CBR expected for A-7 subgrade soils
CBR expected for A-7 subgrade soils
(OP)
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.
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.





RE: CBR expected for A-7 subgrade soils
RE: CBR expected for A-7 subgrade soils
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.
RE: CBR expected for A-7 subgrade soils
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.
RE: CBR expected for A-7 subgrade soils
RE: CBR expected for A-7 subgrade soils
Thank you for your contribution. In fact the results given above are for wet CBRs (four days soak)according the info from testing laboratory.
RE: CBR expected for A-7 subgrade soils
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!
RE: CBR expected for A-7 subgrade soils