California bearing ratio

Objects of California bearing ratio:

The California Bearing Ratio is the ratio expressed in percent force per unit area required to penetrate the soil mass with a standard circular plunger of 50 mm diameter at a rate of 1.25 mm / min.

The penetration ratio of 2.5 and 5 mm is usually determined. When the 5mm ratio is consistently higher than the 2.5mm, the 5mm ratio is used. The table below presents the standard loads received for differential penetration of standard materials with CBR. 100% value.

For Railway Formation purpose, the test is performed on remolded specimens that are compacted dynamically.

The methodology covers the laboratory method for the determination of C.B.R. of remoulded /compacted soil specimens in a soaked state.

Apparatus used to California bearing ratio test:

 

Equipment for CBR testing.

 

fig 1.0

fig 2.0

Cylindrical mould:

Inside dia. 150mm and height 175mm with a detachable perforated base plate of 235mm dia. and 10mm thickness. Net capacity – 2250
ml; conforming to IS-9669:1980 (Reaffirmed-2016).

Collar:

A detachable extension collar of 60 mm height.

Spacer Disc:

148 mm in diameter and 47.7 mm in height along with the handle.

Weights:

One annular metal weight and several slotted weights weighing 2.5 kg each, 147 mm in diameter, with a central hole 53 mm in diameter.

Procedure to conduct California bearing ratio test:

Preparation Of Test Specimen:

1. Remoulded specimen: The test material should pass 19 mm IS sieve and be retained on a 4.75 mm IS sieve. The dry density for a remoulding shall be either the field density or the value of the maximum dry density estimated by the compaction test (Heavy Compaction Test as per IS 2720 (Part-8) – 1983, for Railway Formation). The water content used for compaction shall be the optimum water content or the field moisture as the case may be.

2. Dynamic Compaction: Take a representative sample of approximately 4.5 kg or more of soil and 5.5 kg or more of granular soil and mix thoroughly with water. If the soil is to be compressed to the maximum dry density at maximum moisture, the exact mass of soil required and the required amount of water must be added so that the water content of the soil sample is equal to the maximum determined moisture content.

3. Fix the extension collar and base plate to the mold. Insert a spacer disk to close the base. Place the filter paper on top of the spacer disk.

4. Apply lubricating oil to the inside of the mold. Compact the soil in the mold using a heavy concentration of soil. That means compacting the soil in 5 layers with 55 shots per layer through a 4.89 kg Rammer.

5. Remove the extension collar and carefully trim the compacted soil to the top of the mould, with a straight edge. Any holes developed on the surface of the compacted soil by removing the rough material should be patched with a small amount of material. Remove the perforated base plate, spacer disk and filter paper and record the mould and compact soil sample.

Place a disk of coarse filter paper on top of the perforated base plate, rotate the mold and compacted soil, and clamp the perforated base plate into the mold with compact soil in contact with the filter paper.

6. Place the filter paper on the sample and place the perforated tray on the compacted clay specimen in the mold. Put an annual weight to produce a surcharge equal to the weight of the anchor and pavement, the nearest 2.5 kg.

7. Immerse the mold and weight in a water tank and soak for 96 hours. Mount the tripod on the edge of the mold for the measuring device of the extension and record the initial dial gauge reading.

Observe daily readings versus reading time. Continuous water levels should be maintained in the tank throughout this period.

8. At the end of the soaking period, observe the final reading of the dial gauge and remove the mold from the water tank.

9. Remove the free water stored in the mold and leave the sample to drain for 15 minutes. Remove perforated plate and top filter paper. Weigh the soaked soil sample and record the weight.

Procedure For Penetration Test:

1. Place the axle assembly with the test specimen on the bottom plate of the penetration test machine. To prevent the soil from rolling into the surcharge weigh hole, a plunger should be placed before the 2.5 kg annual weight is placed on the soil surface, then the remaining additional charge.

2. Insert the penetration piston into the center of the specimen with the smallest possible load, but in no case greater than 4 kg, so that the entire connection of the piston is established on the specimen.

3. Set loading and distortion gauges to read zero. Apply the load on the piston so that the penetration rate is approximately 1.25 mm / min.

4. Record load readings in 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10 and 12.5mm penetrations.

5. Raise the plunger and separate the mold from the loading tool. Determine the moisture content by taking about 20 to 50 g of soil from the top 30 mm layer.

Observation And Recording:

Calculation:

1. If the initial portion of the curve is concave upwards, apply correction by drawing a tangent to the curve at the point of the greatest slope and shift the origin. Find and record the correct load reading corresponding to each penetration.

C.B.R. = (PT/PS) X 100

where,

PT = Corrected test load corresponding to the chosen penetration from the load penetration
curve.
PS = Standard load for the same penetration taken from the table above.

2. C.B.R. of specimen at 2.5 mm penetration =

3. C.B.R. of specimen at 5.0 mm penetration =

4. The C.B.R. values are usually calculated for penetration of 2.5 mm and 5 mm. Generally the C.B.R. value at 2.5 mm will be greater than at 5 mm and in such a case/the former shall be taken as C.B.R. for design purpose. If C.B.R. for 5 mm exceeds that for 2.5 mm, the test should be repeated. If identical results follow, the C.B.R. corresponding to 5 mm penetration should be taken for design.

Graph:

Draw graph between Load versus Penetration.

 Plot of Load versus Settlement

Reference

IS 2720(Part 16):1987 Methods of test for soils: Laboratory determination of CBR (second revision). Reaffirmed- Dec 2016.

RDSO report No. RDSO/2009/GE: G-0014 – Guidelines and Specification for Design of Formation for Heavy Axle Load.

Video:

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