Chapter 4 - Seepage through Unsaturated Soils

Chapter 4 - Seepage through Unsaturated Soils

Presenter: Prof. Delwyn G. Fredlund
Title: Chapter 4 - Seepage through Unsaturated Soils
Launching Date & Time: 15th of May 2019 12:00pm GMT

Course: Unsaturated Soil Mechanics

  1. Introduction to Unsaturated Soil Mechanics
  2. State Variables and their Measurement
  3. The Soil-Water Characteristic Curve, SWCC
  4. Seepage through Unsaturated Soils
  5. Shear Strength of Unsaturated Soils
  6. Volume Change & Deformation of Unsaturated Soils


This course is based on notes presented in the book “Unsaturated Soil Mechanics in Engineering practice” published in 2012 by the speaker. The objective of the course is to introduce unsaturated soil mechanics within the framework of classical soil mechanics by incorporating soil suction.

The first lecture introduces unsaturated soils by illustrated practical problems. The link between field and laboratory conditions is established by the “Soil-Water Characterization Curve” (SWCC) also called the soil retention curve.

The second lecture presents the state variables by which the transition from the saturated soil condition to the unsaturated soil condition can be described, in particular the matric suction as the difference between pore-air and pore-water pressures. The measurement of soil suction helps in linking the state of stresses in field and laboratory conditions.

The third lecture details the determination of SWCC in providing information about the amount of water and the energy state in water phase. The experimental measurement of the SWCC is explained and inherent parameters depending on the type of soil are introduced.

The fourth lecture is dedicated to the theory and solution dealing with water flow problems in unsaturated soil. Focus is made on the measurement of unsaturated coefficient of permeability function for design purposes. Steady state and unsteady (transient) state methodologies are explained for the calculation of unsaturated coefficients of permeability. Selected water flow case studies are shown.

Lecture five addresses the determination of shear strength of unsaturated soils. The latter is described by two independent state variables among which soil suction contributes to the cohesive component of soil. Measurement of shear strength of unsaturated soils is conducted by the conventional shear box and triaxial equipment needing modifications to allow air-entry in tested specimens. Using the shear strength parameters of unsaturated soil, current soil mechanics applications: e.g. earth pressures, bearing capacity and slope stability of foundations are formulated.

In the sixth lecture, the theory with related solution of stress deformation problems is investigated. The main objective is to establish the relation between stress state variables and deformation strain state variables for unsaturated soils. In this process, the use of SWCC enables the assessment of the behaviour of unsaturated soil.

Session Question & Answer [CLOSED]

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