What is CBR of a subgrade soil?

The California Bearing Ratio (CBR) is a measure of the strength of the subgrade of a road or other paved area, and of the materials used in its construction. The ratio is measured using a standardized penetration test first developed by the California Division of Highways for highway engineering.

What is effective CBR of subgrade?

Notes: CBR 75% is the maximum permitted value for Effective Subgrade Strength. For the formula to hold, the maximum permissible Equivalent Subgrade Strength is CBR 12%

What should be the CBR value?

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.

What is CBR value of sub-base material?

in subgrade material CBR value should be nearly 15-19% .

What is the minimum value of CBR of subgrade in flexible pavement?

➢The sub-base material should have minimum CBR of 20% for cumulative traffic up to 2 msa and 30% for traffic exceeding 2 msa.

Can CBR exceed 100%?

High quality crushed rock has a CBR over 80. The standard material for this test is crushed California limestone which has a value of 100, meaning that it is not unusual to see CBR values of over 100 in well compacted areas.

How much does the CBR value of subgrade of the pavement should be?

What are the requirements of a subgrade?

A subgrade’s performance generally depends on three of its basic characteristics (all of which are interrelated) as under:

• Moisture content.
• Stabilization with cement or asphaltic binder.
• Strength & Stiffness of Subgrade Soil.
• Construction Works.

What is the CBR test frequency for embankment and subgrade?

One test for every 250 cum of soil. One CBR test for every 3000 cum or closer as and when required by the Engineer.

Is a CBR of 5 good?

A good quality laboratory load frame with a capacity of 10,000lbf (44.5kN) or more that can operate at a strain rate of 0.05in (1.3mm) per minute is suitable for CBR penetration testing.

How do you calculate CBR of soil?

The CBR vlues are usually calculated for penetration of 2.5 mm and 5 mm….Calculation for California Bearing Ratio (CBR) Test.

Penetration depth (1) mm Unit standard load (2) kgf/cm2 Total standard load (3) kgf
2.5 70 1370
5.0 105 2055
7.5 134 2630
10.5 162 3180

What is the density of subgrade?

Generally, adequate density is specified as a relative density for the top 150 mm (6 inches) of subgrade of not less than 95 percent of maximum density determined in the laboratory. In fill areas, subgrade below the top 150 mm (6 inches) is often considered adequate if it is compacted to 90 percent relative density.

What is the CBR value for embankment?

The compaction degree K (K is defined as the ratio of dry density to the maximum dry density, as a percentage) of soils to be used as sub-base embankment fillers should exceed 90%, and the CBR value should be at least 3%.

What is CBR in construction?

CBR Testing (California Bearing Ratio) is primarily undertaken to provide data for road pavement design. It was first developed by the California State Highway Department. It is a penetration test which is used to evaluate the subgrade strength primarily of roads, pavements and foundations.

What does low CBR mean?

A clayey soil generally has a low CBR value (less than 8). Sands are more granular and drain better and will generally have CBR values between 15 and 35. Gravel will have the best CBR values, generally 25 and up. That is why it is suggested to save the best material to cap the subgrade.

Subgrade strength is mostly expressed in terms of CBR, the California Bearing Ratio. Hence, in all, the pavement and the subgrade together must sustain the traffic volume. The subgrade strength owing to its inconsistency or variable nature poses a challenge for the engineer to come up with a perfect design of pavement.

How can I improve my CBR?

Adding lime to soil improves the stability of soil and improves the CBR value. Optimum ratio of adding lime in clayey sand type of soil is 6% [6].

How do you increase CBR value in subgrade?

What is CBR swell?

CBR Swell Plate used in California Bearing Ratio (CBR) testing is a 5.875in (150mm) metal perforated plate with adjustable stem which easily locks in place at the contact end. The stem is also threaded for ease of inserting and removing.

What is Type A fill material?

A Type A material layer of low permeability placed immediately below the pavement subbase or selected material to minimise changes in moisture content in the material below the capping layer. Where a capping layer is provided, the top of this layer will typically be subgrade level.

What is Type A material?

Type A material is roughly 3 to 4-inch (75-100-mm) in size. Type A will have a maximum dimension measured in any direction of 200mm, 100% of Type A TDA will pass through the 100-mm square mesh sieve and a minimum of 95% passing the 75-mm square mesh sieve.

What is the CBR value of a subgrade?

CBR value of the Subgrade / Sub-Base is also a data needed to finalise the thickness of the pavement. When the strength of the subgrade is too low, other means like introducing Geotextiles, ground treatment / improvements also have to be thought of.

What is the minimum CBR for sub-base material?

The sub-base material should have minimum CBR of 20% for cumulative traffic up to 2 msa and 30% for traffic exceeding 2 msa. The material should be tested for CBR at the dry density and moisture content expected in the field. On a California bearing test (CBR) , why do we use the greater CBR (2,5mm or 5mm deeper) instead of the minor one?

What is the value of CBR in soil?

CBR values may vary from less than 1% on soft clays to more than 100% on dense crushed rock samples. CBR test is generally done on compacted soil of around 97% compaction. IS: 2720, Part-XVI covers the laboratory method for determination of California Bearing Ratio (CBR).

What is CBR and why is it percentages?

That’s why the CBR values are percentages, indicating the strength of a granular material in relation to the standard material.