The soil-nailing method of earth retention is the preferred retaining wall option for many cut applications. Advantages of soil-nailed retaining structures include cost, speed of construction, construction flexibility, and aesthetics. Federal Highway Administration (FHWA) Demonstration Project No. 103 developed a comprehensive design and construction manual (FHWA Report SA-96-069R; see Special Note C) for temporary and permanent soil-nailed structures. The FHWA soil-nailing manual contains a detailed design protocol for allowable stress design (ASD) and an early, but incomplete, Load and Resistance Factor Design (LRFD) approach.
The AASHTO Standard Bridge Specifications, the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications do not provide guidance on design and construction of soil-nailed structures. With the absence of AASHTO specifications, some state DOTs are not yet using soil nailed retaining structures. Given the advantages of soil-nailed structures, there is a need to develop standard design and construction specifications for soil-nailed structures for incorporation into the AASHTO LRFD Bridge Design and Construction Specifications.
The objective of this research is to develop recommended LRFD design specifications and LRFD construction specifications for soil-nailed retaining structures.
Accomplishment of the project objective(s) will require at least the following tasks.
TASKS PHASE I (1.) Using existing FHWA guidelines on soil-nailing and other appropriate domestic and international published references, conduct a comprehensive review of current soil-nailing design and construction guidance for both ASD and LRFD specifications. This review shall include examination of the current AASHTO LRFD Bridge Design and Construction Specifications. (2.) Prepare draft LRFD design and construction specifications for soil-nailed retaining structures. Development of the draft specifications should reflect the technical input of industry and other sources. Identify resistance factors that will need evaluation/calibration under Phase II. (3.) Within 6 months of contract award, submit the information developed in Tasks 1 and 2 for review by the NCHRP panel. Provide an updated work plan describing how the research team intends to complete Phase II of the research. The updated work plan shall describe in detail the methodology for accomplishing Task 4. Meet with the panel to discuss the draft specifications, resistance factors, and updated work plan.
PHASE II (4.) Based on the panel's guidance during the Task 3 interim meeting, conduct calibration analyses with respect to current design methods and recommend resistance factors for soil-nailed structures. The recommended resistance factors should provide a range of values to be used based upon the level of detail of the site investigation and the designer's confidence of the accuracy of the site characterization. Use current AASHTO load factors or load factors modified under NCHRP Project 12-55. (5.) Demonstrate the comparability of the LRFD design methodology and the existing ASD design methodology by conducting parallel designs for multiple design scenarios. (6.) Submit a final report documenting the entire research effort. The final report shall include a detailed description of the resistance factor calibration and the Task 5 design comparisons. The final design and construction specifications should be stand-alone appendices and in a format suitable for adoption by AASHTO.
Product Availability: The final report for Project 24-21 has been published as NCHRP Report 701: Proposed Specifications for LRFD Soil-Nailing Design and Construction and is available here.