Soil Mechanics: Consolidation Process

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Questions and Answers

What does the coefficient of (volume) compressibility (mv) primarily help determine?

The change in thickness of soil due to stress (correct)

The total volume of soil

The moisture content of soil samples

The amount of three-dimensional consolidation

Which parameter is NOT needed to calculate in-situ settlement amount (ΔH)?

Change in moisture content (correct)

In-situ increase in applied stress (Δσ)

Coefficient of compressibility (m~v~)

Thickness of the soil layer (H~i~)

What condition must be met for the coefficient of compressibility (mv) to provide useful information?

It must be combined with an increase in applied stress (correct)

It must be determined from field measurements only

It must be calculated using three different soil samples

It must be multiplied by the thickness of the soil

During laboratory tests, which device is used to measure the change in thickness of the soil sample?

Oedometer Signup and view all the answers

Why is the coefficient of compressibility (mv) insufficient on its own for predicting soil consolidation?

It only indicates compressibility without a stress context Signup and view all the answers

What is the process called that occurs when saturated soil reduces in volume over time?

Consolidation Signup and view all the answers

Which condition is necessary for consolidation to take place in saturated soils?

Water must be able to drain out of the soil Signup and view all the answers

During which phase of settlement does water seepage primarily lead to volume reduction?

Primary consolidation Signup and view all the answers

What happens to the soil particles during consolidation?

They rearrange into a closer packed state Signup and view all the answers

What is the result of unloading soil after consolidation?

Heave occurs, leading to volume increase but not fully recovered Signup and view all the answers

Which phase of settlement may involve long-term changes in soil structure?

Secondary settlement Signup and view all the answers

What is heave in relation to soil mechanics?

Increase in volume upon unloading Signup and view all the answers

What is the primary cause of immediate settlement in soil mechanics?

Elastic deformation of soil Signup and view all the answers

What primarily governs the rate of consolidation in soil?

Water movement out of the soil Signup and view all the answers

In which condition will soil consolidate relatively slowly?

Low permeability and undrained Signup and view all the answers

What happens to effective stress in soil when pore water pressure increases in drained conditions?

Effective stress increases Signup and view all the answers

What characterizes an over-consolidated clay?

Previously subjected to a higher consolidating stress Signup and view all the answers

What must occur for over-consolidated soil to consolidate as normally consolidated soil?

The current applied stress must be greater than the maximum historical stress Signup and view all the answers

What is the outcome of increased total stress on soil in undrained conditions?

Effective stress changes very slowly Signup and view all the answers

What role does permeability play in unconfined soils regarding consolidation?

It regulates the consolidation rate Signup and view all the answers

What is the relationship between effective stress and soil strength?

Increased effective stress results in increased strength Signup and view all the answers

Study Notes

Consolidation

Consolidation is the reduction in volume of a saturated soil over time due to stress increase.
Saturated soil contains only soil grains and water (no air).
Consolidation occurs as water drains from the soil.
In unsaturated soil, volume change can occur due to air expulsion (compaction).

Consolidation Stages

Immediate Settlement: A small shape change occurs immediately upon loading, but no volume change. This is recoverable.
Primary Consolidation: Significant volume reduction occurs as water seeps out, and particles rearrange for a tighter packing. This is the major volume change.
Secondary Consolidation: Slow long-term changes in soil structure (plastic deformation) occur. This is usually minor and often ignored in calculations.

Drainage & Consolidation

Consolidation requires water to move out of the soil.
Permeability affects the rate of consolidation.
High permeability soils consolidate quickly in drained conditions.
Low permeability soils can consolidate slowly or not at all in undrained conditions.

Loading and Stress

Loading a soil increases total stress (σ).
In drained conditions, increased pore water pressure (u) leads to water draining out and reducing total stress.
Effective stress (σ') is the stress carried by the soil particles, calculated by subtracting the pore water pressure from the total stress.
An increase in effective stress leads to an increase in soil strength.

Over-Consolidated Clays

Over-consolidated clay has been subjected to a higher stress than currently.
Pre-consolidation stress is the maximum previously applied stress.
Consolidation will only occur if current stress is higher than the pre-consolidation stress.

Prediction of Consolidation

Coefficient of volume compressibility (m_v) is used to determine the one-dimensional consolidation (vertical movement).
It's determined from laboratory tests (e.g., oedometer tests).
The change in thickness of a soil sample (ΔH) due to change in stress (Δσ) is calculated using the formula

mv = ΔH / (H; x Δσ)

Where:
- H₁ is the original thickness of the soil sample
- ΔH is the change in thickness of the sample due to stress increase
- Δσ is the increase in stress applied to the sample
- m_v is the coefficient of volume compressibility

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Description

This quiz covers the concept of consolidation in saturated soils, including its stages and drainage effects. Understand the differences between immediate, primary, and secondary consolidation as well as the role of permeability in affecting the rate of consolidation. Test your knowledge on key principles and terminology in soil mechanics.