CBR TEST

For Railway Formation purpose, the test is performed on remoulded

specimens which are compacted dynamically.
Objective: CBR is the ratio expressed in percentage of force per unit area required
to penetrate a soil mass with a standard circular plunger of 50 mm diameter at the rate
of 1.25 mm/min to that required for corresponding penetration in a standard material.
The ratio is usually determined for penetration of 2.5 and 5 mm. When the ratio at 5
mm is consistently higher than that at 2.5 mm, the ratio at 5 mm is used.

The following table gives the standard loads adopted for different penetrations for the standard
material with a C.B.R. value of 100%.
2. Apparatus Required

CBR Test Apparatus Consisting of Loading machine

with capacity of at least 5000 kg and equipped with a
movable head or base which enables Plunger of 50
mm dia. to penetrate into the specimen at a rate of
1.25 mm/ minute.
 CBR Mould with Base Plate, Stay Rod and Wing Nut
• 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 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.
CBR Mould with Base Plate, Stay Rod and Wing Nut

Perforated bottom of base Plate

2. Apparatus Required
STATIC COMPACTION DYNAMIC COMPACTION

Compacting soil in loading machine (Gradually Compacting soil by specified rammer (Impact Load.)
increasing Load)

DYNAMIC COMPACTION
LIGHT COMPACTION HEAVY COMPACTION

No. of Layers 3 5

RAMMER WEIGHT 2.6 KG 4.89 KG

FALL HEIGHT 31 CM 45 CM

BLOWS/LAYERS 56 56
Procedure Preparation of Test Specimen:
• 1. Remoulded specimens are prepared in the laboratory by compaction. The material used in the
remoulded specimen shall pass 19 mm I.S. sieve. Allowance for large material shall be made by
replacing it by an equal amount of material which passes a 19mm I.S. Sieve but is retained on
4.75 mm sieve.
• 2. 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.
• 3. Dynamic Compaction: A representative sample of the soil weighing approximately 4.5 kg or
more for fine grained soil and 5.5 kg or more for granular soil shall be taken and mixed thoroughly
with water. If the soil is to be compacted to the maximum dry density at the optimum moisture
content, the exact mass of the soil required shall be taken and the necessary quantity of water
added so that the water content of the soil sample is equal to the determined optimum moisture
content.
• 4. Fix the extension collar and the base plate to the mould. Insert the spacer disc over the base.
Place the filter paper on the top of the spacer disc.
 Procedure Preparation of Test Specimen:
• 5. Apply Lubricating Oil to the inner side of the mould. Compact the mix soil in the mould using
heavy compaction. i.e. compact the soil in 5 layers with 55 blows to each layer by the 4.89 kg
rammer.

3rd Layer @56 Blows

2nd Layer @56 Blows

1st Layer @56 Blows

• 6. Remove the extension collar and trim the compacted soil carefully at the
level of top of mould, by means of a straight edge. Any holes developed on
the surface of the compacted soil by removal of the coarse material, shall
be patched with the smaller size material. Remove the perforated base
plate, Spacer disc and filter paper and record the mass of the mould and
compacted soil specimen. Place a disc of coarse filter paper on the
perforated base plate, invert the mould and compacted soil and clamp the
perforated base plate to the mould with the compacted soil in contact with
the filter paper.

• 7. Place a filter paper over the specimen and place perforated plate on the
compacted soil specimen in the mould. Put annular weights to produce a
surcharge equal to weight of base material and pavement, to the nearest
2.5 kg.
Procedure Preparation of Test Specimen:
• 8. Immerse the mould assembly and weights in a tank of water and
soak it for 96 hours. Mount the tripod for expansion measuring
device on the edge of the mould and record initial dial gauge reading.
Note down the readings every day against time of reading. A constant
water level shall be maintained in the tank throughout the period.
• 9. At the end of soaking period, note down the final reading of the
dial gauge and take the mould out of water tank.
• 10. Remove the perforated plate and the top filter paper. Weigh the
soaked soil sample and record the weight
• Procedure for Penetration Test
1. Place the mould assembly with test specimen on the lower plate of penetration testing machine.
To prevent upheaval of soil into the hole of the surcharge weights, 2.5 kg annular weight shall be
placed on the soil surface prior to seating the penetration plunger after which the remainder of
the surcharge weights shall be placed.

2. Seat the penetration piston at the center of the specimen with the smallest possible load, but in
no case in excess of 4 kg so that full contact of the piston on the sample is established.

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

4. Record the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10 and 12.5 mm.
5. Raise the plunger and detach the mould from the loading equipment. Take about 20 to 50 g of soil
from the top 30 mm layer and determine the moisture content.
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 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.
Reference: Guidelines and Specifications for Design of Formation for Heavy Axle Load Report No. RDSO/2007/GE : 0014
Reference: Guidelines and Specifications for Design of Formation for Heavy Axle Load Report No. RDSO/2007/GE : 0014

DESIGN METHODS/PRACTICE IN WORLD RAILWAY SYSTEMS

Various design methods of formation, particularly for blanket thickness, are in use in
different railway systems. These are based on different properties of soil used in
embankment construction which govern the behavior of soil viz. percentage of fines (size
less than 75 microns) present in the soil, CBR value of soil, undrained shear strength Cu
of soil etc. These methods have been described briefly as under :
 Reference: Guidelines and Specifications for Design of Formation for Heavy Axle Load Report No. RDSO/2007/GE : 0014

DESIGN METHODS/PRACTICE IN WORLD RAILWAY SYSTEMS