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auteurMahesa Bhawanin

datumNov 04 2020

The construction industry is a pillar of development in any country and is a marker of economic health. It is also a hotbed for innovation and technological breakthroughs. However, a growing concern is the sustainability of construction activities, from an economic, social, and environmental perspective.

There is a growing global drive to address these concerns in construction to ensure our natural resources are properly allocated and maximum benefit is extracted from any finite resource. To achieve this, a change is needed at all levels in the supply chain from cradle to grave of any project.

In this respect, geosynthetics have received somewhat of a bad rap. Primarily driven by the massive amount of plastic waste generated every year creating frightening statistics such as the 60 to 99 million metric tonnes of mismanaged plastic waste globally in 2015 as reported by Lebreton and Andrady (2019). Added to the fact that it is a product of the oil industry and shares some of this negative stigma. As a result, a notion exists that geosynthetics are not viable in a sustainable future in construction. This is obviously a sweeping statement which does not offer any appreciation for the nuance of its applications.

The fact is that not all geosynthetics are created equal and cover many applications within civil engineering. Geosynthetics incorporated into civil engineering are brought in due to necessity and often part of value engineering exercises. This is common when conventional design and construction techniques would be cost prohibitive. These costs are invariably associated with increased material quantities and construction labour which could be reduced by incorporating geosynthetics.

Another point to consider is that the very element that makes waste plastic such a problem such as its longevity in the environment is a boon for construction. The typical design life of a reinforced soil structure using geosynthetics is around 100 years and pavements are in the order to 15 to 40 years. The typical life of a plastic bottle or bag before it would be considered waste, in comparison, would be a fraction of that. The period of its utility far less than that. Walls and pavements using geosynthetics will also likely exist well beyond its design life requiring the geosynthetics to maintain their performance further than perhaps anticipated.

Even within particular applications, in particular reinforced soil walls and pavements, geosynthetics or specifically geogrids vary greatly in their performance. A well-engineering geogrid used in a well-designed wall or pavement will combine the minimum use of plastic, the maximum life of structure with the least amount of resource such as aggregate, concrete or asphalt. Tensar routinely demonstrates this within their applications by offering value-engineered analyses and high efficiency geogrids. Some example projects are provided in the following passages.

For example, reinforced soils structures such as walls and slopes are common applications for geogrids. These structures are reinforced using uniaxial polymer geogrids where the strength is oriented primarily in one direction. Walls often use concrete finishes and slopes tend to be vegetated. In some cases, erosion control blankets are used to mitigate surface erosion  in the interim before the vegetated surface grows out. The slopes can incorporate large floral features, light features and other aesthetically pleasing and environmentally friendly systems.

By using in-situ soils, the carbon footprint can be significantly reduced and high-performance geogrids can reduce the amount of fill and geogrid needed. For example, the Mansfield Community Hospital modular block wall was constructed using uniaxial geogrids. The use of in-situ fills provided savings of 53% in the overall project cost and a 57% reduction in carbon footprint compared to a traditional reinforced concrete retaining wall (WRAP, 2010).

Another project of note is located in Singapore. Singapore, as an island nation, has a vested interest in maintaining a sustainable and green construction industry. The ABC (Active, Beautiful, Clean) Waters Programme is one such initiative to integrate drainage systems into the local water bodies providing a clean, environmentally healthy and aesthetically pleasing solution (PUB Singapore, 2018). Part of this initiative is the upgrading works along Tampines Avenue 9 with total length of 1.35 km; to transform and upgrade the existing canal system (Ng et al., 2019).

The project includes the construction of a gabion wall system along 168 m of the length from CH10 to CH178 to replace a conventional earth retaining. In support of the ABC Waters programme, a 925m long vegetated slope is constructed along the canal. The vegetated slope serves to control the erosion of the banks during high volume flow conditions and provide a scenic feature to one of Singapore’s most prominent public areas.

Slopes were designed in accordance with EBGEO (Geotecknik, 2012) with a design life of 60 years. The reinforcement of the slope consists of two types of geogrids, a HDPE uniaxial geogrid with a PP biaxial geogrid. A typical section of the slope is shown in Figure 1. The geogrids that form the wraparound are exposed to UV radiation and are protected by a polymer blend that contains 2% of carbon black.

 Slope

                                   Figure 1 Typical section of Bio-Engineering Slope

The HDPE and PP geogrids were advantageous in this application as the apertures would allow vegetation to take root and larger plants could be accommodated by cutting openings. In-situ fills were used in the construction and the structure was immediately vegetated (Figure 2).

 Slope

Figure 2 The Tampines VRSS during construction. The slope is immediately vegetated and installed directly on the geogrid surface.

As for pavements, benefits geogrids have been proven over many years reduction of pavement thicknesses, increased traffic capacity or a combination of these two benefits. Geogrids almost always lower the construction CO2 footprint. This strongly depends on the performance characteristics of the geogrid.

Geogrids using stabilisation such as TriAx reduces the quantity of materials required to for a pavement by typically 30%, and results in material savings and a greener solution. Tensar’s carbon footprint assessment uses publicly available references to quantify estimated embodied carbon from the pavement materials and bespoke data gathered by an independent environmental consultant on the energy required to manufacture the geogrids themselves. This calculator was used to calculate the CO2 footprint improvements in the following case study.

Kawalec et al. (2019) presents the pavement analysis for a project in Poland. They considered a pavement under heavy traffic loadings: 52 million 100kN axles over a 1 km long stretch constructed over a subgrade strength of 25 MPa. This is a relatively soft subgrade and a capping layer was necessary. The design was conducted using mechanistic empirical methodologies. The pavement required excavation and therefore increasing the carbon footprint. The materials were supplied were located between 10 to 25km from the site. The proposed equivalent sections (traditional and stabilised) are presented in Figure 3.

 pavement

 

Figure 3 Comparison between traditional pavement design and alternative stabilised pavement with multiaxial geogrids (Kawalec et al., 2018). Legend: ACWC = Asphalt Concrete Wearing Course, ACBC = Asphalt Concrete Binder Course, ABC = Asphalt Base Course, GBC = Granular Base Course, GSB = Granular Sub-base.

Stabilisation with geogrids resulted in a reduction of thickness of 24.8%, reduction of construction costs by 13.8%, and a reduction of 23.3% carbon emissions. These savings should be assessed on a case-by-case basis and improvements may vary. Geosynthetics differ in their potential improvement and any consideration should be empirically derived at full scale (AASHTO, 2009).

Sustainable construction can benefit from the use of geogrids especially when compared to traditional construction methodologies and are a viable alternative. Reach us for further details on how we can help to reduce your carbon emissions and improve your value proposition without compromising on performance.

 

References

American Association of State Highway and Transportation Officials 1993, 2009 Standard Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures AASHTO R 50-09. Washington, USA.

Ng, C., Ong, C., Jong, H., L.G., L., & Loke, K. (2019). Improving Sustainability through Geogrids in Infrastructure Projects, World Engineers Summit 2019.

Geotechnik, Recommendations for Design and Analysis of Earth Structures using Geosynthetic Reinforcements - EBGEO. John Wiley & Sons, 2012.

J. Kawalec, M. Golos, and P. Mazurowski, ‘Environmental aspects of the implementation of geogrids for pavement optimisation’, IOP Conference Series: Materials Science and Engineering, vol. 356, p. 012018, May 2018.

Lebreton, L., Andrady, A. Future scenarios of global plastic waste generation and disposal. Palgrave Commun 5, 6 (2019).

Public Utilities Board (PUB) Singapore, Active, Beautiful and Clean Waters: Design Guidelines, 4th Edition. PUB Singapore, 2018.

WRAP (Waste and R. A. Programme), Sustainable Geosystems in Civil Engineering Applications. Waste and Resources Action Programme Banbury, UK, 2010.