water squeezed from sample drains freely into chamber Consolidation Oedometer Test loading piston “floats” disc-shaped CLAY SAMPLE water squeezed from sample drains freely into chamber

Load Schedule oedometer and sample are placed in a loading frame time intervals (minutes) double: (0.1, 0.25, 0.5, 1, 2, 4, 8, 16, 30, 60, 120, 240, 1440) over each load cycle loads (N) also double: (61.16, 122.32, 244.64, 489.28, 978.56, 1957.12, 3914.24, 7828.48) Then loads are halved every 24 hours, and a 24 hour strain dial reading recorded oedometer and sample are placed in a loading frame first load applied is 61.16 N from time load deployed, strain dial readings are taken for 24 hours

dial reading versus log time Why Double? There are two types of plots produced from the data: e versus log σ and dial reading versus log time when plotting data it helps to view the plot with equal intervals of x equal spacing on a logarithmic scale is acheived by doubling the interval

Other data: ring dimensions: diameter, (mm), height, H0 (mm) specimen area, A (mm2) water content, initial: w0, final: w1 final dry mass, Ms (g) of sample specific gravity of sample, Gs

Void Ratios Method 1: use final water content, w1 and Gs If Sr = 1 at end of test, e1 = w1Gs Method 1: use final water content, w1 and Gs Change e0-e1 toΔe and you’ve got: Remember this blast from the past? (7.1) Now rearrange to find Δe Since e0 = e1 + Δe Δ H = Initial Dial Reading – Final Dial Reading and H0 = Initial height of the specimen Now with ΔH, H0 and 1+e1, find Δe and e0 = e1 + Δe, find the constant ratio: Then multiply the Δ H for each load increment by this ratio to find the corresponding Δe The void ratio for each load increment is then e0 - Δe

Void Ratios Method 2: use final dry weight, Ms and Gs The height of the specimen at the end of any load increment is H1 = H0 –Initial Dial Reading +Dial Reading at end of load increment Method 2: use final dry weight, Ms and Gs The void ratio, e1 at the end of any load increment is: At end of test, dry mass of specimen = Ms Knowing A and Gs: (7.2)

Compressibility Characteristics The Compression Index, Cc is calculated as the slope between any two points on this linear portion of the Virgin Compression Line: The Expansion Index, Ce is the approximated slope of the expansion part of the e vs logσ’ curve: The Coefficient of Volume Compressibility, mv is defined as the volumetric strain per unit increase in effective stress. The units of mv are m2/MN (i.e., the inverse of pressure) mv can be expressed in terms of void ratio: (7.5) (7.3) or in terms of specimen thickness: (7.4) Their shapes reflect the stress history of the clay These are typical plots of void ratio, e versus effective stress, σ’ for a saturated clay The e vs logσ’ relationship for a normally consolidated clay is nearly linear as shown

Preconsolidation Pressure 5. Bisect tangent and horizontal line through D. 6. Vertical line through intersection of bisector and production of CB is the preconsolidation pressure, σ’c. 4. Draw horizontal line through D. 2. Determine the point, D of maximum curvature on the recompression portion of the curve, AB 3. Draw tangent to curve at D. 1. Produce the straight-line portion of the curve, BC Professor Arthur Casagrande taught Soil Mechanics and Foundation Engineering at Harvard University and developed this empirical method to determine the preconsolidation pressure, σ’c using the e-logσ’ curve.

In-situ e-logσ’ Curve For overconsolidated clays the in-situ condition is estimated by the point G: ( σ’0 , e0) σ’0 is the present effective overburden pressure The in-situ compression line can be define by point E at: ( σ’c , e0) The two compression lines are expected to intersect at a void ratio of 0.42e0 The slope of the in-situ compression line will be slightly greater than that of the virgin compression line produced from testing a disturbed field sample in the lab. The initial void ratio, e0 at the start of the lab test approximates that of the in-situ void ratio