PILE FOUNDATION.

PILE FOUNDATION

UNIT IV PILE FOUNDATION
Types of piles and their function – Factors influencing the selection of pile – Carrying capacity of single pile in granular and cohesive soil – static formula – dynamic formulae (Engineering news and Hileys) – Capacity from insitu tests (SPT and SCPT) – Negative skin friction – uplift capacity- Group capacity by different methods (Feld‟s rule, Converse – Labarra formula and block failure criterion) – Settlement of pile groups – Interpretation of pile load test (routine test only) – Under reamed piles – Capacity under compression and uplift.

DEFINITION Deep Foundations are those
in which the depth of the foundation is very large in comparison to its width. Which are not constructed by ordinary methods of open pit excavations.

Pile Foundations(contd.)
Situations Which Demand Pile Foundation : Sub-soil water table is so high that it can easily affect the other foundations. Load coming form the structure is heavy and non uniform. Where grillage or raft foundations are either very costly or their adoption impossible due to local difficulties. When it is not possible to maintain foundation trenches in dry condition by pumping, due to very heavy inflow of seepage or capillary water. When it is not possible to timber the excavation trenches in the case of deep strip foundation. (strip foundation- spread footing under wall ). When overlay soil is compressible, and water-logged and firm hard bearing strata is located at quite a large depth. When structures are located on river-bed or sea-shore and foundations are likely to be scoured due to action of water. Large fluctuations in sub-soil water level. Canal or deep drainage lines exist near the foundations. In the construction of docks, piers and other marine structures they are used as fender piles.

Types of Piles Based on Their
Function and Effect of Installation

TYPES OF PILES Large displacement piles (driven types)
1. Timber (round or square section, jointed or continuous). 2. Precast concrete (solid or tubular section in continuous or jointed units). 3. Prestressed concrete (solid or tubular section). 4. Steel tube (driven with closed end). 5. Steel box (driven with closed end). 6. Fluted and tapered steel tube. 7. Jacked-down steel tube with closed end. 8. Jacked-down solid concrete cylinder. Large displacement piles (driven and cast-in-place types) 1. Steel tube driven and withdrawn after placing concrete. 2. Precast concrete shell filled with concrete. 3. Thin-walled steel shell driven by withdrawable mandrel and then filled with concrete. Small-displacement piles 1. Precast concrete (tubular section driven with open end). 2. Prestressed concrete (tubular section driven with open end). 3. Steel H-section. 4. Steel tube section (driven with open end and soil removed as required). 5. Steel box section (driven with open end and soil removed as required).

Replacement piles 1. Concrete placed in hole drilled by rotary auger, baling, grabbing, airlift or reverse-circulation methods (bored and castin- place). 2. Tubes placed in hole drilled as above and filled with concrete as necessary. 3. Precast concrete units placed in drilled hole. 4. Cement mortar or concrete injected into drilled hole. 5. Steel sections placed in drilled hole. 6. Steel tube drilled down. Composite piles Numerous types of piles of composite construction may be formed by combining units in each of the above categories, or by adopting combinations of piles in more than one category. Thus composite piles of a displacement type can be formed by jointing a timber section to a precast concrete section, or a precast concrete pile can have an H-section jointed to its lower extremity. Composite piles consisting of more than one type can be formed by driving a steel or precast concrete unit at the base of a drilled hole, or by driving a tube and then drilling out the soil and extending the drill hole to form a bored and castin- place pile.

FACTORS GOVERNING CHOICE OF TYPE OF PILE
Driven displacement piles Advantages 1. Material forming pile can be inspected for quality and soundness before driving. 2. Not liable to ‘squeezing’ or ‘necking’. 3. Construction operations not affected by ground water. 4. Projection above ground level advantageous to marine structures. 5. Can be driven in very long lengths. 6. Can be designed to withstand high bending and tensile stresses. Disadvantages 1. Unjointed types cannot readily be varied in length to suit varying level of bearing stratum. 2. May break during driving, necessitating replacement piles. 3. May suffer unseen damage which reduces carrying capacity. 4. Uneconomical if cross-section is governed by stresses due to handling and driving rather than by compressive, tensile, or bending stresses caused by working conditions. 5. Noise and vibration due to driving may be unacceptable. 6. Displacement of soil during driving may lift adjacent piles or damage adjacent structures. 7. End enlargements, if provided, destroy or reduce skin friction over shaft length. 8. Cannot be driven in conditions of low headroom.

Driven-and-cast-in-place displacement piles
Advantages 1. Length can easily be adjusted to suit varying level of bearing stratum. 2. Driving tube driven with closed end to exclude ground water. 3. Enlarged base possible. 4. Formation of enlarged base does not destroy or reduce shaft skin friction. 5. Material in pile not governed by handling or driving stresses. 6. Noise and vibration can be reduced in some types by driving with internal drop-hammer Disadvantages 1. Concrete in shaft liable to be defective in soft squeezing soils or in conditions of artesian water flow where withdrawable-tube types are used. 2. Concrete cannot be inspected after installation. 3. Length of some types limited by capacity of piling rig to pull out driving tube. 4. Displacement may damage fresh concrete in adjacent piles, or lift these piles, or damage adjacent structures. 5. Noise and vibration due to driving may be unacceptable. 6. Cannot be used in river or marine structures without special adaptation. 7. Cannot be driven with very large diameters. 8. End enlargements are of limited size in dense or very stiff soils. 9. When light steel sleeves are used in conjunction with withdrawable driving tube, skin friction on shaft will be destroyed or reduced.

Bored-and-cast-in-place replacement piles
Advantages 1. Length can readily be varied to suit variation in level of bearing stratum. 2. Soil or rock removed during boring can be inspected for comparison with site investigation data. 3. In-situ loading tests can be made in large-diameter pile boreholes, or penetration tests made in small boreholes. 4. Very large (up to 7.3m diameter) bases can be formed in favourable ground. 5. Drilling tools can break up boulders or other obstructions which cannot be penetrated by any form of displacement pile. 6. Material forming pile is not governed by handling or driving stresses. 7. Can be installed in very long lengths. 8. Can be installed without appreciable noise or vibration. 9. No ground heave. 10. Can be installed in conditions of low headroom. Disadvantages 1. Concrete in shaft liable to squeezing or necking in soft soils where conventional types are used. 2. Special techniques needed for concreting in water-bearing soils. 3. Concrete cannot be inspected after installation. 4. Enlarged bases cannot be formed in cohesionless soils. 5. Cannot be extended above ground level without special adaptation. 6. Low end-bearing resistance in cohesionless soils due to loosening by conventional drilling operations. 7. Drilling a number of piles in group can cause loss of ground and settlement of adjacent structures.

Pile Foundations The term ‘Pile Foundation’ denotes a construction for the foundation of a wall or pier which is supported on piles. Where Used : stratum of required bearing capacity is at greater depth steep slopes are encountered Compressible soil or water-logged soil or soil of made-up type Examples: Piles are used for foundation for buildings, trestle-bridges and water front installations (piers, docks etc ). Advantages: Provides a common solution to all difficult foundation site problems Can be used for any type of structure and in any type of soil

Types of Piles Based on Function
a) Classification based on Function or Use Bearing Piles or End Bearing Piles Friction Piles or Skin Friction Piles Sheet Piles Tension Piles or Uplift Piles Anchor Piles Batter Piles Fender Piles Compaction Piles

TYPES OF PILE FOUNDATION
STEEL PILE

Types of Piles Based on Function (contd)
Bearing Piles Driven into the ground until a hard stratum is reached. Acts as pillars supporting the super-structure and transmitting the load to the ground. Piles, by themselves do not support the load, rather acts as a medium to transmit the load from the foundation to the resisting sub-stratum.

Friction Piles (Floating Piles) Piles are driven at a site where soil is weak or soft to a considerable depth and it is not economical or rather possible to rest the bottom end of the pile on the hard stratum, Load is carried by the friction developed between the sides of the pile and the surrounding ground ( skin friction). The piles are driven up to such a depth that skin friction developed at the sides of the piles equals the load coming on the piles. Skin friction should be carefully evaluated and suitable factor of safety applied, as it is this which is supporting the whole of structure over its head. The load carrying capacity of friction pile can be increased by- increasing diameter of the pile driving the pile for larger depth grouping of piles making surface of the pile rough

Sheet Piles Sheet piles are never used to provide vertical support but mostly used to act as retaining walls. They are used for the following purposes: To construct retaining walls in docks, and other marine works. To protect erosion of river banks. To retain the sides of foundation trenches. To confine the soil to increase its bearing capacity. To protect the foundation of structures from erosion by river or sea. To isolate foundations from adjacent soils.

Figure: Sheet Piles

Anchor Piles Piles are used to provide anchorage against horizontal pull from sheet piling wall or other pulling forces. Batter piles: Piles are driven at an inclination to resist large horizontal and inclined forces. Fender piles: Piles are used to protect concrete deck or other water front structures from the abrasion or impact caused from the ships or barges. Ordinarily made up of timber. Compaction piles: When piles are driven in granular soil with the aim of increasing the bearing capacity of the soil, the piles are termed as compaction piles.

Figure: Under-reamed Piles

Types of Piles Based on Materials
a) Classification based on Materials Timber Piles Concrete Piles Composite Piles Steel Piles Sand Piles

Types of Piles Based on Materials (contd)
Figure: Timber Pile

Types of Concrete Piles
Concrete Piles are of 3 types: Pre-cast Piles Cast in situ Piles Prestressed Concrete Piles

Concrete Piles (contd)
Pre-cast Piles: Reinforced concrete piles, molded in circular, square, rectangular or octagonal form. Cast and cured in the casting yard, then transported to the site of driving. If space available it can be cast and cured near the work site. Driven in similar manner as timber piles with the help of piles drivers. Diameter normally varies from 35cm to 65cm, length varies from 4.5m to 30m.

Pre-cast Piles: Function of reinforcement in a pre-cast pile is to resist the stresses during handling, driving and final loading on the pile rather than strengthen the pile to act as a column. Longitudinal reinforcements usually 20mm to 50mm in diameter, stirrups 6mm to 10mm in dia. For 90 cm length at head and toe, stirrups spacing is 8cm c/c and for remaining intermediate length it is about 30cm c/c. Circular piles are seldom tapered. When tapered piles length is restricted to 12m. A concrete cover of 5cm is maintained throughout, over the main steel bars.

Advantages of Pre-cast Piles: Very effective Simple quality control Improves the entire area Disadvantages of Pre-cast Piles: Limited in length Difficult to transport Not suitable for densely built up area Requires costly equipment

Cast-in-Situ Piles: Cast in position inside the ground. First of all a bore is dug by driving a casing pipe into the ground. Then the soil from the casing is jetted out and filled with cement concrete after placing necessary reinforcement in it. Cast-in-situ piles are of two types: Cased Cast-in-Situ Piles: metallic shell is left inside the ground along with the core Uncased Cast-in-Situ Piles: metallic shell is withdrawn

Advantages of Cast-in-Situ Concrete Piles: Not limited in length Can be cast at any place Requires less equipment Cost is less and is depended on the size Disadvantages of Cast-in-Situ Concrete Piles: Quality control is difficult Load carrying is mostly done through end bearing only Skin frictional resistance is very low.

Figure: Cast-in-Situ Pile

Advantages of Concrete piles: Durability is independent of ground water level. For large size and greater bearing power number of piles required is much less. Can be cast to any length, size or shape. Can be used to marine work without any treatment. Material required for manufacture is easily obtainable. Concrete piles can be monolithically bonded into pile cap which is not possible in wooden piles.

Disadvantages of Concrete piles: Costlier than timber piles. Can not be driven rapidly. Required elaborate tech supervision and heavy driving machines. Must be reinforced to withstand handling stresses.

Prestressed Concrete Piles The greatest disadvantage of large weightt and difficulty in handling of pre-cast pile is eliminated by prestressed concrete piles. The weight is reduced by casting 200mm to 300mm diameter fiber tubes inside the piles at the time of concreting. The pre tensioning cables are subjected to required pull (tension) in the casting bed. The fiber tube is held in position inside the form work and the piles reinforced with pre stressed cables are concreted in a row.

Prestressed Concrete Piles Prestressed concrete piles are provided with lifting hooks at 1/5th ( 0.2L, L= length of pile ) of pile length from each end. Piles length 50 times the thickness →single point pick up More than 50 times the thickness →two point pick up at 0.2L from either end. Piles 500 sq. mm and smaller→ cast solid. Piles over 500 sq. mm may be cast with 200mm to 300mm cored hole (void). Pre stressed piles are always precast.

Advantages of Prestressed Concrete Piles It has greater ability to withstand extremely hard driving. It is more durable in sea water because of absence of crack. It has greater column capacity. It has lesser handling costs because of light weight. It requires lesser pick-up points. It has larger moment of inertia than the conventional piles of same dimension since the concrete is all in compression.

Composite Piles (contd)
Piles of two different materials are driven one over the other, so as to enable them to act together to perform the function of a single pile. This type of composite pile is used with the object of achieving economy in the cost of piling work.

Composite Piles

Selection of Type of Pile
The nature of the ground, where piling operation is to be carried out, determines to a large extent the choice of type of pile to be used. In addition, the other important factors which must be considered in this regard are: The nature of the structure. Loading conditions. Elevation of the ground water level with respect to the pile cap. Probable length of pile required. Availability of materials and equipment. Factors which may cause deterioration of pile. Probable cost of pile.

Causes of Failure of Piles
Load on the pile is more than the designed load. Defective workmanship during casting of the pile. Displacement of reinforcement during casting. Bearing pile resting on a soft strata. Improper classification of soil. Improper choice of the type of pile. Insufficient reinforcement in the pile. Decay of timber piles due to attack by insects. Buckling of piles due to inadequate lateral support. Defective method adopted for driving the pile. Incorrect assessment of the bearing capacity of the pile. Lateral forces not considered in the design of piles.

Limiting End Bearing Resistance (t/m2) as per IS 2911
Bored Piles Driven Piles 1500 Type of Soil Limiting Frictional Resistance (t/m2) Sand 6 Silt Clay 7

Uplift Capacity of Piles
The uplift capacity of a pile is given by sum of the frictional resistance and the weight of the pile (buoyant or total as relevant) as per Section of IS 2911 (Part 1/Sec. 2) 2010. The recommended factor of safety is 3.0 in the absence of any pull out test results and 2.0 with pullout test results. Uplift capacity can be obtained from static formula by ignoring end-bearing but adding weight of the pile (buoyant or total as relevant).

Typical load settlement plot from pile load test
The elastic settlement, (Se) due to the elastic recovery of the pile material and the elastic recovery of the soil at the base of the pile .

Determination of Ultimate Load of pile from Pile Load Test
Single Tangent method Double Tangent Method Log-Log method Rectangular Hyperbola method Vander Veen's method (1953) Maazurkiewicz parabola method (1972)

Single Tangent method Double Tangent Method

Log-Log method Rectangular Hyperbola method

Typical Arrangement of Piles in Groups
Spacing of piles depends upon the method of installing the piles and the type of soil

PILE GROUP EFFICIENCY

CAPACITY OF PILE GROUP Feld’s Rule (Reduces the capacity of each pile by 1/16 for each adjacent pile) Converse-Labarre Formula Block failure criteria

Converse-Labarre Formula for pile group-efficiency

VERTICAL BEARING CAPACITY OF PILE GROUPS EMBEDDED IN SANDS AND GRAVELS
Qgu = n Eg Qu If Eg > 1, then Eg is (2/3) to (¾) for Bored Piles

Pile Groups in Cohesive Soils
Block failure of a pile group in clay soil

Settlement of Pile Group
Total Settlement Elastic Settlement Consolidation Settlement

Semi-Empirical Formulas and Curves
Vesic (1977) S = total settlement, Sp = settlement of the pile tip, Sf = settlement due to the deformation of the pile Shaft.

Qp= point load, d = diameter of the pile at the base, q pu - ultimate point resistance per unit area, Dr = relative density of the sand, Cw = settlement coefficient, = 0.04 for driven piles = 0.05 for jacked piles = 0.18 for bored piles, Qf = friction load, L = pile length, A = cross-sectional area of the pile, E = modulus of deformation of the pile shaft, α = coefficient which depends on the distribution of skin friction along the shaft and can be taken equal to 0.6.

SETTLEMENT OF PILES AND PILE GROUPS IN SANDS AND GRAVELS

Curve showing the relationship between group settlement ratio and relative widths of pile groups in sand (Vesic, 1967)

Curve showing relationship between Fg and pile group width (Skempton, et al., 1953)

IS 8009 (Part 2), Settlement of Pile Group by
Skempton, et al., 1953 Where, Sf = Final Settlement of Pile Group (cm) S1 = Final Settlement of Single Pile (cm) B = Width of Pile Group (cm) s = Ratio of spacing of piles to pile diameter r = No. of rows in the square group

SETTLEMENT OF PILE GROUPS IN COHESIVE SOILS

Settlement of Pile Groups in Cohesive Soil
CASE 1 The soil is homogeneous clay. The load Qg is assumed to act on a fictitious footing at a depth 2/3L from the surface and distributed over the sectional area of the group. The load on the pile group acting at this level is assumed to spread out at a 2 Vert : 1 Horiz slope.

CASE 2 The pile passes through a very weak layer of depth L1 and the lower portion of length L2 is embedded in a strong layer. In this case, the load Q is assumed to act at a depth equal to 2/3 L2 below the surface of the strong layer and spreads at a 2 : 1

CASE 3 The piles are point bearing piles. The load in this case is assumed to act at the level of the firm stratum and spreads out at a 2 : 1 slope.

Consolidation Settlement of Group Piles

NEGATIVE FRICTION

4. In cases, where the piles are driven through a strata of soft clay into firmer soils and the soft clay tends to settle relative to the pile 5. Piles in a clay stratum which undergoes shrinkage settlement

Methods of Mitigating Negative Skin Friction
Coat the surface of the precast pile with thick coat of special bitumenous paint which have been proved to reduce skin friction as much as 90% of the theoretical value. Drive the piles inside a casing. In the top negative friction height, the space between the pile and casing is filled with a viscous material and the casing is withdrwan after installing the pile. Smaller c/s area shaft along pile length compared to base of pile may reduce the negative friction along the pile shaft. However, this is possible only for pure end bearing piles (not depending on shaft resistance).

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