Undrained Shear Strength, Liquidity Index, and Sensitivity
Descdription:
Let us build a relationship between liquidity index and undrained shear strength. The undrained shear strength of a soil at a water content w, with reference to its undrained shear strength at the plastic limit, is obtained from Equation
Putting Gs 5 2.7 and l 5 0.59 PI in the above equation, and recalling that
Clays laid down in saltwater environments and having fl occulated structure often have in situ (natural) water contents higher than their liquid limit but do not behave like a viscous liquid in their natural state. The fl occulated structure becomes unstable when fresh water leaches out the salt. The undistributed or intact undrained shear strengths of these clays are signifi cantly greater than their disturbed or remolded undrained shear strengths. The term sensitivity, St, is used to defi ne the ratio of the intact undrained shear strength to the remolded undrained shear strength:
where i denotes intact and r denotes remolded. By substituting Equationinto we obtain
For values of St . 8, the clay is called a quick clay. Quick clay, when disturbed, can flow like a viscous liquid (LI . 1). Bjerrum (1954) reported test data on quick clays in Scandinavia, which yield an empirical relationship between St and LI as
Summary of Relationships Among Some Soil Parameters from CSM:
Table A.14 (Appendix A) provides a summary of the relationships among some soil parameters from CSM. The approximate expressions for practical use were obtained by a curve fi tting the CSM expressions. The approximate expressions should be used to get a quick estimate during preliminary design. They are accurate to about 10% or less.