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Civil Engineering Construction I(CBE5031)

Foundations

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The British Standard Code of Practice BS8004 defines a foundation as “that part of the structure designed and co9nstructed to be in direct contact with and transmitting loads to the ground”

Definition of Foundation

Shallow foundations are found at a depth of less than 3 m below the finished ground level.

1. Shallow Foundation

1.1 Pad footing

• A pad footing is an isolated foundation to spread a concentrated load.

• Pad footings are generally used to transfer the load from a column, pier or heavy machinery to the ground.

1.1 Pad footing

• The plan shape of a pad footing is usually square, but if the column is close to the site boundary it may be necessary to use a rectangular plan shape of equivalent area.

1.1 Pad footing

• Blinding concrete is a layer of non‐structural concrete of about 50 mm thick laid on the earth.

• It provides a clean and flat platform for steel fixing and formworking, and to prevent contamination of the fresh concrete by soil.

1.1 Pad footing

1.2 Strip Foundation

• A strip foundation is a foundation providing a continuous longitudinal ground bearing.

1.2 Strip Foundation• Strip foundations are used to transfer the load from a wall, or

from a succession of closely spaced piers or columns, to the ground.

• They consist of a continuous ribbon‐shaped strip formed of reinforced concrete.

• Main bars are placed transversely to resist bending while longitudinal bars are used for the continuity of the strip foundation and to bridge soft spots in the soil.

1.3 Raft Foundation• A raft foundation is a foundation continuous in two

directions, usually covering an area equal to or greater than the base area of the structure.

• The structure moves together with the raft foundation when ground movements occurs such that cracking or damage can be prevented.

• A raft foundation is also called a floating foundation.

1.3 Raft Foundation• Raft foundations are useful in the following cases:

– for structures where the column loads and/or soil conditions are such that the resulting footings occupy most of the site;

– where buildings have to be erected on soils susceptible to excessive shrinking, swelling or frost heave;

– where differential settlements are likely to be significant.

• Raft foundations can be subdivided into the following groups:

1.3.1 Solid Slab Raft• Solid slab rafts are suitable for lightly loaded structures such

as small houses. • A solid slab raft consists of a reinforced concrete slab, usually

slightly larger than the area of the building. • Reinforcement in the form of a mesh fabric is provided on

both the top and bottom faces of the slab.

Solid slab Raft (Source: R. Chudley)

1.3.1 Solid Slab Raft• The slab is sometimes stiffened under the peripheral wall

with edge thickening or an edge beam.

Solid slab Raft (Source: R. Chudley)

1.3.2 Beam and Slab Raft• Beam and slab raft foundations are used where poor soil

conditions are encountered. • The slab and beams provide stiffness and prevent the

distortion of the building. • Ground beams are usually designed as down‐stand beam. • The main advantage of the down‐stand beam raft foundation

is that it provides a level surface slab which can form the ground floor of the building.

• Ground beams can also be designed as up‐stand beams with a precast concrete suspended floor at ground level thus creating a void space between raft and ground floor.

1.3.2 Beam and Slab Raft

Beam and slab raft with downstand beams (Source: R. Chudley)

Beam and slab raft with upstand beams(Source: R. Chudley)

1.3.3 Cellular Raft• Cellular raft foundation is suitable where poor soil is

encountered at a shallow depth, and where it would be uneconomical to use slab and beam raft foundations.

• This type of foundation consists of two reinforced concrete slabs linked by internal walls which divide the void into cells. The walls help to spread the load over the raft.

• Openings can be formed in the cell walls allowing the voids to be utilised for the housing of services, as storerooms or for general accommodation.

1.3.3 Cellular Raft• With cellular raft foundations, the columns shall be

positioned at the intersection of the internal walls.

Celleular Raft foundation (Source: R. Chudley)

1.4 Combined foundation• Rectangular footing is used for closely spaced columns.

• Balanced base footing is used for eccentrically load column.

• This often happen in perimeter column where the footing is limited by site boundary.

Balanced base footing (Source: R. Chudley)

1.4 Shallow foundation

Shallow foundations (Source: R. Chudley)

2. Piled Foundation ‐ classifications• Piles can be classified by the way in which they transmit their

loads to the ground and by the way in which they are installed.

End bearing piles (Source: Jufri & Wellman)

2.1.1 Classified by Method of Load TransmissionEnd bearing piles ‐ they act as columns carrying the loads through the overlaying weak subsoils to a firm stratum on which the pile toes rest.

2.1.1 Classified by Method of Load Transmission

• Friction piles ‐ they transfer the loads by the adhesion or friction action of the soil around the perimeter of the pile shaft.

Friction pile (Source: Jufri & Wellman)

2.1.1 Classified by Method of Load Transmission

• Besides supporting a downward load, a friction pile may also be used to support an upward load, which is called an anchorage pile

• In most situations piles work on a combination of the two principles outlined above but the major design consideration identifies the pile class.

2.1.2 Classification by Method of InstallationDisplacement Piles • the piles are forced into the ground, the soil is

displaced downwards and sideways, but material is not actually removed.

• also called driven piles

Displacement pile (Source: Jufri & Wellman)

2.1.2 Classification by Method of InstallationReplacement Piles• a shaft (or hole) is excavated and the soil replaced

with concrete to form a pile.

Replacement pile (Source: Jufri & Wellman)

2.2 Uses of Piled Foundation• as an alternative to other techniques when the following

conditions are encountered: 1. When the load of a structure, cannot be spread sufficiently over the

available ground area without exceeding the allowable bearing capacity of the soil.

2. When settlement due to weak underlying strata is unpredictable (i.e. over mud and poor filled ground).

3. When wind force tends to overturn a tall building, anchor piles are needed on the windward side.

4. When there is need to resist uplift due to the floatation of ground water.

5. To resist lateral loads ‐ e.g. raking piles can used resist the earth pressure behind a retaining wall or the side impact from ships alongside a jetty.

6. To underpin existing foundations.

3. Displacement Piles• Displacement piles are preformed piles made in a factory,

transported to the site, and then driven into the ground.

• Also called ‘preformed piles’ or ‘driven piles’.

• Preformed piles may be made of reinforced concrete or steel.

• ( ‘driven cast‐in‐situ pile’ ‐ 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.)

3.1.1 Percussive driving

c. Hydraulic Hammer• mounted on the pile while operating. • similar to a drop hammer but the weight is raised by

hydraulic jacks. • The weight then falls freely under gravity on to the pile head. • 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

c. Hydraulic Hammer

Hydraulic hammer (Source: Junttan)

3.1.2 Vibratory driving – vibration hammer

• vibration hammer is comparatively silent. • used in driving small displacement piles such as H‐

piles and sheet piles. • consists of a vibrating unit mounted on the pile head

transmitting vibrations down the length of the pile shaft.

• 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.

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.

3.1.3 Piling rigs

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) (left)Pile helmet (Source: Jufri & Wellman)

3.2 Precast prestressed concrete tubular pile• Precast prestressed concrete tubular piles, e.g.

Daido SS pile and Nipon Hime SS Pile, has been wildly used in Hong Kong but they are not obsolete.

Production of Precast PrestressedConcrete PIle

Driving of Precast PrestressedConcrete PIle

3.2 Precast prestressed concrete tubular pile

3.3 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.

3.3 Steel H‐pile

• Strengthening may be carried out by welding plates or angles to the toe of the pile, or a 'rock point' is fitted to the toe when the pile has to penetrate rock.

• Preformed pile shoes for H‐piles are also available.

H‐piles point (Source: Housing Depart)

3.3 Steel H‐pile

• Lengthening of steel H‐pile is by butt‐welding

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,

verticality or desired inclined angle. 4. Place a suitable pile helmet on the pile head then mount

the piling hammer on top of it and start driving. 5. Continue driving. If the whole length of the pile has been

driven into the ground, splice a new length on it by butt‐welding.

6. Continue driving until it renders sufficient resistance, then measure the ‘set’. Finish the driving if the predetermined set is attained. Otherwise repeat from step 5 until it does.

3.4 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 Hiley formula is most commonly used.

• The final set shall be measured as: – 1. penetration per 10 blows (for end bearing piles), or – 2. the number of blows required to produce 25 mm penetration (for

friction piles).

3.5 Advantages of displacement piles:

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 unacceptable noise and vibrations.

3. Pile driving hour is limited by environmental regulations. 4. May break during driving, causing delays and replacement

charges. 5. May suffer unseen damage which reduce carrying

capacity. 6. Displacement of soil during driving may damage adjacent

structure or cause lifting of adjacent piles. 7. Cannot be driven in very large diameters. 8. Cannot be driven in conditions of low headroom.

Driven piles over water displacement pileNecking of displacing pile

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. • it is mostly 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

4.1 Methods and equipment for boring pile shaftsFlight auger (Source: Tomlinson) Cheshire auger (Source: R. Chudley)

Boring Tools

1. The Cheshire auger which has only 1½ to 2 helix turns. The soil is cut by the auger, raised to the surface and spun off the helix to the side of the borehole from where it is removed from site.

2. The continuous auger or flight auger brings the spoil to the surface with spiral motion. The drilling rig must provide sufficient torque to overcome the great friction.

Boring Tools

4. The drilling bucket is designed to withstand the high torque forces developed during penetration of dense and competent strata. The bucket base is fitted with flap doors. Spoil is extracted by drilling with the bucket, raising to the surface, opening the flaps and clearing of the pile bore. Adaptations of the standard bucket are available for coring and for use with bentonite.

5. The coring bucket is used to raise a solid core of rock. 6. The chisel is used when boulder or rock is encountered.

Cheshire auger Bentonite bucket & Coring bucket Chisel

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

4.1.2 Grab boring

• Grabs consist of clamshell buckets which are opened and closed by means of wire ropes.

• They are suspended from mobile cranes. • The grab is dropped on to the soil in an open position and

is then closed. • It is then raised to the surface and emptied at the side of

the shaft

4.1.2 Grab boring

• When boulder is encountered, it can be broken by free falling a chisel on to it.

• The rock fragments are then removed by the grab. • Chisel and grab are often used alternatively to break

through obstructions.

Chisels

4.1.3 Down the hole (DTH) drilling

• DTH is a large size hammer drill 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 these difficulties the ‘down the hole’ drill was developed.

• Its rotary motor remains above ground level while the bit is followed down the hole actuated by its compressed air driven pneumatic hammer

4.1.3 Down the hole (DTH) drilling

• The drill stem rotates in slow motion while the drill bit strikes rapidly.

• The drill debris is blown out by compressed air.

Down the hole drill

4.1.4 Reverse circular drilling

• 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 drill the bed rock sockets for bored piles.

• For boring in rock, rock roller cutters are used.

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

• It consists of a double wall drilling pipe and operates on the principle of air lifting.

• Compressed air is injected through the annular space of the drilling pipe and discharge near the base.

• The rising column of air and water at the centre of the drilling pipe lifts the soil or rock fragments which has been loosened by rotating cutters.

(Air lifting is also commonly use to clean the bottom of a bored pile before concreting.)

Rock roller Cutter Reverse Circulation Drill

4.1.6 Under‐reaming

• The base or toe of the pile can be enlarged to three times the shaft diameter to increase the bearing capacity of the pile.

• The method of enlarging the base is known as under‐reaming or belling.

• Under reaming is achieved by a belling bucket, which is rotated by the kelly bar of the drilling rig.

• The belling buckets are equipped with arms hinged at the top or bottom.

• The arms of the bucket are jacked out to excavate the sides of the shaft and hence form the 'bell'.

4.1.6 Under‐reaming

4.2 Methods of supporting pile shafts

4.2.1 Self‐supported (unsupported)

• For small diameter bored piles and in stable soils, the bored pile shaft may remain unsupported without collapsing.

• a bored pile shaft 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 can be supported by a steel casing.

• To prevent the collapse of the bored shaft, the casing shall be driven ahead of boring.

• The casing can be driven into the ground by a vibration hammer or a casing oscillator.

Casing oscillator Vibration hammer

4.2.2 Supported with temporary casing

• The oscillator consists of a clamp and a pair of hydraulic jack.

• The oscillator clamps itself to the casing with its two hydraulic jacks extend and retract in opposite phrase.

• This oscillation motion helps to cut the soil by the casing shoe and sinks the casing into to ground.

• The steel casing is usually thick and expansive and will be retrieved after concreting.

• Vibrators and the casing oscillators can be used to withdrawn the casing.

4.2.3 Light gauge permanent casing

• Sometimes extraction of the thick temporary casing may be difficult after concreting especially in long piles.

• In these cases, corrugated light gauge permanent casings may be added to overcome the problem.

• The thin permanent casing is cheaper and will be left in place.

• It prevents the thick temporary casing from sticking by the fresh concrete.

• it prevents ‘necking’ and contamination of the fresh concrete by the soil and ground water.

• Corrugated steel casings are often used in long piles and in difficult grounds such as reclaimed land.

4.2.3 Light gauge permanent casing

Light gauge corrugated steel casing

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 shaft 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' holds back the subsoil and ground water.

• This alleviates the need for casing support.

4.2.4 Supported with bentonite slurry

4.3 Examples of Replacement Piles

4.3.1 Bored pile

• Bored pile is the most popular type of pile in Hong Kong• The maximum diameter can exceed 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

support them. • The choice depends on the ground condition and the pile

design. • Combination of these techniques generates various kinds of

piles. • 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

exceed the bottom of the casing to prevent collapse of the borehole.

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

4. If rocks or boulders are encountered, use a chisel and the grab alternatively to break the rocks and remove the fragments.

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

4.3.1.1 Bored pile supported with temporary and permanent casing

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 and lower a permanent corrugated steel casing into the shaft.

8. Lower the reinforcement cage within the permanent steel casing.

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. Remove the temporary steel casing. The pile process is completed.

4.3.1.3 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. Bore within the casing with a hammer grab, and the

borehole is filled with bentonite slurry. 3. Continue boring through the slurry, maintain the slurry level

to within 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 alternatively to break the rocks and remove the fragments.

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

4.3.1.3 Bored pile supported with bentonite slurry

6. Use the chisel and the grab alternatively 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 bore 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 recycled.

10. Remove the temporary casing. The piling process 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 a bored pile except that four concrete guide walls are 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

• piles formed by inserting steel H‐piles in pre‐bored holes sunk into sound rock, and subsequently filling the holes with cement grout.

• In the construction of socketted steel H‐piles, the following should be considered: 1. The prebored holes should be large enough to enable the

installation of steel H‐piles and to allow a minimum cover of 40 mm to the steel H‐piles.

2. A temporary casing should be provided in the pre‐boring process down to rockhead level to prevent soil from falling into the pre‐bored hole;

3. Before the steel H‐pile is inserted in the pre‐bored hole, the hole should be cleaned to ensure that it is free from debris and soil; and

4. The grout should be non‐shrink and have a minimum characteristic strength of 30 MPa at 28 days.

4.3.4 Minipile (Micropile)

• piles having a diameter of less than 300 mm.• 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 used in Hong Kong is:

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

– Grout the borehole with cement or cement sand grout from the bottom of the hole.

– Insert the pile reinforcement into the casing. Normally it consists of 2‐4 numbers of T40‐50 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

4.4.1 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. Material forming pile is not governed by handling or

driving stresses. 6. Can be installed without appreciable noise or vibration. 7. Can be installed in conditions of very low headroom. 8. No risk of ground heave.

4.4.2 Disadvantage replacement piles:

1. Squeezing or ‘necking’ may occur in soft ground where conventional types are used.

2. Water under artesian pressure may pipe up pile shaft washing out cement, special techniques needed for concreting in water‐bearing soil.

3. Concrete cannot be inspected after installation. 4. Drilling a number of piles in group can cause loss of ground

and settlement of adjacent structures.

4.5 Placing Concrete in Piles

4.5.1 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.

4.5.2 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 steel tube suspended in the water by a crane, with a hopper fixed to the top end to receive the concrete. The tube must be watertight, smooth‐bored, diameters of 150 ‐ 200 mm.

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

• Before concreting, the tremie is erected vertically in the pile shaft with the lower end resting on the bottom.

• A travelling plug, formed from plastic bags, foamed plastic or similar material, is placed in the pipe as a barrier between the concrete and water.

• The water in the pipe is displaced as the weight of the concrete forces the plug to the bottom.

• After the pipe has been filled with concrete it is raised off the bottom to allow the concrete to flow.

• concrete should be continually fed into the interior of the initial mass.

• The tremie tube may be raised progressively to maintain the rate of flow of concrete, but keeping at least 3 m embenment in the concrete.

• Concreting is continued until the concrete level reaches 750 mm above the cut‐off level.

• Concreting is completed and the tremie pipe is removed. • The surplus concrete is trimmed before the pile cap is

formed.

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.

civil engineering construction i (cbe5031) - vtctycnw01.vtc.edu.hk/cbe5031/foundations.pdf · with...

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