Quick sand condition occurs when:
The upward seepage pressure in the soil becomes equal to the submerged unit weight of the soil
Understanding the Quicksand Condition in Soil Mechanics
The question asks about the specific condition under which quicksand occurs. The quicksand condition, also known as boiling, is a phenomenon observed in granular soils, typically sands and silts, when there is an upward flow of water.
This upward flow exerts an upward seepage pressure on the soil particles. When this upward seepage pressure becomes significant enough, it reduces the effective stress between the soil particles. Effective stress is the stress transmitted through the soil skeleton by intergranular contacts.
The total stress (\(\sigma\)) at any point in a saturated soil is supported by the pore water pressure (\(u\)) and the effective stress (\(\sigma'\)). This relationship is given by Terzaghi's principle of effective stress:
\(\sigma = \sigma' + u\)
Rearranging this equation, we get the effective stress:
\(\sigma' = \sigma - u\)
Upward Seepage and Effective Stress
In the case of upward seepage, the pore water pressure increases as we move upwards against the direction of gravity. This increased upward pore water pressure counteracts the downward total stress (due to the weight of the soil and any external loads), thereby reducing the effective stress.
Consider a layer of soil. The total stress at the bottom of the layer due to the weight of the soil above it is \(\sigma\). The pore water pressure at the bottom of the layer is influenced by the water levels and the seepage flow. With upward seepage, the pore water pressure gradient is upward, meaning pore water pressure increases in the upward direction relative to hydrostatic conditions.
The upward seepage force per unit volume of soil is equal to the hydraulic gradient (\(i\)) multiplied by the unit weight of water (\(\gamma_w\)), i.e., \(i \cdot \gamma_w\).
The downward submerged weight of the soil per unit volume is the submerged unit weight (\(\gamma'\)).
The Critical Hydraulic Gradient
The effective stress becomes zero when the upward seepage force per unit volume equals the submerged weight per unit volume. This happens when the upward seepage pressure, or more accurately, the upward seepage force per unit volume, balances the submerged unit weight of the soil.
The condition for zero effective stress (\(\sigma' = 0\)) at the base of a soil element is met when the upward seepage force per unit volume equals the submerged unit weight of the soil:
Upward seepage force per unit volume = Submerged unit weight of soil
\(i \cdot \gamma_w = \gamma'\)
The hydraulic gradient at which this occurs is called the critical hydraulic gradient (\(i_c\)):
\(i_c = \frac{\gamma'}{\gamma_w}\)
When the upward hydraulic gradient reaches the critical hydraulic gradient, the effective stress becomes zero. The soil loses its strength and behaves like a liquid, a condition known as quicksand or boiling.
Analyzing the Given Options
Let's evaluate the provided options based on this understanding:
- Option 1: The void ratio of the soil becomes 1.0. The void ratio is a soil property, not the cause of quicksand. While soil structure is important, quicksand is triggered by seepage forces overcoming the soil's submerged weight, not by a specific void ratio value.
- Option 2: The upward seepage pressure in soil becomes zero. If the upward seepage pressure is zero, there is no upward flow or upward seepage force, and the effective stress would be positive (assuming saturated conditions), preventing quicksand.
- Option 3: The upward seepage pressure in soil becomes equal to the saturated unit weight of the solid. This phrasing is unconventional. The quicksand condition relates seepage pressure to the submerged unit weight of the soil mass, not the saturated unit weight of the solid particles.
- Option 4: The upward seepage pressure in the soil becomes equal to the submerged unit weight of the soil. This statement accurately describes the condition where the upward seepage force per unit volume (\(i \cdot \gamma_w\), often referred to in terms of pressure or force balance per unit volume) balances the downward submerged weight per unit volume (\(\gamma'\)), leading to zero effective stress and the quicksand condition.
Therefore, quicksand occurs when the upward seepage pressure (more precisely, the upward seepage force per unit volume) becomes equal to the submerged unit weight of the soil.
| Parameter | Symbol | Description |
|---|---|---|
| Effective Stress | \(\sigma'\) | Stress carried by the soil skeleton |
| Total Stress | \(\sigma\) | Total downward pressure |
| Pore Water Pressure | \(u\) | Pressure of water in soil voids |
| Submerged Unit Weight | \(\gamma'\) | Unit weight of soil minus unit weight of water (\(\gamma_{sat} - \gamma_w\)) |
| Unit Weight of Water | \(\gamma_w\) | Unit weight of water |
| Hydraulic Gradient | \(i\) | Change in head per unit length of flow |
When \(\sigma' = 0\), the soil loses its shear strength and behaves like a fluid. This state is known as the quicksand condition.
Revision Table: Key Concepts for Quicksand Condition
| Concept | Explanation | Relevance to Quicksand |
|---|---|---|
| Effective Stress | \(\sigma' = \sigma - u\). Governs soil strength. | Quicksand occurs when \(\sigma' = 0\). |
| Upward Seepage | Flow of water upwards through soil. | Increases pore water pressure (\(u\)), reducing \(\sigma'\). |
| Seepage Pressure/Force | Upward force exerted by flowing water on soil particles. | Opposes the submerged weight of soil. |
| Submerged Unit Weight (\(\gamma'\)) | Weight of soil solids minus buoyancy force, per unit volume. | Represents the downward weight of soil skeleton that the upward seepage force must balance to cause quicksand. |
| Critical Hydraulic Gradient (\(i_c\)) | Hydraulic gradient at which quicksand occurs (\(i_c = \gamma' / \gamma_w\)). | Direct measure of the flow intensity needed for quicksand. |
Additional Information on Soil Fluidization
The quicksand condition is a specific type of soil fluidization that occurs due to upward seepage. It commonly happens near retaining structures like sheet piles or cofferdams where there's a head difference causing upward flow.
While often associated with sand, quicksand can occur in any granular soil where sufficient upward seepage is present and the effective stress drops to zero. It's important to note that quicksand is not a type of soil, but a condition of a granular soil.
Preventing quicksand involves reducing the upward hydraulic gradient, for example, by lowering the water level outside the excavation, increasing the flow path length, or using filters.
Important Questions from Shear Strength
Field vane shear is the appropriate field test for obtaining the shear strength of which of the following?
Which of following tests is conducted to assess shear strength parameter of the soil.
Assertion (A): In the box shear test, the failure plane is predetermined and horizontal.
Reason (R): The shear stress is applied in the vertical direction.
Select the correct answer from the following:
In a direct shear test, the soil load is subjected to more stress at the _______.
Read the following statements and choose the CORRECT answer.
(i)The unconfined compression test is widely used for determining the consistency of saturated clays and other cohesive soils.
(ii) Unconfined compressive strength is thrice the value of the shear strength of clay soil under drained conditions.
