Lecture 7 - 21 OC Magnitude of Consolidation, C:...

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ρ 21 OC Magnitude of Consolidation, ρ C : Over-consolidated (OC) clay (saturated) for OC Clay σ ' 0 < σ C (or p C ) i.e. the present effective overburden pressure, σ ' 0 , is less than the maximum past pressure the soil has experienced, σ C (or p C ) = preconsolidation pressure Over-consolidation Ratio = OCR = σ ' C / σ 0 History of OC clay : (classic definition) Typical Causes of OC condition: Glaciation Erosion Higher water table (decreases σ 0 ), Temporary lower water table (past σ 0 > present σ 0 ), When an OC soil is loaded, loading occurs back along the unload curve, not the virgin curve, until reaching the preconsolidation pressure. Therefore, an over-consolidated soil is generally a good foundation material since we can add a stress Δσ V = ( σ C - σ 0 ) before getting on to the steeper virgin curve (i.e. smaller change in void ratio). When building on OC soils there are two possible situations depending on the relative magnitudes of the new effective stress and the preconsolidation pressure. If the new effective stress is less than the preconsolidation then loading occurs only along the reload curve. If the new effective stress exceeds the preconsolidation then loading will occur partly along the reload curve and partly along the virgin curve. s Element of soil deposited at P s Pressure increases with continued deposition s Water is squeezed out s Void ratio decreases (e=V v /V s ) s Overburden is removed s External Load Applied P Removed Nature deposits soil (approx 10 6 yrs process) Slope C C (= Compression Index) e 0 σ 0 σ C Void Ratio, e Log Vertical Effective Stress, σ V Final Insitu State Unloading begins
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ρ 22 Highly Over-consolidated (HOC): Since σ ' F does not exceed σ ' C : ( ) + - = - = - - = = 0 V 0 R 0 F R 0 F R 0 F σ ' Δσ σ ' log C σ ' σ ' log C σ ' log σ ' log C ) e - (e Δ e And as before: C 0 ρ e 1 H Δ e Δ H = + - = Therefore: + + = 0 V 0 0 R C σ ' Δσ σ ' log e 1 H C ρ (HOC) C R is typically 10-20% of C C ( σ ' 0 + Δσ V ) σ C Reload with Slope C R (= Recompression index) Virgin Curve with Slope C C (= Compression index) e 0 e F σ 0 Δ e Void Ratio, e Log Vertical Effective Stress, σ V Initial Insitu State Δσ V σ C σ F
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ρ 23 Lightly Over-consolidated (LOC): Consolidation occurs in two steps: 1) Δ e 1 along the reload curve to σ ' C Δ H 1 2) Δ e 2 along the virgin curve to σ ' F Δ H 2 step 1: ( ) - = - = - - = 0 C R 0 C R 0 C R 1 σ ' σ ' log C σ ' σ ' log C σ ' log σ ' log C Δ e + = = + - = 0 C 0 R C1 0 1 1 σ ' σ ' log e 1 H C ρ e 1 H Δ e Δ H step 2: ( ) Δ + - = -
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Lecture 7 - 21 OC Magnitude of Consolidation, C:...

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