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UNIVERSITY OF MAURITIUS

CIVIL ENGINEERING DEPARTMENT

CIVE 2013 Y

Lab. Report :

Determination of CBR by subjecting an unsoaked

soil sample with a penetration apparatus

Submitted by:

HOW YUEN SIONG Yannick Van Pow

1310446

Date: 29th April 2015

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Lab. Report :

Determination of CBR by subjecting

an unsoaked soil sample with a

penetration apparatus

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Table of Contents

1. ABSTRACT .................................................................................................................................... 4

2. INTRODUCTION .......................................................................................................................... 5

Aims and objectives ............................................................................................................................ 5

Literature Review ................................................................................................................................ 5

3. METHODOLOGY ......................................................................................................................... 6

3.2. Initial preparation .................................................................................................................... 6

3.3. Apparatus used ........................................................................................................................ 6

3.4. Sample preparation by dynamic compaction .......................................................................... 6

Health and safety issues ...................................................................................................................... 7

4. RESULTS ....................................................................................................................................... 8

5. DISCUSSION ............................................................................................................................... 10

Limitations ........................................................................................................................................ 10

6. CONCLUSION ............................................................................................................................. 11

7. REFERENCES ............................................................................................................................. 11

8. APPENDIX ................................................................................................................................... 12

Contribution of members .................................................................................................................. 13

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1. ABSTRACT

This experiment examined the load that was required to penetrate a soil at specific depths. A plunger

would penetrate a compacted soil at a constant rate to a certain limit and the load required to do so was

recorded. The ratio of the actual load recorded to a standard load is expressed as the CBR. The latter

gives a representation of the strength subject to loading. The results have shown that at a particular

moisture content, the soil has behaves as sand.

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2. INTRODUCTION

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of

road subgrades and basecourses. It was developed by the California Department of Transportation

before World War II. The CBR test was conducted in accordance to BS 1377 : Soils for civil engineering

purposes : Part 4, Compaction related tests.

Aims and objectives

The aims of this experiment was to determine the CBR ratio of a soil and obtain its dry density.

Literature Review

The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of

standard area. The measured pressure is then divided by the pressure required to achieve an equal

penetration on a standard crushed rock material.

The California bearing ratio test is basically a laboratory penetration test, but it can also be carried out

in situ. It is not easy to obtain reliable reproducible results with wet cohesive soils such as those

commonly found in the UK; the test is much more applicable in tropical and sub-tropical regions where

drier soils occur.

The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads.

The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils

under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled

farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High

quality crushed rock has a CBR over 80. The standard material for this test is crushed California

limestone which has a value of 100.

.

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3. METHODOLOGY

3.1. Material

The CBR test was carried out on material passing the 20 mm test sieve prepared by the lab technicians

as described in 7.6.5 of BS 1377-1:1990. The mass of soil available for test should be about 6kg.

3.2. Initial preparation

900 mL of water was added to the initial soil sample, in order to bring it to the required moisture content,

and the soil was then thoroughly mixed.

3.3. Apparatus used

A cylindrical carrion-resistant, metal mould

Steel Cutting collar to levelled the soils surface

Surcharge weights

Dial gauges to measure penetration and applied forces.

Penetration Plunger

Loading Machine

Metal rammer to compact the soil

A balance capable of reading up to 5 g.

Whatman No.1 filter papers, 150 mm in diameter

A stopwatch readable to 1s

Vice to secure base plate with the mould

Wrench and hammer to secure or release mould components

3.4. Sample preparation by dynamic compaction

The method of dynamic compaction chosen was the 2.5 kg rammer method.

The empty weight of the mould together with the base plate were recorded. The extension collar

was removed (m2)

The extension collar was then tightened with the top of the mould.

Figure 1- General arrangement of apparatus

for CBR test

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A filter paper was placed on top of the base plate.

The mould was placed on the solid floor and the wet soil was compacted in three layers of

approximately equal mass each layer being given 62 blows with 2.5kg hammer equally

distributed and dropped from a height of 300 mm above the soil.

The amount of soil used shall be sufficient to fill the mould, leaving not more than about 6mm

to be struck off when the extension collar is removed

The extension collar was removed and the compacted soil to the top of the mould was carefully

removed by means of a straight edge.

The mould was weighed with the compacting soil (m3)

The mould was placed on the lower plate of the testing machine with top face exposed.

Two annular discs were placed on the soil surface prior to seating the penetration plunger.

The plunger was set under a load so that full contact was established between the surface of the

specimen and the plunger.

The force and strain gauges were set to zero.

The test was started with the plunger penetrating the sample at a rate of 1 mm/min and a timer

put on.

For every penetration of 0.25 mm (up to a total penetration of 7.5 mm), the readings of the force

gauge was recorded.

The plunger was raised and the mould detached from the loading equipment.

A sample of soil was collected from the top layer in order to determine the water content.

The penetration test was then be carried out on the other end of the mould.

The depression left by the plunger was filled and levelled to a flat surface.

The baseplate was removed from the lower end of the mould. It would then be secured to the

top end.

The steps for carrying out the penetration test and water content were the same as described

before.

Health and safety issues

The surcharge weights must be aligned with the plunger so that the plunger penetrates freely in

to the soil.

The rammers casing should be held steady by one hand and not too close to the falling rammer.

Care had to be taken when releasing and securing the mould with the hammer and wrench,

because a lot of force had to be applied.

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4. RESULTS

CBR =force required on test soil

force required for same penetration on standard soil

Table 1

The accepted CBR is 32%.

The full set of Load and penetration data is available in Table 2 of Appendix.

The results have been plotted into a graph for both top and bottom side as shown in Figure 1.

Penetration/mm Load/kN Standard

load/kN CBR (%)

Top side 2.5 3.496 13.2 27

5.0 4.674 20 23

Bottom

side

2.5 4.199 13.2 32

5.0 5.852 20 29

Figure 2- Load vs Penetration graph

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The dry density is calculated by the formula:

=

1+.

The moisture contents calculations are found in Table of Appendix

Table 2

Mass of mould + base plate/g 5620.0

Mass of mould + base plate + soil/g 9935.5

Mass of soil/g 4315.5

Diameter of CBR mould/cm 15.2

Height of CBR mould/cm 12.7

Volume of mould/cm3 2304

, Bulk density/Mg/m3 1.87

, Moisture content % 27.7

, Dry density/Mg/m3 1.46

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5. DISCUSSION

The test was conducted on unsoaked sample. No correction to the graph was required.

Penetrations of 2.5 mm and 5 mm were used for calculating the CBR value. CBR value is taken

with the one with the highest percentage.

The load vs penetration graph showed that the bottom part required more energy to penetrate.

The reasoning behind this is because the soil was placed and compacted in layers, therefore the

bottom part received more compacting blows as compared to the upper layers.

Since all CBR values within 10%, a mean result could have been taken.

The 32% CBR value would indicate that this soil lies between a poorly graded sand to well

graded sand

Surcharge weights prevent upheaval of soil. One disc of 2 kg would simulate the effect of

approximately 70 mm of superimposed pavement.

Limitations

It was difficult to judge to which height soil should be added into the mound such that each

layer would be compacted to one third of the total height.

The soil was conducted on an unsoaked disturbed sample, which would not represent the worst

conditions of the soil.

The soil at the top layer was more prone to moisture loss due to evaporation at the surface.

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6. CONCLUSION

The results have shown that the soil behaves principally as sand. The test has shown that the lower soil

in the mould received more compaction and therefore achieved a slightly higher CBR value.

7. REFERENCES

British Standard Methods of test for Soils for civil engineering purposes Part 4. Compaction-related

tests

Whitlow, R., 2001. Basic Soil Mechanics. 4th ed. Pearson Education

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8. APPENDIX

Table 3 Penetration and force readings

Penetration of plunger/mm Force Device (div) Force on plunger (kN)

Top Bottom Top Bottom

0.00 0 0 0 0

0.25 47 54 0.893 1.026

0.50 82 94 1.558 1.786

0.75 110 120 2.090 2.280

1.00 134 141 2.546 2.679

1.25 148 158 2.812 3.002

1.50 158 173 3.002 3.287

1.75 166 187 3.154 3.553

2.00 172 199 3.268 3.781

2.25 178 211 3.382 4.009

2.50 184 221 3.496 4.199

2.75 189 231 3.591 4.389

3.00 195 240 3.705 4.560

3.25 201 250 3.819 4.750

3.50 206 259 3.914 4.921

3.75 212 268 4.028 5.092

4.00 217 277 4.123 5.263

4.25 223 285 4.237 5.415

4.50 236 293 4.484 5.567

4.75 245 300 4.655 5.700

5.00 246 308 4.674 5.852

5.25 251 315 4.769 5.985

5.50 256 324 4.864 6.156

5.75 261 330 4.959 6.270

6.00 267 344 5.073 6.536

6.25 273 351 5.187 6.669

6.50 278 358 5.282 6.802

6.75 284 364 5.396 6.916

7.00 289 371 5.491 7.049

7.25 294 377 5.586 7.163

7.50 300 384 5.700 7.296

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Table 4

Before test Top Bottom

Mass of tin/g 15.17 15.19 15.05

Mass of tin + soil/g 41.24 35.23 34.44

Mass of tin + dry soil/g 35.56 30.94 30.21

Moisture content (%) 27.9 27.2 27.9

Average moisture content (%) 27.7

Soil Description:

-Condition: Moist

-Colour: Brown 4/4

-Size:

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UNIVERSITY OF MAURITIUS

DEPARTMENT OF CIVIL ENGINEERING

Module Name: Geotechnical Engineering Module Code: CIVE 2013 Y

Student Name & ID: HOW YUEN SIONG Yannick Van Pow - 1310446

Name of Assessor: A. Chan Chim Yuk

Date:

LABORATORY REPORT ASSESSMENT FORM

Marks allocated on a 40/ 60 basis

1. PRESENTATION, STYLE, LANGUAGE (40%)

i. Presentation and style Structure of report: Use of tables, diagrams, illustrations: Clarity: Details: Accuracy: Overall quality:

ii. Language Grammar/ Use of English: Spelling/ Clarity of expression: Overall quality:

GENERAL COMMENTS

MARKS

2. CONTENT (60%) Write Up (1,200 to 1,500 words)

Coverage: Title, Abstract, Introduction, Methodology, Health & Safety issues, Analysis of results, Discussion, Conclusion Level of detail: Accuracy: Use of references: Overall quality: Attainment of Learning Outcomes:

Experiment and data analysis

Professional &Technical Communication

Impact of engineering activity: Health & Safety