CONSOLIDATION ANALYSIS -stuff you should know to complete your lab assignment

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CONSOLIDATION ANALYSIS -stuff you should know to complete your lab assignment. CIE 366: Soil Mechanics Laboratory Originally by Mike St. Pierre, 2007 Modified by Will Manion, 2008. Coefficient of Consolidation (c v ). For every load increment 10 increments -- 10 plots

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### CONSOLIDATION ANALYSIS-stuff you should know to complete your lab assignment

CIE 366: Soil Mechanics Laboratory

Originally by Mike St. Pierre, 2007

Modified by Will Manion, 2008

Coefficient of Consolidation (cv)
• 10 increments -- 10 plots
• Holtz and Kovacs(1981) section 9.4, pg 395
• Use the Casagrande logarithm of time curve fitting method.
• Fig. 9.7 on page 398 shows it nicely.
Coefficient of Consolidation (cv)
• cv (cm2/sec)
• T50 = time factor at U = 50%
• Hdr50 = height of drainage at

U = 50%

• t50 = time at U = 50%
• U = percent of primary consolidation
• From Table 9-1 pg 390
• If Uavg = 50%, T50 = 0.197
Coefficient of Consolidation (cv)Casagrande log time method
• Construct a plot of dial reading vs. log of time
• Figure 9.7, pg 398
• To determine
• Ho = Initial height of specimen
• Rstart = 0 (if you zeroed the dial at the beginning of the test)
• R50 you determine from the plot
• May have to plot data points and complete construction on paper – see next slide
• determine Ro, when U = 0%
• determine R100, when U = 100%
• then average to find R50,
• which then graphically corresponds to t50

Ro

ΔR, the same distance

R1

ΔR

R2

R1 at t1

R2 at t2 (t2 = t1 x 4)

ΔR = R2 – R1

Ro

R100

Ro

R50 = (R100-Ro) / 2

R100

Void Ratio (e) versus Log Stress (σ)
• Determine initial height and final height for load increment
• Calculate the void ratio for each load increment
• Plot e versus log effective stress
• You will have to print the plot and perform the construction on papershow all work
• Holtz and Kovacs (1981) Figure 8.6, pg 296
On paper, determine values from theVoid Ratio (e) versus Log Stress (σ) plot
• Using methodology from Figure 8.6
• Determine the (most probable) preconsolidation pressure (σ’p)
• Steps from Holtz and Kovacs pg 296
• Choose point of maximum curvature (point A)
• Draw horizontal line from point A
• Draw tangent to curve at point A
• Bisect angle made in Steps 2 and 3
• Extend straight line portion of virgin compression curve through bisector line

Minimum Possible

Most Probable

eo

5

2

1

4

Maximum Possible

3

On paper, determine values from theVoid Ratio (e) versus Log Stress (σ) plot
• Determine compression index (Cc)
• Slope of virgin compression curve aka the steep part of curve
• Holtz and Kovacs Figure 8.9 pg 316 graphically
• Holtz and Kovacs Figure 8.7 pg 313 analytically
• Determine recompression index (Cr)