Theory
C.B.R. is widely used for designing the flexible pavements. Flexible pavement are which change their shape to some extent without rapture. The method was developed without in 1928 by California division of highway. The main advantage of this method is simplicity of the test producing C.B.R. test is the penetration test and is used to evaluate the stability of soil.
C.B.R. = load carried by specimen defined / load carried by standard crushed stone at penetration level
Items required
1. Cylindricle mould (CBR) with inside dia. 150 mm and height 175 mm.
2. Penetration piston 50 mm dia. and minimum of 100 mm long.
3. Extension measuring apparatus consisting of (a)perforated plate 148 mm diameter. (b) Adjustable contact head to be screwed over the steam.
4. Loading device
5. Two dial gauge reading to 0.01 mm
Procedure
1. Take a soaked soil sample in a mould and placed the surface weights on the top of the soil specimen.
2. Placed the mould assembly on the penetration test machine.
3. Seal the penetration piston at the center of specimen with the smallest possible load but it no case excess of 4 kg. so that full contact is established between the surface of specimen and piston.
4. Set the stress and strain dial gauge on zero. Apply the load on the penetration so that the penetration rate is approximately at 1.25mm/min record. The load reading at penetration of 00.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 4.5, 5.0, 5.5………10 to 12.5mm. Record the maximum load and penetration. It is occurs for at penetration of less than 12.5mm.
Diagram
Observation
Obs.no. | Penetration (mm) | Standard load in kg | Unit standard load in kg/cm2 |
01 | 2.5 | 1370 | 70 |
02 | 5 | 2055 | 105 |
03 | 7.5 | 2680 | 134 |
04 | 10.0 | 3180 | 162 |
05 | 12.5 | 3600 | 183 |
Calculation
Cross-section area of plungers = π X 52 / 4 = 19.5 cm2
1. Unit st. load = st. load / 19.57 = 1370 / 19.57 = 70 kg/cm2
2. Unit st. load = st. load / 19.57 = 2055 / 19.57 = 105 kg/cm2
3. Unit st. load = st. load / 19.57 = 2630 / 19.57 = 134 kg/cm2
4. Unit st. load = st. load / 19.57 = 3180 / 19.57 = 162 kg/cm2
5. Unit st. load = st. load / 19.57 = 3600 / 19.57 = 183 kg/cm2
Graph
Result
The mean C.B.R. % was found to be 5.06 %
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