- 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.
- PA DOT (Pennsylvania Department of Transportation, and Penn State Center for Dirt and Gravel Road Studies)
- D60 = Diameter when 60% passes, 40% retained
- D30 = Diameter when 30% passes, 70% retained
- D10 = Diameter when 10% passes, 90% retained, AKA the “effective range” which is correlated w/ the permeability of finer grains
- Cu = (Hazen) Coefficient of Uniformity = particle size range ratio: Cu = D60/D10
- Cu < 5 Very Uniform, and may need envelope(geotextile) or an intermediate layer
- Cu = 5 Medium Uniform
- Cu > 5 Non Uniform
- Cc = Coefficient of curvature = measures particle size curve shape: Cc = (D30)^2/(D60xD10)
- Cc from 1 to 3 are typically well graded
- GW/GP (Graded Well/Poor for Gravels); SW/SP (Graded Well/Poor for Sands)
- Well graded soils have a wide range of particle sizes
- The boundary between gravelly and sandy soils is a range, not a point, and not critical for soil behavior.
- 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.
- 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%.
- Soil tests may be needed should surfacing be applied to determine geotextile needs for load bearing capacity and trail longevity
- 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 (%)|
|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:|
|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|
- 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
- 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)