CALIFORNIA BEARING RATIO TEST

DEFINITION:It is the ratio of force per unit area required

to penetrate a soil mass with standard circular piston at the rate of 1.25 mm/min. to that required for the corresponding penetration of a standard crushed rock material.

C.B.R. = (Test load/Standard load) �* 100

AIM : To determine the California bearing ratio of

a compacted soil sample for both soaked and unsoaked condition by conducting a load penetration test in the laboratory. 

APPARATUS: Cylindrical mould of 150mmdia with a

detachable collar and base plate, metal rammer, loading machine, spacer disc, dial gauge, proving ring, surcharge

PROCEDURE:Take about 5 kg of soil and mix thoroughly with

the required water.Dynamic compaction:Fix the extension collar and the base plate to

the mould. Insert the spacer disc over the base . Place the filter paper on the top of the spacer disc.

Compact the soil in 3 equal layers, each layer being given 55 blows by the 2.6 kg rammer in the mechanical compacting machine.

Remove the collar and trim off soil.Turn the mould upside down and remove the

base plate and the displacer disc.Put filter paper on the top of the compacted

soil (collar side)

UNSOAKED CONDITIONPlace the mould assembly with the surcharge

weights on the penetration test machineSeat the penetration plunger at the center of

the specimen so that the plunger just touches

the soil. Set the stress and strain dial gauge to read

zero. Apply the load on the piston so that the penetration rate is about 1.25 mm/min.

Record the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6,7,7.5,8,9, 10 ,11, 12and 12.5 mm.

The load penetration curve is plotted. The loads for 2.5mm and 5mm penetration are taken into consideration for calculating CBR.

If the initial position of curve is concave upwards then correction is applied.

SOAKED CONDITON : Detach the mould from the loading equipment A filter paper is kept on top of soil and perforated

plate on the top of filter paper.Turn the mould upside down and now place a filter

paper and surchargeNow the mould assembly is immersedin water

tank for 3days.Then specimen isallowed drain for 15min by

removing the surcharge and filter paperThe loading is done similar to unsoaked condition

OBSERVATIONS : Proving ring constant = 2.1kg/division Dialgauge L.C. = 0.01mm/division

DIAL GAUGE READING

PENETRATION (mm)

Unsoaked condition

Unsoaked condition

soaked condition

Soaked condition

Proving ring reading

Load (kg)

Proving ring reading

Load (kg)

Graph

CALCULATIONS :Unsoaked condition:CBR at 2.5mm penetration= ( pt/ps )2.5mm

CBR at 5mm penetration=( pt/ps )5mm

Soaked condition:CBR at 2.5mm penetration= ( pt/ps )2.5mm

CBR at 5mm penetration= ( pt/ps )5mm

RESULT :CBR value of given soil sample is found as :Unsoaked CBR value=Soaked CBR value=APPLICATIONS :Determination of thickness of flexible

pavementsStrength of subgrade of pavement