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Foundations

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

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Foundations

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  1. Foundations

  2. 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

  3. 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

  4. 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)

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. Diesel hammer • F10-6 explain steps

  12. Pile Driving Procedures • F10-8 • Impact type pile driver • F 10-7 • Power hammers work on piles not driven straight down

  13. 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

  14. 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

  15. 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

  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

  17. 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

  18. Shoring and Benching • Slope or bench walls away from cut • Takes up more room • Requires more excavation and backfill

  19. 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

  20. 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

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