Over excavation of adverse soils under a gravel road 2

[bbakk]

I am building a Fire Truck Access road (gravel) and the cross section is going to be something like 6" of 3/4" minus over 9" of 1-1/2" minus. I am building this road over adverse soils (saturated garbage fill, maybe clayey silty loom?? with zero percolation properties). My solution to this problem is to over excavate and replace with 4" minus. I can, out of the air, come up with a depth (of 4" minus) that will easily account for the adverse soils but I am tired of over designing (spending too much of the clients money). How does one calculate (and / or which soil property is calculated) the amount of over excavation needed for an economical design. It would be nice to have the calculations to go along with a "hunch". Thanks in advance ...

BRB

 

[CarlB]

There may not be design methods that model "garbage fill clayey silty loom (loam?)", I guess you mean low strength soil with organics. I would suggest use of a geotextile, and possibly a geogrid to reduce amount of gravel fill. I've seen some manufacturer's videos where they place something like 12" gravel over a geogrid/geotextile combo over swamp then drive on it. Your application appears to be a heavy axle load but infrequent use. I assume frost heaving is not a concern. I would recommend at least a geotextile to separate fill from garbage; consider geogrid if subgrade is very weak or gravel costs high. My "hunch" would be to provide geotextile, a section of 18" of 4" minus, then 6" of 3/4" minus. Could replace some of 4" minus with 1.5" minus but don't see the need.

So back to your original question-how to justify a design? There probaly is literature & design methodologies for road design in poor/soft/organic subgrades. Don't know of one offhand but probably can get something from geotextile manufacturers. Usually local experience is heavily relied on in these cases. This may not help but does give you a second opinion!

Carl

 

[jimbo]

bbakk;
By all means use good-quality geotextile fabric as recommended by CarlB.
Your aggregate section will probably suffice on the fabric. I no longer have the means to design the section, but over 40 years of experience says you are on the right track.
Also, tell your client that the subbase condition is unique, and that repairs may be needed later on.

 

[emmgjld]

For very soft and unstable subgrade soils I have done reasonably well using a base geotextile for separation and reinforcement, followed by 12" to 24" (experience dictates) of a coarse,subbase (1-1/2" would suffice, though I usually use coarser material), then a layer of geotextile or geogrid for reinforcement and then 3/4" aggregate 'wear' layer.

I have found that when the lower gravel is 'sandwiched' between the geoproducts, the entire structure will move around but will carry a lot of traffic.

 

[Ron]

Agree with the geotextile approach. If you want to get really geeky and spend more time than its worth to analyze it, you can do so with elastic layer analysis. If you are going without geotextile, limit the vertical strain in the top of the bad soil layer to near zero (this means that the modulus of the soil above the crud is high enough to bridge the poor soil condition). To limit the strain, do a trial and error approach on the thickness of the select fill layer.

If you want to analyze the geotextile condition, you shoot for the lateral stress in the soil layer equal to the fabric strength (or some safety factor on the strength), andd/or the vertical stress equal or less than the punching strength of the fabric, and again use a trial and error thickness approach to get your answer. You will find that very little rock is actually needed over the geotextile, but we usually add more for peace of mind/national fudge factors, etc. After all that you'll probably find that CarlB, jimbo, and emmgjld have pretty much nailed it based on experience (for which there is no analytical substitute!)