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This section is part of: Athanasopoulos, G. A. (2021) "Online Lecture Notes on Soil Mechanics", Geoengineer.org, DOI: https://doi.org/10.48246/GEOENG-EDU-001

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When the soil has a horizontal surface and there is no horizontal variability in its properties, the stresses that are developed within the soil mass which are due to its weight are called geostatic stresses. In this case on a horizontal and on a vertical plane, only normal stresses are acting.

The vertical normal stress, σv, is given by the equation:

If γₜ is constant, and from the equation:

if the γₜ is a function of the depth.

The horizontal normal stress, σₕ, is given by the equation:

where the factor Kₒ is the lateral earth pressure coefficient at rest, and this value varies from values less than 1 (0.4 - 0.5), for normally consolidated soils, to values >1, for overconsolidated soils.

In case of __saturated soil__, the normal stress, σ, which acts on a random plane, consists of two components: the __effective stress__ and the __ pore water pressure__, u:

The sum of the effective stress and the pore water pressure is called __total stress__.

The vertical effective stress, σᵥ, at a certain depth beneath the water table can be calculated in two ways: (a) by finding the total stress at the depth that we are interested in (by using saturated unit weights under the water table) and deduct the pore water pressure, (b) by using the effective unit weights, γₐᵥ, for the soil layers under the water table.

The mechanical behavior of soil (compressibility, strength, lateral earth pressure, bearing capacity) depends on the value of __effective stresses__.

The phenomenon of water buoyancy within capillary piping is observed in soils and the role of the capillary piping is played by the voids in between the soil particles. Naturally, in soils, the phenomenon becomes complicated by the fact that the voids have an irregular and variable shape, dimensions and communication paths. Approximately, the height of the capillary buoyancy above the level of the ground water table can be estimated by the equation:

Where, r is the radius of the capillary piping, which usually is considered smaller than the radius of the soil particles.

The pore water pressure in the zone of capillary pipping is negative (less than the atmospheric) and at a high h₁ above the water table, is given by the equation:

The height of the capillary buoyancy hc in soils is given also by the empirical equation (Hazen, 1930):

Where:

Chapter 7: Earth Pressures7.2. Coulomb Theory (1776)7.1. Rankine Theory (1857)7.3. Graphical Methods7.4. Types of Retaining Structures7.5. Example ProblemsOnline Lecture Notes on Soil MechanicsChapter 2: Stresses in Soils2.1. Normal and Shear Stress2.3. Stresses due to a load surcharge on the Ground Surface2.4. Example ProblemsChapter 9: Bearing Capacity of Shallow Foundations9.1. Bearing Capacity9.2. Example ProblemsChapter 3: Flow of Water through Soil3.1. One Dimensional Flow3.2. Two Dimensional Flow3.3. Graphical Generation of Flow Nets

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