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The California Bearing Ratio (CBR) Test
The main aim of this test is to measure the bearing capacity of subbase, base course material and subgrade, Involving recycled materials to design flexible highways, road and airfield pavements. The results achieved by this test are utilized with empirical curves to determine the thickness of road sub base.
- Spacer Disc
- Metal rammer
- Steel cutting collar
- Surcharge weight
- Dial gauges
- IS sieves
- Penetration plunger
- Loading machine
- Drying oven
- Weighing machine
The sample was prepared in automatic compactor by using 2.5Kg rammer with 62 drops on each layer. The original soil sieved to rid the particles which were greater than 20mm in size. The base plate was attached to the mold and then its diameter and depth were measured. It was being weighed on weighing machine. The mold collar was attached and placed in compacting machine. The mold filled with three different layers; each one was rammed with 62 blows and then removed the collar from the mold, leveled the top soil and weighed the mold base plate.
The mold containing compacted soil was placed on the surface of test machine and then surcharge disc (surcharge weight) was placed on the soil surface.
Then the machine was checked, whether it was set to 1mm/minute speed or not.
The plunger was brought down and passed it into the surcharge disc hole. Care was taken while passing the plunger between the surcharge discs, as it should not touch the soil surface.
The load ring dial gauge was set at zero and seating force of 50N was applied on the sample. After that dial gauge was again reset to the zero.
The penetration dial gauge was set up so that it could touch the rim of the mold and then it was set to zero to note the initial readings.
The machine was started to record the data from 0.25mm until the value of penetration reached 7.5mm.
The mold was taken out from the machine and the soil of approximately 350g was removed below the penetration. The soil was then placed in a pre-weighted tray, then a weight of tray and soil was recorded.
The tray containing soil of approximate 350g was then placed in an oven for drying and next day weight of tray and mass of dry soil was recorded.
Data and Analysis
Analysis and Plotting of Data
= 3.77 / 13
= 0.29 × 100
5 mm –
= 8.45 / 20
= 0.42 ×100
Accepted CBR = 42%
The moisture content is the element that affects the performance of the sub-base. Unfortunately, moisture content varies a lot as water approach from any sources such as rainfall, seasonal movement of the water table and capillary action. The significance of the moisture content is shown by the variation of the CBR values. The following graph shows, how the CBR value of a soil changes with the moisture content.
To demonstrate the examination of design by the CBR method when the sub-grades or base materials are not disturbed by the frost or a distinctive treatment is not the essential consideration in design then consider that a design is to be developed for a road that will need a design index of 5.
Soil Group Classification System
Weak layer in subgrade
GM (Lime rock)
Today there is a vast difference in traffic engineering in industrialized and developing countries due to different construction materials, construction quality, and the tremendous change in the volume and weight of road traffic. Since most counties lie in different tropics or sub-tropics areas, so accordingly their climate condition will also vary. Due to this, some country’s road sub-grade soils are designed on the basis of CBR values determined from soaked samples.
The test was carried out to determine the bearing capacity of subbase, base course material and subgrade to design the flexible highway or road. At the 2.5mm penetration CBR value got 29% and at the penetration of 5.0 mm CBR value got 42%. From the both of the value highest value is 42% of 5mm penetration. Therefore, accepted CBR value would be 42%. The results (accepted CBR 42%), which achieved by the test will be utilized to determine the thickness of road subbase. Paving grade bitumens type of pavement could suffice CBR value of 42% for sub-grade.
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