1. Experiment # 6
Consolidation
ASTM D 1883
Soil Mechanics Lab CE 350

2. Consolidation
It is the process of gradual dissipation of excess pore water pressure under static load which is accompanied by gradual decrease in volume of voids (increase in settlement).
Soil Mechanics Lab CE 350

3. Settlement Immediate Settlement Consolidation Settlement settlement
time
settlement
time
settlement
Soil Mechanics Lab CE 350

4. The Consolidation Process
@t=o
σ=P/A
Total stress
u=P/A
Pore water pressure
σ’=0
Strength of the spring
@t=t
σ=P/A
u=P/A-Δσ
σ’= Δσ
water
spring
@t=∞
σ=P/A u= σ’=σ
Soil Mechanics Lab CE 350

5. The Parameters Compression Index, Cc
used to determine the amount of consolidation.
Coefficient of Consolidation, Cv
used to determine the rate of consolidation.
Soil Mechanics Lab CE 350

6. During consolidation…
 remains the same (=q) during consolidation.
u decreases (due to drainage).
’ increases which is the effective stress carried by the soil particles,
transferring the load from water to the soil.
Due to a surcharge q applied at the GL, the stresses and pore pressures are increased at A.
q kPa GL
saturated clay
q kPa A GL
saturated clay σ u σ’
σ=q
u= σ -Δσ
σ’= Δσ A
Soil Mechanics Lab CE 350

7. Consolidation Test field lab 9P P = 9P/A ≈ 20kpa For the first day
dial gauge 9P GL
Plate load
porous stone
undisturbed soil specimen
Dia = 75 mm
Height = 20 mm
metal ring
(oedometer) P
= 9P/A ≈ 20kpa
For the first day
Soil Mechanics Lab CE 350

8. Date, pressure Time Elapsed time, min. Dial reading, div. Temp. C
Remarks:
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9. Soil Mechanics Lab CE 350

10. Calculate Cv50 Tv = time factor = 0.197 for 50% consolidation
Hd =average length of largest drainage path during the given loading increment.
= H for one way drainage.
= H/ for 2-way drainage.
H = H0 – d50*C
t50 = time elapsed corresponding to d50
Soil Mechanics Lab CE 350

11. Soil Mechanics Lab CE 350

12. Calculate Cv90 Tv = time factor = 0.848 for 90% consolidation
Hd =average length of largest drainage path during the given loading increment.
= H for one way drainage.
= H/ for 2-way drainage.
H = H0 – d90*C
t90 = time elapsed corresponding to d90
Soil Mechanics Lab CE 350

13. To determine Cc ΔH Water Vv Water Vv Solid Vs Solid Vs
Soil Mechanics Lab CE 350

14. Change in thickness of specimen, H (mm)
Pressure, p (kpa)
Initial deformation dial reading at beginning of first loading, d0 (div.)
Deformation dial reading representing 100% primary consolidation, d100 (div.)
Change in thickness of specimen, H (mm)
Change in void ratio e
Void ratio, e
[e = e0  e]
(1)
(2)
(3)
(4)=[(3)(2)] 0.002
(5)=(4)/Hs
(6) = e0 –(5) 20 40 80
160
320
R320
R160
R80
R40
Loading
Unloading
Soil Mechanics Lab CE 350

15. Void Ratio Vs. Log PressureCc Cs
Soil Mechanics Lab CE 350

16. Applications Settlement under foundations Soil Stabilization
Dealing with ground water control
Soil Mechanics Lab CE 350