Long-Term Behavior of Ground Anchors and Tieback Walls (0-1391-1)

The purpose of this project is to advance the state of knowledge on the long-term behavior of ground anchors and make practical recommendations. Two important issues were addressed: long-term movements and corrosion.|For the long-tern movements, some of the main findings from the physical and numerical simulations are: 1. Given the same total anchor length, grouted anchors with a short tendon bond length have higher capacities and lower creep rates than grouted anchors with a long tendon bond length. 2. The capacity of grouted anchors in a stiff clay was found to increase over five years even when under load over the five years. 3. The power law model s1/s2=(t1/t2)^n where s1 and s2 are the anchor movements at time t1 and t2 fit the data well and may be used to predict long-term movements. 4. A delayed failure occurred in triaxial tests on a clay when the stress level was larger than the yield stress of the soil. 5. Long-term horizontal movements can develop in anchored walls if the vertical capacity of the soldier piles is not sufficient to resist the downdrag from the retained soil. 6. The power law model can also be used for the long-term movement of walls. 7. The bending moment profile over the long term tends to shift towards smaller moments at the anchor points. 8. The anchor loads seem to vary very little as a function of time ( plus or minus 10% after five years). 9. An earth pressure coefficient chart is given to design anchored walls for a targeted deflection.|For the corrosion issues, some of the main findings from the laboratory tests and literature review are: 1. Anchor failures have been reported and attributed primarily to a lack of protection and bending stresses. 2. About half of the failures occurred at the anchor head, half within the unbonded length, and very few within the bonded length. 3. The time to failure varied from a few weeks to 31 years. 4. The usual indicators of corrosion were confirmed by laboratory tests: soil resistivity, soil pH. 5. The Electrochemical Impedance Spectroscopy test (EIS) is proposed for measuring the corrosion rate for a given soil. 6. The EIS tests performed in the laboratory showed that the corrosion rate of the steel varies drastically for various protection coatings from 0.34 mm/yr for no protection to 10^-8 mm/yr for fusion bonded epoxy with 3 X 10^-3 mm/yr for grout cover. 7. The time to failure of an anchor can be estimated once the corrosion rate is known.