1. Thoughts on (Efficient) Drilled Shaft Design
Robert Thompson, P.E., D.GE
Dan Brown and Associates, PC
ALDOT Pre-Construction Conference
May 7, 2015

2. Thoughts on Efficient Drilled Shaft Design
Side vs Base Resistance
Point of Fixity – Lateral Analysis
Slurry Effect / Construction Methods
Over Conservative Design?
Flexibility / VE Design

3. Side vs. Base Resistance
Common to neglect one due to belief that they are incompatible – to mobilize base we have to “fail” side resistance
Mobilization of side and base is at different displacements.
Can design for both based on common displacement.
Possible to never mobilize base resistance in rock sockets

4. Base Resistance Only
Common to neglect side resistance because:
Concerns over sidewall quality (construction techniques) in soil, so just easier to neglect.
Inspection requires certain criteria for base resistance in rock (e.g. no seams), so we chase good rock and create a socket that was not included in the design.
Just the way we have always done it.

5. Properties for modeling soil and rock in LPILE and FBPier
Lateral Loading
Properties for modeling soil and rock in LPILE and FBPier
Failure to adjust p-y models to match reality
Inexperienced users
Shaft or Socket length for “Point of Moment Fixity”
Not well defined in AASHTO
Depth for transverse steel – maybe to shaft tip
NOT for determining minimum shaft length

6. • Geotechnical Strength Limit State Overturning failure
Lateral Loading
• Geotechnical Strength Limit State
Overturning failure
• Structural Strength Limit State
Yield in flexure
• Service Limit State
Lateral Deformations

7. • Geotechnical Strength Limit State
Lateral Loading
• Geotechnical Strength Limit State
• Model shaft as linear elastic beam
• Perform analyses to find minimum length for stable solution with (1/φ) x factored loads
• Use φ = 0.67 for Strength I-V Load Cases
• Use φ = 0.8 for Extreme Event Load Cases

8. • Structural Strength Limit State
Lateral Loading
• Structural Strength Limit State
• Compute maximum bending moment using factored loads
• Service Limit State
• Compute deflections using service loads, nonlinear EI for shaft

9. Properties for modeling rock in LPILE and FBPier
Lateral Loading
Properties for modeling rock in LPILE and FBPier
Failure to adjust p-y models
Inexperienced users
Socket length for “Point of Moment Fixity”
Not well defined in AASHTO
Depth for transverse steel – may be to shaft tip
NOT for determining minimum shaft length

10. Construction Methods/Constructability
As designer, why do we need to consider potential construction methods to be used?
Can it be built?
Reinforcement congestion = problems with concrete placement
Are we introducing potential sources of problems?
Side wall roughness, especially in soft rocks (e.g. shale)
Slurry “filter cake” - reduces side resistance?
Polymer slurry – increase side resistance?
Can the contractor achieve a clean bottom
Inspection requirements delaying concrete placement

11. Slurry Effect / Construction Methods
Wet Method - Starter Casing (“Can”)

12. Slurry Effect / Construction Methods
Casing in Starter Hole

13. Slurry Effect / Construction Methods
Wet + Casing

14. What Can Go Wrong?
Voids Around Casing

15. Thoughts on Efficient Drilled Shaft Design

16. Over designed or Under designed?
Not OVER designed but UNDER designed
Not spending enough time on the design
Maybe more conservative (much larger than needed), not OVER designed

17. Maybe NOT Conservative at all
Over Conservative?
Maybe NOT Conservative at all
From a design perspective, yes, but….
Increase risk exposure during construction
Requires larger tools/equipment
More excavation, more chances for error
Inexperienced contractor
Wrong means/methods
Etc.

18. Flexibility / VE Design
Difficult to be flexible with traditional Design-Bid-Build
Value Engineered (VE) Designs
Offers potential for flexibility, potential for cost savings
Owner/designer need to be open to suggestions
Review needs to be thorough but streamlined
Sort out “selling” versus “ a better solution”
Consider pre-bid Alternate Technical Concept (ATC)
Contractor submits confidential pre-bid ATC for review
If approved, can bid ATC or base bid
Adopt Design-Build project delivery

19. What Else?