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Addressing Soft Soil Challenges: Embankment Design for UK Highway Connectivity

Designing an embankment on soft soils for a UK highway, emphasising factors such as construction cost, time, risk, sustainability, and social values.

What is an embankment?

An embankment is a structure typically built by compacting layers of soil, gravel, or other materials to create a raised bank or mound. It serves as support or containment for infrastructure such as roads, railways, and flood defences. Embankments are designed to withstand environmental stresses, maintain stability, and manage water drainage effectively,serving multiple functions  in various engineering projects and flood control systems.

Challenges for construction of embankment?

Constructing embankments presents a host of challenges, such as soft soils’ settlement and instability, selecting suitable materials, addressing environmental concerns, ensuring safety, managing timelines and budgets amidst logistical constraints. For addressing these multifaceted challenges requires a holistic approach that integrates engineering expertise, environmental considerations and innovative design ideas as per site requirements to address embankment construction challenges across countries.

To encourage such innovations among students, the Early Careers Challenge (ECC)  was launched in 2023. It is a competition launched to bridge the gap between industry and education, focusing on ground engineering, sustainability, and digital skills. Participants work in small teams, typically composed of early-career professionals and students, to solve a specific engineering challenge. Mentored by experts from relevant industries, the teams develop innovative solutions to real-world problems, often related to infrastructure development or environmental sustainability.  

In this article, we will analyse one such Early Careers Challenge project in which tasked teams were assigned with designing an embankment on soft soils for a UK highway, emphasising factors such as construction cost, time, risk, sustainability, and social values. This challenge aimed to bridge industry and education while integrating ground engineering, sustainability, and digital skills. Out of 150 applicants, 20 teams were selected for the challenge.

Guided by mentors from Mott MacDonald, a renowned engineering consultancy firm, these teams embarked on a journey to tackle a common geotechnical challenge: the design of an embankment on soft soils for a highway project in the UK. This challenge was not merely an academic exercise; rather, it simulated real-world scenarios, requiring participants to act as consultants to a local authority responsible for improving transportation infrastructure between towns A and B. The task encompassed various considerations, including construction feasibility, cost-effectiveness, risk management, environmental sustainability, and societal impact.

Design Challenge

The challenge centered around designing embankments and bridges for a new highway route, intended to provide a more direct connection between towns A and B. Situated to the south of town B, the proposed route crossed rivers and floodplains, necessitating the construction of new infrastructure. The envisioned highway, designed as a dual carriageway near the UK coast, required embankments reaching heights of up to 8m. Notably, the proposed path traversed a tidal floodplain characterized by soft alluvial soil conditions.l soil conditions, presenting unique challenges for stability and foundation support. Moreover, the route traversed a tidal floodplain, further complicating construction efforts and requiring meticulous planning to ensure resilience against natural forces and environmental impacts.

The winning team and design submission

The winning team for the Early Careers Challenge comprised of Archie Bunney from Systra, Susanna Pahl from the University of Oxford, Ahmed Saadeldein from Bachy Soletanche, Milagros Guerrero Espino from Aecom, and Maria Frantzeskou from Mott MacDonald. They were guided throughout the process by mentors from Mott MacDonald. 

The winning team’s submission is detailed below.

Developing a Geotechnical Ground Model

The initial step in any ground engineering endeavor involves crafting a conceptual ground model. In this instance, the primary focus lies on the embankments flanking the estuary. Despite having only one borehole as a starting point, insights were gleaned and extrapolated through geological assumptions drawn from Fookes et al. (2015), Lord et al. (2002), and professional experiences in chalk ground investigations.

Significantly, the presence of a burial ground and a coniferous forest indicated a zone of weathered chalk, as these features would not be sustainable in soft alluvial clay. It was concluded that assuming a flat, consistent layer of very soft ground extending 10 meters below the entire embankment would be overly conservative and inconsistent with the convex cross-sections typical of alluvial deposits, shaped by river dynamics.

The principal geotechnical concern associated with the proposed project centers on the settlement and deformation risks posed by soft alluvial clay, which could potentially jeopardize the integrity of the embankments. However, other geotechnical risks have been acknowledged within the conceptual site model and taken into account during the conceptual design phase.

Sketch of Design Concept

Considering Alternative Options

In initial discussions leading up to the final presentation, an important question was raised : Is the proposed new road the most sustainable solution?

Drawing from the PAS 2080 reduction hierarchy of avoid, reduce, or change, a range of alternative options were explored. Notably, the table in the bottom right corner illustrates the nearly two-minute variance in the fastest journey times across the four proposed routes.

Alternative Options

Avoid: No Change to Existing Infrastructure (3.1)

Drawing inspiration from the Welsh Government road building policy (Welsh Government, 2023), which refrains from funding projects solely aimed at increasing road capacity or negatively impacting ecological sites, this approach advocates for maintaining the status quo. Instead, we focus on enhancing public transport and incorporating segregated cycle and pedestrian lanes to alleviate congestion and reduce environmental impact.

Reduce: Dualling Existing Single Carriageway (3.2)

Expanding the existing single carriageway offers a viable solution to capacity constraints, mitigating congestion in a cost-effective and expedited manner. This approach, situated on suitable ground, circumvents environmentally sensitive areas such as wetlands and forests. Additionally, its elevated position reduces vulnerability to future sea level rise.

Change: Alternative Dual Carriageway Route (3.3)

Exploring a shorter, more direct route presents the opportunity for reduced travel times and expedited construction, thereby enhancing cost-effectiveness. Spanning a river rather than an estuary for the southern water crossing allows for the implementation of a smaller, less material-intensive culvert. Furthermore, this route sidesteps environmentally sensitive areas, minimizing ecological impact by avoiding wetlands and forests.

Developing and Evaluating Conceptual Designs

The process of developing an embankment on soft soils entails the exploration and analysis of three distinct concept designs, each scrutinized from technical, environmental, and economic perspectives. This thorough evaluation encompasses the investigation of various piling methods suitable for bridge foundations, the assessment of soil improvement techniques, and the consideration of sustainable materials such as expanded geofoam and shredded tire bales. 

Geotechnical Solutions for Three Concept Designs

Concept Design 1: Integrated CFA Piling and CMC Reinforced Embankment Approach

In this concept design, the foundation for the bridge integrates continuous flight auger (CFA) piling with a controlled modulus column (CMC) reinforced embankment. This combination is selected for its potential to enhance stiffness compatibility between the bridge and embankment, offering advantages such as reduced long-term maintenance requirements and a faster construction timeline. However, challenges arise concerning spoil disposal management due to the CFA and CMC methods, as well as relatively high CO2 emissions associated with cement content. The design proposes reinforced steep slopes with geogrids for embankment construction, initially relying on imported soil pending confirmation of the suitability of chalk as a site-won material.

Concept Design 2: Integrated Bored Displacement Piles and VSC Lightweight Embankment Approach

This concept design integrates bored displacement piles with vibro-stone columns (VSC) for soil improvement. Bored displacement piles offer benefits such as quicker installation, reduced costs, and sustainability compared to conventional CFA piles. VSC usage introduces fewer chemicals into the ground, utilizing recycled stones for added sustainability. Challenges include potential drainage pathway creation in contaminated areas and the need for expanded polystyrene geofoam to manage soil instability, leading to an increased embankment footprint.

Concept Design 2

Concept Design 3: Integrated DSM Techniques and Lightweight Embankment Approach

This design utilizes deep soil mixing (DSM) columns for the bridge and transition zone, and trench mixing for the lower embankment zone. DSM and trench mixing eliminate spoil and vibration, minimizing social impact, making it suitable for environmentally sensitive areas. Challenges include dealing with organic materials like peat, limited structural capacity due to reinforcement plunge-ability issues, and significant settlement periods for stabilization. Encased tyre bales are incorporated as a lightweight material for the embankment, enhancing sustainability by repurposing tyres and reducing waste, albeit requiring extra attention for long-term maintenance.

Considering Social and Visual Impacts

The scheme’s meticulous evaluation encompasses both social and visual impacts, drawing inspiration from organizations like Strong Towns and the Dutch government, which prioritize well-designed cities and efficient road infrastructure. This approach emphasizes the distinction between urban environments and transportation corridors: urban spaces are hubs of activity and human-scale interactions, while transportation corridors prioritize swift movement between locations. 

To optimize vehicle speeds and ensure safety for pedestrians and drivers, pedestrian crossings are minimized along the embankment, directing pedestrians to dedicated paths segregated from traffic.Separate levels for pedestrians and cyclists, along with spaced staircases and ramps, provide easy access to surrounding areas. Embankment walls, envisioned to be covered in locally sourced materials, blend seamlessly with the natural environment, offering pedestrians and cyclists scenic views free from vehicle fumes and accident risks. Narrowing lanes as the road approaches town centers fosters a smooth transition from transportation corridor to urban environment, prioritizing pedestrian-friendly spaces.

Moreover, to mitigate environmental impact, biofuels are recommended for construction machinery to minimize chemical leakage. The road design promotes active travel modes such as walking, cycling, and bus usage, optimizing speed and comfort. Implementation of air quality monitors throughout the embankment’s lifespan ensures the assessment of particulate pollution, safeguarding environmental sustainability and public health. This comprehensive approach not only prioritizes efficient transportation but also integrates social considerations and environmental sustainability, reflecting a holistic vision for the development of the embankment.

Sketch of Design Concept

Calculating Carbon Emissions

To assess the carbon impact of each concept design, a variety of carbon calculators were employed, including private tools like the Moata carbon portal by Mott MacDonald and Carbon Instinct by Aecom, as well as publicly available tools such as the National Highways Carbon Calculator and the Government Greenhouse Gas Reporting conversion factors. The carbon assessment encompassed the embankment core and the geotechnical solutions, with the EFFC/DFI carbon calculator used to determine CO2 emissions for the latter. Additionally, lightweight alternative materials for the embankment core, such as lightweight expanded clay aggregate (Leca) and expanded polystyrene (EPS) foam blocks, were evaluated separately. An integrated tool using a Power BI dashboard was created to compare carbon intensity among concept designs, with concept design 2 identified as having the lowest carbon intensity.

Comparing Construction and Maintenance Costs

Construction and maintenance costs were also factored into the evaluation process. Costs were assessed based on material prices and categorized into lowest, moderate, and highest cost categories. Concept design 2 emerged as the cheapest solution for construction and maintenance costs. However, it also had the longest construction and maintenance duration due to integrating multiple techniques and a lengthy settlement period. 

Decision-making Process

To decide on the recommended concept design, the Early Careers team weighted various criteria including carbon emissions, time, technical compliance, operation and maintenance, and environmental and social impact. Each criterion was rated from 1 to 3, with 3 representing the best solution, and the weighted score was calculated. Concept design 2 garnered the most points and was thus recommended for the proposed road. 

Conclusion

The winning team’s submission for the Early Careers Challenge showcased a comprehensive approach to designing an embankment on soft soils for a UK highway. Through careful consideration of geotechnical factors, alternative options, social and visual impacts, carbon emissions, and construction costs, the team arrived at concept design 2 as the recommended solution for the proposed route. However, they also highlighted the potential benefits of dualling the existing route as an even more favourable alternative. This holistic evaluation process exemplifies the importance of integrating technical expertise with sustainability principles to address complex infrastructure challenges effectively.

Source: geplus.co.uk

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