Subgrade CBR to Aggregate Cover Thickness

It is the number that decides whether a haul road needs six inches of rock or eighteen, and getting it wrong costs either money or a road that pumps. The relation used here is the Corps of Engineers cover equation -- the square root of the wheel load times the difference between one over eight point one times CBR and one over pi times the tire pressure -- scaled by a coverage factor that grows with the logarithm of the number of passes. Two consequences follow from the shape of it. Cover falls steeply as CBR rises, so measuring the subgrade rather than assuming it is worth a great deal: assuming CBR 5 on ground that tests at 2 produces a section that pumps and ruts before the first winter. And traffic enters only as a logarithm, so a haul road carrying ten times the passes needs about a quarter more rock rather than ten times as much. THE SENSITIVITY IS REPORTED RATHER THAN ASSERTED, because it is routinely overstated. Going from CBR 3 to CBR 10 at a fixed wheel load does not cut the rock by four -- it cuts it by about half, which is still a very large number across a site and is the honest case for a dynamic cone penetrometer or a plate test at a few locations. Run the comparison here rather than taking a rule of thumb. The correlation to resilient modulus, fifteen hundred times CBR, is the bridge to pavement design, and it is worth knowing its limits: it is reasonable for fine-grained soils in the low CBR range and drifts badly above about CBR 10, where direct modulus testing is the better basis. This flags when the entered CBR is past that. Geosynthetics change the arithmetic more than anything else on the list. A geogrid at the subgrade interface can cut required thickness substantially by improving load spreading, and on very soft ground a separation geotextile is what stops the rock disappearing into the subgrade over the first season -- a failure mechanism with nothing to do with the thickness chosen. THE REDUCTION IS ENTERED, NOT MODELLED: it must come from the manufacturer's design method and the applicable specification. And the honest limit is a threshold, not a curve. Below about CBR 3 the calculation still returns a thickness and the subgrade will deform under construction traffic anyway. Undercut and replace, chemical stabilization, or a designed working platform is the answer there, and this says so rather than returning a number that implies rock alone will work. A screening estimate. Cover charts differ between agencies and between the construction-platform and the pavement-design cases, and none is shipped. CBR itself is a crude index, sensitive to moisture and compaction at the time of testing: a subgrade tested dry in summer can be far weaker in spring, and the design value should reflect the worst credible condition rather than the tested one. It does not design a pavement, address drainage -- which controls subgrade strength more than anything else does -- or evaluate frost susceptibility and frost depth. The geotechnical investigation, the agency's design manual, and the geotechnical engineer govern.

cover thickness = alpha x sqrt( wheel load x ( 1/(8.1 x CBR) - 1/(pi x tire pressure) ) ), with the coverage factor alpha = 0.23 log10(coverages) + 0.15; resilient modulus Mr = 1,500 x CBR; the geosynthetic value is that thickness reduced by an entered percentage.

The Corps of Engineers CBR cover equation by name, with the widely used Mr = 1,500 x CBR correlation, which is reasonable for fine-grained soils at low CBR and drifts above about CBR 10. NO COVER CHART IS SHIPPED; agency charts differ between the construction-platform and pavement-design cases. The geosynthetic thickness reduction is ENTERED from the manufacturer's design method. The geotechnical investigation, the agency's design manual, and the geotechnical engineer govern.

One square root and one logarithm on a CBR the reader measures; no agency cover chart is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: cbr_pct, wheel_load_lb, tire_pressure_psi, coverages, geosynthetic_reduction_pct, alternate_cbr_pct, alpha_factor, thickness_in, thickness_with_geosynthetic_in, resilient_modulus_psi, alternate_thickness_in, sensitivity_ratio

• A screening estimate, not an agency chart charts differ between agencies and between platform and pavement casesthe agency's design manual
• CBR is a crude index sensitive to moisture and compaction at the time of testing; a summer value is not a spring valuethe geotechnical investigation
• The geosynthetic reduction is entered it must come from the manufacturer's design method and the specificationthe manufacturer's design method