Introduction to CBR Testing
The California bearing ratio (CBR) testing is an evaluation of the strength of a ground, base courses, and substrates. The measure was invented by the California Department of Transport has been a vital part of construction, especially the construction of roads and airstrips. A
site test is done to establish the amount of pressure it would take to penetrate aggregate or soil using a standard plunder. This pressure value is then divided by pressure necessary to get equal penetration for the pressure to go through standard crushed rocks.
When CBR testing was developed, the objective was to help in the measurement of load bearing capacity of materials like soil that were used in the construction of roads. It could also be useful in estimating the load bearing capacity of soils beneath paved airstrips or even unimproved airstrips. Basically, the harder a surface becomes, the higher the rating. The test is useful as it helps to determine the thickness of material needed to make a proposed road strong enough to withstand the heaviest vehicle that could use the road.
Procedure of Performing a CBR Test
The procedure required while performing a CBR test includes measuring the pressure needed for a soil sample to be penetrated while using a standard plunder. Harder surfaces will read a higher CBR value and normally, 2% is achieved on clay while some sands could read up to 10%. A sub-base that is of good quality will have a CBR rating between 80 and 100%, and this means such a material or surface is compact and strong enough to withstand a lot of weight.
It is the reason while constructing roads this is put into consideration to ensure the soils and materials sourced match the rating of that particular road. A road that should be used by heavy trucks might require a different kind of material from that designated for cycling.
Particle Size
Something else about CBR testing that you should understand is that it is only possible for materials that have particle size of 20mm. Materials with larger particles than this will usually be subjected to the Plate Bearing Test. The technique used involves using a cylindrical plunder (about 50mm), which is driven into the ground. A four-wheel vehicle is used to provide reaction load and add the force needed to drive the plunder into the material.
Most tests are done up to depths ranging from 500 to 1000mm across 20-30m intervals along the centreline of the construction. It is recommended to have at least three tests for each site to get the best values and to be sure the findings arrived at reflect the truth. Some operators do up to 10 tests in one day and provisional results are provided on the site.
Dynamic Cone Penetrometer CBR Test
CBR tests are also performed using portable hand-held equipment. Different companies use hand held equipment while assessing CBR value and this value is calculated as a value of an empirical test performed on the soil to determine if it is fit to be used for road construction. Such equipment give a continuous record that highlights penetration resistance on each layer with a depth of one meter from the surface.
Impact of CBR in Saturated Clays
Apart from working with dry soils, some areas have wet clay and before a project commences it is always necessary to have measures of the CBR rating of the soil so as to inform on the best material to choose for paving. Several analyses have been done to show how the test performs in clay. The focus was to review if the test would reflect the stiffness of the soil.
Most of the findings showed that CBR shares no consistent correlation with stiffness or strength and thus the recommendations made were to encourage the use of full load penetration together with CBR so as to understand which type of capping layer or subgrade material would be ideal.
Flexible Pavement Design
This is a method applied to help determine the required thickness of a pavement. It relies on two methods for the design using CBR value. The methods are CBR procedure recommended by the state and the CBR method that is recommended by IRC.
Data Required
For flexible pavement design, you need a few variables including:
- The CBR value of soil subgrade
- Base course CBR value
- Sub base course CBR value
- Wheel load (KN or KG)
The wheel load used is picked from one of the three groups available, which are based on traffic conditions and recommendations.
1. For light traffic - 3175 KG
2. For medium traffic – 4082 KG
3. For heavy traffic – 5443 KG
Calculating Thickness
With the values mentioned above, it is now possible to calculate the pavement thickness required. You need the total thickness (T) and the value of the sub base course T(sb). With the two values, you can calculate the recommended thickness of the sub base course (Tsb), which is gotten with the following simple calculation.
Tsb = T – T(sb)
What are the Advantages of CBR Testing?
There are many advantages of CBR testing that make it a necessary procedure to perform before you undertake any construction project that includes building a pavement or road. Here are few you might want to keep in mind.
- No technical experience is required, so the test could be done by anyone with basic skills.
- You can use portable equipment to get the values and this does not require many resources.
- It is applicable to many materials besides working with subgrades.
- You could do testing on samples that act as representatives of water conditions for the future.
- CBR tests are known to adapt to pavement design better than any other method available out there. It is faster and you are able to get more accurate results.
There are many other methods available that could help in measuring the load bearing capacity of a material, but CBR has proved over time to be more reliable. You could get different results while using different methods, but CBR gives a more reliable range of estimation.