Structures Design Guidelines | Jan 2009
3 - Substructure and Retaining Walls |

## 3.6 Drilled Shaft Foundations

### 3.6.1 Minimum Sizes (01/06) 2006 Jan: 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) 2006 Jan: 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 [10.8.1.5] (01/08) 2008 Jan: Deleted reference to SDG 3.5.8 and added specific criteria for drilled shafts.

A. The minimum drilled shaft tip elevation must be the deepest of the minimum elevations that satisfy all axial capacity and lateral stability requirements for the three limit states. The minimum tip for lateral stability requirements must be established by the Structures Engineer with the concurrence of the Geotechnical Engineer. The minimum tip elevation may be set lower by the Geotechnical Engineer to ensure axial compressive and tensile requirements are satisfied and to ensure soft soil strata are penetrated to satisfy post construction settlement concerns.

B. Use the following procedure to establish the Minimum Tip Elevation for lateral stability requirements for each design ground surface (or design scour) elevation:

1.) Establish a high end bearing resistance such that the shaft tip will not settle due to axial forces;

2.) Apply the controlling lateral load cases, raising the shaft tips until the foundation becomes unstable;

3.) Add 5 feet or 20% of the penetration, whichever is less, to the penetration at which the foundation becomes unstable.

Commentary:

The assumed soil/shaft side resistance is not modified using this procedure. It is assumed that the difference in axial resistance predicted during this portion of the design phase versus what is established during construction is due to end bearing only.

### 3.6.5 Load Tests...See SDG 3.5.10

### 3.6.6 Drilled Shaft Data Table (01/09) 2009 Jan: Added E. for Min. Top of Rock Elevation.

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.

E. The Min. Top of Rock Elevation is the highest elevation determined by the Geotechnical Engineer where the material qualities meet or exceed those which are suitable to be included in the rock socket.

1.) In somewhat variable conditions where pilot holes will be required, the Geotechnical Engineer should provide a best estimate of the elevation.

2.) In highly variable conditions where pilot holes will be required, use an asterisk “ * ” in place of the elevation, and refer to Article 3.6.7 C.

### 3.6.7 Plan Notes (01/09) 2009 Jan: Added Notes A through C.; 2004 July: Added section 3.6.7, Plan Notes and wording for pressure-grouted shaft tips.

A. Additional Plan Notes below the Drilled Shaft Data Table:

1.) The Tip Elevation is the highest elevation the shaft tip shall be constructed unless load test data, rock core tests, or other geotechnical test data obtained during pilot holes allows the Engineer to authorize a different Tip Elevation.

2.) The Min. Tip Elevation is required for lateral stability.

3.) Rock encountered above the Min. Top of Rock Elevation is considered unsuitable for inclusion in the rock socket length. The Engineer may revise this elevation based on pilot holes, if performed.

4.) Inspect all shafts considered nonredundant using the SID or an approved alternate down-hole camera to verify shaft bottom cleanliness at the time of concreting. Test all nonredundant drilled shafts using cross-hole-sonic logging (CSL).

B. For Drilled Shaft projects with lateral load tests, add the following to Note 2 above:

The Engineer may revise this elevation based on pilot holes or lateral load tests, if performed.

C. For Drilled Shaft projects in highly variable soil conditions with pilot holes required, refer to Article 3.6.6 E and replace Note 3 above with:

* The District Geotechnical Engineer will provide the Min. Top of Rock Elevation based on the required pilot holes.

D. For Drilled 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 2,000 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) 2006 July: Changed exempted size from up to 72" to from up to 60."; 2006 Jan: Added entire section.

A. Refer to the Soils and Foundations Handbook for special design phase investigation and construction phase testing and inspection requirements for nonredundant drilled shafts.

B. 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.

C. 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.

D. 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.

E. 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.

F. 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 [5.13.4.5.2, 10.8.3.9.3] (01/09) 2009 Jan: 3.6.10 B. Added exemption to double-cage prohibition.; 2008 Jan: A. Clarified min. 6" clear spacing. Added B. Requiring approval for double-cage shafts.; 2007 Jan: Added 3.6.10 Minimum Reinforcement Spacing.

A. For drilled shafts, provide a minimum clear distance between reinforcement of six inches to allow for proper concrete consolidation.

B. Double-cage shafts will not be permitted unless approved by the State Geotechnical Engineer. Inner column cages that develop column reinforcing steel at the top of the drilled shaft are exempted from this requirement.

Commentary: Multiple reinforcing cages in drilled shafts create constructability problems and are highly discouraged. A minimum 12-inch spacing between cages will be required when double cages are proposed for consideration in lieu of a larger diameter shaft.