2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

A Laboratory Investigation into the Effect of Water on the CBR of a local Subgrade soil

Kofi Ampadu

Kwame Nkrumah University of Science and Technology –Kumasi GHANA

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Typical Road Pavement condition

UNSATURATED SUB-GRADE

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

The CBR value

The California Bearing Ratio (CBR) is most common parameter used to describe the subgrade strength

The CBR is the basis of the most popular pavement design method “The CBR Pavement Design Method ”CBR depends on

Water ContentDry DensityTexture

Design CBR-value CBR at expected Dry Density + Stable water contentCBR at expected Dry Density + 4-day Soaked

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Definition of CBR

Load

, kN

max

21 10019.93

P,10013.35

PCBR ⎥⎦⎤

⎢⎣⎡

⎟⎠⎞

⎜⎝⎛ ×⎟

⎠⎞

⎜⎝⎛ ×=

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Test Concept and Methodology

For given Dry Density, How does the CBR change as water content changes?

DR

Y D

EN

SIT

Y

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Test Material Characteristics

1.8016.82.6524494914289

MDD (Mg/m3)

OMC (%)GsPILLClay (%)

Silt (%)

Sand (%)

Gravel (%)

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Initial Test Condition

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Test Results- CBR Water Content Relationship

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

CBR-Water Content

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Water Characteristics

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

CBR-Suction Model

n

e

wasu u

uuCBRCBR ⎟⎟⎠

⎞⎜⎜⎝

⎛ −×=

ue

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

The empirical modeln

e

wa

s

u

uuu

CBRCBR

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

n

s

OMC

4202200

CBRCBR

⎟⎠⎞

⎜⎝⎛=

2.1CBR

CBRs

OMC =

11CBR

CBRs

OMC =

• The empirical model

• For the OMC condition, ua-uw= 2000-2500 kN/m2

• For γdry=1.65-1.71, n=0.45

• For γdry=1.36, n=1.4

2ND INTERNATIONAL CONFERENCE ON MECHANICS OF UNSATURATED SOILS

Conclusion

Drying subgrade lateritic soil from the OMC condition leads to a large increase in CBR

Soaking from the OMC condition leads to a relative reduction in CBR of between 46% and 98%

The change in CBR from drying is 3-7 times larger than the change from soaking.

The CBR and the matric suction appears to be related through an empirical linear log-log model.

Based on laboratory test results of subgrade lateritic soil at water contents between 9.6% and 28.9% and dry densities of 1.36, 1.65and 1.71 Mg/m3 the following conclusions are made from this study: