CALIFORNIA BEARING RATIO (C.B.R) VALUE TEST

CALIFORNIA BEARING RATIO (C.B.R) TEST

The aim of carrying out the California Bearing Ratio (C.B.R) is stated below:

• To determine the California bearing ratio of a soil sample by conducting a load penetration test in the laboratory.
• To evaluate the strength of a subgrade soil, sub base and base course material for design of thickness for highway.

 APPARATUS/DIAGRAMS

The following apparatus were used as specified to carry out the C.B.R test on the sample:

• Mould (150mm Diameter and 175mm Height)
• Steel Cutting Collar
• Spacer Disc
• Surcharge weight (147mm Diameter)
• Dial Gauges
• International Standard (I S) Sieve (20mm)
• Penetration Plunger
• Loading machine Penetration machine

THEORY 

The California bearing proportion (CBR) is a penetration test for evaluation of the mechanical nature of ground, subgrades and base courses underneath new carriageway advancement. It was delivered by the California Department of Transportation before World War II.

The essential site test is performed by estimating the weight required to infiltrate soil or total with a plunger of standard zone. The deliberate weight is then partitioned by the weight required to accomplish an equivalent entrance on a standard pulverized shake material.

The CBR test is described in ASTM Standards D1883-05 (for research facility arranged example). The CBR test is completely depicted in BS 1377: Soils for structural building purposes: Part 4, Compaction related tests, and in Part 9: In-situ tests. The CBR rating was produced for estimating the heap bearing limit of soils utilized for building streets. The CBR can likewise be utilized for estimating the heap bearing limit of unchanged airstrips or for soils under cleared airstrips. The harder the surface, the higher the CBR rating. 


A CBR of 3 likens to worked farmland, a CBR of 4.75 compares to turf or soggy earth, while damp and may have a CBR of 10. Excellent pulverized shake has a CBR more than 80. The standard material for this test is pounded California limestone which has an estimation of 100, implying that it isn't surprising to see CBR estimations of more than 100 in all-around compacted territories. = estimated weight for site soils [N/mm²] = strain to accomplish rise infiltration on standard soil [N/mm²] Significance of CBR test

• Used to assess potential quality of subgrade, sub-base and base course materials. 

• Utilized where the impact of compaction water content on CBR is little, for example, such as cohesion less, coarse grained material.

• To decide the farthest point of swelling of a subgrade or sub base material Factors impacting CBR test result .

PROCEDURES

• The soil sample was sieved in 20mm sieve.
• 6kg  of the soil sample of the specimen was taken
• Water was added to the sample in the amount with the end goal that ideal dampness was achieved (13% of 6000). At that point the dirt and water were blended altogether 
• Spacer plate was put over the baseplate at the base of form and a coarse channel paper was set over the spacer circle 
• The standard CBR was adopted which include three (3) layer and 27 blows for every each layer
• The readied soil sample was blended and separated into three, the shape was cleaned and oil was connected, it was then loaded with 33% of the form with the readied soil. That layer was compacted by giving 27 blow uniformly disseminated blow utilizing rammer of weight 4.89kg. The second layer was filled and the procedure was rehashed. After the second layer, neckline was connected to the form and the procedure was proceeded. After the third layer, neckline was evacuated and overabundance soil was struck off 
• The baseplate was removed and the form rearranged. It was then clipped to baseplate 
• Extra charge weight of 2.5kg was put on the top surface of soil sample
• The form containing sample was put in position on the testing machine. The penetration  plunger was acquired contact with the soil and a load of 4kg (seating load) was connected so contact amongst soil and the plunger was set up 
• The dial readings were changed in accordance with zero and load was connected to such an extent that entrance rate is 1.25mm were noted 
• The load at penetration of 0.5, 1 , 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9 and 10 were noted
• The chart was drawn between the  penetrations and penetration loads and CBR esteem was gotten.

 DATA AND CALCULATION 

                          DATA AND CALCULATION

The following results were obtained while carrying out California Bearing Ratio (C.B.R)

UNSOAKED TEST:

S/N	Top penetration(mm)	Top-           Force Plunger (KN)	on	Reading x 0.032 x 2.931 (machine constant) KN
1	0.50	49		4.60
2	1.00	62		5.82
3	1.50	75		7.03
4	2.00	84		7.88
5	2.50	92		8.63
6	3.00	100		9.38
7	4.00	114		10.69
8	5.00	129		12.10
9	6.00	139		13.04
10	7.00	148		13.88
11	8.00	158		14.82
12	9.00	169		15.85
13	10.00	183		17.16

Table 2.1: Penetration-Dial Reading (Top)

2.5 mm Penetration:

Standard C.B.R Value = 1370kg

= 1370 x 9.81  1000

= 13.44 KN

Converted Force on plunger = 8.63 KN

C.B.R Value

C.B.R Value _{(2.5 mm)} = 64.21%

5.00mm Penetration Test:

Standard C.B.R Value = 2055kg

= 2055 x 9.81  1000

= 20.16 KN

Converted Force on plunger = 12.10 KN

C.B.R Value

C.B.R Value _{(5mm) =} 60.02%

Top penetration(mm)	Converted Force on Plunger(KN)	Standard CBR x 9.81  1000 (KN)	C.B.R Value
2.50	8.63	13.44	64. 21
5.00	12.10	20.16	60.02
			Avg =62.11

BOTTOM PENETRATION:

S/N	Bottom penetration(mm)	Force Plunger(KN)	on	Reading x 0.032 x 2.931 (machine constant) KN
1	0.50	24		2.25
2	1.00	37		3.47
3	1.50	48		4.50
4	2.00	58		5.44
5	2.50	63		5.91
6	3.00	67		6.28
7	4.00	71		6.66
8	5.00	79		7.41
9	6.00	84		7.88
10	7.00	92		8.63
11	8.00	102		9.57
12	9.00	112		10.51
13	10.00	122		11.44

Table 2.1b: Penetration-Dial Reading (Bottom)

2.5 mm Penetration:

Standard C.B.R Value = 1370kg

= 1370 x 9.81  1000

= 13.44 KN

Converted Force on plunger = 5.91 KN

C.B.R Value

C.B.R Value _{(2.5 mm)} = 43.97%

5.00mm Penetration Test:

Standard C.B.R Value = 2055kg

= 2055 x 9.81  1000

= 20.16 KN

Converted Force on plunger = 7.41 KN

C.B.R Value

C.B.R Value _{(5mm) =} 36.76%

Top penetration(mm)	Converted Force on plunger(KN)	Standard CBR x 9.81  1000 (KN)	C.B.R Value
2.50	5.91	13.44	43.97
5.00	7.41	20.16	36.76
			Avg = 40. 36

Adopted C.B.R Value of soil sample = 62

 PRECAUTIONS

1. The holes of the base plate and that of perforated disc was thoroughly cleaned.
2. We ensured that the 27 blows for compaction was distributed evenly over the surface of the soil and The soil was added in layers and each layer was blown to the required number before the next layer
3. The surcharge weight was align with the plunger so that the plunger penetrates freely into the soil.
4. The soil sample was thoroughly mixed before it was been compacted in the mould.
5. We ensured that the soil in the mould touched the collar before placing it in the CBR machine and also, made sure the plunger was touching the soil in the mould before the readings were taken

 OBSERVATION

• During the experiment, it was observed that the load gauge reading increased steadily as the plunger penetrated into the compacted soil in the mould until a stage was reached when the penetration does not add to the load gauge reading, meanwhile this particular stage was quickly reached when the mould was inverted which reflected in the table of values.

• Also, it was observed that the 2.5mm penetration gave the larger CBR value as against the value obtained for the 5mm penetration.

 ERROR

The following errors were experienced during the experiment:

1. Systematic (Determinate) errors: from its name, these are errors whose magnitude and algebraic sum can be determined and eliminated. The following are examples of systematic errors;

• Weighing an overfilled mould the level plane.

• Spillage of soil sample during compaction, reducing the total mass of soil sample.

• Uncertainty inherent in the measuring devices.

2. Random (Indeterminate) errors: these are errors whose magnitude and algebraic sum cannot be easily determined and eliminated.

• Natural variation in measurements.

• Resting on the laboratory table, which affects our reading during the experiment.

• Biased decision during the experiment.

 CONCLUSION 

In conclusion, the CBR value was determined and the value was obtained as follows; for top 60.02% and for bottom 40.36%. Since the CBR value is above the acceptable minimum standard (40), then it can be said to be suitable for use as subgrade and subase since it is about 60%.