Optimization of waste glass, slag, and seashell usage in lime-based hybrid dsm grouts: an experimental and statistical approach

This study investigates the improvement of a low-plasticity silty soil (ML) sourced from the Gölveren region of Kastamonu province, Türkiye, using lime-based hybrid binders partially replaced with glass powder (GP), blast furnace slag (BFS), and seashell powder (SSP) within the Deep Soil Mixing (DSM) framework. The experimental program was designed using the Taguchi L18 orthogonal array method to determine the effect of control parameters and optimum mixture ratios. The unconfined compressive strengths (UCS) of the specimens were measured at 7, 28, 56, and 90 days; in addition, 25- and 50-cycle air-environment freeze–thaw (F–T) durability tests were conducted to simulate the performance of DSM columns under extreme conditions. A 20% binder ratio outperformed 30%: at fixed w/b = 1.2, the higher ultrafine powder dosage raised wetting water demand, leaving insufficient free water for dispersion and causing agglomeration. Maximum UCS values were attained at the end of the 90-day curing period, coinciding with the completion of pozzolanic reactions. In the freeze–thaw tests, the specimens largely preserved their structural integrity after 25 cycles; however, significant strength reductions were observed after 50 cycles, which may be associated with progressive microcracking and weakening of the cementitious bonding network. Analysis of Variance (ANOVA) results demonstrated that the binder ratio and BFS were the most dominant factors in the system. Furthermore, the Random Forest (RF) machine learning model, validated via leave-one-out cross-validation, achieved a robust predictive accuracy of R² = 0.711 and outperformed the linear baseline (Multiple Linear Regression, R² = 0.610), confirming its reliability for hybrid DSM grout design. Microstructural analyses confirmed the strength development and freeze–thaw damage through morphological changes at the micro-scale. This study demonstrates that the hybrid use of industrial and natural wastes offers a high-performance alternative for sustainable ground improvement projects.