FEATURE ARTICLE SPECIAL GEC 7: A New Platform for LRFD Soil Nail Design Geotechnical Engineering Circular (GEC) 7 was recently revised by the Federal High- way Administration (FHWA) to provide up-to-date guidance for the design and construction of soil nail walls (Figure 1). The 2015 version of GEC 7 presents a procedure that is compatible with both existing Allowable Stress Design (ASD) limit equilibrium soil nail analysis/design software and a Load and Resistance Factor Design (LRFD) platform. The latter is consistent with the format for other earth retention structures presented in the AASHTO LRFD Bridge Design Specifications (AASHTO 2014). The main motivation for this update was the development of a LRFD-based pro- cedure. This procedure can be carried into the AASHTO specifications, thus broad- ening the use of rapidly constructed and cost-effective soil nail walls by transpor- tation agencies. The previous version of the manual, published in 2003 (GEC 7 2003), was based solely on the ASD method. Updated GEC 7 The most significant additions and updates to GEC 7 2015 are presented below. • A LRFD platform consistent with AASHTO (2014) is implemented. • A conversion is made of “failure modes” named in the 2003 GEC 7 into “limit states,” which, when applicable, are comparable to those included in AASHTO (2014). • Design considerations for the use of hol- low bar soil nails (HBSNs) are included. • Updated, more thorough corrosion cri- teria for soil nail design are presented. • Recent advances in soil nail materials, construction, and construction Quality Control (QC) and Quality Assurance (QA) aspects are contained. AUTHORS Figure 1. Cover of GEC 7 2015 • Modern seismic principles are described, in accordance with the updated GEC 3 LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations and other leading national documents on the subject. • Procedural and performance-based construction specifications are updated. • Descriptions of similar reinforcing technologies are incorporated. Frequent references are made to the FHWA-sponsored computer program SNAP-2 (Rock and Siel 2014). This program is one of the various numerical tools that are currently available for designing soil nail walls. However, this and other available software developed for soil nail design is based only on ASD methodology. Therefore, LRFD verifica- tions must be performed separately, as detailed within the GEC 7. GEC 7 2015 was written with a focus on soils and soft and weathered rock. In addition, the manual only addresses permanent walls. The use of soil nailing in urban environments as temporary support of excavation is not explicitly included. However, the reader will find some recommended safety factors (for ASD) and resistance factors (for LRFD) to consider temporary stages of permanent structures. The manual does not address the use of soil nails to stabilize landslides. To distinguish between the soil nails specifically addressed in GEC 7 from similar but essentially different systems, a soil nail is defined as follows: “Soil nails are reinforcing, passive elements that are drilled and grouted subhorizontally in the ground to support excavations in soil or in soft and weathered rock that: • Contribute to the stability of earth- resisting systems mainly through ten- sion as a result of the deformation of the retained soil or weathered rock mass. Carlos A. Lazarte, Ph.D., P.E., G.E, Helen Robinson, P.E., and Allen Cadden, P.E., Schnabel Engineering; Ryan R. Berg, P.E., Ryan R. Berg & Associates; Barry D. Siel, P.E., and Silas Nichols, P.E., FHWA DEEP FOUNDATIONS • MAY/JUNE 2015 • 75 ISSUE : LRFD
