Drilled Shafts: Who, When, What, Where How and Why?

1. Drilled Shafts:Who, When, What, Where How and Why? Presented by: W. Tom Witherspoon 23 April 2010

2. Overview • Who:Parties of Interest • When:Project Sequence • What:Drilled Shafts • Where:Applicability • How:Design/Installation • Why:Cost, Time, Quality, Claims, Risk • ADSC-IAFD:What is it?

3. Who? Parties with Interest: • Owners / Developers • Architects • General Contractors • Designers • Geotechnical Engineers • Structural Engineers • Experienced and Inexperienced Designers

4. When?(sequence of non-D/B Projects) Pre-Bid Involvement Post-Bid Involvement Concept (Owner) Exc. & Fdns (Sub & Specialty Cont.) Design (Arch./Eng’r) Site Work and Prep (G.C. & Sub Cont.) Management (Arch./C.M.) Substructure (G.C. & Sub Cont.) Project Const. (General Cont.) Superstructure (G.C. & Sub Cont.) Foundations (Specialty Cont.) Occupancy (Owner)

5. Who by the Numbers • Owners / Developers: 100% • Design: 5-10% • Geotechnical: 1-2% • Construction: 85-90% • Specialty Contractor (Foundations): 10% • QA/QC & Inspection: <5% • Claims: can be more than 100%

6. What? • Drilled and concreted foundation systems • Diameters: >12 inch, <12 ft • More common range: 24 to 60 inch (verify) • Can accommodate a variety of lengths (~100s ft) • Capable of supporting large loads • Axially (xx kip) and Laterally (xx kip) • Installed in soils and rock • Environmentally Friendly • Less noise & vibration than pile driving

7. What? Over- burden Rock

8. What? …Advantages • Reliability – historically proven method • Low noise and minimal vibrations • Can visually inspect bearing stratum • Can monitor vertical alignment • Structural strength in bending • For seismic and wind loading • Eliminate need for pile cap • One drilled shaft in lieu of multiple piles

9. Where? • Variety of subsurface conditions: • In sands, clays and rock • Above or below GWT • Deposits with cobbles • Karst areas • Collapsible soils • Shrink/swell clays • Compressible strata • Soft clays, loose sands • Liquefiable soils

10. Where? • Variety of loading conditions: • Axial loading • Lateral loading • Lateral with axial loading • Uplift loading • Seismic loading • Impact loading • Open areas and low headroom • Difficult access areas • Settlement-sensitive structures

11. Where?

12. Where? …Applicability • Building foundations • Single column per drilled shaft • Transportation structures • Bridge foundations • Retaining walls • Sound walls • Underpinning • Slope stabilization • Seismic loading conditions

13. …Applicability

14. …Limitations • Requires careful QC/QA • Construction method is sensitive to ground conditions • Requires experienced, capable subcontractor • No direct field control of axial capacity • Concerns for structural integrity, rebar placement

15. How? …design elements • Axial Loads are resisted by • Side resistance, RS • Base resistance, RB • Force Equilibrium: • QC + W = RS + RB • Lateral Loads are resisted by • Soil strength • Bending Stiffness of Shaft, EI Axial Design Shown

16. How? …design elements • How are the soil strength and side/base resistances determined?? • Assumptions about soil strength made based on field and laboratory testing • Field tests: SPT, CPT, PMT, DMT, etc. • Lab tests: drained and undrained shear strength tests • Ultimately, develop load-deformation relationships • Strength, resistance and expected behavior is dependent upon soil type (cohesionless, cohesive or rock)

17. How? …design elements • Soil Types: (general terms only) • Cohesionless – sands, gravels, some silts • Cohesive – clays, some silts • Rock – unweathered and weathered rock • Side and Base Resistance = f(soil) • Displacements to mobilize resistance • Side Resistance: • Cohesive and Cohesionless Soils: 0.4 to 0.6 inch • Base Resistance: • Cohesive Soils: ±4% shaft diameter • Cohesionless Soils: ± 10% shaft diameter

18. How? …installation • General Construction Methods • “Dry” Method • Drilling, cage setting, concreting • Installation performed as quickly as possible to minimize the effects of stress relief in the soil or rock • “Wet” Method • Drilling, slurry head, cage setting, concreting • Bond between concrete and rebar not reduced if cage is placed in slurry before concreting • Casing Method • Many procedures can be followed • Casing is removed during concreting

19. How? …Dry Method • Used when boreholes will stand open • Relatively strong cohesive soil or rock with few cohesionless soil seams • Minor water infiltration can be tolerated as long as no sloughing is occurring • Can visually inspect the borehole • Less expensive and faster than wet / casing methods

20. How? …Dry Method Drill Borehole Set Cage

21. How? …Dry Method Place Concrete Completed Shaft

22. How? …Wet Method • Used if caving or water-bearing soils are present • Direct, visual inspection of borehole not possible • Contractor skill and experience are essential • Careful control of the chemical and physical properties of the slurry

23. How? …Wet Method Excavate Borehole under Slurry Set Cage in Slurry

24. How? …Wet Method Completed Shaft Place Concrete with Gravity Tremie / Pump

25. How? …Casing Method • Used when near-surface soils may cave / deep soils or rock are stable and dry • Otherwise, full-depth casing (or wet method) is considered

26. How? …Casing Method Possible Sequence • Drill until caving soil is reached • Introduce slurry (and maintain head) • Drill through caving soil • Insert and seal casing • Continue drilling below casing, if needed • Set cage on bottom of borehole • Place fluid concrete well up into casing • Pull casing slowly and steadily, continuing to add fluid concrete until shaft is completed

27. Why? …The reasons to be interested • Cost • Time • Quality • Claims • Risk

28. ADSC-IAFD • A trade association established in 1964 comprised of people working together to advance all aspects of the industries it serves. • Comprised of • Contractor Members: foundation drilling, anchored earth retention and geo-support contractors • Associate Members: manufacturers and suppliers • Technical Affiliates: engineering and design professionals

29. ADSC-IAFD • Work of the ADSC includes • Establishing standards & specifications for the industries it serves • Promoting ethical practice • Conducting design, construction and inspection seminars worldwide • Developing technical materials • Funding and conducting original research • Providing a forum for the free flow of ideas • Stimulating industry growth • Interfacing with corresponding industries and agencies (FHWA, OSHA, DOTs, etc.)

30. Information about Industry • Get information from Industry Survey to highlight the size of drilled shaft market, the experience of members, safety, training, the quantity performed, etc.

31. Available thru ADSC • Experienced Member Contractors • Training • Regional seminars • In-house seminars • Video • Design, Safety, Personnel • Technical Library • Technical Papers • Design Manuals • Foundation Drilling Magazine • Specifications

32. Recap • Who:All parties involved with the project • When:Get geo-contractor involved as soon as possible during planning/design • What:Drilled Shafts applicable in various loading, ground, environmental conditions • Where: • How: • Why:

33. Thank You forYour Attention!!