California Bearing Ratio (CBR) for determining Shear Strength of subgrade

California Bearing Ratio (CBR) for determining Shear Strength of subgrade
California
Bearing Ratio (CBR) test is a compressive nature penetration test. It was
originally developed by Caltrans i.e. California Department of Transportation
after World War II i.e. in the late 1930s. 

The test is specifically used to
determine the mechanical strength as well as the potential strength of road
subgrades and basecourses materials including the recycled material generally
used for road and airfield pavements. CBR value is a percentage comparison with
the standard crushed rock from California and thus this test is a comparison
test.

The test
is standardized by American Society Of Testing Materials (ASTM) as D1883-05
(this standard is used for laboratory prepared samples or re-molded samples)
and D4429 (for on field soils); and by American Association of State Highway
and Transportation officials (AASHTO) T193; and by British Standard as BS 1377;
and by International Standard (IS) as 2720 (Part XVI).

Main components of Pavement design

The
results obtained by CBR test are used with the empirical curves to determine
the thickness of different layers of flexible pavement like subgrade, subbase,
base courses. This is the most widely used method for the design of flexible
pavement.
CBR value
is used to quantify the response of the pavement foundation and subgrade to
loading. CBR does not provide any data regarding the properties of the soil
except as to compare its resistance to penetration to the base crushed rock’s
resistance to penetration. Although CBR test is empirical and has some
limitations but it is still used around the world due to its low equipment
requirement, ease of performance prediction and history of use.
The two
empirical methods for pavement design used now days are CBR method and Group
Index Method. In CBR method beside many other steps one step is to determined
the CBR value. This test allows the Engineer to design the Capping Layer and
the sub-base layer by determining the strength of the underlying soil.
California Bearing Ratio (CBR) for determining Shear Strength of subgrade
Flowchart for pavement design using CBR method
 

Scope and Objective of the CBR Test

The
Objective of the California Bearing Ratio test is to determine the CBR value
for a soil under consideration as a pavement foundation.
CBR is
generally used for cohesive soils where effect of water content on CBR is more
and generally CBR is determined for a range of water content but it can also be
performed for cohesionless soils or coarse grained materials where the effect
of water content on CBR is small and thus CBR is performed at Optimum Moisture
Content (OMC).
According
to ASTM; CBR is primarily intended for evaluating the strength of materials
having maximum particle sizes less than ¾ inches or 19 mm, but this restriction
is not strict one; if the sample has a material larger than 19 mm; equal amount
of such material be replaced by material of size smaller than 19 mm but from
the same representative sample.
If the
sample to be tested contains much fraction of particles of sizes 3 in (Sieve
No. 4), the CBR results will fluctuate and thus more trials are required to
establish a reliable CBR; thus generally the sample to be tested consists of
particles smaller than 19 mm but larger than 3 in.
CBR test
can be performed for both in soaked conditions or un-soaked conditions; but
mostly soaked conditions is preferred so as to evaluate the material strength
in worst conditions. 

Description of Apparatus

Apparatus
required to perform the CBR test is as follows;
  • Molds:  Cylindrical
    mold with an internal diameter of 6 in. and a height of 7 in. with an extension
    collar of 2 in. height and a perforated base plate.
  • Spacer Disk: A circular disk of metal 5 –
    15/16 in. diameter and 2.416 in. height.
  • Rammer: A rammer of mass 4.54 kg (10 lbs)
  • Apparatus for measuring expansions. This
    consists of a swell plate with adjustable stem and a tripod support for a dial
    indicator.
  • Surcharge weights: Several slotted or split
    metal plates of 149.2 mm; diameter and 5 lb weight.
  • Penetration Piston: A metal Piston of circular
    cross – section having diameter of 1.954 in. Area = 3in2 and
    not less than 4 inches in length.
  • Loading Device: A compression type apparatus
    capable of applying a uniformly increasing load up to 10,000 lb at a rate of
    1.3 mm/min.Soaking Tank: A tank suitable for maintaining
    the water level of 1 in, above the top of specimen.
  • Drying Oven: Oven Capable of maintaining a
    temperature of 110 + 5 0C for drying samples.
  • Moisture content Containers
  • Miscellaneous: Tools such as mixing pans,
    spoons, straightedge, filter paper, balances etc.

Test procedure

  1. Approximately 18 kg soil pass of 19mm sieve and
    retain of sieve no. 4 is taken.
  2. Moisture and dry density curve is obtained
    using the standard AASHTO T 99 or T 180.
  3. Optimum Moisture Content (OPC) is obtained
    from the graph between moisture content and dry density
  4. Prepare the sample by adding optimum moisture
    content and then compact the soil in five layers by applying 10,30 and 65 blows
    respectively in three CBR molds using 10 lb rammer having 18 in. height of
    fall. The compacted densities of the three specimens range from 95 percent to
    100 % of the maximum dry density  already
    determined by the T 180 compaction test.
  5. Soaking: Place the swell plate with adjustable
    stem on the soil sample in the mold and apply sufficient annular weights to
    produce an intensity of loading equal to the mass of sub-base and base courses
    and surfacing above the tested material, but not less than 4.54 kg (10 lbs) .
    Place the tripod with dial indicator on top of the mold and make an initial
    dial reading.
  6. Immerse
    the mold in water to allow free access of water. Place the sample in water for
    96 hours (4 days)
  7. Make a dial reading on soaked specimen and
    calculate swell as a percentage of initial sample height.
  8. Remove the sample from tank and allow to drain
    for 15 minutes.
  9. Penetration Test: Place the mold on the
    loading frame and adjust its potion until the piston is centered on the
    specimen.
  10. Seat the penetration piston with a 44 N (10
    lb) load, and set both the load dial and the strain dial to zero. This initial
    load is considered as the zero load when determining the stress-penetration
    relationship.
  11. Place the surcharge weights on the specimens
    equal to that used during soaking. Apply load at a rate of 1.3 mm / min and
    record the load for penetration of 0.025 in, 0.05 in, 0.075 in, 0.10 in and so
    on up to 0.5 inches.
  12. Stress strain curve: Plot curves between load
    and penetration for each specimen. Apply the corrections to the curves if
    required. Take the readings of load for 0.1 in and 0.2 in. penetration and find
    CBR for both penetrations. The greater values is the required CBR for that
    specimen. Also find the dry density for each specimen.
  13. CBR = Test load value, divided by, the
    standard load, multiplied by 100.
  14. Design
    CBR: it is calculated by plotting a graph between CBR values and dry densities
    of all the three specimens and then calculating the design CBR against value of
    85 % maximum dry density.

   Test results, table ,graphs and calculations

Step 1:
Getting the relationship and graph between moisture content and dry density
California Bearing Ratio (CBR) for determining Shear Strength of subgrade
Moisture Density Relationship
Dry
Density (lb/cft)
Sample No.
Moisture Content (%)
Dry Density (lb/cft)
1
2
3
4
5
Step 2: Finding the density of the three
samples each of 10 blows, 30 blows and 60 blows
Step 3: Load v/s Penetration Graph
California Bearing Ratio (CBR) for determining Shear Strength of subgrade
Load Penetration Curve
For sample made with 10 blows
Sr.No
Load (lbs)
Penetration (mm)
1
0.5 mm
2
1.0 mm
3
1.5 mm
4
2.0 mm
5
2.5 mm
6
3.0 mm
7
3.5 mm
8
4.0 mm
9
4.5 mm
10
5.0 mm
11
5.5 mm
12
6.0 mm
13
6.5 mm
14
7.0 mm
15
7.5 mm
16
8.0 mm
17
8.5 mm
18
9.0 mm
19
9.5 mm
20
10 mm
For sample made with 30 blows
Sr.No
Load (lbs)
Penetration (mm)
1
0.5 mm
2
1.0 mm
3
1.5 mm
4
2.0 mm
5
2.5 mm
6
3.0 mm
7
3.5 mm
8
4.0 mm
9
4.5 mm
10
5.0 mm
11
5.5 mm
12
6.0 mm
13
6.5 mm
14
7.0 mm
15
7.5 mm
16
8.0 mm
17
8.5 mm
18
9.0 mm
19
9.5 mm
20
10 mm
For sample made with 60 blows
Sr.No
Load (lbs)
Penetration (mm)
1
0.5 mm
2
1.0 mm
3
1.5 mm
4
2.0 mm
5
2.5 mm
6
3.0 mm
7
3.5 mm
8
4.0 mm
9
4.5 mm
10
5.0 mm
11
5.5 mm
12
6.0 mm
13
6.5 mm
14
7.0 mm
15
7.5 mm
16
8.0 mm
17
8.5 mm
18
9.0 mm
19
9.5 mm
20
10 mm

Summary of CBR Test and conclusion

Sample No.
Compaction Effort
Dry Density (lb/cft)
CBR value (%)
1.
10
113
14.5
2.
30
121
28
3.
65
125
34.5
Maximum
Dry density _____________________ (lb./cft)
95% of
maximum dry density________________(lb/cft)
Resultant
CBR or Designed CBR __________________ (lb/cft)
California Bearing Ratio (CBR) for determining Shear Strength of subgrade
CBR – Density Relation

Limitations

The
laboratory and field compaction methods are not identical, however, comparative
tests indicate that reasonable correlation of results can be obtained from
field compact materials and samples compacted under similar conditions in the
laboratory.
Because added strength to highly stabilized surfaces such as
asphaltic concrete is neglected, the assumption of a completely saturated
subgrade condition sometimes results in a too conservative factor of safety.
Because many of the procedures are of an arbitrary nature, you
must run the test to exact standards in order for the design tables to be
valid.

Conclusion:

California
bearing ratio is a widely used method to design the flexible pavements, beside
all the limitations it is easy to perform and still does not need any big
instruments etc. The value of the CBR test is then compared with the following
table to get the quality of material from which the required thicknesses and
other parameters are decided.
CBR VALUE
SUBGRADE STRENGTH
COMMENTS
3%
and less
Poor

Capping is required 
3%
– 5%
Normal
Widely
encountered CBR range capping considered according to road category
5%
– 15%
Good
“Capping”
normally unnecessary except on very heavily trafficked roads
.
Saad Iqbal
Hi there, I am Saad Iqbal from Pakistan. I am an enthusiastic blogger, passionate content creator, construction geek, and a creative graphic designer. Connect with me at my social channels.