Chapter 1 — Introduction and Sources¶
1.1 What soil mechanics is¶
Soil is a particulate, three-phase material — solid particles, water, and air — whose behaviour depends on the interaction between the phases. Soil mechanics studies how that material responds to stress, water, and time. It underpins every geotechnical calculation: bearing capacity, settlement, slope stability, earth pressure, and seepage.
1.2 The three phases and why they matter¶
Almost every soil property is explained by the three phases:
- The solid skeleton carries the effective stress and gives strength and stiffness.
- Water in the pores carries pore pressure and governs consolidation and seepage.
- Air makes the soil unsaturated, adding suction and apparent cohesion.
The relationship between the phases — expressed by void ratio, water content, and degree of saturation — is the starting point of every analysis.
1.3 Why this reference uses public-domain sources¶
The classic soil-mechanics textbooks are copyrighted. This reference is instead built from the FHWA NHI Soils and Foundations Reference Manual and the USACE Engineering Manuals — U.S. Government works in the public domain, free to reuse, adapt, and translate. They are written for practising engineers and form the basis of real design in the United States.
1.4 Total and effective stress — the central idea¶
The single most important concept in soil mechanics is Terzaghi's principle of effective stress:
where \(\sigma\) is total stress, \(u\) is pore-water pressure, and \(\sigma'\) is effective stress — the stress carried by the solid skeleton. Strength, stiffness, and volume change depend on effective stress, not total stress. This idea threads through every later chapter.
1.5 Key takeaways¶
- Soil is a three-phase material; its behaviour follows from the phases.
- Effective stress \(\sigma' = \sigma - u\) governs strength, stiffness, and volume change.
- This book is built entirely from public-domain FHWA and USACE manuals.
- Soil parameters carry uncertainty — good engineering bounds it.