Trail Surfacing

1. If aggregate materials are going to be used for the repair or surfacing of trails, consider the preceding table and corresponding graph as general guidelines for materials.
   • Definitions:
     1. PA DOT (Pennsylvania Department of Transportation, and Penn State Center for Dirt and Gravel Road Studies)
     2. D60 = Diameter when 60% passes, 40% retained
     3. D30 = Diameter when 30% passes, 70% retained
     4. D10 = Diameter when 10% passes, 90% retained, AKA the “effective range” which is correlated w/ the permeability of finer grains
     5. Cu = (Hazen) Coefficient of Uniformity = particle size range ratio: Cu = D60/D10
        1. Cu < 5 Very Uniform, and may need envelope(geotextile) or an intermediate layer
        2. Cu = 5 Medium Uniform
        3. Cu > 5 Non Uniform
     6. Cc = Coefficient of curvature = measures particle size curve shape: Cc = (D30)^2/(D60xD10)
        1. Cc from 1 to 3 are typically well graded
     7. GW/GP (Graded Well/Poor for Gravels); SW/SP (Graded Well/Poor for Sands)
        1. Well graded soils have a wide range of particle sizes

2. The boundary between gravelly and sandy soils is a range, not a point, and not critical for soil behavior.
3. The minimum depth of compacted aggregate should be three times the largest stone diameter. However, PA DOT recommends a 3 inch compacted minimum for the TSA mix.
4. When ordering materials, request a “Plasticity Index” using ASTM D4318 to show the range of soil water content where the soil exhibits plastic properties: (0-3)- Non-plastic, (3-15) – Slightly plastic, (15-30) – Medium plastic, >30 – Highly plastic. Less than 6 is preferred. Many DOTs strive for a plasticity index and the liquid limit of the minus No. 40 less than 6% and 25%.
5. Soil tests may be needed should surfacing be applied to determine geotextile needs for load bearing capacity and trail longevity
6. For longevity also consider compacting aggregates at optimum moisture content (table below):

Typical compacted densities and optimum moisture contents for USCS soil types (after Carter and Bentley, 1991).
Soil Description	USCS Class	Compacted Dry Unit Weight	Optimum Moisture Content (%) for compaction
(lb/ft3)	(kN/m3)
Gravel/sand mixtures:
well-graded, clean	GW	125-134	19.6-21.1	8-11
poorly-graded, clean	GP	115-125	18.1-19.6	11-14
well-graded, small silt content	GM	119-134	18.6-21.1	8-12
well-graded, small clay content	GC	115-125	18.1-19.6	9-14
Sands and sandy soils:
well-graded, clean	SW	109-131	17.2-20.6	9-16
poorly-graded, small silt content	SP	94-119	15.7-18.6	12-21
well-graded, small silt content	SM	109-125	17.2-19.6	11-16
well-graded, small clay content	SC	106-125	16.7-19.6	11-19

References:

• US Dept. of Agriculture, USFS: Environmentally Sensitive Road Maintenance Practices for Dirt and Gravel Roads
• Penn State Center for Dirt and Gravel Road Studies: Trail Surface Aggregate (TSA) Technical Bulletin PA DOT 2014
• Uelph Turfgrass Institute, Dr. R.W. Sheard: Drain Envelopes, Sports Turf Management 3/99
• HQ Dept. of Army, Materials Testing, The Unified Soil Classification System (USCS), FM 5-472 NAVFAC MO 330 AFJMAN 32-1221(I)
• Carter and Bentley, Correlations of Soil Properties, 1991

More:

• Gravel Roads Maintenance and Design Manual
• What is Good Gravel?
• Road Aggregate 101
• Plasticity requirements of aggregates used in base and sub base course
• Driving Surface Aggregate (DSA) and plasticity index

7. Gravel Base Course and Surfacing Table: recommended size distributions (see#1 1-7 for explanations)

8. Graph: Recommended size distributions (see#3 for explanation)