2a Foundation

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Shallow foundations

2 - Foundation

The British Standard Code of Practice BS8004 defines a foundation as 'that part of

the structure designed and constructed to be in direct contact with and transmitting

loads to the ground'

1 Shallow Foundation (quick revision)

1.1 Pad footing

1.2 Strip footing

1.3 Raft footing

Solid Slab Raft

Beam and Slab Raft

Cellular Raft

1.4 Combined footing

2. Piled Foundation

2.1 Classification of Piles

2.1.1 Classification by Method of Load Transmission

End bearing piles

Friction piles

2.1.2 Classification by Method of Installation

Displacement Piles

Replacement Piles -


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2/27Diesel hammer (Source: R. Chudley)

3. Displacement Piles

Displacement piles are preformed piles made in a factory, transported to the site, and

then driven into the ground. They are also called preformed piles or driven piles.

Preformed piles may be made of reinforced concrete or steel.

(There was a pile type called driven cast-in-situ pile. It was formed by driving a

shell or a casing with a closed end into the ground and then casting the annular space

with concrete. This type of pile is now obsolete)

3.1. Piling equipment for displacement piles

Displacement piles are generally driven into the ground by one of the following

methods:

1. Percussion

2. Vibration

3.1.1 Percussive driving

Piles are driven into the ground by the

impact of heavy pile hammers. There

are several types of pile hammers:

a. Drop hammer

b. Diesel hammer

(These two types of piles are now

obsolete.)

Drop hammer


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Hydraulic hammer (Source: Junttan)

c. Hydraulic Hammer

A hydraulic hammer unit is mounted on the pile

while operating. It is similar to a drop hammer but

the weight is raised by hydraulic jacks. The weightthen falls freely under gravity on to the pile head.

Hydraulic jacks are so powerful that the weight can

be up to 18 tonnes. The stroke height is about 1-2 m.

The blow rate depends on the stroke height and

ranges from 40 to 100 blows per minutes.

Most hydraulic hammers are equipped with silencers

nowadays. They produce less noise than diesel

hammers and they do not emit exhaust fumes. They

are suitable for hard drive.

3.1.2 Vibratory driving vibration hammer

Vibration hammer is comparatively silent. It is used

in driving small displacement piles such as H-piles

and sheet piles. The equipment consists of a

vibrating unit mounted on the pile head transmitting

vibrations down the length of the pile shaft. These

vibrations are in turn transmitted to the surrounding

soil, reducing its shear strength enabling the pile to

sink into the subsoil under its own weight and also

that of the vibration hammer.

Vibration Hammer


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Crane supported leader

Piling frame(Source: R. Chudley)

3.1.3 Piling rigs

A piling rig has the function of guiding the pile at its correct alignment from the stage

of first pitching in position to its final penetration. It also carries the hammer and

maintains it in position co-axially with the pile.

3.1.4 Helmets

The head of displacement piles must be protected

from damage during driving. The protection consists

of resilient packing which is held in position by a

steel helmet. The resilient packing distributes the

impact load from the piling hammer evenly on to the

pile head.

The helmet should fit loosely around the pile, so that

the pile may rotate slightly without inducing torsion

on the pile.

Pile helmet (Source: R. Chudely)

Pile helmet (Source: Jufri & Wellman)


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3.2 Precast prestressed concrete tubular pile

Precast concrete piles used in Hong Kong are prestressed and tubular in shape. The

outside diameters of the piles are about 400-600 mm. They have a nominal bearing

capacity in the range up to 3500 KN.

Each pile length has integral steel joint plates (end plates) cast onto ends of the pile.

Lengthening is done by butt welding of the end plates.

The disadvantages of these piles are that they

produce great noise and vibration during driving

the piles. Moreover, most Hong Kong soils are

coarse, containing boulders and rocks which are

not too suitable for large diameter displacement

piles.

Driving of Precast PrestressedConcrete Pile

Steel end plate Guide ring Butt welding(Source: Jufri & Wellman)

Reinforcement of precast prestressed tubular pile (Source: Nippon Hume)


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Oslo Point(Source: Tomlinson)

Strengthening bywelding of angles(Source: Tomlinson)

3.3 Preformed Steel piles

3.3.1 Hollow steel piles

Two main types of hollow steel piles: tubular and box sections. Box piles are

normally driven with closed ends (now obsolete) with internal drop hammers. Open

ended tubes can be driven with vibration hammers. Hollow Steel piles have a high

resistance to lateral loading and buckling and good energy-absorbing properties.

They are particular suitable for structures subjected to lateral and impact loading such

as jetties and dolphins.

3.3.2 Steel H-pile

Steel H-pile is strong and tough. The displacement of the soil is little compared with

other types of displacement piles. It is widely used in HK.

Driving steel piles generally do not require pile shoes. In hard conditions, the toe of

the pile may be strengthened. Strengthening may be carried out by welding steel

plates or angles to the toe of the pile Preformed pile shoes for H-piles are also

available. In ground conditions where boulders exist, Oslo point may be used.

H-pile point(Source: Housing Department)


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Lengthening of steel H-pile is by butt-welding

Lengthening

Lengthening of steel H-pile is done by butt-welding. Splicers may be used to help

splicing.

Corrosion Protection

Steel preformed piles may deteriorate as a result rusting. Corrosion protection is of particular importance for steel piles because they are embedded in the ground and

become inaccessible for maintenance. A protective coating shall be applied on the

steel piles before being driven into the ground.

The surfaces of steel piles to which protective coatings will be applied shall be

prepared by blast cleaning.

(Source: General Specification for Civil Engineering Works)

Splicer


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3.4 Procedure of driving a displacement pile

1. Prepare a piling rig and a piling hammer.

2. Put marks on the pile length at 500 mm interval to indicate the length being

driven.3. Pitch the pile in the piling rig. Check its alignment and verticality.

4. Place a suitable pile helmet on the pile head. Mount the piling hammer on top of

it and then start driving.

5. If the whole length of the pile has been driven into the ground, splice a new

length on it, then continue the driving.

6. Repeat step 5 until it renders significant resistance against driving.7. Perform the set measurement. If the predetermined set is attained, the piling

process is completed. Otherwise repeat from step 5 until it does.

3.5 Measurement of set

Set related to the ultimate bearing capacity of the pile can be determined by pile

driving formulae by considering the parameters of the pile, the soil and the ground

condition. There are many pile driving formulae, among which the Hiley formula is

the most commonly used one.

The final set shall be measured as:

1. penetration per 10 blows, or

2. the number of blows required to produce 25 mm penetration.

Measurement of Set


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Necking of replacement pile

3.6 Advantages and disadvantages of displacement piles

3.6.1 Advantages

1. Where large numbers of piles are to be installed in easy driving conditions, it

can be more economic.

2. Projection above ground level advantageous to marine structures.

3. The pile can be inspected for quality and soundness before driving.

4. Construction operations not affected by ground water.

5. Not liable to squeezing or necking.

3.6 Disadvantages of displacement piles

1. Transporting the complete length of pile through narrow and/or congested

streets may be difficult.

2.

The driving process, which is generally percussion, can cause unacceptablenoise and vibrations.

3. Pile driving hour is limited by environmental regulations.

4. May break during driving, causing delays and replacement charges.

5. Displacement of soil during driving may damage adjacent structure or cause up

lifting of adjacent piles.

6. Cannot be driven in very large diameters.

7. Cannot be driven in conditions of low headroom.

Driven piles for marine structure


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Cheshire auger (Source: R. Chudley)Flight auger (Source: Tomlinson)

4. Replacement Pile

Replacement pile is also called 'non-displacement pile'. It is formed by boring a pile

shaft and replacing the soil with in situ concrete. In Hong Kong, it is commonly

called bored pile.

4.1 Methods and equipment for boring pile shafts

4.1.1. Rotary boring

Rotary boring involves a drilling rig which may be mounted on a mobile crane or a

truck. The drilling rig consists of a telescopic or extendable kelly bar on which a boring tool is attached.


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There are various types of boring tools:

1. Cheshire auger The auger has only a few helix turns. It has to go up and down

repeatly to cut the soil and bring the spoil to the ground surface.

2. Continuous auger (flight auger) It has a long helix. The spoil is continuously brought to the ground surface by the spiral motion. However, the drilling rig

must provide sufficient torque to overcome the great friction.

3. Drilling bucket It is designed to withstand the high torque forces developed

during penetration of dense strata. The spoil is temporarily stored in the bucket to

reduce the friction created. The frequency of up and down motions can also be

reduced as compared with that of the Cheshire auger.4. Coring bucket It is used to raise a solid core of rock.

5. Chisel - It is used break through boulder or rock.

Cheshire auger Drilling bucket Coring bucket Chisel (Source: Tomlinson)


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Button bit

4.1.3 Down the hole (DTH) drilling

A down the hole drill (DTH) is a large size hammer drill. It is equipped with a button

bit that can drill holes into rock with diameters up to 750 mm and to depths of several

hundred meters.

A conventional hammer drill becomes less efficient as the length of borehole

increases because of the loss of energy in drill stem. To overcome this difficulty the

down the hole drill was developed. Its rotary motor remains above ground level

while the pneumatic hammer is followed down the hole. The drill stem rotates in

slow motion while the drill bit strikes rapidly. The drill debris is blown out by

compressed air.

Pneumatic Hammer

Down the hole drill


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4.1.4 Reverse circulation drilling (RCD)

A reverse circulation drilling rig can drill at a fast rate in a wide range of ground

conditions including rocks. In Hong Kong, it is commonly use to form the bed rock

sockets for bored piles.

The Reverse circulation drill sits on the temporary steel casing while operating. The

Compressed air is injected through the air delivery pipe and discharge at the base of

the hollow drill stem. The rising column of air and water at the centre of the drill

stem lifts the soil and rock fragments which have been loosened by the drilling bit.

(It is termed as air lifting. Air lifting is also commonly used to clean the bottom of a

bored pile before concreting.)

Rock roller Cutter Reverse Circulation Drill


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4.1.5 Under-reaming

The base of a bored pile can be enlarged to three times the shaft diameter to increase

the bearing capacity of the pile. The method is known as under-reaming which is

done by a belling bucket.

The belling bucket is lowered to the bottom a borehole by a kelly bar with the cutter

in retracted position. It is then rotated by the kelly bar and the cutter is jacked out to

form the bell.

Belling bucket in retracted position (Source: R. CIRIA)

Belling bucket in extended positionSource: CIRIA

Belling tool for rock


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4.2 Methods of supporting boreholes

4.2.1 Self-supported (unsupported)

For small diameter bored piles and in stable soils, the borehole may remain

unsupported without collapsing. However, a borehole without support is not

recommended in Hong Kong.

4.2.2 Supported with temporary casing

In unstable or water-bearing ground, the bored pile shaft should be supported by a

steel casing to prevent collapse. The casing can be driven into the ground by a casingoscillator (or a vibration hammer).

The oscillator consists of a collar clamp and a pair of hydraulic jacks. The collar

clamp grips the casing. The two hydraulic jacks extend and retract repeatly inopposite phrases. This oscillation motion helps to cut the soil by the casing shoe and

sink the casing into to ground.

The steel casing is usually thick and expensive and will be retrieved after the

borehole is concreted. The casing oscillators (or vibrator hammer) the can be used to

withdraw the casing. In a long pile the casing may be stuck by the concrete and the

extraction becomes difficult. Moreover, temporary steel casings which are in contact

Casing oscillator Vibration hammer


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Light gauge corrugated steel casing

with concrete shall be withdrawn before the initial set of the concrete has taken place.

Otherwise, it shall be left in place.

To overcome this problem, it is advisable to extract the casing in stages, i.e., the

casing is extracted as concreting progresses upwards. However, it is important that a

sufficient concrete pressure head shall be maintained within the steel casing to ensure

that the pressure from external soil and ground water would not cause necking of the

pile.

4.2.3 Permanent casing

Sometimes a permanent casing will be lowered into a temporary cased borehole. The

permanent casing is made of light gauge corrugated steel sheet. It is much cheaperthan the thick temporary casing.

The permanent casing prevents the expensive temporary casing being stuck by the

fresh concrete. It also reduces the risk of necking of the pile after extraction of the

temporary casing and prevents the fresh concrete being contaminated by soil and

ground water. Corrugated steel casings are often used in long piles and in difficult

grounds such as reclaimed land.

Extraction ofcasing

Earth andgroundwater

pressure

Freshconcrete

pressure

Necking of replacement pile


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4.2.4 Supported with bentonite slurry

Bentonite is a kind of clay. When mixed with the correct amount of water bentonite

slurry produces thixotropic properties, it gives a liquid behaviour when agitated and a

gel structure when undisturbed.

During boring, the borehole is filled with bentonite slurry. The boring action stirs the

slurry so it remains liquid state. But the slurry penetrates slightly into the subsoil and

forms a soft gel or so called 'filter cake' at the interface of the excavation sides.

Hydrostatic pressure caused by the slurry thrusting on the 'filter cake' that prevents

the borehole from collapse. This method is not suitable for soil with high

permeability.

4.3 Examples of Replacement Piles

4.3.1 Bored pile

Bored pile is perhaps the most popular type of pile in Hong Kong currently. The

maximum diameter can be over 3 m and the pile length can be over 100 m. It can be

designed to carry loads up to 30000 kN.

There are various techniques for boring pile shafts and supporting them. The choices

of the methods and their combinations depend on the ground condition and the pile

design. Some typical examples are as follows:

4.3.1.1 Bored pile supported with temporary and permanent casing

Procedure

1. Pitch a temporary steel casing in the correct position and drive it into the ground

with a casing oscillator.

2. Bore within the casing with a hammer grab. Boring shall not go beyond the bottom of the casing to prevent collapse of the borehole.


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Boring supported with bentonite slurry

3. If rocks or boulders are encountered, use a chisel to break them and then use the

grab to remove the fragments.

4. Extend the steel casing and drive it into the ground if necessary.

5. Repeat steps 2 to 4 until the founding rock is reached.

6. Place a reverse circulation drill on top of the steel casing. Form the socket of

desired depth into the bed rock with a roller cutter. The debris is removed by air

lifting.

7. Remove the RCD. (Lower a permanent casing into the borehole.)

8. Lower the reinforcement cage into the borehole.

9. Check the rate of ingress of ground water. If it does not exceed 0.3 L/s, place

high slump concrete into the shaft immediately after it is pumped dry.

Otherwise, fill the shaft with tremie concrete.

10. Extract the temporary steel casing as the concreting is progress upward. When

the concrete level reaches the desired lever, the piling process is completed.

Boring supported with temporary casing


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4.3.1.2 Bored pile supported with bentonite slurry

Procedure

1. Pitch a steel collar casing in the correct position and drive it into the ground with

a vibration hammer.

2. Fill bentonite slurry within the casing and excavate the borehole with a hammer

grab.

3. Maintain the slurry level to about 500 mm below the top of the casing during the

whole piling process.

4. If rocks or boulders are encountered, use a chisel and the grab alternately to

break the rocks and remove the fragments.

5. Repeat steps 2 to 4 until the founding rock is reached.

6. Use the chisel and the grab alternately to form a socket in the rock.

7. Lower the reinforcement cage into the borehole through the slurry.

8. Insert an air lift pipe to the bottom of the borehole. Clean the bottom of the

borehole with air lifting. Slurry loaded with soil particles shall be pumped to a

settling tank, stained and recycled.

9. Fill the pile shaft with tremie concrete. Displaced slurry shall be retrieved.

10. Stop concreting with the concrete reaches 1 m above the cut-off level. Remove

the collar casing and the piling is completed.

4.3.2 Barrette

By definition, a barrette is exactly the same as a bored pile supported with bentonite

slurry. However, a barrette is often referred as a pile in rectangular cross section. Its

formation is the same as that of a bored pile except that four concrete guide walls are

(Source: General Specification for Civil Engineering Works)


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used instead of the collar casing. The guide walls are about 150 mm thick, arranged

as a topless and bottomless box with internal dimension the same as the barrette

section.

4.3.3 Socketted steel H-piles

A socketted steel H-pile is a replacement pile with an H-pile in the core. It is formed by pre-drilling a borehole which is temporary supported with a steel

casing. The borehole shall be of adequate size to allow a minimum grout cover

of 40 mm to the H-pile.

The borehole shall be sunk to sound rock level and the bottom of the borehole

shall then be cleaned by air lifting.

A H-pile is inserted into the borehole and the annular space is subsequently

filled with cement grout.

The temporary casing can then

be withdrawn.

Formation of guide walls Excavation of barrette

Socketted steel H-piles


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4.3.4 Bored pile over water

P i l i n g o v e r

W a t e r


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4.3.5 Minipile (Micropile)

Minipiles are defined as piles having a diameter of less than 300 mm. Generally they

range in shaft diameter from 50 to 250 mm, with working loads in the range of 50 to

500 kN. There are many ways of forming minipiles. A typical method currently usedin Hong Kong is:

1. Drill a borehole of about 150 mm diameter with a rotary drilling rig and line the

borehole with a steel casing.

2. Grout the borehole with cement slurry from the bottom of the hole.

3. Insert the pile reinforcement into the casing. Normally it consists of 2-4 numbers

of T40 or T50 bars.(The steel casing is left in place to enhance corrosion protection.)

The principal use of minipiles is for underpinning work or in steep slope where large

piling machine cannot access.


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4.4 Placing Concrete in Piles

4.4.1 Concreting by trunking

In boreholes where the rate of ingress of water does not exceed 0.3 L/s, the piles shall

be dried immediately before concrete is placed. Then the concrete can be placed

using a readily workable mix (slump 100 mm , which is self-compacting but does

not segregate) through a trunking.

4.2.2 Concreting under water (tremie method)

If the excavations for piles are supported by bentonite slurry or if the rate of ingress

of water exceeds 0.3 L/s, concrete shall be placed by a tremie. A tremie is a steeltube suspended in the water by a crane, with a hopper fixed to the top end to receive

the concrete. The tube shall be watertight, smooth-bored, diameters of 150 - 200 mm.

The tremie concrete shall be of high workability (minimum 150 mm slump).

Procedure:

Erect the tremie pipe (with hopper) vertically and resting on the bottom of the

pile shaft.

Place a traveling plug (formed with foamed plastic in plastic bag) into the tremie

pipe.

Feed high slump concrete into the hopper to push the traveling plug downward. When the plug reaches the bottom, slightly raise the tremie pipe to let the plug

and the concrete to flow out.

Continuously feed fresh concrete into the tremie pipe. Pump away displaced

water/bentonite slurry.

Raise and remove sections of the tremie pipe progressively to maintain the

concrete flow rate, but keeping at least 3 m embedment of the tremie pipe in the

concrete.

Continue concreting until the concrete level reaches 750 mm above the cut-off

level.


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Remove the tremie pipe. The concreting process is completed.

(The surplus concrete shall be trimmed away after hardening.)

C o n c r e t

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b y t r e m

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4.5 Advantages and Disadvantages of Replacement Piles

45.1 Advantages of replacement piles:

1. Soil or rock removed in boring can be inspected for comparison with site

investigation data.

2. Can be installed in very large diameters.

3. Can be installed in very long length.

4. End enlargements up to three diameters are possible

5. Can be installed without appreciable noise or vibration.

6. Can be installed in conditions of very low headroom.

7. No risk of ground heave.

4.5.2 Disadvantage replacement piles:

1. Drilling a number of piles in group can cause loss of ground and settlement of

adjacent structures.

2. Squeezing or necking may occur in soft ground that reduces the bearing

capacity of the pile.

Reference:1. Pile design and construction practice, fourth edition (1994), M.J. Tomlinson, E & FN Spon.2. Introduction to civil engineering construction, third edition (1995), Roy Holmes, The college of

estate management.3. Construction Technology, second edition (1987), R. Chudley, Longman Scientific & Technical.4. Civil Engineering Construction IV (1991), S.A.R. Jufri & R.J. Wellman, Hong Kong

Polytechnic.5. General Specification for Civil Engineering Works (1992), Hong Kong Government.

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