The CBR (California Bearing Ratio) is a measure of the strength of soil under controlled moisture and density conditions. It is widely used in geotechnical engineering to design pavements, foundations, and embankments. The CBR is expressed as a percentage of the bearing capacity of a standard crushed rock material.
CBR Value | Soil Strength |
---|---|
0-3 | Very weak |
4-7 | Weak |
8-15 | Moderate |
16-30 | Strong |
>30 | Very strong |
CBR testing involves applying a load to a soil sample and measuring the resulting deformation. The CBR value is calculated based on the load required to cause a specific deformation. Advanced testing methods include:
Test Method | Description |
---|---|
Static CBR** | Load is applied at a constant rate |
Dynamic CBR** | Load is applied in a series of blows |
Repeated Load CBR** | Load is applied repeatedly at different magnitudes |
CBR is a critical factor in pavement design. A pavement with a low CBR may experience excessive deformation and premature failure. The American Association of State Highway and Transportation Officials (AASHTO)** estimates that pavement failure costs the U.S. economy over $60 billion annually.
Story 1:
Problem: A road in a rural area was experiencing excessive rutting and cracking.
Solution: CBR testing was performed, revealing a low CBR value. The soil was replaced with a higher quality material and compacted to the required CBR.
Result: The pavement has been performing well for several years, with no signs of distress.
Story 2:
Problem: A construction site needed to build a temporary access road on weak soil.
Solution: CBR testing was used to determine the required thickness of the pavement. The access road was built with a thick layer of crushed rock to meet the CBR specifications.
Result: The access road provided safe and reliable access to the construction site, enabling the project to be completed on schedule.
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