estimation of cbr value using dynamic cone penetrometer
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Post on 16-Jan-2015
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DESCRIPTION
The subbase/base thickness of pavement is governed by the CBR value of the subgrade soil along with some other parameters such as traffic intensity, climatic conditions, etc. Dynamic cone penetration test (DCPT) value conducted in the field can be used to estimate the CBR value provided a suitable relationship exists between CBR and DCPT value.TRANSCRIPT
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K.S.Gill*, A.K. Choudhary **, J.N.Jha*,S.K.Shukla****Guru Nanak Dev Engg. College, Ludhiana,India
**National Institute of Technology, Jamshedpur, India***School of Engineering,Edith Cowan University,Perth Australia
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IntroductionExperimental WorkField and Laboratory TestTest Procedure and sample preparationResult and DiscussionConclusionsReferences
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Design of new flexible pavements/ Rehabilitation of existing pavements:
Needs accurate estimation of CBR Requires simulation of worst possible
environmental condition to be faced by by highway during its design life
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Insitu California Bearing Ratio (CBR) test: Expensive Relatively slow to conduct Difficult to conduct insitu soaked CBR test Misleading results obtained if piston tip rests on small
stone particle or pebbles Repeatibility of result doubtful
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Laboratory California Bearing Ratio test:• CBR specimens tested at field mosture content and
density tend to give higher CBR value in laboratory than those obtained in the field (particularly for granular soil)
• Difference is due to the confining effect of rigid mould in laboratory tests
Highway Engineers generally donot favour CBR tests
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Dynamic Cone Penetration Light and Portable Repeatability is considerably higher Suitable co-relation if developed between DCPT and
CBR value can offer an attractive means of determining insitu CBR at a comparative speed and ease of operation
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DCP Test conducted : Insitu and soaked conditions(Widening and Strengthening of Existing Road)
• 8 km Stretch of the left bank of Sidhwan Canal passing through sotheren part of Ludhiana city
• 8 locations marked at an interval of 1 km Laboratory soaked CBR tests on the specimen moulded
at insitu density
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Hammer correction factor is unity for 8 kg DCP Index(DCPI)or reading is defined as the
Penetration depth in mm per a single drop of hammer Depth of penetration in the present case is 800mm Stress induced due to wheel load becomes negligible
beyond this depth
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DCP test conducted on existing subgrade surface (Three test for each location)
Number of blows counted for 800mm penetration Soil resistance = Penetration (mm)/Blow Penetration for first blow is discounted since the
imprint area of cone tip for the first blow smaller than that of subsequent blows
Soaked Insitu DCP test : Conducted during Monsoon
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Soaked CBR test: Specimen prepared at insitu density Specimen prepared at different compaction level by
varying number of blows No. of blows selected= 15, 25, 35, 65
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Chainage(Km)
Insitu dry density
(kN/m3 )
Field moisture
content (%)
Optimum moisture
content (%)
Maximum dry density
(kN/m3 )
Compaction Level
4.25 19.20 2.4 9.5 19.70 97.46
5.25 18.80 3.2 9.5 19.70 95.43
6.25 17.55 3.4 9.6 19.55 89.76
7.25 16.95 3.8 9.6 19.50 86.92
8.25 18.50 2.9 9.6 19.50 94.87
9.25 18.00 3.1 9.4 19.75 91.13
10.25 16.85 3.9 9.5 19.55 86.18
11.25 16.80 3.9 9.5 19.60 85.71
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Chainage(Km)
Sand content(%) Liquid limit (%) Plasticity Index
4.25 70.0 16.5 NP
5.25 70.0 16.5 NP
6.25 68.5 17.0 NP
7.25 68.0 17.0 NP
8.25 66.5 17.2 NP
9.25 64.5 17.7 NP
10.25 65.0 17.5 NP
11.25 63.5 17.9 NP
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Chainage(Km)
Soaked Lab CBR at insitu
density (%)
Insitu CBR (%) based on
DCPT
Soaked insitu CBR (%) based on
DCPT
Insitu DCPI(mm/Blow)
4.25 10.94 14.40 7.20 14.70
5.25 10.59 13.50 6.75 15.55
6.25 8.95 12.10 5.87 17.15
7.25 8.50 10.80 5.40 19.00
8.25 9.94 13.24 6.68 15.83
9.25 10.12 12.95 6.41 16.20
10.25 7.61 10.24 5.09 19.90
11.25 7.61 10.20 5.07 19.98
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Variation of CBR under different condition is expressed by a term CBRI defined by
CBRI1=CBRLS/CBRDCPS
CBRI2=CBRDCP/CBRDCPS
Where CBRLS is the laboratory soaked CBR value at in situ densityCBRDCP is DCP based in-situ CBR value at field moisture content and in-situ density and CBRDPCS is the DCP based in situ CBR value under soaked condition.
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Compaction level :%age compaction in the field with respect to the maximum dry density.
Y=0.0007x+1.4646 Y= -0.0015x+2.1465
X= compaction levelY= CBRI
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The CBR value of uniform soils having similar characteristics can be determined quickly and with adequate accuracy using the DCPT results.
Once the correlation is established between CBRI and compaction level or in-situ density for tests conducted under different conditions, soaked CBR value in the field can be determined very quickly by conducting the in-situ DCPT for existing conditions and using the CBRI value for that particular condition.
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Similarly laboratory soaked CBR value can be evaluated after establishing a correlation between CBRI ( CBRLS/CBRDCPS ) and compaction level .
For construction of new embankments or strengthening of existing pavements, DCPT will be a very useful tool for evaluating the strength of sub grade in terms of CBR value.
It may helpful in enhancing highway construction quality control, ensuring long-term pavement performance, stabilityand achieving more uniform structural property.
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Thankyou ..........................