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?

<p>Oedometer (C)</p> Signup and view all the answers

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

<p>It only indicates compressibility without a stress context (A)</p> Signup and view all the answers

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

<p>Consolidation (B)</p> Signup and view all the answers

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

<p>Water must be able to drain out of the soil (D)</p> Signup and view all the answers

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

<p>Primary consolidation (D)</p> Signup and view all the answers

What happens to the soil particles during consolidation?

<p>They rearrange into a closer packed state (C)</p> Signup and view all the answers

What is the result of unloading soil after consolidation?

<p>Heave occurs, leading to volume increase but not fully recovered (B)</p> Signup and view all the answers

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

<p>Secondary settlement (B)</p> Signup and view all the answers

What is heave in relation to soil mechanics?

<p>Increase in volume upon unloading (A)</p> Signup and view all the answers

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

<p>Elastic deformation of soil (C)</p> Signup and view all the answers

What primarily governs the rate of consolidation in soil?

<p>Water movement out of the soil (A)</p> Signup and view all the answers

In which condition will soil consolidate relatively slowly?

<p>Low permeability and undrained (A)</p> Signup and view all the answers

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

<p>Effective stress increases (A)</p> Signup and view all the answers

What characterizes an over-consolidated clay?

<p>Previously subjected to a higher consolidating stress (C)</p> Signup and view all the answers

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

<p>The current applied stress must be greater than the maximum historical stress (C)</p> Signup and view all the answers

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

<p>Effective stress changes very slowly (C)</p> Signup and view all the answers

What role does permeability play in unconfined soils regarding consolidation?

<p>It regulates the consolidation rate (B)</p> Signup and view all the answers

What is the relationship between effective stress and soil strength?

<p>Increased effective stress results in increased strength (B)</p> Signup and view all the answers

Flashcards

Consolidation

The process of reducing volume in saturated soil due to water draining out over time. Soil particles get closer and the soil becomes stronger and stiffer.

Saturated Soil

A type of soil containing only soil particles and water, with no air present.

Heave

The increase in volume of a soil after unloading due to the recovery of elastic strain, but not the full recovery of particle re-arrangement.

Immediate Settlement

The immediate, small settlement of a soil upon loading due to shear strain, but not a change in volume.

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Consolidation Settlement

The significant reduction in volume of soil as water drains out and particles rearrange under load, a major part of consolidation.

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Secondary Settlement

A slow, long-term change in soil structure, leading to further settlement after primary consolidation.

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Compaction

The process of reducing volume in unsaturated soil by expelling air under load, rather than water.

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Drained Condition

The state of a soil where the pores are filled with water, allowing for consolidation to occur.

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Consolidation Rate

The rate at which water leaves the soil. Higher permeability equals faster consolidation.

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Drained Soil

A soil in which water can easily move out.

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Undrained Soil

A soil in which water cannot easily move out.

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Total Stress

The total force pressing down on the soil.

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Pore Water Pressure

The pressure of the water within the soil pores.

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Effective Stress

The force that soil particles exert on each other. It increases as the soil becomes more compacted.

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Overconsolidated Clay

A clay that has been compressed by a higher force than it currently experiences.

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Normally Consolidated Clay

A clay that has not been subjected to a significant force during its history.

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Coefficient of Compressibility (mv)

A measure of how much a soil's volume changes in response to a change in applied stress, determined through laboratory tests.

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ΔH (Change in thickness)

The change in thickness of a soil sample due to an increase in applied stress.

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Δσ (Change in stress)

The increase in applied stress on a soil layer, usually due to the weight of a building or construction.

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Hi (Initial thickness)

The in-situ thickness of the soil layer before any applied stress is added.

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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 (mv) 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
    • mv is the coefficient of volume compressibility

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Related Documents

Consolidation 1 - Notes - Nottingham Trent University PDF

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.

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