Effect Of Ocr And Confining Pressure On Soilproperties

Table of Contents
1. EFFECT OF OCR AND CONFINING PRESSURE ON SOIL PROPERTIES..........2
1.1Effect of OCR on Soil Strength Parameter ( c-Φ SOIL )......................................2
 Effect of OCR on the Elastic modulus of soil:..........................................................4
1.2Effect of confining Pressure on Stress-Strain Behaviour (Cohesionless Soil).............5
 Effect ofconfining pressure on the Elastic modulus of soil:........................................7
2.YOUNG'S MODULUS OF SOIL......................................................................................8
2.1Introduction.............................................................................................8
2.2Factors influencing Elastic Modulus ofsoil......................................................8
2.3Assessment of Elastic Modulus of soil............................................................8
2.3.1California Bearing Ratio (CBR)...............................................................9
2.3.2Standard Penetration Test (SPT)............................................................10
2.3.3Static Cone Penetration Test(SCPT).......................................................11
2.4Relation Between soil Elastic Modulus (Es) and Depth......................................13
3.REFERENCES.................................................................................................................15
REFERENCES

EFFECT OF OCR AND CONFINING PRESSURE ON SOILPROPERTIES
Following gives the effect of OCR on the strength parameters of the cohesivesoil and confining pressure on the non cohesive soil.
1.1 Effect of O CR on Soil Strength Parameter ( c –Φ SOIL )
OCR of soil is defined as the highest stress experienced by the soil divided by the current stress. Following are the chart showing the effect of OCR on the ratio of the maximum deviator stress to the confining stress, angle of internal friction and cohesion of the soil (fig. 1.1).Fig 1.1 Effect of OCR on Soil Strength Parameters.
Sivadass, T., Lee, C.Y. & Karim M.S.A, 2003, “ Behavior of a tropical residual soil”, ISBN 905809 604 1.

In the present study, all soil samples were tested after being subjected to different initial consolidation pressure. The initial consolidation pressures (σ 0’) applied to the soil sample were50kPa, 200kPa and 400kPa. It was found thatwith increase of over consolidation ratio increases, the maximum deviator Stress/σc’ ratio (σc’=confining pressure), angle of internal friction and cohesion value increases. This effect is verified by two separate experiments. Experiments were carried on the normally consolidated soil and Over consolidated soil. Please note that for Normally consolidated soil OCR value is 1 and for overconsolidated soil the OCR >1.

 fig 1.2 Failure envelope for (a) Normally consolidated and (b) over-consolidated clay from consolidated drained triaxial test. (Das B.M., “Advanced Soil Mechanics”)

The value of c is obtained by drawing a common tangent to Mohr’s circle which is Mohr-coulomb’s envelope. In figure 1.2(a), it is clear that in normally consolidated clay the cohesion is zero. But whenover consolidated clay as shown in  figure 1.2(b) the failure envelope makes an intercept with the vertical giving cohesion value. Thus as the OCR value increase, the increase in the cohesion value is observed.

Fig 1.3 Failure envelope of soil with a pre-consolidation pressure σ c’.
(Das B.M., “Advanced Soil Mechanics”)

The change in the angle of internal friction is verified in thefollowing experimental observation. Please refer figure 1.3. If a soil is initially consolidated by an encompassing chamber pressure of σc=σc’, and allowed to swell under reduced chamber pressure of σ3=σ3’, the specimen will beover-consolidated. The failure envelope obtained from the consolidated drained triaxial tests of this type of specimen has two distinct branches. Portion ‘ab’ of the envelope has...