Effective and Neutral Stress MCQ Quiz - Objective Question with Answer for Effective and Neutral Stress - Download Free PDF

Explanation:

• Effective stress controls the shear strength of soil by determining the resistance offered by soil particles to sliding. As effective stress increases, the interparticle contact force increases, improving the soil’s ability to resist shear. This relationship is defined by the Mohr-Coulomb strength criterion.
• Compressibility of soil is directly influenced by effective stress, as it governs how much the soil consolidates under load. When effective stress increases, voids between soil particles decrease, leading to settlement. This is particularly important in clayey soils under long-term loading.
• Permeability is indirectly affected by effective stress, especially in fine-grained soils. As effective stress increases, the soil becomes denser, reducing the size of pore spaces. Smaller pore spaces lower the rate at which water can flow through the soil.
• Volume changes in soils under loading or unloading are dependent on changes in effective stress. Higher effective stress compresses the soil, while a decrease can lead to expansion or swelling. This behavior must be considered in foundation and earthwork designs.
• In saturated conditions, total stress is divided between pore water pressure and effective stress. Only the effective portion contributes to soil stiffness and strength. Understanding this is critical in analyzing submerged or water-logged soils.
• All essential soil behavior—strength, settlement, and permeability—is governed by effective stress rather than total stress. Engineers rely on effective stress to predict soil response under different loading conditions. It is the central concept in soil mechanics.

Additional InformationShear Strength

• Shear strength depends directly on effective stress because only the soil grains (not pore water) resist shearing. As effective stress increases, the soil structure becomes more stable and resistant to failure.

• In saturated soils, a rise in pore water pressure reduces effective stress, leading to a decrease in shear strength, which is critical in slope failures and bearing capacity analysis.

Compressibility

• Compressibility refers to the reduction in soil volume under applied stress and is controlled by effective stress. When effective stress increases, soil particles are pressed closer together, reducing voids and causing settlement.

• In fine-grained soils like clay, compressibility is high and time-dependent due to low permeability, which delays drainage and extends consolidation time.

Permeability

• Permeability is affected indirectly by effective stress, as higher effective stress compacts the soil, reducing pore size and thus the ease of water flow.

• This effect is more prominent in cohesive soils, where small changes in structure from increased effective stress can significantly reduce permeability.

Explanation:

Effective stress: 

• Effective stress can be defined as the stress that keeps particles together. In soil, it is the combined effect of pore water pressure and total stress that keeps it together.
• The effective stress in a soil mass not subjected to external loads is computed from the unit weights of soil and water, and the depth of groundwater table.
• Effective stress(\(\overline\sigma\)) = Total Stress(\(\sigma\)) - Pore water pressure(\(u\))
• Due to rise in water table pore water pressure increases, which in turn decreases the effective stress.

• For no flow condition, the total stress , Pore water pressure and effective stress diagram is shown below:

​

Explanation:

Quicksand Condition – Explanation

• Quicksand is a phenomenon in cohesionless soil, where the soil loses its shear strength due to upward water flow.

• This condition occurs when water pressure increases, causing the soil particles to lose contact and behave like a fluid.

• The upward flow of water creates a quick condition, leading to instability and lack of soil strength.

• This phenomenon is commonly observed in foundations or excavations where water is flowing upward.

• Quicksand is not a specific type of soil, but rather a condition affecting the soil's behavior.

• It is different from sand acting as a filter, uniformly graded sand, or conditions focusing on water flow through soil.

• The correct answer is option 3: "The condition when cohesionless soil loses its shear strength due to upward flow water."

Concept:

Critical hydraulic gradient (ic): The quicksand / boiling condition occurs at a critical upward hydraulic gradient typically around 1.0 for many soils, when the seepage force just balances the buoyant weight of an element of soil.

At the critical conditions, the effective stress is equal to zero.

\({i_c} = \frac{{G - 1}}{{1 + e}}\;{\rm{OR\;}}\left( {{\rm{G}} - 1} \right) \times \left( {1 - {\rm{n}}} \right)\)

Calculations:

Critical hydraulic gradient

 n = 40%, G = 2.5

 G - 1 = 1.5

\(\begin{array}{*{20}{c}} {{i_c} = \left( {{\rm{G}} - 1} \right) \times \left( {1 - {\rm{n}}} \right) = 1.7 \times \left( {1 - {0.40}{}} \right) = 1.02} \end{array}\)

Explanation:

Effect on Effective Stress During Downward Flow Through Soil

When water flows downward through a soil mass, it exerts a force that reduces the effective stress due to the upward drag of pore water. The effect is explained using effective stress principles.

Concept of Effective Stress (Terzaghi's Principle)

The total stress (σ) in soil is given by:
$$\sigma =\gamma H$$

where H is the depth of the soil, and γ is the unit weight of soil.

The effective stress (σ') is given by:
σ′=σ−u

where u is the pore water pressure.

Effect of Downward Flow on Effective Stress

1. Seepage Forces in Downward Flow

   • When water flows downward, it increases the pore water pressure.

   • The seepage force acts in the direction of gravity, reducing the effective stress.

2. Mathematical Representation
   The effective stress equation, considering the hydraulic gradient (i), becomes:
   σ′= γ_sat × H − (u−iHγ_w​) = γ_sub ×  H +  iHγ_w

_​So, effective stress is increased by  iHγw in case to downward seepage

1. where γ_w is the unit weight of water

Concept:

The effective stress concept was developed by "Terzaghi". A fully saturated soil relates three types of stress:

1. Total Stress
2. Pore Pressure( Neutral Stress)
3. Effective Stress

Total Stress(\(\sigma\)) on a plane within soil mass is the force per unit area of soil mass transmitted in the normal direction across a plane.

Pore pressure(u) is the pressure of the water filling the void space between solid particles.

Effective stress(\(\bar \sigma\)) is defined as equal to the total stress minus the neutral pressure.

The relation between total stress, pore pressure, and effective stress is as follows:

\(\bar \sigma = \sigma -u\)

where \(\bar \sigma\)= effective stress, \(\sigma\)= total stress, u= pore pressure

Explanation:

Given:

γ_w = 10 kN/m³, γsat = 20 kN/m³, γ_b = 18 kN/m3

Effective stress at a depth of 10 m below the ground level:

γ_eff = γ_b × 3 + γ_sub × 7

γeff = 18 × 3 + (20 - 10) × 7

γeff = 124 kN/m³

Explanation:

In the process of loading, initially, as we increase the load the pore water pressure increases, due to which pore water escapes out from the voids of the soil.

Hence, the pore water pressure in the soil sample of the consolidometer test is maximum at the center and when the primary consolidation is completed, which leads to the expulsion of pore water from the voids of the soil stops then the excess pore water process developed due to the application of load reduced to zero.

The effective stress principle was given by Terzaghi. It is the physical interaction between water and soil solids.

This concept was assumed to be applied to fully saturated soil and relates 3 types of stresses. i.e. total stress, Pore water pressure and effective stress.

Mathematically, σ̅ = σ - U

• The effective stress diagram of soil for submerged and saturated soil is different because the buoyancy effect is taken into account in the case of submerged soil.
• In submerged soil, the pore water pressure is equal to the hydrostatic pressure, which means that the effective stress is reduced due to the upward force of the water.
• Hence the effective stress in the zone of submerged soil will become 0

Concept:

For rigid footing:

When a rigid footing rests on a soil then the settlement will be uniform, but the contact pressure will be variable, which depends upon the type of soil underneath it.

For different soil the variation will be:

For flexible footing:

In case of flexible footing contact pressure will be uniform for all type of soil but the settlement will be variable.

Explanation:

Capillarity in soils:

Soil water can exist in three zones as shown below:

(i) Zone-I (Submerged zone): This zone exists below the groundwater table. The soil in this zone is in submerged condition and pore pressure is hydrostatic i.e. positive.

(ii) Zone-II (Capillarity Zone):

• If gravity was the only force, acting on the percolating water and taking it downward, then the soil above the water table would be completely dry. But it is not so in actual practice.
• Soil in this zone is completely saturated upto some height above the water table. This phenomenon of rising water in soil is known as capillarity in soils.
• Capillary water rises against gravity and is held by the surface tension. Therefore the capillary water exerts a tensile force on soil and the resulting negative pressure of water (capillary pressure) creates attraction between the particles.

(iii) Zone-III (Zone of saturation):

• Above the capillarity zone and upto a certain height, there exists a zone called the zone of aeration, in which the soil is able to retain small water droplets, surrounded on all sides by air.

Concept:

Relationship between total stress, effective stress, and pore water pressure of lake water:

Effective stress is given by,

σ̅  = s - u

Where, σ = Total stress and u = pore water pressure

Calculation:

Given,

h = 5 m, γ_sat = 20 kN/m³ and γ_w = 9.81 kN/m³

Total stress (s) = γ_sat × h = 20 × 5 = 100 kN/m²

Pore water pressure (u) = γ_w × h = 9.81 × 5 = 49.05 kN/m²

Effective stress, σ̅  = s - u = 100 - 49.05 = 50.95 kN/m²

σ̅  ≈ 51 kN/m²

Concept:

A pore pressure parameter greater than one is associated with a loose structure, either in sand or clay, which collapses upon load application.

On the other hand, if we have a sample of sand or clay which tends to expand upon loading, negative pore pressure can be developed.

If a sample of dense sand is subjected to shear, it tends to expand. This is due to the soil particles trying to ride on each other during sliding. The expansion under shear is termed as “Dilatancy”.

In case of saturated dense sands if there is sudden application of stress in the field, the soil does not get time to expand and negative pore water pressure develop.

In case of loose sands, they try to compress by bringing their grains closer together. This results in reduced effective stress and reduction in corresponding shear strength.

If fine sands in the saturated condition are subjected to sudden loading, the tendency towards volume reduction causes pore water pressure to increase instantaneously. This causes a large decrease in the shear resistance. 

So dense sand and over-consolidated clay would anticipate negative pore pressure when subjected to shearing.

Explanation:

Effective stress is the ratio of force at the contact of particles of soil to the total area. It cannot be obtained practically but we can calculate the effective stress by measuring total stress and pore water pressure as:

Total stress (σ) = effective stress (σ’) + pore water pressure (u)

Effective stress (σ') = Total stress (σ) - pore water pressure (u)

If we increase the ground water table then value of pore water pressure increases and effective stress decreases.

If we lowering the groundwater table below ground then the value of pore water pressure decreases and effective stress increases. 

Concept:

Effective Stress (σ’): The pressure transmitted by the grain to grain contact between soil particles.

Pore water pressure (or Neutral Pressure) (u): The pressure transmitted by the water molecule to water molecule which are present in pores. As this pore water pressure does not have any measurable influence on mechanical soil property it is also called Neutral Pressure. Normally saturated pore water exerts positive pore water pressure whereas capillary water exerts negative pore water pressure.

Total Stress (σ): Effective Stress (σ’) + Pore water pressure (u).

Note: When Soil is completely saturated by capillarity then pore water pressure is negative

And vary from zero at water table to γw× z at any depth z from water table.

Effective stress (σ ‘) = Total stress (σ) – pore pressure (u)

 Say pore pressure at pt A = - X

So Effective Stress = Total Stress - ( - X)

Effective Stress = Total Stress +  X

The effective stress diagram for given diagram is, 

Concept:

Piezometric head (H) = Pressure head (h’) + datum head (z)

At critical condition,

Uplift pressure = downward pressure

\({\gamma _w}h' = {\gamma _w}Gt + h{\gamma _w}\)

Calculation:

Given,

H = 10 m, G = 2.5

Standing water depth above the floor (h) = 2 m

h’ = 10 - 3 = 7 m

\({\gamma _w}h' = {\gamma _w}Gt + h{\gamma _w}\)

\(\begin{array}{l} {\gamma _w}h' = {\gamma _w}Gt + h{\gamma _w}\\ 7{\gamma _w} = 2.5{\gamma _w} \times t + 2{\gamma _w}\\ t = \frac{5}{{2.5}} = 2m \end{array}\)

∵ t is the minimum thickness required. Hence answer will be 2 m.