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1 CHAPTER 1 Background Transportation agencies use a variety of metal-reinforced reinforced earth structures [i.e., mechanically stabilized earth systems in geotechnical applications, including soil and rock (MSE)] that may consist of steel strips, welded wire fabric, wire reinforcements, ground anchors, and tiebacks. These systems mesh, or soil nails. Type I elements are not prestressed, and support retaining walls, bridge abutments, approaches, and load is transferred to the elements as the structure deforms highway embankments, and they also stabilize roadway cuts during construction, and throughout its service life. Type II and fills. Corrosion is known to have an impact on service life, reinforcements are active elements that are prestressed dur- and engineers, faced with the task of allocating budgets to reha- ing installation and include ground anchors (strands and bars) bilitate aging facilities, need reliable techniques to estimate that and rock bolts. Due to the application of prestress, the loads in remaining service life. Service life estimates for new systems active systems are controlled and involve more certainty com- need to be evaluated, and consideration of metal loss in the pared to passive systems. In general, Type II reinforcements design needs to be consistent with the reliability-based approach consist of relatively high-strength steel and a higher level of adopted in the AASHTO Load and Resistance Factor Design corrosion protection compared to Type I reinforcements. A (LRFD) Bridge Design Specifications. NCHRP Project 24-28 significant difference between element types is that Type I addresses these needs by developing a database to document the elements are often designed by including sacrificial steel to performance of earth reinforcement systems, and by perform- account for metal loss due to corrosion, whereas Type II ele- ing the statistical and reliability analyses necessary to consider ments do not include sacrificial material and the durability of service life within the context of reliability-based design. the corrosion protection system controls the design life. The objectives of NCHRP Project 24-28 are to (1) assess and improve the predictive capabilities of existing computational Details of Type I Reinforcements models for corrosion potential, metal loss, and service life of metal-reinforced systems used in geotechnical engineering Most steel reinforcements for MSE structures are hot-rolled applications; (2) develop methodology that incorporates the steel strips, welded wire grids, or bar-mat grids manufactured improved predictive models into an LRFD approach for the from cold-drawn wire as depicted in Figure 1. Standard sizes design of metal-reinforced systems; and (3) recommend addi- of reinforcements, steel grades, and details of galvanization tions and revisions to the AASHTO LRFD specifications to have evolved, and current practices differ from those employed incorporate the improved models and methodology. Current prior to approximately 1978. Type I reinforcements are man- design specifications (AASHTO, 2009) incorporate metal loss ufactured from mild steel and, currently, steel strips are man- models, but these models have limited application with respect ufactured from ASTM A-572, Grade 65 steel. Prior to 1978 to reinforcement type and fill conditions. Results from this steel strips were manufactured from Grade 36 steel, described study serve to broaden the recommendations for metal loss by ASTM A-446. Grids are manufactured from Grade 80 cold- modeling, and describe effects of fill quality and reinforce- drawn wire in accordance with ASTM A-82, and deformed ment type on performance and service life. welded wire is sometimes used, as described by ASTM A-496. Bar mats are often configured with between two and five lon- gitudinal wires, whereas welded wire mesh may have 10 or Earth Reinforcements more longitudinal wires per unit. For the purpose of this study, metal-reinforced systems Most often the reinforcements include hot-dip galvanizing are broadly categorized into two types. Type I reinforcements for corrosion protection that is applied in accordance with are passive elements used in the construction of metallically ASTM A-123 for wire- or strip-type reinforcements. Minimum

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2 (a) Ribbed Strips (b) Welded Wire Mesh (c) Figure 1. Examples of metallic reinforcements used in MSE construction: (a) bar-mat grid reinforcements, (b) hot-rolled steel strip reinforcements, and (c) welded-wire grid reinforcements. requirements for thickness of zinc coating depend on the thick- reinforcement samples. These data reflect a mean zinc coat- ness and size of the reinforcements. AASHTO specifications ing thickness of approximately 150 m. However, these data require a minimum of 86 m per side for MSE reinforcements, are very limited and more data describing the variation of the as described in ASTM A-123. However, for reinforcements thickness of zinc coating are needed to obtain a reasonable installed prior to 1978 and galvanized in accordance with distribution of measurements. ASTM A-525 or A-641, the specified initial thickness of zinc Soil nails are steel bars with diameters ranging between coating was less than the current specifications of 86 m and 1 and 1.5 inches that are inserted into a 4-inch to 12-inch ranged between 17 m and 30 m. diameter drill hole and surrounded by grout. Often Grade 60 The hot-dip process provides good coverage of zinc along deformed, reinforcing steel bars are employed for soil nails the surface, but the distribution of thickness is difficult to but high strength prestressing steel bars (e.g., Grade 150) are control. Thus, the mean thickness of the zinc coating neces- sometimes used. Soil nails may be galvanized or epoxy coated sarily exceeds the AASHTO minimum requirements of 86 m. and sometimes are encapsulated similar to the corrosion pro- This ensures a low probable occurrence of a spot with less tection systems described for Type II reinforcements in the than the minimum requirement. From the standpoint of next section. However, there are several new developments in reliability analyses it is important to recognize that zinc soil nails that may not be fully encapsulated, if at all, includ- thickness is a variable. Sagues et al. (1998) and Rossi (1996) ing self-drilling and self-grouted nails, screwed-in nails, and report measurements of zinc coating thickness from frequent dynamically inserted nails (i.e., inserted using a nail gun or intervals along the lengths of a limited number of strip-type sonic method).