California Bearing Ratio Test ReportHighways & Traffic Engineering

Ivan ThomsonOlisanwendu Ogwuda

Group 5BN0903A

11NOV2004

Table of Contents

Introductioniii

Objective of the Experiment 1

Testing Procedures & Apparatus2

Tables of Data, Calculations & Graphs3

Discussion of Results 8

Conclusions 9

Bibliography11

ii

Introduction

This report is the result of the California Bearing Ratio laboratory test carried out by Group 5 in the soils lab (Baxter building) at the University of Abertay-Dundee on 28 OCT 2004.

This report is primarily concerned with the data gathered by Group 5. However, as required, data and graphs from Group 4 and Group 6 are included and discussed.

The members of Group 5 were:

Bruce ShearerIvan ThomsonRichard Todd

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Objective of the Experiment

The objective of the California Bearing Ratio test is to determine the CBR value for a soil under consideration as a pavement foundation. This value is a percentage comparison with the standard crushed rock from California. Thus this test is a comparison test.

The CBR value is used to quantify the response of the pavement foundation and subgrade to loading1.

The standard crush rock from California values are as follows:

Standard Crushed Rock from California2

Load (kN) 13.24 19.96

Penetration (mm)

2.5 5.0

It should be noted that this test was created by the California Division of Highways in the 1930’s and as such is an empirical test and does not provide any data regarding properties of the soil except as to compare its resistance to penetration to the base crushed rock’s resistance to penetration.

The test remains in existence around the world due to its low equipment requirements, easy of performance and history of use.

It is important to realize that the CBR test is but one step in the road pavement foundation design process; the test allows the roadEngineer to design the capping layer (if needed) and the sub-baseLayer by determining the strength of the underlying soil.

By knowing this the Engineer can determine if thisstrength is adequate to handle the desired road designor if additional procedures need to be done to increasethis strength.

1 University of Abertay Dundee, Sub grade and Unbound Pavement Foundation, pg 2.2 University of Abertay Dundee, Subgrade and Unbound Pavement Foundation, pg 3.

Map of the pavement foundation design process*.

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Testing Procedures & Apparatus

Apparatus:*

Loading machine (a loading press) Prepared soil sample in CBR mould with collarSurcharge weightsScalesCBR mould wrenchesSteel (or Brass) ramming rodMisc. lab equipment such as a tray or bucket to contain the soil*

Testing Procedure Carried Out:

Step:

1. Determined the mass of soil needed to fill the mould by calculation using the formula:

2. Poured the sample into the mould whilst ramming with the steel rod.

3. Placed filter paper and 50mm compaction plug on top of soil.4. Place mould into compression machine and applied load until

top of plug was flush with the top of the mould collar.5. Removed the sample from compression machine, compaction

plug and filter paper.6. At this point the sample was prepared and ready to be placed

into the CBR loading press.7. Mould was placed into CBR loading press and the surcharge

weights were placed on top.8. Seated the CBR plunger on top of the soil sample and began

the process of applying the load at a rate of 1mm per minuet.9. Recorded the dial gauge readings for every .25mm of

penetration up to a maximum of 7.5mm.10. Removed the sample from the loading press, placed a

top cap on the sample, flipped it over and removed the bottom cap. This effectively flipped the sample over without removing it from the mould.

* CBR Equipment, Pavement Design – Foundation Design*Google Images [online], Photographs of CBR equipment, http://images.google.com/images?hl=en&lr=&q=CBR+test

Top -Typical CBR loading Press. Middle – CBR moulds, wrenches, plungers, etc. Bottom – Diagram of a CBR apparatus*.

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11. Repeated the CBR test upon the same sample (except it was the bottom instead of the top of the sample being loaded).

3

Tables of Data, Calculations & Graphs

4

5

6

6

7

Discussion of Results

Group 5;

The graph for Group 5 shows fairly close lines with only one line needing a minor correction.

The CBR values were;

This value (48.29) indicates that the soil was at the top end of a ‘Well Graded Sand’ and is a moderately strong soil.

Group 4;

The graph for Group 4 shows a very large difference in the results between test 1 and test 2 with one line needing a correction.

The CBR values were;

This value (98.19) indicates that this soil is a ‘Well Graded Sand’ to a ‘Sandy Gravel’ and is a fairly strong soil.

Group 6;

The graph for Group 6 shows a very large difference in the results and is kind of inverted compared to the graphs of Group 5 and Group 4. This inverted nature of the graph indicates there was higher moisture content in the soil (13.7% - almost 4 times as high as Group 5) and shows that as the load increased the soil offered more resistance. However, overall this soil offered little as far as a Max CBR value.

The CBR values were;

According to this value (8.05) this places the soil in the ‘Sandy Clay’ range and is a weak soil.

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Figure 2 – Diagram showing the direction of the force applied to the sample.

Conclusions

Group 4 and Group 5 had very similar results were the soil offered a decent amount of resistance to the initial loading but that this resistance decreased with increased loading. The soils for these two groups were moderate in strength to fairly strong.

Group 6, however, had a soil that offered little or no resistance to loading initially but that resistance rapidly increased as the load continued to increase. However, this soil was significantly weak when compared to the soils of the other two groups.

The major difference between the soil for Group 6 and the other groups appears to be due to more of the voids in the soil being filled with water rather than air. Air is much more compressible than water and once the air was pushed out (or at maximum compression) the water began to offer rapidly increasing resistance to loading.

The soil for Group 6 may have been much more representative of a soil in moisture equilibrium.

The soils for Groups 4 and 5 would require less compaction than the soil for Group 6 to achieve identical strengths.

Since the sample was compacted in the compacting machine only once and was tested in the CBR machine twice (once on the A face {see figure 2} and once on the B face) there will be different layers of compaction within the test sample.

The samples appear to compress much more (have more penetration) on the first test and less after the sample has been flipped over and retested on test 2. This would indicate that there are many more voids in test 1 than test 2 since many of the voids were removed during test 1.

Additional. After test 1 (and before test 2) the soil will be at it’s highest compaction along the horizontal plane at point A and the amount of compact will decrease until reaching it’s lowest compaction along the horizontal plane at point B. In other words the compact will steadily decrease from point A to point B.

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After test 2 was performed the sample will have high compression along the horizontal planes at both points A & B and lowest compaction in the center.

Test one face of the sample then testing on the opposite face simulates the forces the sample will face since it will have a constant upward force upon it as will as a constant downward force.

Additionally, the downward force will include periodic increases and decreases in the force as vehicles move over it. This will have duel effects on the sample as the upward force will increase to deal with the increase in force in the downward direction. In real-life situations the sample will be subjected to constant ‘squeezing and release’ time and time again at random intervals as traffic moves over it.

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Bibliography

Craig, R.F. 2004. Craig’s Soil Mechanics. 7th ed. London: Spon. ISBN 0-415-32703-2

Cover picture: CBR Testing Machine Picture: ELE International [online]. Available from: http://www.ele.com/geot/images/24-9150.jpg [Accessed on 11 Nov 2004]

Google.com [online]. Available from: http://www.google.com [Accessed 28 Nov 2004]

Google Images [online]. Available from http://images.google.com/ [Access 28 Nov 2004]

Pavement Design [online]. Available from: http://www.dur.ac.uk/~des0www4/cal/roads/pavdes/pavfound.html [Accessed 11 Nov 2004]

The Idiot’s Guide to Highways Maintenance [online]. Available from: http://www.highwaysmaintenance.com/cbrtext.htm [Accessed on 11 Nov 2004]

Napier University – School of Built Environment – Projects [online]. Available from: http://sbe.napier.ac.uk/projects/compaction/chapter7a.htm [Accessed on 11 Nov 2004]

University of Abertay Dundee. Unknown. Subgrade and Unbound Pavement Foundation [Class information sheet provided by Mr. Ogwuda].

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