The Art of Crushed Stone Trails

By Danna Lambert, Arrowhead Trails, Inc.

Arrowhead Trails, Inc. has built almost 400 miles of natural surface trails since 1995. Their specialty is machine-built, 48” wide trails for mountain bikers, runners, and hikers. Even though 95% of their new construction is with native soils, they are receiving more requests for soft surface, accessible trails built with 3/8” minus crusher fines; or crushed stone, cinder or rock dust as it is called in other areas.

crushed stoneWhether it’s a trail to school in an urban area or a core loop of your open space trail system, crushed stone trails provide a user-friendly, all-season surface for all types and ages of visitors. If built properly crushed stone trails can meet the American with Disabilities Act (ADA) Accessibility Guidelines. ADA trails often service an incredibly diverse range of visitors including: hikers, bikers, runners, strollers, mobility impaired, visually impaired, and wheelchairs.

Accessible crushed stone trails should be designed and constructed at grades less than 8% grade to promote accessible use. Trail tread grades of up to 10% for short distances are allowed but difficult for most visitors seeking an ADA trail to enjoy. Overall trail grade averages of less than 6% will provide the most user-friendly experience and offer the most sustainable natural trail surface if compacted crusher fines are to be used.

Many parks and open space areas are constructing accessible, soft surface trails into their trail systems. In areas with long periods of rain or snow and clay or silt based soils, park infrastructure trails are often surfaced with concrete or crusher fines to minimize muddy trail conditions and reduce facility maintenance.

High clay content soils typically cause trails to be slick and muddy when wet. They also take longer to dry out since their extra fine particles don’t give up water easily. Trail treads surfaced with 4-6 inches of compacted fines over a landscape fabric can eliminate many of the problems associated with soils and climate. Landscape fabrics or geotextiles are also the key to preventing vegetation from growing into the trail and preventing commingling of the crusher fines with the natural soils.

Since crusher fine trails usually cost in excess of $10.00 per linear foot it is important to construct them right the first time. Success relies on getting the right materials and installing them correctly. Mistakes often result in poor compaction, soft surface conditions, non-accessibility, and inevitably costly repairs.

Crusher fines in their purest form have no soil mixed in, they are pure crushed stone. Gravel and crusher fines differ from one another in that gravel is screened to remove the fines, which contain the natural binders/cements. Gravels remain loose because they have dead air space within the structures that allow them to drain well and resist compaction.

Crusher fines retain their inherent soil cements and binders which promote soil compaction. Fines that contain too many rounded particles (like some decomposed granites) are more difficult to interlock and often yield a loose and unconsolidated surface. Angular particles like andesite, dolomite, and certain types of granite can easily be wetted and compacted to meet the ADAAG.

A good indication of the strength of a rock binder is the hardness of the parent rock. The harder the source rock, the stronger the binders will be. Crushed rock contains the original rock cements and binders within the rock dust. These binders, combined with water and then compacted with a vibratory roller or plate compactor should produce a solid, compacted surface that resists significant deformation from hiking boots and mountain bike tires. We frequently see baby strollers and road bikes using our crushed stone trails.

A sieve analysis for trails, using 3/8” minus crusher fines, typically describes the material with the following specifications:

If the surface of the trail becomes loose and un-compacted over time it can often be reshaped, wetted and compacted again, (as long as the fines have not sifted to the bottom and the larger particles floated to the top). Poor compaction can be the result of a variety of influences such as: lack of fines to bind particles together, improper wetting and compacting, lack of angularity, lack of precipitation, trail grades greater than 6%, and/or inadequate amounts of natural soil cements in the parent material. Some “refreshing” of trails material is required on a routine basis. Trail tread grades over 6% will require significantly more maintenance since they tend to unravel or erode over time.

In summary, the best crusher fines for trails exhibit three critical characteristics. The rock source is crushed into irregular angular particles that interlock and bind into a firm matrix. The material has particles ranging from dust to a specified maximum particle size in order to mechanically bind the matrix (ex. 3/8” minus). Lastly, the material must retain all of its original binders in order to be recompacted to a firm surface after shaping, wetting and vibratory compaction.