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Retaining walls play a critical role in modern construction, helping to manage elevation changes, prevent soil erosion and create stable land formations by transforming steep terrains into usable spaces.

The concept of reinforced soil structures has existed for centuries, with ancient civilisations using stone and timber to stabilise slopes and build terraces. However, modern segmental retaining walls (SRWs) emerged in the late 20th century when engineers developed high-strength concrete blocks with interlocking features.

The introduction of geosynthetic reinforcements, such as geogrids, in the 1980s significantly advanced SRW technology, allowing for the construction of taller, more robust structures capable of withstanding increased loads. Over the years, SRWs have evolved with improvements in material strength, design methodologies and environmental considerations, making them a go-to solution for engineers and specifiers seeking efficiency and long-term durability.

This module is designed to support architects, housebuilders, engineers, local authorities and specifiers in selecting the correct retaining wall for their projects. Readers will gain a comprehensive understanding of SRWs, including their benefits, limitations, construction methods and relevant technical certifications to ensure informed decision-making in real-world applications.

Learning objectives

• Understand the principles of retaining wall design, including key factors such as site conditions, loading requirements and material selection.
• Gain insight into the benefits and applications of SRWs for residential developments, including their construction methodology, aesthetic versatility and structural performance.
• Knowledge of best practice for planning, design and installation, ensuring compliance with relevant standards.

Types of retaining walls

Choosing the right retaining wall is essential to ensuring stability, longevity and cost-effectiveness. The best solution depends on factors such as soil conditions, site constraints, loading requirements and overall project objectives. Below is an overview of the primary types of retaining walls, their construction methods, benefits and key considerations.

Gravity retaining walls

Gravity retaining walls rely on their substantial mass to counteract the lateral earth pressures the retained soil exerts. Constructed from materials such as stone, concrete or heavy masonry units, these walls are often designed with a slight backwards lean, known as a batter, to enhance stability.

They are particularly effective for lower-height applications due to their inherent stability and straightforward construction methods. However, as wall height increases, the base width must proportionally expand to maintain stability, which can consume valuable space and materials.

Reinforced soil retaining walls

Reinforced soil retaining walls integrate the wall’s mass with the reinforced soil’s strength to resist lateral earth pressures. This is achieved by embedding reinforcing elements, such as geogrids or geotextiles, into the soil behind the wall face, creating a composite mass that acts cohesively. Geogrid is a high-strength geosynthetic material that extends into the backfill.

This method allows for the construction of taller walls capable of supporting greater loads than traditional gravity walls. The flexibility in design and material selection makes them suitable for a wide range of applications, from highway embankments to commercial developments. However, installing reinforcement layers requires meticulous planning and execution to ensure structural integrity.

Cantilever retaining walls

Cantilever retaining walls are constructed from reinforced concrete and feature an internal stem connected to a base slab, forming an inverted T or L shape. This design efficiently transfers lateral earth pressures into vertical forces on the foundation. Cantilever walls are advantageous when space is limited, as they occupy less area than gravity walls. They are suitable for medium-height applications, typically up to 5m. However, they require precise engineering and construction practices, including substantial concrete footings below frost depth, to ensure durability and performance.

Embedded retaining walls

Embedded retaining walls, such as sheet pile walls or bored pile walls, are driven or cast deep into the ground to retain soil. They are particularly effective in soft soil conditions or tight spaces where traditional wall types are impractical. Sheet pile walls, made from materials such as steel or vinyl, are driven into the ground and are ideal for temporary or permanent structures requiring minimal space. Bored pile walls involve drilling and casting concrete piles in situ, providing robust support in challenging soil conditions. However, both types often necessitate specialised equipment and expertise, potentially increasing project costs.

Segmental retaining walls

SRWs are a subset of gravity retaining walls that use modular, interlocking concrete units to retain soil. These dry-stacked units rely on their mass and interlocking mechanisms for stability, making them a flexible and adaptable solution for various site conditions. They can function as gravity walls for lower heights or be reinforced with geosynthetics to construct taller structures, integrating principles from reinforced soil retaining walls.

The modular nature of SRWs allows for aesthetic versatility, with a range of colours, textures and finishes to complement project designs. They are commonly used in residential landscaping, commercial developments and infrastructure projects. Their dry-stack, mortar-free construction enables rapid installation, even in varying weather conditions, with skilled crews achieving approximately 40m2-50m2 daily.

Proper design and installation are crucial to ensure long-term performance, particularly in addressing drainage and soil characteristics. Compliance with standards such as BS EN 771-3:2011, which specifies requirements for aggregate concrete masonry units, is essential to ensure quality and durability.

In the subsequent section, we will delve deeper into the considerations of SRWs, providing a comprehensive understanding of the specification process at each design stage.

Concept design

In the concept design phase, it is essential to grasp the fundamentals of SRWs, assess preliminary costs and determine their applicability to specific projects. Although uncommon, engaging with manufacturers at this early stage can provide valuable insights into retention options, ensuring informed decisions as the project progresses.

While material costs for SRWs may appear higher initially than alternatives, the expedited construction timeline and reduced labour requirements often result in overall cost savings. Additionally, the potential to reuse on-site materials as backfill can further enhance economic efficiency. However, challenges such as market unfamiliarity can lead to design hesitancy or reluctance from clients and contractors. Moreover, SRWs often require specialised design input early on, necessitating collaboration with structural engineers to develop project-specific solutions.

Understanding these aspects during the concept design phase ensures that SRWs are appropriately considered and aligned with project goals and site-specific conditions.

Developed design

The developed design stage refines the technical specification and aesthetic details of SRWs. This phase ensures the solution integrates seamlessly with the project’s overall design, meeting structural and visual requirements.

Reviewing standard details and existing installations can provide a clearer understanding of how SRWs will perform in practice. Previous case studies can offer valuable insights into construction methods, material choices and design refinements that improve efficiency on site.

Detailed decisions regarding colour, texture and layout come into focus at this stage. While functionality remains the priority, the visual impact of retaining walls should not be overlooked – particularly in residential and public spaces.

For larger retaining walls, banding or patterning can break up the visual mass and enhance the design. Depending on the project’s intent, the use of colour variation can help the wall blend into the landscape or create a striking architectural feature.

With more detailed design work in progress, several factors need to be addressed at this stage:

• Co-ordination with other site elements – the retaining wall must integrate with adjacent structures, pathways, drainage systems and soft landscaping features.
• Structural and geotechnical input – where walls exceed certain heights or retain significant loads, early engagement with engineers helps refine the design to meet performance requirements.
• Construction detailing – the buildability of the chosen system should be carefully reviewed, including base preparation, drainage provisions and any reinforcement requirements.

By thoroughly addressing these elements at the developed design stage, project teams can refine their approach to ensure that the selected SRW system meets practical and aesthetic objectives.

Technical design

At the technical design stage, the focus shifts to the detailed engineering and construction methodology required to bring SRWs from concept to reality. This involves refining structural considerations, ensuring compliance with industry standards and co-ordinating the retaining wall design with other site elements such as steps, fencing and landscaping.

To ensure structural integrity, engineering consultants with expertise in retaining wall design can develop site-specific preliminary designs. These initial designs help determine the most appropriate construction method and are often sufficient for planning applications.

All SRWs should be manufactured in accordance with BS EN 771-3:2011 – Specification for masonry units, designed in accordance with BS 8006-1:2010+A1:2016 – Code of practice for strengthened/reinforced soils and other fills, and designed following BS EN 1997-1:2004 Eurocode 7: Geotechnical design (A1:2013). Ensuring adherence to these standards provides confidence in the system’s performance.

Geogrid reinforcement is the preferred approach for SRWs that are more than 1m in height or require additional stability. Key factors influencing the design of a geogrid-reinforced wall include wall height, soil conditions and the anticipated surcharge. The geogrid layers are placed between block courses at regular intervals, typically every four courses, and must be installed in the correct orientation to ensure optimal strength.

A concrete backfill approach can be used instead of geogrid reinforcement for sites where excavation space is limited or where minimising ground disturbance is critical. This involves filling the void behind the blocks with a no-fines concrete mix (concrete that doesn’t contain fine aggregates such as sand or crushed stone), forming a monolithic structure that resists lateral pressures.

A no-fines concrete backfill wall typically requires excavation of 40% of the overall wall height, compared with 70% for a geogrid-reinforced wall. This makes it a preferred choice in boundary-constrained sites or areas where excavation and soil removal would be costly.

In challenging environments where traditional retaining methods may not be feasible – such as steep slopes, restricted boundaries, or areas with existing structures – geopins and soil nail stabilisation systems can provide additional support. These systems involve the insertion of small-diameter pins or nails into the retained soil, reinforcing the ground and preventing movement.

Integrating these advanced solutions into the design allows SRWs to be successfully implemented even in complex site conditions.

Other considerations

It is essential to consider various factors that could influence the final implementa