Marginal Fills - The Sustainable and Innovative Solution to New Zealands overuse of aggregates in embankments, slopes and walls

Most practitioners and engineers who currently design walls and slopes still prefer to work with high quality and well graded granular fill as backfill. However, as we move towards a Net Zero world, engineers can make a significant difference and act to help reduce carbon emissions within projects and therefore address climate change.

We also know that the infrastructure required to get to Net Zero needs to be resilient and financeable. Thus, considering marginal fills or recycled spoil generated as part of construction activities shall become the new normal for the creation of walls and slopes.

Marginal fills typically have high silt and/or clay contents which, when loaded, have the potential to generate excess pore water pressures in the structural backfill. Poor drainage in the structural fill reduces the available strength of the fill, thus reducing the bond between the fill and the geogrid reinforcement. Therefore, to use marginal fill efficiently, adequate drainage must be provided in the reinforced soil structure. By using a geocomposite that combines reinforcement and drainage into geogrid sustainable and environmentally friendly slopes and walls can be designed and constructed. Between 2015 and 2017, an innovative design methodology and approach for the construction of reinforced slopes and walls, with low-permeability fills, was successfully used in the UK. Now is the time to bring it to Aotearoa!

The use of marginal fills on site can yield significant cost and environmental savings for a project. In the UK, any material leaving a construction site is considered waste and must be disposed of accordingly. This is expensive, as waste is subject to a £94.15/ton (2020 prices) landfill tax in addition to the cost of excavation and transportation of the soil. Thus, there is significant interest in utilizing material, once considered unsuitable, in site works. Low-permeability marginal fills can be used to construct reinforced slopes and walls, granted that adequate drainage is provided within the reinforced fill (Fukuoka, 1998; Kempton et al., 2000; Naughton et al., 2001). Without adequate drainage, excess pore water pressures would typically build up in the fill during construction. Pore water pressure reduces the internal shearing resistance of the fill, as well as the
interface shear strength between the reinforcement and the fill (O’Kelly & Naughton, 2008 and Clancy & Naughton, 2011).

Between 2015 and 2017, several reinforced slopes and walls, up to 17 m in height, were constructed in the UK using excavated marginal fill from construction activities on site. The reinforced soil structures were constructed using a system consisting of a combined reinforcement and drainage geosynthetics product, referred to herein as the draining geogrid and a facing unit, to form the face, thus eliminating the need for a temporary shutter/formwork during construction. A drainage layer to the rear of the reinforced block was generally required (which can be either a geocomposite drain or a free draining granular chimney
drain). The function of the back drain is to prevent the flow of water from the retained fill into the reinforced soil block and to remove water from the draining geogrid. The draining geogrid was specifically designed for usage with low-permeability marginal fills. In presenting the case histories later in this paper, emphasis will be placed on the properties of the fill, the design and construction of the structures, and the quality control procedures implemented on site.

Taken from Nicola Brusa's (business partner of Sam Best) paper "Drainage Reinforced Geocomposite for Marginal and Cohesive Slopes and Walls". Full paper available upon request.