Finely crushed compacted rock is a popular trail surface improvement throughout America.
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.
by Lois Bachensky, 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:
Sieve Size % Passing
| Particle Size | % of Passing |
|---|---|
3/8" | 100% |
#4 | 90 - 100% |
#8 | 55 - 80% |
#16 | 40 - 70% |
#30 | 25 - 50% |
#200 | 6 - 15% |
If the gradation of crusher fines does not meet the 6% passing the #200, clay fines may be added and mixed with the aggregate to do the job.
Crusher fines will have exactly the same color as the rock from which they are ground. The color should either match or complement the native stone and surroundings of the site, but color is of secondary importance to the structural characteristics of the fines. If the crusher fine surface needs to be patched in the future, the fines added should be from the same rock source or the colors may not match.
Crusher fines are not expensive, but the cost of delivery can equal or exceed the cost of the material. An 8' wide contractor built crusher fine trail in the Denver area costs between $4 and $5 per foot, not including the cost of site preparation and infrastructure such as retaining walls, and bridges. This compares with $12 to $15 per linear foot for concrete. The fines cost about $3.00 per cubic yard delivered in the metro area.
The fines weigh approximately one ton per cubic yard before compaction. When determining quantities, calculate the cu. yds. needed for the length, width and depth of surfacing, and then add 20 to 30% to compensate for compaction.
Also, consider ordering and stockpiling additional fines for future maintenance since it is often difficult to match the colors and composition from other sources.
For Crusher FinesSubgrade, slope, curves, and other components should be designed by engineers to the same standard as a paved trail surface. Special attention should be given to drainage toensure all water is conveyed away from or underneath the trail. Concrete is recommended for areas where erosive flows are unavoidable.
Underlying soils need to be analyzed to determine soil suitability. Certain clays, organic soils, and high moisture soils require special preparation, such as placement of a geotextile. The fabric helps prevent fines from mixing with soft soils below and helps control damage from vegetation.
1. Keep crusher fines from becoming saturated with water.
2. Prevent concentrated flows of runoff from reaching crusher fine surfaces.
3. Quickly and efficiently drain crusher fine surfaces before water can form a concentrated flow across the fines.Grades
In general, when using crusher fines, grades should be kept as minimal as possible. Grades above 5% should be used only where absolutely necessary, but should not exceed 8%. Grades steeper than 8% may require a harder, more stable surfacing material.
To prevent washouts for long stretches of trail on grades, dips or grade breaks should be designed into the trail. The steeper the trail, the more drainage features will be required. If grades consistently greater than 5% are required, consider using a different type of surfacing material.
Curves
If bicycle traffic is using the crusher fines trail and speeds may exceed 15 MPH, avoid curves of less than 50' radius, and switchbacks of less than 35' radius. Radii tighter than these may cause bicyclists to lose control on loose crusher fine surfaces. Wherever bike paths curve on a grade, provide long sightlines and a transition zone at the top and bottom of the grade.
Outslope or Crown
The crusher fines trail should be crowned to drain water at 2% or outsloped at 2%. This will ensure surface water sheds from the surface rather than penetrates into the surface.
Minimal Cross Slope
If the crusher fines trail is crossing a flat area with no cross slope, the trail needs to be raised slightly above the surrounding ground to ensure the water drains off the trail surface. If there is some cross slope, the pitch of the trail surface should be in the same direction as the slope. This preserves the natural drainage patterns at the site.
A ditch above the trail may be needed if concentrated or heavy flows can reach the trail from the upslope area. Ditches on both sides of the trail may be needed when the trail is crowned and goes through a wet area.
Crusher Fines For Accessible Trails
Since crusher fine trails are not always smooth enough or hard enough, they do not fulfill all the requirements of a fully accessible trail. To make the surface harder and smoother, lime or some other stabilizing agents may be added to the crusher fines so that it will set up harder and remain that way for longer periods of time.
For accessible trails, try to keep the outslope and crown to 2% maximum. In locations where surface pitch could divert a wheelchair into a dangerous place, the cross slope should be as close to 0% as possible.
Selecting a Crusher Fines Trail Construction Method
One method of placing the crusher fines involves excavation of a trench, and backfill with crusher fine material. Prior to placing the crusher fines, a 5" deep trench should be cut slightly wider than the desired width of the trail. Adequate excavated material should be placed along the edges of the cut to use later as backfill. Drainage collection ditches and schedule 40 plastic pipe may then be placed before laying the crusher fines. To avoid maintenance problems associated with pipes plugging up, consider using concrete lined swales or dips to move water across the trail.
Underlying soils should be analyzed to determine the need for geotextiles. Certain clays, organic soils, and high moisture soils most likely will require placement of a non-degradable geotextile. The fabric will help prevent the crusher fines from mixing with the soft soils below. The geotextile is easilyhand laid using utility knives for cutting and wire staples for securing. If needed, a growth inhibitor such as "Casoron G-4 or G-10" may be applied.
After the fabric is placed, the crusher fines are spread and smoothed with shovels, mcleods and other hand tools. Leveling bars may be used to smooth the surface to a 2% cross slope toward the downhill side for drainage or the surface may be crowned to drain to both sides of the trail. The crusher fines should be spread to a depth necessary to meet the desired compacted crusher fine thickness. (For example, spread 7" to 8" deep to get a 5" compacted depth)
After initial smoothing and compacting, the trail edges are back-filled and dressed smooth. Finally, the trail surface is re-compacted with rollers or vibratory compactors. During the compaction process, the crusher fines should have some moisture to help "cement" the material when it dries. To ensure adequate moisture, fines may be sprayed with water during the crushing process to give them 4 to 5% water content. If this is not possible, and fines are dry at the time of compaction, use a very fine mist type hose and spray the fines sparingly. Using too much water will cause the crusher fines to become mushy or run off. The disturbed edges should be raked smooth and seeded.
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