Preliminary Investigation of Railway Embankment Settlement Control for the Chabahar–Zahedan–Mashhad Line in the Seasonal Mohammadabad-e Alam Wetland

The passage of the Chabahar–Zahedan–Mashhad railway project through the seasonal Mohammadabad-e Alam wetland, located north of Qaen, may cause irreversible hazards due to saturation of the embankment bed. Therefore, this study aims to estimate the railway embankment settlement under dry and saturated wetland conditions, propose effective settlement-control measures, and perform a technical and economic comparison of the selected methods. Initially, various techniques for reducing settlement were briefly reviewed, among which replacement, chemical stabilization with lime, and deep mixing were identified as feasible alternatives. Subsequently, a site investigation was carried out; hand-dug pits were excavated at critical locations to collect both disturbed and undisturbed samples, field tests were performed in these pits, and two borrow-material sources were identified and sampled. Laboratory tests were then conducted to determine the physical and mechanical properties of the wetland bed, borrow materials, and improved soils. Numerical modeling using the finite element method was employed to estimate the embankment settlement under different conditions, followed by technical and economic evaluations of the proposed approaches. The numerical results revealed that settlement under dry condition falls within acceptable limit, whereas the saturated condition exceeds this threshold. Both replacement and lime-stabilization reduced settlement to allowable level, whereas the deep mixing was technically ineffective. Although lime-stabilization proved technically superior, replacing the unsuitable wetland soil with suitable borrow material was found to be the more economically viable option according to cost estimates. • Applicable techniques for settlement reduction were determined for the critical zone of Section No. 21 of the Chabahar–Zahedan–Mashhad railway line (seasonal Mohammadabad-e Alam wetland). • Geotechnical characteristics of the wetland bed were identified through field investigations and laboratory testing. • Embankment settlements under dry and saturated conditions were computed using finite element numerical modeling and compared with the allowable limit. • Settlement reduction methods were numerically analyzed and technically assessed. • Selected improvement techniques were compared in terms of implementation cost.