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Background: Clay-basedand geosynthetic liners are generally used as hydraulic barrier in solid waste disposal facilities, mining tailing ponds, and soil-based wastewater treatment technologies, avoiding leaching of hazardous compounds to subsoil and groundwater. Water treatment sludge (WTS) is a water treatment plant (WTP) residue which due to hydraulic properties seems to decrease the permeability in WTS:soil mixtures and may become an alternative material to produce sustainable waste-based liners. Objectives: This research aims to characterize and analyse physical, chemical and mechanical parameters of a WTS, a soft soil and four mixtures WTS: soil following 05:95%, 10:90%, 15:85%, 20:80% ratios. Therefore, evaluate the best ratio for producing waste-based liners for civil engineering applications. Methods: The geotechnical characterization performing –particle size distribution, specific surface, specific gravity, Atterberg limits, and Normal Proctor compaction; chemical composition due to oxides analyses through X-ray fluorescence (XRF), mineralogical description by X-ray diffraction (XRD) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) coupled for imaging; and mechanical behavior performing - –oedometric consolidation, consolidated undrained (CU) triaxial, and falling head permeability. Tests were conducted for all mixtures, the soil, and WTS, with pointed out exceptions. Results and discussion: The results showed that the fine-grained WTS filled the soil voids but rearranging soil particles, thus, compacted dry unit weight decreased with WTS addition, probably due to its chemical composition with high amounts of aluminium and silica. The compressibility of the compacted mixtures did not differ significantly compared to the soil, while the shear strength analysis demonstrated a reduction in cohesion, and an increase in the effective internal friction angle proportional to WTS addition. Hydraulic conductivity increased with WTS until 10% of residue introduction, decreased for 15%, and continued to decrease for 20%, reaching optimum permeability in 15%. Conclusion: The incorporation of WTS can improve or just do not interfere at soil’s properties to be used as liner material in solid wastes storage facilities, mining ponds and soil-based wastewater treatment technologies. Furthermore, 15% of WTS (15:85% mixture) incorporation provided the best results meeting the hydraulic conductivity requirement for liner materials, i.e., equal, or lower than 10-9 m/s. The reuse of WTS for this purpose would allow producing a new added-value material in the scope of circular economy.

Biomass ashes (BA) have been intensively studied as amendments for soil in earthworks. This paper aimed to geotechnically characterize BA from pines and olive trees compared to the soil from Castelo Branco, Portugal. Namely, granulometry, specific gravity, Atterberg limits and optimal compaction values were obtained and analyzed in order to valorize the residue incorporated into soils. This work is part of broader efforts to develop an alternative material that can be used in hydraulic barriers as liners and for soil reinforcement. Thus, BA can contribute to reductions in weight and plasticity, and filling properties. Further studies are needed, particularly mechanical and hydraulic performance tests. Keywords: biomass ashes, geotechnical and mechanical properties, residue valorization, soil reinforcement, liner material

Foram escolhidos vários locais para a investigação do comportamento mecânico de um solo residual granítico da Covilhã: o campo de futebol da Associação Desportiva da Estação (AD.E.); o novo campo de futebol da Câmara Municipal da Covilhã (C.M.C.) e o local de Santo António. Nestes locais foi feita uma caracterização geotécnica, ensaios de campo e colheita de amostras deformadas e indeformadas. Com as amostras colhidas foi feita a classificação laboratorial, foram efectuados ensaios edométricos, de caixa de corte directo e triaxiais. Através dos ensaios laboratoriais foram determinadas as propriedades de compressibilidade, os parâmetros de resistência ao corte e o comportamento tensão - deformação. Foram identificadas duas superficies de cedência a partir das variações de rigidez nas curvas tensão - deformação, resultantes da rotura do cimento estrutural existente neste tipo de solos residuais graníticos. O efeito das sucções no comportamento destes solos foi também discutido.