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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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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
