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7 Table 4. Recommended sampling protocol for electrochemical testing of MSE wall fill (Elias et al., 2009). During Preconstruction Range General Construction of min Description No. resistivity Sample Comments (-cm) Samples (-cm) Interval (yd3) >10,000 Crushed rock 1 / 31 NA NA 1. pH outside the and gravel, specified limits is not <10% passing allowed for any sample. No. 10 sieve 2. Backfill sources shall 5,000 to Sandy gravel 3 / 61 <2,000 4,000 / 2,0001 be rejected if min 10,000 and sands measured for any sample <5,000 Silty sands and 5 / 101 <1,000 2,000 / 1,0001 is less than 700 -cm, Cl- clayey sand, > 500 ppm or SO4 > 1,000 screenings ppm. 3. For materials with min < 5,000 -cm, for Cl- and SO4 shall be less than 100 ppm and 200 ppm, respectively. 1 Number of resistivity tests / number of tests for pH, Cl-, and SO4. used to remove material from approximately 2 feet beyond environment is immediate and unaffected by the presence of the edge of the stockpile. Particular emphasis on sampling a corrosion protection system or grout cover surrounding the needs to be placed at sites where different reinforced fill reinforcements. Corrosion protection measures include the sources and/or types are being considered, and each source use of coatings, protective sheaths, passivation with grout, should be sampled as described in Table 4. and encapsulation. Thus, the estimated metal loss is applica- Differences in the electrochemical properties of the soil fill ble to unprotected portions of the installation and is a con- can adversely affect corrosion rates and contribute to more servative estimate for portions of the reinforcements that are severe and localized occurrences of metal loss. In instances passivated by grout or otherwise protected from corrosion. where more easily compacted (e.g., open-graded) material is The appropriate parameters for use in estimating metal placed adjacent to the wall face, significant differences in the loss are based on the corrosiveness index of the surrounding soil fill conditions may exist with respect to position along the earth. According to the recommendations described in With- reinforcements. For cases where reinforcements are not elec- iam et al. (2002), the parameters "k" and "n" for use in Equa- trically isolated (e.g., metallic facing), variations of backfill tion (1) are adjusted relative to soil/rockmass conditions as types along the height of the wall may also have a significant summarized in Table 5. The constant "n" is taken as one for effect on corrosion rates of metallic reinforcements. simplicity and considering the relatively short time frame (<20 years) inherent to most of the observations used to develop the table. Ground conditions in Table 5 are described Type II Reinforcements as average, corrosive, or highly corrosive based on electro- Since the integrity of the corrosion protection system is chemical characteristics of the surrounding material that may known to have a significant effect on service life, condition be soil, rock joint infill, or groundwater. Average conditions and assessment must focus on obtaining information on the sys- corrosive conditions refer to relatively neutral (pH > 5) and min tem's integrity. Properly installed grease and sheathing, and greater than 2,000 -cm, or 700 -cm < min < 2,000 -cm, protection at the anchor head assembly, can provide sub- respectively. Highly corrosive conditions are acidic (pH < 4), stantial benefits on service life. Equation (1) and correspond- ing parameters from Table 5 should be applied to those systems where protection is questionable; otherwise corrosion Table 5. Recommended parameters for cannot occur. For high strength steel reinforcements, corro- service life prediction model for Type II sion processes may also include hydrogen embrittlement or reinforcements (Withiam et al., 2002). SCC. Equation (1) does not apply to these types of corrosion Ground Conditions processes and, for these cases, the end of service is considered Parameter Highly to be when the corrosion protection is compromised. Average Corrosive Corrosive Equation (1) is applied to estimate metal loss of Type II k (m) 35 50 340 reinforcements assuming that attack from the surrounding n 1.0 1.0 1.0