Estimation of CBR value using Dynamic Cone Penetrometer

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

IntroductionExperimental WorkField and Laboratory TestTest Procedure and sample preparationResult and DiscussionConclusionsReferences

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

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

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

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

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

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

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

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

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

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

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

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.

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

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.

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.

Thankyou ..........................