The California Bearing Ratio (CBR) test is one of the most widely used and trusted methods for assessing the strength and stability of soil and sub-base materials, playing a vital role in the design of roads, pavements and airfields across the world. In this article, Matt Hartnup, Geotechnical Laboratory Manager at Lucion Ground Engineering, takes a look at the history behind the CBR test, how it's carried out, and why it continues to be such a valuable tool for engineers and developers alike.
In 1921, the Federal Aid Highway Act was launched in the US and kickstarted a massive program to upgrade the national infrastructure over the next few decades. A big part of this program was to pave and upgrade more than 73,000 miles of roads. A quick reliable and cheap method of testing was therefore needed to assess the soil and base stability – The California Bearing Ratio Test. Later this method, also known as a CBR Test, became increasingly popular and trusted and was crucially used to help design and construct hundreds of airfields quickly during WW2. Soon after it became adopted around the world as the standard test method for road and pavement design.
CBR testing was first devised in the late 1920’s by the California State Highway Association as a means to measure the resistance of a material to penetration by a plunger of known weight and surface area. Credit for the method is often attributed to Omer James “Pappy” Porter, a senior engineer for the California Division of Highways.
How the CBR Test Works
The lab test can be used on a variety of soil types and can be carried out on either undisturbed material, sampled on site in a CBR mould with cutting collar attached, or else remoulded in the lab to a standard method as detailed in BS1377-2:2022.
The procedure typically follows these steps:
- Sample preparation – The specimen is either taken as an undisturbed sample on site, or remoulded in the laboratory in accordance with BS1377-2:2022.
- Soaking (if required) – If there’s a history of flooding in the locale, the sample may be soaked before testing, as this can give an indication of potential swelling with moisture increases.
- Set-up – The prepared specimen is placed on a test frame fitted with a penetrating plunger, a force-measuring device and a dial gauge to measure penetration. Surcharge weights are then applied to the mould to simulate the effect of the overlying layer thickness.
- Testing – The plunger is pushed into the soil at a rate of around 1mm per minute, with the force required to maintain this rate measured at suitable intervals. The test typically runs for around 6–8 minutes.
- Repeat on the base – Once the top of the sample has been tested, it’s flipped over and the base is tested in the same way.
- Reporting – A CBR percentage value is calculated for both the top and base of the sample. If both values are within 10% of the mean, an overall average is also reported.
Interpreting the Results
Once testing is complete, a graph is drawn plotting force against penetration. The force values at two pre-determined points (2.5mm and 5mm penetration) are then expressed as a percentage of the historical standard force to give the CBR value (%).
It’s interesting to note that the two historical factors of 13.24 and 19.96 used to calculate CBR% at 2.5mm and 5mm penetration exist because these are the original known resistance benchmarks used when the test was first devised:
- 1000 lb/in² (13.24 kN) at 2.5mm penetration
- 1500 lb/in² (19.96 kN) at 5mm penetration
Sometimes corrections need to be applied on more sandy materials, which can develop a ‘lazy S’ curve. This is accounted for during the quality control phase of reporting results for our clients.
The advantages of the CBR test are that it is quick, simple and cost-effective, giving an instant value. When used alongside field data and other test results, a reliable picture of a material’s characteristics can quickly be built up and incorporated into an investigative report.
Choose Lucion for CBR Testing
As part of our unwavering commitment to the highest standards of quality, our lab team here at Lucion Ground Engineering Ltd are constantly validating our results and practices against other technicians, equipment and external labs to maintain total confidence in our testing.
You can be certain that when you ask for a CBR test from us, your testing will be carried out in our UKAS accredited laboratory by highly experienced and dedicated technicians, giving you complete confidence in your project results. Our technicians have decades of experience between them, and this kind of insight is invaluable. We are very proud of what we do as Lucion’s specialist geotechnical and geoenvironmental service and lab provision.
Speak to our team about your next project and how our specialist geotechnical lab can help you start off on the right foundation.
