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Drilled Shaft


									Drilled Shafts

               Manoochehr Zoghi, Ph.D., P.E.
                     Professor & Chair
       Department of Civil & Environmental Engineering

              CEE 434 – Geotechnical Design
                    20 October 2008

I. Introduction

II. Advantages and Disadvantages

III. Construction Methods

IV. Design Methods

V. Examples
                               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.)

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.

     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.
 88-story, high-rise building
 supported on a combination raft
 and drilled shaft foundation

Petronas Towers, Kuala Lumpur
High rise building construction
in Chicago, where large diameter
drilled shafts are used widely
Drilled shafts are often chosen for
tower structures to resist uplift loads
and for constructability
Smaller diameter drilled shafts
are used for residential structures,
landslide repair, and other
A view of elevated I-71 downtown from the I-471 6th St. exit.
1. 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.

2. 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.

3. 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.

II. Advantages and Disadvantages


•Minimizes pile cap needs
•Slightly less noise and reduced vibrations
•Easily adaptable to varying site conditions
•High axial and lateral loading capacity


•Extremely sensitive to construction procedures
•Not good for contaminated sites
•Lack of construction expertise
•Lack of Qualified Inspectors

       III. Construction Methods


         Dry Shaft Construction Process

Wet shaft Construction Process


 •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

               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
                                                Rock Auger
           Earth Auger                                                                   "Belling tool"

Single flight, single cut earth auger.   Single flight, single cut rock auger. The blades extend outward,
                                                                               excavating a "bell" shape bottom.

                     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.

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.

V. Examples

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