Foundations

• Foundation supports weight of structure– Includes soil and rock under foundation– Building construction described by foundation

type• Slab on grade• Crawl space• Basement – provides useful space if dry

Spread footings

• Most common type of foundation– Square or rectangular pad which spreads

building load over an area• Load is less than bearing capacity of soil

– F10-2• Isolated footing – support single column• Wall footing – supports wall• Combined footing – support multiple columns or walls• Mat –supports entire building, uses a heavily

reinforces slab

Spread footings

• Mat –supports entire building, • uses a heavily reinforces slab• Floating slab – mat foundation where weight of soil

removed = weight of building (building floats on soil)

Piles

• Column driven into soil– Common types

• Timber – inexpensive, easy to cut & splice, no special handling

– Max length = 100’, load carrying limited, pile ends may splinter, subject to insects and decay

– Can use pressure treated– Good underwater

• Precast concrete piles– Come in almost any size and shape – round, square ,

octagonal

Piles

• Precast concrete piles– Come in almost any size and shape – round, square ,

octagonal– High strength, no decay– Heaviest type of pile, brittle, no tensile strength– Need careful handling, hard to cut & splice

• Cast in Place Piles– Shell piles – steel shell driven into ground and then

concrete poured in– Steel serves as additional reinforcement– Types = uniform taper, step taper and straight– Light, easy to handle, easy to cut & splice

Piles

• Steel Piles– Capable of heavy loads– Driven to great depths, easy to cut & splice, – Common types = H-piles, pipe piles– High cost

• Composite piles• Made from 2+ materials

– Timber bottom, steel top– Won’t decay

• Bulb piles

• Bulb Piles– Franki Piles or pressure injected footings– Special form of cast in place pile with an enlarged base

for increased load bearing

• F 10-4• Minipiles, micro piles

– Small diameter piles (2-8 inches)– High capacity (to 60 ton)– Used where there is restricted access or headroom to

underpin or temp support a building– Usually placed as a steel casing with reinforcing then

whole thing is grouted to soil

Pile driving

• Drop hammer – ancient pile driver F10-5– Pile driver is usually crane driven– Pile driver consists of a

• Drop weight• Leads – guide for weight• Pile driving consists of placing a pile in the lead• Dropping the weight• Lifting weight• Repeat• Must have a stop block to keep weight on leads

• Power Hammer Pile Drivers– Use a working fluid to drive hammer– Steam or compressed air was first type used– Hydraulic is newer version– Single acting hammer

• Fluid lifts weight which then falls down

– Double acting hammer• Fluid lifts weight and drive weight down• Hammer Lighter than single acting

• Diesel hammer– F10-6 explain steps

• Pile Driving Procedures– F10-8

• Impact type pile driver– F 10-7

• Power hammers work on piles not driven straight down

Piers & Caissons– Pier – reinforced concrete column constructed below

ground surface• Drilled piers – holes drilled in cohesive soils are filled with a

slurry until concrete is poured• Holes drilled in non-cohesive soil has a liner which can be

pulled as concrete is placed

• Caisson structure used to provide all around lateral support to an excavation– Pneumatic caisson – air or watertight structures open

at bottom to permit excavation• Air pressure keeps water & soil out• Must take safety into account – bends• Brooklyn Bridge

Stability of Excavation

• Slope stability– Cohesive and non cohesive soils– F 10-10, 10-11

• Embankment failure – Based on soil type and angle of internal friction– Safe depth range 5 -18 ft

• OSHA – anything over 4’ must be guarded• Failure modes F 10-12• Stability effected by weather, ground water, loads on banks

Stability of Excavation

• Stability of Cut Bottom – Bottom can heave due to weight of soil on

sides• Usually seen when in a cut

– Boiling or piping• Occurs when water pressure moves soil up from

bottom of cut• F 10-16

Preventing Bank Failure

• Need to strengthen soil or hold it back– To hold back use columns or piles across slip

plane– Soil reinforcement F 10-17– Dewatering

Protecting Excavations and Workers

• OSHA has regulations– Protection can be sloping or benching side

walls– Supporting sidewalls with shoring– Trench boxes– Exception is when in stable rock

Shoring and Benching

• Slope or bench walls away from cut– Takes up more room – Requires more excavation and backfill

Shoring and Shielding

• Laterally support side walls of cut– Timber shoring F 10-18– Aluminum Hydraulic shoring F 10-19– Lagging – horizontal sheet piles– Sheet piling– Trench shields F10-20– Slurry trench – F 10-21

Dewatering

• Remove water from excavation– Well point F 10-22– Vacuum well F 10-23

• Pressure grouting– F 10-24– Grout is pumped into soil and fills voids

creating a stronger soil bond