Test Method: California Bearing Ratio (CBR) BS 1377: part 4: Cl 7.0:1990 AMD 13925.
Sample preparation.
Checks weather sample having identification number. This test shall be carried out on material passing 20 mm test sieve. If the fraction retained on 20mm is greater than 25% the test is not applicable.
After bringing to the required moisture content the soil shall be thoroughly mixed and shall normally be sealed and stored for at least 24hr.
The required mass of test specimen shall be calculated with formula. (V(100+w)*MDD*required compaction)/100.
V= Volume of container, w= moisture content of soil. The initial mass shall be measured to the nearest 5g.
Pour the weighed soil slowly into the mould while tamping it with the steel rod (16mm dia and 600mm long). Uniform pressure should be given at the time of tamping.
Testing Procedure.
When all the soil is added, level off its surface which should then be about 5mm to 10mm above the top of the mould body.
Place a filter paper on the soil surface followed by the 50mm thick spacer plug.
Place the mould assembly in the compression device and apply a load to the sample until the top of the plug flush with the collar. Hold the load constant for at least 30 s.
Attach a perforated swell plate to the CBR mould in place of the normal plate.
Apply water repellent gels like petroleum jelly to the screw threads of the collar and mould.
Place the mould assembly in the empty soaking tank. Place a filter paper on the top of the sample followed by the perforated swell plate. Fit the required number of annular surcharge discs.
Mount a dial gauge.
Fill the tank with water just below the top of the mould extension collar. Start the timer when the water has just covered the baseplate.
Record the initial reading of dial gauge. It should be recorded in suitable intervals.
Record the time taken for water appears on the top of the sample. If not happened within 3 days, pour little water over there.
After 96 hr. (4days) soaking, take off dial gauge after recording final reading. Drain the water and keep the sample for 15min.
After replacing perforated plate to metal plate and trimming of sample its ready to penetration test.
CBR machine should have a cylindrical metal plunger with having nominal cross section area of 1935 mm2, Specified diameter of 49.65+-0.10mm, 250mm long.
The machine shall be capable of applying at least 45 KN. The unloaded machine approach speed shall be 1.2+-0.2 mm/min.
Place the mould with baseplate containing sample, Place the appropriate annular surcharge on the top of the sample. Apply a seating force to the plunger, depending on the expecting CBR value, For CBR value up to 5% apply 10 N, For CBR value from 5% to 30% apply 50N, For CBR value above 30% apply 250 N.
Reset the force measuring device and penetration gauge to read zero and start the test so that the plunger penetrates the sample at a nominal rate of 1 mm/ min.
Record the reading of the force gauge at intervals of penetration of 0.25mm up to 7.5mm.
Remove the mould from the machine and remove the baseplate from the lower end to the top end and repeat the test.
After completing the penetration test determine the moisture content.
Plot a graph with force and penetration.
Calculate CBR value for 2.5 and 5.0 reading. For 2.5, (Force *100)/13.2 and 5.0, (Force *100)/2