SCC for Drilled Shafts

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CIDH Piles Design for Constructability
By Dan Brown, P.E., Ph.D. Dept. of Civil
Engineering, Auburn University Dan Brown and
Associates, PLLC
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CIDH Piles (Drilled Shafts)

• 12 to 12 dia.
• Avoids noise vibration associated with pile
  driving
• Small footprint single shaft vs pile group
• Capable of extremely large loads (axial
  lateral)
• Most effective where good bearing stratum present

Over- burden
Rock
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Outline

• Overview of LRFD Design Approach
• Overview of CIDH Construction
• Issued Relating to Design for Constructability

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New Design Considerations

• AASHTO Code LRFD Design
• 2008 FHWA Manual - Overview
• Design for Lateral Loading
• Design for Axial Loading

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Load and Resistance
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LRFD Design Approach
General Form of Equation
where fi resistance factor for
resistance component i Ri nominal value of
resistance component i gi load factor for
load component i Qi nominal value of load
component i

gt Fn
For axial resistance to DL LL
ASD approach
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Planning and Preliminary Design
Foundation Type Selection
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Foundation Design
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Construction
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Design for Lateral Loading

• Geotechnical Strength Limit State
• Sufficient embedment to resist overturning
  failure use (factored loads)/(f0.67)
• Structural Strength Limit State
• Yield in flexure
• Servicability Limit State
• Lateral Deformations

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Applications with Large Lateral Loads
Single Column Piers with Monoshaft Foundations
Monoshaft Foundations Used by Caltrans (Caltrans
Seismic Design Criteria, Version 1.4, June, 2006)
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Applications with Large Lateral Loads
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Design for Axial Loading

• Geotechnical Strength Limit State
• Plunging failure
• Structural Strength Limit State
• Structural failure
• Servicability Limit State
• Settlements or Axial Displacement

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Generalized Behavior Under Axial Load
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Dilation at Rock/Shaft Interface
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General CIDH Pile Construction Methods

• Dry Method
• Wet Method
• Casing Method
• Combinations

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Dry Method
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Casing Method w/ slurry starter hole
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Casing Method w/ casing advanced ahead of hole
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Casing Installation
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Wet Method with slurry
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Managing Polymer Slurry
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Polymer Slurry

• Doesnt hold sand in suspension
• Use settling basin for desanding
• Sensitive to pH
• Sometimes difficult to remove silt-sized particles

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Bottom Cleanout Tools
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Wet Method with full length casing
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Circulation Drilling
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Design for Constructability Issues

• Site Investigation
• Casing Requirements
• Contract Scheduling
• Rebar Cage
• Concrete Mix
• Inspection NDT

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Site Investigation Construction

• Construction challenges must be addressed
• Rock Strength
• Groundwater
• Sequence of Construction
• Boulders/Cobbles

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Site Investigation - GBR

• Geotechnical Baseline Report (GBR)
• Defines geotechnical conditions as basis for bid
• Reduces contingency costs in Bidding
• Provides Basis for resolution of differing site
  condition claims
• Common Practice in Tunneling Industry

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Rock Excavation
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Casing

• Contractor needs flexibility in casing elevations
  top bottom

Cutting teeth used to penetrate soft rock
achieve seal
Elevated casing used to overcome artesian head
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Importance of Sealing the Casing
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Scheduling

• Avoid conflicts with adjacent activity
• Construct deeper elements first
• Minimize setups (esp. over water)

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Essentials of Rebar Cage Design for
Constructability

• Clear Spacing Between All Bars ? 5 D of CA (5
  minimum window is current Caltrans std)
• Use 6 Design Cover
• Bundle Bars if Necessary
• Tie Transverse Steel Every Lap
• Locate Splices As Deep As Possible
• Avoid Out-Hooks
• Use roller centralizers

Bundled Bars
Whats wrong with these pictures?
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Benefit of 6 Design Cover
With 6 Designed Cover CIDH Cage can be moved
3 in any direction and maintain 3 minimum
cover
Shaft Tolerance 3 on location
3
9
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Flow through Congested Cages is Difficult
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CongestedRebar Cage
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Type I Type II Column Splice

• Issues
• Depth of column steel (currently 3 column
  diameters)
• Need for space between cages (currently 9)
• Need to make shaft cutoff at base of column
  reinforcement,
• construct splice in the dry
• Openings in two cages will never line up

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WashDOT std design
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Effect of Larger Shaft
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Design of Reinforcement for Constructability

• Large, heavy cages pose construction challenge
• Concrete Flow
• Lifting
• Splicing
• Time

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Design of Reinforcement for Constructability

• Effect of permanent steel liner

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Design of Reinforcement for Constructability

• Use of permanent steel liner (CISS) for design
  offers advantages
• Increased Strength
• Increased Ductility
• Reduce rebar

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Concrete Requirements
Flowable, Cohesive Mix
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Control Slump Loss
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Initiation of Concrete with Tremie Placement
Potential Problems

• Inadequate control of initial charge
• Non-continuous delivery to tremie
• Concrete flows out into shaft, inertia carries
  concrete down
• Slurry head gt concrete in tremie and breach can
  occur

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Consider Constructability in Drilled Shaft Design
Specs

• Maximum aggregate size no more than 1/5 clear
  space between rebar use pea gravel mix if
  necessary
• Bundle the rebar for heavy reinforcement
• Slump of 8 /- 1 for workability
• Retarder to maintain workability for duration of
  pour (including pulling of casing)
• Concrete for drilled shafts should be
  specifically designed for this application!

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Need for Workability and Passing Ability
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Need for Passing Ability
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Slump Flow (left) and L-Box (right) Measurements
of Workability and Passing Ability
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Need for Retention of Workability
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Exposure of Trapped Laitance Attributed to
Inadequate Workability
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Structural Defect Produced by Interruption in
Delivery of Concrete to Site Photo Courtesy
of Caltrans
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Trapped Laitance Bleeding in Center of Shaft
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Concrete Temperature
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Concrete Ingredients for Quality Construction

• Use rounded gravel aggregates rather than crushed
  stone
• Use No. 67 or 78 aggregate gradation rather than
  No. 57
• Use sand to total aggregate ratio in the range of
  0.44 to 0.50
• Use water reducing and hydration control
  admixtures

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Concrete Ingredients for Quality Construction

1. Use fly ash and/or slag to increase cementitious
   materials content and reduce the portland cement
   content
2. Use 56 day or 90 day strength specifications in
   lieu of 28 day for concretes with high dosage of
   fly ash or GGBF slag
3. Use viscosity modifying admixtures (VMA) as
   needed to help control bleeding

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Concrete Ingredients for Quality Construction

1. Consider the temperature of the mixture when
   selecting admixture dosages to meet workability
   requirements and delay setting
2. Control the fresh concrete placement temperature
   to less than 80F (75F)
3. Utilize Type II cement along with fly ash or slag
   to control heat of hydration and reduce potential
   for delayed ettringite formation

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Inspection NDT

• Shaft bottom cleaning inspection
• Weighted tape or camera
• Airlift or pump is generally more effective
• Use cleanout bucket in clean granular soil
• Issues with fine sand or silt
• its probably the slurry!

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End Bearing Cleanliness

• It is possible to get a reasonably clean base
  under water or slurry
• It is not possible to get a perfectly clean base
  in any circumstances
• Attempts to require a dry hole for end bearing
  may be counterproductive

Lumber River Test Shaft
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End Bearing in Sands

• Cohesionless sands may never indicate sound base
• Base grouting can be very effective to enhance
  base resistance QA

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Inspection NDT

• PVC tubes are prone to debonding
• Stabilizers to avoid damage during handling
• Use warm water, not cold
• Avoid congestion from cage tubes
• Note CSL with metal tubes is standard elsewhere

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Small Imperfections May be Detected
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Keys for Constructability

• Understand available construction techniques and
  equipment
• Good subsurface information
• Rebar cage essentials
• Concrete mix workability
• Appropriate QA/QC with well-trained inspectors

Try to Make it Easy to Build Correctly!
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Take a Break!