Deep Foundation MCQ Quiz - Objective Question with Answer for Deep Foundation - Download Free PDF

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.

Concept:

The given pile is a friction pile hence all the load is carried through friction only.

Static pile load formula is given as-

QU = QPU + Qf

where,

QU = Ultimate pile load capacity,

QPU = qp × Ab ; Ultimate point bearing capacity.

Qf = fsAS ; Frictional pile load capacity.

where,

fs  = α cu

cu  =  Cohesion averaged over the pile length, α = Adhesion factor, qp = Effective stress at the base of pile, Ab = Base area of pile.

As = Surface area of pile.

Given:

α = 0.7, cu = 4t/m2, FOS = 2.5, Pile diameter (φ) = 30 cm = 0.3 m

Thus, by using static pile load formula –

QU = QPU + Qf

⇒ QU = 0 + fs­AS

⇒ QU  = (α CU)As

⇒ QU  = (0.7 × 4)× (π ∅ l)

⇒ QU  = (0.7 × 4)× (π × 0.3× 10)

⇒ QU  = 26.389 t

Thus,

Safe load = \(\frac{{{{\rm{Q}}_{\rm{U}}}}}{{{\rm{FOS}}}}{\rm{\;}} = {\rm{\;}}\frac{{26.389}}{{2.5}}{\rm{\;}} = {\rm{\;}}10.55{\rm{\;t}}\)

Important Points

Static pile load formula assumes that both point bearing capacity and frictional capacity develop its full value. But both do not mobiles to full capacity simultaneously as for developing full frictional resistance movement of the pile required is small but for developing point bearing moment of pile required is large. Thus when full point bearing develops friction falls below its peak value.

Adhesion factor (α) depends upon adhesion b/w soil and pile. Softer is the clay, larger is the value of α.

Explanation:

Pile Foundations

Pile foundations transfer loads from structures to hard strata deep underground. Understanding the behavior and interaction of piles with the surrounding soil is crucial for their design and application. Here, we analyze the given statements:

• Load-carrying capacity: The capacity of a pile is influenced by both the skin friction along the pile shaft and the point resistance at the pile tip.

• Negative skin friction: This occurs when the surrounding soil settles more than the pile, exerting a downward drag on the pile.

• Group efficiency: In sandy soils, the efficiency of closely spaced driven piles may exceed 100% due to the densification of the sand around the piles.

Analyzing the Given Statements

1. Statement 1: The load-carrying capacity of a pile depends upon both the skin friction and point resistance.

   • True. The capacity of a pile is a combination of the skin friction along the pile shaft and the point resistance at the pile tip.

2. Statement 2: The negative skin friction on a pile develops when the soil surrounding it settles less than the pile.

   • False. Negative skin friction occurs when the soil settles more than the pile, not less.

3. Statement 3: The group efficiency of driven piles in sand at a close spacing may be greater than 100%.

   • True. In sandy soils, closely spaced driven piles can densify the surrounding sand, resulting in a group efficiency greater than 100%.

Explanation:

Choosing the Most Appropriate Excavation Technique

When selecting an excavation technique for a substructure requiring deep excavation, especially in areas with high water tables and sandy soil, several factors must be considered. These include soil stability, water control, and the depth of excavation. Given these conditions, the techniques can be analyzed as follows:

1. Trench excavation with timber shoring

   • This method involves using timber to shore up the trench walls, providing support against collapse.

   • While it offers some support, it may not be sufficient for deep excavations, especially in sandy soils with high water tables where soil can easily collapse or become unstable.

2. Hydraulic shoring with sheet piling

   • This method uses hydraulic pressure to drive sheet piles into the ground, creating a watertight barrier that prevents soil collapse and water ingress.

   • It is highly effective for deep excavations in sandy soils and areas with high water tables, providing robust support and control over water intrusion.

3. Open cut excavation with sloped sides

   • Open cut excavation involves sloping the sides of the excavation to prevent collapse. This method is simple and cost-effective.

   • However, it requires a large area and is not practical for deep excavations in urban settings. Moreover, it may not effectively control water in high water table areas.

4. Caisson construction

   • Caissons are watertight structures used for constructing foundations in underwater or high water table conditions. They are highly effective for very deep excavations.

   • However, this technique is often more complex and expensive compared to other methods. It is generally used for specialized projects like bridge piers.

Conclusion

Given the high water table and sandy soil conditions, the most appropriate excavation technique for constructing a substructure requiring deep excavation is Hydraulic shoring with sheet piling. This method provides the necessary support and water control, ensuring stability and safety during the excavation process. Therefore, the correct answer is option 2.

Explanation:

Selecting Piling Method for Dense Urban Environment

When installing a deep foundation in a dense urban environment with limited space and strict vibration control requirements, the choice of piling method is crucial. The correct method should minimize noise, vibration, and disruption to the surrounding structures and environment. Here's an analysis of the options provided:

1. Diesel hammer piling (Not suitable)

   • This method generates significant noise and vibrations, which are unsuitable for dense urban areas with strict vibration control requirements.

   • It can disturb nearby structures and residents, making it an inappropriate choice.

2. Continuous flight auger piling (Most suitable)

   • This method is relatively quiet and produces minimal vibrations, making it ideal for urban environments with strict vibration control.

   • It allows for the installation of piles with minimal disruption to the surrounding area and structures.

3. Drop hammer piling (Not suitable)

   • This method involves dropping a heavy weight onto the pile, generating substantial vibrations and noise.

   • It is not appropriate for areas with strict vibration control requirements due to its disruptive nature.

4. Vibratory hammer piling (Not suitable)

   • This method uses vibration to drive the pile into the ground, which can cause significant vibrations in the surrounding area.

   • It is not suitable for environments where vibration control is critical, as it can disturb nearby structures and residents.

In flexible footings pressure is uniform.

In rigid footings settlement is uniform.

Contact pressure distribution of Rigid footing over cohesive soil:

For rigid footings, the settlement has to be uniform over the contact area. Since, a flexible footing produces a dish shaped pattern in clay soil; the contact pressure must be more near the edge of the loaded area and less near centre, in order to produce a uniform settlement.

Contact pressure distribution of rigid footing over granular soil:

For a rigid footing, where the settlement has to be uniform, the contact pressure is more near the centre then near the edges.

Concept:

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

Q_F = α × C_U × A_F

⇒ Q_F = α × C_U × π × D × L

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

QFG = α × CU × AFG

⇒ Q_FG = 1 × C_U × (3S + D) × 4L

Where, Q_F = capacity of a single pile, Q_FG = Capacity when piles act as a group,

A_F = Area for friction of single pile, C_U = Unconfined compressive strength of soil, 

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

A_FG = Area under friction for a group of piles

Calculation:

Given:

α = 0.7, N = 16

For finding the optimum spacing between the pile, we can say that

Capacity of single pile × Number of piles = Group capacity of piles

⇒ N × Q_F = Q_FG

⇒ N ×  α × CU × π × D × L = 1 × CU × (3S + D) × 4L

⇒ 16 × 0.7 × π × D = (3S + D) × 4

⇒ 8.796 × D = 3S + D

⇒ S = 2.5988D ≈ 2.6D

∴ The optimum spacing between the piles is 2.6D

Explanation:

• The bearing capacity of a single or group of piles shall be determined from test loading. It is the most direct method for determining safe load on pile and it is more reliable on account of its being in-situ test.
• The load test on a concrete pile shall not be carried out earlier than 28 days of its casting.
• Initial test shall be carried on test pile which is not used as working pile and Routine tests shall be carried out as a check on working pile.
• Routine test shall be one-half percent to two percent of the total number of piles or as specified, applicable to vertical and lateral load.
• Load Test shall generally conform to the provision made in IS 2911 (Part IV) which provides guidelines for the determination of safe loads and conducting of different types of tests. 

Additional InformationThe allowable load on a single pile may be obtained as per IS code provision. Allowable load is obtained as per one of the following:

• The allowable load on pile may be taken as 50% ultimate load at which total settlement of the pile is (1/10)^th of diameter.
• Allowable load on piles is (2/3)^rd of ultimate load at which total settlement is 12 mm.
• Allowable load on pile is (2/3)rd of ultimate load at which net plastic settlement is 6 mm.

Concept:

Efficiency of pile is given by,

\(\eta = \frac{{{P_{ug}}}}{{{P_{us}}\;}}\)

P_us = capacity of single pile

P_ug = capacity of group

Calculation:

\(0.75 = \frac{{{P_{ug}}}}{{6 \times 150}} \Rightarrow {P_{ug}} = 675kN\)

Explanation:

The functions of a pile cap are:

1. To distribute horizontal and vertical loads equally over the pile group and thus over a greater area of bearing potential.

2. To laterally stabilize individual piles thus increasing the overall stability of the group.

3. To provide the necessary combined resistance to stresses set up by the superstructure and/or ground movement.

Explanation:

1. Bored cast in situ pile are typically poured in place and transfer the load through end bearing and skin friction.
2. While driven pile are driven straight in and transfer the load through friction and/or bearing.

Important Points

Types of pile:

i) End bearing Pile: Piles which transfer the structural load to a hard and relatively incompressible stratum 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.

v) Tension pile: Used to anchor down the structures subjected to hydrostatic uplift forces or overturning forces

vi) Sheet pile: Used for retaining earth or water

Explanation:

Shifting and tilting of well foundation:

• When the well is moved away horizontally from the desired position, it is called shifting of the well foundation.
• When the well is sloped against vertical alignment, it is called tilting of the well foundation.

Prevent shifting and tilting:

• The diameter of well curb should be more than the external diameter of steining. Generally, a difference of 40 to 80 mm is recommended.
• The well steining should be symmetrically placed over the curb.
• The outer surface of the well curb and steining should be smooth.
• All the sides should be uniformly dredged.
• The cutting edge should be uniformly thick and sharp.

Limitations:

• The maximum tilt allowed in case of well foundation is 1 in 60.
• The shift in well foundation should not be more than 1 % of depth of sunk.
• Beyond the above limits, well foundation is considered as dangerous and in such a case, remedial measures to rectify shifting and tilting should be followed.

Concepts:

Cyclic Pile Load test:

The cyclic load test helps to estimate how much load is taken up by the end bearing of the pile and how much by skin friction. The main objective of the cyclic load test is to separately determine the point bearing and friction bearing capacities of a pile.

Plate bearing or Load test:

The plate load test is a field test, which is performed to determine the ultimate bearing capacity of the soil and settlement under a given load.  

The modulus of subgrade reaction is calculated by using the relation K = P/Δ; where K is the modulus of subgrade reaction, p applied pressure, Δ is the settlement estimated using the Plate load test.

Pressure meter test:

It is used to determine the stress-strain relations of in-situ soil from which elastic constants are calculated.  It is mostly used for hard clays and dense sands.

Standard penetration test

It is used to determine the relative density, bearing capacity, and settlement of granular soil. 

Concept:

Classification of the pile foundation:

Piles may be classified in a number of ways based on different criteria:

Concept:

Pile Spacing:

As per IS 2911 (Par1/Sec1):2010, Cl. 6.6.

• The spacing of piles means the center-to-center distance between adjacent piles.
• The minimum center to centre spacing of pile is considered from the following three aspects –

1. Practical aspects of installing the piles
2. Diameter of the pile (d)
3. Nature of the load transfer to the soil and possible reduction in the load capacity of piles group

From practical consideration, pile spacing should not be less than any of the above spacings.

Hence, centre-to-centre distance between adjacent piles from practical spacing ≮ 3.0d