Effective stress
The effective stress can be defined as the stress, depending on the applied tension and pore pressure , which controls the strain or strength behaviour of soil and rock (or a generic porous body) for whatever pore pressure value or, in other terms, the stress which applied over a dry porous body (i.e. at ) provides the same strain or strength behaviour which is observed at ≠ 0.[1] In the case of granular media it can be viewed as a force that keeps a collection of particles rigid. Usually this applies to sand, soil, or gravel, as well as every kind of rock and several other porous materials such as concrete, metal powders, biological tissues etc.[1] The usefulness of an appropriate ESP formulation consists in allowing to assess the behaviour of a porous body for whatever pore pressure value on the basis of experiments involving dry samples (i.e. carried out at zero pore pressure).
History
In 1962, work by Jeremiah Jennings and John Burland examined the applicability of Terzaghi’s effective stress principle to partly saturated soils.[6] Through a series of experiments undertaken at the University of the Witwatersrand, including oedometer and compression tests on various soil types, they showed that behaviours such as volume changes and shear strength in partly saturated soils do not align with predictions based on effective stress changes alone. Their findings showed that the structural changes due to pressure deficiencies behave differently from changes due to applied stress.[7][8][9][6]
Description
Effective stress (σ') acting on a soil is calculated from two parameters, total stress (σ) and pore water pressure (u) according to:
Typically, for simple examples
Much like the concept of stress itself, the formula is a construct, for the easier visualization of forces acting on a soil mass, especially simple analysis models for slope stability, involving a slip plane.[10] With these models, it is important to know the total weight of the soil above (including water), and the pore water pressure within the slip plane, assuming it is acting as a confined layer.[citation needed]
However, the formula becomes confusing when considering the true behaviour of the soil particles under different measurable conditions, since none of the parameters are actually independent actors on the particles.[citation needed]
Consider a grouping of round quartz sand grains, piled loosely, in a classic "cannonball" arrangement. As can be seen, there is a contact stress where the spheres actually touch. Pile on more spheres and the contact stresses increase, to the point of causing frictional instability (dynamic friction), and perhaps failure. The independent parameter affecting the contacts (both normal and shear) is the force of the spheres above. This can be calculated by using the overall average density of the spheres and the height of spheres above.[citation needed]
If we then have these spheres in a
The concept of effective stress truly becomes interesting when dealing with non-
Two extremes of this effect are quicksand, where the groundwater gradient and seepage force act against gravity; and the "sandcastle effect",[12] where the water drainage and capillary action act to strengthen the sand. As well, effective stress plays an important role in slope stability, and other geotechnical engineering and engineering geology problems, such as groundwater-related subsidence.
References
- ^ .
- ^ Terzaghi, Karl (1925). Erdbaumechanik auf Bodenphysikalischer Grundlage. F. Deuticke.
- ^ Terzaghi, Karl (1936). "Relation Between Soil Mechanics and Foundation Engineering: Presidential Address". Proceedings, First International Conference on Soil Mechanics and Foundation Engineering, Boston. 3, 13–18.
- ^ "Vertical stress in the ground". fbe.uwe.ac.uk. Archived from the original on June 18, 2006.
- ^ ISBN 978-0-7277-3982-7, retrieved 2023-04-11
- ^ .
- .
- .
- Science Direct.
- ^ "Geo-Engineering at Durham University".
- ^ "Groundwater". fbe.uwe.ac.uk. Archived from the original on September 2, 2006.
- ^ "Capillary Aging of the Contacts between Glass Spheres and a Quartz Resonator Surface" (PDF). home.tu-clausthal.de. Archived from the original (PDF) on May 30, 2008.
- Terzaghi, K. (1925). Principles of Soil Mechanics. Engineering News-Record, 95(19-27).