ue to advantages in economics, constructability, and aesthetics, Lessons the construction of mechanically stabilized earth (MSE) walls is Dnow commonplace. An MSE wall consists of Learned , reinforcement, and facing to retain earth and support overlying structures (Figure 1). Thirty- to forty-foot high walls are not uncom- problems and solutions mon. Reinforcement often consists of geogrids encountered by practicing or steel reinforcement strips, while the facing structural commonly consists of segmental precast con- crete units, baskets, metallic panels, or geosynthetic facing. There are many different MSE wall construction materials, making it more important for Contractors and design Engineers Figure 1: Typical MSE Wall Cross-Section. to understand how the products work with the ® remainder of the system. afterthought. The plans show a bold black line at For various reasons, some systems fail and require the property line to represent the costly repair (Figure 2). Based on lessons learned with the label “retaining wall – to be designed by from case studies, the authors discuss common others,” with a 30-foot change from the top pitfalls of MSE wall design and construction, in to the bottom of the wall. The exposed side of Copyrightthe form of a hypothetical the wall will be visible from local neighborhoods case study. and a shopping center. The marsh is just outside Mechanically Stabilized the limit of the retaining wall. It’s Just a The Contractor submits a bid that utilizes an Earth Walls MSE wall, and engages a Fabricator who provides Retaining Wall… proprietary masonry blocks for MSE systems. The Pitfalls in Design Don’t Sweat It! Fabricator works with a design who is not local to the site, but regularly designs the proprietary and Construction An Owner selects an affordable but complex site MSE system. The Fabricator and his design Engineer to construct a outlet with four, one-story assemble their standard design plans using subsur- By Scott J. DiFiore, P.E. and buildings. The multiple-acre site slopes signifi- face information and recommendations from the Bryan P. Strohman, P.E. cantly frommagazine west to east, with more than 40-feet geotechnical report. They do not visit the site, discuss S Tof grade R differential, U and a Cmarsh at Tthe lower U the subsurface R conditions E with the Geotechnical elevations covering less than 10 percent of the site. Engineer, review the site topography and utilities The Owner engages a Geotechnical Engineer to: with the Civil Engineer, or discuss whether soil drill one boring beneath each building, perform backfill will be imported or reused from the site Scott J. DiFiore, P.E. is a soil gradation tests, and prepare a report with development work. The design plans provided to senior project manager at site , building , and generic the Contractor contain notes stating that others Simpson Gumpertz & Heger retaining wall recommendations. Site , from are responsible to check the below Inc., Waltham, Massachusetts. the limited boring data, vary from clayey the wall, the settlement of the wall, and the slope He can be reached at to sandy . stability of the wall and retained backfill. [email protected]. The Owner hires a Civil Engineer to layout the The Owner hires the Geotechnical Engineer at roadways, parking lot, buildings, and site utili- the start of construction to perform periodic site Bryan P. Strohman, P.E. is a ties. Site utilities include pressurized waterlines, inspections. However, the Geotechnical Engineer Senior Staff II – Structures at sanitary sewer lines, and a storm pipe was not involved with the project through the Simpson Gumpertz & Heger system to collect runoff from the roadway, parking course of the site or retaining wall design. The Inc., Waltham, Massachusetts. lot, and roof drainage. The retaining wall is an Geotechnical Engineer assigns his recently pro- He can be reached at moted professional engineer (Inspector) to monitor [email protected]. site and retaining wall construction, who has no experience with MSE walls. The Inspector visits the site daily and diligently conducts compaction test- ing on the onsite clayey fill placed in the reinforced zone of the wall. The onsite soils vary significantly as the project progresses, due to the large amount of earthwork required, and the Inspector obtains new laboratory compaction data to correlate with the field-density tests. The Inspector observes water bearing utilities installed in the retained backfill, and storm drain manholes along the top of the wall Figure 2: Costly repairs to failed MSE wall. at the edge of the pavement. The Fabricator and

10 December 2012 STRUCTURE magazine • Settlement: and undergo immediate settlement, whereas fine-grained soils (clays and ) are susceptible to long- term settlement (consolidation). The Geotechnical Engineer did not perform consolidation tests, and no one predicted the amount of wall settlement. If the wall settles over the long-term, the pavement, utilities, Figure 3: Water collecting in clay backfill buildings, and other site features will immediately behind MSE wall facing. go along for the ride. • Geogrid-Soil Interaction: The geogrids his design Engineer provided no specific wall interact with the soil through . drainage systems in the design. The Contractor Together, the geogrids and soil act constructs the wall from the specified modular as a gravity structure. Geogrid and ® blocks and geogrid reinforcement, and the reinforcing manufacturers regularly clayey site soils. test for interface friction with sand and The project team is primarily focused on the backfill, but not always for silts building, utility, and pavement construction. and clays, which have different behavior. After all, the project is a retail center. But, as the The reinforcing could also be more project developed, the MSE wall became its own susceptibleCopyright to creep in clayey soils. significant construction project. It is up to 30-feet • Compaction: Fine-grained soils require

high, next to an established neighborhood, with visit www.STRUCTUREmag.orgADVERTISEMENT Information, - For Advertiser special attention to compaction water sources in the wall backfill, supports build- methods and moisture content, more ing foundations and a parking lot. Treating the so than sands and gravels which are MSE wall as “incidental” to the project could easier to compact. Poor compaction be disastrous. Is the project team giving the wall results in weaker soils, reducing soil- its due consideration? reinforcement interface friction and The Imperfect Site increasing wall pressures. • Soil Migration: As water flows through Much of the easily-developed land has already soil, it collects fine-grained particles been taken; this site has its development chal- magazineand transports them. If the wall lenges. The primary developmentS consistsT of Rdesign U did not includeC filter T fabric orU R E four, simple one-story retail structures. Just other protective measures against soil bring the site to grade, install spread footings migration and erosion, the soil mass will and pavement, and the development is off and lose volume and strength, resulting in running. What has the project team neglected additional wall pressures and settlement. that could come back to haunt them? • Water Retention: Compared to sands Subsurface Data and gravels, fine-grained soils are orders of magnitude less permeable. By performing only four borings over a multi- Water readily collects on and within acre site, the team focused on the buildings, fine-grained soils Figure( 3), which, but neglected ancillary development which (a) introduces groundwater and includes significant earthworks and soil reuse. seepage forces, (b) reduces soil strength Retaining wall construction is near the edge (increasing wall pressures), (c) reduces of the marsh, which likely consists of unsuit- interface friction between backfill able compressible soils. Could unsuitable soils and reinforcing, and (d) in colder extend beyond the marsh and into the MSE environments introduces freeze-thaw wall area? Backfilling over the unsuitable soils cycles in soils at and below ground will result in settlement. What about other surface and just behind the facing, unclassified soils outside the building foot- which can create frost-jacking pressures. prints? Incomplete soil characterization could cause construction problems or project delays. Water, Enemy of the Wall Soil Backfill Water comes from a variety of sources (Figure Clayey soils dominate the site, which are 4 ), including precipitation, leaky utilities, intended for reuse. MSE walls ideally use irrigation in landscaped areas, and ground- sand and gravel backfill in the reinforced zone. water. Unless actively managed by the project How do clayey soils impact MSE wall design team, all sources can have detrimental effects and construction? on MSE walls. continued on next page

STRUCTURE magazine11 December 2012 Open landscape areas are often located immediately at the top of the wall behind a curb line, or perhaps in islands set back from the wall. Irrigation lines and sprinkler systems, just like other water bearing utilities, Figure 4: Water sources include: (A) groundwater, are susceptible to leakage. (B) pressurized water lines, (C) storm drain • : Rainwater and snow melt systems, (D) infiltration through open landscape infiltrate the ground through open areas, and (E) runoff through pavement cracks. landscape areas and act on the MSE wall. Rainwater also penetrates cracks Figure 5: Drainage swale at top of wall intended • Utilities: Water-bearing utilities are in pavement. Significant storms can to collect water. However, open landscaping behind commonly placed directly within produce many inches of water in the swale allows water to enter soil mass. or behind the MSE wall, including course of just a day. pressurized water lines, storm • Groundwater: Natural groundwater hydrostatic or seepage pressures. When water drains and sewer lines. MSE walls fluctuates over time and during pressures are large® enough, performance or are flexible structures and expected different seasons. What happens during stability problems occur. Drainage systems are to move laterally as they reach an a flood? Is there a perched water table? particularly important where fine-grained soil equilibrium condition. Can the Site characterization, prior to design, backfill is used, since water does not readily utilities accommodate the anticipated should identify groundwater elevations flow through such soils. movement without damage? What if and potential fluctuations during Reference manuals by the Federal Highway they leak due to design or installation seasonal Copyrightand extreme weather events. Administration and National Concrete Masonry deficiencies? It is reasonable to expect MSE walls are typically designed without Association have provisions for wall drain- that utilities will leak during their consideration for hydrostatic or seepage age with varying site conditions. External lifetime, possibly catastrophically, pressures. This design approach is acceptable and internal drainage systems, usually used which introduces seepage or provided that the design and construction in combination, collect and divert water hydrostatic pressures. effectively manages all water sources. It is away from the wall system. External systems • Landscaping: Most development essential to identify water sources and divert typically include drainage swales and imper- projects include a landscaping water away from the wall. Collection/reten- meable barriers to collect and divert surface component that requires irrigation. tion of water within the wall backfill adds runoff (Figure 5 ). Internal systems typically include chimney and blanket drains at the rear and bottom of the reinforced zone, magazinerespectively, with drain pipes and periodic S T R U C T U outletsR through E the wall. Has the Wall Received Its Due Attention? The retaining wall? It’s just a line on the drawings; incidental to the project, right? Engineers and contractors design and install them every day; they must be routine. As described above, challenges exist with site topography, soil condi- tions, and water sources. Each project requires several play- ers – Owner, Geotechnical Engineer, Civil Engineer, Contractor, Fabricator, Contractor/Fabricator’s Engineer–but many times each member may not have a full understanding of their role and responsibilities.

ADVERTISEMENT - For Advertiser ADVERTISEMENT Information, visit www.STRUCTUREmag.org Project Delivery In this hypothetical case, the wall design flowed down to the Contractor, who passed it down to the Fabricator and his design Engineer. The Fabricator’s Engineer designs many walls and follows his typical protocol. He often assumes a “typical” soil type (sands and gravels), continued on page 14 STRUCTURE magazine12 December 2012 Long-Term Stability players, and the MSE wall taking a back seat to the buildings, it is common for everyone Due to the limited subsurface investigation, to simply “assume” that the wall will be taken no strength testing was performed on the care of. It’s a bad assumption. Vigilance foundation and backfill soils. What are the is required by all project team members, appropriate soil strengths for design, and from the Owner, to his Geotechnical and how do they compare with the assumed Civil Engineer, to Contractor, Fabricator, parameters from the design drawings? It may Fabricator’s design Engineer, and the site be appropriate to consider residual strengths Inspector. It is important to know how in some cases. buildings, utilities, or other adjacent con- Quality Assurance and Quality Control struction could affect the MSE walls. It is also critical to verify the soils and drainage The Inspector is prepared to make regular are appropriate for the wall, and that all visits to the site, but has no MSE wall experi- relevant stability checks have been addressed. Figure 6: Vertical wall with no batter, leaning ence. The Contractor has not installed this Periodic meetings with experienced person- over adjacent roadway. Construction with inward specific facing block before, and has not nel, both during project development and batter could alleviate this performance problem. talked to the Fabricator. What are the con- ® during construction, are effective in vetting nection details at the wall facing? At what with “typical” strength properties and gra- out and coordinating the potential pitfalls. elevations are the geogrids installed? Has dation, to be verified by “others” during MSE walls can be cost-effective, aesthetically the Contractor installed the drainage con- construction. All parties often assume that pleasing, and a technically sound approach for trols? Regular, proactive project meetings everything is fine, since the Contractor’s permanent earth retention on a site, provided are required to vet these issues; if not, they team has done this many times before. But, that they are given due consideration during often remainCopyright unresolved. this site is going to use clayey backfill, which design and construction. When the project differs from the design assumptions and will team considers the retaining wall “incidental alter the reported factors of safety. How does Putting it All Together to the project,” and they lightly treat the site this impact wall performance? topography, soil type, water management, or In this hypothetical project, there are many project team coordination, the entire team Soils and Stability players, each with a different contract iden- may be faced with performance problems or tifying their responsibility. But with so many The Fabricator’s Engineer has a note on even a wall failure.▪ the drawings: “Fabricator and its Engineer accept no responsibility for settlement, bear- ing capacity, and global stability of existing soils and subsurface water mitigation.” magazineReferences The Owner and his Engineers,S if theyT have R U C T U R E limited experience, may miss this critical United States Department of Transportation (USDOT) and Federal Highway exclusion and neglect to act on it. Without Administration (FHWA). (2001). Mechanically Stabilized Earth Walls and Reinforced design consideration of these potential fail- Soil Slopes Design and Construction Guidelines, Publication No. FHWA-NHI-00-043, ure modes, the wall is at risk. Washington, DC. (FHWA 2001). Wall Performance National Concrete and Masonry Association (NCMA). (2010). Design Manual for Segmental Retaining Walls, Third Edition, Herndon, VA. (NCMA 2010). MSE walls are flexible and will move laterally, particularly during construction. The designer Naval Facilities Command. (1986). Foundations and Earth Structures, Design must consider this movement and design the Manual 7.02, Alexandria, VA. (NAVFAC 7.02). wall with sufficient batter to compensate. Building a vertical wall, which eventually leans outward, results in a perceived failure even if it is technically stable (Figure 6). Miscellaneous Obstructions At this site, there may be utilities, manholes, parking lots, guardrails, and light poles that penetrate the soil reinforcement. Such obstructions result in additional loads (e.g., impact on guardrails) or decreased resistance (e.g., damage to reinforcing from installa- tion). Did the design Engineer consider these obstructions? Has the Civil Engineer looked at the MSE wall plans, and considered how the civil improvements affect the wall? Are the Contractor and Owner’s Engineer expe- rienced enough to acknowledge these issues if encountered during construction?

STRUCTURE magazine14 December 2012