The economy and effectiveness of tiebacks for support of temporary excavations has led to their increased use for support of permanent excavations. To better understand the characteristics and performance of tiebacks, a field observation program was undertaken.
Various characteristics of over 900 tiebacks exhumed from an excavation along I-90 in Mercer Island, Washington were observed in the field. The geometric spacing of the tiebacks appeared to be quite uniform. Centering of the anchor tendon in the augered hole was generally good, although some instances of significantly off-center anchor tendons were observed. Significant corrosion was not observed on any of the anchor tendons that could be examined in the field. Evidence of grease leakage at the end of the trumpets was observed at a number of tieback locations. While physical disturbance of the tiebacks during excavation was undoubtedly responsible for much of the observed leakage, leakage was also observed at a number of tiebacks that appeared to be undisturbed.
On the basis of the observations made in the field, no specific changes in the current WSDOT design methods and specifications for tiebacks appear to be necessary.