In place CBR

[72shortbox]

Has anyone hear of an in place CBR test
I am trying to get a desing value to used, but the material is a decomposed rock. the bag sample run in the lab destroy the natural structure of the soil bringing the CBR value down to 2. I know the material is much stronger in the field. any suggestions

 

[ishvaaag]

I am an architect (I mean, these tests only rarely fall under my authority, and in fact only once in my life I have had some professional concern on specified compaction levels, through modified Proctor checks) yet I always thought of the CBR as performed in the field with a manual or so penetrometer. I look a reference and confirmes what I remembered

ASTM D4429

or California Bearing Ratio Test (CBR).

A piston is pushed unto the soil, readings of bearing stress taken at penetration 0.1 and 0.2 in, each divided by some standard reference and multiplied by 100 and the lower of both is the CBR.

So you must be referring to some other laboratory test on compaction on which of course some correlation to CBR values will exist.

Look such ASTM standard or read

Practical Foundation Engineering Handbook
Robert Wade Brown, editor
Mc Graw Hill
p. 5.202

 

[grafter]

Basford,

In-situ CBR tests are common practice in the UK for geotechnical site investigations and suggest you read the relevant British Standard for In-situ testing of soils for background information (think it'BS1377art 9 - I can email you a copy if you like). Check the US Army Corps of Engineers web site.....they developed the CBR test after all.

In the field the CBR equipment is mounted on the back of a JCB excavator and the piston pushed into the subgrade. As a guide the test is performed at proposed formation level and for good practice 0.5 m lower. I anticipate that you are a consulting engineer. I suggest that you procure a site investigation contractor to conduct the test, any contractor worth his salt will put you right. Further, it would help if you expanded your soil description, is the parent rock 'decomposed' to the structure of a soil? If so is the soil matrix granular or cohesive? You should determine the location of the groundwater table for your design.

Regards,

Brendan.

 

[grafter]

Ishvaaag,

For your records, laboratory CBR tests are also common practice in the UK and, I would imagine, around the globe. You were referring to standard compaction test (Proctor) which is used to control density of soils placed in the field for earthworks control, which is a different matter from obtaining laboratory CBR values for highway pavement design.

Regards,

Brendan.

 

[72shortbox]

Thank you for your help
I contracted a testing firm and had the ASTM D4429 test run is several locations. The results were over 2 time the values measured in a remolded lab test

 

[Sean2]

Please note that the CBR test is only suitable for material with a particle size of 20 mm (2 cm or 0.75 in) or less. This is because of the plunger size used in the standard test.

Refer to Section 4.3 of British Standard BS1377art 9:1990 (and Amendment No. 1 to the Standard).