California Bearing Ratio C.B.R
The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05") per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.). Figure Below is a sketch of a typical CBR sample.
CBR Sample
| where: | x | = | material resistance or the unit load on the piston (pressure) for 2.54 mm (0.1") or 5.08 mm (0.2") of penetration |
| y | = | standard unit load (pressure) for well graded crushed stone | |
| = | for 2.54 mm (0.1") penetration = 6.9 MPa (1000 psi) | ||
| = | for 5.08 mm (0.2") penetration = 10.3 MPa (1500 psi) |
Table below shows some typical CBR ranges.
Typical CBR Ranges
Typical CBR Ranges
| General Soil Type | USC Soil Type | CBR Range |
| Coarse-grained soils | GW | 40 - 80 |
GP | 30 - 60 | |
GM | 20 - 60 | |
GC | 20 - 40 | |
SW | 20 - 40 | |
SP | 10 - 40 | |
SM | 10 - 40 | |
SC | 5 - 20 | |
| Fine-grained soils | ML | 15 or less |
CL LL < 50% | 15 or less | |
OL | 5 or less | |
MH | 10 or less | |
CH LL > 50% | 15 or less | |
OH | 5 or less |
AASHTO T 193: The California Bearing Ratio
ASTM D 1883: Bearing Ratio of Laboratory Compacted Soils
Source : training.ce.washington.edu
Other source :
The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation.
The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO T193.
The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High quality crushed rock has a CBR over 80. The standard material for this test is crushed California limestone which has a value of 100.
 | = CBR [%] |
 | = measured pressure for site soils [N/mm²] |
 | = pressure to achieve equal penetration on standard soil [N/mm²] |
source : wikipedia
CBR Component :
- Swell Plate : Contact end of the stem is easily locked in place with a knurled nut.
- Swell Tripod Attachment : Metal tripod supports dial gauge for measuring the amount of swell during soaking. Attachment is used with swell plate. Order dial indicator separately.
- Dial Indicator : Dial indicator has 1.000" operating range, graduated in 0.001" divisions, clockwise movement and revolution counter.
- Cutting Edge : Machined from seamless tubing with a sharpened edge to enable undisturbed samples to be taken in the field, cutting edge is plated for rust resistance. Cutting edge has 6" (152mm) ID and is 2" (51mm) high. Recess in upper section allows edge to be mounted at either end of the Compaction Mold or CBR Mold with Perforated Base mold to facilitate sample removal in the field.
- Filter Screen :
- Filter Paper : used to separate spacer disc and soil in the CBR mold during compaction operation or over the top surface of the soil when the compaction operation is completed.
- Surcharge Weight : Used in the application of surcharged loads on the soil‘s surface during soaking and penetration. Rust-resistant, plated annular disc weighs 5 lbs. (2.3kg), 5-7/8" (149mm) OD with a 2-1/8" (54mm) ID hole in center.
- Slotted Surcharge Weight
- Spacer Disc : Disc is used as a false bottom in a soil mold during the compaction process.
- Penetration Piston : CBR Penetration Piston has 3 sq. in. (19.35cm2) base area and is about 7-1/2" (191mm) long. Designed for use in conjunction with weights Surcharge Weight and Slotted Surcharge Weight to apply penetration surcharge loads.
- Dial Indicator Bracket : Bracket used to attach a dial indicator to the penetration piston.
- Swivel Base : for mechanical jack
source : humbolt
TYPE OF CALIFORNIA BEARING RATIO
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