Investigation and Implications of Corrosion Aggressiveness on MSE Wall Stability

1. Investigation and Implications of Corrosion Aggressiveness on Mechanically Stabilized Earth Wall Stability By John D. Thornley, P.E. Hattenburg Dilley & Linnell, LLC March 30th, 2010

2. What are MSE Walls???

3. Glenn – Bragaw Interchange Anchorage, AK

4. Outline • Background • I-515/Flamingo MSE walls in Las Vegas • I-15/Cheyenne MSE walls in Las Vegas • Historic case studies • Science of corrosion • Corrosion mechanisms • Predictive models • Nevada case history – revisited • NDOT MSE wall inventory and implications • Recommendations – Both for Nevada and other DOTs • Conclusion

5. Background – Flamingo • Accidental discovery of corroded reinforcements – 2004 • 20-year old walls • Steel w/o galvanization • Walls look “fine” from the outside • No obvious distress • Largest of 3 walls was replaced with cast-in-place tie-back wall • McMahon & Mann investigation 2004-2005

6. Original MSE Wall at Flamingo http://www.hilfiker.com/rseshadowpanel/img/rseshadow02.html

7. Background – Cheyenne • Accidental discovery of corroded reinforcements – 2008 • Only 9 Years old • This time reinforcements were galvanized • Walls look “fine” from the outside • No obvious distress • 100’ section of 1 of 7 walls replaced

8. MSE Wall at Cheyenne

9. Background (cont.) • Caltrans – Mariposa MSE wall • High rates of corrosion • Led to the investigation of other Caltrans walls • South African Tweepad Mine MSE walls • Wall failure at another mine • High rates of corrosion after 18 months • Monitored for 8 years • Removed and replaced

10. Science of Corrosion • Causes • Localized Corrosion • Micro-irregularities on metal surface • Galvanic Corrosion • Differential aeration or saturation level • Results • Pitting losses • Uniform losses

11. Science of Corrosion (cont.) • Soil Characteristics (AASHTO 2007 Specifications) • Resistivity • ≥3,000 ohm-cm • Salt Content • Chlorides ≤ 100ppm • Sulfates ≤ 200ppm • pH • 5 to 10 • Degree of Saturation

12. Historical Corrosion Limits* *Respective Standard Specifications for Highway Bridges (AASHTO) and Standard Specifications for Road and Bridge Construction (NDOT Silver Book) a There are no references to retaining walls in NDOT editions before 1986, these requirements were found in material test data (Contract 1918, July 1982) b There are no references to MSE walls prior to this AASHTO edition

13. Predicted Corrosion Rates

14. Nevada Case History – revisited • Welded Wire Fabric was not galvanized • Significant loss in cross sectional area measured • Backfill was identified as corrosive • A variety of test methods were used • Soil reinforcement corrosion is directly related to aggressive backfill

15. Statistical Analysis of Flamingo Reinforcements * Area loss calculations are based on a nominal original cross sectional diameter of 0.298 inches, as specified by Hilfiker Retaining Walls

17. Predicted Corrosion Rates

18. Statistical Analysis of MSE Backfill • Data from the 2005 investigation by MMCE was compared to original data from backfill approval in 1985 *Data has been converted to AASHTO T-288 equivalent resistivity

19. Common Resistivity Test Methods • Nevada T235B • Conductivity of water from saturated soil is measured • AASHTO T-288 • Soil resistivity is measured in a 2-pin soil box • ASTM G57 • Soil resistivity is measured in a 4-pin soil box

21. Consequences of Corrosion Critical Locations for Tensile Failure • There are critical locations where loss of cross sectional is more detrimental • The transition between the active and resistant zones is a primary concern

22. Consequences of Corrosion (cont.) • Wall stability was assessed for the two remaining Flamingo MSE walls using LRFD analysis • Corrosion rates are estimated from power loss models based on statistical analysis of measured reinforcements Prediction of Failure

23. Statistical Analyses Performed • Evaluation of direct measurement data • Prediction of future losses based on current data • Anova for backfill electrochemical data • Comparisons between original approval data and sampled backfill data • Resistivity test method correlation development • Nevada T235B and AASHTO T-288 methods • Development of future sampling procedures • Number of samples required to account for backfill variation

24. NDOT MSE Wall Inventory • NDOT has 154 MSE walls at 39 locations (as of 2009) • District 1 (Las Vegas area) • 90 walls at 18 locations • First wall constructed in 1981 • District 2 (Reno area) • 61 walls at 19 locations • First wall constructed in 1974 • District 3 (Elko Area) • 3 walls at 2 locations • First wall constructed in 1996

27. Recommendations for Nevada DOT • Two categories • Existing MSE walls • Further investigation should be performed • Begin with better characterization of actual backfill properties • Future MSE walls • Modify approval practices • Use AASHTO T-288 instead of Nevada T235B soil resistivity method • Modify backfill acceptance practices • Begin monitoring corrosion • Install direct and indirect observation devices

28. Lessons for Other State DOTs • Evaluate sampling and testing regimes • How do you know corrosion has not occurred? • Direct and indirect observation of steel reinforcements and coupons • Prevention costs less than mitigation • MSE walls cost ~$45/square foot (sf) • Nevada Cast-in-place Tie-back mitigation cost ~$60/sf • New focus on Geotechnical Asset Management (GAM) • Critical structural components should be monitored in a comprehensive management plan

29. Conclusions • Statistical analysis of MSE wall corrosion data shows: • Aggressiveness of backfill in Nevada is not characterized well • MSE wall service life is directly affected by the use of aggressive backfills • There are likely to be more MSE walls in Nevada that are experiencing high rates of corrosion • These walls need to be investigated in order to better quantify the problem • What assurances do you include in your design and construction practices to reduce the effects of corrosion?

30. Acknowledgements • Raj Siddharthan, PhD, PE of University of Nevada, Reno • J. Mark Salazar, PE of Nevada Department of Transportation • Barbara Luke, PhD, PE of University of Nevada, Las Vegas Further Reading • TRB 10-0480 • NDOT Research Report 2009-03

31. Thank YouQuestions???