Art and science truly combine in today’s retaining wall systems. Civil engineers must design increasingly taller structures that not only support slopes, buildings, or roads, but also enhance the aesthetics of the project. The retaining wall can be a work of art itself, making an architectural statement; or it may disappear into the natural elements surrounding it, whether vegetation, soil, or rock. According to manufacturers, the most significant trends influencing the use of segmental, mechanically stabilized earth (MSE), gravity, and geocellular retaining walls include new project delivery and design methods, a focus on green or sustainable design, and wall aesthetics.
“One trend is the increasing use of design-build contracting for highways, with compressed schedules, bids based on 30-percent complete plans, and construction beginning well before design completion,” said Robert A. Gladstone, P.E., executive director of the Association for Metallically Stabilized Earth (AMSE), the trade association for suppliers of steel-reinforced MSE structures. “Time is saved from concept to completion and the project benefits economically, but wall suppliers must bid on preliminary plans and develop final plans and construction details using incomplete information. Steel-reinforced MSE walls fit well into these compressed schedules because of their segmental, premanufactured components and rapid, predictable construction.”
David P. McKittrick, co-founder and vice president of EarthTec, Inc., warned that this trend can impact project risk. “Public bodies increasingly turn to design-build and public-private partnerships to procure transportation infrastructure,” he said. “These innovative procurement methods transfer significant risks from the public authority to the design-build entities. These entities are looking to push these risks down to their suppliers and subcontractors while looking to these key suppliers to provide innovation and value.”
McKittrick said that designers need to decide who is going to take what risks and how to manage the interfaces between them. “Where a retaining structure is supported by ground improvement, how should this interface be managed?” he asked. “Who is responsible for what? How are the risks shared and covered? Is it sensible to consider a complete design-build solution where all the risks can be placed with a single party?”
A second trend to watch, according to Gladstone, is the transition — mandated by the Federal Highway Administration (FHWA) by 2010 — from allowable stress design (ASD), based on safety factors, to Load and Resistance Factor Design (LRFD), based on reliability analysis. “Both structural and geotechnical engineering are increasingly using LRFD methods,” he said. “LRFD-designed MSE walls have about the same amount of earth reinforcement as ASD-designed walls, so there is no significant economic difference between ASD and LRFD.”
But, the transition could require some design changes. “FHWA’s directive to states to adopt AASHTO LRFD design provides an opportunity for states to re-evaluate their own specifications and selection process for retaining walls,” said McKittrick.
Gladstone said that the Coherent Gravity design method, added to the 2009 AASHTO LRFD Bridge Specifications, has been preferred by many state departments of transportation for three decades because it rigorously and accurately models the behavior of steel-reinforced MSE walls. AMSE recently funded research to develop LRFD load and resistance factors for the Coherent Gravity design method.
Green building and sustainability initiatives comprise a third important trend highlighted by retaining wall manufacturers that impacts selection of wall type as well as construction. According to Bryan Wedin, P.E., chief design engineer with Presto Geosystems, the Clean Water Act, green building initiatives, and a desire for vegetated walls are influencing the use of cellular confinement systems for retaining walls. “The cellular confinement system’s highly permeable front fascia is a natural, low-impact development (LID)/best management practice (BMP) for reducing stormwater runoff and provides onsite stormwater management,” he said. “The outer fascia allows rain water to fall on the horizontal soil terrace and infiltrate through the exposed cells, thereby maximizing water collection.”
Additionally, he said, cellular confinement retaining walls offer contributions to LEED green building credits for reducing site disruption, reducing the heat island effect, and for stormwater quality and quantity control.
“Desire for vegetation in channels and culvert outfalls in place of rip rap has increased the use of cellular confinement retaining walls for these applications,” Wedin said. “New bioengineered techniques such as wrapping a naturally degrading coir fabric over the front fascia allow vegetated channels to withstand higher flows while protecting the outer cells from topsoil loss. The system’s versatility also allows a mixture of infill types to address flow conditions [through use of] aggregate or concrete grout in lower layers up to the high water line, [with] topsoil/vegetation in upper layers.”
Sustainability and green building also are driving the use of segmental retaining walls (SRWs), according to Karen A. Nelson, P.E., manager of engineering services for Versa-Lok. The company introduced Versa-Green, a plantable wall system that allows structurally stable tall walls made with concrete units and geogrid-soil reinforcement to be plantable. Drip-irrigation can be incorporated into the walls so they can be converted to year-round vertical gardens, she said.
Additionally, SRW soil-reinforced structures can often use native soils sourced onsite, Nelson said. “SRWs are composite gravity wall systems, with over 90 percent of wall system mass being the soil between geogrid layers,” she said. “Also, the SRW concrete units at the face of the wall systems are typically made locally, and thus require little shipping, improving the LEED credits available for building with SRWs.”
The push for sustainability is leading many authorities to investigate recycled or secondary fill materials for MSE walls, according to EarthTec’s McKittrick. “A few authorities are already using recycled concrete as backfills for MSE structures,” he said.
Peter L. Anderson, P.E., Northeast regional manager for The Reinforced Earth Company, suggested that civil engineers consider use of recycled materials such as bottom ash and crushed concrete, both of which are in plentiful supply. Civil engineers may also consider use of lightweight backfill materials such as expanded shale, foamed concrete, and pumice to reduce structure weight and the resulting bearing pressure on foundation soils.
Use of suitable backfill material for construction of MSE walls is important to structural performance in all applications and weather conditions, said Anderson. There is abundant suitable backfill material for MSE walls in nearly all 50 states, including any combination of fine to coarse sand, gravel, and crushed stone with a maximum 15 percent passing the #200 sieve, he said. Use of finer backfills — as much as 35 percent passing the #200 sieve, as implemented extensively in the early 1980s — resulted in undesirable wall deformations, which ultimately led to the current 15-percent fines limitation in the AASHTO specifications.
“The performance of a MSE wall is largely influenced by the quality of the backfill used in the structure,” agreed McKittrick. “Well-graded, free-draining materials perform well in highway embankments and can be handled in wet weather conditions.”
Likewise, soil within an SRW’s reinforced zone must be of proper quality and placed and compacted by proper methods to ensure the wall’s performance, said Versa-Lok’s Nelson. “This is particularly important with tall walls,” she advised, “where post-construction settlement of the fill within the reinforced soil mass can be a concern if not properly addressed in the specification for soils and wall installation procedures.”
Of course, much depends on site conditions for both wall stability and backfill drainage. “Design responsibilities must be assigned correctly,” said AMSE’s Gladstone. “The MSE supplier’s responsibility is limited to the MSE design it produces, the internal stability of the retaining wall, and the quality of the manufactured MSE wall materials. Only the representative of the owner, typically the geotechnical engineer, can evaluate the site conditions and the adequacy of the foundation soils to support the proposed retaining wall. The geotechnical engineer must be responsible for analyzing bearing capacity, estimating the magnitude of settlement that the wall may be subjected to, and evaluating site global stability, including the effect on the site of the proposed retaining structure. Surface drainage for the overall project site, as well as subsurface drainage for the retaining wall(s), must be designed by and be the responsibility of the project civil engineer.”
According to Reinforced Earth’s Anderson, wall failures are caused by a large variety of circumstances, including global instability, excessive settlement, rupture of a nearby water main, poorly designed friction connections of block walls, poor construction practices, failure to control stormwater runoff during construction, use of poor durability backfill, and inappropriate backfill specifications, among other reasons.
Geocellular walls may be a viable alternative in poor soil conditions, according to Presto Geosystems’ Wedin. “Due to their relatively light weight, cellular confinement walls are advantageous when subgrade soils are soft or compressible,” he said. “Cellular confinement walls can tolerate reasonable differential settlement without loss of system integrity and are often installed in less than ideal soil environments. When reinforcement is not feasible due to site limitations or availability of suitable backfill materials, cellular confinement walls can be designed as gravity walls. The cellular wall materials are compact for transporting and offer an excellent option for difficult-to-access or remote locations.”
Of course, structural performance, an engineer’s primary concern, is only part of a successful retaining wall design. It also has to look good. “One of the most prominent aspects that is influencing the use of retaining walls today is that engineers and owners are becoming more concerned with aesthetics,” said Jacob Manthei, marketing director for Redi-Rock International. And flexible wall systems that allow designers to change the setback — and therefore the batter — and to design corners and curves easily help achieve attractive structures.
“We are finding that owners/developers want something that looks like a rock, that can maximize usable space, and that is economical,” Manthei said. “Our ability to build tall gravity walls that have design flexibility and also look great is important for both engineers and owners.”
The company has noticed a trend toward retaining wall products that look more like natural rock. Redi-Rock currently has two architectural faces — Limestone and Cobblestone, both of which have as much as five inches of texture. This year it is introducing a texture called Ledgestone that has greater texture than its Cobblestone or Limestone faces. “Our research and development department has become very good at developing natural-looking textures using architectural-grade precast concrete,” Manthei said.
To provide flexibility to change wall setback, Redi-Rock has introduced product lines that create zero-batter walls, 4-degree batter walls, and a 9-inch setback line that creates 26.6-degree batter walls. Also, Redi-Rock planter blocks can be inserted in a wall to change the setback and allow taller gravity walls to be built, the company said.
Versa-Lok’s Mosaic system includes segmental units of differing heights and differing widths, that, along with increased availability of blended colors and weathered units, provides a high-end aesthetic comparable to natural stone, yet at a much lower cost, Nelson said. “While many such appealing looks were available to the landscape market, the commercial civil engineering market previously had more limited, expensive options for tall, structural walls,” she said. “Despite the random look and varying height of the units, there is level coursing every 10 inches in the Mosaic wall unit layout that allows incorporation of geogrid layers, and thus allows for construction of aesthetically pleasing, random-looking walls in excess of 40 feet tall.”
Retaining wall manufacturers offer various tools to help civil engineers design earth retention structures, including design manuals; guide specifications, engineering calculations, and detail drawings; proprietary and non-proprietary software programs for both wall design and global stability analysis; project evaluation and preliminary design services; and case studies. For SRWs, the National Concrete Masonry Association offers a design manual and associated software — SRWall 4.0.
Tools, however, are just one piece of the design puzzle. “There is no substitute for experience,” McKittrick said. “The tools on the market generally address analysis and sometimes are only functional for a single type of wall system. Where failures have occurred, these haven’t generally been caused by poor analysis, but more often by important aspects of design — global stability, for example — getting missed altogether.”