PILE FOUNDATIONS

      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).

 

Figure 1-1 End bearing piles

Figure 1-2 Friction or cohesion pile

 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)

 

Figure 1-3 under-reamed base enlargement to a bore-and-cast-in-situ pile

 (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 .

 

Figure 1-4 a) concrete pile connecting detail. b) squared pre-cast concert pile

 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.

  

 

                                                                                                                                    Hercules type of pile joint

  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

 

a) X- cross-section

b) H - cross-section

c) steel pipe

 

Figure 1-6 Steel piles cross-sections

 

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).

  

Figure 1-7 Protecting timber piles from decay:
a) by pre-cast concrete upper section above water level.
b) by extending pile cap below water level

 

(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:

                          

A micropile installation

 

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.

 

 

 

5. LOAD ON PILES

 

Introduction:

                                       Piles can be arranged in a number of ways so that they can support load imposed on them. Vertical piles can be designed to carry vertical loads as well as lateral loads. If required, vertical piles can be combined with raking piles to support horizontal and vertical forces.

                                      often, if a pile group is subjected to vertical force, then the calculation of load distribution on single pile that is member of the group is assumed to be the total load divided by the number of piles in the group. However if a group of piles is subjected to lateral load or eccentric vertical load or combination of vertical and lateral load which can cause moment force on the group which should be taken into account during calculation of load distribution.

 

Pile arrangement

Normally, pile foundations consist of pile cap and a group of piles. The pile cap distributes the applied load to the individual piles which, in turn,. transfer the load to the bearing ground. The individual piles are spaced and connected to the pile cap or tie beams and trimmed in order to connect the pile to the structure at cut-off level, and depending on the type of structure and eccentricity of the load, they can be arranged in different patterns. Figure 1 below illustrates the three basic formation of pile groups.

 

 

a) PILE GROUP CONSIST OF ONLY VERTICAL PILES

b) PILE GROUP CONSIST OF BOTH VERTICAL AND RAKING PILES

c) SYMMETRICALLY ARRANGED VERTICAL AND RAKING PILES



Q = Vertically applied load
H = Horizontally applied load

 

 

 

Figure -1 Basic formation of pile groups

 

 

 

LOAD DISTRIBUTION

To a great extent the design and calculation (load analysis) of pile foundations is carried out using computer software. For some special cases, calculations can be carried out using the following methods.For a simple understanding of the method, let us assume that the following conditions are satisfied:

The pile is rigid

The pile is pinned at the top and at the bottom

Each pile receives the load only vertically (i.e. axially applied );

The force P acting on the pile is proportional to the displacement U due to compression

P = k.U

……………………….1

Since P = E.A

E.A = k.U

……………………….2

 

……………………….3

 

where:

P = vertical load component

k = material constant

U = displacement

E = elastic module of pile material

A = cross-sectional area of pile

Figure 2: load on single pile

The length L should not necessarily be equal to the actual length of the pile. In a group of piles, If all piles are of the same material, have same cross-sectional area and equal length L , then the value of k is the same for all piles in the group.

Let us assume that the vertical load on the pile group results in vertical, lateral and torsion movements. Further, let us assume that for each pile in the group, these movements are small and are caused by the component of the vertical load experienced by the pile. The formulae in the forthcoming sections which are used in the calculation of pile loads, are based on these assumptions.

 

5.1 Pile foundations: vertical piles only

Here the pile cap is causing the vertical compression U, whose magnitude is equal for all members of the group. If Q (the vertical force acting on the pile group) is applied at the neutral axis of the pile group, then the force on a single pile will be as follows :

………………….4

 where:

Pv = vertical component of the load on any pile from the resultant load Q

n = number of vertical piles in the group (see fig3.4)

Q = total vertical load on pile group

 

If the same group of piles are subjected to an eccentric load Q which is causing rotation around axis z (see fig 3.1); then for the pile i at distance rxi from axis z:

           ………………………….5

Pi = force (load on a single pile )

Ui = displacement caused by the eccentric force (load) Q

rxi = distance between pile and neutral axis of pile group;

rxi positive measured the same direction as e and negative when in the opposite direction.

e = distance between point of intersection of resultant of vertical and horizontal loading with underside of pile (see figure 3.8)

 

The sum of all the forces acting on the piles should be zero

……………………..6

 

Figure 3-2 Moment

 

If we assume that the forces on the piles are causing a moment M about axis z-z then the sum of moments about axis z-z should be zero (see figure 3.1 a& b)

 

 

…………………7

 

from e.q. 3.2 we see that

 

MZ =  MZ

 

………………………...8

 

applying the same principle, in the x direction we get equivalent equation.If we assume that the moment MX and MZ generated by the force Q are acting on a group of pile, then the sum of forces acting on a single pile will be as follows:

………………...9

 

if we dividing each term by the cross-sectional area of the pile, A, we can establish the working stream  :

 5.2 Pile foundations: vertical and raking piles

To resist lateral forces on the pile group, it is common practice to use vertical piles combined with raking piles (see figure3-5) The example below illustrates how the total applied load is distributed between the piles and how the forces acting on each pile are calculated.

Figure 3-5 Load distribution for combined vertical and raking piles

 To derive the formulae used in design, we first go through the following procedures:-

1. Decide the location of the N.A of the vertical and the raking piles in plan position. (see example below).
2. Draw both N.A till they cross each other at point c, this is done in Elevation and move the forces Q, H& M to point c (see fig.3.5 elevation).
3. Let us assume that the forces Q &M cause lateral and torsional movements at point c.
4. Point c is where the moment M is zero. Y is the moment arm (see fig. 3.5)

Figure 3.6 shows that the resultant load R (in this case Q) is only affecting the vertical piles.

 

Figure 3-6

  

n = number of vertical piles

m = number of raking piles

Pv =

As shown in figure 3.6 the lateral force, H, is kept in equilibrium by the vertical and the raking piles.

∑ H = 0=> H-m× Psinα = 0

∑ V = 0 => m× Pr x cosineα - n× Pv = 0

where:

Pr = H/(m sinα)

Pv = H/(n tanα )

Figure 3-7

 

NB : The horizontal force, H, imposes a torsional force on the vertical piles.

 

Sum of forces on a single pile = Q + H + M

as a result of Q: Pvi = Q/n

as a result of H: Pvi = - H/(n tan )

as a result of H: Pri = + H/(m sin )

as a result of moment M:

ri measured perpendicular to the N.A of both the vertical and raking piles

 

 

 

5.3 Symmetrically arranged vertical and raking piles

Just as we did for the previous cases, we first decide the location of the neutral axis for both the vertical and raking piles.

Extend the two lines till they intersect each other at point c and move the forces Q & H to point C. (see fig.11)

Figure 3-11symmetrically arranged piles

In the case of symmetrically arranged piles, the vertical pile I is subjected to compression stress by the vertical component Pv and the raking pile Pr is subjected to tension (see figure 3.11 - 12)

 

Figure 3-12

Figure 3-13

 

Pv = k (U)

pr = k (U cosα ) = PV cosα

∑ V = 0 => Q – n× Pv - m × Pr cos.α = 0

Pr = Pv cosα => Pv =

The symmetrical arrangement of the raking piles keeps the lateral force, H, in equilibrium and its effect on the vertical piles is ignored.

With reference to figure 3.13 Horizontal projection of forces yield the following formulae.

∑ H = 0 , therefore,

 

Figure 3-14

NB the lateral force H imposes torsional stress on half of the raking piles.

5.3.1 Example on installation error

Until now we have been calculating theoretical force distribution on piles. However during installation of piles slight changes in position do occur and piles may miss their designed locations. The following example compares theoretical and the actual load distribution as a result of misalignment after pile installation.

Figure 3-16 Example on installation error

 

Before installation (theoretical position) see fig.3-16

Q = 500 kN and MX = 500 × 0.3 = 150

MZ = 500× 0 = 0

Q/n = 500/6 = 83.3 kN

Pi = Q/n ± ( Mz × rxi)/ ∑ r2xi

∑ r2xi = 0.72 × 3 = 0.73 = 2.94 m2

And Pi = 83.3 - (150/2.94)× rxi

P1,2,3 = 83.3- (150/2.94)× 0.7 = 47.6 kN

P4,5,6 = 83.3 + (150/2.94) × 0.7 = 119 kN

 

After installation

Displacement of piles in the X-X direction measured, left edge of pile cap as reference point (see figure 3.17)

Figure 3-17 piles after installation

 

Therefore, The new neutral axis (N A) for the pile group:

(0.5+0.6+0.4+2.0+2.1+1.7) 1 = 6 e  e = 1.22 m

The new position of Q = 0.29 m

Thus, M = 500 x (0.29) = 145 kNm

Measured perpendicular to the new N.A, pile distance, ri, of each pile:

ri1 = 1.22-0.5 = 0.72

ri2 = 1.22-2.0 = -0.79

ri3 = 1.22-0.6 = 0.62

ri4 = 1.22-2.1 = -0.88

ri5 = 1.22-0.4 = 0.82

ri6 = 1.22-1.7 = -0.49

∑ r2xi = 0.722 + 0.792 + 0.622 + 0.882 + 0.822 + 0.492 = 3.2 m

 

pile

Q/N (kN)

45.3 (rxi)

sum of forces on each pile



Pi = Q/n + ( Mz x rxi)/ ∑ r2xi

1

83.3

45.3 (-0.72)

51

2

 

45.3 (0.79)

49

3

 

45.3 (-0.62)

55

4

 

45.3 (0.88)

123

5

 

45.3 (-0.82)

47

6

 

45.3 (0.49)

105

6..SINGLE PILE DESIGN

6.1 End bearing piles

If a pile is installed in a soil with low bearing capacity but resting on soil beneath with high bearing capacity, most of the load is carried by the end bearing.

In some cases where piles are driven in to the ground using hammer, pile capacity can be estimated by calculating the transfer of potential energy into dynamic energy . When the hammer is lifted and thrown down, with some energy lose while driving the pile, potential energy is transferred into dynamic energy. In the final stage of the pile's embedment,On the bases of rate of settlement, it is able to calculate the design capacity of the pile.

For standard pile driving hammers and some standard piles with load capacity (FRsp,), the working load for the pile can be determined using the relationship between bearing capacity of the pile, the design load capacity of the pile described by: FRsp �  n� FSd and table 5-1

where: FS

 

Table 6-1 Bearing capacity of piles installed by hammering

hammer

DROP HAMMER (released by trigger)

drop hammer (activated by rope and friction winch

 

 

cross-sectional area of pile

cross-sectional area of pile

 

fall height

0.055m2

0.073m2

fall height

0.055m2

0.073m2

3 TON

0.3

0.4

0.5

420 kN

490

560

450 kN

520

590

0.4

0.5

0.6

390 kN

450

520

420 kN

480

540

4 TON

0.3

0.4

0.5

470

540

610

510

590

680

0.4

0.5

0.6

440

510

550

480

550

610

5 TON

0.3

0.4

0.5

580

670

760

640

740

840

0.4

0.5

0.6

550

610

670

600

660

730

d = design load for end bearing.

The data is valid only if at the final stage, rate of settlement is 10 mm per ten blow. And pile length not more than 20 m and geo-category 2 . for piles with length 20 - 30 m respective 30 - 50 m the bearing capacity should be reduced by 10 res. 25%.

 6.2 Friction piles

Load on piles that are driven into friction material, for the most part the weight is carried by friction between the soil and the pile shaft. However considerable additional support is obtained form the bottom part.

In designing piles driven into friction material, the following formulas can be used

………………………… 6.1

where: qci = consolidation resistance

* can be decided using table 10-4

Ab = end cross-sectional area of the pile

Ami = shaft area of the pile in contact with the soil.

should be � 1.5 for piles in friction material

qcs = end resistance at the bottom of the pile within 4 � pile diameter from the end of the pile

Figure 6-1 Friction Pile

6.3 Cohesion piles

Piles installed in clay: The load is carried by cohesion between the soil and the pile shaft. Bearing capacity of the pile can be calculated using the following formula for pile installed in clay.

………………………… 6.2

Where:

a i = adhesion factor for earth layer

cudci = undrained shear strength of clay.

Ami = area of pile shaft in contact with the soil.

The adhesion factor  is taken as 0 for the firs three meters where it is expected hole room and fill material or week strata. For piles with constant cross-sectional area the value of  can be taken as 1.0 and for piles with uniform cross-sectional growth the value of  can be taken as 1.2 .

Figure 6-2 Cohesion Pile

6.4 Steel piles

Because of the relative strength of steel, steel piles withstand driving pressure well and are usually very reliable end bearing members, although they are found in frequent use as friction piles as well. The comment type of steel piles have rolled H, X or circular cross-section(pipe piles). Pipe piles are normally, not necessarily filled with concrete after driving. Prior to driving the bottom end of the pipe pile usually is capped with a flat or a cone-shaped point welded to the pipe.

Strength, relative ease of splicing and sometimes economy are some of the advantages cited in the selection of steel piles.

The highest draw back of steel piles is corrosion. Corrosive agents such as salt, acid, moisture and oxygen are common enemies of steel. Because of the corrosive effect salt water has on steel, steel piles have restricted use for marine installations. If steel pile is supported by soil with shear strength greater than 7kPa in its entire length then the design bearing capacity of the pile can be calculated using the following formulas. Use both of them and select the lowest value of the two:

 

………………………… 6.3

 

………………………… 6.4

 

Where: m = correction factor

ESC = elasticity module of steel

I = fibre moment

fyc characteristic strength of steel

A = pile cross-sectional area

Cuc = characteristic undrained shear strength of the soil.

6.5 Concrete piles

Relatively, in comparable circumstances, concrete piles have much more resistance against corrosive elements that can rust steel piles or to the effects that causes decay of wood piles, furthermore concrete is available in most parts of the world than steel.

Concrete piles may be pre-cast or cast-in place. They may be are reinforced, pre-stressed or plain.

6.5.1 Pre-cast concrete piles

These are piles which are formed, cast to specified lengths and shapes and cured at pre casting stations before driven in to the ground. Depending up on project type and specification, their shape and length are regulated at the prefab site. Usually they came in square, octagonal or circular cross-section. The diameter and the length of the piles are mostly governed by handling stresses. In most cases they are limited to less than 25 m in length and 0.5 m in diameter. Some times it is required to cut off and splice to adjust for different length. Where part of pile is above ground level, the pile may serve as column.

If a concrete pile is supported by soil with undrained shear strength greater than 7 MPa in its entire length, the following formula can be used in determining the bearing capacity of the pile :

 

………………………… 5.5

 

………………………… 5.6

 

Where: Nu = bearing capacity of the pile, designed as concrete column

Esc = characteristic elasticity module of concrete

Ic = fibre moment of the concrete cross-section ignoring the reinforcement

Cuc = characteristic undrained shear strength of the soil in the loose part of the soil within a layer of 4.0 m

5.6 Timber piles (wood piles)

Timber piles are frequently used as cohesion piles and for pilling under embankments. Essentially timber piles are made from tree trunks with the branches and bark removed. Normally wood piles are installed by driving. Typically the pile has a natural taper with top cross-section of twice or more than that of the bottom.

To avoid splitting in the wood, wood piles are sometimes driven with steel bands tied at the top or at the bottom end.

For wood piles installed in soil with undrained shear strength greater than 7kPa the following formula can be used in predicting the bearing capacity of the pile:

………………………… 5.7

Where: = reduced strength of wood

A = cross-sectional area of the pile

If the wood is of sound timber, (e.g. pinewood or spruce wood with a diameter > 0.13m), then (reduced strength) of the pile can be taken as 11MPa.

Increase in load per section of pile is found to be proportional to the diameter of the pile and shear strength of the soil and can be decided using the following formula:

 

where: Am, = area of pile at each 3.5 m section mid point of pile

Cm = shear strength at each 3.5m section mid point of pile

dm = diameter of pile at each 3.5 m section mid point of pile

Pmi = pile load at the middle of each section 

7. DESIGN OF PILE GROUP

Introduction

Group action in piled foundation: Most of pile foundations consists not of a single pile, but of a group of piles, which act in the double role of reinforcing the soil, and also of carrying the applied load down to deeper, stronger soil strata. Failure of the group may occur either by failure of the individual piles or as failure of the overall block of soil. The supporting capacity of a group of vertically loaded piles can, in many cases, be considerably less than the sum of the capacities the individual piles comprising the group. Grope action in piled foundation could result in failure or excessive settlement, even though loading tests made on a single pile have indicated satisfactory capacity. In all cases the elastic and consolidation settlements of the group are greater than those of single pile carrying the same working load as that on each pile within the group. This is because the zone of soil or rock which is stressed by the entire group extends to a much greater width and depth than the zone beneath the single pile (fig.6-1)

Figure 7-1 Comparison of stressed zone beneath single pile and pile group

 

7.1 Bearing capacity of pile groups

Pile groups driven into sand may provide reinforcement to the soil. In some cases, the shaft capacity of the pile driven into sand could increase by factor of 2 or more.

But in the case of piles driven into sensitive clays, the effective stress increase in the surrounding soil may be less for piles in a group than for individual piles. this will result in lower shaft capacities.

Figure 7-2 Under axial or lateral load, In a group, instead of failure of individual piles in the group, block failure (the group acting as a block) may arise.

Figure 7-2 Block failure

In general ,the bearing capacity of pile group may be calculated in consideration to block failure in a similar way to that of single pile, by means of equation 4-1,but hear As as the block surface area and Ab as the base area of the block or by rewriting the general equation we get:

................................(7.1)

 

where:

As, surface area of block

Ab = base area of block (see fig.7-3)

Cb, Cs= average cohesion of clay around the group and beneath the group.

Nc = bearing capacity factor. For depths relevant for piles, the appropriate value of Nc is 9

Wp and Ws = weight of pile respective weight of soil

 

In examining the behavior of pile groups it is necessary to consider the following elements:

·         A free-standing group, in which the pile cap is not in contact with the underlying soil.

·         A "piled foundation," in which the pile cap is in contact with the underlying soil.

·         pile spacing

·         Independent calculations, showing bearing capacity of the block and bearing capacity of individual piles in the group should be made.

·         Relate the ultimate load capacity of the block to the sum of load capacity of individual piles in the group ( the ratio of block capacity to the sum of individual piles capacity) the higher the better.

·         In the case of where the pile spacing in one direction is much greater than that in perpendicular direction, the capacity of the group failing as shown in Figure 6-2 b) should be assessed.

 7.1.1 Pile groups in cohesive soil

For pile groups in cohesive soil, the group bearing capacity as a block may be calculated by mans of e.q. 4-5 with appropriate Nc value.

7.1.2 Pile groups in non-cohesive soil

For pile groups in non-cohesive soil, the group bearing capacity as a block may be calculated by means of e.q. 4-7

7.1.3 Pile groups in sand

In the case of most pile groups installed in sand, the estimated capacity of the block will be well in excess of the sum of the individual pile capacities. As a conservative approach in design, the axial capacity of a pile group in sand is usually taken as the sum of individual pile capacities calculated using formulae in 4-8.

Pile spacing and pile arrangement

In certain types of soil, specially in sensitive clays, the capacity of individual piles within the a closely spaced group may be lower than for equivalent isolated pile. However, because of its insignificant effect, this may be ignored in design. Instead the main worry has been that the block capacity of the group may be less than the sum of the individual piles capacities. As a thumb rule, if spacing is more than 2 - 3 pile diameter, then block failure is most unlikely.

It is vital importance that pile group in friction and cohesive soil arranged that even distribution of load in greater area is achieved.

Large concentration of piles under the centre of the pile cap should be avoided. This could lead to load concentration resulting in local settlement and failure in the pile cap. Varying length of piles in the same pile group may have similar effect.

For pile load up to 300kN, the minimum distance to the pile cap should be 100 mm

for load higher than 300kN, this distance should be more than 150 mm.

In general, the following formula may be used in pile spacing:

End-bearing and friction piles: S = 2.5( (d) + 0.02)L 

...............7.2

 

Cohesion piles: S = 3.5( (d) + 0.02 ) L

...............7.3

where:

d = assumed pile diameter

L = assumed pile length

S = pile centre to centre distance (spacing)

 

 

 

8.PILE INSTALATION METHODS


8.1 Introduction

The installation process and method of installations are equally important factors as of the design process of pile foundations. In this section we will discuss the two main types of pile installation methods; installation by pile hammer and boring by mechanical auger.

In order to avoid damages to the piles, during design, installation Methods and installation equipment should be carefully selected.

If installation is to be carried out using pile-hammer, then the following factors should be taken in to consideration:

·         the size and the weight of the pile

·         the driving resistance which has to be overcome to achieve the design penetration

·         the available space and head room on the site

·         the availability of cranes and

·         the noise restrictions which may be in force in the locality.

 

8.2 Pile driving methods (displacement piles)

Methods of pile driving can be categorized as follows:

1.      Dropping weight

2.      Explosion

3.      Vibration

4.      Jacking (restricted to micro-pilling)

5.      Jetting

6.      Dropping weight

7.      Explosion

8.      Vibration

9.      Jacking (restricted to micro-pilling)

10.  Jetting

 

8.2.1 Drop hammers

A hammer with approximately the weight of the pile is raised a suitable height in a guide and released to strike the pile head. This is a simple form of hammer used in conjunction with light frames and test piling, where it may be uneconomical to bring a steam boiler or compressor on to a site to drive very limited number of piles.

There are two main types of drop hammers:

·         Single-acting steam or compressed-air hammers

·         Double-acting pile hammers

 

1.Single-acting steam or compressed-air comprise a massive weight in the form of a cylinder (see fig.8-1). Steam or compressed air admitted to the cylinder raises it up the fixed piston rod. At the top of the stroke, or at a lesser height which can be controlled by the operator, the steam is cut off and the cylinder falls freely on the pile helmet.

2.Double-acting pile hammers can be driven by steam or compressed air. A pilling frame is not required with this type of hammer which can be attached to the top of the pile by leg-guides, the pile being guided by a timber framework. When used with a pile frame, back guides are bolted to the hammer to engage with leaders, and only short leg-guides are used to prevent the hammer from moving relatively to the top of the pile. Double-acting hammers are used mainly for sheet pile driving.

 

 

8.2.2 Diesel hammers

Also classified as single and double-acting, in operation, the diesel hammer employs a ram which is raised by explosion at the base of a cylinder. Alternatively, in the case of double-acting diesel hammer, a vacuum is created in a separate annular chamber as the ram moves upward, and assists in the return of the ram, almost doubling the output of the hammer over the single-acting type. In favourable ground conditions, the diesel hammer provide an efficient pile driving capacity, but they are not effective for all types of ground.

8.2.3 Pile driving by vibrating

Vibratory hammers are usually electrically powered or hydraulically powered and consists of contra-rotating eccentric masses within a housing attaching to the pile head. The amplitude of the vibration is sufficient to break down the skin friction on the sides of the pile. Vibratory methods are best suited to sandy or gravelly soil.

Jetting: to aid the penetration of piles in to sand or sandy gravel, water jetting may be employed. However, the method has very limited effect in firm to stiff clays or any soil containing much coarse gravel, cobbles, or boulders.

 8.3 Boring methods ( non-displacement piles)

8.3.1 Continuous Flight Auger (CFA)

An equipment comprises of a mobile base carrier fitted with a hollow-stemmed flight auger which is rotated into the ground to required depth of pilling. To form the pile, concrete is placed through the flight auger as it is withdrawn from the ground. The auger is fitted with protective cap on the outlet at the base of the central tube and is rotated into the ground by the top mounted rotary hydraulic motor which runs on a carrier attached to the mast. On reaching the required depth, highly workable concrete is pumped through the hollow stem of the auger, and under the pressure of the concrete the protective cap is detached. While rotating the auger in the same direction as during the boring stage, the spoil is expelled vertically as the auger is withdrawn and the pile is formed by filling with concrete. In this process, it is important that rotation of the auger and flow of concrete is matched that collapse of sides of the hole above concrete on lower flight of auger is avoided. This may lead to voids in filled with soil in concrete.

The method is especially effective on soft ground and enables to install a variety of bored piles of various diameters that are able to penetrate a multitude of soil conditions. Still, for successful operation of rotary auger the soil must be reasonably free of tree roots, cobbles, and boulders, and it must be self-supporting.

During operation little soil is brought upwards by the auger that lateral stresses is maintained in the soil and voiding or excessive loosening of the soil minimise. However, if the rotation of the auger and the advance of the auger is not matched, resulting in removal of soil during drilling-possibly leading to collapse of the side of the hole.

Figure 8-2 CFA Process

 

8.3.2 Underreaming

A special feature of auger bored piles which is sometimes used to enable to exploit the bearing capacity of suitable strata by providing an enlarged base. The soil has to be capable of standing open unsupported to employ this technique. Stiff and to hard clays, such as the London clay, are ideal. In its closed position, the underreaming tool is fitted inside the straight section of a pile shaft, and then expanded at the bottom of the pile to produce the underream shown in fig. 8-3.Normally, after installation and before concrete is casted, a man carrying cage is lowered and the shaft and the underream of the pile is inspected.

 

Figure 8 -3 a)hydraulic rotary drilling equipment b) C.F.A, c)undrreaming tool open position

 

 

8.3.3 C.H.D.P

Figure 8-4, Continuous helical displacement piles: a short, hollow tapered steel former complete with a larger diameter helical flange, the bullet head is fixed to a hallow drill pipe which is connected to a high torque rotary head running up and down the mast of a special rig. A hollow cylindrical steel shaft sealed at the lower end by a one-way valve and fitted with triangular steel fins is pressed into the ground by a hydraulic ram. There are no vibrations.

Displaced soil is compacted in front and around the shaft. Once it reaches the a suitably resistant stratum the shaft is rotated. The triangular fins either side of its leading edge carve out a conical base cavity. At the same time concrete is pumped down the centre of the shat and through the one-way valve. Rotation of the fins is calculated so that as soil is pushed away from the pile base it is simultaneously replaced by in-flowing concrete. Rates of push, rotation and concrete injection are all controlled by an onboard computer. Torque on the shaft is also measured by the computer. When torque levels reach a constant low value the base in formed. The inventors claim that the system can install a\ typical pile in 12 minute. A typical 6m long pile with an 800mm diameter base and 350mm shaft founded on moderately dense gravel beneath soft overlaying soils can achieve an ultimate capacity of over 200t. The pile is suitable for embankments, hard standing supports and floor slabs, where you have a soft silty layer over a gravel strata.

Figure 8 -4 C.H.D.P

.

           

 


                                   9. LOAD TEST ON PILES

9.1 Introduction

Pile load test are usually carried out that one or some of the following reasons are fulfilled:

·         To obtain back-figured soil data that will enable other piles to be designed.

·         To confirm pile lengths and hence contract costs before the client is committed to over all job costs.

·         To counter-check results from geotechnical and pile driving formulae

·         To determine the load-settlement behaviour of a pile, especially in the region of the anticipated working load that the data can be used in prediction of group settlement.

·         To verify structural soundness of the pile.

Test loading: There are four types of test loading:

·         compression test

·         uplift test

·         lateral-load test

·         torsion-load test

the most common types of test loading procedures are Constant rate of penetration (CRP) test and the maintained load test (MLT).

 9.1.1 CRP (constant rate of penetration)

In the CRP (constant rate of penetration) method, test pile is jacked into the soil, the load being adjusted to give constant rate of downward movement to the pile. This is maintained until point of failure is reached.

Failure of the pile is defined in to two ways that as the load at which the pile continues to move downward without further increase in load, or according to the BS, the load which the penetration reaches a value equal to one-tenth of the diameter of the pile at the base.

Fig.9-2, In the cases of where compression tests are being carried out, the following methods are usually employed to apply the load or downward force on the pile:

A platform is constructed on the head of the pile on which a mass of heavy material, termed "kentledge" is placed. Or a bridge, carried on temporary supports, is constructed over the test pile and loaded with kentledge. The ram of a hydraulic jack, placed on the pile head, bears on a cross-head beneath the bridge beams, so that a total reaction equal to the weight of the bridge and its load may be obtained.

9.1.2 MLT, the maintained increment load test

Fig.9-1, the maintained increment load test, kentledge or adjacent tension piles or soil anchors are used to provide a reaction for the test load applied by jacking(s) placed over the pile being tested. The load is increased in definite steps, and is sustained at each level of loading until all settlements has either stop or does not exceed a specified amount of in a certain given period of time.

Figure 9-1 test load arrangement using kentledge

 

 

Figure 9-2 test being carried out

 

 

 

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