Utilization of coconut shell residual in green roof: hydraulic and thermal properties of expansive soil amended with biochar and fibre including theoretical model

The study intends to explore hydraulic and thermal properties of expansive soils treated with fibre, biochar and biochar–fibre mix. Both fibre and biochar are derived from coconut shell, which is highly common in coastal regions around the world. Besides, benefits, limitations and engineering feasibility of these geomaterials in green roofs are explored. Theoretical framework for thermal–hydraulic analysis is proposed based on mass conservation and the first law of thermodynamics. Heat capacity, thermal conductivity, water retention curve, crack intensity factor (CIF) and saturated and unsaturated hydraulic conductivities of four kinds of soils are evaluated and compared. Characterizations of geomaterials are also investigated via thermal mass loss, micro-structure, surface area and functional groups identification. Both biochar and fibre admixtures contribute to improvement in soil heat capacity and saturated and unsaturated hydraulic conductivities. Biochar enhances saturated and residual water contents of expansive soil by 10% and 8%, respectively. Also, biochar decreases soil thermal conductivity and CIF by 31% and 5%, respectively, while fibre decreases soil-saturated and residual water contents by 15% and 29%, respectively, and reduces soil thermal conductivity and CIF by 21% and 50%, respectively. Soil–biochar–fibre composite is also recommended due to low air-entry value, acceptable water-holding capacity and limited crack propagation. The study fills the knowledge gap of how soil thermal–hydraulic properties are affected due to biochar and/or fibre admixture. It is recommended to pay more attention on production and utilization of biochar derived from coconut shell currently utilized for fibre extraction.