One of the common applications of geofoam blocks is the construction of retaining wall backfills. Two different geofoam block application concepts have been defined for retaining wall backfill construction (Horvath, 2004; 2010). The first is the Zero Earth Pressure (ZEP) concept, while the second is the Reduced Earth Pressure (REP) concept. Detailed information on these concepts is presented below.
Zero Earth Pressure (ZEP) Concept
According to the ZEP concept, the entire retaining wall backfill is constructed using geofoam blocks. Geofoam blocks are self-supporting materials; therefore, lateral loads acting on a retaining wall constructed with geofoam blocks in accordance with the ZEP concept can be reduced to negligible levels. Similarly, in addition to retaining walls, lateral loads acting on bridge abutments can also be reduced to negligible levels by constructing approach embankments partially or entirely with geofoam blocks. However, the primary purpose behind the development of the ZEP concept extends beyond simply reducing lateral loads acting on a retaining wall.
If a retaining wall is not required to resist any additional design loads other than the effectively “zero” lateral load generated by the geofoam blocks, there is no need to construct the retaining wall itself. Therefore, the main objective of the ZEP concept is to enable the construction of fills with vertical faces, i.e. 90 degrees to the horizontal, without the need for a retaining wall. By eliminating the need for a retaining wall, the ZEP concept can provide advantages in terms of both initial project cost and construction time.
An essential requirement for constructing a vertical fill without a retaining wall in accordance with the ZEP concept is that the soil slope adjacent to the geofoam blocks must be stable. This condition can be achieved in two ways. The first is to provide a “safe” soil slope inclination. In this context, a “safe slope” refers to a slope inclination at which the soil remains stable under static and seismic design loads without the use of mechanical stabilization measures such as permanent passive anchors or prestressed ground anchors. Before constructing geofoam block fill against an existing soil slope, excavation, including benching where necessary, is carried out to reduce the slope inclination to a stable value. For newly constructed fills, the soil slope adjacent to the geofoam blocks should be designed at a stable inclination (Figure 1). Details of the first retaining wall backfill constructed in Türkiye according to the ZEP concept were published by Özer et al. (2018), and its performance during and after construction was evaluated based on instrumentation and monitoring data. Following the first application reported by Özer et al. (2018), the use of the ZEP concept in retaining wall applications has become increasingly widespread in Türkiye (Figure 2).
Where slope stability is to be provided using mechanical methods, the slope is first stabilized using permanent ground anchors or another suitable mechanical slope stabilization technique, after which geofoam block installation can begin (Figure 3). An example of this type of application is the State Route 264 lane-widening project completed in Arizona, USA, in 2006 (Figure 4). In sections of this project characterized by weak ground conditions, it was decided to construct the new roadway embankment using geofoam blocks in order to eliminate the need for ground improvement. The fill, constructed with a vertical face at 90 degrees to the horizontal, was built using geofoam blocks without a retaining wall, with only a precast reinforced concrete facing installed at the front for protection. To enable construction of the geofoam block fill without a retaining wall, the slope of the existing trapezoidal roadway embankment was first stabilized using soil nails (Figure 4a). Following soil nailing, the geofoam block roadway embankment was constructed in accordance with the stages described in the article Construction of Highway Embankments Using Geofoam Blocks (Figure 4b–h).
Reduced Earth Pressure (REP) Concept
There may be conditions in which application of the ZEP concept is not feasible for engineering or economic reasons. For example, where lateral movement of the adjacent soil slope cannot be permanently prevented, or where constructing the entire fill using geofoam blocks would be more expensive than constructing a retaining wall, a wall-free geofoam fill may not provide a suitable solution. However, where a retaining wall is required, a more economical retaining structure can be designed using the REP concept.
In the REP concept, highly compressible geofoam blocks are placed between a rigid retaining wall and conventional compacted soil backfill as a “compressible inclusion” (Figure 5). Lateral earth pressure from the soil backfill causes lateral deformation of the compressible inclusion, allowing the lateral stresses within the soil backfill to approach the active earth pressure condition. Ertuğrul and Trandafir (2011) investigated the effect of compressible inclusion thickness on lateral stresses acting on a 0.7 m high rigid wall through laboratory model tests and reported that, when the compressible inclusion thickness was equal to 28% of the wall height, the lateral stresses acting on the wall were very close to Rankine active earth pressure values. Ertuğrul and Trandafir (2013) further reported that compressible inclusions also reduce lateral earth pressures acting on cantilever retaining walls, although the magnitude of the reduction is inversely related to wall flexibility.
REFERENCES
- Ertuğrul, O. L., Trandafir, A. C. (2011). “Reduction of Lateral Earth Forces Acting on Rigid Non-Yielding Retaining Walls by EPS Geofoam Inclusions”, ASCE, Journal of Materials in Civil Engineering, 23(12): 1711–1718.
- Ertugrul, O. L., Trandafir, A. C. (2013). “Lateral Earth Pressures on Flexible Cantilever Retaining Walls with Deformable Geofoam Inclusion”. Engineering Geology, 158: 23–33.
- Horvath, J. S. (2004). “Geofoam Compressible Inclusion: The New Frontier in Earth Retaining Structures,” Geotechnical Engineering for Transportation Projects, GeoTrans 2004, ASCE, 1925–1934.
- Horvath, J. S. (2010). “Lateral Pressure Reduction on Earth-Retaining Structures Using Geofoams: Correcting Some Misunderstandings”, Proc., ER2010: Earth Retention Conference 3, ASCE, Reston, VA.
- Özer, A. T., Akınay, E., Türer, E. (2018). “Use of Geofoam Blocks in Retaining Wall Applications”, 17th National Conference on Soil Mechanics and Geotechnical Engineering with International Participation (ZMGM17), Istanbul University, Istanbul, September 26–28.