FEATURE ARTICLE subject to tensile strain. Furthermore, all the strands of a traditional tieback are embedded into one single bond zone, and have the same total length. LDCAs consist of an anchorage, high Instrumenting the test anchors Research on LDCA Technology An experimental research effort was undertaken by Schnabel Engineering, Skyline Steel and Berkel & Company Contractors on Load Distributive Compressive Anchors (LDCA). LDCAs were developed and first used in South Korea. This research effort was aimed at developing design, installation and testing criteria suitable for U.S. practice. Further information can be found in the paper “Development of Removable Load Distributive Compressive Anchor Technology,” presented at the ASCE Metropolitan Section Geotechnical Group Seminar in May 2014. Since the completion of the research report, LDCAs have been used successfully for several projects in the U.S. including Hudson Yards in Manhattan and Maryland Public Health Building in Baltimore. AUTHORS LDCA vs. Traditional Tiebacks A prestressed grouted ground anchor or tieback is a structural element installed in soil or rock used to transfer an applied tensile load to the surrounding soil, typically behind the active wedge of a retaining wall or slope stability application. Tiebacks derive their geotechnical capacity from bond (shear strength) at the grout/ground interface along the bond length. In traditional tiebacks the free stressing length is established by using a smooth polyethylene sheath over the intended free stressing length of the tendon. The bond zone is established by embedding the bare tendon into the surrounding grout. Thus, the mobilization of the bond stress starts at the proximal end of the bond length, and the grout surrounding the strand in the bond zone is density polyethylene (HDPE) sheathed strand tendons, and an aluminum anchor body, or end fixture, at the distal end. Each strand is fitted with one end fixture. LDCAs also derive their capacity from bond at the grout/ground interface. However, because the strands are sheathed, they do not transfer their load to the surrounding grout through bond, but rather through end bearing, or compression, between the end fixture and the grout body above. Thus, the grout is subject to significant compressive strain. The end fixtures may be spaced along the bond zone in a manner that prevents significant concentration of shear stresses along the bond zone. This promotes overall efficiency of the element, which may result in higher load capacity for LDCAs than traditional tiebacks in identical soil, especially in soils susceptible to creep. For both types of tiebacks, the Aerial photograph of project site distribution of shear stress is not uniform along the bond length because the magnitude of shear stress mobilized at the LDCA technology can be utilized in municipalities that have imposed restrictions on the use of tiebacks grout/ground interface depends on the magni tude of relat ive inter face displacement. In tiebacks subject to relatively small loads, the maximum shear stress develops near the proximal end of the bond zone, and shifts toward the distal end under progressively increasing loads. In LDCAs, this trend is reversed with the point of maximum shear stress shifting toward the proximal end of the bond zone as the tendon load increases. Carlos Englert and Jesús Gómez, Ph.D., P.E., D.GE, Schnabel Engineering; Chris Wilkinson, P.Eng., Skyline Steel; Vaughn Godet, P.E., Berkel & Company Contractors DEEP FOUNDATIONS • JULY/AUG 2014 • 69
