Subgrade Preparation

[martin888888]

We are constructing a 10" gravel roadway over existing surface. No cutting of existing ground will occur. Is it normal to run a roller over the existing ground and get our 95% compaction before placement of the rock?

 

[Ron]

Yes, but I would require higher than 95%.

 

[cvg]

You say no cutting, but you really should strip the topsoil and then scarify the subgrade, add water and then compact. Nothing wrong with 95% compaction, that is fairly standard, especially for a gravel road.

 

[oldestguy]

I like to use a simple rule that may or may not require undercutting, compaction and the like.

Before placing the gravel, loaded trucks should not make a rut over 2 inches deep. Otherwise, you may well find the resulting job has 18" thickness gravel at the rut and 2" of gravel beside the rut.

If in frost type country any high degree of compaction is likely lost over the winter.

Also on stripping topsoil, I ask why. Let's say you have 4 feet of topsoil, as at some areas in the states.

My take is that topsoil tends to soften somewhat quicker when wet than non-organic soil. I've done many a job with only stripping sod and all works great, with my 2 inch rule. By the way, I think stripping topsoil is sometimes just a habit, with no reason for it, and not always a good habit.

 

[msucog]

you should probably clarify "standard" or "modified" Proctor when throwing out 95% compaction...just a suggestion

 

[cvg]

oldestguy's suggestion is valid, it is a type of very cheap geotechnical investigation. However, if you are going to spend the money to build the road, I would recommend you spend a little more and have a geotech visit the site, collect samples, analyze and then give you a recommendation.

 

[CJ4]

For what will the road be used? What kind of stone are you placing?

In most places I've worked, there was no density testing of roadways in cut since the soil was consolidated. However, there was always a proofroll done to determine if there were any areas of organic material, plastic clays or other unsuitable material. I would highly recommend doing this at a minimum. Another option if this is a lightly used road is to put down geogrid before the stone to bridge any soft areas.

If you're not cutting the road, however, there will be additional static loads placed on the subgrade due to the new loading (10" of stone) and you will probably get differential settling. If you are just putting down a gravel road (ABC or similar), there shouldn't be enough to matter.

Geotechnical testing is great when you are doing a pavement design for a road. However, it is only a general sampling of the roadway, typically every 500 feet or so, and proofrolling and undercutting is still required for localized areas of unsuitable soil.

Any undercut material should be replaced with compacted fill. Depending on the type of soil and the roadway use, I typically recommend 95% of the standard Proctor up to the last foot. The last foot should be at 100% of the standard Proctor and the roadway base should be at 100% of the modified Proctor. In areas of deep cut you can lower these standards for the bottom lifts to bridge soft areas.

CJ

 

[fattdad]

O.K. I'll jump in. . .

I have a basic problem "compacting" a natural soil subgrade. In the absence of geotechnical data, you may find that the natural soil is pretty strong (i.e., suited for subgrade support), but the natural moisture content is over "optimum." Now what's the contractor obligated to do? Scarify and dry?

If you just tell the contractor to compact (and it's naturally at 93 percent compaction), he'd likely do what's required and turn the site into mud. For what purpose?

A proper geotechnical study should provide a basis to design a pavement on the soil as it exists. If the natural soil is that soft, then an undercut or soil raft (i.e., with geotextile/grid) may be needed. If the soil has some natural strength, then I'd just do a proofroll with a fully loaded 10T dump truck and address those areas that rut, weave or become otherwise disturbed. After some surgical repairs, I'd then go in with my pavement section - in this case using dense-graded aggregate.

I'll point out that an aggregate road will be prone to infiltration and the water will perch on the subgrade soils (i.e., 10-in below the driving surface) and you may likely get post-construction disturbance irrespective of whether you compact the subgrade or proofroll in advance. As a engineering measure, you may want to consider open-graded aggreagate, drainage relief and a top surface of dense-graded aggregate.

Then again, I don't know the overall objectives of your project.

f-d

¡papá gordo ain’t no madre flaca!