Foundations Foundation supports weight of structure Includes soil

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 – F 10 -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 F 10 -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 – F 10 -6 explain steps

• Pile Driving Procedures – F 10 -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 F 10 -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