Sustainable Stabilization of Expansive Soil for Subgrade Applications Using Waste Glass Powder and Steel Slag

Expansive soils pose a significant challenge to civil engineering practice, frequently causing structural damage and constraining road infrastructure development in Ethiopia. Pavements and light structures constructed on such soils are particularly vulnerable to premature failure due to excessive settlement and recurring volume changes. This study investigated the improvement of expansive soil for subgrade applications through stabilization using steel slag (SS) and waste glass powder (WGP). Laboratory experiments were conducted with glass powder and steel slag contents ranging from 5% to 20% at 5% increments. Waste glass powder, sourced from discarded drinking glass and construction waste, was finely ground and evaluated for its chemical composition and pozzolanic activity prior to use. Although steel slag is an effective stabilizing agent, its limited availability and high cost restrict its widespread application; therefore, a blended SS–WGP stabilization approach was examined as a more economical and sustainable alternative. Soil samples were collected from a road section at a depth of 1.5 m, and comprehensive laboratory tests were performed to determine the physical and mechanical properties of both untreated and treated soils. According to the AASHTO classification system, the natural soil was classified as A-7-5, while the Unified Soil Classification System (USCS) categorized it as CH. The results demonstrated that increasing SS and WGP contents significantly enhanced the maximum dry density (MDD), California Bearing Ratio (CBR), and unconfined compressive strength (UCS), while reducing the liquid limit (LL), plasticity index (PI), free swell index (FSI), optimum moisture content (OMC), California Bearing ratio (CBR), CBR swell Unconfined compressive Strength (UCS). Specifically, the PI decreased from 39.52% to 8.40%, FSI from 102.7% to 35.5%, and OMC from 25.82% to 21.05%. Conversely, MDD increased from 1.385 g/cm³ to 1.65 g/cm³, CBR improved from 2.26% to 9.35%, CBR swell drops 9.74% 1.58% and UCS rose from 24.26 kPa to 125.38 kPa at the optimum blend of 20% SS and 15% WGP. Overall, the combined use of steel slag and waste glass powder proved to be an effective and sustainable method for enhancing the engineering performance of expansive soils for subgrade applications.