An Integrated Soil Assessment Framework for Preliminary Subgrade Characterization

Abstract

Reliable identification of unfavorable subgrade conditions is essential for supporting geotechnical site characterization and infrastructure planning. Conventional investigations commonly evaluate physical, mechanical, or classification parameters separately, whereas engineering behaviour is controlled by the interaction among multiple soil characteristics. This study proposes an Integrated Soil Assessment Framework that combines physical properties, engineering classification, mechanical response, and mineralogical information into a unified interpretation of subgrade soil characteristics. The framework was applied to an industrial development area in East Java, Indonesia, using laboratory and field investigations, including natural water content, grain-size distribution, Atterberg limits, hydraulic conductivity, California Bearing Ratio (CBR), sand cone testing, Unified Soil Classification System (USCS), AASHTO classification, and X-ray diffraction (XRD) analysis. The integrated interpretation identified sampling locations exhibiting weak, moderate, and strong indications of unfavorable subgrade characteristics based on the consistency among multiple engineering indicators. Although all investigated locations satisfied the minimum CBR and compaction requirements, several sampling locations consistently exhibited relatively high plasticity, low hydraulic conductivity, unfavorable AASHTO classifications, and the occurrence of montmorillonite, indicating that satisfactory mechanical performance alone does not necessarily represent favorable subgrade conditions. Spatial interpretation further revealed that these indications occur locally rather than uniformly across the study area, supporting the importance of incorporating spatial variability into preliminary site characterization. The proposed framework provides a practical and transferable approach for integrating complementary geotechnical datasets to improve engineering interpretation and support preliminary geotechnical investigations in heterogeneous soil environments.