Bearing Capacity MCQ Quiz in తెలుగు - Objective Question with Answer for Bearing Capacity - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Explanation

Dilatancy correction:

It is to be applied when N’ 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’ > 15) using the equation

Given,

N value = 35

Corrected  value for dilatancy is 

N'' = 15 + 10 = 25

∴ Corrected value N'' = 25 

Explanation

Given,

 Width of footing = 5 m

 And the water table is 20 m below the ground surface

In the given zone Water table is below the base of footing at distance b, from the base, correction factor is given by 

But we know that the water correction factor cannot be greater than 1. So, the correction factor for water table will be 1

Concept:

Plate load test: 

• The plate load test is a field test, which is performed to determine the ultimate bearing capacity of the soil and the probable settlement under a given load.
• This test is very popular for the selection and design of the shallow foundation.

Procedure:

• Excavate test pit up to the desired depth. The pit size should be at least 5 times the size of the test plate (Bp).
• At the center of the pit, a small hole or depression is created. The size of the hole is the same as the size of the steel plate.
• A mild steel plate is used as a load-bearing plate whose thickness should be at least 25 mm thickness and size may vary from 300 mm to 750 mm. The plate can be square or circular. Generally, a square plate is used for square footing and a circular plate is used for circular footing.
• A column is placed at the center of the plate. The load is transferred to the plate through the centrally placed column.
• The load can be transferred to the column either by the gravity loading method or by the truss method.
• At least two dial gauges should be placed at diagonal corners of the plate to record the settlement. The gauges are placed on a platform so that it does not settle with the plate.
• Apply seating load of .7 T/m2 and release before the actual loading starts.
• The initial readings are noted.
• The load is then applied through the hydraulic jack and increased gradually. The increment is generally one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity or any other smaller value. The applied load is noted from the pressure gauge.
• The settlement is observed for each increment and from the dial gauge. After increasing the load-settlement should be observed after 1, 4, 10, 20, 40, and 60 minutes and then at hourly intervals until the rate of settlement is less than .02 mm per min. The readings are noted in tabular form.
• After completing the collection of data for a particular loading, the next load increment is applied and readings are noted under new load. This increment and data collection is repeated until the maximum load is applied. The maximum load is generally 1.5 times the expected ultimate load or 3 times of the expected allowable bearing pressure.

Concept:

Rankine's formula provides guidance on the minimum depth of foundation based on the bearing capacity of the soil.

​

Therefore the depth of the foundation can be expressed as,

​

Where Df = Minimum depth of foundation, qu = Ultimate bearing capacity of the soil

Calculation:

Intensity of loading (qu) =180 kN/m2,

Unit weight = 20 kN/m3 and angle of internal friction (ϕ) = 30°

Depth of foundation = 

Depth of foundation (Df) = 1.0 m

Hence, According to Rankine's formula, the minimum depth of foundation is 1.0 m.

Explanation:

NOTE: We have to find the incorrect statement from the given statements.

Standard Penetration Test: Standard penetration test can be used to determine:

• Relative density of sand
• Angle of internal friction (ϕ)
• Unconfined compressive strength of clay
• Ultimate bearing capacity on the basis of shear criteria
• Allowable bearing pressure on the basis of settlement criteria
• Load carrying capacity of pile
   

For standard penetration test:

(i) It is performed as a guideline in IS: 2131-1981 in the clean hole of 55 to 150 mm in diameter.

(ii) A thick wall split-tube sampler, 50.8 mm outer diameter and 35 mm inner diameter, is driven into the undisturbed soil at the bottom of the hole under the blows of a 65 kg drive weight with 75 cm free fall.

(iii) The minimum open length of the sampler should be 60 cm. It is first driven through 15 cm as a seating drive and is further driven through 30 cm or until 100 blows have been applied.

(iv) The number of blows required to drive the sampler 30 cm beyond the seating drive, is termed the penetration resistance N or SPT number.

The final SPT number is related to friction angle and relative density. which is given in the table

Concept:

Coorection in SPT

Observed SPT = No

First-I: SPT no. after correction for overburden

N1 = C1 N0

Second-II: Dilatancy correction

SPT after correction for water table

As water table is above the test level and SPT Number after corrected for overburden is greater than 15. So this correction is required

Calculation:

Given data,

N₁ = 25

Dilatancy correction

 = 20 

Hence, The corrected value of N is 20.

Concept:

Variation of contact pressure and settlement of footing on different types of soil is as follows:

1. Flexible footing:

Case-I: Resting on Sand

• Settlement: Maximum at edges and minimum at the center
• Contact Pressure: Uniform

Case-II: Resting on Clay

• Settlement: Minimum at edges and maximum at the center

• Contact Pressure: Uniform

  ​

2. Rigid footing:

Case-I: Resting on Sand

• Settlement: Uniform

• Contact Pressure: Zero at edges and maximum at the center

Case-II: Resting on Clay

• Settlement: Uniform

• Contact Pressure: maximum at edges and minimum at the center

Explanation:

Foundation Contact Pressure: Generally loads from the structure are transferred to the soil through the footing. As a reaction to this load, soil exerts upward pressure on the bottom surface of the footing which is termed as contact pressure.

On different types of footing:

1) Under flexible footing: For flexible footing on cohesive soil, the settlement is maximum at the center of footing and minimum at the edges which form a bowl-like shape as shown in the below figure.

When a flexible footing is laid on the Cohesionless soil, the settlement at the center becomes minimum while at edges it is maximum which exact opposite case of the settlement of flexible footing over cohesive soil.

2). Under Rigid footing: For rigid footings resting on cohesive soils, the settlement is uniform but contact pressure varies. At edges contact pressure is maximum and at the center, it is minimum which forms an inverted bowl shape as shown in the below figure.

If the rigid footing is resting on Cohesionless soils, the contact pressure is maximum at the center and gradually reduces to zero towards edges. The settlement is uniform in this case also.

Important Points

The distribution of contact pressure under different types of footings on different types of soils are given below:

Concept:

Cyclic pile load test method is used in the case of the initial test to separate skin frictional resistance and point bearing load separately for a single pile of uniform diameter.

Alternate loading and unloading shall be carried out at each stage. Each loading and unloading stage shall be maintained for at least 15 min. The subsequent elastic rebound in the pile should be measured accurately by dial gauges or LVDTs.

From the graph, we can find Point resistance (R_p) and Frictional resistance (R_f)

Point Resistance will be represented by a straight line.

Additional Information

Different types of tests and their purposes conducted on soil

Explanation:

Confusion Points

For cohesive soil like clay, the net BC is 0, as the value of Nq is 1 and the term Nq - 1 becomes 0, but the question is asking about bearing capacity, which means ultimate BC, not net BC. So, statement 3rd is false

The ultimate bearing capacity of strip footing is given by;

Effect of width and water table on bearing capacity:

Case1: For Cohesionless soil ( sand, C=0)

The ultimate bearing capacity for cohesionless soil is given by;

Conclusion:

• In the case of sand ultimate bearing capacity is proportional to the width of the foundation means Bearing capacity of Cohesionless soil increases with an increase in the width of the foundation.

Additional Information If the water table rises to ground level

or 

If the water table rises to ground level in sand ultimate bearing capacity approximately reduces to 50%.

Case 2: For cohesive soil (clay):

For pure clay, the angle of internal friction, ϕ = 0° 

For ϕ = 0°, N_c = 5.7, N_q = 1 and N_γ = 0

Ultimate bearing capacity is given by;

⇒ 

q_u = 5.7 C + γ'D_f

qnu = 5.7 C + γ'D_f - γ'D_f = 5.7 C

Conclusion:

• In the case of clay bearing capacity is independent of the width of footing.