CBR test on subgrade soil

The California Bearing Ratio (CBR) test (IS:2720 Part 16) measures the strength of subgrade soil by the resistance to penetration of a plunger and is used with IRC:37 and NRS 2070 to find the pavement thickness.

Preparation of dynamically compacted specimen

1. Take about 5-6 kg of soil, pass through the 20 mm sieve (material above 20 mm is replaced by an equal weight of 4.75-20 mm fraction).
2. Find the optimum moisture content (OMC) and maximum dry density (MDD) by the standard/heavy compaction (Proctor) test; the specimen is prepared at OMC and at the required field density (generally 97% of MDD of heavy compaction).
3. Mix the soil with the calculated water.
4. Mould: 150 mm dia and 175 mm high, with collar and a base plate; the spacer disc (148 mm dia, 47.7 mm high) is placed at the bottom with filter paper above it.
5. Compact the soil dynamically in the mould in five layers (heavy compaction: each layer with 56 blows of a 4.89 kg rammer falling 450 mm; for standard compaction: 3 layers of 56 blows with a 2.6 kg rammer from 310 mm).
6. Remove the collar, trim the top surface level, take out the spacer disc, and invert the mould on the base plate with filter paper.
7. For soaked CBR: place surcharge weights (2.5 kg annular weights, total 5 kg, representing the pavement weight) on the top, and immerse in water for four days while measuring the swell with a dial gauge.

Method of conducting the test

1. After soaking, drain for 15 minutes and place the mould with the surcharge weights under the plunger of the loading frame.
2. Seat the plunger (50 mm diameter) under a small load of 4 kg; set the load and penetration dials to zero.
3. Apply the load so that the plunger penetrates at 1.25 mm per minute.
4. Record the load at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10 and 12.5 mm.
5. Plot the load-penetration curve; if the curve is concave upward at the start, correct by shifting the origin.
6. Read the loads at 2.5 mm and 5.0 mm penetration.

$$\text{CBR} = \frac{\text{Test load}}{\text{Standard load}}\times 100\%$$

Standard loads for crushed stone: 13.34 kN (1370 kg) at 2.5 mm and 20.0 kN (2055 kg) at 5.0 mm. The CBR at 2.5 mm is normally adopted; if the CBR at 5 mm is greater, repeat the test, and if the same, adopt the higher value.

Take the average of three specimens; the design CBR is the soaked value at the field density.