There is no specific engineering standard or guideline formally nominated in the Project Design Criteria. Consequently, the earthwork design has been developed based on drawing upon past project experience, recognized industry best practices, and, where relevant, established guidelines published by the Ministry of Railways, Government of India. These reference documents have been selectively applied to enhance the performance and reliability of the proposed design.

The key design and performance criteria for embankments, as agreed upon with the stakeholders, are summarized in Table 1 . These criteria align with the relevant guidelines issued by the Ministry of Railways, Government of India, ensuring consistency with national best practices and facilitating long-term performance and durability of the earthwork structures.

The Historical seismicity data for Bangladesh and its adjoining regions indicate that the country is seismically vulnerable. This vulnerability stems from the tectonic interaction between the northward-moving Indian Plate and the Eurasian Plate, resulting in active geological deformation across the region. Over the past 100 years, more than 1000 earthquakes with a magnitude (M) ≥ 4.0 have been recorded within Bangladesh and nearby areas, highlighting the region’s significant seismic activity. According to the Earthquake Zoning Map provided in the Bangladesh National Building Code (BNBC-1993), study project is situated within Seismic Zone 2 ( Figure 1 ), indicating a moderate level of seismic risk that must be considered in the engineering design and structural detailing of project components.

It is widely recognized in both international and national geotechnical engineering practice that flexible structures—such as earth embankments, soil nail walls, and mechanically stabilized earth (MSE) systems—are capable of accommodating lateral displacements without significant structural distress. Consequently, the horizontal seismic design coefficient for such structures is typically taken as half of the peak horizontal ground acceleration (PGA), in accordance with established guidelines such as those from the Federal Highway Administration (FHWA) and Australian Standard AS 5100.2.

Based on this principle, and considering the PGA value of 0.17 g for the project site (as per BNBC-1993), the adopted horizontal seismic design coefficient for embankment stability analysis is:

k_h = 0.5 × 0.17 g = 0.085 g (1)

This reduced coefficient reflects the deformable nature of the embankment structure while ensuring compliance with recognized seismic design approaches for geotechnical systems.

The earthwork design presented in this report has been developed based on borehole logs and laboratory test results available at the time of design. The subsoils encountered along the proposed alignment have been classified primarily on the basis of mechanical particle-size grading, index properties, and Standard Penetration Test (SPT-N) values. The mechanical grading and plasticity characteristics of the subsurface materials have been derived from laboratory testing, and the material descriptions are consistent with those presented in the borehole logs. Sub-soil investigations were conducted through 378 boreholes, with depths ranging from 16 m to 35.5 m along the alignment. The predominant strata encountered comprise Clayey Silt, Silty Clay, and Sand, with compressible layers typically varying between 5 m and 10 m in thickness. A series of laboratory tests was carried out, including moisture content, specific gravity, grain size distribution, hydrometer analysis, Atterberg limits, and relative density. In addition, strength and stiffness characteristics were evaluated through Unconfined Compression (UC), Consolidated Drained Direct Shear (DS), Unconsolidated Undrained Triaxial (UU), Consolidated Undrained Triaxial (CU), and One-Dimensional Consolidation tests. While the classification of soils based on mechanical grading and index testing may appear elementary, these tests capture the key geotechnical properties that govern soil behavior in practical engineering contexts. It is acknowledged that the mineralogical, chemical, and geological origin of soils can influence their mechanical response. However, as supported by the foundational work of Schofield and Wroth (1968), these effects are generally reflected adequately in index test results, which are widely accepted as reliable indicators of engineering behavior in design practice. Based on the one-dimensional consolidation tests, the design values of C_v, C_ce, and C_re are obtained. Parameters estimated using empirical correlations with average initial void ratio (e₀), moisture content (MC), and Atterberg limits (liquid limit, LL, and plasticity index, PI) are also investigated and compared for the setting of adopted design values. Secondary compression/creep consists of the residual primary consolidation settlement together with the subsequent creep settlement. The modified secondary compression index adopted in this project is based on the average value recommended by Mesri and Godlewski (1977), i.e., C_αε (NC) = 0.03 × C_cε. To comply with the design performance criteria, long-term creep settlement was mitigated through the application of an additional surcharge load to the foundation soils. Geotechnical design parameters are provided in Table 2 in accordance with NAVFAC [33] .

Groundwater has been observed at or near the existing ground surface across the project site. This condition is primarily attributed to the extensive network of canals and water bodies in the vicinity, which exerts a significant influence on the local groundwater regime. Given this observation, and in the absence of data indicating sustained seasonal variation, the groundwater level is conservatively assumed to coincide with the existing ground level for the purpose of the embankment design. This approach ensures that the design remains robust under potentially saturated conditions, particularly in stability and settlement assessments.

The following nomenclature ( Figure 2 ) has been adopted throughout this report to describe the key components, parameters, and features associated with the embankment design over soft ground conditions:

The assessment of embankment-induced settlement has been conducted using established one-dimensional consolidation theory, with the choice of method tailored to the soil type and its stress-strain response characteristics. Two analytical approaches are considered, based on the relationship between the final effective stress (σ′_f) and the pre-consolidation pressure (σ′_p):

1) m_v Method (Elastic Compression Method)

This approach is applicable to:

Settlement is estimated using:

This method assumes minimal plastic deformation and is typically used in over-consolidated soils or conditions where applied loads remain within the elastic range of the soil structure.

2) C_cε-C_rε-OCR Method (Elasto-Plastic Compression Method)

This approach is suitable for:

Settlement is evaluated using:

This method accounts for pre-consolidation effects and is generally applied in soft compressible soils where significant plastic deformation is expected under the applied loading.

The global stability of the approach embankments has been evaluated under both temporary and permanent loading conditions, considering the various stages of construction and anticipated soil strength development. The stability assessments address the following critical scenarios:

To ensure compliance with the required factors of safety (FoS), reinforcement measures such as high-strength geotextiles and/or stabilizing berms may be employed. In cases where higher tensile capacity is required, multiple geotextile layers can be incorporated into the design.

The global stability analysis was performed using a limit equilibrium approach, implemented via SLOPE/W (Version 2007), a commercial geotechnical analysis software. The analysis involves:

The minimum calculated Factor of Safety for each design condition must meet or exceed the values stipulated in the Project Design Criteria, ensuring both short-term and long-term embankment stability.

The ultimate tensile capacity of the geotextile reinforcement is determined as twice the working (allowable) load, in accordance with conventional geotechnical design practice. Material properties of embankment are shown in Table 3 .

Liquefaction is the sudden, temporary loss of shear strength in saturated, loose to medium dense, granular sediments subjected to ground shaking. Liquefaction generally occurs when seismically induced ground shaking causes pore water pressure to increase to a point equal to the overburden pressure. Liquefaction can cause foundation failure of buildings and other facilities due to the reduction of foundation bearing strength. The potential for liquefaction depends on the duration and intensity of earthquake shaking, particle size distribution of the soil, density of the soil, and elevation of the groundwater. Areas at risk due to the effects of liquefaction are typified by a high groundwater table and underlying loose to medium-dense, granular sediments, particularly younger alluvium and artificial fill. Due to presence of loose to very loose sand along the project alignment and high-water level, liquefaction potential for ground under seismic loads has been evaluated. The liquefaction potential assessments presented in the factual report on Geotechnical Investigation works were adopted. The permanent ground deformation due to liquefaction was assessed based on Hynes-Griffin and Franklin (1984). In this method, the yield acceleration is first estimated, then the permanent displacement of the embankment is calculated.

General embankment fill refers to the material layer positioned beneath the prepared sub-grade. It is classified as an open specification material, meaning it is not rigidly defined but should be locally sourced and possess adequate strength and stability. The material must be capable of being compacted to meet the required density and California Bearing Ratio (CBR) standards. Importantly, it must be free from any unsuitable materials outlined in Section 4.5.4, which are strictly prohibited from use in embankment construction. Material properties of separator geotextile and high strength geotextile are shown in Table 4 and Table 5 .

Drainage is a critical factor in maintaining the stability of embankments and cuttings in railway construction. Proper management of rainwater, particularly during the monsoon season, is essential to protect the embankment from potential failure. Water must not be allowed to flow along the track, as this can lead to ballast contamination and erosion of the formation. Prolonged water stagnation on the formation is also undesirable. Therefore, the drainage system must be designed to efficiently prevent water accumulation and ensure rapid water discharge. The sub-ballast layer is constructed with a transverse slope to facilitate the runoff of surface water toward the sides and down the embankment. Longitudinal drains are installed at the toe of the embankment to channel the water toward the nearest natural watercourses. Typical cross-sections for embankment are shown from Figures 3-5 .

Exposed sloping surfaces are vulnerable to surficial erosion caused by surface runoff, which can lead to the formation of erosion gullies. These gullies may compromise the soil matrix, reduce the excess width, and result in the steepening of slopes. The severity of erosion depends on several factors, including soil type, climatic conditions, topography, and slope length. To mitigate such erosion, two categories of control measures are typically considered:

Engineered Systems

Among various engineered solutions, the commonly employed methods include geojute, polymer grids, and hydroseeding. This section focuses on geojute, which is recognized as the most cost-effective and economical option. Geojute, a biodegradable material made from jute yarn with a coarse, open-mesh structure, is especially suited for areas experiencing significant erosion. As it degrades, geojute promotes the establishment of vegetation, enhancing slope stability. When applied during the dry season, initial watering should be ensured to support seed germination and plant growth. Typical cross-sections for embankment considering dredge sand are shown in Figures 6-8 .

Bio-Technical Solutions

This method involves establishing vegetation on exposed slopes, making it particularly suitable for soil types containing a clay fraction. The approach includes grading the slope to prepare it for sowing seeds or planting root strips of locally available creeping grasses. These grasses typically develop roots penetrating 50 - 75 mm into the slope, acting as natural soil anchors and significantly enhancing resistance to erosion. For existing embankments, turfing and planting have been adopted. Following discussions with Bangladesh Railway (BR) officials, and based on the observed absence of significant erosion on current embankments, this method has been selected. Turfing and planting will be incorporated into the design drawings accordingly.

Materials classified as unsuitable shall not be used in the construction of embankment fill. Unsuitable materials include topsoil, peat, and other highly organic soils; logs, stumps, and combustible materials; as well as soluble substances such as gypsum and rock salt. Additionally, expansive soils, free-draining materials prone to scouring, very fine sands, cohesionless silts, organic clays, and highly dispersive soils are also deemed inappropriate. Any material exhibiting a free swell index greater than 3%, soluble content exceeding 3%, or organic matter exceeding 5% by weight of dry soil is considered unsuitable for embankment construction. Furthermore, collapsible soils present in the foundation are classified as unsuitable and must be appropriately treated prior to embankment placement. Typical cross-sections for cutting section are shown in Figure 9 and Figure 10 .

The presence of a compressible silty clay layer at the embankment foundation along the project alignment necessitates ground improvement measures to mitigate post-construction settlement and to ensure slope stability both during and after construction. The proposed ground improvement methods include surcharge loading (preloading), with the use of prefabricated vertical drains (PVDs) to expedite the consolidation of soft clay layers. Two distinct types of ground treatment have been proposed, as detailed below.

Type A ground treatment is designed for areas where embankments are to be constructed over weak, compressible layers of soft to firm clay or silty clay, which are underlain by stiff clay or medium-dense sand. These untreated compressible soils are inadequate for supporting embankments and would result in excessive settlement.

This treatment involves the installation of Prefabricated Vertical Drains (PVDs) arranged in a triangular grid at 2.0-meter center-to-center spacing, extending to a depth of up to 16 meters. A geotextile-wrapped sand drainage blanket will be placed at the base of the embankment to collect groundwater expelled during soil consolidation under surcharge loading. The collected water will be directed to low points and removed via designated drains.

Type A ground treatment is designed to achieve a minimum of 95% primary consolidation through preloading before the railway tracks are opened to traffic. The rate of embankment fill placement will be controlled to allow for gradual strength gain in the compressible subsoil. To ensure embankment stability during construction, additional stabilization measures, including high-strength geotextile reinforcement and temporary berms, have been implemented.

Type B ground treatment is applicable in areas where:

In this case, the use of Prefabricated Vertical Drains (PVDs) is not required. However, a drainage blanket enclosed within a separator geotextile will be provided to ensure effective drainage and facilitate the discharge of groundwater generated during the consolidation process. Material properties of PVD are shown in Table 6 .

Representative geotechnical models for each design section were developed based on the following considerations:

Cutting Conditions: Due to the limited subsurface data available for cut sections, two generalized ground profiles were assumed for design purposes: 1) firm clay or medium dense sand, and 2) stiff clay or dense sand.