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

Explanation:

i) End bearing Pile: Piles which acts as columns and transfer the structural load to a hard and relatively incompressible stratum at a greater depth such as rock or dense sand are known as end-bearing piles. These piles derive the required bearing capacity from end bearing at the tip of the pile.

ii) Compaction piles: These piles are used to compact loose granular soil to increase its bearing capacity. Compaction piles do not carry the load and hence they can be of weaker material. Sand piles can be used as compaction piles.

iii) ) Fender piles and dolphins: Fender piles and dolphins are used to protect waterfront structure from the impact of any floating object or ships.

iv) Anchor piles: These piles are generally used to provide anchorage against horizontal pull from sheet piling.

Explanation:

Explanation:

Negative skin friction

It is the phenomenon in which soil surrounding the pile settles more than the settlement of the pile.

This condition occurs when the soil surrounding the pile is loose or soft, under such conditions friction on the pile is downwards which reduces the load-carrying capacity of the soil.

Negative skin friction is caused when

1. Increase in surcharge over surrounding soil due to recently deposited clay layer.
2. Lowering of the groundwater table.
3. Disturbances due to seismic activities and other dynamic effects.

Concept:

Pile load test: 

• Pile load test is the only direct method for determining the allowable loads on the piles and is considered to be the most reliable due to the fact that it is an in-situ test.
• Pile load test provides very useful results for design for cohesionless soil but in case of clays.

The design load on a single pile for the initial test is taken as least of the following:

• Two-thirds (2/3) of the maximum test load at which the total settlement is 12 mm.
• Fifty percent of the maximum load at which the total displacement is 10% of the pile diameter in the case of uni­form-diameter piles.
• Fifty percent of the maximum load at which the total settlement is 7.5% of the bulb diameter in the case of under-reamed piles. 

Hence maximum test load on a working pile should not increase one and a half times or (3/2) the design load.

Concept-

Corrections to be applied-

a) Dilatancy correction/ water table correction-

Dilatancy correction is to be applied when N’ obtained after overburden correction, exceeds 15 in saturated fine sands and slits.

N” = 15 + 0.5 (N’ – 15)

Where N” = final corrected value to be used in design charts.

If N’  15, N” = N’

From several investigations, it is proven that the penetration resistance or N's value depends on the overburden pressure. If there are two granular soils with relative density same, a higher ‘N’ value will be shown by the soil with higher confining pressure.

With the increase in the depth of the soil, the confining pressure also increases. So the value of ‘N’ at shallow depth and larger depths are underestimated and overestimated respectively.

Hence, to account for this the value of ‘N’ obtained from the test is corrected to a standard effective overburden pressure.

b) Overburden Correction:

The corrected value of ‘N’ is

\(N' = N \times \frac{{350}}{{\sigma + 70}}\)

σ = Overburden Pressure (It should be less than 280 N/mm²)

N = Measured SPT No

Calculation:

Given Data,

Standard Penetration Test (SPT) gave an average count of 35 in fine sand below the water table.

N” = 15 + 0.5 (N’ – 15)

N” = 15 + 0.5 (35 – 15) = 25

N’ > 15 is an indication of dense sand. In such a soil, the fast rate of application of shear through the blows of a drop hammer is likely to induce negative pore water pressure in saturated fine sand under the undrained condition of loading. Consequently, a transient increase in shear resistance will occur, leading to an SPT value higher than the actual one.

Concept: -

Under-Reamed Piles: -

As per IS2911:1980 (part 3), cl.2.20, under-reamed piles are bored cast in situ or bored compaction concrete pile with an enlarged bulb made by either cutting or scooping out the soil or by any other suitable process.

As per IS 2911: Part III (Some other recommendations)

1. The diameter of under-reamed piles may vary from 2 to 3 times the stem diameter depending upon the feasibility of construction. For Bored cast in situ under-reamed piles the bulb diameter normally be 2.5 times while for compaction piles it is 2 times.

2. For piles up to 30 cm diameter, spacing of bulbs should not be greater than 1.5 times the diameter of bulb. For piles of diameter greater than 30cm, spacing can be reduced to 1.25 times the stem diameter.

3. The top most bulb should be at a minimum depth of 2 times the bulb diameter. In expansive soils, it should not be less than 1.75 m below ground level. The minimum clearance below underside of pile cap embedded in ground and the bulb should be a minimum 1.5 times the bulb diameter.

Explanation:

Strap Footing:

• A strap footing is a type of combined footing, consisting of two or more column footings connected by a concrete beam. In other words, when the independent footings of two columns are connected by a beam, it is called a strap footing.
• This type of beam is called a strap beam. It is used to distribute the weight of either heavily or eccentrically loaded column footings to adjacent footings.
• It is used to transfer the moment between two adjacent footings.

Under-reamed Piles

• Under reamed piles are bored cast-in-situ concrete piles having one or more bulbs formed by enlarging the pile stem.
• These piles are best suited in soils to restrict damage due to volume changes of swelling soils.

Floating Raft

• The main principle of a floating foundation is to balance the weight of removed soil with a structure of the same weight which causes zero settlement to the structure.
• It is also called the Balanced foundation.
• Soils which are having good shear strength but having a problem with large settlements and differential settlements under heavy loads in soft clays or silts.
• It is also used where soils which are having low shear strength and there is no hard layer of soil at reasonable depth. 

Combined Footing:

• The combined footings comprise of a connecting beam between the columns integrally cast with a slab on either side of the connecting beam.
• In the case of columns located close to the property line, footings cannot be extended on one side. To overcome this problem of non-availability of space near the exterior column, the footings of the exterior and interior columns are combined by using a connecting beam and trapezoidal-shaped slab. Due to the soil pressure, the slab bends transversely while the connecting bend longitudinally between the columns.

Concept:

The load-carrying capacity(Friction) of a single pile driven in clay is given by

QF = α × CU × AF

⇒ QF = α × CU × π × D × L

The load-carrying capacity(Friction), when piles act as a group is given by

QFG = α × CU × AFG

⇒ QFG = 1 × CU × (2S + D) × 4L

Where, QF = capacity of a single pile, QFG = Capacity when piles act as a group,

AF = Area for friction of single pile, CU = Cohesion

α = Adhesion factor, D = Diameter, L = Length of the pile,

AFG = Area under friction for a group of piles

Calculation:

Given,

Diameter of pile, D = 250 mm = 0.25 m, Length of pile, L = 12 m

Undrained cohesion, Cu = 30 kN/m²

Adhesion factor, α = 0.9

The load-carrying capacity(Friction) of a single pile driven in clay is given by;

QF = α × CU × π × D × L

Q_F = 0.9 × 30 × π × 0.25 × 12 = 254.469 kN

For 9 piles,

Q_F = 9 × 254.469 = 2290.22 kN

Q_F ≈ 2290 kN

Concept:

Negative skin friction is usually a downward shear drag acting on a pile or pile group because of downward movement of surrounding soil relative to the piles. This shear drag movements are anticipated to occur when a pile penetrates into compressible soil layer that can consolidate.

Factors that cause negative skin friction on piles and pile group

1. Newly placed fill material on compressible soil before the completion of consolidation.

2. If fill material is loose cohesion-less soil.

3. When fill material is deposited over layer of soft soil or peat.

4. Lowering groundwater which increases the effective stress causing consolidation of soil with resultant settlement and friction force being developed on the pile.

Important Points

Effect of negative skin friction on piles and pile groups

1. Negative skin friction contributes to the uneven settlement of piles or pile group.
2. For piles in compressible soils where pile capacity is contributed by both point resistance and shaft adhesion, the problem of negative skin friction should be considered a settlement problem.
3. In bearing piles where the settlement of the pile is negligible, negative skin friction becomes a pile capacity problem.

Concept:

Negative skin friction: It is usually a downward shear drag acting on a pile or pile group because of the downward movement of surrounding soil relative to the piles. Such a force can exist under one of the following conditions:

1. If a fill of clay soil is placed over a granular soil layer into which a pile is driven, the fill will exert a downward drag force on the pile due to consolidation.
2. If a fill of granular soil is placed over a layer of soft clay, it will induce the process of consolidation in the clay layer and thus exert a downward drag on the pile.
3. Lowering of the water table will increase the vertical effective stress on the soil at any depth, so it can induce negative skin friction on piles.

Now axial capacity of a pile is summation of upward reaction due to bearing at base and net upward skin frictional resistance. As, the negative skin friction (acting downward) lowers the net skin resistance, it in turn reduces the axial capacity of piles.

A schematic diagram of negative skin friction on piles is shown below:

∴ Negative skin friction on a pile under vertical compressive load acts downwards and reduce the load-carrying capacity.