Soil Mechanics Laboratory
Test to determine the California CBR load factor
1. Introduction
This test is used in designing the thickness of layers in road construction. And its purpose is to determine the bearing capacity of the section.
2. Test theory
For any amount of penetration into the soil, the ratio of the amount of load to the standard load for the same amount of penetration in the standardized sample is called the California load ratio or CBR number.
Usually, the amount of load is read for penetration of 2.5 mm to be used as a measurement standard, but if the penetration distance is between 2.5 mm and 5 mm, the results will be larger than what was obtained for penetration of 2.5 mm. The largest value obtained is considered as the CBR number. The compaction of the sample is done in 5 layers with a 4.5 kg hammer and 56 blows.
3. Conditions and equipment needed
1. CBR device
2. Modified compression format
3. 10-pound hammer
4. Iron blade
5. Jack
6. Oil
7. Caliper
4. Description of the experiment
The test is carried out in two ways: dry and saturated. The following method is dry.
To prepare the soil sample, we first take 5.5 kg of soil. According to the optimal moisture percentage determined in the previous tests (10%), we add this amount of moisture and then pour the soil into the pre-oiled proctor mold in 5 layers and compact each layer under 56 strokes. . The volume of the mold is measured by a caliper before pouring the soil. Now we place the mold under the jack and place the disc on the mold so that the load column passes through the hole between these two discs and reaches the soil surface. Then we read the change in the length of the incoming load in terms of pounds for the desired values, the time of the end of the test is when the needle of the gauge returns in the meantime. Then, according to the obtained values, we draw the stress-length change curve and modify it in such a way that we read the amount of stress per 0.1 inch and then obtain the CBR from the following equation:
P0.1: Correction pressure at 0.1 inch
P: standard pressure which is equal to
The reason for the correction of stress is due to the presence of concavity or the presence of breaks in the curve, which causes changes in the origin.
In fact, CBR:
1- Comparison of calculations done with the standard mode
2- The amount of indentation and the cross-sectional area compared to the standard state
5. Calculations
Mold diameter: 6 inches
Mold height: 12 inches
The amount of water added to the soil:
The moisture content of the two samples taken:
6. Interpretation of the results
Considering that in the existing relations and graphs and as observed, with increasing pressure, penetration increases almost linearly. This process continues until the force reaches the bearing capacity of the soil. Since then, the incoming load has caused a great increase in infiltration, practically the soil can no longer tolerate it, henceforth, the soil continues to infiltration with less load. According to the diagram, you can see this effect.
Regarding the CBR number, the closer this number is to 1, the better performance this soil will have.
7. Errors in the test
Inaccuracy in properly pounding the soil inside the mold
Inaccuracy in gauge readings
The loading speed is not the same as the standard loading speed
Inaccuracy in cleaning the mold before testing
Error in measuring the dimensions of the mold and also weighing the soil
8. Application of on-site testing
All the soil mechanics tests are basically to determine the capacity of a sample, the CBR test is one of these tests, through which the maximum pressure can be calculated for the soil of a soil or the soil of the site. For example, to build the foundation of a building, it is necessary to know the maximum pressure that we can apply to the soil, so that we can determine the dimensions of the foundation.
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