Sub grade soil is an integral part of the road pavement structure as it provides the support to the pavement from beneath. The main function of the sub grade is to give adequate support to the pavement and for this the sub grade should possess sufficient stability under adverse climatic and loading conditions. Therefore, it is very essential to evaluate the sub grade by conducting tests.
The tests used to evaluate the strength properties of soils may be broadly divided into three groups:
- Shear tests
- Bearing tests
- Penetration tests
Shear test
Shear tests are usually carried out on relatively small soil samples in the laboratory. In order to find out the strength properties of soil, a number of representative samples from different locations are tested. Some of the commonly known shear tests:
1. Direct shear test,
2. Triaxial compression test,
3. Unconfined compression test.
California Bearing Ratio (CBR)
This test was developed by the California Division of Highway as a method of classifying and evaluating soil-sub grade and base course materials for flexible pavements.
CBR is defined as the ratio of force per unit area that is required to penetrate a soil mass with a piston of 1.25mm/min corresponding to that required for the penetration of a standard material.
CBR = Test load/Standard load * 100
Apparatus Required:
- Steel Cutting Collar
- Dial Gauges
- Weights
- IS Sieves
- Penetration Plunger
- Disc and loading machine
- Cylindrical Mold and Rammers
Procedure:
The procedure for conducting the CBR test is as follows:
- Firstly, an undisturbed sample specimen is prepared by attaching a cutting edge to the mold and pushing it gently in the mold to remove the soil on the outside.
- The density of the sample is determined and is remolded at Proctor’s maximum density or Optimum moisture content. The sample must be such that it must pass through a 20mm IS sieve but must be retained on a 4.75mm IS sieve.
- About 4.5kg to 5.5 kg of the sample is mixed with water. The extension collar and base plate are then fixed to the mold and the disc is placed over the base plate.
- The soil mix is then compacted in several layers by a rammer. Generally, for light compaction, the soil is compacted in three layers by 55 blows from a 2.6kg rammer.
- The collar is then removed and the soil is trimmed. The mold is then turned over and the base plate and disc are removed.
- The mold is then weighed and bulk density, as well as dry density, is determined.
- Then the mold along with the surcharge weight is placed on the penetration test machine.
- The load is applied to the piston such that the rate of penetration is 1.25mm/min.
- The load readings at penetrations of 0.5,1.0,2.0,2.5,3.0,4.0,5.0,7.5,10 and 12.5 mm are recorded.
- The mold is then detached from the loading equipment and moisture content is determined. C
Plate Bearing Test
Plate bearing test is used to evaluate the support capability of sub-grades, bases and in some cases, complete pavement. Data from the tests are applicable for the design of both flexible and rigid pavements. In plate bearing test, a compressive stress is applied to the soil or pavement layer through rigid plates relatively large size and the deflections are measured for various stress values.
plate load test
