By Paul C. Frankenberger and Robert Lozano
This article will focus on a geosynthetic strip mechanically stabilized earth (MSE) wall case study using discrete geosynthetic strips with large precast facing panels (Figure 1), an innovation of the original MSE invention. The system is the Highway Innovative Technology Evaluation Center/National Transportation Product Evaluation Program reviewed GeoMega Wall by The Reinforced Earth Co.
Modern-day MSE wall technology dates to the invention by Henri Vidal, who first published his research in France in 1963. The Terre Armée-patented system was galvanized ribbed steel soil reinforcement bolted to precast concrete facing panels. The system has evolved over the past 40 years, is regularly in use worldwide today, and is recognized as one of the top engineering achievements of the 20th century. With its inception, using the typical MSE wall design methods, the maximum tensile force in the reinforcing strips is distributed at each reinforcement layer into the number of reinforcements per unit of facing surface. The reinforcement length is then checked behind the line of maximum tension with respect to the available frictional capacity.
The first recorded use of polymeric strips as reinforcement for an MSE structure was a 4,844-square-foot (450-m2) wall located near Poitiers, France. The wall was designed by Terre Armée in 1969 and constructed in 1970 using woven polyester strips (TAI 1982). Polymeric strips were fully introduced into the European market in the mid-1970s. Instrumentation and full research followed; in 1977, a full-scale and fully instrumented trial MSE wall was constructed at the Transport and Roads Research Laboratory in Berkshire, United Kingdom, followed by a fully functional instrumented bridge abutment constructed in Carmarthen Southern Bypass in 1981. These two abutments were the first on a major road in the U.K. to utilize polymeric reinforcement. Implementation and adoption of the geosynthetic strips technology grew rapidly thereafter in Europe and the Middle East but was practically unknown in North America until introduction to the market in 2005.
Applications along rivers, lakes and coastal areas are common for MSE walls. The MSE technology offers aesthetically pleasing structure, long service life, considerable savings in both material and construction costs, and flexibility to differential settlement and earthquake loading. In marine environments, an MSE wall provides desirable features such as resistance to severe loading conditions from flooding, rapid drawdown, tidal fluctuations, storms, ice and overtopping in extreme wave conditions. The combination of articulated concrete facing panels and earth reinforcements forms a flexible structure that can support large loads and provide permeability to efficiently accommodate large variations in water level.
Project description
The project is located at 5400 North Pearl Street in Tacoma, Wash., an area exposed to harsh weather and sea conditions. The engineer proposed an MSE wall concept with continuous geosynthetic reinforcements to be constructed at Point Defiance peninsula to expand an existing marina (Figure 2) as part of a $1 billion redevelopment. The project site consists of hundreds of feet of loose slag mine tailings placed in the waterfront area as part of the ASARCO lead and copper smelter ore-processing plant from the turn of the 20th century, now a Superfund site. The height of the wall is 16.40 feet (5.0 m), and the length is 426.51 feet (130.0 m). The geometry is complex, with many bends and corners. The top of the wall includes traffic barrier and portions with coping and fence post. In one location the wall design requires an MSE abutment to carry a 24.93-foot (7.60-m) span gangway extending down to the boat docks.
The MSE design had to consider geosynthetic strip orientation within the MSE backfill to accommodate multiple obstructions, such as buried utilities, light poles, electrical vaults, 4.49-foot (1.37-m) diameter storm drain and a geomembrane cap cover located at top of wall. The use of the geosynthetic strip (discrete reinforcement strip) proved advantageous over continuous geosynthetic reinforcement to accommodate the obstructions. The geosynthetic strip MSE wall was selected on this project because of its technical advantages for use in seawater and other electrochemical aggressive environments. The geosynthetic strip MSE system provided the following primary characteristics:
- Conventional MSE design characteristics for internal and external stability
- High coefficient of friction between geosynthetic strips with granular backfill
- High resistance to chemical and biological degradation
- Not affected by marine environments
- Reliable connection to the wall facing
Design
MSE wall components
- The geosynthetic strip MSE wall consists of three major components:
- Large precast concrete facing panels to provide wall facing and wave action armor.
- Geosynthetic strips manufactured with high-tenacity, multifilament polyester (PET) yarns placed in tension and coated by direct extrusion with linear low-density polyethylene (LLDPE). The coating provides a continuous sheathing, maintaining the dimensional integrity by encasing and protecting the yarns from construction-induced damage, environmental exposure and dimensional stability while the polyester yarns are the load-carrying elements.
- Connection recess embedded in the facing panel composed of smooth-surface, blow-molded, polyolefin, omega-shaped sleeve. When the geosynthetic strip is threaded through the sleeve, the connection recess provides a fully synthetic integral mechanical connection.
The polymeric strip used for the structure has a nominal strength of 11.24 kip (50 kN) and a nominal width of 2 inches (50 mm). Quality control in production of the strip will yield a 100% certainty that the values tested for the strip are higher than the nominal value used for design. Polymeric properties of polyester yarns follow the requirements of the American Association of State Highway and Transportation Officials (AASHTO) for molecular weight and carboxyl end groups. The allowable design strength of the geosynthetic strip is based on material-specific testing used to derive reduction factors (AASHTO–NTPEP 2017). The long-term performance of geosynthetic reinforcement is influenced by factors such as time, temperature, load, installation conditions and environmental exposure. These factors, specific to polymer type and product, directly affect long-term performance and durability, and must be determined and considered in design when using geosynthetics.
Research
Andrawes et al. (1978) originally defined the differences between the relative extensibility of the reinforcement inclusions. Inextensible inclusions are those that “have rupture strains which are less than the maximum tensile strains in the soil without inclusions, under the same operational conditions,” and “extensible inclusions are those that have rupture strains larger than the maximum tensile strains in the soil without inclusions, under the same operational conditions.” This has been the basis for the separation of types of reinforcements in AASHTO.
It was determined that the pullout resistance of extensible and inextensible inclusions is mobilized differently (Segrestin and Bastick 1996). However, when compared to inextensible, ribbed steel strips in pullout tests, 2-inch (50-mm) polyester strips mobilized the dilatational frictional behavior in an analogous manner to ribbed steel strips (Lozano and Sankey 2013). Still, as reported by Anderson et al. (2012), geosynthetic strips develop frictional resistance along the length of reinforcement quite differently from inextensible steel strip reinforcements, as evidenced by the lack of trailing end displacement on the polyester strip.
MSE walls have been constructed with instrumentation to further understand the tensile load and pullout resistance in the reinforcement layers. Although geosynthetic strip reinforcements have been in use worldwide for at least four decades, there is limited instrumented structure data. One of the reasons for this is the complexity in attaching the right type of instrumentation directly to polyester fibers in the geosynthetic strip. Due to the two-part composition of a geosynthetic, it has proven extremely difficult to install strain gauges that can withstand the rigors of earth-moving construction without substantially altering the reinforcement geometry and stiffness properties. As such, out of the hundreds of structures constructed, only two relevant case histories are available in the literature with sufficient data in the instrumentation: the St. Remy wall and the Christiana wall.
The structure located in St. Remy, France, was designed and supplied by Freyssinet Inc. in the late 1980s and reported around 1993 by Schlosser et al. (1993). It was constructed with 3.5-inch (90-mm) wide strip reinforcements with a tensile strength per strip of 22.48 kip (100 kN) with a heavy LLDPE sheath. These were the predecessors of the current generation of geosynthetic strip and were nearly twice as wide and twice as strong as the most commonly used geosynthetic strips for construction of MSE structures worldwide. This wall was designed to have a trapezoidal reinforced zone with a lower zone width of 84% of the height, a middle zone width of 95% of the height, and an upper zone width of 127% of the height. The St. Remy wall had a vertical reinforcement spacing of 2.62 feet (0.8 m) and a horizontal reinforcement spacing of 1.64 feet (0.5 m) on average, resulting in a tributary area of 4.31 square feet (0.4 m2) per pair of geosynthetic strip. For reference, a typical MSE currently constructed in the U.S. using geosynthetic strips of this height would have a vertical reinforcement spacing of 2.46 feet (0.75 m) and a horizontal reinforcement spacing of 2.46 feet (0.75 m), resulting in a tributary area of 6.22 square feet (0.578 m2) per pair of soil reinforcement strips.
The most recently instrumented wall located in Christiana, Del., for the Delaware Department of Transportation (DelDOT) was instrumented as required by DelDOT for confirmation of design parameters/methodologies. A study of the collected data was performed by Dov Leshchinsky and others and presented at the 2015 TRB Annual Meeting in Washington, D.C. (Luo et al. 2015). This wall consisted of ParaWeb 2D 6.74 kip (30 kN) and ParaWeb 2D 11.24 kip (50 kN) soil reinforcements. Pairs of reinforcements were placed at a vertical spacing of 2.5 feet (0.76 m) and a horizontal spacing of 2.5 feet (0.76 m), resulting in a tributary area of 6.22 square feet (0.578 m2).
These two structures are the basis for validation of the design approaches not clearly stated in AASHTO, since for the U.S., polymeric strip reinforcement is a relatively new development (reintroduced in 2005), requiring the suppliers of this system to provide sufficient evidence for design.
AASHTO
The MSE wall follows AASHTO load and resistance factor design (LRFD) and Federal Highway Administration (FHWA) simplified design method for geosynthetics (AASHTO 2015 and NHI 2009) with supplemental data for geosynthetic strip reinforcement type regarding lateral stress coefficient (Kr/Ka) and pullout resistance factor (f *), as shown in Figures 3 and 4 (Lozano et al. 2017).
FIGURE 4 Pullout envelope of f * for design of MSE wall with geosynthetic strip
With geosynthetic strip tensile strength calculations, the design assumes an independent connection to the panel face with the strength of two strips per connection. The geosynthetic strip MSE wall reinforcement is connected to the face by looping the reinforcement through the connector such that the movement of one reinforcement affects the other as in a pulley system. The tensile breakage calculated in the design assumes that the strength of the pair of strips is equal to twice the strength of the strip itself. To achieve this concentric loading, all the slack is removed from the reinforcement during construction as described below.
Construction
Installation method
The facing panel is cast off-site in a precast facility with in-house quality-control measures. A typical facing panel consists of four omega-shaped connector inserts embedded in the panel, as shown in Figure 5. The openings for the geosynthetic strip recess are covered with tape and caps to prevent concrete, dirt and water from entering the recess during precasting and storage.
An unreinforced concrete leveling pad is poured to ensure even panel placement. Facing panels are set at a slight batter toward the backfill. A batter of 1.5 inches (38 mm) over 5 feet (1.5 m) is generally used as a starting point. Coarse backfill such as crushed stone may require less batter, as was done on this project. If finer backfill such as sand is used, more batter may be required. The wall batter is monitored and adjusted according to field conditions.
The geosynthetic strip is threaded through the recess opening cast in the back of the precast facing panel after the panels are set in the wall. The opening is protected during precast operations by a removable cap. The geosynthetic strip is unrolled in the field and cut to twice the reinforced length shown in the design plus 3.3 feet (1 m) to account for the length inside the recess. Each geosynthetic strip is pulled through the connection to form two parallel equal-length reinforcing strips extending into the MSE fill zone perpendicular to the facing panel (Figure 6).
Tensioning the geosynthetic strip is very important to maintain wall alignment. Before backfilling, the geosynthetic strip is anchored by securing the strip free end with a trench, nail, staple or soil pile.
MSE backfill
The geosynthetic strip MSE wall consists of compacted select granular backfill interlayered with geosynthetic reinforcing strips that are connected to facing elements. The structure relies on the interaction between the frictional soil, as the geosynthetic strip resists stresses produced within the soil mass. This composite mass retains compacted random backfill beyond the reinforced zone. The properties of both the select and random backfills have a significant impact on the design of the geosynthetic strip MSE wall. Specifications for select backfill should meet the following guidelines:
- Well drained
- Not prone to postconstruction movement/settlement
- Durable and does not break down or change its properties during construction
- Not aggressive to geosynthetics
If on-site backfill does not meet these guidelines, imported select backfill may be required or modifications to the design and construction procedures may be considered after careful assessment by the engineer and contractor. On this project, imported select backfill conforms with the requirements shown in Table 1.
Using the low fines aggregate backfill allowed MSE wall construction in the rain. Installation of the MSE wall began during late fall of 2017. Battling constant rainfall during construction became a normal occurrence on the project. Furthermore, tidal influence was expected. At the time of setting the bottom row of panels, the water level reached the top of the leveling pad. Backfill placement followed conventional MSE wall construction allowing rubber-tired vehicles to operate directly on the exposed geosynthetic strips. An example of backfill placed on top of the geosynthetic strip is shown in Figure 7.
Further project applications
The first geosynthetic strip reinforced MSE wall with integrated connections of this type was put in service in 2005 in Morzine, France. Since then more than 400 geosynthetic strip MSE walls have been constructed in 33 countries. The geosynthetic strip MSE wall has applications in all environments, especially those aggressive to galvanized-steel reinforcement. In the U.S., a tall 46-foot (14-m) high geosynthetic strip MSE wall was constructed for a dam raising at the Los Vaqueros Reservoir in northern California (Hardianto et al. 2013). A geosynthetic strip MSE wall was specified by the owner to avoid the potential for metal loss by-products to leach from backfill into the reservoir.
The Florida Department of Transportation (FDOT) has implemented restrictions related to MSE wall location and nearby marine environments. With this standard in place, the geosynthetic strip MSE wall satisfies the restriction and allows MSE walls to continue to be constructed where steel reinforcements would not be feasible. FDOT has embraced this technology and continues to take advantage of the architectural possibilities available with the precast facing panels of the geosynthetic strip MSE wall system.
Polymeric strip reinforcement has been utilized in the Port of Miami infrastructure supporting tunnels, in Virginia for light-rail applications, several projects for highway applications in the Midwest, and internationally in extremely tall walls (>98 feet [30 m]) for supporting the infrastructure needed for crushers, dumps and roll-off/roll-on ramps in mines, 88.6-foot (27-m) tiered walls for a highway in Croatia, a 33-foot (10-m) mixed abutment in Fort Carson, Colo., support walls for the ring foundations in Marrero, La., and the list of applications in challenging conditions grows every day. The latest project now in construction is the 141-foot (43-m) high wall supporting roadway as part of infrastructure expansion in the Dubai/Abu Dhabi region.
Conclusion
Linear geosynthetic reinforcing strips were used on the Point Defiance project to enhance the MSE wall design to meet the challenge of a saltwater marine environment. The results of research and monitored MSE walls have led to a design basis using AASHTO design models for this type of discrete reinforcement. As with any geosynthetic, tensioning the geosynthetic strip in construction is important to maintain the wall alignment. Using geosynthetic strip allowed for reinforcement to easily splay around obstructions in the MSE wall, such as light poles, electric conduits and drainage media, an advantage over other traditional geosynthetic reinforcements. Geosynthetic strip is a relatively new development in the U.S. but well-known and commonly used for more than 40 years worldwide. It can be used in confidence in projects where, due to environmental conditions or restrictions on backfills, the use of metallic reinforcement may be difficult.
Acknowledgments
The authors would like to acknowledge the following partners involved in this project: Atkinson Construction, KLB Construction, Jacobs Engineering and Wilbert Precast.
References
AASHTO. (2015). LRFD bridge design specifications, 6th edition, American Association of State Highway and Transportation Officials, Washington, D.C.
AASHTO–NTPEP. (2017). Report REGEO-2015-01-Reinforced Earth Co. GeoStrap 5—Laboratory evaluation of geosynthetic reinforcement. September.
Anderson, P. L., Gladstone, R., and Sankey, J. E. (2012). “State of the practice of MSE wall design for highway structures.” Proc., ASCE GeoCongress 2012 Conf., ASCE, Reston, Va.
Andrawes, K. Z., Al-Hasani, M. M., and McGown, A. (1978). “Effect of inclusion properties on the behavior of sand.” Géotechnique, January 28(3), 327–346.
Berg, R., Christopher, B., and Samtani, N. (2009). “Design and construction of mechanically stabilized earth walls and reinforced soil slopes.” FHWA-NHI-10-024 and 025, Volumes I and II, NHI Course Nos. 132042 and 132043.
Hardianto, F. S., Lozano, R., Sankey, J. E., and Hughes, D. K. (2013). “GeoStrap™ reinforced MSE wall for dam expansion.” Proc., GeoCongress 2013, ASCE, Reston, Va., 543–554.
Lozano, R., Brabant, K. P., Truong, K., and Anderson, P. L. (2017). “Evaluation of lateral stress ratio (Kr/Ka) for geosynthetic strip reinforcements in mechanically stabilized earth walls.” Proc., 19th Int. Conf. on Soil Mechanics and Geotechnical Engineering, Seoul, South Korea.
Lozano, R., and Sankey, J. (2013). “Pullout performance of polymeric strip reinforcements used in mechanically stabilized earth walls.” Proc., GeoMontreal 2013, 66th Canadian Geotechnical Conf, Montreal, Que., Canada.
Luo, Y., Leshchinsky, D., Rimoldi, P., Lugli, G., and Xu, C. (2015). “Instrumented MSE wall reinforced with polyester straps.” Transportation Research Board.
Schlosser, F., Price, D., and Hoteit, N. (1993). “Instrumented full scale Freyssisol-Websol reinforced soil wall.” Proc., Conf. for Soil Reinforcement Full Scale Experiments of the 80’s, Nov. 18–19.
Segrestin, P., and Bastick, M. (1996). “Comparative study and measurement of the pull-out capacity of extensible and inextensible reinforcements.” Proc., Int. Symposium on Earth Reinforcement, Fukuoka, Kyushu, Japan, A. A. Balkema, 139–144.
Terre Armée Internationale (TAI). (1982). “Study of the Tergal webbing used as reinforcing strip for the reinforced earth wall of Beaulieunear Poitiers,” Report No. IR 1, February, unpublished.
Paul Frankenberger, P.E., is business development manager for The Reinforced Earth Co. in Laguna Hills, Calif.
Robert Lozano is senior geotechnical engineer–corporate geosynthetic specialist with The Reinforced Earth Co. in Sterling, Va.
SIDEBAR: Project Highlights
Geosynthetic strip MSE wall
Owner: Metro Parks Tacoma
Location: Tacoma, Wash.
General Contractor; Atkinson Construction
MSE Wall Subcontractor: KLB Construction
Design Engineer: Jacobs Engineering
Geosynthetics Product: GeoStrap
Geosynthetics Manufacturer: The Reinforced Earth Co.