Investigation of water retention functions of artificial soil-like substrates for a range of mixing ratios of two components

Abstract

Purpose Urban greening is politically fostered as an adaptation strategy to climate change. Therefore, the demand for fertile planting substrates increases. Such substrates are usually mixed from mined geogenic resources but should rather be produced from recycled materials. Furthermore, their hydraulic properties should be designed according to their application, e.g., by optimizing the mixing ratio of their components. Therefore, this study introduces an approach to investigate the water retention curves (WRC) of soil-like substrates as a function of the mixing ratio of two recycled components: exemplarily for green waste compost (GWC) and ground bricks (GB) in the fraction of sand. Materials and methods Seven mixing ratios for GWC and GB, 0/100, 18/82, 28/72, 37/63, 47/53, 68/32, and 100/0 have been packed to mixture-specific densities using a newly constructed packing device. The packing density resulted from applying six strokes with a constant momentum of 5.62 × 10 −3 N s m −2 that was chosen according to the German green roof guideline. Thus, a standardized compaction was assured. The WRCs were measured using the simplified evaporation method in five replicates for each of the seven mixtures. A set of water retention models was parameterized and analyzed in regard to their suitability to represent the full range of binary mixtures. Results and discussion The newly constructed packing device enables to pack cylinders reproducibly. The densities in the cylinders for the mixtures varied from 0.64 g cm −3 (GWC/GB = 100/0) to 1.35 g cm −3 (GWC/GB = 0/100) with a coefficient of variation less than 1.3%. The simplified evaporation method delivered homogeneous results for all five replicates of the investigated mixtures. The WRC of the seven mixtures is the result of a complex combination of the pore systems of GWC and GB. The multi-modal water retention models of Peters, Durner, and Iden are principally suitable to describe soil-like substrates that are rich in organic matter. The models PDI (van Genuchten) and PDI (Fredlund-Xing) best described the WRCs for the full range of mixing ratios according to the quality criterion RMSE. Conclusions The study delivers a template how to prepare and analyze soil-like substrates regarding their WRCs using the simplified evaporation method. Complemented by total porosity and measurements at pF > 4, it is a suitable method to gain high-resolution WRCs of soil-like substrates. Available water retention models are capable to describe the hydraulic behavior of binary mixtures over the full mixing ratio. Therefore, it would be possible to model the WRC of binary mixtures as a function of their mixing ratio.

Towards the Circular Soil Concept: Optimization of Engineered Soils for Green Infrastructure Application

“…Other parameters for soil structural performance (e.g., infiltration rate, pore volume, water-holding capacity) will be essential for the applicability and application decisions of engineered soil alternatives. Future research should address the water-retention curves additional to the grading curves, to assure the long-term functionality of the optimized soil alternatives for applications in sustainable rainwater management [69].…”

Performance of experimental bioretention cells during the first year of operation

“…The function of the BC relies on the performance of the biofilter soil layer which is formed, in many cases, by specifically designed constructed soil (Gulbaz and Kazezyilmaz-Alhan, 2017;Hatt et al, 2009) mostly composed of sand, various types of topsoil and organic material. The ratio of these components affects the resulting retention properties (Willaredt and Nehls, 2021) hydraulic conductivity of the mixture, which predetermines the infiltration rate in the bioretention cell and the overall functioning of the biofilter layer (Zhang and Chui, 2017). Gulbaz and Kazezyilmaz-Alhan (2017) showed that a high sand ratio decreases the arrival time of a peak flow bioretention system and increases the outflow peak whereas high clay and silt contents increase the arrival time and decrease the peak of the outflow of the bioretention system.…”

Predicting Water Retention Curves for Binary Mixtures – Concept and Application for Constructed Technosols

“…These constructed Technosols can be engineered from locally available valuable mineral and organic waste. This is considered a sustainable path to meet the demand (Prado et al, 2020;Deeb et al, 2020;Fabbri et al, 2021), as it reuses materials that would otherwise be land filled and decreases the degradation of fertile natural soil resources and other geogenic materials (Willaredt and Nehls, 2021). Tams et al (2022) showed in a life cycle analysis, that the use of recycled brick particles instead of expanded clay reduces the CO2 footprint of the substrate layer by 50 % in an extensive green roof.…”

“…Tams et al (2022) showed in a life cycle analysis, that the use of recycled brick particles instead of expanded clay reduces the CO2 footprint of the substrate layer by 50 % in an extensive green roof. The composition of waste materials and the processing (Ulrich et al, 2021) are the most important design levers to manipulate the properties according to their targeted application (Rokia et al, 2014;Fields et al, 2018;Willaredt and Nehls, 2021). Most UGI addresses the re-establishment of soil function related to the regulation of the water cycle (Grabowski et al, 2022).…”