Introduction to Soil Mechanics and Shear Strength

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Introduction to Soil Mechanics and Shear

Learning Objectives
• Understand different soil types. • Understand the basics of soil shear strength. • Understand what conditions dictate the type of shear strength
that should be considered.

• Soil – an un-cemented aggregate of rock and mineral grains and decaying organic matter (solid particles) with liquid and/or gas occupying the void space between the solid particles.

Properties of Solid Particles

• Particle size



Clay -- Silt -- Sand -- Gravel -- Cobbles -- Boulders





0.075mm 4.75 mm 75mm 350mm

A quantitative measure of soil particles.

• Particle shape
Rounded -- Subrounded -- Subangular -- Angular A qualitative measure of soil particles.


Engineering Soil Classification
• The purpose:
• Common language for naming soils. • Mixtures of particles that vary in mineralogy
and particle size. • Categorize soils based on their engineering
properties and characteristics. • Based partly on grain size. • Based partly on index properties.

Silts and Clays
• Particles smaller than we can detect by visual or sieve analysis methods.
• Silts are very fine bulky particles like sands – they are rock and mineral fragments – they are reasonably inert.
• Clays are minerals that are the product of chemical weathering of feldspar, ferromagnesian, and mica minerals.
• Clay particles are very small and are chemically active. • Silts and Clays exhibit different engineering behavior.


The Plasticity Chart

How do we determine shear strength of soil and rock?
• Direct Shear • Triaxial Shear • In-situ testing (Vane Shear, CPT, SPT correlations, Schmidt
Hammer) • Correlations • Back-analysis of existing landslide geometry.

Most Important Concept: Effective Stress
• Effective Stress is arguably the most important concept in soil mechanics.
• It dictates the relationship between water pressure and the mobilized stress in a soil matrix.
• Most simply, it is defined as: Effective Stress = Total Stress – Pore Water Pressure 𝝈𝝈′ = 𝝈𝝈 − 𝒖𝒖
• Effective Stress = Actual Contact Forces between Soil Grains • Total Stress = The total weight of soil and water within a column. • Pore Water Pressure = The buoyant forces pushing grains apart.

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Introduction to Soil Mechanics and Shear Strength