Monitoring Saves a Site

by Donald R. McMahon, (M.ASCE), Geotech Engr.; Goldberg-Zoino Assoc., Buffalo, N.Y.,
Donald B. Abrams, (A.M.ASCE), Chf. Engrg. Geologist; Goldberg-Zoino Assoc., Buffalo, N.Y.,

Serial Information: Civil Engineering—ASCE, 1985, Vol. 55, Issue 5, Pg. 59-61

Document Type: Feature article


A comparison is made of two supported excavations that were similarly designed and constructed. Both excavation support systems consisted of a soldier pile and lagging sheeting wall supported by three levels of struts or tiebacks. The excavation depths were both about 40 feet (12.2 m), one excavation was made in a silty sand, and the support system performed adequately. However, when the same design and construction procedures were used to construct the excavation support system in a sandy gravel, problems ensued causing concern for the safety of workers and surrounding structures. Tiebacks in both excavations were installed by augering through the sheeting wall and pressure grouting as the auger was withdrawn. They were prestressed to 95% of the design capacity to limit ground movements. During the augering for middle level tiebacks in the sandy gravel excavation, large amounts of soil sloughed down from behind the excavation support system, onto the auger flights, leaving large voids. Load losses were observed in the upper level tiebacks as the support system deflected away from the excavation. As the excavation was advanced and lower level tiebacks were installed, it was feared that development of full active earth pressures might overstress the lower level tiebacks and lead to a progressive catastrophic failure. Instead, repairs were made to the support system, the construction techniques were modified, and the project was completed safely.

Subject Headings: Excavation | Tieback (geotechnical) | Occupational safety | Walls | Sandy soils | Gravels | Structural safety | Comparative studies

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