Stabilization of Clayey Soils with Eucalyptus Ash from Artisanal Kilns: An Experimental Evaluation in San Agustín (Huancayo)

In the Andean region, clay subgrades with low bearing capacity are common; in San Agustín (Huancayo, Peru), artisanal brick kilns generate eucalyptus ash that is currently discarded. This study evaluates eucalyptus ash from artisanal brick kilns (CEPLA) as a stabilizer for a local clay, quantifying its effects on physical properties, compaction, bearing capacity, and shear strength. Soil–ash mixtures were prepared at 0, 10, 20, and 30% by dry mass and tested for grain-size distribution (sieve analysis), Atterberg limits, modified Proctor—reporting maximum dry density (MDD) and optimum moisture content (OMC)—California Bearing Ratio (CBR) with 96 h soaking and expansion, and consolidated drained direct shear (DST, CD) to obtain cohesion (c′) and friction angle (φ′). The natural soil was classified as CL under the USCS, with CBR = 3.16% and expansion = 4.8% after soaking; CEPLA was granular with non-plastic fines. Adding ash decreased MDD and increased OMC without changing the USCS class. The 20% mix performed best, with CBR = 9.13% (≈3× the natural value) and expansion = 1.6%, alongside increases in c′ (0.100 → 0.169 kgf/cm²) and φ′ (25.97° → 29.90°). DST served as a mechanical characterization complementary to CBR. The findings provide experimental evidence to valorize a local waste as a soil stabilizer and suggest a preliminary dosage of 20% for low-plasticity clay subgrades in Andean settings. Limitations include the short-term laboratory scope, possible kiln-to-kiln compositional variability, and the lack of chemical characterization, wet–dry durability tests, and field-scale validation. Practically, the CBR gain and swelling reduction are compatible with low-volume roads using conventional compaction adjustments, while the use of CEPLA supports circular-economy objectives, reduces waste, and may lower costs relative to commercial binders.