3.6 Drilled Shaft Foundations

3.6.1 Minimum Sizes (01/06) Jan 2006 -  included language regarding  non-redundant  shafts.

The minimum diameter size for drilled shafts is 36-inches except that nonredundant shafts as defined in SDG 3.6.9 must be no less than 48-inches in diameter.  Shafts for bridge widening or under miscellaneous structures (i.e. sign structures, mast arms, high-mast light poles, noise walls) are exempt from this requirement.

3.6.2 Downdrag

A. Show the downdrag load on the plans.

B. For drilled shaft foundations, “downdrag” is the ultimate skin friction above the neutral point (the loading added to the drilled shaft due to settlement of the surrounding soils) minus the live load.

C. Scour may or may not occur as predicted; however, do not discount scourable soil layers to reduce the predicted downdrag.

3.6.3 Resistance Factors [10.5.5] (01/06) Jan 2006 -  1. deleted including 1/3 end bearing reference  for rock socket load testing. 2. Added column for differentiate  factors for redundant and non-redundant shafts. 3. added commentary.

Delete LRFD Table 10.5.5-3 and substitute SDG Table 3.5 for drilled shafts.

Table 3.5 Resistance Factors for Drilled Shafts (Bridge Foundations)

Loading

Design Method

Construction

QC Method

Resistance Factor,φ

Redundant

Non-redundant6

Compression

For soil: FHWA alpha or beta method1

Std Specifications

0.60

0.50

For rock socket: McVay's method2

neglecting end bearing

Standard Specifications

0.60

0.50

For rock socket: McVay's method2

including 1/3 end bearing

Standard Specifications

0.55

0.45

For rock socket: McVay's method2

Statnamic Load Testing

0.70

0.60

For rock socket: McVay's method2

Static Load Testing

0.75

0.65

Uplift

For soil: FHWA alpha or beta method1

Std Specifications

Varies1

Varies1

For rock socket: McVay's method2

Std Specifications

0.50

0.40

Lateral3

FBPier4

Std Specifications

or

Lateral Load  Test5

1.00

0.90

1. Refer to FHWA-IF-99-025, soils with N<15 correction suggested by O'Neill.

2. Refer to FDOT Soils and Foundation Handbook.

3. Extreme event.

4. Or comparable lateral analysis program.

5. When uncertain conditions are encountered.

6. As defined in SDG 3.6.9.

Commentary:  LRFD resistance factors are based on the probability of failure (Pf) of an element or group of elements resisting structural loads.  When resistance factors were calibrated, the state of practice utilized redundant drilled shaft foundations, therefore, the design Pf for each drilled shaft is larger than the design Pf for the entire bent or pier because multiple drilled shafts would have to fail before the bent or pier could fail. In a nonredundant foundation, the Pf for each foundation element should be the design Pf for the entire bent or pier because of the consequence of failure. Therefore, the resistance factor for nonredundant foundation element shall be smaller than that of the redundant foundation units. Until resistance factors are properly calibrated for nonredundant foundations, reduce the values provided in Table 3.5 of the FDOT Structures Design Guidelines by 0.10.  In addition, the design engineer should also evaluate the number of load tests required in order to qualify using the resistance factor for load test; fewer load tests may be adequate for a fairly uniform subsurface project site but multiple load tests may be necessary at a site with variable subsoil conditions. Sometimes load test may be required at each different soil profile if a representative soil profile of the site cannot be obtained.

3.6.4 Minimum Tip Elevation See SDG 3.5.7

3.6.5 Load Tests...See SDG 3.5.10

3.6.6 Drilled Shaft Data Table

A. For projects with drilled shafts, include in the plans, a Drilled Shaft Data Table. See SDME EX-8.

B. For items that do not apply, place "N/A" in the column but do not revise or modify the table.  

C. Round loads up to the next ton. Round elevations down to the nearest foot.

D. The “Drill Shaft Data Table” is not required in the Geotechnical Report; however, the Geotechnical Engineer of Record must review the information shown on the plans for these tables.

3.6.7 Plan Notes (07/04) July 2004 - Added section 3.6.7, Plan Notes and wording for pressure-grouted shaft tips.

For Drill Shafts with pressure-grouted tips, add the following note:

 

NOTE: A.H. Beck Foundation Company, Inc. owns U.S. Patent No. 6,371,698 entitled “Post Stressed Pier.” You are advised that the Department has, in any case, obtained a patent license agreement with A.H. Beck Foundation Company, Inc. that provides royalty free use of U.S. Patent No. 6,371,698 in the design, manufacture and construction of the post-grouted drilled shafts on this Department project, and no royalties will be asserted by A.H. Beck Foundation Company, Inc. against the Department, the prime contractor, subcontractors, manufacturers, or suppliers as to the post-grouted drilled shafts for this project. For more information as to U.S. Patent No. 6,371,698, contact:

A.H. Beck Foundation, Inc.

5123 Blanco Road

San Antonio, Texas 78216

Phone (210) 342-5261

3.6.8 Construction Joints

For drilled shafts used in bents located in water containing more than 2000 ppm chloride, detail the shaft to extend without a construction joint a minimum of 12 feet above the Mean High Water elevation or bottom of the bent cap, whichever is lower.

Commentary: It is preferred that taller shafts extend to the bottom of the bent cap without a construction joint.

3.6.9 Nonredundant Drilled Shaft Foundations(07/06) July 2006 Changed exempted size from up to 72" to from up to 60."
Jan 2006 - Added entire section.

Refer to the Soils and Foundations Handbook for special design phase investigation and construction phase testing and inspection requirements for nonredundant drilled shafts. Nonredundant drilled shaft foundations consist of bridge bents or piers consisting of one column with three or fewer drilled shafts, or two columns with one or more of the columns supported by only one or two drilled shafts, or as those shafts deemed nonredundant per  AASHTO LRFD Article 1.3.4.  Shafts for bridge widening when the substructure is attached to the original structure, and shafts up to 60-inches in diameter installed to support miscellaneous structures (i.e. sign structures, mast arms, high-mast light poles, noise walls) are exempt from these requirements.

For nonredundant drilled shafts:

A. Add a note to Foundation Layout Sheet requiring additional pilot holes at nonredundant shaft locations when the original design phase borings are insufficient.  See Soils and Foundations Handbook for requirements.  Require the pilot holes to be performed two weeks prior to shaft excavation.  Direct additional pilot holes during construction, where shafts are lengthened or shaft locations are modified.  

B. Add a note to Foundation Layout Sheet requiring that all nonredundant drilled shaft locations be inspected using a Shaft Inspection Device (SID) to ensure shaft cleanliness at the time of concrete placement.

C. Add a note to the Foundation Layout Sheet requiring all nonredundant drilled shafts to be integrity-tested using cross-hole-sonic logging (CSL.)

3.6.10 Minimum Reinforcement Spacing (01/07) Jan 2007 - Added 3.6.10 Minimum Reinforcement Spacing.

For Drilled shafts, ensure a minimum reinforcement spacing of six inches to allow for proper concrete consolidation.