The benefits of reinforcing slopes with Green Terramesh

Green Terramesh is an environmentally friendly modular system used for soil reinforcement in mechanically stabilised earth slopes and embankments. Its purpose is to prevent erosion, provide soil reinforcement and assist with slope stability – all while blending in with the natural surroundings.

By Thivaska Padayachee & Vernita Govender*

The concept was initially introduced in Malaysia in the late 1970s when a structure was erected to support a road embankment in a highly constrained area. It was composed of a vertical ‘skin’ of rock-filled gabion mesh units that were fastened to the structural fill of the embankment with horizontal steel strips. From there, Maccaferri’s Green Terramesh System (GTMS) continued to evolve to meet a range of modern day engineered solutions.

During construction, Terramesh units are placed in horizontal layers during the filling and compaction of a structural embankment. The main unit is fabricated from continuous, heavily galvanised and PVC coated steel wire, which is woven into a double-twisted hexagonal mesh configuration. Attached to the inside facing is a geosynthetic or biodegradable erosion control blanket (ECB), a welded steel panel, and two steel brackets preformed to a 70-, 60- or 45-degree slope angle (or inclined front face) to enhance vegetative growth. The ECB retains the backfill soil and permits a vegetative cover to establish rapidly. It may consist of a permanent geosynthetic threedimensional geomat (hydraulic application), or a biodegradable 100% coconut fibre biomat (dry application).

The welded steel panel placed behind the ECB adds strength and provides support to the facing, while holding the blanket in place during construction. In turn, the top section of the main wire mesh is used to anchor and connect the unit to the one above via the specific lacing technique.

Applications

GTMS can be used in various applications, such as: an alternative to near-vertical retaining structures; protection in river, stream banks and shoreline situations; and development projects on steepened landscape sites.

As an environmentally engineered solution, GTMS is ideal where a greened aesthetic is required in addition to immediate and long-term protection from surface erosion and geotechnical failures. Installing a continuous series of mesh panels ensures rigidity of the front face.

GTMS units are supplied cut to specified measurements, as determined by the design. Therefore, no cutting is required on-site.

Vegetation and drainage

Various vegetation techniques can be used with this system, including live stakes, rooted plants, seed and hydroseeding. These vegetation techniques are incorporated after the installation of the structure. The best approach is to match the planted vegetation with naturally occurring plants in the area. This will ensure that the final landscaped result is indigenous and water wise. Plus, there’s a much greater certainty that the new vegetation will take hold.

As with any retaining system, adequate drainage at the rear of the reinforcement should be installed where seepage of surface water is a factor. It is also recommended to step the units back 100 mm to 150 mm to reduce the velocity of the water flowing down the front face.

Design software for Green Terramesh

Since the system is a mechanically stabilised earth structure, it is important to verify the stability of the slope prior to construction. To determine this, MacStars W software is one of Maccaferri’s most popular programs. It is mainly used to design and check the stability of reinforced and unreinforced structures.

The designs generated are mainly based on the shear strength of the soil below and

M4 INFRASTRUCTURE REINSTATEMENT WITH GREEN TERRAMESH

The Department of Transport together with eThekwini Municipality faced a major challenge when periodic heavy rainfall, coupled with a burst underground concrete pipeline, resulted in a major embankment failure and partial collapse of a section of the M4 highway.

Interventions included a new HDPE underground stormwater pipe, while the upper and lower embankment slopes were reinforced with Green Terramesh in conjunction with Maccaferri’s ParaGrid geogrid reinforcing solution.

Reinstatement in progress on a section of the M4 Vegetation starting to take hold on the completed GTMS installation

Benefits Features

Structurally sound Computer software and technical support to assist in the design of structures ensure a structurally sound engineered solution. Excellent connection mechanisms eliminate the risk of failure between the reinforcement and the front face In situ materials Besides the 0.3 m deep wedge behind the front face, in situ material can be used to backfill the structure Sand-only solution Useful when soil is the only material available for construction Immediate, long-term erosion protection GTMS itself gives immediate structural stability, which grows stronger with time as the vegetation is established Easily greened More conducive to greening due to the sloped front face

Ecological protection Provides aquatic, riparian cover and wildlife habitat value

Traps sediment Helps to slow water velocities near banks and trap sediment

Durability

Galfan and PVC coated wire products ensure high performance and durability Versatility Individual units allow for easy design and construction of a sitespecific solution Easy to install A straightforward system of combining individual units to form a reinforced structure. Unskilled staff can quickly and efficiently install units on-site, under skilled supervision Cost-effective In situ construction minimises the cost associated with using imported materials. Furthermore, the modularity of the unit makes the construction time cost-effective, while the durability gives an outstanding life-cycle cost benefit

Environmentally friendly

Flexibility Structures can be naturally integrated with the surrounding environment by using in situ materials combined with various bioengineering techniques. Plant life can be incorporated into the design to accelerate vegetation The structure’s flexibility allows its use even in poor soils, as it can adapt to foundation settlements with moderate deformations

Representation of a Green Terramesh system

Top tail

Coconut fibre blanket

Steel bracket

Welded steel panel

Anchor Components of a Green Terramesh system economical operation surrounding the structure in terms of its frictional angle, weight and cohesion parameters. The software also enables the user to model the exact strata of the area with and without a water table in the case of a river or heavy rainfall area.

There are many load configurations that can be used for the different load cases, along with a wide variety of reinforcing elements to select from and design with.

The MacStars W software produces designs in accordance with the guidelines prescribed in the BS Code – ‘British standard codes of practice for reinforced soils and other fills’. The design considers both the ultimate and serviceability limit state principles and runs the design analysis using the coherent gravity method.

Internal and external stability

It is important to note that there are key rules that need to be followed when determining the structure’s minimum embedment depth and reinforcement lengths. For the structure to be rendered stable, the MacStars W software analyses the internal and external stability. This includes rupture of the reinforcements within the structure, and any sliding, overturning and bearing pressures exerted on to the structure and the foundation.

Slopes and embankments often require additional soil reinforcement for extra strength. This is most common in cases where a steeper slope is desired or in the reconstruction of a failed slope. Flexible reinforcement enables the structure to rest at a much steeper angle than its natural form and provides stability to the structure by transferring the destabilising forces from the active zone to the resistant zone with an adequate bond tensile stiffness.

Conclusion

Green Terramesh has proven to be a costeffective and popular solution worldwide and has recently been widely adopted in South Africa to reinstate failed embankments and slopes washed away during recent floods.

For the rehabilitation of vegetated slopes and embankments, GTMS provides lasting stability with the inclusion of high-strength geosynthetic grid reinforcement. Vegetating the front face serves two key purposes: the first is short-term protection, while the second is to achieve that legendary Green Terramesh effect.

*Thivaska Padayachee and Vernita Govender are graduate civil engineers at Maccaferri Africa.