Introduction:
In order to analyze the California Bearing Ratio (CBR) of a soil, we perform a CBR test during the laboratory section. CBR test is the penetration penetration test for evaluating the mechanical strength of the road subgrades and also the basecourses, after obtaining the CBR value, we will know that whether it needs to have the capping layer between subgrade and the subbase or not. Normally, Normally, when CBR value for a soil is lower than 15%, both capping layer and subbase layer are required, while when CBR values for a soil is larger than 15%, we can choose to use capping+subbase capping+subbase or subbase only. In this lab section, we measure the force and penetration when a cylindrical plunger of cross sectional area 1935
2
is made to penetrate the soil at 1mm/min rate to
a maximum depth of 7.5mm. For reference, the standard curve was obtained by the Californian originators of the
tests from tests on crushed limestone, li mestone, a very good base material. The loads required producing penetrations of 2.5 mm and 5.0 mm are expressed as percentages of standard loads of 13.24 kN and 19.96 kN respectively. The higher of these percentages is the CBR value of the soil.
Apparatus:
CBR test machine
2.5 kg rammer
CBR mould (including mould body, body, extension collar and base plate)
C-spanner
Base plate tool
Straight edge
Procedures
- Weigh the mould body with base plate. - Divide the prepared quantity of soil into three portions. - Attach the extension collar to the mould. (Figure 5) - Stand the mould assembly on solid base. - Place the first portion of soil into the mould and compact it using the 2.5 kg rammer,
so that after 62 blows the layer occupies about one-third of the height of the mould. Ensure that the blows are evenly distributed over the surface. - Repeat using the other two portions of soil in turn (62 blows for each layer).
(Figure 6) - Remove the extension collar by using the C-spanner and trim the soil level with the
top of the mould by the steel straightedge. (Figure 7 – 7 – 10) 10) - Weigh the mould, soil and base plate.
centrall y on the testing machine. (Figure 11) - Place the mould with the sample centrally - Place the appropriate surcharge discs on top of the sample. - Apply a seating force to the plunger, depending on the expected CBR value, as
follows. For CBR value up to 5% apply 10 N For CBR value from 5% to 30%, apply 50 N For CBR value above 30% apply 250 N - Reset the reading of the force-measuring device to zero(because the seating force is
not taken into account during the test). - Start the machine and record readings of the force gauge at intervals of penetration
of 0.25 mm, to a total penetration not exceeding 7.5 mm , then stop the machine. - Remove the base plate by using the Base plate tool and C-spanner. (Figure 12) - Take out the soil by the Extruder. (Figure 13) - Take a sample of the soil to determine the moisture content. (Figure 14 & 15)
Result
Data
Density
CBR value
Discussion
According to the graph of the result, it is not a very smooth curve of the plotted graph. Therefore, adjustment of the graph should be done in order to have a more accurate result. Also, there is a difference between the standard CBR value and the experimental value, the possible errors are listed as below: 1.
Air void appears in the soil In realistic, honestly, the volume of the air void cannot be neglected. No matter how the compaction is perfect, the air void still exists. Moreover, the size of the aggregates is different, the gap between the a ggregate becomes void. Therefore, the effect of the air void in the experiment cannot c annot be determined correctly and hence affects the result.
2.
Chemical composition of aggregate to the soil Since there is mineral contains in the soil, for such quartz, plagioclase, microcline these minerals are relatively stiffer than others. Then, the stiffness and the strength of the soil will change. It is chemical composition of the aggregate to the soil that cannot be determined. Thus, the result is affected. Moreover, the standard of the CBR value used in laboratory is the British standard. As the soil is different everywhere in the world, the characteristic of the soil is also different. Therefore, the value of the soil is different from the experimental result since the soil used is not the same type.
3.
The moisture content of the soil
According to the graph, as the moisture content increase, the CBR value decreases. The moisture content in the experiment is only 9.72%, which is relatively low. Therefore, the experimental value should be higher than the standard value. 4.
Visual error of getting results There may be mistakes to have an incorrect data. As the gauge reading is changing every second, the reaction time of human may lead the mis taken of the experiment result.
5.
Not smooth surface of the soil The CBR value is measured the penetration of plunger and the load that is recorded. Also, the compaction of the soil should be exactl y perpendicular to the soil surface to ensure the compaction is satisfied. As the surface of the soil is not smooth or irregular in shape, the penetration of the soil cannot be recorded correctly and hence affects the result.
Discussion on Safety Regulation 1
Sandals are not allowed.
2
Do not smoke, eat or drink in the laboratories.
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Do not use audio headphones while carrying out experiments.
4
Do not leave things lying around and keep work areas tidy and clear.
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Do not run or play games in the laboratory.
6
For an un-conventional testing, the risk assessment should should be performed by the lecturer concerned, and the arrangement and advice should be sought from the Senior Technician or the Lecturer In-charge of the laboratory.
7
Students can operate the equipment in the laboratory, for the job it is intended intended to do, only (i) after receiving proper training, and (ii) with the arrangement and permission from the laboratory technician.
8
Follow exactly the safe way of using the equipment. Do not take short cuts to save time.
9
When using any equipment, know how to stop it in an emergency.
10
Do not attempt to try out equipment alone.
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Do not leave tests or experiments unattended.
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Inform the technician at once if you you think an equipment/instrument is not working properly or is unsafe.
13
Do not not wear dangling chains or loose loose clothing which could could get caught in moving parts. Keep long hair tucked under a cap or tied back.
14
Mind your your fingers when when using using rotary machines or or compacting equipment.
15
Beware of leaking coal gas.
16
Protective gloves gloves must be worn worn when handling heated bitumen or solvent. solvent.
17
When heating bitumen, make sure it is free of of water.
18
Organic solvents solvents are potentially toxic, it should be used only only under under the exhaust hood.
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All solvents containing vessels should always be covered.
20
Keep solvents away from heat, sparks or open flames. (Exposure of the vapour to naked flames or red-hot surface may produce “TOXIC GASES”.)
From the above safety regulations, there are some rules that are important for students to follow. The main and important rules are listed as follows: 8. Follow exactly the safe way of using the equipment. Do not take short cuts to save time Many students do not follow the steps of the experiment in order to save time for the earlier complete the laboratory section. However, it is really dangerous for the safety of the laboratory and the experimental result is totally affected. Therefore, no short cuts should try for student and follow all the steps in laboratory sheet to complete the experiment. 10. Do not attempt to try out equipment alone. Since the experiment is a group project. Never do the experiment alone. It is very dangerous for the student doing the experiment alone as accident easily happened when the student having the experiment carelessly. A team work is needed for having the experiment. 14. Mind your fingers when using rotary rotar y machines or compacting equipment. In the experiment, a 2.5 kg hammer is used to have a compaction to t he soil. However, make sure to use the hammer carefull y. Otherwise, it is easily easil y to hurt the finger when having the compaction to the soil.
Conclusion
In the experiment, we finally found that the CBR values of soil in 2.50mm and 5.00mm are 25.66% and 25.32% respectively respect ively.. Generally, the CBR value at 2.50mm penetration will be greater than that at 5.00mm penetration and is such case takes the value at 2.50mm as the CBR value. If the CBR value corresponding to a penetration of 5.00mm exceeds that of 2.50mm, we need to repeat the test. In this c ase , the value is acceptable. The larger of the CBR at 2.50mm and 5.00mm deformation will be taken as the design CBR. So the actual CBR value of soil is 25.66%. So when we use these soils which CBR value is 25.66% to make the subbase layer, we can choose to use capping+subbase capping+subbase or subbase.
