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

Explanation:

A fully compensated raft foundation (also called buoyant raft) is designed so that the weight of the soil excavated for the foundation equals the weight of the building and the foundation combined.

• This compensation reduces the net increase in stress on the soil, minimizing settlement.

• It is especially useful in soft or compressible soils where minimizing settlement is critical.

• The idea is that the removal of soil weight balances the load, so the foundation does not cause additional stress on the soil below.

Additional InformationRaft Foundation (or mat foundation) is a large, continuous slab supporting multiple columns and walls, spreading the load over a wide area. It is used when soil bearing capacity is low or loads are heavy.

Types of Raft Foundations

• Rigid Raft: Assumes the raft acts as a stiff plate, distributing loads uniformly.

• Flexible Raft: Assumes the raft bends and deflects under loads, distributing unevenly.

• Fully Compensated Raft: Designed so that the weight of the soil excavated equals the weight of the building and foundation.

Fully Compensated Raft Foundation

• Also known as buoyant foundation or compensated foundation.

• The purpose is to neutralize the additional load on the soil by removing an equivalent weight of soil during excavation.

• Reduces settlement by minimizing changes in stress on the underlying soil.

• Commonly used in soft, compressible soils or areas with high water tables.

• Helps prevent excessive consolidation and uneven settlement.

Explanation:

• Grip length refers to the embedment depth of the well below the maximum scour level.

• It is the depth provided to ensure stability of the well foundation against:

  • Overturning

  • Sliding

  • Uplift

  • Lateral loads (especially in bridge foundations)

• Scour is the removal of soil around a foundation due to flowing water.

• The maximum scour level represents the lowest level the riverbed may reach due to extreme flow conditions.

• The well must penetrate below this level to remain stable even during floods.

 Additional Information

Well Foundation: A deep foundation type used mostly for heavy loads, especially for bridges, piers, and waterfront structures. It consists of a large-diameter hollow cylindrical well sunk into the ground to a suitable depth.

• The cutting edge is the sharp bottom edge of the well, designed to cut through soil or rock during sinking.

​Grip length is important because:

• It provides skin friction between the well’s cylindrical surface and the surrounding soil, enhancing stability.

• It resists uplift and lateral loads acting on the foundation.

• Ensures that the well foundation remains stable under various load conditions, including scour and soil movement.

Sinking process: The well is gradually sunk by excavating soil inside the well or by adding weights, and the cutting edge helps penetrate the soil or rock below.

Design considerations include:

• Grip length should be sufficient to provide adequate friction and prevent sliding.

• The length also affects the settlement and bearing capacity of the foundation.

• Must consider scour depth if located in water bodies; foundation depth should be below maximum scour level to prevent exposure.

Explanation:

As per IS 1080:1985 – Code of Practice for Design and Construction of Shallow Foundations in Soils, the minimum depth of foundation depends on the type of soil to ensure stability against settlement, loss of bearing capacity, and seasonal moisture variation.

For Sand (800 mm minimum):

• Sand is a non-cohesive soil that is sensitive to disturbances and has low water retention.

• A minimum depth of 800 mm is recommended to ensure the foundation lies below the zone affected by surface weathering, erosion, and seasonal moisture fluctuation.

• This also helps mobilize sufficient bearing capacity from the deeper layers.

For Clay (900 mm minimum):

• Clay is a cohesive soil that is highly sensitive to shrinkage and swelling due to moisture content changes.

• A foundation placed at least 900 mm deep ensures it is below the active zone of seasonal moisture movement.

• This minimizes the risk of settlement or cracking due to volume changes in the clay.

 Additional Information

• The depth of foundation is the vertical distance between the ground surface and the base of the foundation.

• It must be adequate to:

  • Transfer loads safely to the ground.

  • Prevent damage from soil movement, frost, or erosion.

  • Avoid bearing failure and differential settlement.

These minimum depths are general guidelines, and actual design may require greater depths depending on load, local conditions, and soil investigation reports

Explanation:

• A cantilever sheet pile is a retaining structure that resists earth pressure without the use of anchors. Its stability comes from:

• The lateral resistance of soil on the embedded (passive) side of the pile.

• The sheet pile bends due to the lateral earth pressure, and the soil in front of the lower embedded portion provides the necessary passive resistance to balance the moment and force.

 Additional Information

• A cantilever sheet pile is a type of retaining wall made of interlocked steel sheet piles driven vertically into the ground.

• It does not use anchors or braces for support.

• Stability is achieved by embedding the sheet pile sufficiently deep into the soil so that the passive earth pressure on the embedded portion balances the active pressure from the retained soil.

• The soil on the embedded side provides lateral resistance, preventing overturning and sliding.

• Cantilever sheet piles are commonly used for shallow retaining structures or where anchoring is not feasible.

• For larger heights or loads, anchored sheet piles or other support systems are preferred.

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.

Ultimate bearing capacity for a strip footing is

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

qu = (π + 2)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, 

Ultimate bearing capacity of strip footing, 

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

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

Bearing Capacity Calculation for Sandy Soils

Settlement of plate in clayey soil:

Settlement of plate in sandy soil:

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:

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

N₀ - SPT value after overburden correction

Calculation:

Given: N₀ = 21

⇒ 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: 

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

Calculation:

Given:

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