Basics of Soil Mechanics- Oedometer Test

Question 1

What does the Oedometer test provide information about and define what this means?

Answer 1

Soil compressibility

• The capacity of the soil to deform and change its volume (without breaking) under the effect of mechanical loads

Question 2

What are the 4 key objectives when performing the Oedometer test?

Answer 2

1. Overconsolidation ratio OCR
2. Coefficient of consolidation C_v
3. Compression index C_c and swelling index C_s
4. Preconsolidation stress σ’_vp

Question 3

What is step 1 in the Oedometer test?

Answer 3

• The soil sample is placed inside a stiff metallic oedometric ring and sandwiched between two porous stones.
• A top cap is then placed on top of the upper porous stone and the whole system is fixed to a supporting frame.

Question 4

What is the diameter and height of the oedometric ring?

Answer 4

Diameter = 50mm

Height = 20mm

Question 5

What does the top cap do?

Answer 5

Centers the applied vertical load

Question 6

What is step 2 in the Oedometer test?

Answer 6

• Loading is applied in steps with a geometric progression with a ratio of 2, i.e. 20 kPa, 40 kPa, 80 and so on up to arbitrary maximum stress.
• During loading vertical displacement of the top cap are recorded (i.e. settlements)

Question 7

How long does each loading step last and why?

Answer 7

24 hours

• Each loading step is maintained constant until primary consolidation is completed.
• This means that the pore water overpressure generated by the loading are fully dissipated and the total stress applied on the soil sample is converted into effective stress.

Question 8

What is step 3 of the Oedometer test?

Answer 8

After loading, the soil sample is unloaded with a number of unloading steps which are half of the loading steps.

Question 9

What is the minimum testing time for the Oedometer test?

Answer 9

10 days

Question 10

Sketch a graph for the results of the Oedeomter test?

Answer 10

Question 11

What is the formula for void ratio?

Answer 11

e = V_v/V_s

V_v= Volume of voids

V_s= Volume of solid

Question 12

What is the formula for effective vertical stress?

Answer 12

σ’_{v = F/A}

_{F = Vertical force applied}

_{A = Cross-sectional area of sample}

Question 13

Why is there a sudden change in the loading part of the curve?

Answer 13

Soils keep the memory of the maximum stress that they have sustained in their tensional history. This maximum stress is named pre-consolidation stress σ_(v,p)^′ and it can be estimated from an oedometric test

Question 14

What is the maximum stress also called?

Answer 14

This maximum stress is named pre-consolidation stress σ’_v,p and it can be estimated from an oedometric test.

Question 15

How do we estimate the pre-consolidation stress?

Answer 15

The pre-consolidation stress is estimated with a graphic construction on the oedometric curve. Draw two tangent lines on the loading part of the curve and where they intersect is the pre-consolidation stress.

Question 16

Define the overconolidation ratio (OCR)?

Answer 16

The OCR is the ratio between the pre-consolidation stress σ’_v,p and current effective stress σ’_v,0.

Question 17

What is the OCR when the soil is normally consolidated?

Answer 17

1

Question 18

What is the OCR when the soil is overconsolidated?

Answer 18

>1

Question 19

How can we calculate the compression index C_c?

Answer 19

The compression index Cc is the slope of the virgin line.

Question 20

How can we calculate the Swelling index C_s?

Answer 20

The swelling index Cs is the slope of a swelling line.

Question 21

How do we calculate the slop of the virgin line?

Answer 21

Question 22

How do we calculate the slope of the swelling line?

Answer 22

Question 23

How many times higher is the compression index than the swelling index?

Answer 23

10 times higher

Question 24

For an OCR of 1 what is the compressibility equal to?

Answer 24

A normally consolidated soil (OCR=1) has compressibility equal to the compression index Cc for any given increase of vertical effective stress.

Question 25

For an OCR >1 what is the compressibility equal to?

Answer 25

• An overconsolidated soil (OCR>1) has compressibility equal to the swelling index Cs as long as the applied effective vertical stress is lower than the pre-consolidation stress σ’_v,p.
• If the vertical effective stress grows above the pre-consolidation stress, then the compressibility becomes equal to the compression index Cc

Question 26

What is the formula to calculate the coefficient of consolidation C_v?

Answer 26

Where:

• E_Œd = Oedometric Young elastic modulus
• k =
• γ_w = Unit weight of water

Question 27

What is the formula to calculate the time factor?

What is the difference when we have a permeable surface at the top and an impermeable surface at the bottom compared to having a permeable top surface and a permeable bottom surface?

Answer 27

Permeable, permeable = t ~ L/4 as H=L/2

Permeable, impermeable = t ~ L² as H=L