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

Concepts:

The maximum pressure a soil can withstand without undergoing settlement in excess of the permissible value for the structure is called allowable bearing capacity or net safe bearing pressure or safe bearing pressure.

The maximum pressure a soil can withstand without the occurrence of shear failure is called ultimate bearing capacity of soil (q_u).

The net pressure at the base of foundation in excess of initial overburden pressure which a soil can withstand without shear failure called net ultimate bearing capacity.

Or

Q_nu = q_u­  - Overburden pressure(q)

When factor of safety is applied on the net ultimate bearing capacity, then it is called net safe bearing capacity (q_ns).

Safe bearing capacity: q_s = q_ns + Effective overburden pressure

Explanation:

Contact Pressure

Loads from the structure are transferred to the soil through the footing. As a reaction to it, soil exerts upward pressure on the bottom surface of the footing which is termed as contact pressure.

For flexible footing:  Contact pressure is uniform.

For Rigid footing:  Settlement is uniform

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

According to IS 1904 - 1986, the permissible values for settlement of isolated foundation is given in following table.

It can be seen that the maximum settlement of isolated foundation on clayey soil is 75 mm.

Important point:

The settlement of a shallow foundation can be divided into two major categories:

(a) Elastic, or immediate, settlement 

(b) consolidation settlement

The immediate settlement is given as:

\({{\rm{S}}_{\rm{i}}}{\rm{ = q}}{{\rm{B}}^{\rm{}}}\left( {{\rm{1 - }}{{\rm{\mu }}^{\rm{2}}}} \right)\frac{{\rm{I}}}{{\rm{E}}}\)

Where

B = Width of footing, q = Applied pressure, μ = Poison’s ratio and I = Influence factor.

The primary settlement can be given as:

\({{\rm{S}}_{\rm{p}}}{\rm{ = }}\frac{{{\rm{CH}}}}{{{\rm{1 + e}}}}{\rm{log}}\frac{{{\rm{(\sigma ' + \Delta \sigma ')}}}}{{{\rm{(\sigma ')}}}}\)

Where

C = Compression index, H = Height of soil strata, e = Initial void ratio and σ’= Effective stress.

Plate load test:

Plate load test is performed to determine soil bearing capacity and settlement.

It is a semi-direct method to measure the allowable pressure of soil by inducing a given amount of settlement. The load on the plate is applied by making use of a hydraulic jack. The reaction of the jack load is taken by a cross beam or a steel truss anchored suitably at both the ends. The settlement of the plate is measured by a set of three dial gauges of sensitivity 0.02 mm placed at 120° apart. The dial gauges are fixed to independent supports which do not get disturbed during the test.

Shape and size of the plate load test are given below:

• Square or circular in shape
• Thickness > = 25 mm
• Size may vary from 300 to 750 mm. therefore, the maximum size is 750 mm
• For clayey and silty soils and for loose to medium dense sandy soils with N < 15, 450 mm square plate is used
• For dense sandy or gravelly soils(15 < N < 30) three plates of sizes 300 mm to 750 mm is used depending  on reaction loading and maximum grain size
• Side of plate = 4 × size of particles

Explanation:

Standard penetration test (SPT):

• It is used to find the resistance of penetration of a sampling spoon and this resistance is empirically correlated with some engineering properties of soil such as density index or relative density, consistency by grouping them as soft, very soft, etc, bearing capacity of the soil.

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Option 2 is incorrect because

SPT test is generally not used for finding the undrained shear strength of clay as there are huge variations in N  Value on small variation in water content, and hence the test results are not very reliable in the case of clay. So we prefer an unconfined compression test or CPT test in the case of clay.

Option 4 is incorrect because

SPT is a dynamic test and hence measures the undrained shear strength , of loose sand, as there is no time available for dissipation of the pore water pressure so it cannot measure the drained shear strength of soil

Test procedure:

• First, the split spoon sampler is allowed to sink under its own weight, it is then seated 15 cm with the help of a hammer, weighing 63.5 kg and falling through a height of 75 cm.
• Then sampler is further driven by 30 cm i.e total of 45 cm seating is done from the starting.
• But first 15 cm of the drive is considered as seating drive and is ignored for calculations and resistance for the last 30 cm is penetration resistance (N).
• If the split spoon sampler is driven less than 45 cm (total), then penetration resistance for the last 30 cm is taken.

Additional Information

Correction for the observed value of N

1) Correction for overburden: 

\(N_{o}=N\frac{350}{70+σ }\)

Where N_o = Corrected value of N, N = Observed value of SPT number, σ = Effective pressure at the level of test σ \(\ngtr\) 280 kN/m²

2) Dilatancy correction:

The value obtained after overburden correction is further corrected for dilatancy if the stratum consists of fine sand and silt below the water table and is given by

N_F = 15 + 0.5(N_o - 15)

Explanation:

Plate Load Test:

• It is a field test, which is performed to determine the ultimate bearing capacity of the soil and settlement under a given load.
• This test is done for the selection and design of the shallow foundation.
• For performing this test, the plate is placed at the desired depth, then the load is applied gradually and the settlement for each increment of the load is recorded.

The limitations of the plate load test are:

1. It has a limited depth of influence. It could only give the bearing capacity of soils with depths up to two times the diameter of the bearing plate.
2. It may not provide information on the potential for long-term consolidation of foundation soils.
3.  There is a scale effect as the size of the test plate is smaller than the actual foundation. For example, the bearing capacity of sands and gravels increases with the size of the footing.
4. To gain access to the test position, excavation is carried out which causes significant ground disturbance. The change in ground stress leads to the change of soil properties which the test is planned to investigate.
5. In the case of cohesive soils, this test does not give the ultimate settlement.

Explanation:

Given,

bulk unit weight (γ_b) = 20 kN/m3

Undrained shear strength (S = C) = 25 kPa

Bearing Capacity N_c = 5.7, General Shear Failure

We know q_u = CN_c

q_u = γ D_f = 25 × (5.7) 

20 × D_f = 142.5

D_f = 7.124 m

Concept:

For Cohesive soil or clay,

\(\frac{{{S_f}}}{{{S_p}}} = \frac{{{B_f}}}{{{B_p}}}\)

But for Cohesionless soil or sand

\(\frac{{{S_f}}}{{{S_p}}} = {\left[ {\frac{{{B_f}}}{{{B_p}}}\left( {\frac{{{B_p} + 0.3}}{{{B_f} + 0.3}}} \right)} \right]^2}\)

Where,

Bp = Size of the square bearing plate

Bf​ = Size of square bearing footing

Sp = Settlement of bearing plate

Sf = Settlement of bearing footing

Calculation:

Given data: 

Here, in question given that it is cohesive soil.

Size of square bearing plate, Bp = 30 cm 

Size of square bearing footing, Bf = 90 cm

Settlement of bearing plate, Sp = 2 cm

\(\rm\frac{{{S_f}}}{{{S_p}}} = \frac{{{B_f}}}{{{B_p}}} \Rightarrow \frac{{{S_f}}}{{2}} = \frac{{90}}{{30}} \Rightarrow {S_f} = 6\ cm\)

Hence, Settlement of a square footing = 6 cm

Explanation:

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 hour. 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:

Standard Penetration Test is used to determine:

• Relative density or density index for cohesionless soil 
• The angle of shearing resistance of cohesionless soils
• Allowable Bearing Pressure based on settlement criteria and Shear criteria
• Unconfined compressive strength of Cohesive soils.

The standard penetration test is an in-situ test that is coming under the category of penetrometer tests carried out in boreholes.

The test will measure the resistance of the soil strata to the penetration undergone. A penetration empirical correlation is derived between the soil properties and the penetration resistance. 

The test is most commonly useful for cohesionless soil but in clayey and gravely soil, it yields poor representative of true soil conditions.