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How is a CBR Test performed?

How is a CBR Test performed?

The CBR test is performed by measuring the pressure required to penetrate a soil sample with a plunger of ordinary area. The measured pressure is then divided by the pressure required to realize an equal penetration on a typical gravel material. The harder the surface, the upper the CBR value. Typically, a worth of twenty-two equates to clay, while some sands may have a CBR value of 10%. top quality sub-base will have a worth of between 80-100% (maximum). Know more detail about this topic click here this site cbr testing, plate testing

The CBR test is administered on soils with a maximum particle size of 20mm. (Note: For material greater than 20mm please see Plate Bearing Tests). The technique involves driving alittle cylindrical plunger (approx 50mm) into the bottom at a consistent rate, employing a four wheel drive vehicle because the reaction load to supply the force.

Tests are normally administered at surface level or at depths of between 500-1000mm in 20-30m intervals along the proposed construction cent reline. A minimum of three tests are usually administered at each site. On a typical site, and assuming surfaces are prepared, up to 8-10 tests are often administered during a day, by one operator, with provisional results available on site.

Scope of CBR Testing

The laboratory CBR test consists essentially of preparing a sample of soil during a
cylindrical steel mould then forcing a cylindrical steel plunger, of nominal diameter
50 mm, into the sample at a controlled rate, whilst measuring the force required to
penetrate the sample. A pictorial view of the overall test arrangement is shown in Figure 5.1.1.
CBR values may vary from but 1% on soft clays to over 150% on dense crushed
rock samples.

Preparation of remoulded samples for the CBR test are often made in several ways.
However, commonly used methods are described here:

(1) Static compression
(2) Dynamic compaction by
(a) using 2.5 or 4.5 kg rammer and
(b) using vibrating hammer.


The CBR test is administered on material passing a 20mm test sieve. If soil
contains particles larger than this the fraction retained on 20mm shall be removed and
weighed before preparing the test sample. If this fraction is bigger than 25% of the
original sample the test isn’t applicable. The moisture content of the specimen or
specimens are often adjusted as necessary following the procedure given in Chapter 4.
The moisture content used is generally to the Optimum Moisture Content (OMC), but
obviously this will be varied to suit particular requirements.


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