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Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls (2012)

Chapter: Chapter Three - Collection of Mechanically Stabilized Earth Wall Data

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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
×
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
×
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Suggested Citation:"Chapter Three - Collection of Mechanically Stabilized Earth Wall Data ." National Academies of Sciences, Engineering, and Medicine. 2012. Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls. Washington, DC: The National Academies Press. doi: 10.17226/22721.
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10 At perhaps its most basic level, effective asset management consists of three components: (1) data collection; (2) data assessment and interpretation; and (3) taking action consis- tent with asset performance goals. Each of these three com- ponents is constrained by available resources. This chapter will focus on the data collection component. TYPES OF DATA CONTAINED IN WALL INVENTORIES/DATABASES Not all data are helpful in meeting asset management goals. Rather, the appropriate data must be collected—data that can be reliably quantified and assessed so that meaningful conclu- sions regarding performance can be drawn. In practice, data collection often focuses on potential symptoms of adverse performance and is obtained during field investigations and inspections. Alzamora and Anderson (2009) provide a review of MSE wall performance issues based on their expe- rience with FHWA. They particularly identified geometry/ wall layout, obstructions, wall embedment, surface drainage, backfill placement and compaction, panel joints, leveling pad, and durability of facing as potential problem areas. Consis- tent with their findings, most data collection efforts currently undertaken relate to the condition and performance of these particular elements. Several agencies have developed guidance manuals and/ or inspection forms for gathering post-construction wall per- formance data. Examples of some of these materials devel- oped by FHWA (DeMarco et al. 2010b), Nebraska (Jensen and Arthur 2009; Nebraska Department of Roads 2009), Ohio (Ohio Department of Transportation 2007), and Utah (Bay et al. 2009) are provided as examples in web-only Appendix E. There are also MSE wall inspection manuals that focus on installation/construction issues (e.g., New York State Department of Transportation 2007), but these usu- ally do not explicitly address long-term wall performance. A feature common to several of the above-cited manu- als is the use of photographs illustrating the nature of a par- ticular feature needing identification (such as a sand cone in front of a wall joint, indicative of backfill migration) and/or quantification of its severity (minor verses major amounts of concrete degradation). The picture and the man- uals themselves serve a calibration purpose when multiple individuals are involved in data collection; without a com- mon baseline, data scatter can be excessive, particularly when the metric is subjectively quantified (i.e., not directly measurable). Perhaps the best documented, large wall inventory pro- gram in the United States is FHWA’s Wall Inventory Pro- gram (WIP). Extensive guidance and discussion concerning data collection methods are presented in the WIP Procedures Manual (DeMarco et al. 2010b). The WIP Procedures Man- ual emphasizes that “collected wall data must be accurate, concise and descriptive.” Photographic documentation dur- ing data collection efforts is also encouraged. For MSE wall types, data collection and rating focuses on the following primary wall elements: wire/geosynthetic facing, concrete panels, manufactured block, wall foundation materials, and wall drains. Applicable secondary wall elements include road/shoulders, upslope, downslope, and lateral slope. Rather than being numeric in nature or measurement-based, the con- dition data collected for each wall element consist of a written “condition narrative,” which is “a concise, descriptive narrative of element condition sufficient to characterize severity, extent, and urgency of element distresses” (DeMarco et al. 2010a). To help ensure consistency, these narratives use terminology and definitions consistent with the types of potential distress described in Figure 1. As seen in this figure, element ratings reflect observa- tional wall condition data relative to four distress categories: corrosion/weathering, cracking/breaking, distortion/deflection, and lost bearing/missing elements. These narratives are later converted to a numerical “condition rating” ranging from 1 to 10 using the descriptions shown in Table 6. This process is sub- jective, and rating variances among inspectors are reported to be within plus-or-minus two rating points for a given element. In the FHWA WIP, a general wall performance rating is also determined along with the element condition ratings. This rating scheme is shown in Table 7. Use of the wall performance rating is illustrated using the following example from the WIP procedures manual (DeMarco et al. 2010b, p. 101): For example, an MSE wall with a geogrid-wrapped face shows little sign of specific element distress (geogrid and backing geo- textile are largely unweathered, drains are working, etc.). How- ever, the wall is differentially settling at one end, as evidenced by a 3- to 6-inch vertical sag extending full-height in the wall face. A tension crack has begun to open at the top of the wall just beyond the estimated length of reinforcements, further indi- cating a global or external wall failure mechanism is actively chapter three COLLECTION OF MECHANICALLY STABILIZED EARTH WALL DATA

11 FIGURE 1 Element condition narrative guidance (DeMarco et al. 2010b).

12 developing. The inspecting engineer describes the overall wall performance as ‘low,’ providing appropriate narrative describ- ing the state of global distress, and rates the wall performance at a ‘4’ per the rating definitions. As discussed in the next chapter, these element condition rat- ings combined with the wall performance ratings create an overall wall performance rating ranging from 5 to 100, and these ratings are used in assessment management decisions. Although not quite as detailed as the FHWA WIP just pre- sented, Brutus and Tauber (2009) have also developed a guide to asset management of earth structures. They indicate that con- ditions listed here could be indicative of wall stress or deterio- ration, and recommend that the precise vertical and horizontal locations where these conditions are observed should be docu- mented. Brutus and Tauber also suggest that a severity or prior- ity rating such as (1) low, (2) moderate, (3) high, or (4) urgent be assigned as conditions are assessed in the field. Rating Rating Definition 9 to 10 Excellent No-to-very low extent of very low distress. Defects are mi nor, are within the normal range for newly constructed or fabricated elements, and m ay include those resulting from fabrication or construction. In practice, ratings of 9 to 10 are only given to elem ents with very minor to no distress whatsoever—conditions typically seen only shortly after wall construction or substantial wall repairs. 7 to 8 Good Low-to-moderate extent of low severity di stress. Distress does not significantly compromise the element function, nor is there significant severe distress to major structural components. In practice, ratings of 7 to 8 indicate highly functioning wall elem ents that are only beginning to show the first signs of distress or weathering. For exam ple, a ten-year-old soldier pile wall ma y have m oderately extensive m inor surface corrosion on piles where protective paint has weathered and peeled, and m ay have wood lagging beginning to split. Distresses are very low overall, present over a m odest am ount of the wall, and do not require im me diate or near-term attention. 5 to 6 Fair High extent of low severity distress and/or low-to-m edium extent of m edium to high severity distress. Distress present does not com prom ise element function, but lack of treatment may lead to impaired function and/or elevated risk of elem ent failure in the near term . In practice, ratings of 5 to 6 indicate functioning wall elem ents with specific distresses that need to be m itigated in the near-term to avoid significant repairs or element replacement in the longer term. For exam ple, num erous anchor struts holding MSE wire facing elem ents in place are be ginning to break due to corrosion and suspected over-stressing of the connections at the tim e of construction. Although the overall functi on of the reinforced earth wall is not in jeopardy, failing wall facing baskets are allowi ng faci ng fi ll to spill out. If several overlying baskets experience this isolated element failure, significant wall face sag and deformation may result at the top of the wall, eventually im pacting the overlying guardrail installation. This ele me nt should be inspected carefully along the entire wall and repaired as needed to forestall further facing basket deterioration. 3 to 4 Poor Medium-to-high extent of medium-to-high severity distress. Distress present threatens elem ent function, and strength is obviously com prom ised and/or structural analysis is warranted. The element condition does not pose an immediate threat to wall stability and closure is not necessary. In practice, a rating of 3 to 4 indicates marginally functioning, severely distressed wall elements in jeopardy of failing without element repair or replacem ent in the near-term . For exam ple, mo rtar throughout a historic stone ma sonry wall is cracked, spalling, highly weathered, and often m issing. Individual stone blocks are mi ssing from the wall face, and adjacent blocks show signs of outward displacem ent. Although not an immediate threat to overall wall stability, stone block replacement and repointing throughout the wall in the near-term are necessary to forestall rapid wall deterioration. 1 to 2 Critical Medium-to-high extent of high severity distress. Element is no longer serving intended function. Element performance is threatening overall stability of the wall at the time of inspection. In practice, a rating of 1 to 2 indicates a wall that is no longer functioning as intended, and is in danger of failing catastrophically at any time. For example, a 15-ft- tall cast-in-place concrete cantilever wall has a large open horizontal crack running the full length of the wall at the base of the stem . Vertical cracks are also beginning to open up in the wall face. Water is seeping from most wall cracks, and is running from the basal horizontal crack at several locations . The wall face has rotated outward, resulting in a negative batter of several degrees. The overlying guardrail is highly distorted above the wall and the adjacent roadway is showing significant settlement above the retained fill. The wall is in imminent danger of fa iling catastrophically, requiring the overlying roadway be closed to all traffic until the wall can be replaced or retained soil backslope can be stabilized. Source : DeMarco et al. (2010b). TABLE 6 NUMERICAL CONDITION RATING DEFINITIONS FOR WALL ELEMENTS IN FHWA WALL INVENTORY PROGRAM

13 • Joint spacing • Condition of “v-ditch” (i.e., drainage way at top of wall) • Coping deterioration • Drainage runoff • Drainage at the front of the wall. A rating scale ranging from zero to 9 (consistent with most bridge assessment procedures) is provided to describe the extent or severity of each feature. For example, with respect to loss of backfill, the following ratings descriptions are used: (zero)—backfill loss has resulted in significant settlement of the v-ditch or roadway or has affected wall inclination or alignment; (3)—significant areas/quantities of backfill loss are visible; (6)—backfill loss is occurring, but only minor areas/ quantities of backfill loss are visible; and (9)—no visible evi- dence of backfill loss. Numeric rating descriptions are unique to each type of feature or condition being assessed and can be found in the materials in Appendix E (web-only). The MSE wall inspection program in Ohio has focused data collection activities on observed problems, particularly sand leaking from joints, settlement of panels (largely from erosion of underlying support), uncontrolled drainage, and deteriorating panels (Narsavage 2006). The inspection pro- gram focuses on 23 potential symptoms (e.g., signs of water flow along the base of the wall) associated with wall joints, wall facing, drainage, and conditions at the top of the wall (see inspection form in web-only Appendix E). Condition ratings consist of simple “yes” or “no” responses. After its first inspection effort completed in 2006, Ohio reported that of the state’s 339 inspected walls, nearly one-third exhib- ited backfill migrating through wall joints and 13% exhibited some type of erosion problem. Utah’s MSE wall data collection largely follows the Ohio model. As shown on the inspection form provided in (web- only) Appendix E, data collection efforts focus on features and conditions believed to affect or reflect wall performance; namely, drainage, wall joints, wall facing, conditions at top of wall, foundation conditions and external stability, corrosion • Wall or parts of it out of plumb, tilting, or deflected • Bulges or distortion in wall facing • Some elements not fully bearing against load • Joints between facing units (panels, bricks, etc.) are misaligned • Joints between panels are too wide or too narrow • Cracks or spalls in concrete, brick, or stone masonry • Missing blocks, bricks, or other facing units • Settlement of wall or visible wall elements • Settlement behind wall • Settlement or heaving in front of wall • Displacement of coping or parapet • Rust stains or other evidence of corrosion of rebar • Damage from vehicle impact • Material from upslope rockfall or landslide adding to load on wall • Presence of graffiti (slight, moderate, heavy) • Drainage channels along top of wall not operating properly • Drainage outlets (pipes/weepholes) not operating properly • Any excessive ponding of water over backfill • Any irrigation or watering of landscape plantings above wall • Root penetration of wall facing • Trees growing near top of wall. Another data collection/wall inspection process has been developed by the Nebraska Department of Roads. In this meth- odology (Nebraska Department of Roads 2009), the MSE wall features that are assessed are: • Wall tilting • Structural cracking • Facial deterioration • Bowing of the wall • Panel staining • Exposure of fabric • Loss of backfill • Erosion in front of wall • Erosion in back of wall Rating Rating Definition 7 to 10 Good to Excellent No com binations of elem ent distresses are observed indicating unseen problem s or creating significant perform ance proble ms . No history of rem ediation or repair to wall or adjacent elem ents is observed. 5 to 6 Fair Som e observed global distress is not associated with specific elements. Some element distress com binations are observed th at indicate wall com ponent problems. Minor work on prim ary elements or major work on secondary elements has occurred improving overall wall function. 1 to 4 Poor to Critical Global wall rotation, sliding, settlement, and/or overturning are readily apparent. Combined elem ent distresses clearly indicate serious stability problem s with com ponents or global wall stability. Major repairs have occurred to wa ll structural elements, though functionality has not im proved significantly. Severe distresses are apparent on adjoining roadways. Source : DeMarco et al. (2010b). TABLE 7 WALL PERFORMANCE RATINGS

14 Wall panels shall be checked for cracking, spalling, other forms of deterioration, and collision damage. • Drainage systems through or along MSE walls should be inspected to verify water is free flowing into and out of the appropriate facility. • Ensure that weep holes are free draining. • Inspect all inlets to verify water is draining into the inlet, and flowing freely to the inlet and out of the outlet. Examine inlets for cracks. • Inspect visually or use down hole cameras (as appropriate) for all culverts and pipes contained or having portions in, behind, or above the MSE wall mass and for pipes or culverts which run above, adjacent to, or outlet through the MSE walls to ver- ify pipes are free draining and water is flowing through (and not under or around) the pipe. Examine drainage pipes for cracking or damage with emphasis on areas where water may flow, or is flowing, into the MSE wall soil mass. Inspect outlet ends to verify free drainage or for evidence of migration of fill or other material. • Inspect swales above the MSE wall. Verify rock fall or other materials (trees, etc.) are not blocking, redirecting, or restrict- ing the flow of water through the drainage ditch above the MSE wall to the appropriate receptacle. • Inspect collection and outlet basins to verify water is draining freely. Look for any signs of infiltration or migration of mate- rial which may prevent water from draining from the wall. • Identify inappropriate appearance of water along the base of the wall (i.e., if water is appearing when weather conditions have been particularly dry). Note areas where there is inappropriate collection and/or lack of drainage for water along the length of the MSE wall. • Note erosion of soil along the base of the wall exposing or undermining the leveling pad. In the Pennsylvania methodology, observed conditions are then translated into ratings (shown in Table 8) that are assigned to the following MSE wall elements/items: • Anchorage • Backfill • Wall conditions such as bulging, joint conditions, dete- rioration of face panels, connection of the backs, etc. • Panels • Drainage • Foundation • Parapets. Data collection and inspection schemes are inherently rooted in the experience and judgment of their developers. In the city of Seattle, Washington, for example, instances of and degradation, impact and collision, and miscellaneous issues. As in Ohio, condition ratings consist of simple “yes” or “no” responses; however, the extent of the symptom/issue is quantified as a percentage of the total wall. Some of Utah’s inspection queries relate directly to two-stage MSE walls, which are widely used in the state. The Pennsylvania DOT (PennDOT; see Pennsylvania Department of Transportation 2010) has a well-defined retain- ing wall inspection program conducted in conjunction with its bridge inspection program. (Bridge and retaining wall data are maintained in the same management system.) The program involves all walls, not just those at bridges. One wall element receiving particular focus in PennDOT’s inspection process is a button-head connection present in some first-generation MSE walls, because the cold-formed button head details were found to develop micro-cracks that contributed to the failure of the button head. The following directives relating to MSE walls are specified in the PennDOT inspection manual: Mechanically Stabilized Earth (MSE) retaining walls should be inspected for evidence of wall movement. • Examine barrier and moment slab for evidence of movement as well as the MSE wall for evidence of bulging, bowing, or panel offset. • Perform a survey if movement is suspected to compare to ini- tial inspection data to gauge amount of movement. • Examine the roadway above MSE walls for indications of fail- ing pavement or tension cracking. These may indicate a loss of fill. For MSE walls in front of sloping backfill, the crest of the embankment should be investigated for soil stress or failure, both of which may indicate settlement or wall movement. The joints between panels of MSE walls are to be inspected and examined for loss of backfill, change in spacing, and indications of settlement. The specification requirement for joint spacing is a maximum three-quarters of an inch. • Inspect walls for evidence of backfill loss (piles of aggregate at the base of the wall). • Indicate visibility of backfill or fabric behind the panel through joints. • Examine for evidence of damage to the geotextile fabric, if visible. • Look for variation in joint spacing. Note vegetation growing in joints. • Vertical slip (expansion joints) used on long lengths of walls should be investigated similar to panel joints. The initial spac- ing at the slip joint should be determined from design, shop, or as-built drawings. Rating Rating Definition 8 Good condition. No apparent problems. 6 Satisfactory condition. Structural elements sound. Localized drainage problems, settlement, staining, washing of fines from backfill material. 4 Poor condition. Localized buckling, deteriorated face panels, joint problems, major settlement, ice damage. 2 Critical. Major structural defects, components have moved to point of possible collapse. TABLE 8 PERFORMANCE RATINGS ASSIGNED TO WALL ELEMENTS IN PENNSYLVANIA INSPECTION/ASSESSMENT PROCESS

15 Other examples of using new technologies to monitor the performance of MSE walls include the incorporation of fiber-optics into geosynthetic reinforcement (Lostumbo and Artieres 2011). Various structural health monitoring tools now being built into bridges can readily be adapted for retaining walls. New technologies such as these will become increasingly more common in wall performance data collec- tion and assessment efforts. The general state of practice with respect to which MSE wall features or components are examined during data col- lection activities, based on survey respondents, is shown in Table 9. Only three of the 17 respondents to the associated survey question reported having some type of inventory. Responses suggest that the wall features or conditions most frequently examined by agencies are wall plumbness, bulg- ing or distortion of the wall facing, and cracking of facing elements. As can be seen subsequently in Table 16, these features/conditions correlate well to those distress/failure modes which are believed most important or significant rela- tive to wall performance. Eight of the 11 responses pro- vided as “other” features were simple declarations that the particular respondent did not collect any such data. Two more adverse retaining wall performance were observed to accom- pany (or even be manifest as) excess wall tilt. Consequently, wall tilt measurements using a digital protractor are a princi- pal component of Seattle’s inspection program (Molla 2009). To help ensure comparable and consistent data, tilt mea- suring stations are permanently established on many walls. Another example of how experience affects data collection activities is the scope and frequency of inspections speci- fied for MSE walls in Pennsylvania. An in-depth inspection including a three-dimensional spatial survey of the wall is required every 10 to 15 years. This requirement arises largely from global stability and creep concerns stemming from local geologic conditions in the state—more particularly along Route 22/322 in Lewistown Narrows, where one of the longest MSE walls in the United States has been con- structed. PennDOT has also implemented new technology as part of its data collection efforts. In 2008 and 2009, Lidar technology using a fixed-wing aircraft was used to assess the amount of creep that the Lewiston Narrows wall was expe- riencing. Unfortunately, the goal of 0.10 ft (30 mm) proved difficult to confirm because of the low altitude required within the canyon. The technology may be retried using a helicopter instead. Only Agencies with Inventories All Respondents to Particular Question Response Number Percent Number Percent Wall plumbness 2 67 5 29 Bulging or distortion of wall facing 2 67 5 29 Alignment and spacing of joints between facing elements 2 67 4 24 Cracking of facing elements 2 67 5 29 Damage to corners of facing elements 2 67 4 24 Damage from vehicular impact 1 33 3 18 Settlement along line of wall 1 33 4 24 Settlement behind wall 1 33 4 24 Distress in ground or pavement in front of wall 1 33 2 12 Distress in ground or pavement behind wall 1 33 3 18 Displacement of coping or parapet 2 67 3 18 Rust stains or other external evidence of corrosion 1 33 3 18 Functionality of drainage/catch basin 1 33 2 12 Functionality of internal drainage features (e.g., weepholes and piping) 1 33 2 12 External erosion 2 67 3 18 Internal erosion of backfill 1 33 2 12 Changes to wall geometry (e.g., excavation at toe, add surcharge load) 1 33 3 18 Vegetation growth 0 0 1 6 Internal corrosion/degradation of reinforcement 1 33 2 12 Other (specify) 0 0 11 65 TABLE 9 MSE WALL FEATURES AND/OR CONDITIONS ASSESSED AS PART OF DATA COLLECTION AND MONITORING ACTIVITIES (multiple responses possible)

16 conducted “in the absence of any special condition or cir- cumstance that makes it prudent to inspect more often”). Selection of an inspection interval for a specific wall involves considerations of any known occurrence of adverse perfor- mance; wall age (older walls may require more frequent inspections); presence of questionable backfill (that may lead to settlement or internal corrosion concerns); and occur- rence of flooding, earthquake, or vehicle damage. Principles of risk management dictate that walls whose failure would produce significant consequences are candidates for more frequent inspection. When survey respondents were asked, “Which of the following statements best describes your agency’s MSE wall performance monitoring activities?” (as shown in Table 10), the overwhelming response was that such activities were gen- erally in response to specific instances of adverse performance. The remainder indicated that assessments were performed, but not always including all MSE walls in their inventory. This appears to suggest that, contrary to the practices and recom- mendations previously discussed, the frequency of monitoring activities appears to be largely driven by resource availability and/or in response to incidents of adverse performance. Table 11 summarizes some interrelationships between those agencies that have reported the establishment of MSE wall inventories, the extent of those inventories, and the nature of their ongoing monitoring activities. As can be seen in this table, more than half of the agencies reporting MSE wall inventories only monitor their walls in response to known incidents of adverse performance. Just over one- quarter of agencies having inventories regularly inspect or assess most or all of those walls. From these data, it appears that once MSE wall inventories are initially developed, additional information relative to ongoing performance is generally either not collected or not assessed for most walls. (As pointed out previously, there is no uniform standard for designating and counting MSE walls). COLLECTION OF CORROSION AND DEGRADATION DATA A distinguishing feature of MSE walls relative to other retain- ing wall types is the reinforcement in the retained soil mass. The stability of the wall depends on the integrity of the reinforce- of these responses indicated that feature assessment was only performed in response to observed wall distress, while the remaining response clarified that wall features were exam- ined as part of their bi-annual bridge inspection activities. FREQUENCY OF FIELD INSPECTIONS AND MONITORING ACTIVITIES The condition and performance of MSE walls vary over time. Because of this, it is important that data collection and assessment activities be conducted routinely. According to the NBIS, bridges are inspected at two-year intervals. Some agencies have adopted similar two-year inspection intervals for retaining walls. Other agencies such as New York City require privately owned retaining walls to be inspected every five years. Kansas typically assesses its MSE walls at three- year intervals, whereas Oregon’s plan calls for inspection of “good” walls of all types every five years, and “fair” or “poor” walls more often. Between 1986 and 1997, California had established five- to ten-year inspection intervals for MSE wall elements, particularly internal reinforcement elements. PennDOT takes a tiered approach, with a “routine” wall inspection every five years and an “in-depth” inspection (which includes a three-dimensional survey for MSE walls more than 100 ft long and more than 20 ft high) at either 10- or 15-year intervals. Unscheduled “special” inspections are to be performed after a significant event, such as a vehicular collision, extreme weather, or indication of wall movement. Similarly, the FHWA’s WIP directs that all walls should be inspected on a maximum 10-year cycle, and walls having performance issues are subject to more frequent inspection and assessment work, particularly those subject to “qualify- ing emergency relief events” such as a landslide or flood. PennDOT defines a routine inspection as “a close visual and hands-on examination of retaining walls and their drainage systems without traffic control. Those portions which cannot be accessed safely from beyond the edge of pavement are viewed using binoculars and/or a digital camera.” In con- trast, an in-depth inspection consists of “a close visual and hands-on examination of retaining walls and their drainage systems. Use of down-hole cameras or visual inspection of larger pipes is required for the drainage system.” Based on their study, Brutus and Tauber recommend a five-year interval for routine inspections (i.e., inspections Response Number Percent Reactive to reported incidents of adverse performance 32 73 Irregular inspection/assessment of some MSE walls 3 7 Regular inspection/assessment of some MSE walls 4 9 Irregular inspection/assessment of most or all walls in inventory 1 2 Regular inspection/assessment of most or all walls in inventory 4 9 TABLE 10 BEST DESCRIPTION OF AGENCY’S MSE WALL PERFORMANCE MONITORING ACTIVITIES

17 thickness loss, as well as decreases in tensile strength. With electrochemical methods, potential and polarization resis- tance measurements are made and correlated with dimen- sions of the reinforcement. In Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, a principal reference in the United States regarding the degradation and corrosion of MSE wall reinforcement, Elias et al. (2009) advise that “given the advan- tages, utilization of remote electrochemical methods is highly recommended with at least some coupons buried for retriev- als to confirm results.” Their provided rule of thumb regard- ing installation is two locations spaced at least 200 ft (60 m) apart for MSE walls 800 ft (250 m) or less in length and three locations for longer walls. At each location, corrosion should be monitored at a minimum of two depths. For extractible cou- pons (i.e., inspection wires), Caltrans has developed a typical layout of 18 clustered coupons to be periodically extracted (see appendix in Fishman and Withiam 2011). Caltrans has also developed a set of extraction guidelines (California Depart- ment of Transportation 2004). With respect to frequency of assessing corrosion, Elias et al. (2009) recommend that potential and polarization resis- tance measurements (owing to their sensitive nature) be made monthly for the first three months, bi-monthly for the next nine months, and annually thereafter. This recommended frequency is significantly greater than the frequency at which other wall inspection and data collection activities occur (as described in the previous section). Extractible coupons are typically removed at five- to 15-year intervals, depending on the number of coupons installed. In California’s typical ment, which can be either relatively extensible geosynthetic materials or inextensible metallic straps or meshes. Because of the reinforcement’s criticality, many MSE performance assess- ments focus on the reinforcement, which can be challenging since the reinforcement is buried and not directly observable. Also problematic is corrosion, which is a rate process affected by multiple factors. If certain other factors are assumed, wall age might serve as a proxy parameter for corrosion and remain- ing service life. However, premature failures illustrate potential shortcomings of relying on such assumptions. Several U.S. state agencies have undertaken reinforce- ment corrosion studies. Table 12 presents a brief summary, slightly expanded from that prepared and presented by Fish- man and Withiam (2011) of these various efforts. It can be noted that the corrosion issues reported in Nevada resulted from a now-outdated backfill specification rather than cur- rent AASHTO backfill specifications, and care must be taken when interpreting adverse performance of walls constructed using early design methods. Detailed descriptions of the corro- sion monitoring activities of California, Florida, New York, and North Carolina are presented in Elias et al. (2009). It is interesting to note the correlation between agencies that have developed MSE wall inventories and those that have expe- rienced MSE wall corrosion issues (and have subsequently developed monitoring programs). Corrosion monitoring of steel reinforcement is typically accomplished by either retrieval of buried coupons or non- destructive electrochemical methods. With exhumed coupons, corrosion can be assessed by determining weight and section Agency Num ber of Walls Percent Walls in Inventory Best Description of Monitoring Activities Alberta, Canada 3 00 10 R eactive to reported incidents of adverse perform ance California 4 00 75 R eactive to reported incidents of adverse perform ance Colorado 800 60 R eactive to reported incidents of adverse perform ance Kansas 300 50 Regular inspection/assessment of most or all walls in inventory Minnesota 300 60 Reactive to reported incidents of adverse performance Missouri 899 100 Reactive to reported incidents of adverse performance Nebraska —1 10 Regular inspection/assessment of most or all walls in inventory New York 635 100 Regular inspection/assessment of most or all walls in inventory 2North Carolina 75 97 Regular inspection/assessment of most or all walls in inventory North Dakota 100 100 Irregular inspection/assessment of most or all walls in inventory Ontario, Canada 500 100 Regular inspection/assessment of some MSE walls Tennessee 1000 50 Reactive to reported incidents of adverse performance Utah 700 80 Reactive to reported incidents of adverse performance Wisconsin 400 85 Reactive to reported incidents of adverse performance 1Data missing. TABLE 11 RELATIONSHIPS BETWEEN THOSE AGENCIES WITH MSE WALL INVENTORIES, THE SCOPE OF THOSE INVENTORIES, AND NATURE OF ONGOING MONITORING ACTIVITIES

18 State Description California Has been installing inspection elements with new construction since 1987, and has been perform ing tensile strength tests on extracted elements. Some electrochemical testing of in-service reinforcements and coupons has also been performed. Linear polarization resistance (LPR) and EIS tests were performed on inspection elements at selected sites as part of NCHRP Project 24-28 and results compared with direct physical observations on extracted elements. Florida Program focused on evaluating the impact of saltwater intrusion, including laboratory testing and field studies. Coupons were installed and reinforcements were wired for electrochemical testing and corrosion monitoring at 10 MSE walls. Monitoring has continued since 1996. Georgia Began evaluating MSE walls in 1979 in response to observations of poor performance at one site located in a very aggressive marine environment incorporating an early application of MSE technology. Exhumed reinforcement samples for visual examination and laboratory testing. Some in situ corrosion monitoring of in-service reinforcements and coupons at 12 selected sites using electrochemical test techniques was also performed. Kentucky Developed an inventory and performance database for MSE walls. Performed corrosion monitoring including electrochemical testing of in-service reinforcements and coupons at five selected sites. Nevada Condition assessments and corrosion monitoring of three walls at a site with aggressive reinforced fill and site conditions. Exhumed reinforcements for visual examination and laboratory testing; performed electrochemical testing on in-service reinforcements and coupons. A total of 12 monitoring stations were dispersed throughout the site providing a very good sample distribution. New York Screened inventory and established priorities for condition assessment and corrosion monitoring based on suspect reinforced fills. Two walls with reinforced fill known to meet department specifications for MSE construction are also included in program as a basis for comparison. Corrosion monitoring uses electrochemical tests on coupons and in-service reinforcements. North Carolina Initiated a corrosion evaluation program for MSE structures in 1992. Screened inventory and six walls were selected for electrochemical testing including measurement of half-cell potential and LPR. This initial study included in-service reinforcements, but coupons were not installed. Subsequent to the initial study, NCDOT has installed coupons and wired in-service reinforcements for measurement of half-cell potential on MSE walls and embankments constructed since 1992. LPR testing was also performed at approximately 30 sites in cooperation with NCHRP Project 24-28. Ohio Concerned about the impact of their highway and bridge de-icing programs on the service life of metal reinforcements. Performed laboratory testing on samples of reinforced fill but did not sample reinforcements or make in situ corrosion rate measurements. Oregon Preliminary study including (1) a review of methods for estimating and measuring deterioration of structural reinforcing elements, (2) a selected history of design specifications and utilization of metallic reinforcements, and (3) listing of MSE walls that can be identified in the ODOT system. Utah Extracted 22 wire coupons from one- and two-stage MSE walls all approximately 11 to 12 years old. Galvanization thickness was found to still be greater than initial specified values. Data to provide baselines for future assessments. After Fishman and Withiam (2011). TABLE 12 SUMMARY OF US STATE MSE WALL CORROSION ASSESSMENT PROGRAMS

19 year intervals for a minimum of four retrievals, or one-third the expected life of the facility. The state of practice for assessing degradation and corro- sion in MSE walls, as indicated by 14 survey participants who provided specific responses, is shown in Table 13. Three of these respondents indicated that they have their own MSE wall inventories. Based on the information presented in this table and in Table 12, it appears that a minority of agencies assesses corrosion of metallic MSE wall reinforcement, and none sys- tematically assess degradation of geosynthetic reinforcement. installation, coupons are removed and inspected after five, ten, 20, 30, 40, and 50 years. For geosynthetic reinforcement, the primary performance issue is polymer degradation. At present, the only effective means of assessment is retrieval of buried specimens. The assessment process involves successive retrieval and testing of samples to determine both mechanical and chemical proper- ties. Strength and elongation (i.e., creep) properties can then be extrapolated to predict future performance. Elias et al. (2009) recommend that sampling and testing occur at five- to seven- Only Agencies with Inventories All Respondents to Particular Question Response Number Percent Number Percent Do not currently assess 2 67 12 86 Linear polarization resistance (LPR) for metallic 0 0 1 7 Extractible coupons for metallic 1 33 2 14 Exhumation for geosynthetic 0 0 0 0 Other (specify) 0 0 0 0 TABLE 13 METHOD(S) CURRENTLY USED BY AGENCIES TO ASSESS DEGRADATION/CORROSION OF REINFORCEMENT (multiple responses possible)

Next: Chapter Four - Assessment and Use of Mechanically Stabilized Earth Wall Data »
Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 437: Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls explores methods to assess the long-term performance of mechanically stabilized earth walls. For the purposes of the report, "long-term" denotes the period of time from approximately one year after the wall is in service until the end of its design life.

The report focuses on state and federal agency wall inventories and highlights methods of inspection and assessment of wall conditions.

Mechanically stabilized earth walls are retaining walls that rely on internal reinforcement embedded in the backfill for stability.

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