Soil Mechanics: Shear Strength of Soil

Questions and Answers

The ability of soil deposit to support an imposed loading or of a soil mass to support itself is governed by ______ strength of the soil.

shear

Shear strength refers to the resistance along a plane that passes between or along a long particle ______.

surfaces

The problems related to shear strength include the ______ capacity of soil for foundation design.

bearing

Shear strength and related ______ of a foundation is conventionally studied in laboratory by testing samples.

deflection Signup and view all the answers

When the stress applied on soil exceeded the shear strength of soil, the ______ of soil particles upon each other causes change in volume.

sliding Signup and view all the answers

Part of soil may move or get displaced with respect to ______ media when its shear strength is exceeded.

surrounded Signup and view all the answers

The shear strength developed either due to compression or ______ forces.

tension Signup and view all the answers

What is the optimal amount of water for soil compaction?

A little bit of water Signup and view all the answers

What is the primary goal of soil compaction?

To reduce air-void volume in soils as much as possible Signup and view all the answers

What happens when too much water is added to soil during compaction?

Compaction is inhibited Signup and view all the answers

What is the primary purpose of the Standard Proctor Test?

To determine the maximum dry unit weight of the soil Signup and view all the answers

Why is it important to control soil water content during compaction?

To achieve the optimal compaction effort Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The compaction energy applied to the soil Signup and view all the answers

What is the relationship between compaction energy and soil water content?

Compaction energy increases with increasing soil water content Signup and view all the answers

How is the compaction effort calculated in the Standard Proctor Test?

By using the formula: compaction effort = (number of drops x weight of hammer) / volume of soil Signup and view all the answers

What is the ideal outcome of optimal soil compaction?

Minimum air-void volume Signup and view all the answers

What is the range of optimum water content for sandy silt (sub-base material)?

8-12 Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

lb-ft/ft³ Signup and view all the answers

What is the number of times the hammer is dropped onto the confined soil for each lift?

What is the purpose of dividing the soil mold into three lifts?

To facilitate testing of different soil layers Signup and view all the answers

What is the name of the test developed during World War II by the U.S.?

Modified Proctor Test Signup and view all the answers

How many times is the compaction process repeated from dry to wet?

What is the total number of blows applied to each layer?

25 blows Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Soil compaction energy Signup and view all the answers

What is the purpose of the standard Proctor test?

To find the optimal water content for soil compaction Signup and view all the answers

What is the relationship between the void ratio and the degree of saturation?

e = w / (Gs * S) Signup and view all the answers

What is the Zero-Air-Void (ZAV) curve used for?

To represent the fully saturated condition of soil Signup and view all the answers

What is the purpose of finding the Optimum Moisture Content (OMC) in soil compaction?

To optimize the water content for maximum dry density and stability Signup and view all the answers

What is the unit of the compaction energy in the Modified Proctor test?

Foot-pounds per cubic foot Signup and view all the answers

What is the formula for calculating the dry unit weight of soil (γd) during compaction?

γd = γw / (1 + w / c) Signup and view all the answers

What is the primary objective of increasing soil density through compaction?

Increase density, strength, and bearing capacity Signup and view all the answers

What is the process of increasing soil unit weight by reducing soil void ratio?

Compaction Signup and view all the answers

What is the purpose of laboratory work in soil compaction?

To obtain maximum dry density and optimum water content Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The weight of the hammer Signup and view all the answers

What is the relationship between compaction energy and soil water content?

Compaction energy increases with increase in soil water content Signup and view all the answers

How is the compaction effort calculated in the Standard Proctor Test?

By counting the number of blows per lift Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

Foot-pounds per cubic foot Signup and view all the answers

Why is it important to control soil water content during compaction?

To achieve optimal density and strength Signup and view all the answers

What is the ideal outcome of optimal soil compaction?

Maximum dry density and strength Signup and view all the answers

What is the primary purpose of the Standard Proctor Test?

To determine the maximum dry density of soil Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The weight of the hammer Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

lb-ft/ft3 Signup and view all the answers

What is the range of optimum water content for sandy silt (sub-base material)?

8-12% Signup and view all the answers

What is the purpose of controlling soil water content during compaction?

To achieve the maximum dry density of soil Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What happens to the water films around soil particles as the water content increases?

They start to replace soil particles in the mold Signup and view all the answers

What is the purpose of the Optimum Moisture Content (OMC) in soil compaction?

To achieve maximum dry density Signup and view all the answers

What is the relationship between compaction energy and soil water content?

Increased water content requires less compaction energy Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The compaction energy applied Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases linearly Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

Foot-pounds per cubic foot Signup and view all the answers

Why is it important to control soil water content during compaction?

To achieve optimal dry density Signup and view all the answers

What is the primary purpose of the Standard Proctor Test?

To determine the optimum water content for soil compaction Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The Modified Proctor Test uses a heavier hammer Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the relationship between compaction energy and soil water content?

Compaction energy increases with increasing water content Signup and view all the answers

What is the purpose of finding the Optimum Moisture Content (OMC) in soil compaction?

To achieve the maximum compaction of soil Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

ft-lbs/ft³ Signup and view all the answers

What is the primary objective of increasing soil density through compaction?

To increase the stability of soil structures Signup and view all the answers

What is the purpose of drawing the Line of Optimum in soil compaction?

To identify the optimum water content for soil compaction Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The Modified Proctor Test uses a heavier hammer Signup and view all the answers

What happens to the dry density of soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What is the relationship between compaction energy and soil water content?

Compaction energy increases with decreasing water content Signup and view all the answers

What is the primary objective of increasing soil density through compaction?

To decrease the soil's void ratio Signup and view all the answers

What is the purpose of laboratory work in soil compaction?

To determine the soil's optimum water content Signup and view all the answers

What is the primary goal of determining the Optimum Moisture Content (OMC) in soil compaction?

To achieve the maximum dry unit weight Signup and view all the answers

What is the relationship between the void ratio and the degree of saturation?

They are inversely proportional Signup and view all the answers

What is the purpose of the Zero-Air-Void (ZAV) curve?

To represent the fully saturated condition of the soil Signup and view all the answers

What is the formula for calculating the dry unit weight of soil (γd) during compaction?

γd = γw / (1 + w/c) Signup and view all the answers

What is the primary objective of increasing soil density through compaction?

To increase the bearing capacity of the soil Signup and view all the answers

What is the purpose of laboratory work in soil compaction?

To simulate field conditions in the laboratory Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The amount of compaction energy applied Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

foot-pounds per cubic foot Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

lb.ft/ft3 Signup and view all the answers

What is the primary difference between the Standard Proctor Test and the Modified Proctor Test?

The number of blows applied to each layer Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the purpose of the Standard Proctor Test?

To determine the optimum water content for soil compaction Signup and view all the answers

What is the range of optimum water content for sandy silt (sub-base material)?

8-12% Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What is the purpose of controlling soil water content during compaction?

To achieve the maximum dry density of soil Signup and view all the answers

What happens to the dry density of the soil when the water content is at the optimum?

It becomes maximum Signup and view all the answers

What is the purpose of the Standard Proctor Test?

To determine the optimum water content for soil compaction Signup and view all the answers

What is the primary purpose of the Standard Proctor Test?

To determine the optimal water content for compaction Signup and view all the answers

What is the effect of increasing compaction energy on soil density?

It increases soil density Signup and view all the answers

What happens to the dry density of soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

Compactive effort Signup and view all the answers

What is the purpose of controlling soil water content during compaction?

To achieve the optimal density Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The compaction energy applied Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

ft-lbs Signup and view all the answers

What is the purpose of controlling soil water content during compaction?

To achieve maximum density Signup and view all the answers

What happens to the water films around soil particles as the water content increases?

They become thicker Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

ft-lbs/in³ Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What is the range of optimum water content for sandy silt (sub-base material)?

8-12% Signup and view all the answers

What is the unit of measurement for the compactive effort in the Modified Proctor Test?

lb-ft/ft³ Signup and view all the answers

What is the purpose of drawing the Line of Optimum?

To determine the optimum water content for compaction Signup and view all the answers

What happens to the dry density of the soil when the water content is below the optimum water content?

It increases rapidly Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The compactive effort applied Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What is the purpose of controlling soil water content during compaction?

To achieve the maximum dry density of the soil Signup and view all the answers

What is the effect of entrapped air on the compaction curve?

It shifts the curve to the right Signup and view all the answers

What is the purpose of finding the Optimum Moisture Content (OMC) in soil compaction?

To achieve the desired density of the soil with minimum compaction effort Signup and view all the answers

What is the relation between the void ratio and the degree of saturation?

The void ratio decreases with an increase in the degree of saturation Signup and view all the answers

What is the purpose of the Standard Proctor Test?

To determine the optimal water content for soil compaction Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the unit of measurement for the compaction effort in the Standard Proctor Test?

Foot-pounds per cubic foot Signup and view all the answers

What is the difference between the Standard Proctor Test and the Modified Proctor Test?

The Modified Proctor Test uses a higher compaction energy Signup and view all the answers

What is the purpose of controlling soil water content during compaction?

To achieve the desired density of the soil with minimum compaction effort Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What is the Zero-Air-Void (ZAV) curve used for?

To represent the fully saturated condition of the soil Signup and view all the answers

What is the degree of compaction that can be achieved when all air voids have been removed?

S=1 Signup and view all the answers

What is the purpose of controlling the water content during compaction?

To increase the dry density of the soil Signup and view all the answers

What is the term used to describe the energy applied to the soil to achieve the desired density?

Compaction energy Signup and view all the answers

What is the purpose of the Standard Proctor equipment?

To compact the soil to the desired density Signup and view all the answers

How is the compactive effort calculated in the Standard Proctor test?

By measuring the number of blows applied to the soil Signup and view all the answers

What happens to the dry density of the soil when the water content is above the optimum water content?

It decreases Signup and view all the answers

What is the unit of measurement for the compactive effort in the Standard Proctor test?

kJ/m^3 Signup and view all the answers

What is the ideal outcome of optimal soil compaction?

Maximum dry density Signup and view all the answers

What is the purpose of finding the Optimum Moisture Content (OMC) in soil compaction?

To achieve the maximum dry density Signup and view all the answers

What happens to the entrapped air in the soil as the water content increases?

It increases Signup and view all the answers

Study Notes

Shear Strength of Soil

Shear strength of soil determines its ability to support imposed loading or its own weight.

Definition of Shear Strength

Shear strength refers to the resistance along a plane that passes between or along long particle surfaces, but not through the particles.

Importance of Shear Strength

It is a crucial characteristic of soil that affects several aspects of foundation design and construction.
Shear strength is important in determining:
- Bearing capacity of soil for foundation design
- Ultimate bearing capacity of a soil for foundation design
- Lateral pressure against retaining walls, sheeting, or bracing
- Friction of slopes, hills, slides, cuts, embankments, and earth dams

Development of Shear Strength

Shear strength is developed due to compression or tension forces, commonly caused by compression.
When the applied stress exceeds the shear strength of soil, it leads to:
- Sliding of soil particles upon each other
- Change in volume
- Part of the soil may move or be displaced with respect to the surrounding media
- Shear failure of soil

Soil Compaction

The goal of soil compaction is to reduce air-void volume Va in soils as much as possible.
Compaction is done by dropping a hammer 25 times onto the confined soil, which is divided into three lifts, and each lift is compacted 25 times.

Standard Proctor Test

The test measures the maximum dry unit weight and optimum water content of a soil.
The test involves compacting the soil by dropping a hammer, and the process is repeated 4-6 times from dry to wet conditions.
The density is at its maximum at the optimum water content, and it does not increase any further.

Modified Proctor Test

The test was developed during World War II by the U.S.
The test is used to measure the maximum dry density and optimum water content of a soil.

Optimum Water Content (w/c)

The range of optimum w/c varies depending on the soil type: 6-10 for sand (sub-base material), 8-12 for silty sand (sandy silt), 12-16 for silt, and 15-22 for clay.

Compaction Curve

The curve shows the relationship between the water content and dry density of a soil.
The curve has three stages: below optimum water content, at optimum water content, and above optimum water content.

Zero-Air-Void (ZAV) Curve

The curve represents the fully saturated condition of a soil (S=100%).
The curve shows the maximum dry density that can be achieved for a given water content.

Importance of Soil Compaction

Compaction increases density, strength, and bearing capacity of a soil.
Compaction reduces permeability and compressibility of a soil.

Laboratory Work

Laboratory tests are used to obtain the maximum dry density and optimum water content of a soil.
The results are used to guide field compaction.

Field Compaction

Field compaction is done by contractors using rollers and compactors.
The compaction execution is checked in the field using several methods.

Soil Compaction

There are two types of soil compaction tests: Standard Proctor and Modified Proctor.

Soil Types and Properties

Fine material: Standard Proctor test uses a 4 * 4.6 hammer with 12 drops per layer, and Modified Proctor test uses a 4 * 4.6 hammer with 18 drops per layer.
Coarse material: Standard Proctor test uses a 6 * 4.6 hammer with 12 drops per layer, and Modified Proctor test uses a 6 * 4.6 hammer with 18 drops per layer.
Sand (sub-base material): range of optimum water content is 6-10%.
Silty sand (sandy silt): range of optimum water content is 8-12%.
Silt: range of optimum water content is 12-16%.
Clay: range of optimum water content is 15-22%.

Proctor Test Results

The test results in a curve that shows the relationship between water content and dry density.
The maximum dry unit weight is achieved at the optimum water content.
The unconfined compressive strength of the soil can also be measured.

Compaction Curve

The curve shows the effect of water content on dry density.
Below the optimum water content, the density increases as the water content increases.
At the optimum water content, the density is at a maximum and does not increase further.
Above the optimum water content, water starts to replace soil particles in the mold, and the density decreases.

Zero-Air-Void Curve (ZAV)

The curve represents the fully saturated condition (S=100%).
It shows the relationship between the degree of saturation and the void ratio.

Goal of Compaction

The goal is to find the appropriate amount of water (optimum water content) and the maximum dry density for a soil to be used in the field.
The test helps to determine the compactive effort required to achieve the maximum dry density.

Soil Compaction

There are two types of soil compaction tests: Standard Proctor and Modified Proctor.

Soil Types and Properties

Fine material: Standard Proctor test uses a 4 * 4.6 hammer with 12 drops per layer, and Modified Proctor test uses a 4 * 4.6 hammer with 18 drops per layer.
Coarse material: Standard Proctor test uses a 6 * 4.6 hammer with 12 drops per layer, and Modified Proctor test uses a 6 * 4.6 hammer with 18 drops per layer.
Sand (sub-base material): range of optimum water content is 6-10%.
Silty sand (sandy silt): range of optimum water content is 8-12%.
Silt: range of optimum water content is 12-16%.
Clay: range of optimum water content is 15-22%.

Proctor Test Results

The test results in a curve that shows the relationship between water content and dry density.
The maximum dry unit weight is achieved at the optimum water content.
The unconfined compressive strength of the soil can also be measured.

Compaction Curve

The curve shows the effect of water content on dry density.
Below the optimum water content, the density increases as the water content increases.
At the optimum water content, the density is at a maximum and does not increase further.
Above the optimum water content, water starts to replace soil particles in the mold, and the density decreases.

Zero-Air-Void Curve (ZAV)

The curve represents the fully saturated condition (S=100%).
It shows the relationship between the degree of saturation and the void ratio.

Goal of Compaction

The goal is to find the appropriate amount of water (optimum water content) and the maximum dry density for a soil to be used in the field.
The test helps to determine the compactive effort required to achieve the maximum dry density.

Soil Compaction

Objective: increase density, strength, bearing capacity, and reduce permeability and compressibility
Laboratory work: obtain maximum dry density and optimum water content
Field compaction: executed by contractors using rollers and compactors

Compaction Process

Definition: increasing soil unit weight by reducing the soil void ratio using mechanical devices
Modified Proctor Test: compacting soil in five layers with a 10 Lbs hammer, 18 inches drop height, and 25 blows/layer
Modified Proctor Energy: 56,250 ft-lb/ft³, calculated by (25 blows/layer) * (5 layers) * (10 lbs) * (1.5 ft) / (1/30) ft³

Comparison of Standard and Modified Proctor Tests

Mold size: 1/30 ft³ for both tests
Standard Proctor Test: 12" height of drop, 5.5 lb hammer, 3 layers, 25 blows/layer, and energy 12,375 ft-lb/ft³
Modified Proctor Test: 18" height of drop, 10 lb hammer, 5 layers, 25 blows/layer, and energy 56,250 ft-lb/ft³

Compaction Procedure in Lab

Method: removing air voids to achieve a degree of compaction (S=1)
Water content: wW / vt vw

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Description

Learn about the shear strength of cohesive and cohesionless soils, and its significance in supporting imposed loading and self-support. Understand laboratory and field tests for measuring shear strength.