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

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

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

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.

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

• 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

• 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

• 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

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

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

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

Slurry Effect / Construction Methods Casing in Starter Hole

Slurry Effect / Construction Methods Wet + Casing

What Can Go Wrong? Voids Around Casing

Thoughts on Efficient Drilled Shaft Design

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

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.

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

What Else?