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Soil Modification or Stabilization

Soil Modification or Stabilization

(OP)
I have a project where we plan to use a dredged marine soil as structural fill and plan to use lime or cement to modify/stabilize it for use. I've performed a number of tests on the native material (hydrometer, proctor, atterberg limits, and moisture content) to classify it and to be able to quantify improvements through the addition of the admixtures. The material consists of 35% silt, 35% sand, 20% clay, and 10% gravel, LL=38, PL=21, PI= 17, SL=18, optimum moisture for compaction is 13.5%, and the natural moisture is about 40%.

Soil modification is not a common procedure in our area, so I'm looking to develop a test mixture program to evaluate the proper mixture percentages. I'm unsure where to start. If I'm using cement what percentage should I start at and what increments should I continue forward with? If I'm using lime are the percentages different? Is there a specific mixing and mellowing procedure (ASTM or other)? I'm hoping to come up with an optimum mixture percentage for each and then perform a CBR to evaluate the strength. Are there any other test I could or should be performing to evaluate the effectiveness of the additives?

Appreciate the input.

RE: Soil Modification or Stabilization

What is your strength goal? Using cement will create a material with an unconfined compressive strength of at least 100 to 150 psi, even with small amounts in the 2 to 3 % by dry unit weight.

The material itself doesn't seem that bad, just high moisture. Dewater it and check a few CBRs.

RE: Soil Modification or Stabilization

Ask soil improvement contractors or look at cement companies. Normally they develop charts for qu/CBR vs cement amount for various type of soils and water contents.

RE: Soil Modification or Stabilization

Hi OP,

I have been involved in project in Christchurch New Zealand where liquefaction is an issue. One foundation Option given in the MBIE Guidelines was a densified crust, this involved excavating mixing with cement/lime and recompacting. More or less what you are proposing. One question, what is the stabilised material required to support? A subbase for a road or a building platform for a structure?

The soils you have in your area are a little different from what they tested in the MBIE fields test, probably a lot more cohesive. This may mean you might have to use a combination of lime and cement. Essentially you should do a range of percentages i.e. starting at 3%,5% and 8% and perform some remoulded CBRs if you have the luxury. You could alternatively do a field test and do three separate 3m x 3m x 1m dp stabilised areas and test after 7 and 28 days. All depends on how big the job is. An extract from the MBIE guidelines below. See link attached for full document http://www.building.govt.nz/assets/Uploads/buildin...

Reinforced Cement Stabilised Crust Method Statement (excavate, mix and
replace) (Type G2a)

This method is generally suited to clean sand to sandy silt sites where the water table is at
least 1.0m below ground level. For sites with a higher water table, temporary dewatering
may be required.
The cement stabilised crust is to be a minimum of 1.2m deep (below foundation elements)
over the house footprint and extend at least 1m beyond the house perimeter foundation
line. It may be necessary to batter the sides of the excavation, and provide a drainage sump
to remove ground water for the duration of the excavation, filling and compaction work.
This method may not be used where water inflows cannot be controlled to prepare a ‘dry’
base of excavation.
The minimum cement content required for stabilisation is 8% (by dry unit weight).
Alternatively, laboratory testing can be used to determine the required minimum cement
content. Following are the minimum laboratory strength/stiffness to be achieved at 7 days
(or 28 days at the discretion of the Design Engineer):
• UCS > 1 MPa and initial tangent Young’s Modulus of 250 MPa; or
• CBR > 25.
Where the water table is close to the base of the excavation and it is difficult to fully
compact the first layer of fill, an increase in cement content to 10% is recommended and
compaction should be undertaken using a static roller.

RE: Soil Modification or Stabilization

(OP)
Thanks for the input. I think the highest priority is to dry the soil to a compactable moisture. There is no specific area this soil is currently destined for. It would likely be placed under parking or warehouse construction. I guess I'm looking to determine is what the quickest way of drying the soil using an additive.

RE: Soil Modification or Stabilization

If there are any coal burning electric generating plants there burning high calcium coal, that fly ash generated is similar to quick like in moisture grabbing capability.

RE: Soil Modification or Stabilization

Just read the ASTM and follow the standards for testing cement-stabilized materials.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Soil Modification or Stabilization

Is this a hydraulic dredging job or somehow excavating and hauling sloppy material to be disposed of somewhere? Will the water content be increased in the process?

If hydraulic dredging, I'd contact contractors that have hydraulic dredging experience. Chances are the fines will tend to stay in suspension while coarser material settles out. How you arrange the disposal area can greatly affect this and the resulting material that you want to use. Can you waste the fines somewhere and make use of the coarser fractions more rapidly? Are there restriction on disposal the dredge water? How about mud problems if not hydraulic?

RE: Soil Modification or Stabilization

Fattdad, do you know the ASTM number?

RE: Soil Modification or Stabilization

Also check the amount of salt that you can choose the right stabilizing materials.

RE: Soil Modification or Stabilization

Thanks for the ASTM references.

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