Construction and Initial Performance of a Full-Scale Excavation Supported by Spiralnail Groutless Soil Nails

Spiralnails are hollow steel pipes extruded and twisted to form a square helical shape. They are driven rather than drilled, and thus they do not require cement grout to develop a bond with the soil, and provide immediate internal stability improvement to a soil mass. Because effective and economical designs require an improved understanding of individual and group spiralnail behavior, a 6-m (20-ft) tall, top-down vertical excavation was constructed using spiralnails in a well-characterized, engineered fill. The spiralnail reinforcement design used existing soil nail design methods to achieve an acceptable factor of safety using nail lengths and spacing comparable to those common for a conventional grouted nail design. The objective of the project was to measure the performance of the spiralnail reinforced soil mass in order to evaluate the applicability of existing soil nail design methods to spiralnail design. The paper describes the design, construction, soil characterization and properties, instrumentation, and presents the measured load-displacement behavior of the spiralnails and observed deformation of the reinforced soil mass. A preliminary analysis of load-displacement data is presented, and calculated spiralnail equivalent bond stress values are provided. Based on the measured deformations and using an empirical correlation, the spiralnail-reinforced soil mass performance is compared to a grouted nail-reinforced soil mass.