PILE FOUNDATIONS

By: The Civil Engineer on 19:47

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      Piles are relatively slender  shafts, cylindrical in shape, driven or bored into the ground to the required depth. Piles are used to carry vertical loads through weak soil to dense strata having high bearing capacity. In normal ground conditions, they can resist large uplift and horizontal loads, hence can be used as foundations of multistoried buildings, transmission line towers, retaining walls, bridge abutments.

      Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity The main types of materials used for piles are Wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Piles are classified depending upon type of soil, pile material and load transmitting characteristics.

   Pile foundations have been used as load carrying and load transferring systems for many years.

In the early days of civilisation from the communication, defence or strategic point of view villages and towns were situated near to rivers and lakes. It was therefore important to strengthen the bearing ground with some form of piling.

Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones.

In 1740 Christoffoer Polhem invented pile driving equipment which resembled to days pile driving mechanism. Steel piles have been used since 1800 and concrete piles since about 1900.

The industrial revolution brought about important changes to pile driving system through the invention of steam and diesel driven machines.

More recently, the growing need for housing and construction has forced authorities and development agencies to exploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techniques of pile installation

Function of piles

As with other types of foundations, the purpose of a pile foundations is:

a)    to transmit a foundation load to a solid ground

b)    to resist vertical, lateral and uplift load

A structure can be founded on piles if the soil immediately beneath its base does not have adequate bearing capacity. If the results of site investigation show that the shallow soil is unstable and weak or if the magnitude of the estimated settlement is not acceptable a pile foundation may become considered. Further, a cost estimate may indicate that a pile foundation may be cheaper than any other compared ground improvement costs.

In the cases of heavy constructions, it is likely that the bearing capacity of the shallow soil will not be satisfactory, and the construction should be built on

pile foundations. Piles can also be used in normal ground conditions to resist horizontal loads. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.

CLASSIFICATION OF PILES.

Classification of pile with respect to load transmission and functional behaviour

·         i) End bearing piles (point bearing piles)

·         ii) Friction piles (cohesion piles )

·         iii) Combination of friction and cohesion piles

i) End bearing piles

These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile (see figure 1.1). The pile behaves as an ordinary column and should be designed as such. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion. But sometimes, the soil surrounding the pile may adhere to the surface of the pile and causes "Negative Skin Friction" on the pile. This, sometimes have considerable effect on the capacity of the pile. Negative skin friction is caused by the drainage of the ground water and consolidation of the soil. The founding depth of the pile is influenced by the results of the site investigate on and soil test.

ii) Friction or cohesion piles

Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile (see fig 1.2).

 ii-a) Cohesion piles

These piles transmit most of their load to the soil through skin friction. This process of driving such piles close to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are some times called compaction piles. During the process of driving the pile into the ground, the soil becomes moulded and, as a result loses some of its strength. Therefore the pile is not able to transfer the exact amount of load which it is intended to immediately after it has been driven. Usually, the soil regains some of its strength three to five months after it has been driven.

ii-b) Friction piles

These piles also transfer their load to the ground through skin friction. The process of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known as floating pile foundations.

iii) Combination of friction piles and cohesion piles

An extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay. The pile is driven far enough into the lower material to develop adequate frictional resistance. A farther variation of the end bearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base. A similar effect is produced with bored piles by forming a large cone or bell at the bottom with a special reaming tool. Bored piles which are provided with a bell have a high tensile strength and can be used as tension piles (see fig.1-3)

 

 (I)Classification of pile with respect to type of material

·         i) Timber

·         ii) Concrete

·         iii) Steel

·         iv) Composite piles

i) Timber piles

Used from earliest record time and still used for permanent works in regions where timber is plentiful. Timber is most suitable for long cohesion piling and piling beneath embankments. The timber should be in a good condition and should not have been attacked by insects. For timber piles of length less than 14 meters, the diameter of the tip should be greater than 150 mm. If the length is greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is essential that the timber is driven in the right direction and should not be driven into firm ground. As this can easily damage the pile. Keeping the timber below the ground water level will protect the timber against decay and putrefaction. To protect and strengthen the tip of the pile, timber piles can be provided with toe cover. Pressure creosoting is the usual method of protecting timber piles.

ii) Concrete pile

Pre cast concrete Piles or Pre fabricated concrete piles : Usually of square (see fig 1-4 b), triangle, circle or octagonal section, they are produced in short length in one metre intervals between 3 and 13 meters. They are pre-caste so that they can be easily connected together in order to reach to the required length (fig 1-4 a) . This will not decrease the design load capacity. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Pre stressed concrete piles are also used and are becoming more popular than the ordinary pre cast as less reinforcement is required .

 

 The Hercules type of pile joint (Figure 1-5) is easily and accurately cast into the pile and is quickly and safely joined on site. They are made to accurate dimensional tolerances from high grade steels.

  

  Driven and cast in place Concrete piles

Two of the main types used in the UK are: West's shell pile : Pre cast, reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel and driven into the ground after a concrete shoe has been placed at the front of the shells. Once the shells have been driven to specified depth the mandrel is withdrawn and reinforced concrete inserted in the core. Diameters vary from 325 to 600 mm.

Franki Pile: A steel tube is erected vertically over the place where the pile is to be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. When the required depth has been achieved the tube is raised slightly and the aggregate broken out. Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in position and more dry concrete is placed and rammed until the pile top comes up to ground level.

iii) Steel piles

Steel piles: (figure 1.4) steel/ Iron piles are suitable for handling and driving in long lengths. Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. They can be easily cut off or joined by welding. If the pile is driven into a soil with low pH value, then there is a risk of corrosion, but risk of corrosion is not as great as one might think. Although tar coating or cathodic protection can be employed in permanent works.

It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years. Normally the speed of corrosion is 0.2-0.5 mm/year and, in design, this value can be taken as 1mm/year

iv) Composite piles

Combination of different materials in the same of pile. As indicated earlier, part of a timber pile which is installed above ground water could be vulnerable to insect attack and decay. To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is installed under the ground water level (see figure 1.7).

  

(II)  Classification of pile with respect to effect on the soil

A simplified division into driven or bored piles is often employed.

i) Driven piles

Driven piles are considered to be displacement piles. In the process of driving the pile into the ground, soil is moved radially as the pile shaft enters the ground. There may also be a component of movement of the soil in the vertical direction. 

Figure 1-8 driven piles

 ii) Bored piles

Bored piles(Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced. Piles can be produced by casting concrete in the void. Some soils such as stiff clays are particularly amenable to the formation of piles in this way, since the bore hole walls do not requires temporary support except cloth to the ground surface. In unstable ground, such as gravel the ground requires temporary support from casing or bentonite slurry. Alternatively the casing may be permanent, but driven into a hole which is bored as casing is advanced. A different technique, which is still essentially non-displacement, is to intrude, a grout or a concrete from an auger which is rotated into the granular soil, and hence produced a grouted column of soil.

There are three non-displacement methods: bored cast- in - place piles, particularly pre-formed piles and grout or concrete intruded piles.

The following are replacement piles:

Augered

Cable percussion drilling

Large-diameter under-reamed

Types incorporating pre caste concrete unite

Drilled-in tubes

Mini piles

III) Special Types Of Piles:

1)      Micropiles:

Micropiles, also called mini piles, are used for underpinning. Micropiles are normally made of steel with diameters of 60 to 200 mm. Installation of micropiles can be achieved using drilling, impact driving, jacking, vibrating or screwing machinery.

Where the demands of the job require piles in low headroom or otherwise restricted areas and for specialty or smaller scale projects, micropiles can be ideal. Micropiles are often grouted as shaft bearing piles but non-grouted micropiles are also common as end-bearing piles.

2)      Tripod Piles

The use of a tripod rig to install piles is one of the more traditional ways of forming piles, and although unit costs are generally higher than with most other forms of piling, it has several advantages which have ensured its continued use through to the present day. The tripod system is easy and inexpensive to bring to site, making it ideal for jobs with a small number of piles. It can work in restricted sites (particularly where height limits exist), it is reliable, and it is usable in almost all ground conditions.

3)      Sheet Piles

Sheet piling is a form of driven piling using thin interlocking sheets of steel to obtain a continuous barrier in the ground. The main application of steel sheet piles is in retaining walls and cofferdams erected to enable permanent works to proceed.

4)      Shoulder Piles

A soldier pile wall using reclaimed railway sleepers as lagging.

Soldier piles, also known as king piles or Berlin walls, are constructed of wide flange steel H sections spaced about 2 to 3 m apart and are driven prior to excavation. As the excavation proceeds, horizontal timber sheeting (lagging) is inserted behind the H pile flanges.

The horizontal earth pressures are concentrated on the soldier piles because of their relative rigidity compared to the lagging. Soil movement and subsidence is minimized by maintaining the lagging in firm contact with the soil.

Soldier piles are most suitable in conditions where well constructed walls will not result in subsidence such as over-consolidated clays, soils above the water table if they have some cohesion, and free draining soils which can be effectively dewatered, like sands.

Unsuitable soils include soft clays and weak running soils that allow large movements such as loose sands. It is also not possible to extend the wall beyond the bottom of the excavation and dewatering is often required.

5)      Suction Piles

Suction piles are used underwater to secure floating platforms. Tubular piles are driven into the seabed (or more commonly dropped a few metres into a soft seabed) and then a pump sucks water out the top of the tubular, pulling the pile further down.

The proportions of the pile (diameter to height) are dependent upon the soil type: Sand is difficult to penetrate but provides good holding capacity, so the height may be as short as half the diameter; Clays and muds are easy to penetrate but provide poor holding capacity, so the height may be as much as eight times the diameter. The open nature of gravel means that water would flow through the ground during installation, causing 'piping' flow (where water boils up through weaker paths through the soil). Therefore suction piles cannot be used in gravel seabeds.

Once the pile is positioned using suction, the holding capacity is simply a function of the friction between the pile skin and the soil, along with the self-weight and weight of soil held within the pile. The suction plays no part in holding capacity because it relieves over time. The wall friction may increase slightly as pore pressure is relieved. One notable failure occurred (pullout) because there was poor contact between steel and soil, due to a combination of interal ring stiffeners and protective painting of the steel walls.

6)       Adfreeze Piles

In extreme latitudes where the ground is continuously frozen, adfreeze piles are used as the primary structural foundation method.

Adfreeze piles derive their strength from the bond of the frozen ground around them to the surface of the pile. Typically the pile is installed in a pre-drilled hole 6"-12" larger then the diameter of the pile. A slurry mixture of sand and water is then pumped into the hole to fill the space between the pile and the frozen ground. Once this slurry mixture freezes it is the shear strength between the frozen ground and the pile, or the adfreeze strength, which support the applied loads.

Adfreeze pile foundations are particularly sensitive in conditions which cause the permafrost to melt. If a building is constructed improperly, it will heat the ground below resulting in a failure of the foundation system.

Another ongoing concern for adfreeze pile foundations is climate change. As the climate warms, these foundations lose their strength and will eventually fail.

The hierarchial chart representation given below can be used  for a quick understanding of pile classification:   

Advantages and disadvantages of different pile material

Wood piles:

Advantages:

+ The piles are easy to handle

+ Relatively inexpensive where timber is plentiful.

+ Sections can be joined together and excess length easily removed.

Disadvantages:

-- The piles will rot above the ground water level. Have a limited bearing capacity.

-- Can easily be damaged during driving by stones and boulders.

-- The piles are difficult to splice and are attacked by marine borers in salt water.

Prefabricated concrete piles (reinforced) and pre stressed concrete piles. (driven) affected by the ground water conditions.

Advantages:

+ Do not corrode or rot.

+ Are easy to splice. Relatively inexpensive.

+ The quality of the concrete can be checked before driving.

+ Stable in squeezing ground, for example, soft clays, silts and peats pile material can be inspected before piling.

+ Can be re driven if affected by ground heave. Construction procedure unaffected by ground water.

+ Can be driven in long lengths. Can be carried above ground level, for example, through water for marine structures.

+ Can increase the relative density of a granular founding stratum.

Disadvantages:

-- Relatively difficult to cut.

-- Displacement, heave, and disturbance of the soil during driving.

-- Can be damaged during driving. Replacement piles may be required.

-- Sometimes problems with noise and vibration.

-- Cannot be driven with very large diameters or in condition of limited headroom.

Driven and cast-in-place concrete piles

Permanently cased (casing left in the ground)

Temporarily cased or uncased (casing retrieved)

Advantages:

+ Can be inspected before casting can easily be cut or extended to the desired length.

+ Relatively inexpensive.

+ Low noise level.

+ The piles can be cast before excavation.

+ Pile lengths are readily adjustable.

+ An enlarged base can be formed which can increase the relative density of a granular founding stratum leading to much higher end bearing capacity.

+ Reinforcement is not determined by the effects of handling or driving stresses.

+ Can be driven with closed end so excluding the effects of GW

Disadvantages:

-- Heave of neighbouring ground surface, which could lead to re consolidation and the development of negative skin friction forces on piles.

-- Displacement of nearby retaining walls. Lifting of previously driven piles, where the penetration at the toe have been sufficient to resist upward movements.

-- Tensile damage to unreinforced piles or piles consisting of green concrete, where forces at the toe have been sufficient to resist upward movements.

-- Damage piles consisting of uncased or thinly cased green concrete due to the lateral forces set up in the soil, for example, necking or waisting. Concrete cannot be inspected after completion. Concrete may be weakened if artesian flow pipes up shaft of piles when tube is withdrawn.

-- Light steel section or Precast concrete shells may be damaged or distorted by hard driving.

-- Limitation in length owing to lifting forces required to withdraw casing, nose vibration and ground displacement may a nuisance or may damage adjacent structures.

-- Cannot be driven where headroom is limited.

-- Relatively expensive.

-- Time consuming. Cannot be used immediately after the installation.

-- Limited length.

Bored and cast in -place (non -displacement piles)

Advantages:

+ Length can be readily varied to suit varying ground conditions.

+ Soil removed in boring can be inspected and if necessary sampled or in- situ test made.

+ Can be installed in very large diameters.

+ End enlargement up to two or three diameters are possible in clays.

+ Material of piles is not dependent on handling or driving conditions.

+ Can be installed in very long lengths.

+ Can be installed with out appreciable noise or vibrations.

+ Can be installed in conditions of very low headroom.

+ No risk of ground heave.

Disadvantages:

-- Susceptible to "waisting" or "necking" in squeezing ground.

-- Concrete is not placed under ideal conditions and cannot be subsequently inspected.

-- Water under artesian pressure may pipe up pile shaft washing out cement.

-- Enlarged ends cannot be formed in cohesionless materials without special techniques.

-- Cannot be readily extended above ground level especially in river and marine structures.

-- Boring methods may loosen sandy or gravely soils requiring base grouting to achieve economical base resistance.

-- Sinking piles may cause loss of ground I cohesion-less leading to settlement of adjacent structures.

Steel piles (Rolled steel section)

Advantages:

+ The piles are easy to handle and can easily be cut to desired length.

+ Can be driven through dense layers. The lateral displacement of the soil during driving is low (steel section H or I section piles) can be relatively easily spliced or bolted.

+ Can be driven hard and in very long lengths.

+ Can carry heavy loads.

+ Can be successfully anchored in sloping rock.

+ Small displacement piles particularly useful if ground displacements and disturbance critical.

Disadvantages:

-- The piles will corrode,

-- Will deviate relatively easy during driving.

-- Are relatively expensive.