Application of LRFD Principles for Deep Foundations in Oregon: Phase 1

Trevor Smith, Portland State University



Project Sponsor Champion: Bruce Johnson, ODOT State Bridge Engineer .
 The Oregon DOT has routinely relied on the use of the Wave Equation Analysis of Piles (WEAP) software to dynamically model the bridge pile driving installation operation.  Either from the end of initial driving (EOID) when the pile reaches design penetration, or a later Beginning of Restrike (BOR) by the driving hammer, the ODOT has become proficient at establishing axial capacity based on WEAP results.  This removed the need for more costly dynamic or full scale load testing and eliminated delays on pile driving for most bridge foundation construction.  This has served Oregon well and a history of satisfactory application has been established. The State of Oregon has now embarked upon a major cracked and deficient bridge reconstruction program which is ‘fast tracked’ to replace originally 365 bridges across the state on key highway routes, all in addition to the regular budgeted bridge work. Thus a bridge construction surge will likely entail a large number of high capacity pile foundations, driven in all soil types across the state.
By rapid implementation of Load Resistance Factor Design (LRFD) principles (developed by structural engineers) the FHWA is requiring adoption by AASHTO code, and state DOT’s are required to respond.  This creates a precedent in the area of geotechnical policy, communication and dissemination of knowledge. Beginning April 1, 2007 all new ODOT bridges will be required to be designed using LRFD, including all bridge foundations. The WEAP method has been in use in ODOT for over 15 years, using a factor of safety of 2.5 applied to the ultimate bearing capacity. The new AASHTO LRFD code specifies a resistance factor (Ø) of 0.40. The Ø factor of 0.40 relates to an equivalent factor of safety of about 3.5 (assuming an "average" load factor of 1.40). This will result in a much more conservative pile foundation designs for projects using the wave equation. In establishing code minimums AASHTO "default values" were used for all soil input values in the wave equation to minimize variability from local engineering judgment. A review of the data in the NCHRP report indicates that the wave equation method generally under predicts bearing capacity when using EOID data. Dictated by Oregon soils, and the present use of WEAP for capacity, ODOT is left very exposed and likely to see high and unnecessary increases in cost for it’s foundations without any improvement in confidence, safety or reliability by the new AASHTO requirements, which are not locally specific.

The Phase 1 objectives of this research are to: attempt to fully articulate the magnitude of the anticipated cost increase for Oregon, nurture a high level of geo-competency in pile foundation statistics and reliability concepts, build regional support for a procedure to gain acceptance of raising the resistance factor Ø locally, prepare concise LRFD deep foundation implementation feedback of the state’s needs to AASHTO, and coordinate and pool experience of other affected states and establish implementation policies.

Two principal activities are proposed to build support for increasing the Ø value.  Firstly, the ‘state of practice’ in Oregon as it relates to soil variations, WEAP experience and pile performance will be documented. Second, a soil strength property parametric reliability analysis by WEAP for a typical bridge pile foundations is performed to quantify the effect of known standard deviations from Oregon Soils.  It is anticipated that both these activities, and a stakeholder dialogue, will lead to supporting preliminary revision in the Ø value which respects the superior BOR technology over the dynamic driving formulae.  Finally the cost savings from these preliminary recommendations will be applied to a ODOT selected case study of a completed bridge to establish Ø = 0.4 and the Ø > 0.4 foundation costs.  Example; the proposed new Interstate 5 Columbia River Crossing (CRC) has a possible range of foundation costs between $300M- $800M and application of this possible ultra conservative flaw in the AASHTO code may increase foundation costs between $10M to $50M for driven piles, if selected.

Project Details

Project Type:
Technology Transfer
Project Status:
End Date:
January 31,2008
UTC Grant Cycle:
OTREC 2007
UTC Funding: