoving operations is a major emissions contributor, and produces considerably more emissions compared to on-road vehicles such as cars. For example, the amount of particulate matter from a bulldozer with a 175 hp engine is nearly 500 times more than that of a new car (EPA 2005). Accordingly, appropriate site practices need adopting in order to reduce emissions. For efficient operations, cycle times (loading, hauling, dumping, empty returning and, in some cases, queuing) of equipment such as scrapers need close scrutiny.A long scraper load time translates to a larger payload, but slower travel times and diesel engines working harder; this increases fuel use (and emissions) and cost per scraper cycle. However, since fewer scraper cycles are needed to transport a given volume of material, the overall fuel use and cost of a hauling op...