Road Construction Soil Tests

Soil tests to check strength of soil

Following test can be mostly used when we want to check the strength of soil in civil engineering. 

  • CBR 
  • Field Density 

California Bearing Ratio (CBR)

The CBR is a measure of the resistance of a sample of soil to penetration under controlled conditions of density and moisture content. It is defined as the percentage of load required to force a piston into the testing sample to the standard load required to produce the same effect on a standard material (crushed California limestone).

The soaked CBR test is a measure of the strength of soil of the paving material when saturated, which is the worst condition expected during the life of the  road . For a soaked CBR value, the compacted sample is soaked for four days before testing.




The object in compacting a pavement is to improve its properties, in particular to increase its strength of soil and bearing capacity, reduce its compressibility and decrease its ability to absorb water. Compaction tests are carried out to determine whether the density of the pavement achieved during compaction complies with requirement of the specification.

Maximum Dry Density and Optimum Moisture Content

For any soil aggregate, or crushed rock there is a moisture content at which, for a given compactive effort, maximum dry density is achieved. This moisture content is referred to as the optimum moisture content.

To determine the degree of compaction of a paving material, a laboratory test is carried out to find the maximum density which can be achieved by compacting the material in a mold with a given number of blows of a standard hammer.

Samples of the test material are compacted at varying moisture contents and the dry density calculated. Results are graphed and the M.D.D. and O.M.C. determined.


Field Density


To test the compaction being achieved by the rollers, a field density test is carried out. One method of doing this consists of digging a hole, measuring its volume by filling it with sand of known density, weighing the material taken from the hole and finding its moisture content. The density measured in the field is then compared with that achieved in the laboratory after subtracting the weight of moisture in both materials.

A second method is to use a nuclear moisture density meter. Using this method the dry density of the material is calculated from the “count” read from the gauge of the meter which is calibrated for each material.