Soil Characteristics and Behavior – Transcript Notes

Introduction and learning context

• The session focuses on soil today, with plans to move into computational aspects later. The progression starts with basic descriptions of soil characteristics and then numbers in upcoming weeks.

• Big picture: rocks weather into soils; weathering processes create soil deposits. We’ll classify soils by origin and describe how these origins influence soil behavior.

• Learning outcomes highlighted:

  • Describe how soil deposits form from different processes.

  • Understand how formation processes inform soil behavior in engineering contexts.

Types of soils by origin (basic classifications)

• Transported soils: soils derived from rock masses far away, transported by natural forces and deposited elsewhere. Example given: rock from the Southern Alps flowed down a river and was deposited as soil.

• Residual soils: soils that form in place from weathering of the local rock; chemically degraded in situ to form soil. Example: common in Auckland.

• Organic soils: soils formed from organic material (e.g., trees, logs) that have been inundated with water over time, creating a problematic soil type.

• The slides emphasize studying the characteristics of these deposits and how origin informs behavior.

Context and references

• Encouragement to consult textbooks for deeper reading:

  • Greg, Mechanics, first chapter, first section. Start with rock versus soil and weathering concepts.

• Key point: rock is an aggregate of minerals that is strong and well-balanced; soils are typically products of weathering, forming from rock grains that become finer.

Rock vs soil: fundamental definitions

• Rock: an aggregate of one or more minerals, usually strong and intact.

• Soil: an aggregate of mineral grains (and possibly rock fragments) that is the product of weathering; soils are in general a smaller-scale, weathered counterpart of rock.

• Practical test for soil: pick up a handful and shake it; if grains separate easily, that separation indicates a soil rather than solid rock.

• Everyday relevance: most people have encountered soils, e.g., beach sand, farm soils; surface soils are what you plant in and interact with routinely.

Composition and particle size (how soils are broken down)

• Soils are made up of a range of particle sizes, analogous to sedimentary rocks in terms of grain size categorization.

• Size-based progression (from largest to smallest):

  • >200 mm: boulders (not considered soil)

  • Boundary around 60 mm: cobbles to the right are still not as easily handled as soil; the range left of 60 mm is increasingly soil-like.

  • Gravel to sand: as grains get smaller, soils become finer; sands are grains smaller than 2 mm (described as classic engineering sand when grains are <2 mm).

  • Silts and clays: the boundary between silt and clay is complex and will be discussed further; the exact boundary is not fixed.

• Organic soil: mentioned as a special category with no fixed size boundary, resulting from organic matter accumulation.

• Clay minerals: tiny plate-like minerals that define clay and influence its classification; mineralogy drives whether a grain behaves like clay.

How soil differs from other materials (the three-phase nature)

• Unlike solids like steel, concrete, wood, or rock, soil is a three-phase material:

  • Solid grains (the mineral/rock particles)

  • Pore fluid (water, possibly oil)

  • Pore air (gas) occupying voids between grains

• Consequence: the solid–fluid–gas interactions create highly complex behavior; the presence and state of water and air within pore spaces strongly influence mechanical response.

• Relation to behavior: the properties and arrangement of the solid phase are not enough to predict behavior—the fluids in the voids play a crucial role.

Factors that control soil behavior (friction, stress, and pore pressure)

• Soils are frictional materials; the interaction between solid grains governs movement

• The friction between grains depends on the normal (compressive) force pressing them together; higher normal force generally increases frictional resistance

• Pore pressure effect: liquids (water) in pores can push grains apart, reducing shear strength; this is captured by effective stress concepts

• A simple takeaway: soil behavior is governed by both the solid grain contacts and the state of pore fluids within the voids

• The lecturer notes that his next lecture will cover stress in confined soil, reinforcing the importance of confinement and stress conditions on soil behavior

Sign conventions and tensile strength in soils

• In soil mechanics, there is generally no tensile strength; soils do not sustain significant tensile loads like some structural materials

• Sign convention: in geotechnical engineering, compression is treated as positive;

  • This differs from some structural engineering conventions where tension is often treated as positive

• The practical implication: design and analysis in geotech focus on compression and confinement, not tensile strength of soils

Nonlinear plastic behavior and modulus concepts in soil

• Soil exhibits nonlinear plastic behavior: it does not respond linearly to loading; stiffness (modulus) changes with strain

• Young’s modulus (E) is not a central or constant parameter for soils; it is often poor or not used as a constant value

• Shear modulus (G) is commonly used in soil mechanics, and G is a function of shear strain (γ)

  • As shear strain increases, G tends to decrease

• Stress–strain behavior in soil: represented by a curve of shear stress (τ) versus shear strain (γ)

  • At low γ: τ increases with γ with a certain slope (modulus)

  • Peak strength: the maximum τ achieved before failure

  • Post-peak behavior: after reaching the peak, the material often exhibits plastic behavior where additional strain does not produce proportional increases in strength, and strength may decline or plateau

• Summary: soil behavior is characterized by nonlinear plasticity, with a strain-dependent modulus and a peak strength followed by post-peak softening or yielding

Key relationships and concepts to remember

• Friction and normal force:

  • Simple form: F_f = BCdot; B5 N (illustrative friction relation; actual μ may vary with path and conditions)

• Effective stress and pore pressure:

  • Terzaghi principle: \sigma' = \sigma - u

  • Here,

  • σ is the total stress,

  • u is the pore water pressure, and

  • σ' is the effective stress that actually governs soil strength and deformation

• Three-phase soil: solid grains + pore water + air | This combination dictates how grains move past one another and how stress is carried

• Sign convention in geotechnical engineering:

  • Compression is positive; tension is not typically used as a primary descriptor for soil behavior

• Shear response and modulus:

  • Shear stress: au

  • Shear strain:  (gamma)

  • Tangent shear modulus: G = \frac{d\tau}{d\gamma} (function of γ)

• Elastic vs plastic response in soils:

  • Not linear elastic; transitions to plastic behavior beyond yield or peak strength

  • Modulus is not constant; it changes with strain level

Practical implications and real-world relevance

• Origin of soils affects behavior:

  • Transported soils (e.g., alluvial) may have different textural/structural properties than residual soils formed in place

  • Organic soils tend to have unique degradation characteristics and higher compressibility

• Pore water and drainage are critical:

  • High pore pressure during loading (e.g., rapid loading, flood conditions) can reduce effective stress and shear strength, influencing foundation design, slope stability, and earthworks

• Sign conventions matter for engineers:

  • Understanding that soils primarily experience compression and do not rely on tensile strength helps align analysis with real behavior

• Nonlinear and time-dependent aspects (to be covered later):

  • The current discussion foreshadows time-dependent behavior (creep, consolidation) and rate effects that will be explored in future lectures

Connections to foundations and prior knowledge

• Builds on the idea that rock is strong and uniform, while soils are heterogeneous and capable of large deformations due to porosity and fluid interactions

• Links to basic mechanics concepts:

  • Friction and normal force relations

  • Stress, strain, and modulus concepts, adapted for soils (G and its dependence on γ)

• Bridges rock-weathering concepts to practical soil behavior in geotechnical engineering

References and suggested readings

• Greg, Mechanics, Chapter 1, Section 1 (for foundational concepts on rock vs soil and weathering)

• Holtz and Kovacs (older edition): statements about soil as a frictional material and the role of grain interactions in soil behavior

Reminders for upcoming sessions

• Tomorrow (or next session): delve into time-dependent behavior of soil (creep, consolidation, rate effects) and its implications for design and analysis

• Be prepared to connect soil behavior to numerical or computational methods discussed in the upcoming weeks