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1.3.2. Second Part of Effective Stress Principle
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Today, let's discuss total stress in soils. Total stress is the weight of everything above a point in the soil, right? This includes soil, water, and any loads above. The equation for total stress at depth z is σ = γ.Z, where γ is the unit weight of the soil. Can anyone tell me why total stress increases with depth?
Is it because more weight is above as we go deeper?
Exactly! More mass means more weight. Now, let's consider total stress below a water body. What do we need to add to account for the water?
We have to add the weight of the water above, right?
Correct! The formula for that is σ = γ.Z + γw.Zw, where γw is the unit weight of water. Great job understanding the concepts!
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Now, let's talk about pore water pressure. Can anyone explain what this term means?
It's the pressure exerted by water in the soil's pores.
Right! We calculate it using u = γw.h. What influences pore water pressure the most?
The depth below the water table, I think!
Exactly! And common conditions, like seepage flow, can alter it as well. Can someone summarize how pore water pressure relates to effective stress?
Effective stress is total stress minus pore water pressure!
Spot on! Great recap. Effective stress is vital for understanding soil behavior!
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Let’s now explore effective stress further. What do we mean by effective stress?
It's the stress that actually contributes to soil strength, right?
Correct! It’s not something we can measure directly because it’s a conceptual value. Instead, we compute it with the equation σ' = σ - u. Why is that computation important?
Because it helps us understand soil stability during different loading scenarios!
Exactly right! And remember, if total stress increases due to added load, pore water pressure will also increase initially. This might lead to drainage and a rise in effective stress. Can someone explain what happens above the water table?
Pore pressure might become negative due to capillary effects!
Wonderful! This shows how varied soil conditions can complicate effective stress.
Overview
Short Summary
This section explains the effective stress principle in soil mechanics, focusing on total stress, pore water pressure, and their interrelationships.
Medium Summary
The section elaborates on the fundamentals of the effective stress principle introduced by Karl Terzaghi, detailing how total stress and pore water pressure influence effective stress in saturated soils. The impact of water table levels and soil types on effective stress computations are also discussed.
Detailed Summary
Effective Stress Principle in Soil Mechanics
The effective stress principle is a crucial concept in soil mechanics, valid for saturated soils, as established by Karl Terzaghi in 1936. The principle articulates that the effective stress (c3'e0) at any point in a soil mass is determined by subtracting the pore water pressure (u) from the total stress (σ):
Understanding Total Stress
The total vertical stress (c3) at a point below the ground surface aggregates all the weights of soil, water, and any surface loads acting on it. The formula for total vertical stress at depth z is:
- Total Stress at Depth: c3 = b3.Z where b3 is the unit weight of the soil.
In saturated conditions below a water table, total stress is enhanced by the weight of the water column overlying the soil:
- Total Stress Below Water Level: c3 = b3.Z + b3w.Zw where b3w is water unit weight and Zw is the depth of water above.
Pore Water Pressure
Pore water pressure (u) refers to the pressure exerted by water within soil pores, computed as:
- Pore Water Pressure: u = b3w.h where h is the depth below the water table.
This pressure plays a significant role in effective stress because it varies with depth and water table conditions. The water table marks the boundary where pore water pressure is zero; below that, it remains positive, while above it can become negative in saturated partly porous sections.
Implications of Effective Stress
The effective stress, given by: c3' = c3 - u, is the stress that contributes to soil behavior—enhancing compression and shearing resistance. It can't be directly measured but can be computed considering the parameters discussed. An increase in total stress results in a corresponding rise in pore water pressure before any volume change occurs, advocating drainage and an eventual rise in effective stress. Notably, above the water table, saturated soil experiences negative pore pressures (capillary action). Thus, in saturated soils, the complex relationship between pore water pressure and effective stress affects soil stability and behavior significantly.
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Create a free accountAt any point in a soil mass, the effective stress (represented by σ') is related to total stress (σ) and pore water pressure (u) as:
σ' = σ - u
Detailed Explanation
Effective stress is a crucial concept in soil mechanics that reflects the stress experienced by the soil skeleton. It is determined using the formula σ' = σ - u. Here, σ is the total stress acting on the soil, which includes the weight of the soil and any additional loads, and u is the pore water pressure, which is the pressure of water in the soil's voids. By subtracting the pore water pressure from the total stress, we calculate the effective stress, which is what actually contributes to the strength and stability of the soil.
Examples & Analogies
Think of a sponge in water. When you press down on the sponge (total stress), the water inside it pushes back (pore water pressure). If you want to know how much the sponge itself (the soil) can hold without being deformed, you need to consider how much pressure is coming from the water inside. The effective stress would be the pressure exerted by the sponge after accounting for the water's pressure.
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Create a free accountAll measurable effects of a change of stress, such as compression and a change of shearing resistance, are exclusively due to changes in effective stress.
Detailed Explanation
When stress applied to the soil changes, the effects seen in the soil, such as how much it compresses or how it resists shear forces, are exclusively determined by changes in effective stress. This means that any variation in how the soil responds to forces acting upon it comes from the effective stress that is created by the balance between total stress and pore water pressure.
Examples & Analogies
Imagine filling a balloon with air. If you increase the air pressure (total stress), but some of the air escapes (reducing effective stress), the balloon may not expand or can even contract if the air escapes quickly enough. This situation mirrors how soil behaves: the effective stress dictates the soil's strength and deformability under changing conditions.
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Create a free accountIf the total stress is increased due to additional load applied to the soil, the pore water pressure initially increases to counteract the additional stress. This increase in pressure within the pores might cause water to drain out of the soil mass, and the load is transferred to the solid grains. This will lead to the increase of effective stress.
Detailed Explanation
When an additional load is applied to saturated soil, the total stress increases. As a response, the pore water pressure also increases to balance this change. Initially, this can lead to the soil becoming temporarily weaker as water is pushed out of the way. However, once the water drains, the load is fully supported by the solid grains of the soil, which results in an increase in effective stress. This process illustrates how soil can manage and adapt to changes in loading conditions over time.
Examples & Analogies
Imagine jumping on a wet sponge. At first, the sponge compresses and the water inside resists your weight, leading to a soft landing. As some of the water drains out, the sponge gets firmer and better able to support your weight. Eventually, you feel more stable as the load is carried by the sponge's structure rather than just the water within it, similar to how soil redistributes stresses.
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Create a free accountAbove the water table, when the soil is saturated, pore pressure will be negative (less than atmospheric). The height above the water table to which the soil is saturated is called the capillary rise, and this depends on the grain size and the size of pores.
Detailed Explanation
In areas above the water table, saturated soil can exhibit negative pore water pressure due to tension within the water in the soil. This tension occurs because the water is 'pulling' on the soil grains. Capillary rise is the phenomenon where water moves upwards through small soil pores against gravity, influenced by the size of those pores. Smaller pores promote higher capillary action, which can impact soil moisture content and behavior significantly.
Examples & Analogies
Consider how a paper towel absorbs water. When you dip one end of a dry paper towel into water, the water travels up the towel because of capillary action. Similarly, in soils with very fine particles, water can be drawn up from a deeper source, even against the force of gravity, which illustrates how soil moisture dynamics work above the water table.
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Total Stress:
Represents the weight above a soil point, increasing with depth.
- Pore Water Pressure:
Pressure from water in soil pores that influences effective stress.
- Effective Stress:
A critical measure of soil strength, computed as total stress minus pore water pressure.
- Water Table:
The level below which soil is saturated with water.
- Saturated Soil:
A soil condition where all voids are filled with water affecting pore pressures.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
When calculating total stress at a depth of 5 meters with a soil unit weight of 18 kN/m³, the total stress is σ = 18 kN/m³ * 5 m = 90 kN/m².
If the water table is 3 meters below a point and the pore water pressure is computed as u = 9.81 kN/m³ * 3 m = 29.43 kN/m², this illustrates how pore pressure is calculated.
Memory aids
To find stress that’s effective, subtract with glee, / From total stress, the pore pressure, you see!
Imagine a sponge under a heavy load, the sponge represents the soil, holding both water and weight. The heavier the load, the more water pressure builds up until it drains, just like how effective stress works in soils!
Flash Cards
Glossary
Total Stress
The sum of the weight of soil, water, and surface load above a given point in the soil.
Pore Water Pressure
The pressure exerted by water in the pores of the soil, dependent on depth and water table conditions.
Effective Stress
The stress that contributes to soil behavior, derived from total stress minus pore water pressure.
Water Table
The natural level of ground water, marking the transition between saturated and unsaturated soil.
Saturated Soil
Soil where all voids are filled with water, impacting pore water pressure and effective stress.