Bearing Capacity MCQ Quiz - Objective Question with Answer for Bearing Capacity - Download Free PDF

Explanation:

1. Black cotton soil (also known as expansive soil) is least suitable for shallow foundations because it has high shrinkage and swelling characteristics, especially when exposed to moisture.
2. These soils expand when wet and shrink when dry, causing instability in foundations and leading to cracks or shifting.

 Additional Information

1. Silty clay: While silty clay has low bearing capacity, it does not exhibit the same expansive behavior as black cotton soil, making it relatively more stable for shallow foundations.

2. Fine sand: Fine sand has good drainage properties, but its bearing capacity may be lower than coarser sands. However, it is generally suitable for shallow foundations if properly compacted.

3. Coarse sand: Coarse sand is suitable for shallow foundations as it has good load-bearing capacity and excellent drainage properties.

Explanation:

1. A pile cap is a large concrete slab placed on top of a group of piles in a deep foundation system.
2. Its primary function is to distribute the loads from the superstructure evenly across all the piles.
3. This helps in achieving a uniform load transfer and ensures the piles work together as a collective unit, reducing the likelihood of uneven settlement.

 Additional Information

Pile Foundation

1. Load Distribution: The pile cap ensures that the load from the structure is properly distributed across multiple piles, preventing excessive loading on any single pile.

2. Foundation Stability: It helps maintain the overall stability of the foundation by managing the transfer of both vertical and horizontal loads.

3. Pile Grouping: Pile caps are used in pile groups where several piles are arranged close together to support a structure, especially when the bearing capacity of the soil at shallow depths is insufficient to carry the load.

4. Construction: Typically, pile caps are made from reinforced concrete and may be designed with additional steel reinforcement based on the load requirements.

5. No Impact on Aesthetic Appearance: The pile cap's primary function is structural, and while it may be covered or concealed depending on the design, it is not intended to improve the aesthetic appearance of the building.

Explanation:

1. A raft foundation is a large slab that supports the building and spreads the load across the entire foundation area. This helps in evenly distributing the weight across the soil.

2. It is particularly useful when the soil is weak or highly variable in terms of load-bearing capacity, as it minimizes differential settlement.

3. By spreading the load, it ensures uniform settlement, reducing the risk of cracks or structural issues in the building.

Additional InformationPile Foundations:

1. Deep Foundation
   A pile foundation is a type of deep foundation that transfers the load of a structure to deeper, more stable soil or bedrock, often used when the surface soil is not strong enough to support the load.

2. Types of Piles
   Piles can be pre-cast concrete, steel, or timber and can either be driven (hammered into the ground) or bored (drilled and filled with concrete).

3. Used in Weak Soils
   Pile foundations are commonly used in areas with weak or compressible surface soils, such as swamps, soft clay, or loose sands, to provide stability to buildings and structures.

4. Load Distribution
   Piles transmit the load of the structure through weak soil layers and into stronger soil or rock beneath. This helps in resisting both vertical loads and lateral forces such as wind or seismic activity.

5. Types of Pile Foundations
   There are different types of pile foundations based on the method of installation: friction piles (rely on friction between the pile surface and surrounding soil) and end-bearing piles (transfer load to a firm rock or soil layer at the pile tip).

Explanation:

1. Settlement refers to the vertical downward movement of a structure's base, which occurs due to the compression of the soil beneath the foundation or due to the weight of the structure itself.

2. This is a natural response to loading and can vary depending on the soil type, moisture content, and other environmental factors.

Additional Information

1. Deformation: A general term for the change in shape or size of a material or structure due to applied forces, but not specifically referring to the downward movement of a foundation.

2. Deflection: Refers to the displacement or bending of structural elements under load, typically in beams or slabs, rather than the movement of the base.

3. Depression: Refers to a general lowering or sinking, but it is not commonly used in structural engineering to describe the movement of the foundation.

Explanation:

1. Settlement of foundations due to water level fluctuation is primarily caused by seasonal swelling and shrinking of expansive soils.
2. These soils, such as clay, change their volume with moisture content.
3. When the water level fluctuates, it can cause the soil to either expand (when the water content increases) or shrink (when the water content decreases), leading to differential settlement in foundations.

Additional InformationTypes of Settlement:

1. Uniform Settlement:

   • Occurs when the foundation settles uniformly across its entire area.

   • Usually occurs when the soil beneath the foundation is homogenous and the loading is evenly distributed.

   • This type of settlement generally does not cause major issues unless it results in excessive deformation.

2. Differential Settlement:

   • Occurs when different parts of the foundation settle by different amounts.

   • Often caused by variations in soil properties, uneven loading, or water fluctuations in the soil.

   • Differential settlement can cause structural damage, such as cracks in walls or misalignment of structural elements.

3. Immediate Settlement:

   • Occurs quickly after the load is applied, usually due to the compression of loose or soft soils.

   • This is typically a small, quick settlement that happens as the soil adjusts to the load.

4. Consolidation Settlement:

   • Takes place over a longer period as the soil particles compress due to the weight of the structure.

   • Common in fine-grained soils, like clay, which experience a gradual reduction in volume when water is expelled under load.

Ultimate bearing capacity for a strip footing is

\({q_u} = C{N_c} + \gamma {D_f}{N_q} + 0.5\gamma B{N_\gamma }\)

For pure clay, N_c = 5.14, N_q = 1 and Nγ = 0 (∵ assuming smooth footing)

Footing is on the ground surface i.e. D = 0

q_u = c_uN_c

qu = 5.14 c_u

Explanation:

Black cotton soil is an expansive soil having clay mineral (montmorillonite) which is responsible for the excessive swelling and shrinkage characteristics of the soil.

Note:

To construct the structure under this type of soil, an under-reamed pile is been provided. These piles are taken to depths below the zone of seasonal variation in moisture content.

Under-reamed pile

• An under-reamed pile is a special type of bored pile which is provided with a bulb at the end.
• The usual size of such piles is 150 to 200 mm shaft diameter and 3 to 4 m long.

Concept:

According to Tarzaghi,

Ultimate bearing capacity of circular footing, 

\({q_u} = 1.3C{N_c} + q{N_q} + 0.3Dγ {N_γ }\)

Ultimate bearing capacity of strip footing, 

\({q_u} = C{N_c} + q{N_q} + 0.5Bγ {N_γ }\)

For square footing, ultimate bearing capacity,

qu = 1.3 CNc + γDfNq + 0.4 γBNγ

Where,

C = cohesion 

Nc, Nq, Nγ = Bearing capacity factors

q = overburden pressure = γDf

Df = depth of the footing, B = width of footing, D = diameter of circular footing

γ = unit weight of soil

Calculation:

Given,

D = B

Surface footing ⇒ Df = 0 ⇒ q = 0

Purely cohesionless ⇒ C = 0

\(Ratio = \frac{{{q_{u,circular}}}}{{{q_{u,strip}}}} = \frac{{0.3D\gamma {N_\gamma }}}{{0.5B\gamma {N_\gamma }}} = 0.60\)

Concept:

Plate Load Test:

It is a field test to determine the ultimate bearing capacity of soil and the portable settlement under a given loading.

Bearing Capacity Calculation for Clayey Soils

\(Ultimate\: bearing\: capacity\:[q_u(f)] = Ultimate\: load\: for\: plate\:[q_u(p)]\)

Bearing Capacity Calculation for Sandy Soils

\(Ultimate\: bearing\: capacity\:[q_u(f)] = {Width\: of\: pit (Bf) \over Size\: of\: Plate (Bp)}\times q_u(p)\)

Settlement of plate in clayey soil:

\(​\dfrac{S_P}{S_F} = \dfrac{B_P}{B_F}\)

Settlement of plate in sandy soil:

\(\dfrac{S_P}{S_F} = \left(\dfrac{B_P(B_F + 0.3)}{B_F(B_P + 0.3)}\right)^2\)

Where

Sf = settlement of foundation

Sp = settlement of plate

Bf = width of footing/foundation

Bp = width of plate

Calculation:

Given data,

Width of plate(Bp) = 35 cm, S_P = 2 cm

Width of footing(Bf) = 85 cm

Settlement of footing(S_F) in clayey soil:

\(​\dfrac{S_P}{S_F} = \dfrac{B_P}{B_F}\)

\(​\dfrac{2}{S_F} = \dfrac{35}{85}\)

S_F = 4.85 cm

Concept;

Dilatancy correction:

It is to be applied when N_o obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when N_o > 15) using the equation

\(\rm N= 15 + \frac{1}{2}\left( {N_0-\;15} \right)\)

N₀ - SPT value after overburden correction

Calculation:

Given: N₀ = 21

\(\rm N= 15 + \frac{1}{2}\left( {21-\;15} \right)\)

⇒ N = 18

SPT test can be conducted to determine:

a) Relative Density of sands

b) Angle of internal friction

c) Unconfined compressive strength of clays

d) Ultimate bearing capacity on the basis of shear criteria

e) Allowable bearing pressure on the basis of settlement criteria

In this test, the split spoon sampler is driven by dynamic mechanism of hammer. This test is conducted either at every 2 to 5 meter interval or at the change of stratum.

Note:

The weight of the hammer is 63.5 kg.

The height of free fall is 750 mm or 75 cm.

The inner and outer diameter of the sampler is 35 mm and 50.5 mm respectively.

Explanation:

SPT test can be conducted to determine:

a) Relative Density of sands

b) Angle of internal friction

c) Unconfined compressive strength of clays

d) Ultimate bearing capacity on the basis of shear criteria

e) Allowable bearing pressure on the basis of settlement criteria

In this test, the split spoon sampler is driven by dynamic mechanism of hammer. This test is conducted either at every 2 to 5 meter interval or at the change of stratum.

Note:

The weight of the hammer is 63.5 kg.

The height of free fall is 750 mm or 75 cm.

The inner and outer diameter of the sampler is 35 mm and 50.5 mm respectively.

Concept:

A. True: The proportioning of a footing is typically governed by its bearing capacity. Bearing capacity refers to the ability of the soil or rock to support the loads applied to the ground. Ensuring that the footing can adequately distribute the load to prevent excessive settlement or failure is crucial in foundation design.

B. True: Friction piles are often referred to as 'Floating piles.' Unlike end-bearing piles, which transfer loads to a strong soil or rock layer deep below, friction piles transfer load to the surrounding soil along their length through skin friction. Since they don't rely on a firm layer at the bottom and derive their support from the soil along their sides, they are sometimes called floating piles.

Both statements are true.

Explanation:

Terzaghi's Bearing Capacity Theory:

Bearing capacity of soil: 

\({q_u} = C{N_c} + γ {D_f}{N_q} + 0.5γ B{N_γ }\)

Where,

C = Cohesion, D_f = Depth of footing, B = Width of footing, γ = unit weight of footing, N_c, N_q, N_γ = Bearing capacity factors

Assumptions:

1. The soil is homogeneous and isotropic and its shear strength is represented by Coulomb's equation.
2. The strip footing has a rough base, and the problem is essentially two-dimensional.
3. The elastic zone has straight boundaries inclined at ψ = ϕ to the horizontal, and the plastic zones fully develop.,
4. Pp consists of three components, which can be calculated separately and added, although the critical surface for these components is not identical.
5. Failure zones do not extend above the horizontal plane through the base of the footing, i.e. the shear resistance of soil above the base is neglected and the effect of soil around the footing is considered equivalent to a surcharge σ = γ × D [γ = unit weight of soil, D = Depth of foundation

Concept:

The area of footing (A_f) is given by

\(\rm A_f = \rm\frac{{Total~ load}}{{{Safe ~bearing ~capacity}}}\)

Calculation:

Given:

Total load = 1750 kN, Safe bearing capacity = 200 kN/m²

\(\rm A_f = \rm\frac{{Total~ load}}{{{Safe ~bearing ~capacity}}}\)

\(A_f = \frac{{1760}}{{200}} = 8.8\;{m^2} \approx9 m^2\)