Classification of piles based on execution method

Pile Foundation: Execution methods

Piles can be executed either

- by driving or by boring

Driven piles are those formed by driving precast piles and thosemade by casting concrete in a hole formed by driving.

Bored piles are those formed by casting concrete in holepreviously bored or drilled in the subsoil.

Driven Piles Drilled/Bored Piles

Pile driving is the process by way of which a pile is forced ordriven into the ground without excavation or boring

Driven piles are considered to be displacement piles. In theprocess of driving the pile into the ground, soil is moved radiallyas the pile shaft enters the ground. There may also be acomponent of movement of the soil in the vertical direction.

There are several types of pile driving method in the pilingconstruction, type of pile driving method used is depending onthe condition of soil and location of the site. However, differentpiling machine will be used with different pile driving methodstoo.

Driven piles

Generally Piles are driven by means of a hammer supported bya crane or by a special device known as a pile driver.

The hammer is guided between two parallel steel membersknown as leads.

The leads are carried on a frame in such a way that they can besupported in a vertical position of an inclined position.

Driving hammers are of the following types:

1. Drop Hammer

2. Single acting hammer

3. Double acting hammer

4. Diesel hammer

5. Vibratory hammer

Driven Piles

Driven piles are either wood, reinforced concrete, or steel.

Wooden piles are made from trunks of tall trees.

Concrete piles are available in square, octagonal, andround cross-sections. They are reinforced with rebar andare often prestressed.

Steel piles are either pipe piles or some sort of beamsection (like an H-pile).

Driving piles, as opposed to drilling shafts, isadvantageous because the soil displaced by driving thepiles compresses the surrounding soil, causing greaterfriction against the sides of the piles, thus increasing theirload-bearing capacity.

Bored piles (Replacement piles) are generally considered to benon- displacement Piles: a void is formed by boring orexcavation before piles is produced.

Piles can be produced by casting concrete in the void.

Some soils such as stiff clays are particularly amenable to theformation of piles in this way, since the bore hole walls do notrequires temporary support except cloth to the groundsurface.

Bored piles

In unstable ground, such as gravel the ground requirestemporary support from casing or bentonite slurry.Alternatively the casing may be permanent, but driven into ahole 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 augerwhich is rotated into the granular soil, and hence produced agrouted column of soil.

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

Also called drilled piers or Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles.Rotary boring techniques offer larger diameter piles than any other piling method andpermit pile construction through particularly dense or hard strata. Constructionmethods depend on the geology of the site. In particular, whether boring is to beundertaken in 'dry' ground conditions or through water-logged but stable strata - i.e.'wet boring'.

Boring is done until the hard rock or soft rock layer is reached in the case of endbearing piles. If the boring machine is not equipped with a rock auger, then socketingof the hard rock layer is done with the help of a heavy chisel which is dropped from aheight of about 1.5 metres (depends on the weight of the chisel and designrequirements) by suspending it from a tripod stand attached to a winch crane. Thesocketing is carried out until the desired depth within the rock layer has beenattained. Usually, the required depth within the rock layer is considered to be equal tothe diameter of the pile in hard rock layers and is taken to be equal to 2.5 times thediameter of the pile in soft rock layers.

'Dry' boring methods employ the use of a temporary casing to seal the pile borethrough water-bearing or unstable strata overlying suitable stable material. Uponreaching the design depth, a reinforcing cage is introduced, concrete is poured in thebore and brought up to the required level. The casing can be withdrawn or left in situ.

Drilled/Bored piles

'Wet' boring also employs a temporary casing through unstable ground and is usedwhen the pile bore cannot be sealed against water ingress. Boring is thenundertaken using a digging bucket to drill through the underlying soils to designdepth. The reinforcing cage is lowered into the bore and concrete is placed bytremmie pipe, following which, extraction of the temporary casing takes place.

The reinforcement cage may need to be lapped with another cage if the depth ofthe pile exceeds 12 metres as that is the standard length of reinforcement bars ofdiameter 16mm and above.

In some cases there may be a need to employ drilling fluids (such as bentonitesuspension) in order to maintain a stable shaft. Rotary auger piles are available indiameters from 350 mm to 2400 mm or even larger and using these techniques,pile lengths of beyond 50 metres can be achieved.

Such piles commonly fail due to the collapse of the walls of the shaft resulting inthe formation of a reduced section which may not be able to bear the loads forwhich it had been designed. Hence at least a third of piles in projects with a largenumber of piles are tested for uniformity using a "Pile Integrity Tester". This testrelies on the manner in which low intensity shock waves are affected as they passthrough the pile and are reflected to judge the uniformity and integrity of the pile.A pile failing the integrity test is then subjected to a pile load test

Drilled Piles

Bored pile is a cast-in-place concrete pile where the bored piles have tobe cast on construction site.

Bored piling is cast by using bored piling machine which has speciallydesigned drilling tools, buckets and grabs, its used to remove the soiland rock. Normally it can be drilling into 50metres depth of soil. Theadvantage of bored piling is its drilling method, little vibration andlower noise level.

Usually bored pile is used for those tall buildings or massive industrialcomplexes, which require foundations which can bear the load ofthousands of tons, most probably in unstable aor difficult soilconditions.

The method of drilling bored pile is different from RC Square pile orspun pile which are using driving method, the piling machine to be usedwill be different too.

Bored piling works required specialist bored piling contractor to followup instead of hiring general piling contractor.

There is a simple step-by-step method on how to drill the bored pile inconstruction site.

How To Drill Bored Pile?

Bored Piles Construction How Bored Piles Are Drilled

1.The first step is to drill a hole of the require diameter into theground.2.When the design depth or foundation bearing layer isreached, drilling stopped.3.The hole is then cleaned.4.Steel reinforcement cage is lowered into the hole.5.Concrete is then poured into the hole until the designed cut-off level of the pile.6.After Step5, the construction of bored pile is completed.Thus it can be seen that bored piles are constructed by firstdrilling into ground. Drilling is one of the most important stepin bored pile construction. Only successful drilling can lead tothe next construction step to be carried out. Thus drillingrequires experience and the use of the right tools andequipment.

1) Adjust the piling machine on the pile axis

and haul with the auger to the drilling

place.

2) Start drilling until the auger is filled.

3) Return from the drilling level to the top of

the pile hole4) Swing to the uploading area.

5) Unload the dirt in the unloading area 6) Swing back to the top of the hole

7) Relocate the machine and start steps 1 to

6 Repeat steps 2 to 7 until the pile is

completely drilled.8 ) Start erecting rebar cage using a crane.

9) Use funnel for dry method and tremie

for wet method. Start pouring the concrete

and finish the pile

The drilling method is depending on

the condition of soil, piling contractor

has to do soil investigation and decide

which drilling technology has to be

carried on. Piling contractor decide

the correct drilling technology and

minimize disturbance of the

surrounding soil. For cohesionless

soils such as sands, gravels, silts etc,

whether its under the water table ornot, the pile bore hole must be

supported using steel casing or

stabilizing muds such as bentonite

suspension. After these, reinforcement

bar will be put into the bore hole and

concrete will be poured into the bore

hole.

Drilling methods for bored piles

Bored piles can be drilled by using anauger ( see attached picture of an

auger) that is powered mechanically.

There will be an auger attached to a

the tip of a telescopic Kellybar.

Telescopic means the Kellybar isextensible, there by enabling drilling to

greater depths.

Drilling rigs can be crane mountedor as a single unit known as rotary

drilling rig. Both of which is discussed

in greater details below.

Crane Mounted Drilling Rig

The drilling rig is a power unit that rotates the Kellybar which in turn rotates the auger. The power unit is normally diesel powered. The

Kellybar is slotted through the power unit at the front end known as

the Table. Horizontal rotation of the diesel engine is converted to vertical rotation which rotates the Kellybar.

There is also a pair hydraulic arms at the table. These arms canclamped to the Kellybar and are use to raise or lower the the

it slowly over a short lenght. Rotation can be either anticlockwise or

clockwise. The whole power unit, Kellybar and auger assembly is

then mounted onto a base crane. Drilling starts by pressing and rotating the auger at the same time.

Once the initial hole is formed and depending on the soil material,the weight of the Kellybar pushes the drilling deeper.

The Kellybar is raised and lowered into the hole by a separate liftingcable of the base crane. This enables the Kellybar and auger to belifted up quickly to the surface to dispose of the drilled material andlater lowered back again to the hole to continue drilling. Thisprocesses is repeated until the desired depth is reached.

The drilling rig can also be mounted onto

a heavy truck like what is shown in the

picture.

Name of Parts

1 KellyBar

2 Auger

3 Diesel Powered Drilling Rig

4 Base Crane

5 Table

6 Hydraulic arms

Crane Mounted Drilling Rig

A crane mounted rig requires on siteassembly and requires the help ofanother crane to mount and set up.As for rotary drilling rigs, there is nosuch requirement.

Once mobilized to site it is just amatter of raising up the leader ,attached the auger and drilling canstart.

A compact design in whichthe drilling rig is permanently

mounted onto a crawler unit.

The drilling rig is hydraulic

powered giving it a very

powerful drilling capability.

The drilling process is the

same as what is mentioned

above.

However rotary drilling unitis easy to deploy. There is

no setting up required

compared to crane mounted

drilling rig.

Rotary Drilling Rigs

Drilling by Using Continuous Flight Auger

Drilling of bored piles can also becarried out by using continuous

flight auger (CFA). CFA is used

when bored piles depths are fixed

and the diameter not larger than

1500mm. The construction of

bored piles using CFA is different

from the auger and Kellybar

drilling.

The major difference is this, whenthe bored pile desired depth is

reached, the hole can be filled by

injecting concrete down the hollow

stem of the CFA. In this way the

bored pile is concreted as the

auger is slowly raised.

After completing the concreting and the auger had been removed, areinforcing cage is pushed down the shaft while the concrete has notset. It is not uncommon to push down cage length of 12m. Thus itcan be seen that the final steps of bored piles construction by CFA isdifferent from the Kellybar and auger process.

However if the drilled hole depth is shallow and the ground stable (the drilled hole does not collapse), the CFA can be completelyremoved. In this case, the concreting can be carried out like those ofthe Kellybar and auger method.

Modern drilling rigs can drill bored piles of diameters ranging from600mm to 4000mm. Due to the convenience of the rotary drillingrigs, they have become more common nowadays as compared tothe crane mounted drilling rigs. Rotary drilling rigs are self-mountingand also easily transportable. On the whole this capability whichreduces cost of mobilization.

2.5 m diameter off shore piles for Del Pan BridgeOff-shore piles for LRT-1 bridge crossing

Pasig River

Driving of steel casing for Aloragat

Bridge bored piles

Sometime referred to as drilled piers, bored piles are cast-in-place piles

ranging from 600mm to 6000mm in diameter with depth that can reach down

to 100 meters. Bored piles are installed by first removing the soil by a

drilling process and then constructing the pile by placing concrete in the

hole. The simplest form of construction consists of drilling an unlined or

unprotected hole and filling it with concrete. Complications that may arise

such as difficult ground conditions and the presence of ground water have

led to the development of special drilling technologies. The choice of the

correct drilling technology must be done in a way as to minimize

disturbance of the surrounding soil. For cohesionless soils (sands, gravels,

silts), whether under the water table or not, the pile borehole must be

supported using steel casing or stabilizing muds such as bentonite

suspension.

Piles may be used to support pad, strip or raft foundation. Normally, pile foundations consist of pile cap and a group of

piles. Where a group of piles is used to support a column or

pier base.

The individual piles are spaced and connected to the pile capor 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.

The load from the column or pier is transmitted to the pilesthrough a concrete pile cap which is cast over the pile. The

pile cap distributes the applied load to the individual piles

which, in turn,. transfer the load to the bearing ground.

To provide structural continuity the reinforcement of the pilecaps through starter bars protruding from the top of the cast in

situ piles or through reinforcement exposed by breaking off

the top concrete from precast piles.

Grouping of piles and Pile caps:

Q = Vertically applied load

H = Horizontally applied load

Figure 2-1 Basic formation of pile groups

Figure bellow illustrates the three basic formation of pile groups.

Piled WallVertical And Raked Piles

Pile GroupsSingle Piles

SELECTION OF THE TYPE OF PILE

Different types of piling have been devised to suit different groundconditions.

Hence the nature of the ground, where the piling operation is to becarried out, determines to a large extent the choice of type of pile tobe used.

The other important factors in this regard:- 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.

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