California Bearing Ratio (CBR)

The California Bearing Ratio or CBR test (desk five-27) is an oblique measure of soil electricity based totally on resistance to penetration by a standardized piston moving at a standardized charge for a prescribed penetration distance (discern 5-12). CBR values are commonly used for dual carriageway, airport, automobile parking space, and different pavement designs based on empirical nearby or organization particular methods (i.E., FHWA, FAA, AASHTO). CBR has also been correlated empirically with resilient modulus and a ramification of other engineering soil houses. cbr testing uk

CBR isn’t always a fundamental fabric property and thus is incorrect for direct use in mechanistic and mechanistic-empirical design tactics. But, it’s miles a pretty clean and inexpensive test to perform, it has a protracted history in pavement design, and it is reasonably nicely correlated with more essential residences like resilient modulus. Therefore, it remains used in practice.

Table 5-27. California Bearing Ratio (CBR).
Description The California Bearing Ratio or CBR is an indirect measure of soil strength based on resistance to penetration.
Makes use of in Pavements
Direct input to some empirical pavement layout strategies
Correlations with resilient modulus and different engineering houses
Laboratory dedication AASTHO T 193 or ASTM D 1883. CBR is based on resistance to penetration via a standardized piston transferring at a standardized price for a prescribed penetration distance (figure five-12). CBR is defined because the ratio of the burden required to motive a sure intensity of penetration of a piston into a compacted specimen of soil at a few water content and density, to the usual load required to obtain the same intensity of penetration on a wellknown pattern of crushed stone (usually limestone). Usually soaked situations are used to simulate predicted long time situations in the subject.
The CBR test is run on 3 identically compacted samples. Each series of the CBR test is administered for a given relative compaction and moisture content. The geotechnical engineer have to specify the situations (dry, at most advantageous moisture, after soaking, ninety five% relative compaction, and so on.) beneath which each check have to be carried out.

Subject size ASTM D 4429. Take a look at process is similar to that for laboratory determination.
Observation maximum CBR trying out is laboratory based totally; as a consequence, the consequences might be enormously dependent on the representativeness of the samples examined. It’s also crucial that the checking out conditions be actually said: CBR values measured from as-compacted samples at optimal moisture and density conditions can be extensively extra than CBR values measured from similar samples after soaking, as an instance.
For area size, care need to be taken to make sure that the deflection dial is anchored properly outside the loaded vicinity. Subject dimension is made at the field moisture content material whilst laboratory trying out is commonly completed for soaked situations, so soil-specific correlations among subject and laboratory CBR values are frequently required.

Usual Values See table 5-28. For AASHO avenue test, CBR ≈ a hundred for the granular base layer and about 30 for the granular subbase.
Figure five-12. California Bearing Ratio test device (http://www.Ele.Com/geot/cali.Htm).

Near up photograph of the California Bearing Ration tool referenced in table five-27.

Desk five-28. Standard CBR values (after U.S. Navy Corps of Engineers, 1953).
USCS Soil elegance area CBR
GW 60 – 80
GP 35 – 60
GM forty – 80
GC 20 – 40
SW 20 – forty
SP 15 – 25
SM 20 – 40
SC 10 – 20
ML 5 – 15
CL 5 – 15
OL four – 8
MH four – 8
CH three – 5
OH 3 – five

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