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38 70 60 50 CR (m/yr) 40 30 20 10 0 NH I-93 NC I-240 Bruceton PA Anchors (SB) Mine (NR) Figure 17. Ranges of corrosion rate measurements for Type II reinforcements. surrounding rockmass (i.e., the electrolyte). Thus, these mea- Resin Grout surements demonstrate that resin grout can effectively protect Although resin grout may provide a barrier from corro- the surface, assuming the surface is adequately covered. Much sion, there is strong evidence to suggest that areas of the sur- higher corrosion rates are evident from the NIOSH SRCM. face may not be covered and vulnerable to corrosion. Even for This correlates well with the harsh environmental conditions fully grouted rock bolts it is likely that a gap exists behind at this site that includes pH ranging between 2.5 and 3.5 and the anchor plate that is vulnerable to corrosion. Prestress- sulfate concentrations between 800 ppm and 7000 ppm. Also, ing tends to cause resin grout to crack, compromising its abil- roof bolts installed at the SRCM may not have a gap behind ity to act as an effective barrier to corrosion. Poor coverage the anchor plate similar to the rock cut installations such that has been observed, both from the results of NDT and direct LPR measurements reflect corrosion rates near the proximal observations (Fishman et al. 2005; Fishman, 2005). Results end where moisture and oxygen are more prevalent. Data in from sonic echo testing imply that often the degree of cover- Figure 17 also confirm that the non-restressable anchors tested age afforded by the grout is relatively low, or grout quality is at the I-99 17th Street Exit ramp in Altoona, PA, may not poor (voids and cracks exist). This is consistent with Comp- be adequately protected by portland cement grout behind ton and Oyler (2005) who reported that the resin grout only the anchor plate and localized corrosion is occurring at an covered approximately 60% of the surface for fully grouted average rate of 5 m/yr. roof bolts that were exhumed for observation. Fishman (2005) Grout quality and the potential for gaps are indicated from also reported incomplete coverage in the bonded zone of results of sonic echo and ultrasonic testing. These tests indi- grouted end-point anchorages exhumed at the site of the cate that gaps often exist behind the bearing plate, even for Barron Mountain Rock Cut. fully grouted installations (Withiam et al., 2002). Results from Kendorski (2003) estimates the design life of unprotected sonic echo testing also provide information on remaining pre- rock reinforcement systems is approximately 50 years. Results stress and this may also correlate with conditions along the from this study demonstrate that, in instances where the design bonded or anchorage of the reinforcements. Since these con- load of rock reinforcements is based on pullout resistance, ditions may deteriorate with respect to time, these measure- the design life may be longer than 50 years, depending on site ments are also useful to interpret service life and durability of conditions. rock bolts. Specific details for rock bolt and ground anchor Other factors, such as loss of prestress, may also affect the installations, data from condition assessment, data interpre- service life of rock bolts with end-point anchorages. Results tation, and reliability analysis relative to durability and service from sonic echo tests, that were confirmed from lift-off test- life, are described in the following sections. ing, indicate that a relatively high proportion (approximately 30%) of resin-grouted bolts with end-point anchorages have lost significant levels of prestress at the Barron Mountain Rock Bolts Rock Cut (Fishman, 2004; Fishman, 2005). However, this Sites with rock bolts described in Table 24 do not incorpo- may not be as much of a problem for fully grouted elements. rate corrosion protection measures other than grout, there- Loss of prestress may be indicative of poor grout cover along fore details of the condition and type of grout are particularly the bonded zone, or due to weathering of rock beneath the important. bearing plate for end-point anchorages.
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39 Knowledge of surface area in electrical contact with earth catcher Cut are fully grouted, and results from sonic echo is required to reconcile corrosion rates from LPR measure- tests indicates that the bolts that were sampled for testing are ments. This surface area is more difficult to determine for rock approximately 10 feet long. bolt installations compared to MSE (i.e., Type I) reinforce- LPR measurements depicted in Figure 17 demonstrate cor- ments. First, the length of the bolt and the grouted length need rosion rates for sections of rock bolts surrounded by grout to be estimated, and then the amount of coverage afforded are relatively low. Although LPR measurements are useful to along the grouted length must be assessed. Installation details assess corrosion along grouted areas, more information is for rock bolts are not readily available from construction plans, needed to assess corrosion rates for exposed sections (i.e., not and details related to bolt length and the length of the bonded surrounded by grout) of the reinforcements. Direct observa- zone must be obtained from field notes when available. The tions of exposed portions of exhumed reinforcements indi- length of the bond zone can also be estimated from knowledge cate that corrosion in these vulnerable areas is much higher of the lock-off load, drill hole diameter, and by estimating than that indicated via LPR measurements. Thus, critical the allowable bond stress at the grout/rock interface. How- locations that may control design life include the gap behind ever, another utility of sonic echo testing is the confirming the anchor plate or other exposed areas. or obtaining of missing information about the geometry of the Metal loss of exposed portions of the reinforcement behind installation. Data from sonic echo tests have been used in this the anchor plate, or other areas, may be expressed using the study to verify bolt lengths and bond lengths that may then Romanoff equation as be used to estimate the surface area of the rock bolts in con- tact with the surrounding rock in order to reconcile corrosion m m 0.8 X = A t ( yr ) side (23) rates from LPR measurements. yr side The rock bolts installed at Barron Mountain were only grouted along the bonded length (i.e., end-point anchor- where t is time in years. Table 25 is a summary of metal loss ages), thus the free/stressing length is more vulnerable to cor- measurements obtained from rock reinforcements that have rosion. This is confirmed by direct visual observations of been exhumed from sites located in the United States, Swe- bolts that were exhumed from the site as reported by Fishman den, Finland, and England. These data are useful to assess the (2005), and is evident in the results from sonic echo tests per- variability associated with the constant A that appears in formed on more than 50 rock bolts from this site. This is use- Equation (23). ful since direct observations of metal loss from portions of Based on the data in Table 25, A in Equation (23) has a mean reinforcements that have been retrieved can be compared to value of 60 m/yr, a standard deviation of 40 m/yr, and can corrosion rates measured in situ with LPR. Based on results be approximated with a lognormal distribution. The data in from sonic echo testing, the bond lengths of rock bolts along Table 25 represent site averages. However, data from six rein- the southbound barrel of I-93 at the Barron Mountain Rock- forcements retrieved from the Barron Mountain Rock Cut Cut range between 3 and 13 feet. Rock bolts at the Beau- were analyzed and rendered a mean of 66 m/yr for A. Also, Table 25. Summary of metal loss measurements from rock bolts and ground anchors. Site Country Type Age X A (yrs.) (m) (m/yr) Barron Mountain Rock Cut, I- USA Resin-grouted 32 880 55 93 NB1 bar Barron Mountain Rock Cut, I- USA Resin-grouted 33 1498 91 93 SB2 bar State Route 52, Ellenville, NY3 USA Expansion shell 20 1690 154 Pyhasalmi4 Finland Split set 1.5 63 46 Pyhasalmi4 Finland Split set 1 28 28 Hemmaslahti4 Finland Split set 2 43 25 Hemmaslahti4 Finland Swellex 1.5 90 65 Kerretti4 Finland Swellex 2 56 32 FIP Case 45 Sweden Ground anchor 26 445 33 Dovenport Royal Dockyard6 England Ground anchor 22 750 63 1 Fishman (2005). 2 NCHRP 24-28. 3 Withiam et al. (2002). 4 Lokse (1992). 5 FIP (1986). 6 Weerasinghe and Adams (1997).
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40 measurements of metal loss from steel elements embedded in For this example, it is assumed that the design capacity of fills with resistivity between 3,000 -cm and 10,000 -cm as the rock bolt is based on the resistance mobilized along the discussed in the resistance factor calibrations for Type I rein- bonded length and not the structural capacity of the bolt. The forcements render a value for A equal to 54 m/yr. Further- structural resistance (tensile strength) remaining at the end more the statistics generated from Table 25 are relatively close of the design life must equal the design load enforced on the to the nominal metal loss expressed by Eq. (3), which applies system during installation. If the difference between the lock- to plain steel buried in a wide range of environments. Thus, the off load and the original structural capacity of the bolt is high statistics rendered from the data in Table 25 appear to be rea- enough, then enough structural capacity may be available at sonable. The statistical variation of metal loss represented by the end of the design life to sustain the design loads even these parameters can be used to calibrate resistance factors for though metal loss may not be explicitly considered during LRFD similar to that for Type I (MSE) reinforcements. How- design. A nominal resistance, Tnominal, equal to 40 kips, is used in ever, there are some notable differences as described in the this example similar to the Barron Mountain and Beaucatcher following example: Rock Cuts described in Table 24. The reinforcements are The example resistance factor calibration considers that assumed to be solid bars made from Grade 150 steel that has Type II reinforcements are not redundant and failure of a sin- a guaranteed ultimate tensile strength (GUTS) of 150 ksi. The gle element could mean that a block of rock is loosened, lead- GUTS is considered as the nominal strength and F ult (the sta- ing to a system failure. For the purpose of this example a target tistical variable for ultimate strength of steel) is considered to reliability index, T, equal to 3.1 corresponding to pf 0.001 is have a normal distribution equal to 1.05 times the nominal and adopted. This is consistent with past geotechnical design prac- COV equal to 0.1 similar to that described by Bounopane et al. tice for foundations as described by Withiam et al. (1998). (2003) and the statistics used to describe the variation of yield The load bias, Q, used in the resistance factor calibration strength for Type I reinforcements. Equation (23) is used to presumes that rock bolts are actively loaded and prestressed compute metal loss, X , where the parameter A is varied statis- during installation; and prestress is verified via lift-off testing tically according to a lognormal distribution with A = 60 m as described by PTI (2004). Thus, the uncertainty relative to and A = 40 m. the design load is much less compared to Type I reinforce- The calibration is performed considering bolts with a 1-inch ments, whereby the loads are passive and transferred to rein- initial diameter, Di, and 50-, 75- and 100-year design lives. forcements as the system deforms. Design loads are enforced Table 26 is a summary of the computed bias, resistance fac- upon the reinforcements during installation, and PTI (2004) tor, and probability of occurrence for each design life. These recommends that verified lock-off loads be within 5% of results indicate that for this example metal loss does not have design specifications. For the purpose of this example, R = 1 a significant impact on performance for design lives of 50 and and COV = 10% are used, and considered to be conservative 75 years, but should be taken into account for service lives in estimates. excess of 75 years. The results from this example depend on The resistance bias is computed as follows: the selected values for Tnominal, Di, and Fult. If these inputs vary then the results depicted in Table 26 do not apply. The pur- A Fult pose of this example is to demonstrate the approach and R = c (24) identify the input needed for a complete calibration. More Tnominal data are needed to assess typical design scenarios before a more complete calibration can be performed. D2 Ac = (25) Current AASHTO specifications specify = 0.8 relative to 4 rupture resistance for high-strength steel reinforcements. Assuming that metal loss is not considered in design but D2 = Di - 2 X for 2X < Di (26) using the same statistical properties for the remaining vari- D2 = 0 for 2X Di ables renders pf < 0.0001. The 75-year case shown in Table 26, Table 26. Results from reliability-based calibration considering the yield limit state for rock bolts. tf R pf (years) Distribution 50 2.36 0.33 Weibull 1.0 < 0.0001 75 2.22 0.42 Weibull 1.0/0.80 0.001/0.0001 100 2.14 0.55 Weibull 0.55 0.001