2005 AASHTO BRIDGE COMMITTEE AGENDA ITEM 40 (REVISION 2)

regarding the estimation of downdrag and how it is applied as discussed in those articles now apply to drilled shafts as well.

Article 10.8.2.2 "Settlement" – This article has been updated to be consistent with the recommendations provided by O'Neill and Reese (1999), as the current article is based on Reese and O'Neill (1988). In general, the procedures are the same, except the 1999 procedures include additional load transfer curves and procedures for soils classified as a gravel, and for a new class of materials termed Intermediate Geo Materials (IGM's), which are transitional materials between soil and rock.

Article 10.8.3.5 "Nominal Axial Compression Resistance of Single Drilled Shafts" – Commentary has been added to provide specific guidance on the use of design methods other than those specified in this article, as allowed by Article 10.1. The specific procedures recommended in this article have been updated from the Reese and O'Neill (1988) to be consistent with O'Neill and Reese (1999). Several older methods for shafts in sand were also eliminated from this article. The biggest change is the method for estimating resistance of shafts in rock. A subarticle has also been added that addresses Intermediate Geo Materials (IGM's). See Allen (2005) for a more detailed summary of the differences between the 1988 and 1999 drilled shaft resistance methods. Additional guidance has also been provided in this article on the effect of permanent casing on shaft side resistance.

Article 10.8.3.5.3 "Shafts in Strong Soil Overlying Weak Compressible Soil" – This is a new article that addresses the case of a strong bearing stratum of limited thickness overlying weaker soil.

Article 10.8.3.5.6 "Shaft Load Test" – This article has been expanded to address load testing of smaller scale shafts and the use of the Osterberg load cell for load testing. In addition, more detailed guidance has been added to address how load test results should be applied to design.

Article 10.8.3.6.3 "Cohesionless Soil" – The η factors for shaft group resistance in this article have been modified from the current specifications to be consistent with the shaft group efficiency results summarized by O'Neill and Reese (1999) – the main change is that η now equals 1.0 at a shaft spacing of 4.0 diameters rather than 6.0 diameters. Commentary has also been added to explain this.

ANTICIPATED EFFECT ON BRIDGES:

Changes to the resistance factors could make foundation designs more or less conservative, which could affect the overall bridge design. However, the changes are not anticipated to be large. These changes were necessary to make the reliability of the foundations, as much as possible, to be more consistent. The reliability of current, and past, foundation design was considered in the final selection of resistance factors. See FHWA NHI report by Allen (2005) for specific justification for the changes.

REFERENCES:

Allen, T. M., 2005, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Design, FHWA, Publication No. FHWA-NHI-05-052, February 2005.

Barker, R. M., Duncan, J. M., Rojiani, K. B., Ooi, P. S. K., Tan, C. K. and Kim. S. G. (1991). Manuals for the Design of Bridge Foundations. NCHRP Report 343, TRB, National Research Council, Washington, DC.

Hannigan, P.J., G.G.Goble, G. Thendean, G.E. Likins and F. Rausche 2005. "Design and Construction of Driven Pile Foundations" - Vol. I and II, Federal Highway Administration Report No. FHWA-HI-05, Federal Highway Administration, Washington, D.C.

Munfakh, G., Arman, A., Collin, J. G., Hung, J. C.-J., and Brouillette, R. P. (2001). Shallow Foundations Reference Manual. Publication No. FHWA-NHI-01-023, Federal Highway Administration, Washington, D.C.

O'Neill, M. W. and Reese, L. C. (1999). Drilled Shafts: Construction Procedures and Design Methods. Report No. FHWA-IF-99-025, Federal Highway Administration, Washington, D.C.

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period. Over 700 comments were received from 15 states and 3 industry organizations, the proposed Section 10 amended, and responses provided to each agency or organization who provided comments. A second opportunity for review of the amended version of Section 10 was provided to those who commented, during February 2005, and the proposed Section 10 was further amended to address the comments received, to produce the final draft agenda item.

Key changes to Section 10 in the 2004 AASHTO LRFD Design Specifications resulting from this rewrite are summarized as follows:

Article 10.4 – This article has been rewritten to reflect the guidance provided in FHWA Geotechnical Engineering Circular GEC-5. More definitive guidance is provided in the setting up of the geotechnical field investigation to define the subsurface conditions for foundations and walls (note that Section 11 does not address geotechnical investigation requirements for walls – therefore, all of the geotechnical investigation requirements for foundations and walls are proposed to be included in Article 10.4), but some flexibility in the required number and depth of borings to adapt to the specific site conditions, based on local experience, has been retained in this article. The required objectives of the geotechnical investigation are more clearly laid out. The role of geophysical testing to better define the subsurface conditions and to allow the number of borings to be reduced as appropriate is now described. The objectives of the laboratory and in-situ field testing programs are now more clearly laid out, and the detailed lists of test procedures provided in the current specifications have been removed, but included in the specification instead by reference to CEG-5, which contains a very thorough treatment of laboratory and in-situ testing. Minimum requirements and detailed guidance on the selection of soil and rock design properties from the laboratory and in-situ field testing programs has been added to this article, again based on the information provided in GEC-5. For common in-situ tests such as the Standard Penetration Test (SPT), the correlations used to determine soil properties (e.g., shear strength) that were used as part of the calibration of resistance factors (for those factors in which reliability theory was used) are provided. A new article on the determination of rock properties has been added to this article.

Article 10.5.2.2 "Tolerable Movements and Movement Criteria" – The key change to this article is to eliminate "Design horizontal movements at the service limit state should not exceed 1.5 in.," and replace it with text that provides criteria for determining the movement criterion to be used and where to apply it. The magnitude of displacement that should be used will depend on the structure geometry, for example, column height, bridge seat width, etc., as a "one size fits all" approach does not work. Furthermore, movement criteria for other foundation types have been consolidated here rather than being split up among other articles for specific foundation types.

Article 10.5.2.2 "Overall Stability" – This article has been modified to reference Article 11.6.3.4, to eliminate redundancy in the specifications.

Article 10.5.3 "Strength Limit State" – As previously written, this article applied fairly well to footing foundations, but did not apply to pile or shaft foundations. This article has been broken out into three subarticles to more clearly address each foundation type.

Article 10.5.5 "Resistance Factors" – As previously written, this article was written to address only the strength limit state. This article is now broken up into sub-articles to provide clear guidance on resistance factors for service and extreme event limit states. Explanation as to the basis of the resistance factor values provided was lacking in the previous specification. Commentary has been added to help the user of the specifications better understand how the current resistance factors were derived, how they relate to current LFD or ASD practice, and if they are to be adjusted for local or regional practices or site specific considerations, how to go about doing that. The resistance factors for footings are in general the same as they were in the previous specifications, but the resistance factor table for footings has been simplified. For piles, the resistance factors have been widely recognized as being too conservative, and recommendations in the previous specifications to combine the resistance factors for static pile analysis methods with dynamic pile analysis methods is not consistent with the approach in current AASHTO Standard Specifications and national practice in general. To be consistent with the approach in the AASHTO Standard Specifications regarding pile design, it is proposed to tie the resistance factor selection directly to the method used to estimate pile resistance in the field. Furthermore, the results provided in NCHRP Report 507, which contain the results of a detailed calibration based on an extensive database for deep foundations, have been used to provide more accurate resistance factors, also including more commonly used methods for estimating pile

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capacity than what was previously included in this article. The relationship between resistance factor selection and the various aspects of strength limit state pile design has been clarified, especially with regarding to pile length estimation versus estimating the size of the pile foundation needed to support the factored loads. For shafts, several of the methods listed are out of date or are earlier versions of the Reese and O'Neill (1988) method. Outdated shaft design methods have been removed from the specifications, and new resistance factors consistent with past LFD/ASD practice have been included. Work is on-going to determine reliability based resistance factors for shafts. Regarding the new sub-article on resistance factors for extreme event limit states, some clarification regarding how to apply resistance factors for various scour events (e.g., refer to Article 2.6) has been provided. In general, however, resistance factors for service and extreme event limit states are unchanged from previous specifications. As part of the rewrite effort, an FHWA report has been produced that summarizes the data and logic used to establish the foundation design resistance factors proposed in this agenda item (see Allen, T. M., 2005, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Design, FHWA, Publication No. FHWA-NHI-05-052, February 2005).

Article 10.6.1.3 "Effective Footing Dimensions" – This article has been moved to much farther up front in the footing design articles, since the effective footing dimensions affect settlement and bearing resistance calculations in addition to the determination of whether or not the footing meets eccentricity requirements.

Article 10.6.1.4 "Bearing Stress Distributions" – The technical requirements have in general not changed, but the guidance provided is clearer and better tied into Article 11.6.3.2, where more specific guidance on how to calculate the stress distribution is given.

Article 10.6.2.4.1 "Settlement of Cohesionless Soils" – The Hough Method, used widely nationwide and recommended in FHWA manuals, has been added as an acceptable method for estimating settlement. Detailed procedures are provided.

Article 10.6.3.1.2 "Theoretical Estimation" – The theoretical method for estimating strength limit state bearing resistance of footings currently in the specifications (the Rational Method – Barker, et al., 1991) is proposed to be replaced with the general bearing capacity method as presented by Munfakh, et al (2001). The Rational Method does not allow soils with both cohesion and friction, a common occurrence, to be analyzed regarding bearing resistance. Some states have found that this inability has caused the Rational Method to produce overly conservative bearing resistance values. The general method as presented by Munfakh, et al. (2001) is consistent with traditional bearing capacity theory, and does allow soils with both cohesion and friction to be evaluated. In addition, there has been a great deal of concern and confusion regarding the use of load inclination factors regarding their applicability to traditional bearing resistance determination. The proposed specifications and commentary attempt to clarify when inclination factors should be used and clarify what is current national practice regarding the use of inclination factors. Clearer guidance is also provided regarding bearing resistance of footings where two distinct soil layers are present.

Article 10.6.3.1.4 "Plate Load Tests" – More detailed guidance on plate load tests and how they should be used has been added.

Article 10.6.4 "Extreme Event Limit State Design" – This is a new article that provides general considerations for design for this limit state, in anticipation that more detailed guidance will be provided later once new LRFD seismic provisions are developed. Furthermore, this article ties into Article 11.6.5 so that design footings in general is consistent with design of footings that support walls. This consistency is missing in the current specifications.

Article 10.7 "Driven Piles" – This portion of Section 10 has been completely rewritten to be more consistent with national design practice in its organization, flow, and technical content based on review of the AASHTO Standard Specifications (2002), FHWA pile design manuals, and discussions with representatives from various state transportation departments.

Article 10.7.1.5 "Determination of Pile Loads" – This article (including sub-articles) has been pulled together from other existing articles, and expanded to address forces due to expansive soils, and conceptually how to address the effect on or by nearby structures. In the current (2004) specifications, while downdrag loads are addressed, they are not specifically addressed regarding how they are to be handled with respect to service, strength, and extreme event

limit states. More detailed guidance is provided in the proposed specifications for the service (Article 10.7.2.5) and strength (Article 10.7.3.6) limit states regarding application of downdrag forces. For extreme events, only conceptual guidance is given until such time that LRFD seismic design specifications are more fully developed. The specific guidance on estimating downdrag loads in Article 10.7 in the current specifications has been moved to Article 3.11.8. Therefore, Article 10.7.1.5 refers to Article 3.11.8 for specific guidance on determining downdrag loads.

Article 10.7.2. “Service Limit State Design” – This article (and sub-articles) has been expanded to address overall stability, to be consistent with changes to Sections 3 and 11 made in 2003 to make overall stability a service limit design consideration, and to add lateral squeeze as a potential service limit state consideration.

Article 10.7.2.3 “Settlement” – The pile group stress distributions available have been expanded to address various layered soil profile situations. The method to estimate the group settlement magnitude has not been changed, however.

Article 10.7.2.4 “Horizontal Pile Foundation Movement” – This article combines together several existing articles that deal with this subject, and has been expanded somewhat to provide clearer explanation on how to conduct lateral load/deflection analyses, focusing primarily on the P-y curve approach. Strain wedge theory has been added as an alternative approach for lateral analysis of short stiff piles. Group reduction factors have been updated to reflect current FHWA manual guidance (Hannigan, et al., 2005).

Article 10.7.3 “Strength Limit State Design” – This article has been expanded and rewritten to identify all elements of the strength limit state pile design process.

Article 10.7.3.1 “Point Bearing Piles on Rock” – This article has been rewritten to be consistent with the practice of several DOT’s who routinely drive piles to bedrock, and to include practical considerations of design and installation.

Article 10.7.3.2 “Pile Length Estimates for Contract Documents” – This has been expanded from the original article in the current specifications. The current specifications only briefly mention this key aspect of pile foundation design. This article now explains how to estimate the pile penetration depth required to obtain the desired nominal resistance, and how to relate the compression resistance determined using a dynamic method during driving to the results from a static analysis method, attempting to maintain a consistent level of reliability for both the dynamic and static determination of pile resistance. The proposed provisions were developed through consultation with 15 state DOT’s who were actively involved in the development or review of this agenda item, and represents a consensus of those states.

Article 10.7.3.3 “Nominal Axial Resistance Change after Pile Driving” – This is a new article. The current specifications do not address this issue. The guidance provided in conceptual in nature and emphasizes the need for restrike testing if significant soil setup or relaxation is anticipated. Time dependent axial resistance changes are a common occurrence, and can affect interpretation of testing and pile driving results.

Article 10.7.3.5 “Scour” – Scour is not addressed in the current specifications. This article was added to address this key failure mechanism.

Article 10.7.3.6 “Downdrag” – This article provides significantly expanded guidance on how to deal with downdrag loads for strength limit state design. Handling downdrag in design has been an area of confusion in the past. The guidance provided in this article attempts to provide clarity on how to account for downdrag in the strength limit state.

Article 10.7.3.7 “Determination of Nominal Axial Pile resistance in Compression” – This article provides significantly expanded guidance on the assessment of axial pile resistance in compression, providing detailed guidance on the use of pile load tests for design, dynamic testing with signal matching, wave equation analysis, and driving formulae. The resistance factors recommended in Article 10.5 for each of these methods were developed with certain assumptions as to how such methods are used. The guidance provided in this article provides needed description of how each of these methods should be used to justify the use of the specified resistance factors, and to

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be consistent with how they were calibrated. Various state DOT’s were consulted to make sure the use of the methods described in this article is consistent with widespread practice.

Article 10.7.3.7.5 “Static Analysis” – The Nordlund/Thurman method for piles in sand was added within this article. This method is in widespread use nationally and is recommended in FHWA manuals.

Article 10.7.3.11 “Nominal Horizontal Resistance of Pile Foundations” – This article has been rewritten and updated to be consistent with Article 10.7.2.4 (see previous comment on Article 10.7.2.4), and to conceptually address the issue of pile foundation fixity. Detailed recommendations on how to address pile fixity are not available at this time.

Article 10.7.3.12 “Pile Structural Resistance” – This article has been rewritten to provide clearer reference to articles in other sections of the LRFD specifications where structural design recommendations can be found. Furthermore, in the current specifications, an approximate empirical approach was recommended as the primary method to determine the effective pile length and the depth of fixity. Routine computer modeling using P-y analyses demonstrates that the empirical method provided in the current specifications does not produce results that are consistent with the P-y approach. Therefore, to be consistent with Article 10.7.3.11 and related articles, the empirical approach has been identified for use in preliminary design only, and that the approach provided in Article 10.7.3.11 should be used for final design.

Article 10.7.4 “Extreme Event Limit State” – This is a new article that provides conceptual guidance to address pile design for seismic and other extreme event limit states. Detailed guidance cannot be provided, however, until a new LRFD seismic specification is developed. Scour as an extreme event is also addressed in this article and is tied to the check flood.

Article 10.7.6 “Determination of Minimum Pile Penetration” – This article has been expanded from the original article to more fully address all the design issues that affect the selection of a minimum pile penetration requirement. This article also addresses what pile design requirements should be included in the construction contract documents.

Article 10.7.7 “Determination of R_ndr Used to Establish Contract Driving Criteria for Bearing” – This is a new article that specifies how to select the nominal driving resistance required to meet all the applicable limit states. How to make this selection is not clear in the current specifications.

Article 10.7.8 “Drivability Analysis” – The current specifications only address driving stress criteria, and furthermore are not fully written as a LRFD criteria. This article presents these driving stresses more clearly in LRFD format, provides more detailed reference to other articles where the appropriate resistance factors can be found, provides more detailed guidance on how to apply these criteria as part of a drivability analysis, and how the drivability analysis should be used as a part of the pile foundation design process.

Article 10.8.1.2 “Shaft Spacing, Clearance, and Embedment into Cap” – The spacing where interaction effects should be evaluated has been increased based on experience and the guidance regarding this issue clarified.

Article 10.8.1.3 “Shaft Diameter and Enlarged Bases” – A portion of this article that was originally in the specifications (in particular rock socket diameter) has been moved into the commentary.

Article 10.8.1.5 “Drilled Shaft Resistance” – Specific considerations and limit states for determining the shaft resistance required have been added (the current specification refers to the parallel pile design article, which, because Article 10.7 has been rewritten, no longer applies). In addition, specific shaft construction issues and techniques that affect the shaft design have been added (the current article provides only a general mention of constructability issues).

Article 10.8.1.6 “Determination of Shaft Loads” – In the current specifications, this article only addresses downdrag loads. This article has been expanded somewhat to address other load issues that are applicable to shafts, specifically application of the dynamic load allowance article in Section 3. For downdrag, this article has been revised to refer to Article 3.11.8 and parallel provisions in Article 10.7.1.5.1. Therefore, the changes made

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regarding the estimation of downdrag and how it is applied as discussed in those articles now apply to drilled shafts as well.

Article 10.8.2.2 "Settlement" – This article has been updated to be consistent with the recommendations provided by O'Neill and Reese (1999), as the current article is based on Reese and O'Neill (1988). In general, the procedures are the same, except the 1999 procedures include additional load transfer curves and procedures for soils classified as a gravel, and for a new class of materials termed Intermediate Geo Materials (IGM's), which are transitional materials between soil and rock.

Article 10.8.3.5 "Nominal Axial Compression Resistance of Single Drilled Shafts" – Commentary has been added to provide specific guidance on the use of design methods other than those specified in this article, as allowed by Article 10.1. The specific procedures recommended in this article have been updated from the Reese and O'Neill (1988) to be consistent with O'Neill and Reese (1999). Several older methods for shafts in sand were also eliminated from this article. The biggest change is the method for estimating resistance of shafts in rock. A subarticle has also been added that addresses Intermediate Geo Materials (IGM's). See Allen (2005) for a more detailed summary of the differences between the 1988 and 1999 drilled shaft resistance methods. Additional guidance has also been provided in this article on the effect of permanent casing on shaft side resistance.

Article 10.8.3.5.3 "Shafts in Strong Soil Overlying Weak Compressible Soil" – This is a new article that addresses the case of a strong bearing stratum of limited thickness overlying weaker soil.

Article 10.8.3.5.6 "Shaft Load Test" – This article has been expanded to address load testing of smaller scale shafts and the use of the Osterberg load cell for load testing. In addition, more detailed guidance has been added to address how load test results should be applied to design.

Article 10.8.3.6.3 "Cohesionless Soil" – The η factors for shaft group resistance in this article have been modified from the current specifications to be consistent with the shaft group efficiency results summarized by O'Neill and Reese (1999) – the main change is that η now equals 1.0 at a shaft spacing of 4.0 diameters rather than 6.0 diameters. Commentary has also been added to explain this.

ANTICIPATED EFFECT ON BRIDGES:

Changes to the resistance factors could make foundation designs more or less conservative, which could affect the overall bridge design. However, the changes are not anticipated to be large. These changes were necessary to make the reliability of the foundations, as much as possible, to be more consistent. The reliability of current, and past, foundation design was considered in the final selection of resistance factors. See FHWA NHI report by Allen (2005) for specific justification for the changes.

REFERENCES:

Allen, T. M., 2005, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Design, FHWA, Publication No. FHWA-NHI-05-052, February 2005.

Barker, R. M., Duncan, J. M., Rojiani, K. B., Ooi, P. S. K., Tan, C. K. and Kim. S. G. (1991). Manuals for the Design of Bridge Foundations. NCHRP Report 343, TRB, National Research Council, Washington, DC.

Hannigan, P.J., G.G.Goble, G. Thendean, G.E. Likins and F. Rausche 2005. "Design and Construction of Driven Pile Foundations" - Vol. I and II, Federal Highway Administration Report No. FHWA-HI-05, Federal Highway Administration, Washington, D.C.

Munfakh, G., Arman, A., Collin, J. G., Hung, J. C.-J., and Brouillette, R. P. (2001). Shallow Foundations Reference Manual. Publication No. FHWA-NHI-01-023, Federal Highway Administration, Washington, D.C.

O'Neill, M. W. and Reese, L. C. (1999). Drilled Shafts: Construction Procedures and Design Methods. Report No. FHWA-IF-99-025, Federal Highway Administration, Washington, D.C.

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