Lecture 15 - DF1 DEEP FOUNDATIONS Types of Deep Foundations...

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DF1 DEEP FOUNDATIONS: Types of Deep Foundations: o Prefabricated – usually driven, concrete, steel, timber, composite o Cast-in Place Concrete - drilled shafts, bored piles, high capacity, low vibration o Caissons – prefabricated box sunk into ground down to bearing stratum (Brooklyn Bridge), rarely used now, often confused with drilled shafts o Specialty - auger cast, pressure injected, mandrel-driven thin shell, step taper, anchors Categorize Based on Resistance and Load: o Axial Load – compression (most common) or tension/uplift (towers, anchors, overturn) o Lateral Load - walls, bridge piers, wharves o End Bearing (“bearing”) - often rock at tip, side shear may be small o Floating - typically >50% of resistance in side shear o Most piles have combination of end bearing and side shear resistance (figure from Bowles, 1996)
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DF2 Why Use a Deep Foundation? o Transmit structural load through weak upper layers to a deeper “bearing” stratum o Transmit loads through water (bridges, docks, wharves, offshore structures) o Resist uplift, overturn, or lateral loads (bridge piers, towers, signs, basements under GWT) o Control settlement or heave under spread footings or mats o Support and isolate machines (presses, lathes, mills, turbines) and heavy loads (vaults) o Allow for future excavation, potential scour under bridges, or liquefaction under structures o Support retaining walls on soft ground Why Not Use a Pile Foundation? o More expensive than spread footings o Mobilize special equipment (hammers, cranes, drills rigs, compressors, etc.) o Risk ground movements next to existing structures o Significant ground vibrations during construction Parts of a Pile, Loads: PREFABRICATED PILES : Types: o Displacement - solid or closed tip, soil moves/densifies, greater setup, will penetrate soft rock, difficult driving in dense gravels, tills and sands, potential ground heave/movement o Non-displacement - minimal cross section, less ground displacement and driving vibrations, easier to drive and extract, typically lighter, h-piles, pipe (may plug!), lower capacity o Timber – tapered, 12”-20” dia., borer attack, rot above GWT, treated less popular, light loads, flexible, easily damaged in hard driving, may be jetted o Concrete – mostly prestressed (~1000 psi) to resist tension during driving, reinforced found under older structures, square, octagonal, sometimes voided, difficult to splice, reasonable corrosion resistance, heavy, 12” to 36” width solid, up to ~84” dia. pipe (5”-6” thick wall!) o Steel - 8”-14” width h-pile (t w = t f , square), 8”-84” dia. pipe pile, poor corrosion resistance, often used under structures away from coastline, esp. temporary structures & false work, easier to splice and transport, good strength, relatively lightweight, often driven to rock o Composite – concrete with steel stinger, plastic/steel has corrosion resistance, strong plastics now, questions about longevity for plastics
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Lecture 15 - DF1 DEEP FOUNDATIONS Types of Deep Foundations...

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