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DETERMINATION OF CBR VALUE

EXPERIMENT 4

INTRODUCTION

The California bearing ratio (CBR) is a penetration test conducted both in laboratory as well as

insitu for e!aluation of the strength and stability of a soil sa"ple for the construction of roads

or highways# It was de!eloped by the California Depart"ent of Transportation# The results of

the test are e$pressed in %ercentage and are relati!e results# The results are obtained for ' ""

penetration and ' "" penetration#

The CBR !alue is deter"ined by

CBR *+,

"easured pressure for site soils *N-"".,

pressure to achie!e e/ual penetration on standard soil *N-"".,

T01 B23IC CBR T13T

This consists of causing a plunger of standard area to penetrate a soil sa"ple4 (this can be in the

laboratory or on site)# The force (load) re/uired to cause the penetration is plotted against

"easured penetration4 the readings noted at regular ti"e inter!als#

This infor"ation is plotted on a standard graph4 and the plot of the test data will establish the

CBR result of the soil tested#

The test is fully co!ered in

B3 5677 3oils for ci!il engineering purposes%art 84Co"paction related tests#

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T01 R123ON :OR T01 CBR T13T

;e are deter"ining the resistance of the subgrade4 (i#e the layer of naturally occurring "aterial

upon which the road is built)4 to defor"ation under the load fro" !ehicle wheels# 1!en "ore

si"ply put4


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1. @ >g of the soil sa"ple was weighed in a tray and "i$ed with A99 "l of water# ?oisture

content was ta>en before "i$ing water (Tin 18.35g *Tin 3a"ple, 53.12g)

2. The "ass of the "ould was recorded together with the base plate# (5559g)

3. The "ould was fitted with collar4 filter paper placed at botto" and then filled with the soil

sa"ple#

4. 2 first layer was put and co"pacted using the 2.5 kgra""er gi!ing 27e!enly distributed

blows on the surface#

5. The co"paction was carried out in 6 e/ual layers pro!ided at the end the "ould was filled

abo!e @"" when the collar is re"o!ed#

6. The collar was then re"o!ed and the surplus co"pacted soil tri""ed with the steel

straightedge#

7. The "ass of the "ould4 base plate and co"pacted soil was recorded (@A6 g)

8. The "ould with the sa"ple was placed in the testing "achine with the top face e$posed#

9. 3urcharge weights of 2277.11gand 22.15gwere added and the gauge set to 9 ""#

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10. The "achine was turned on and readings of force were ta>en at e!ery 9#&'""

penetration up to 7#' ""#

11. Then the "ould was re"o!ed and a portion of the top surface was ta>en for "oisture

content# The hole left by the plunger was then fitted with co"pacted soil#

12. The base plate was then re"o!ed and fi$ed at the top of "ould4 thus penetration on the

botto" part was now recorded using the sa"e procedure fro" step A to step 55#

13. The readings were tabulated in the for" gi!en and graphs of load (EN) against

penetration ("") were plotted for the top and botto" face and CBR calculated using the

standard graph also#

OB31RF2TION3 2ND C2GCUG2TION

?ass of soil @ >g

?ass of "ould ''A' g

?ass of "ould Base plate sa"ple @A6 g

?ass of sa"ple co"pacted @A6 ''A' 89A g

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B!"k #$%&'* +, -498235

- 1.778 Mg/3

?oisture ;eight of sa"ple ;eight of dry sa"ple

68#77 &8#@&

59#5' g

The initial dry density,Hd(in Mg/m3), of the sample is calculated from the equation

Dry Density4 Hd Hb-(5w) where w is the "oisture content (+)

Hd

5#77A -

(59#85&&)

1.259

Mg/3

T2BG1 5

Before Test 2fter Test

Top Gayer Botto" layer

;eight of tin-g 5A#6' 5A#&7 5A#9

;t of sa"ple tin-g '6#5& 6A#98 '9#'&

;eight of sa"ple-g 68#77 5#77 65#@&

;t of tin dry sa"ple-g 8 6' 85#@&

;eight of dry sa"ple-g &8#@& 58#6& &&

?oisture -g 59#5' '#8' A#9

?oisture content4 w (+) 85#&6 6A#9@ 6#57

Dry density4 Hd(?g-"6) 5#&@ 5#& 5#&A

T2BG1 &

%enetration of plunger :orce

"" Top gauge Botto" gauge Top-EN Botto" EN

9#&' 59#& #& 9#5A 9#57A

9#'9 56#9 #A 9#&'& 9#59

9#7' 5'#9 5A 9#&5 9#&8A

5#99 5A#9 5@#9 9#68 9#659

5#&' 5#9 5#& 9#6@ 9#67&

5#'9 &5#9 & 9#897 9#8&7

5#7' &6#6 &@#9 9#8'& 9#'98

c &'#9 &A#9 9#8A' 9#'86

&' &7#9 69#& 9#'&8 9#'A@

'9 66#9 9#'@6 9#@89

' 65#9 6'#A 9#@95 9#@'

6#99 66#9 6A#' 9#@89 9#787

6#&' 68#' 85#9 9#@@ 9#7'

6#'9 6@#' 86#' 9#79A 9#A88

6#7' 6A#' 8@#9 9#787 9#A&

8#99 89#9 8A#& 9#77@ 9#6'

8#&' 8 '9#& 9#A5' 9#78

8#'9 86#' '' 9#A88 5#95

8#7' 8'#' '8#A 9#AA6 5#9@6

'#99 87#' '@#' 9#&& 5#9@

'#&' 8#9 'A#& 9#'5 5#5&

'#'9 '5#9 @9#' 9#A 5#578

'#7' '6#9 @' 5#9&A 5#&56

@#99 ''#9 @8#' 5#9@7 5#&'5

@#&' '@#' @@#6 5#9@ 5#&A@

@#'9 'A#' @A#5 5#56' 5#6&5

@#7' @9#9 79#9 5#5@8 5#6'A

7#99 @ 75#A 5#&96 5#66

7#&' @6#' 76#' 5#&6& 5#8&@

7#'9 @@#9 7'#9 5#&A9 5#8''

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JR2%0

F0 T0 0'"

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F0 B00/ 0'"

R13UGT3

The

standard

Goad fro"

B3 for

'"" and

'#9""

penetration

is

interpolated#

The !alues at penetration ' and '#9 ""4 obtained fro" the plotted graph of Goad-EN against

%enetration ("") were tabulated below and the CBR !alue calculated#

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3tandard Goad for '"" penetration 55#' (57#@ K 55#')-8

56#9&' EN

3tandard Goad for '#9"" penetration (57#@ &&)-&

5# EN

T2BG1 6

(5) 2t Top of speci"en

P$%$'0%// %## "0#kN L0# "'$#kN CBR

2.5 13.25 .563 4.33

5. 19.9 .922 4.63

(&) 2t botto" speci"en

P$%$'0%// %## "0#kN L0# "'$#kN CBR

2.5 13.25 .64 4.92

5. 19.9 1.96 5.51

CBR + for '"" penetration at top Goad applied - 3tandard load

(9#'@6 - 56#9&') $ 599

8#66+

3a"e procedure applies for '#9""4 hence the table abo!e being filled#

R13UGT3

CBR !alue at ' "" (8#66 8#&)

&

8#@6+

5 (%$$& 1 )

CBR !alue at '#9 "" (8#@6 '#'5)

&

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'#97+

5 (%$$& 1 )

CONCGU3ION

The soil tested has an al"ost good CBR !alue and can be considered to ha!e a good strength# In

general a soil of CBR less than &+ is of poor /uality and cannot be used in a road construction#

R1:1R1NC13

http--www#highways"aintenance#co"-cbrte$t#ht"

http--www#tpub#co"-content-engineering-58979-css-58979L&77#ht"

B3 5677859

A
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