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Drilled Shafts

Drilled Shafts. Manoochehr Zoghi, Ph.D., P.E. Professor & Chair Department of Civil & Environmental Engineering. CEE 434 – Geotechnical Design 20 October 2008. Outline. Introduction Advantages and Disadvantages Construction Methods Design Methods Examples. I. Introduction . What?

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Drilled Shafts

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  1. Drilled Shafts Manoochehr Zoghi, Ph.D., P.E. Professor & Chair Department of Civil & Environmental Engineering CEE 434 – Geotechnical Design 20 October 2008

  2. Outline Introduction Advantages and Disadvantages Construction Methods Design Methods Examples

  3. I. Introduction • What? • “A deep foundation that is constructed by placing fluid concrete in a drilled hole.” • Why? • Selection of the foundation system is generally based upon: • Loads to be imposed • Site subsurface materials • Special needs (high lateral capacity, etc.) • Cost • Drilled shafts (also called caissons, drilled piers or bored piles) have proven to be a cost effective, excellent performing, deep foundation system, that is utilized world-wide. Typically they are used for bridges and large structures, where large loads and lateral resistance are major factors. Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  4. What types of structures are supported on drilled shafts? Drilled shafts are used as foundations for all types of structures, including buildings, bridges, towers, industrial facilities, and others. They are also used for earth retention systems.

  5. 88-story, high-rise building supported on a combination raft and drilled shaft foundation system Petronas Towers, Kuala Lumpur

  6. High rise building construction in Chicago, where large diameter drilled shafts are used widely

  7. Drilled shafts are often chosen for tower structures to resist uplift loads and for constructability

  8. Smaller diameter drilled shafts are used for residential structures, landslide repair, and other applications

  9. A view of elevated I-71 downtown from the I-471 6th St. exit.

  10. The process first involves drilling a cavity. Drill depths normally range from about seven to 30 feet, depending on design requirements. Pre-drilling allows you to see the soil between the borings, ensuring that the piers are engineered to reinforce the right soils. Layers of aggregate are then introduced into the drilled cavity in thin lifts of one-foot compacted thickness. A patented beveled tamper rams each layer of aggregate using vertical impact ramming energy, resulting in superior strength and stiffness. The tamper densifies aggregate vertically and forces aggregate laterally into cavity sidewalls. This results in excellent coupling with surrounding soils and reliable settlement control. Following installation, RAP elements reinforce slopes and embankments, support shallow foundations, floor slabs and tank pads. The footing stresses are attracted to the stiff RAP elements, resulting in engineered settlement control. http://www.geopier.com/index.asp?id=11

  11. II. Advantages and Disadvantages • Advantages • Economics • Minimizes pile cap needs • Slightly less noise and reduced vibrations • Easily adaptable to varying site conditions • High axial and lateral loading capacity • Disadvantages • Extremely sensitive to construction procedures • Not good for contaminated sites • Lack of construction expertise • Lack of Qualified Inspectors Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  12. III. Construction Methods How? Dry Shaft Construction Process Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  13. Wet shaft Construction Process Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  14. Wet-vs.-Dry • Wet is more expensive • Wet requires more contractor expertise • Wet requires more equipment • Wet is when there is more than 12" of accumulated water in the • bottom of the shaft (typically) • Wet precludes visual inspection of the bottom of the shaft by the • inspector Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  15. Cased Shaft Construction Process Drill- hole is advanced using slurry through the caving soils Case- casing is then installed through the caving soils and drilling continues to desired depth Clean- slurry and cuttings removed from the hole Position- rebar cage is positioned in the hole Place- concrete is placed. If temporary casing, casing slowly withdrawn as concrete level in hole rises Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  16. Rock Auger Earth Auger "Belling tool" The blades extend outward, excavating a "bell" shape bottom. Single flight, single cut earth auger. Single flight, single cut rock auger. Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  17. IV. Design Methodology End Bearing Drilled shafts can be designed as "End Bearing" meaning the load is carried by the base or "end" of the shaft. Friction Shafts design for having their load dissipated throughout the materials they are formed into are called "Friction" shafts. The site subsurface soils the shaft are installed into "grab" the sides of the shaft, much like when you step in mud and try to pull your foot out. Source: http://www.fhwa.dot.gov/infrastructure/tccc/tutorial/shafts/fhcha02.htm

  18. V. Examples

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