Abstract from presentation delivered by Giulia Lugli of Maccaferri at the 19th International Conference on Soil Mechanics and Geotechnical Engineering in Seoul, Korea between September 17 and 22, 2017.  The presentation was part of the TC218 workshop.


Reinforced soil retaining walls offer economic advantages over conventional retaining walls. The cost advantage increases with the height of the wall. However, the cost of reinforcement can be as great as about 25% of the cost of the wall. Several studies addressed the possibility of further reducing the total cost of a RS wall by optimizing the reinforcement strength and pattern along the wall.

The hybrid RSS presented are modular systems which combine a primary reinforcement (typically polymeric high strength geogrids) and wire mesh units (wire mesh gabion units or wire mesh vegetated external facing units) with a tail of steel mesh reinforcement that plays the role of “secondary reinforcement” and “facing units”. The primary reinforcement provides the tensile strength needed to ensure global stability; while the facing units provides the local stability at the face, and prevents from local mechanism of direct sliding, pullout or rotational failure. The static and deformation behavior of hybrid MSE walls depends on reinforcement stiffness, arrangement, and quantity (as a function of stiffness, length and vertical spacing). Numerical results showed that mixed reinforcement configurations with reduced stiffness toward the wall top did not result in significantly larger lateral wall displacements. Moreover, a 50% reduced length of every other reinforcement layer was found to be the best method to reduce the reinforcement supply requirement while maintaining wall serviceability and performance. Finally, a vertical spacing of the primary reinforcement equal to 2-3 times the standard spacing represents an optimal distribution. Three case histories of hybrid MSE walls are presented below.

From 2013, a total of twenty walls have been built in Izmir as part of a large petrochemical refinery project. The retaining structures were designed using gabion facing units, with a tail consisting of 3 m long double twisted wire mesh, and coated PET high strength uniaxial geogrids. The 81,000 sm retaining walls are located in a first-degree seismic zone where PGA is 0.75 g. A section of the wall has been instrumented and centrifuge and numerical modeling were studied by an international Research Group, including Dr. J. Collin, and Professors D. Leshchinsky, J. Han, B. Tanyu, H. Ling. The research demonstrates that hybrid structures behave in accordance with traditional reinforced earth theory when the vertical spacing of the reinforcement is 1 meter. The AASHTO 2014 Standards were modified to allow for a 1 m vertical spacing of the soil reinforcement.

The construction of the new airport in the Sikkim region of India involved massive cutting and filling earth works. The retaining walls are located in a seismic zone IV, according with Indian guidelines. Recently an earthquake of 6.9 magnitude hit the area but no visible damages have been identified. A total of 1.7 km of RSS up to 68m high have been built in this project: the systems are a combination of vertical gabion and vegetated facia and high strength PET geogrids. On top other 12m of backslope.

Tana Toraja is located in the South Sulawesi province of Indonesia. The new airport runway is 2 km long and approximately 210 m wide. The main technical issues involved the presence of clay shale foundation soils, the construction to be performed and finished during the rainy season and the high seismicity of the area (0.3g PGA). The selection criteria for this project leads the choice on a Hybrid RSS solution with 300 kN/m geogrids every meter and sloped vegetated system used on top where the vertical spacing in the top layers was increased to 2m.

Hybrid RSS provide significant advantages if compared to traditional retaining structures such as flexibility, high permeability, environmental friendliness and economy. RSS, if properly designed and carefully executed, can be built also on clay shales and using cohesive materials as backfill.