Numerical Analysis Of Unsuitable Tropical Red Subgrade Soils For Assessing Undercut Depth: A Case Study Of Merewa-Seka Road Project

Abstract

Unsuitable tropical red subgrade soils form in situ by the intense weathering of parent material in tropical and sub-tropical climatic environments. They cover a wider area of the Western part of Ethiopia, including the project under study. Due to the absence of an adequate classification system, they are well known to be misunderstood and are deemed unsuitable, considering only index properties without assessing the engineering properties of the soils. This leads to a conservative treatment approach with an undercut depth suggested for expansive soils in their application for flexible pavements. The aim of this study is to investigate the geotechnical properties and deformation characteristics of unsuitable tropical red subgrade soils under vehicular loading. After modeling the subgrade soils, the researcher examined the degree of weakness present in these soils and determined the appropriate undercut depth using numerical solutions in the PLAXIS 2D software. In this study, three different categories (1, 2, and 3) of subgrade were selected, and disturbed and undisturbed samples collected and investigated using field and laboratory test programs. The test programs included determination of moisture content, Atterberg limits, specific gravity, grain size distribution, moisture-density relation, California bearing ratio, dynamic cone penetration, triaxial shear strength, consolidation parameters, free swell index, swelling pressure, geochemical properties, and X-ray diffraction analysis. The numerical analysis program used cyclic (dynamic) pulse loading on top of flexible pavement and the Hardening-Soil Model with Small-Stiffness Material Model for simulating the behavior of unsuitable subgrade soils. The subgrade soils were classified as fine grained that are inactive and have a non-expansive nature. The mineral analysis indicated the presence of dominant minerals like quartz, chloritoid, heulandite, and kaolin, and chemical analysis revealed that the soils are true laterites. In addition, category-1 subgrade soil was found to have adequate bearing capacity and strength that could be used for subgrade without treatment (having a soaked CBR value of 10.2%, a cohesion value of 78.3 kPa, and a friction angle value of 17.11o ), while categories 2 and 3 were found to be inadequate, requiring treatment (having a soaked CBR value of 7.2% and 2.5%; a cohesion value of 40.71 kPa and 32.64 kPa; and a friction angle value of 12.11o and 10.56o , respectively). Moreover, from in-situ testing, DCP values were determined to be 23 mm/blow, 25 mm/blow, and 49 mm/blow for categories 1, 2, and 3, respectively. The numerical deformation analysis revealed that 60 cm and 100 cm undercut depth treatments are adequate for categories 2 and 3, respectively. The effectiveness of undercut treatment in relation to deformation resulting from numerical simulation and the comparison of this study’s findings with the current practice in Ethiopia are also presented in this paper. Furthermore, conducting additional geotechnical investigations and numerical analysis on a large number of unsuitable tropical red subgrade soils is recommended in order to obtain reliable and comprehensive information on how they behave.