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Geology Rocks! Breaking Down Gravel with Ian Madden

You have wobbled and skittered on it, gotten it stuck in your cleats, probably cursed it a few times, but what do you really know about gravel? In preparation for the 2023 Cycle Oregon GRAVEL ride, here is an introduction to all things gravel from a geological perspective.

With the exception of water, gravel is the most widely used natural resource in modern society. Gravel literally underpins everything we build, and huge amounts are mined, transported and used every year. In 2018, Oregon mines produced nearly 40 million tons of gravel, which works out to 10 tons per Oregonian. That is enough to fill 2.4 million dump trucks. What did you do with your 10 tons?

breadwinner gravel bike

From a geologists perspective, gravel is an aggregation of rock fragments whose size is greater than .078 inches. Fragments between .078 and .157 inches are called (if these numbers seem odd, its because they are actually defined in metric units) granules, between .157 and 2.5 inches pebbles, between 2.5 inches and 10 inches cobbles and greater than 10 inches boulders. So if the fragments are in the pebble size range, it would be called a pebble gravel, though in most natural deposits of gravel, there are usually a mixture of sizes. The range and variability of sizes is called sorting, and a well -sorted gravel would have fragments in a narrow size range, while a poorly sorted gravel would have a mixture of all sizes. In addition to size and sorting, the other defining characteristic of a body of gravel is rounding. If the fragments all have sharp points and edges, they are called angular, and if they are smooth , without points and edges they are called rounded. These three characteristics , size, sorting and rounding define how a gravel will behave when used in some sort of construction or engineering application, like roads.

Gravel is either mined by excavating a natural deposit, like the gravel from old river beds or it is produced by mining and crushing hard rock like basalt, granite or limestone. In both cases the gravel is run through a series of different sized screens to sort it into different sizes which are then blended to produce the desired product. Crushed rock is always very angular, but natural gravel has typically been rounded by centuries spent rolling along the bottom of a river. Different applications call for different gravel characteristics. Poorly sorted angular crushed rock can be compacted into a hard layers, as the angular fragments interlock and smaller pieces fill in the spaces between fragments. A well sorted-well rounded gravel however would not compact and would remain loose with lots of open space between the grains, which is terrible for a road surface, but perfect for drainage.

gravel bikes

Many types of rock are mined for the production of gravel for roads, but in Oregon the most common is basalt. Basalt is a hard, dense black rock that forms from the cooling of molten magma either on the surface as a lava flow, or underground, as a dike or sill. Basalt is very common throughout most of Oregon, which is important because the majority of the cost of producing gravel for building roads is transportation. Gravel is heavy, so road builders like to find sources close to their construction projects. There are currently over 1000 rock quarries permitted in Oregon, and at least as many small pits used for construction of forest roads.

There are several important factors in designing and constructing a gravel road. The road must be able to support the weight of vehicles, and the gravel surface must be drained, smooth and cohesive. A well-constructed road starts with removal of surface soil and compaction of the subsoil. A base layer of large gravel is laid down and compacted to provide drainage and to support loads. The surface gravel layer has to be a mix of angular pebble gravel with the right mix of finer fragments to compact into a smooth layer that will hold together. The shape of the road is very important, with a pronounced crown, meaning that the center is high and slopes smoothly down towards the edges. Over time, the passage of vehicles at higher speeds causes washboards to develop, so frequent regrading and application of gravel may be needed.

When everything is done right, a gravel road can provide a smooth ride at high speeds for both vehicles and bicycles.

Ian Madin has worked as a geologist with the Oregon Department of Geology and Mineral Industries for nearly 30 years. In his spare time he enjoys riding with Cycle Oregon and sharing stories about the incredibly cool geology of Oregon.

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