Integrated cold resistance subgrade system utilizing oil shale waste and XPS insulation for sustainable infrastructure in seasonal frozen regions

To address the environmental concerns of oil shale waste (OSW) accumulation and improve road engineering sustainability, this paper proposes a novel cold resistance structure (CRS) incorporating extruded polystyrene (XPS) insulation plates and OSW-modified soil. OSW primarily consists of two components: residual semi-coke from retorting processes and combustion-derived ash residues. The improper disposal of accumulated OSW poses significant environmental risks. Following a comprehensive feasibility assessment, this study identifies the application of OSW in highway subgrade construction as an eco-friendly solution that achieves triple objectives - waste valorization, pollution mitigation, and alleviation of material shortages in road infrastructure. Targeting the freeze-thaw challenges prevalent in northeast China's road structures, the CRS system combines XPS insulation technology with OSW-modified subgrade soil through three key phases. First, the optimal XPS plate thickness was determined using thermal resistance equivalence principles. Second, controlled freeze-thaw experiments employing a specialized unidirectional testing system evaluated the CRS's frost resistance through triplicate comparative trials. Third, field validation involved constructing a CRS test road and conducting in-situ assessments of bearing capacity and dynamic stress responses, with conventional sand-gravel subgrade sections serving as controls. Environmental impacts and economic viability were systematically analyzed. Results demonstrate that the CRS system reduces the subgrade freezing depth by up to 52.8%, limits surface water migration by over 60%, and decreases dynamic stress amplitudes by more than 50% compared to conventional structures. The effective stress buffering depth of the XPS insulation is equivalent to an 89.75 cm thick gravel layer, while subgrade deflection is reduced by 21%. Additionally, the CRS system achieves a 43% reduction in material cost per kilometer and enables the reuse of over 9300 tons of solid waste, offering both economic and environmental benefits.