1. Which soil properties are commonly measured during a consolidation test?
Compression Index, Cc
Swelling Index, Cs
Preconsolidation Pressure, pσ
Coefficient of Consolidation, Cv
2.To what kinds of soils is the consolidation test applicable?
Fine-grained soils
3. Which strain and stress components vary during a consolidation test?
Strain = Void ratio (e)
Stress = Vertical stress (vσ)
4. What is the relation between volumetric strain and void ratio? Derive this relation.
Volumetric strain =
5. Which is the most common system of coordinates to report the compressibility of soils?
(Logarithm of Effective Vertical Stress, Void Ratio)
6. What are the preconsolidation pressure and overconsolidation ratio?
Preconsolidation pressure (pσ) is the maximum effective past pressure that a soil has experienced.
Overconsolidation ratio (OCR) is the ratio of the preconsolidation pressure to the presenteffective stress OR pOCR=σσ.
7. Indicate: i) Recompression Line; ii) Virgin Compression Line; and iii) Swelling Line in the e-logσplot.This plot shows the answers.
8.From the figure below, describe how to find the preconsolidation pressure (pσ).
1. Define point a at which the e-logσplot has a minimum of radius of curvature.
2. Draw a horizontal line ab.
3. Draw the line actangent at a.
4. Draw the line ad, which is the bisector of the angle bac.
5. Project the straight-line portion ghto intersect adat f. The abscissa of fis pσ.
9. What are normally consolidated and overconsolidated clays?
Normally consolidated clays are the clays that have never been subjected to an effective stress greater than the present effective stress (OCR = 1).
Overconsolidated clays are the clays that have been subjected to an effective stress greater than the present effective stress (OCR > 1).
10. Are there approximate relations to calculate the compression index in terms of the liquid limit?cC =0.009 LL-10
11. What is the major application of Cc, Csand pσdetermined from a consolidation test to the geotechnical engineering problems?
Cc, Cs and pσ are used in the determination of final settlement caused by an increment of effective vertical stress in a clay layer until the end of consolidation process.
12. How to calculate the dimensionless time factor (vT) for a given time t?
vv2drCtT=H
where: vC is coefficient of consolidation; and drH is drainage length.
13. What is the major application of Cv determined from a consolidation test to the geotechnical engineering problems?
Cv is used to determine the time factor Tv at the time t for a clay layer with the thickness H. Then the average degree of consolidation (U) is determined. With the known final settlement until the end of consolidation process and U, one can determine the settlement that occurs by consolidation process at the time t. In other words, Cv is used to determine the time-dependent settlement during consolidation process.
14. What is the basic relation between Tvand U,for U < 60 %?
2vπ U%T=4 100
15. What methods are commonly used to determine the coefficient of consolidation (Cv)?
1. Logarithm-of-Time method OR t50-method; and
2. Square-Root-of-Time method OR t90-method.
16. What are t50and t90?
For a given incremental loading, t50 and t90 are the times since the start of consolidation until the average degree of consolidation of test specimen (U) reaches 50% and 90 %, respectively.
17. From time-settlement curve below, describe how to determine t50?
1. Extend the straight-line portions of primary and secondary consolidations to intersect at A. The ordinate of Ais d100.
2. Select times t1and t2such that t2= 4t1. Let the difference in the deformation from t1to t2equal to x.
3. Draw a horizontal line DE such that the vertical distance BD is also equal to x. The ordinate of line DE is d0.
4. Locate F on the time-settlement curve such that the vertical distance between F and line DE is equal to the vertical distance between F and A. The ordinate of F is therefore d50 and the corresponding abscissa is t50.
18. From time-settlement curve below, describe how to determine t90?
1. Draw a line AB through the early portion of the curve.
2. Draw a line AC such that 1.15OCOB. Line AC intersects the curve at D. The abscissa of D gives the square root of time for 90 % consolidation, 90t.
19. How to determine the initial void ratio (e0) and the current void ratio (e) for a consolidation test?
1. Measure the dry mass of total specimen, Ms.
2. Determine the solid volume from the known specific gravity as: ssswMV=Gρ
3. Determine equivalent height of solids as:ss H = V A where: A = specimen area
4. Initial void ratio is obtained as:0s0sH -He=H where: H0= initial height of specimen
5. Current void ratio is obtained as:ssH-He=Hwhere: H = current height of specimen
20. How to calculate the coefficient of consolidation (Cv) from t50 and t90?
From t50:2drv500.197HC=tFrom t90:2drv900.848HC=t
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