California Bearing Ratio (CBR) Geotechnical engineering

Laboratory Determination of California Bearing Ratio (CBR)

The California Bearing Ratio (CBR) is a key parameter in geotechnical engineering used to evaluate the strength and load-bearing capacity of soil. This test, specified under IS 2720 (Part 16) – 1987 (Re-affirmed 2002), involves measuring the resistance of a soil sample to penetration by a standard plunger. The CBR value, expressed as a percentage, compares the force required to penetrate a soil sample with that needed for a standard material under identical conditions. This test is performed on both natural and compacted soils under soaked and un-soaked conditions, with results compared against standard test curves.

California Bearing Ratio test procedure 

Apparatus Required

1. CBR Mould: Equipped with a detachable perforated base plate.
2. Spacer Disc: Comes with a removable handle.
3. Collar: 50 mm in height.
4. Penetration Plunger: 50 mm in diameter.
5. Surcharge Masses: Annular and slotted, each weighing 2.5 kg.
6. Rammer: 2.6 kg with a 310 mm drop for standard Proctor results and 4.89 kg with a 450 mm drop for modified Proctor results.
7. Straight Cutting Edge.
8. Loading Machine: 50 kN capacity with a calibrated proving ring to which the plunger is attached.
9. Penetration Measuring Dial Gauge: 0.01 mm accuracy.
10. Soaking Tank.
11. Swelling Gauge: Includes a perforated plate with an adjustable extension stem.

Mould and Soil Specifications

• Mould Dimensions:
  • Diameter: 150 mm
  • Height: 175 mm
• Soil Specimen Dimensions:
  • Height of CBR soil specimen: 125 mm
• Soil Requirements:
  • Particle Size: Should pass through a 19 mm sieve. Particles larger than 19 mm should be replaced with particles between 4.75 mm and 19 mm.

Procedure

1. Weigh the Empty Mould.
2. Prepare the Mould: Place the spacer disc and a filter paper inside the mould. Attach the collar over the mould.
3. Compaction: Add water to the soil specimen and compact it according to either the Standard or Modified Proctor test procedures.
4. Finish Compaction: After compaction, remove the collar and level the surface with a cutting edge.
5. Weigh the Compacted Specimen: Detach the base plate, remove the spacer disc, and weigh the mould with the compacted specimen to determine the bulk density.
6. Moisture Content: Take a sample for moisture content determination to calculate the dry density.
7. Prepare for Testing: Place a filter paper on the perforated base plate. Fix the mould upside down on the base plate so that the surface previously in contact with the spacer disc is now facing upwards (for un-soaked conditions).
8. Soaking (if applicable): For soaked conditions, fix the adjustable stem and perforated plate on the compacted soil, add a 2.5 kg surcharge load, and place the setup in the soaking tank for four days. Measure swelling after four days.
9. Drainage: Remove the mould from the tank and let it drain.
10. Penetration Testing: Place the specimen under the penetration piston, add a total surcharge load of 4 kg, and set the load and deformation gauges to zero. Apply the load at a rate of 1.25 mm per minute and record the load at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0, and 12.5 mm.
11. Post-Test: Remove the plunger and determine the water content of the soil. Plot the load versus deformation curve.

Calculations

Expansion Ratio:

$$\text{Expansion Ratio}=\frac{d_f-d_s}{h}$$

Where $d_f$ is the final dial gauge reading (after 96 hours), ds​ is the initial dial gauge reading, and $h$ is the initial height of the specimen.

California Bearing Ratio (CBR):
$$\text{CBR}=\frac{P_{\mathrm{T/}}}{P_S}\times 100$$

Where PT​ is the corrected load at the chosen penetration from the load-penetration curve, and PS​ is the standard load for the same depth of penetration.

For most cases, the CBR value at a 2.5 mm penetration is used for design purposes. If the CBR value at 5 mm penetration is higher, the test should be repeated, and if identical results are obtained, the CBR value at 5 mm penetration should be used.

Correction to Load vs. Deformation Curve: If the curve is convex upwards, apply a correction by joining the tangent at the point of maximum slope to the curve. The corrected curve will be this tangent plus the convex portion of the original curve, with the strain origin shifted to the point where the tangent intersects the horizontal axis.

Correction to Load vs. Deformation Curve

This comprehensive approach ensures accurate evaluation of soil strength, critical for designing and constructing stable structures. 

Parameter	Unit	Value
Maximum Dry Density of the Specimen	g/cc
Optimum Moisture Content	%
Weight of the Empty Mould	g
Weight of the Soil Specimen and Mould Before Soaking	g
Volume of the Soil Specimen	cm³
Bulk Density of the Specimen	g/cc
Water Content of the Specimen	%
Dry Density of the Specimen	g/cc
Diameter of Plunger	cm
Area of Plunger	sq.cm
Deformation Rate	mm/minute
Proving Ring Constant	kN/division
Standard Pressure for 2.5 mm Penetration	kg/cm²
Standard Pressure for 5 mm Penetration	kg/cm²

Unsoaked Condition

Dial Gauge Reading	Penetration (mm)	Load (division)	Load (kN)	Pressure (kg/cm²)	CBR (after correction)
0
50
100
150
200
250
400
500
750
1000
1250

soaked Condition

Parameter	Unit	Value
Maximum Dry Density of the Specimen	g/cc
Optimum Moisture Content	%
Weight of the Empty Mould	g
Weight of the Soil Specimen and Mould Before Soaking	g
Volume of the Soil Specimen	cm³
Bulk Density of the Specimen	g/cc
Water Content of the Specimen	%
Dry Density of the Specimen	g/cc
Weight of the Soil Specimen and Mould After Soaking for 96 Hours	g
Weight Gain Due to Absorption of Water	g
Diameter of Plunger	cm
Area of Plunger	sq.cm
Deformation Rate	mm/minute
Proving Ring Constant	kN/division
Standard Pressure for 2.5 mm Penetration	kg/cm²
Standard Pressure for 5 mm Penetration	kg/cm²

Dial Gauge Reading	Penetration (mm)	Load (division)	Load (kN)	Pressure (kg/cm²)	CBR (after correction)
0
50
100
150
200
250
400
500
750
1000
1250