Reuse of Tunisian excavated material into composite soil for rainwater infiltration within urban green infrastructure

Abstract

Excavated material or soil is one of the most abundant types of waste produced by civil works. The study draws a methodology for assessing through selected standard tests the environmental, geotechnical, and agronomical features of excavated material to highlight the most ecological way to reuse it. In view of making a constructed soil to be implemented within green infrastructure part of new urban landscape, imperative skills of soil or material are referred to as SWOFI: safety, workability, fertility, infiltrability, and guidelines are derived accordingly. As a practical case of the application of the methodology, a non-cohesive sedimentary parent rock with a sandy loam texture excavated at Bou Argoub in Tunisia has been characterised. This excavated material is a negligible source of pollution hazard by heavy metals. It classifies in a category very sensitive to compaction and strongly influenced by the clay content, so difficult to reuse in road applications. It is very poor in organic matter and too much alkaline, so grape marc waste is used to improve its fertility, thus making a composite soil that prefigures a constructed soil. This allows the valorisation of local agriculture waste throughout the designed strategy.Novelty resides in the selection of tests and the association of technics issued from usually separated disciplines and the integrated workflow of the soil at three stages of the value-chain: excavated-homogenised, amended, then repacked at a given bulk density as a composite soil. Immediate compactibility changes as a linear function of repacked bulk density BD; from the literature, this result is assumed true in the composite soil as well as the excavated material. Based on required intervals of permeability for the projected use in green infrastructure requiring rainwater infiltration, a decision-making table is applied with, as input, the most probable value of permeability determined from pedotransfer function as topsoil or as subsoil. Combining pollution hazard assessment, geotechnical improvement, agronomical amendment, an innovative approach leads to pedological and hydrological functionality for constructed soil, a nature-based solution.