One - dimensional consolidation

Question 1

True or False
In soil consolidation, inter-particle forces cannot increase until the water within the soil escapes because water is incompressible.

Answer 1

True

Question 2

When coarse granular soil like sand is loaded, it shows a fully drained response, meaning the change in effective stress Δσ′ equals the change in total stress Δσ, and the change in pore water pressure Δu is _______.

Answer 2

Zero

Question 3

What happens in fine-grained soils under undrained conditions when loaded?
A) Immediate increase in effective stress
B) No change in effective stress initially
C) Decrease in total stress

Answer 3

B) No change in effective stress initially

Question 4

What is the purpose of an oedometer test in soil mechanics?

Answer 4

The oedometer test is used to obtain information on the consolidation settlement of soil (how much the soil compresses vertically) and the rate at which this consolidation occurs.

Question 5

True or False
During consolidation, an increase in pore pressure is referred to as excess u, which causes a transient flow towards the soil’s free-draining boundaries.

Answer 5

True

Question 6

In the oedometer test, the vertical displacement of the soil sample is measured using _______ gauges.

Answer 6

dial

Question 7

Why is it important that the area of loading in a consolidation test is large compared with the thickness of the soil layer?
A) To ensure uniform distribution of stress
B) To minimize edge effects and ensure one-dimensional consolidation
C) To increase the rate of consolidation

Answer 7

B) To minimize edge effects and ensure one-dimensional consolidation

Question 8

True or False
The oedometer test setup typically includes a container with a sample, a confining ring, and a base with a porous stone that is not submerged in water.

Answer 8

False (The base has a porous stone, and the container is filled with water after the first load to allow for drainage)

Question 9

In the context of consolidation, Δu represents the change in _______ pressure.

Answer 9

pore water

Question 10

How does the mechanism of consolidation differ between coarse granular and fine-grained soils?

Answer 10

In coarse granular soils, the response is fully drained, allowing immediate stress transfer to soil particles. In fine-grained soils, the response is initially undrained, preventing stress transfer until pore water pressures dissipate, which then increases effective stress over time.

Question 11

True or False
In an oedometer test, the initial pressure applied is typically equal to the in-situ effective stress.

Answer 11

True

Question 12

During an oedometer test, loads are applied on a daily basis, each load _______ the previous one.

Answer 12

doubling

Question 13

Why is the dissipation of excess pore water pressures assumed to be complete after 24 hours in an oedometer test?
A) Because the soil fully settles
B) To standardize test duration
C) To match typical daily load application intervals

Answer 13

C) To match typical daily load application intervals

Question 14

What does the plot from oedometer test results typically show?

Answer 14

he plot typically shows the void ratio (e) or change in height (ΔH) at the end of each load increment against the corresponding effective stress.

Question 15

True or False
Plastic clays show higher compression in oedometer tests compared to high porosity clays due to slower pore pressure dissipation.

Answer 15

True

Question 16

In the context of soil mechanics, the pre-consolidation pressure (
σ_c’) refers to the highest past effective stress that a clay has been subjected to, often due to events like _______.

Answer 16

Melting of ice sheets, erosion of upper layers, or rise in groundwater table

Question 17

What is the coefficient of volume compressibility (m_v)?
A) The amount of stress required to cause a unit strain
B) The volume change per unit volume per increase in effective stress
C) The rate of strain under constant stress

Answer 17

B) The volume change per unit volume per increase in effective stress

Question 18

The slope of the linear portion of the e-log σ_v′ line, known as the compression index, indicates the soil’s tendency to compress under load.

Answer 18

True

Question 19

What does the compression index tell us about a soil’s properties?

Answer 19

The compression index provides a measure of a soil’s compressibility under loading conditions; higher values indicate a soil is more compressible and vice versa, reflecting the soil’s stress history and inherent characteristics.

Question 20

The calculation of settlement in an oedometer test for a soil layer of thickness
H and an increase in effective stress Δσ_v’ is given by m_vΔσ_v’H = p_∞, where
p_∞ represents the _______.

Answer 20

long-term settlement

Question 21

What are typical values of m_v?

Answer 21

• 1 × 10^{-3} m^2/kN for soft clay
• 0.1 × 10^{-3} m^2/kN for stiff clay

Question 22

What is unloading due to?

Answer 22

• melting of ice sheets
• erosion of upper layers
• rise in ground water table (affecting u rather than σv)

Question 23

What is H?

Answer 23

H is the stratum thickness

Question 24

True or False
Plastic clay, due to its longer dissipation time for pore water pressures and greater magnitude of compression, exhibits higher compression than high porosity, low plasticity clay under the same loading conditions.

Answer 24

True