Mechanical and microstructural investigation on clayey subgrade soil stabilised with wood biomass ash: mix design optimisation and curing effects

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Abstract

The disposal of wood biomass ash (WBA) from wood chip combustion represents an increasing challenge in Italy, where biomass-fuelled power plants widely use locally sourced wood chips. Despite its potential, comprehensive information on the use of WBA as a stabilizing agent for subgrade soils is still lacking, and the optimal ash and water contents for maximising in-situ performance remain under-investigated. This study investigates the use of WBA to stabilise low- to medium-plasticity clayey soils. The effects of moisture content, WBA type, and WBA dosage on compressive performance were first evaluated and compared with hydrated lime treatment. The evolution of mechanical, volumetric, and plasticity properties was then analysed during curing. Results show that a water content about 2% below the optimum Proctor value and WBA dosages corresponding to those ensuring a mixture pH of 12.4 maximise compressive strength. WBA treatment improves bearing capacity, reduces swelling and plasticity, and enhances strength over time through flocculation and cementitious bonding mechanisms. Mineralogical and thermogravimetric analyses confirmed the formation of cementitious phases responsible for these improvements, supporting the suitability of WBA for subgrade stabilisation.

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