What I Learned About Gravel
A few months ago I started volunteering with a community trail restoration group that works on some of the walking paths near where I live. The trails had fallen into disrepair over several years — erosion, root exposure, drainage problems, the usual things that happen when foot traffic and weather conspire against a maintained surface. I signed up mostly because I like walking those trails and figured I should help take care of them. I did not expect to develop strong opinions about gravel.
But here we are. I now know more about aggregate materials than I ever thought I would, and I'm going to write about it, because I spent the time learning it and it seems like a waste not to put it somewhere.
The first thing I learned is that "gravel" is not really a single thing. It's a general term for loose, granular rock material, but the specific type of gravel you use matters enormously depending on the application. For trail work, the considerations are different than for a driveway, which are different from a drainage bed, which are different from a construction subbase. The volunteers on the project who had done this before were very particular about what got used where, and once they explained why, it made complete sense.
The most common classification system for aggregate materials is based on particle size. At the finest end you have stone dust, sometimes called screenings or crusher fines, which is essentially powdered rock with particles smaller than about a quarter inch. It compacts very well and creates a smooth, firm surface. We used a fair amount of this as a top layer on some of the flatter trail sections. It packs down nicely underfoot and sheds water reasonably well if the grade is right. The downside is that heavy rain can wash it away if drainage isn't handled properly, and it can get muddy in sustained wet conditions.
Next up in size is what most people picture when they think of gravel: particles roughly between three-eighths of an inch and three-quarters of an inch. This is sometimes sold as "pea gravel" when the stones are rounded and smooth, or as "crushed stone" when the pieces are angular and have rough faces from being mechanically broken. The distinction between rounded and angular turns out to be very important. Rounded pea gravel looks nice and feels pleasant to walk on, but it doesn't lock together well. It shifts underfoot and migrates out of place over time. Angular crushed stone, by contrast, interlocks when compacted. The irregular faces grip each other, creating a more stable surface. For trail work, angular material is almost always preferred.
There's a product called "crusher run" or "dense grade aggregate" that we used extensively on the project. It's a blend of crushed stone and fine material, typically ranging from about three-quarters of an inch down to dust. The mix of sizes is the key: the larger pieces provide structure while the fines fill in the gaps and bind everything together when compacted. It's sometimes referred to by numerical designations depending on the region — #411, #21A, QP (quarry process) — but the idea is the same. You spread it, wet it lightly, compact it with a plate compactor or a roller, and it forms a surprisingly hard, durable surface. Most of the main trail sections we restored used some variant of this approach.
Larger aggregate, in the range of one to two inches, serves a different purpose. We used it primarily as a base layer in areas with drainage problems. Laid down in a trench or a low area and covered with landscape fabric and finer material on top, the larger stone creates channels for water to move through without destabilizing the walking surface above. This is essentially how a French drain works, and we installed several along sections of trail where water had been pooling and creating boggy, impassable stretches. The improvement was dramatic. Areas that had been ankle-deep mud for as long as anyone could remember were suddenly dry and walkable within a few days of the work being completed.
I also learned about the different types of rock that gravel is made from, which affects its performance. Limestone gravel is very common and relatively inexpensive. It compacts well and the calcium carbonate content means it can actually cement itself together slightly over time as it weathers, which is a nice bonus for trail surfaces. Granite is harder and more durable but doesn't bind as well. Traprock, which is a catch-all term for various dark igneous rocks like basalt, is extremely hard and resistant to wear. Recycled concrete is sometimes used as an aggregate material as well, and it performs reasonably well, though it can have a high pH that affects nearby soil and plants.
One thing that surprised me was how much the source quarry matters. Two batches of "the same" product from different quarries can behave quite differently depending on the parent rock, the crushing process, and the gradation. The project coordinator mentioned that they had learned through trial and error which local suppliers provided material that worked best for their purposes, and they stuck with those suppliers even when slightly cheaper options were available. Consistency matters more than you'd think.
We also dealt with some erosion control work that involved riprap, which is very large stone — six inches to two feet or more — placed along slopes and water channels to prevent washout. Placing riprap is more art than science. You want the pieces to fit together somewhat, with the flat faces oriented to deflect water flow, and you want to avoid creating gaps large enough for water to undercut the installation. A couple of the more experienced volunteers were genuinely skilled at this. They'd eye a stone, rotate it a few times, set it down, and it would lock in like a puzzle piece. I mostly carried stones and handed them over.
The whole experience gave me a new appreciation for how much thought goes into surfaces we walk on without thinking. Every well-maintained trail, every parking area, every gravel road represents a set of decisions about material, gradation, compaction, and drainage. Someone chose that specific gravel for a reason. It wasn't random.
The trail project is ongoing. We go out every few weekends when weather permits and work on the next section. It's good physical work and the group is pleasant. Nobody talks too much. We show up, move rock and dirt for a few hours, and go home. The trails are noticeably better than they were a year ago. I've started paying attention to gravel everywhere I go, which is probably not something I should admit, but there it is.
If you're involved in any kind of outdoor maintenance work and you haven't thought much about aggregate materials, I'd encourage you to look into it. It's one of those subjects that seems impossibly mundane until you understand it, and then you can't stop noticing it. I'm not sure that's a recommendation exactly, but it's honest.
— Kevin Vadala