Laboratory CBR Seating loads. 1

[dirtman85]

Hi everyone, i'm a soils technician from England working to BS1377 standards.
We are trying to develop a method to accurately judge the CBR value of recompacted soils to pre-determine seating loads. We have trid a hand penetrometer but found it to be too inaccurate. Any suggestions?

 

[Ron]

Penetration tests don't correlate well to CBR, usually because of the larger aggregate in the specimen. You can get reasonable correlation in sands, clayey sands, and silty sands.

The surcharge for a CBR test is based on the pavement section loads. It should be the weight of the base and surface sections, but not less than 4.5kg.

 

[dirtman85]

Thanks for your response Ron. The seating loads I am referring to are the KN loads (in units)for specific load rings.

eg: for CBR values 8%-40% using a 10KN load ring we apply a seating load equivalent to 10N (6 units).
The problem lies in accurately estimating the CBR% to determine which load ring and/or seating load to apply.

 

[Ron]

[tt]OK...in general, I would use the following ranges:

Material CBR Value
Sand 5-20
Clayey Sand 10-30
Silty Sand 10-25
Stabilized Sand (with gravel) 20-35
Stabilized Clayey Sand (with gravel) 30-50
Graded aggregate base 50-80[/tt]

 

[BigH]

We have found that using well graded crushed stone base course that the CBR values are well in excess of 100 - sometimes they were determined to be 150+. But, then, again, if the CBRs are >100, who really cares?
With respect to the surcharge load - alluded to by dirtman85, he is correct in his definition as per the ASTM/AASHTO standards. On one job I was on, we required some 34 kg to hang on the surcharge weight - and the "commercially" available equipment only allowed 10kg. So it was necessary to manufacturer specific hangers to permit the 34 kg to be placed. Now, normally, it just wouldn't be done (I presume by 90% or more of the testing companies - in this case, it was necessary to do so to achieve the CBR minimum requirements for subgrade).

 

[Ron]

BigH..I agree. We often see graded aggregate bases with CBR values in excess of 100. Here in Florida, we use a bastardized CBR known as the LBR (Limerock Bearing Ratio...the tests are almost identical)". It must have been a California-Florida ego thing back in the 1950's! Anyway, the values can get quite high and you're right again...most testing labs put a 10-lb surcharge weight (not even 10kg) on and go for it, but then that's conservative so they are protecting their liability!

 

[dirtman85]

Typically we apply 8.5kg surcharge on laboratory CBRS and 9.2kg in-situ. We often find engineers are unclear as to their proposed pavement load design and even more unclear what a CBR test is about, they simply want a percentage!!
Most in-situ tests are requested on the subgrade which has particles far in excess of the recommended tolerance. We always try to convince clients to go for a plate load test in these circumstances instead to allow for the particle sizes and convert it to a CBR percentage using the modulus of subgrade reaction, typically this is declined due to cost, you can but try!!!