Gravel Road Surfacing
The North Dakota DOT maintains only PAVED ROADS. As such, the DOT uses gravel for pavement base and
shoulder material. They typically specify:
Cl 5 Gravel - drainable base material that is placed beneath a paved surface. Water that passes
through pavement cracks enters the Cl 5 base. Since the Cl 5 base has limited fine material, water
easily drains to the outer edge of the roadway rather than progressing down into the subgrade. By
keeping the subgrade (i.e., natural soil foundation) dry, the NDDOT maximizes the roadway's load-carrying capacity.
Cl 13 Gravel - shouldering material for highways. Cl 13 shoulder material is essentially a Cl 5
material with more fine material allowed (i.e., a dirty Cl 5). The fine material provides a
reduction in water passage and allows sensible utilization of pit materials.
Local roadway gravel surfacing requires modifications to the typical pavement base and shouldering
gravel. Gravel without binder leads to wash boarding, dust, and float - all critical safety risks.
Adding a binder can create a quality gravel surfacing material for local gravel roads.
Quality gravel surfacing includes a binder material to hold the stone and sand together. Clay is a
natural binder that can bind the stone and sand into a gravel matrix. Generally, fines may be clay
or silt. Clay has good binder properties, silt does not. Clay has good engineering strength
properties, silt does not. A gradation test tells us the amount of rock, sand, and fines. A Pl test
(Plasticity Index - clay value) - this tells us how cohesive or 'sticky' the fine material is and
, as such, how well it will perform to hold the rock and sand together. Combined, the tests help tell
us how well the gravel will function as a surfacing material. See the spec info below.
Specifying and testing gravel are key to ensuring that you get the correct gravel for your gravel
road driving surface.