Information Circular 46. Aggregate Resources Inventory of the Seven-County Metropolitan Area, Minnesota

Abstract

Construction aggregates are sand, gravel, and crushed rock-bulk granular materials that are used in building and landscaping projects of all sizes and kinds. Most of the highest quality aggregate is used in the manufacture of concrete and top-grade asphalt paving. Aggregates of lower quality are used as fill, base-course for roads, and for a myriad of other purposes. Aggregate quality is determined by the mechanical and chemical properties of the constituent rock particles. In very general terms, the best aggregates for high-end uses contain particles that are strong (resist abrasion and fracturing), chemically inert (do not decompose, swell, or shrink on exposure to air, moisture, or road chemicals; do not react adversely with cement materials), and are of optimum size and shape for the specific engineering requirements. High-strength concrete for heavy-duty use such as highways and airport runways requires aggregate composed of particles that are strong and inert, and also have broken faces; i.e. they are not round and smooth. This broken shape enables the particles to lock up mechanically with one another rather than roll under stress, and improves the durability of the paving. Construction aggregate producers and their largest customers in the construction sector have recognized for many years that the aggregate resources available for mining within the seven- county metropolitan area are rapidly diminishing. The ultimate reason for this is urbanization, which on the one hand increases the demand for construction aggregates, and on the other, tends to remove aggregate-bearing lands from production through land development and zoning decisions that preclude mining. When sources of aggregate are eliminated locally, and become more remote from places of need, the costs of construction rise significantly. This is mainly because of the increased cost associated with aggregate transportation. Cost increases are felt most acutely in large projects such as freeway or airport runway construction that require huge volumes of high-quality aggregate for concrete. Local decision-makers have become increasingly aware of aggregate-resource issues over the past few decades. Most counties and townships are substantial purchasers of aggregate materials for road building and other purposes, and are therefore sensitive to aggregate costs. Many are also involved in the controversies between neighbors and aggregate producers over the noise, dust, truck traffic, and other environmental impacts (real or perceived) associated with aggregate- mining operations. In Minnesota, including the seven-county metropolitan area, the powers to regulate aggregate mining and associated industrial operations reside largely at the county, city, and township level. Issues of land-use planning and regulation that apply to the construction aggregates industry need to be resolved. Government entities, the aggregate industry, and citizens of the seven-county metropolitan area all require dependable information on the physical distribution of aggregate resources and the probable economic lifespan of the local resource base. This report and the companion geological maps on which it is based (Meyer and MossIer, 1999) were prepared to meet that need.