Laboratory Determination Of California Bearing Ratio

STANDERD

• IS: 2720 (Part 16) 1979

DEFINITION

• California bearing ratio is the ratio of force per unit area required to penetrate in to a soil mass with a circular plunger of 50 mm diameter at the rate of 1.25 mm/mm to the Standard force for the same depth of penetration.

APPARATUS

• Moulds 2250 cc capacity with base plate, stay rod and wing nut confirming to 4.1, 4.3 and 4.4 of IS: 9669-1980.
• Collar conforming to 4.2 of IS: 9669-1980.
• Spacer Disc conforming to 4.4 of IS: 9669-1980.
• Metal rammer conforming to IS: 9189-1979.
• Expansion measuring apparatus with the adjustable stem, perforated plates, tripod confirming and to weights conforming to 4.4 of IS: 9669-1980.
• Loading machine having a capacity of at least 5000 kg and equipped with a movable head or base that travels at a uniform rate of 1.25 mm / mm for use in forcing the penetration plunger in to the specimen.
• Penetration plunger conforming to 4.4 of IS: 9669-1980.
• Dial gauge two numbers reading to 0.01 mm.
• IS sieves 37.50 or 22.50 or 19 mm and 4.75 mm.
• Miscellaneous apparatus such as mixing bowl, straight edge, scales, soaking tank, drying oven, filter paper, dishes and calibrated measuring jar.

PROCEDURE

• There are two types of methods in compacting soil specimen in the CBR moulds
  i.              Static Compaction method.
  ii.            Dynamic Compaction method.
• The material used in the above two methods shall pass 19 mm sieve for fine grained soils and 37.50 mm sieve for coarse materials up to 37.50 mm.
• Replace the material retained on 19 mm sieve by an equal amount of material passing 19 mm sieve and retained on 4.75 mm sieve
• Replace the material retained on 37.50 mm sieve by an equal amount of material passing 37.50 mm sieve and retained on 4.75 mm sieve.

Dynamic Compaction

• Take representative  sample of soil weighing approximately 6kg and mix thoroughly at OMC.
• Record the empty weight of the mould with base plate, with extension collar removed (m₁).
• Replace the extension collar of the mould.
• Insert a spacer disc over the base plate and place a coarse filter paper on the top of the spacer disc.
• Place the mould on a solid base such as a concrete floor or plinth and compact the wet soil in to the mould in five layers     of approximately equal mass each layer being given 55 blows with 4.90 kg hammer equally distributed and dropped from a height of 50 mm above the soil.
• The amount of soil used shall be sufficient to fill the mould, leaving not more than about 6 mm to be struck off when the extension collar is removed.
• Remove the extension collar and carefully level the compacted soil to the top of the mould by means of a straight edge.
• Remove the spacer disc by inverting the mould and weigh the mould with compacted soil (m₂).
• Place a filter paper between the base plate and the inverted mould.
• Replace the extension collar of the mould.
• Prepare two more specimens in the same procedure as described above.
• In both the cases of compaction, if the sample is to be soaked, take representative samples of the material at the beginning of compaction and another sample of remaining material after compaction for the determination of moisture content.
• Each sample shall weigh not less than 50 gms.
• Place the adjustable stem and perforated plate on the compacted soil specimen in the mould.
• Place the weights to produce a surcharge equal to• the weight of base material and pavement to the nearest 2.5 kg on the perforated plate.
• Immerse the whole mould and weights in a tank of water allowing free access of water to the top and bottom of specimen for 96 hours.

Test for Swelling

• This test is optional and may be omitted if not necessary.
• Determine the initial height of specimen (h) in mm.
• Mount the expansion-measuring device along with the tripod on the edge of the mould and record the initial dial gauge reading (ds).
• Keep this set up as such undisturbed for 96 hours noting down the readings every day against the time of reading.
• Maintain a constant water level through out the period of soaking.
• Note the final reading of the dial gauge at the end of soaking period (d_f).

Calculations for Swelling
Expansion ratio = -(- (d_f ds)/ h)x100

ds=Initial dial gauge reading in mm

d_f=final dial gauge reading in mm

h=initial height of specimen in mm

Penetration Test

• After 96 hours of soaking take out the specimen from the water and remove the extension collar, perforated disc, surcharge weights and filter paper.
• Drain off the excess water by placing the mould inclined for about 15 minutes and weigh the mould.
  Place the mould on the lower plate of the testing machine with top face exposed.
• To prevent upheaval of soil in to the hole of surcharge weights, place 2.5kg annular weights on the soil surface prior to seating the penetration plunger after which place the remainder of the surcharge weights.
• Set the plunger under a load of 4kg so that full contact is established between the surface of the specimen and the plunger.
• Set the stress and strain gauges to zero.
• Consider the initial load applied to the plunger as the zero load.
• Apply the load at the rate of 1.25 mm/mm.
• Take the readings of the load at penetration of 0, 0.5, 1.0, 1.5, 2.0,2.5,3.0,4,5,7.5, 10 and 12.5.
• Raise the plunger and detach the mould from the loading equipment.
• Collect the sample of about 20 to 50 gms of soil from the top 30 mm layer of specimen and determine the water content in accordance with IS: 2720 (Part 4)1973.
• Examine the specimen carefully after the test is completed for the presence of any over size soil particles, which are likely to affect the results if they happen to be located directly below the penetration plunger.

CALCULATION OF CBR FROM LOAD PENETRATION CURVE

• Plot the load penetration curve in natural scale, load on Y – axis and penetration on X — axis as shown below
  
• If the curve is uniformly convex upwards although the initial portion of the curve may be concave upwards due to  surface irregularities make correction by  drawing tangent to the upper curve at the point of curvature
• Take the intersection point of the tangent and the X — axis as the origin.
• Calculate the CBR values for penetration of 2.50 mm and 5.00 mm.
• Corresponding to the penetration value at which CBR is to be desired, take the corrected load values from the load penetration curve and calculate the OR from the equation.

California Bearing Ratio =

PT X Cf

——————-x 100.
PS

PT=  Corrected unit test load corresponding to the chosen penetration from load
penetration curve
PS= Total standard load for the same depth of penetration, which can be taken from the Table: 1.
Cf= Proving ring calibration factor.
Table: 1 Standard loads at specified penetrations

Penetration depth (mm)	Unit Standard load Kgf/ cm2	Total Standard load (Kg)
2.50	70	1370
5.00	105	2055
7.50	134	2630
10.00	162	3180
12.50	183	3600

REPORT

• Report the CBR value to the nearest second decimal.
• Take the average of three test specimens as the CBR value of the test.
• Generally, the CBR value at 2.50 mm penetration will be greater than that at 5.00 mm penetration and in such case take the value at 2.50 mm as the CBR value.
• If the CBR value corresponding to a penetration of 5.00 mm exceeds that of 2.5 mm,repeat the test.
• If the identical results follow, take the value corresponding to 5.00 mm as the C.B.R. value.

PRECAUTIONS

• Clean the holes of the base plate and that of perforated disc thoroughly.
• Align the surcharge weight with the plunger so that the plunger penetrates freely in to the soil.

LABORATORY DETERMINATION OF CALIFORNIA BEARING RATIO
IS: 2720 (Part 16) 1979

 

Description	Test1		Test2		Test3
	Before soaking	After soaking	Before soaking	After soaking	Before soaking	After soaking
DETERMINATION OF MOISTURE CONTENT AND UNIT WEIGHT
Mould No.
Volume of the mould (V)
Weight of the mould (m1)
Weight of soil taken in gms
Weight of water added in gms
Weight of mould + Compacted soil in gms (m2)
Weight of compacted soil in gms m3=(m2-m1)
Wet Density in grn/cc Ү b =m3 I V
Container No.
Weight of container (W1)
Weight of container + wet soil (W2)
Weight of container + dry soil (W3)
Weight of water ( W2- W3)
Weight of dry soil ( W3- W1)
W2-W3 Moisture Content, W=————x 100 W3- W1
Dry density in gms/cc Үd =  Ү b /(l+W/100)
DETERMINATION OF SWELLING
Initial dial gauge reading
Final dial gauge reading
Percentage Swelling

 LABORATORY DETERMINATION OF CALIFORNIA BEARING RATIO
IS: 2720 (Part 16) 1979

 

MDD :		OMC :	Surcharge weight :
Penetration in mm	Specimen 1	Specimen 2	Specimen 3
	Dial gauge reading	Dial gauge reading	Dial gauge reading
0
0.50
1.00
1.50
2.00
2.50
3.00
4.00
5.00
7.50
10.00
12.50
Results from the Graph	Specimen No	CBR at 2.5 mm	CBR at 5.0 mm
	1
	2
	3
Average CBR Value :
CBR Value of the Sample:
Specified Limits :	Soaked CBR value for Subgrade is 10%
	Soaked CBR value for GSB is 30% (Min.)
Remarks :