Spec. for mixing soils?

[Davidmat]

Is anyone aware of a spec or proceedure for mixing two types of soils together? The existing fill soils at our project site are too silty to use as a common borrow. If we were to mix them with a state standard gravel borrow (50/50 ratio), this soil sould be re-used. In addtiion, these existing soils are contaminated; therefore, keeping them on site is prefferred.

 

[Ron]

The "specifications" are end result. If for instance you are looking for a certain stability value, then you can mix soils to achieve such. If you are looking for a certain gradation, you can also mix to meet that.

Using gradation curves, you can optimize the gradation to achieve whatever end result you want (given enough time and money!!)

 

[Focht3]

Davidmat:

Mixing is a real headache - unless you're only dealing with a few cubic yards of material. This will be very slow and costly. Tell your client to plant a new money tree - he'll need it.

Have you thought about using a silt-sized additive like Portland cement or Class C fly ash? These are much easier to mix than gravel; you can use an agricultural-type discing unit to blend your mixture. This won't work with gravel - it will ruin the discs.

What are the contaminants in the soil?

 

[BigH]

I agree with Focht3. Gravel is not a keen idea. We are using a flyash and f-m sand mix for embankment and then disk-harrowing for mixing. It works well - assuming your site soils are silts and not clays (i.e., basically granular silts).

 

[Ron]

The purpose of mixing either cement or fly ash into fine soils is so that their hydration/cementation will stabilize the materials. If you have contaminants, particularly organic solvents or actual organics, the hydration process will be severely impeded, thus negating the effects of cement or fly ash.

The gravel does not rely on chemistry...it is a mechanical stabilization. It can be easily mixed with the right equipment. It's pretty common in my area.

 

[Davidmat]

The soils to be mixed are generally granular with high silt contents (~25% silts). The soils contain elevated levels of metals and semi-volatile organic compounds (SVOCs) above Rhode Island Department of Environmental Management (RIDEM) Standards.

 

[BigH]

Ron:
FYI: We are not mixing flyash into the sand to stabilize it -actually it is the other way around. We have abundant flyash deposits. We are using 85% flyash (about 40% sand sized particles) and 15% fine to medium sand. For your information we are getting soaked CBR values in excess of 10 with this mix. For subgrade we are mixing 25% sand and 75% flyash - CBR soaked is >15. The only reason we aren't using flyash alone is that contractually, the contractor must mix soil and flyash. The "auditors" might make hay if we were not mixing something with the flyash. Flyash alone has the requisite soaked CBR. We chose 15% for embankments because, well, it can't be argued that it isn't a mix - we figured at 10% or less, one might argue that we were just paying lip service. Contract over-rules technical yet again.