Soil Mechanics
Soil mechanics is the branch of geotechnical engineering that applies the principles of mechanics and hydraulics to the engineering behaviour of soils, studying their strength, deformation, permeability, and consolidation under various loading conditions. Founded by Karl Terzaghi in the early 20th century, it provides the theoretical and experimental framework for analysing slope stability, bearing capacity, settlement, earth pressure on retaining walls, and seepage through embankments. In India, soil mechanics principles underpin all foundation design codes including IS 1904, IS 6403, and IS 8009.
Key Formula
Shear strength = effective cohesion + effective normal stress × tan(effective friction angle)
LaTeX: \tau_f = c' + \sigma' \tan\phi'
| Symbol | Meaning | Unit |
|---|---|---|
| \tau_f | Shear strength of soil at failure | kPa |
| c' | Effective cohesion intercept | kPa |
| \sigma' | Effective normal stress on the failure plane | kPa |
| \phi' | Effective angle of internal friction | degrees (°) |
Worked Example
Problem
A sandy clay soil has effective cohesion c' = 10 kPa and effective friction angle φ' = 28°. A laboratory shear box test applies an effective normal stress of 80 kPa. Calculate the shear strength at failure.
Solution
Step 1: c' = 10 kPa, φ' = 28°, σ' = 80 kPa. Step 2: tan(28°) ≈ 0.5317. Step 3: τ_f = c' + σ' tan(φ') = 10 + 80 × 0.5317 = 10 + 42.5 = 52.5 kPa.
Answer
Shear strength at failure τ_f = 52.5 kPa
Key Soil Properties and Their Engineering Significance
| Property | Symbol | Typical Range | Significance in Design |
|---|---|---|---|
| Cohesion (effective) | c' | 0–50 kPa | Contributes to shear strength independent of stress |
| Friction angle | φ' | 25°–45° | Determines strength increase with depth |
| Void ratio | e | 0.3–1.5 | Controls settlement and permeability |
| Liquid limit | LL | 20–100% | Indicates plasticity and clay content |
| Compression index | Cc | 0.1–1.0 | Controls magnitude of consolidation settlement |
| Coefficient of permeability | k | 10⁻⁹ to 10⁻² m/s | Controls drainage rate and pore pressure |
Interactive Tools
GeoTechniEasy Soil Mechanics Tools
Online calculators for Mohr-Coulomb, consolidation, and bearing capacity
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Bearing Capacity (soil)
The bearing capacity of soil is the maximum load per unit area that a soil can support without undergoing shear failure, excessive settlement, or instability below a foundation. The ultimate bearing capacity (q_u) is the stress at which the soil fails in shear, while the safe bearing capacity (q_s) is q_u divided by a factor of safety, and the allowable bearing capacity also accounts for permissible settlement. Terzaghi's bearing capacity equation, later extended by Meyerhof, Hansen, and Vesic, expresses q_u as a function of soil cohesion, surcharge, foundation width, and soil friction angle, forming the basis of IS 6403 in India.
Foundation (civil engineering)
A foundation is the lowest part of a structure that transfers all superstructure loads safely to the underlying soil or rock, ensuring stability against settlement, sliding, and overturning. Foundations are broadly classified as shallow foundations (spread footings, combined footings, raft/mat foundations) when the depth of embedment is small relative to width, and deep foundations (piles, caissons, well foundations) when loads must be transferred to deeper, stronger strata. The design of foundations requires knowledge of both the structural loads imposed from above and the geotechnical properties of the soil below, making it an interdisciplinary activity bridging structural and geotechnical engineering.
Structural Load
A structural load is any force or collection of forces that acts on a structure, causing internal stresses, deformations, or displacements within the members. Loads are classified by their nature (static or dynamic), their source (gravity, wind, seismic), and their duration (permanent or transient). Accurate load estimation is the foundation of structural design, ensuring that every member can safely resist the demands placed on it throughout the life of the structure.
The term "soil mechanics" was coined by Karl von Terzaghi (1883–1963), who published "Erdbaumechanik" (Earth Construction Mechanics) in 1925, establishing the discipline. "Soil" comes from Latin "solium" (seat) or "solum" (ground, base), while "mechanics" derives from Greek "mēkhanikē" (the art of making machines), from "mēkhanē" (device, contrivance).