paving over structural horticultural soil 1

[versaengine]

I am a geotech working for a landscape architect.
He wants to use a designed "structural soil" that is root friendly for trees to be planted within and adjacent to paved areas- usually residential fancy driveways and parking, includes precast concrete interlocking pavers, Portland concrete-usually exposed aggregate or decorative stamped or stone laid on a concrete slab.

The soil is to consist of 2 different recipes = sand plus...

The sand= natural river sand/gravel (USDA size criteria) = coarse sand(47%) and fine gravel(43%)with a trace of fine to medium sand. Uniform gradation, Cu=2.2.

Mix #1 = add 8% by dry weight colloidal high plastic clay and 2% organic matter e.g., peat moss

Mix #2 = add 2%-4% peat moss, no clay

I want to determine either the standard proctor moisture density curve or the minimum and maximum density (for use with relative density evaluation)with the goal being to confirm the mix is adequate for pavement support. They want to place the material at the equivalent of 85% standard proctor and 2 to 3 ft deep below pavement. My role is to make sure this is adequate for support.

How would folks establish the compaction characteristics and subgrade support, Proctor or Min-max Density? CBR?

 

[Ron]

Generally the Proctor value is used for structural applications such as buildings and pavements. The max-min (relative density) is OK for correlations to SPT, but not really practical for construction quality assurance.

Providing root growth friendly soils under pavements is asking for trouble. You are encouraging root growth under the pavement, which will cause cracking and faulting of concrete or asphalt pavement sections. I would not recommend this action.

As for your materials, both would be better served for the Modified Proctor, not the standard Proctor. Further, 85% of standard Proctor is very low and additional compaction from traffic, construction, and variations in the water table will cause differential settlement and potential rutting. The requirement is too low, even at 2 feet below the pavement.

Isolate your pavement from the horticulture...you'll have a better performing pavement and the adjacent plant growth won't suffer.

 

[oldestguy]

Be aware of the expansion and shrinkage that occurs should the clay you use be bentonite (montmorillonite). If this material is not restrained, it can swell to 16 times its original volume. In a mix it can shrink or expand, depending on available moisture (tree roots are great for removing moisture) and you may not like the results.