Foundations

Purpose – To transfer weight of structure (including live loads) to the ground within allowable settlement criteria

Foundation Failure

• Leaning Tower of Pisa

Foundation Shape & Size

• Dependant on type of structure, weight, soil bearing capacity & sensitivity of structure

• Pressure under foundation

= structure weight + imposed loads

area of foundation * F.O.S

Note effect of wind or horizontal loading

Mass Concrete Pile Cap

45

Reinforced Concrete Pile Cap

ReplacementDisplacement

Driven castinsitu

Permanentcasing

Temporarycasing

Pre-Formed

Concrete Timber Steel

UnsupportedSupported

Permanent

Temporarysupport

CasingDrilling mud

Or water

Types of Bearing Pile

Driven Cast Insitu• No soil removed

• Can cause vibration and noise

Driven Cast Insitu Pile Rig

Driven Piles

Replacement - Rotary Bored Piling• 300mm to 2400mm

diameter• Up to 50M depth• Auger ‘screws’ into

ground, breaking up soil• Auger withdrawn with

soil• Back spun to release • May require temporary

casing

Continuous Flight Auger (CFA)

• Auger not removed during drilling – supports bore sides

• When design depth reached concrete delivered through centre of auger to base of bore and auger removed at same rate as concrete infill

• Reinforcement cage added to wet concrete• Quiet• Excavated soil to be disposed of

Continuous Flight Auger (CFA)

CFA Piling

Auger Pile Walling1. Contiguous Piling – small gap between piles

2. Secant Piling – piles interlocked

Reinforced Non Reinforced

Contiguous Pile Wall (with capping beam)

Vibro Replacement• 1. Vibrator penetrates weak soils under action

of vibrations and 'compressed air' jetting medium and forms a hole to design

• 2. After being held at depth for a short time, the vibrator is withdrawn and a charge of stone is placed into the hole.

• 3. The vibrator is reintroduced into the hole, the stone is compacted, forced out and tightly interlocked with the surrounding ground.

• 4. by adding successive charges of stone and compacting each one, a column of very compact stone is built up to ground level.

Vibro Replacement

Dynamic Deep Compaction• With DDC, the ground is subjected to repeated

surface tamping using a heavy steel and concrete weight.

• Typically the tamper weighs between 5 and 20 tonnes, dropping in free fall from heights of up to 25 metres.

• The tamper is dropped a set number of times on a grid pattern over the site to form a pass. Two to five passes on a site, dependent on soil type and condition, can be required.

• The imprints formed at each drop position are infilled with granular material after each pass.

Dynamic Deep Compaction

Health & Safety Issues

• Heavy plant manoeuvring on site – use banksman

• Other plant eg concrete deliveries need to work close by

• Overhead working on many applications• Noise of driven piles• Deep excavations to be protected• Piles designed on basis of site investigation

Environmental Issues

• Noise from driven piles – ear protectors, neighbours

• Vibration from driven piles• Removal of excavated material from

replacement piling• Protection of groundwater, particularly

drilling mud (bentonite)• Diesel storage