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43 rosion protection included in the installation. Many rock forcements described in the current AASHTO specifications bolts are only protected by grout and a lack of coverage may is recommended for computing the nominal amount of sac- occur as a gap behind the anchor plate where grout is lost to rificial steel. This recommendation is based upon approxi- the surrounding rock mass during installation, or from rem- mately 1,000 measurements of metal loss and corrosion rate nants of plastic cartridges inherent to resin-grouted installa- from samples of galvanized reinforcements and coupons, cal- tions. However, the design load is often based on the pullout culation of the resistance bias and corresponding statistics, resistance rather than yield of the reinforcement, and metal and reliability-based calibration of the resistance factor for loss does not appear to be significant for design lives of 50 or LRFD considering the yield limit state. The current AASHTO 75 years. For a 100-year design life, the rupture limit state model is necessarily conservative, and corresponding resis- likely controls the performance and resistance factors appro- tance factors correlate to an acceptably low probability of priate to this design have been calibrated. The calibration uses occurrence for a 75- or 100-year design life. The resistance fac- approximately 70 observations of metal loss from sites located tors listed in Table 27 are recommended for use with LRFD in the United States, Scandinavia, and the United Kingdom. considering the yield limit state, and using the AASHTO metal Compared to MSE reinforcements (Type I), rock bolts are loss model to compute the nominal resistance at the end of the not necessarily redundant so a target reliability index of approx- design life. Different resistance factors are recommended for imately 3.1 rather than 2.3 was used for the calibration, cor- good versus high quality fill, strip or grid reinforcements. The responding to pf 0.001. protocol for sampling and electrochemical testing of wall fill, Ground anchors for permanent installations generally have described in Table 4, is recommended to assess fill quality and a Class I, double corrosion protection system including a trum- if the materials meet the criteria for good or high quality fills. pet head assembly to protect the area behind the anchor head. Although the calibrations resulted in different resistance There have not been any observations of poor performance factors depending on use of the simplified or coherent gravity when Class I corrosion protection measures are incorporated methods for computing nominal load, use of the same resis- with proper detailing and workmanship during installation. If tance factor is recommended for either method. This is based the anchor head does not include a trumpet head assembly that on the fact that the difference is relatively small, and similar is filled with grease, the area near the anchor head may be vul- designs are rendered when the same resistance factors are nerable to corrosion. If acidic conditions or high chloride applied as described from the results of the example problem concentrations prevail, then the service life may be severely described in Appendix F. Also, the simplified method was compromised from hydrogen embrittlement or SCC along originally calibrated to render similar results compared to the exposed portions of the anchor. coherent gravity approach with the traditional allowable stress methods of design. Furthermore, the load bias associated with the coherent gravity method (D'Appolonia, 2007) may Recommended Resistance Factors not consider the effect of reinforcement depth, and the cali- for LRFD bration appears to favor reinforcements placed within the top Current practice (LRFD) for the design of metallic rein- 20 feet of the wall. The bias factor for reinforcements located forcements for MSE applications is to ensure that reinforce- below this depth is expected to be less, corresponding to cal- ments maintain enough yield resistance to keep the probability ibrated resistance factors that are higher and closer to those that overstress occurs (i.e., probability of occurrence, pf) obtained with respect to the simplified method. below acceptable limits throughout the design life of the facil- The factors recommended for use with good quality fill ity. Galvanized reinforcements are recommended and sacri- and galvanized reinforcements are based on results presented ficial steel is included in the cross section to compensate for in Table 17 from calibrations performed with respect to the the expected loss of steel subsequent to depletion of the zinc simplified method of analysis. The recommended value of coating along the surface. Provided that the reinforced fill 0.65 corresponds well with values of computed considering material meets AASHTO criteria for electrochemical param- a 75-year design life. Designs considering a 100-year design eters, the metal loss model for galvanized metallic MSE rein- life are likely to result in relatively thicker reinforcements, Table 27. Summary of recommended LRFD strength reduction factors for galvanized reinforcements. Metal Backfill Metal Loss Design Life Resistance Factors () Type Quality Model (Years) Strip Grid Galvanized Good AASHTO 75 or 100 0.65 0.55 Galvanized High AASHTO 75 or 100 0.80 0.70
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44 therefore, the lower resistance factors of 0.55 and 0.60 com- good or high quality fills are used during construction. The puted for thinner reinforcements are not recommended; = following equations are recommended to estimate nominal 0.65 corresponds to thicker reinforcements with S = 6 mm. sacrificial steel requirements for good and high quality fills: For galvanized grid-type reinforcements embedded within good quality fill the recommended value of = 0.55 corre- For good quality fill: sponds to most of the values presented in Table 17, and is m m lower than the maximum of = 0.6 computed for W7 and X = 80 × t 0.8 ( yr ) side (27) W9 size wires and a 75-year design life. yr side The factors recommended for use with high quality fill and For high quality fill: galvanized reinforcements are based on results presented in Table 18 from calibrations performed with respect to the sim- m m X = 13 × t ( yr ) side (28) plified method of analysis. The recommended value of 0.80 for yr side strip-type reinforcements corresponds well with the value com- puted for 4-mm-thick reinforcements and a 75-year design life, For good quality fill, only a 50-year design life is considered and is generally less than the values computed considering a corresponding to a nominal sacrificial steel requirement of 100-year design life, although it is a little higher (by 0.05) than 1,829 µm per side according to Equation (27). Thus, a total of the value of 0.75 computed for the thicker reinforcements (S = 3.66 mm of sacrificial steel is required considering metal loss 6 mm). The recommended value of = 0.8 is considered rea- from all surfaces (additional diameter for round elements and sonable because using higher quality fill results in designs with additional thickness for strip-type elements). This is consider- relatively thinner reinforcements, and will most likely be con- ably higher than the current AASHTO requirement of 0.82 mm sidered in conjunction with longer design lives. For galvanized for a 50-year design life with galvanized reinforcements. grid-type reinforcements embedded within high quality fill, the A 75-year design life is considered when high quality fill is recommended value of = 0.70 is equal to or lower than most used in construction corresponding to a nominal sacrificial steel of the values presented in Table 18, but is 0.05 higher than the requirement of 975 µm per side according to Equation (28), or approximately 1.95 mm added to the diameter or thickness values of 0.65 computed for W11 and W14 size wires and design of an element. This is also higher than the current AASHTO lives of 75 years. However, this is not considered to be a signif- requirement of 1.42 mm for a 75-year design life with galva- icant difference, so use of = 0.70 is considered to be a reason- nized reinforcements. For both cases, calibrated resistance fac- able representation of the results presented in Table 18. tors of approximately 0.35 were computed considering use of Current AASHTO specifications prescribe resistance factors the simplified or coherent gravity methods. Design efficiency of 0.75 for strip-type reinforcements and 0.65 for grid-type factors for the case of plain steel reinforcements are less than reinforcements with rigid facing units (see Chapter 1, Table 7). half of those realized for the case of galvanized steel reinforce- These resistance factors apply with respect to use of either the ments (efficiency factor 0.2 compared to 0.5). simplified or coherent gravity methods to compute reinforce- Use of materials for reinforced fill that do not meet current ment loads. The resistance factors recommended in Table 27 AASHTO requirements is not recommended. However, data are similar to these in the current AASHTO specifications for exist in the literature from several sites in which special studies walls constructed with higher quality reinforced fill materials. were conducted to access the condition and remaining service Based on the information shown in Table 27, resistance factors life at sites where marginal quality fill was used, often inadver- should be reduced by 0.15 for fills that meet current AASHTO tently. These data are used to assess the conservatism inherent criteria for electrochemical parameters, but not by a very wide to existing models for computing nominal sacrificial steel margin (i.e., good fill). Alternatively, the same resistance fac- requirements and to calibrate resistance factors for the yield tors could be used with the understanding that the probability limit state. Marginal quality fills are described herein as hav- of occurrence for the case of good fill conditions is greater com- ing 5 < pH < 10 and 1,000 -cm < min < 3,000 -cm. Only pared to when high quality fills are used in construction. the use of galvanized reinforcements and design lives (tf) less Use of plain steel (i.e., not galvanized) reinforcements is than 50 years are considered with respect to use of marginal not recommended. However, data on the performance of fills. The following equation is recommended for computing plain steel reinforcements were analyzed, statistics on metal nominal sacrificial steel requirements: loss were generated, and resistance factors for use in LRFD were calibrated. The objectives of the study are to present dif- m m X = ( t f - 10 yrs ) × 28 (29) ferences with respect to design with galvanized reinforce- side yr side ments. Only fill materials that meet current AASHTO criteria for electrochemical parameters were considered, however, Application of Equation (29) presumes that zinc coating the performance of steel reinforcements depends on whether with a minimum required thickness of 86 µm per side will be