To determine the CBR value for soaked soil sample.

OBJECT :To determine the CBR value for soaked soil sample.

OBJECTIVE AND SCOPE

The California bearing ratio test is penetration test meant for the evaluation of sub grade strength of roads and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers. This is the most widely used method for the design of flexible pavement.

STANDARDS

ASTM D1883 – 16

EQUIPMENTS

CBR mold with detachable perforated base plate, Spacer disc, Collar, Rammer 4.89 kg, straight cutting edge, CBR machine, Surcharge weight (10 lbs.).

THEORY

California Bearing Ratio (CBR) is defined as the ratio between the load sustained by the soil sample at a specific penetration of a standard plunger and the load sustained by the standard crushed stones at the same penetration. Tests are performed out on natural or compacted soils in water soaked or un-soaked conditions and the results so obtained are compared with the curves of standard test. The C.B.R. values are usually calculated for penetration of 2.5 mm and 5mm. Generally, the C.B.R. value at 2.5 mm will be greater that at 5 mm and in such a case/the former shall be taken as C.B.R. for design purpose. If C.B.R. for 5 mm exceeds that for 2.5 mm, the test should be repeated. If identical results follow. the C.B.R. corresponding to 5 mm penetration should be taken for design.

PROCEDURE

1. Take the weight of empty mold.
   
2. Keep the spacer disc on the base plate and a filter paper on the disc and fix the mold to the base plate with the disc inside the mold and the attach the collar over the mold.
   
3. Add water to the specimen and compact it in accordance to Standard proctor test or modified proctor test.
   
4. After compaction, remove the collar and level the surface using cutting edge.
   
5. Detach the base pate and remove the spacer disc.
   
6. Take the weight of mold + compacted specimen and determine the bulk density of the specimen.
   
7. Take sample for moisture content determination and hence find the dry density
   
8. Place filter paper on the perforated base plate.
   
9. Fix the mold upside down to the base plate so that surface of the specimen which was downwards in contact with spacer disc during compaction is now turned upwards on which the penetration test is to be performed.
   
10. The load and deformation gauges shall then be set to zero.
    
11. Load shall be applied to the plunger into the soil at the rate of 1.25mm per minute.
    
12. Reading of the load shall be taken at the penetrations of 0.5, 1.0,…..12.5mm
    
13. Remove the plunger and determine the water content of the soil.
    
14. Plot load versus deformation curve.
    

Standard Loads at Specific Penetration

OBSERVATION

Diameter of plunger = 5cm

Area of plunger = 19.634cm2

Sample N0. 01

Volume of Mold =

Density Determination

Moisture Content

Sample N0. 02

Mold Specification:

Diameter of Mold =

Height of Mold =

Volume of Mold =

Density Determination

Moisture Content

Sample N0. 03

Diameter of Mold =

Height of Mold =

Volume of Mold =

Density Determination

Moisture Content

Review Questions

1. The weight of surcharge is?
2. In CBR test the value of CBR is calculated at?
3. What is swelling of soil?