4.2 TOTAL STRESS, PORE WATER PRESSURE AND EFFECTIVE STRESS: UNDERSTANDING SOIL BEHAVIOR SIMPLIFIED
Understanding how soils behave under loads is critical in geotechnical engineering. Engineers design foundations, retaining walls, and embankments based on how soil responds to stress.
Water may influence the nature of the mineral surface chemically and consequently affect the bonding forces between adjacent soil grain. This kind of interaction between the soil solid and water is called chemical interaction.
The other kind of interaction is a physical interaction between solid and water.
Volume of the soil skeleton as a whole can change due to rearrangement of soil particles into new position mainly by rolling and sliding due to forces acting between the particles This physical interaction is studied when we study the effective stress concept: given by Terzaghi.
The effective stress concept applies to a fully saturated soil and relates three key concepts to help us grasp soil behaviour: Total Stress, Neutral Stress (Pore Water Pressure), and Effective Stress.
Let’s break them down.
1. Total Stress (σ):
Total stress is the overall stress applied to a soil mass, including both the weight of the soil above and any external loads like buildings, vehicles, or water. It is a physical parameter which can be measured by suitable arrangement, such as by pressure cells.
Formula:
· σ
· σ
When there is imposed load on soil mass as shown below, the total stress value at point O is given by:
Example:
Think of a sponge on a table. If you place a book on top of it, the sponge feels the weight of the book. Similarly, soil at a certain depth "feels" the weight of the soil above plus any extra load.
· Soil under a thick embankment or tall building has high total stress.
· Soil in a shallow garden bed has low total stress.
2. Pore Water Pressure (u):
Pore water pressure is the pressure exerted by water trapped in the soil pores. Soils often contain water in their tiny voids, which can support some of the total stress.
u = gw * h2
A case when force F has been acting on soil since long ago.
Pore water pressure is also called a neutral stress because it acts on all sides of the particles, but does not cause particles to press against adjacent particle. It has no shear component.
It is measured by inserting a stand pipe at the point under investigation and observing the height up to which the water rises in the stand pipe. Thus it is a measurable quantity. It is measured using a Piezometer or a stand pipe.
Example:
Imagine a wet sponge. When you press on it, water in the sponge pushes back. That “push-back” is like pore water pressure in soil.
· After heavy rainfall, water fills the soil pores, increasing pore pressure.
· In fully saturated soils like clay, pore water can carry most of the load temporarily.
3. Effective Stress (σ’):
Effective stress is the stress actually carried by the soil skeleton, i.e., the solid particles themselves. It determines soil strength and deformation.
σ
Where:
these two are physical parameters.
Even though effective stress is not a measurable quantity, it is very important parameter in soil mechanics, because effective stress is a parameter on which compressibility, consolidation, settlement, shear stress and bearing capacity depends. These parameters do not depend on total stress directly.
The principle of effective stress is valid for coarse grained soils and clayey soils both.
CALCULATION OF EFFECTIVE STRESS:
Soil strength comes from the friction and interlocking of soil particles, not the water. If water carries all the stress, soil behaves weakly.
Example:
Picture standing on wet sand at the beach:
· Your weight (total stress) pushes down.
· Water between the sand grains (pore pressure) temporarily supports some weight.
· The sand grains themselves carry only the remaining stress (effective stress), which determines if you sink or stay firm.
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