It is based on the type of Road. You should specify it as minimum acceptable CBR varies with the type of road (as Traffic varies).

Basically, Sub-grade is required 4% and Sub-base is actually required 30% 4 days soaked CBR.

@janak parmar

@md. sharior alam

thankyou for ur response

G'day

I don't believe that there is minimum acceptable CBR value for subgrade - the subgrade is the natural material so you work with what is there. If it is weak (low strength) subgrade, you need more pavement material over the top of it. It may be a geogrid and rock to bridge the soft subgrade before placing pavement layers. Essentially you place layers of increasing strength over the softer lower layers until you can support the higher strength material. The big challenge with low strength subgrade is whether you can construct and compact the layers above it.

In Queensland, for higher order roads a soaked CBR of 80 % is used for base, 45% for subbase and 15% for selected fill (lower subbase). For lower order roads, soaked CBR of 60% for base, 30%/35% for subbase and 10%/15% for selected fill (lower subbase) could be appropriate. The depth of each material is based on the traffic and subgrade CBR.

The other question when specifying CBR is what level of compaction you use in the mould.

@greg stephenson thank you fr you detailed response...

Greg's answer is the best one, I think.

@behzad kalantari ty fr ur response

I think it depends on the traffic loads and the type of pavement you want to develop. There is no minimum acceptable subgrade. Some researches suggest its 4% for subgrade. But all depends on strengthening the subgrade by various techniques.

@jasim nisar ty fr ur respnse

Dear, a ceptable CBR value depends on the classification of the roadway. A minimum suitable value is 4 percent for subgrade, 30 to 45 percent for subbase and 80 percent for base course as per the SCRB specification adopted in IRAQ.

@ Saad Sarsam thankyou v much fr ur response..

for subgrade it should be minimum 3% and for subbase it should be 30%.

@Ganesh Shivaji Ingle thankyou for your response

CBR value depend on type of soil used in sub-grade.

generally less then 5.

@Rajshekhar Rathod thankyou fr ur answer

Dear Najam kindly go through IRC 37-2018 which will give you detail guidelines for subgrade & Subbase recommended values, design.

Rajshekhar Rathod Thank you for your response

Dear Najam, I would like to inform you that according to Egyptian code, Min soaked value (10-15%) for sugrade, (25-40%) for subbase and 80% for Base. Fill more than 2 meter Min soaked CBR is 8%, fill in general Min soaked CBR is 10%.

@Mahmoud Enieb Thank you for your timely response

Hello Najam,

If you are looking to improve subgrade, stabilization is recommended for a value of 6 or less. Depending on your pavement design, the higher the CBR, the less stone you would need to build your pavement section. Well designed subgrade can save you on aggregates and concrete/asphalt. See this link:

https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/

@Brendan Daly Thankyou for your response....highly appreciated...

Dear Najan,

there's no easy or quick answer to your question.

Firts, when talking about CBR we refer to 4-day soaked CBR, which is the typical value used in pavement design (other conditions are possible but it's a long story).

If you want a quick one, then 3% is the value below which the test's validity is questionable.

But I suggest you don't stop to that simple answer and think about the reasons for your question: are you looking for a subgrade CBR value to determine an indicative pavement thickness? And if so, what type of material are we talking about? Materials like sand have a CBR of between 10 and 15%, although CBR is generally limited to 12% for pavement design. Clayey subgrades have a CBR that may vary from 3% to 15%, but then you may want to consider swelling potential in your pavement design. Similar considerations apply to expansive soils.

The soil type also dictates the conditions under which the CBR test should be undertaken in terms of density and moisture content.

Also, what type of traffic your road is likely to carry? Is your subgrade gonna stay dry or do you anticipate interactions with groundwater?

I hope the above helps you realise that your question requires a very articulated answer and perhaps a bit more context.

Bye!

Luca.

Luca Noferini Thankyou for the detailed response, indeed your observations and explaination is of value and better for understanding the real issue.

Based on AASHTO specifications, the minimum CBR value for subgrade soil is 5% and subbase is 30%.

Subgrades can have low CBR values as long as they are in soil groups that are classified as suitable for supporting a pavement layered structure. On the other hand, subbases ma have a CBR around 30 or 40 .

Dear K. M. N. Saquib Wani

It depends on the skill of the road designer that how to build the road whatever the value of CBR. Firstly, wheel loads should be known along with the volume of traffic . At least , to know, whether it is a village road, State Highway or National Highway. after that, from the data available from the above, to calculate thickness of the pavement and then to design the various intermediate layers with the materials available in the locality. CBR value of the Subgrade is also a data needed to finalise the thickness of the pavement. Therefore, there is nothing like minimum value of CBR but Fairbrother (2011), divided the CBR of forest road materials to three categories as: weak (CBR < 40), average (40 < CBR < 60), and good (CBR > 60)

Dear K. M. N. Saquib Wani ,

Different agencies recommend different CBR values. According to Standard Nasional Indonesia (SNI), the recommended CBR value for subgrade is 4% (soaked condition) and 6% (unsoaked condition). Regards

Dear K. M. N. Saquib Wani

Subgrades can have low CBR values as long as they are in soil groups that are classified as suitable for supporting a pavement layered structure. On the other hand, subbases ma have a CBR around 30 or 40 .

Secondly,

Aggregates in Sub-base layers are not selected based on CBR. MORTH code specifies aggregate properties based on

Sourangshu Ghosh Thanks for your response......

As recommended by many international standards, the minimum subgrade strength is about CBR = 5% and for subbase materials CBR greater than or equal to 30%.

Based on AASHTO specifications, the minimum CBR value for subgrade soil is 5% and subbase is 30%.