CBR expected for A-7 subgrade soils

[Mercycity]

Dear Colleagues,
I am perusing a soil testing report for material collected along the road center line. One things came to surprise is that all the soil classified to A-7 sub grade group have CBRs ranging between 7-18% at 95 Mod AASHTO density and CBRs increase to 10 - 28% at 98 Mod AASHTO density. According to this classification, these soils are clay soils and my expectation is to have a CBR of 0% at 90% density. The soil lab conducted the testing is not giving me a satisfactory answer. I am almost stuck how to continue with pavement design using these data. I am seeking help from anybody who have an idea or have met this kind of problem before I continue seeking more clarification from the soil Lab conducted the tests.

 

[BigH]

Is it possible to give LL, PL and gradation of the material? I'm rusty on what A-7 means . . .

 

[Mercycity]

Dear BigH

A-7 subgrade classification group is based on Highway Research Board (HRB). It is Ammerican System of classifying soils. The bases of classificaton of A-7 is that material passing 0.075mm sive must be grater than 36%, liguid limit must be greater than 41% and plasticity Index must be grater than 11%. The results of one of the sample retrieved from the report was 59% passing 0.075mm, liquid limit of 55%, Plascticity Index of 31% and CBR of 11 and 18% at 95 and 98 Mod AASHTO density respectively. My expectation is to have a 0% CBR of such type of soils.

 

[BigH]

Mercycity - I am familiar with the system (how it works) but don't use it as such. I don't think that you should expect the CBR to be zero - that it "water". I would expect, depending on the moisture content of the material to have low CBRs (if saturated or near saturated) - say in the order of 4 or less - this comment as per in situ subgrade material.

Fills placed at or near optimum moisture contents, I can see that the CBRs are higher - one thing is that the lab might not be giving soaked CBRs but unsoaked values. By test method, they should soak the mould for 4 days before determining the CBR - but perhaps they are not doing that. Check this out.

 

[emmgjld]

I would expect a CBR range of 1.5 to 3 for the soils I am familiar with.

 

[Mercycity]

Dear BigH,

Thank you for your contribution. In fact the results given above are for wet CBRs (four days soak)according the info from testing laboratory.

 

[fattdad]

A-7 soils include sandy lean clay and sandy silt. The CBRs that you cite are reasonable for these soils. CBR value of less than 2 are reasonable also.

Did the lab give you the penetration curves?

Did the lab publish the 0.1 and 0.2 values?

Was the 0.2 value higher than the 0.1 value (if so the test should have been rerun)?

Have you looked at the moisture and density of the molded sample? Was it compacted at the optimum moisture content or was it compacted on the dry side of optimum?

I'd re-read the CBR proceedure and make sure that the lab performed to that standard. If they did and you got corrected CBR values (i.e., to correct for any initial concavity) then I'd take the results as good.

f-d

¡papá gordo ain’t no madre flaca!

 

[molerat2210]

We see CBRs as low as 0.2 to about 4 for similar material. We use the 3-point method (10, 25 and 56 blow samples). AMRL had a proficiency sample last year that was aroud 0.2. Not sure how other labs come up with crazy high number all the time.