How to Build a Rock Crusher?

Building Crusher Fines Trails

Finely crushed compacted rock is a popular trail surface improvement throughout America.

By Lois Bachensky, USFS
USDA Forest Service

Crusher fines are small particles of crushed rock. Generally, they are the leftovers from rock crushing operations, but at times the rock can be ground especially to make the crusher fines. To make a good trail surfacing material, they should have a range of particle sizes from a fine dust up to a specified 3/8" maximum particle size. With proper subgrade preparation and drainage, the crusher fines trail should remain stable for many years in all weather conditions.

A crusher fine trail combines the rustic feeling of a natural surface trail with a surface type that's durable (but not concrete or asphalt). The natural gravel-like surface feels more like a trail than a hard surfaced path and fits in well with primitive settings.

An excellent alternative for medium to high use trails, crusher fines can be used for mountain bike paths, hiking and running trails, and when properly constructed, for accessible trails. Generally, crusher fine trails are more suitable to mountain bikes than road bikes, and may cause some difficulty for the physically-challenged.

Water, drainage, existing soil types, and the types of usage are the primary considerations for designing and constructing crusher fine trails. Crusher fines are highly susceptible to washouts from running water, particularly if fines become saturated such as during spring snowmelt.

Crusher fines are available in various stone types, colors, and particle sizes, but not all crusher fines are suitable for trails. Tradeoffs may need to be made between the surface smoothness and erosion resistance, between colors and rock types, and between choice and availability.

The rock must be crushed into irregular and angular particles to allow interlocking into a tight matrix. The more angular the particles, the better. Rounded particles like pea gravel or decomposed granite never mechanically lock together.

The crushed rock must have adequate fines and some natural binders in order to cement the particles together after the fines are moistened, compacted, and allowed to dry. The fines, when laid to a depth of 4 to 5 inches, should bind to each other in a consolidated slab which is porous yet resistant to water falling on the surface.

Particle size for crusher fines on trails should be 3/8" minus. Fines from granite or other suitable hard stone works best. The ideal particle size distribution is one where there are enough small particles to completely fill the voids between the larger ones. One good distribution to use is: