Earth-Retaining Structure Solutions

Earth-retaining structures form a critical part of below-ground construction, providing stability to excavations and resisting the lateral pressures exerted by surrounding soils and groundwater. They are designed to maintain changes in ground elevation, prevent soil collapse, control erosion, and protect adjacent infrastructure.  Common forms of earth-retaining structures include reinforced concrete (RC) retaining walls, contiguous and secant piled walls, sheet pile systems, basement walls, and diaphragm walls.

These structures play a vital role in civil engineering projects, supporting roads, buildings, bridges, embankments, and underground facilities whilst ensuring the stability and safety of both temporary and permanent works.  However, their performance depends not only on sound structural design, but also on the effectiveness of the waterproofing strategy employed to protect both the retaining structures themselves, and in many cases, the spaces they enclose.

As earth-retaining structures can be continuously exposed to moisture, groundwater, and hydrostatic pressure, the selection and installation of an appropriate waterproofing system is essential.  Effective waterproofing manages or prevents water ingress, protects internal environments, mitigates deterioration caused by moisture-related infiltration, and enhances the long-term durability and service life of the structure.  Consequently, the waterproofing strategy is a key consideration in the design, construction, and maintenance of earth-retaining structures.

When designing waterproofing solutions for earth-retaining structures, key considerations include:

• Soil type(s), permeability and stability, site geology, latent contamination, ground gases, flood risk & likely water table characteristics
• Performance level, client expectations and end-use of internal environment
• Selection of Type A barrier protection, Type B integrally water-resisting structures, or Type C drained protection systems, or combined waterproofing protection derived from these systems when used together to lower risk (see below)
• Structural movement and settlement
• Construction and movement joints
• Service penetrations
• Accessibility for future maintenance and repairs
• Compatibility of waterproofing systems and components
• Long-term durability and life-cycle performance

BS 8102:2022, Protection of Below Ground Structures Against Water Ingress, provides recommendations and guidance on methods for managing or preventing the entry of water from external sources into structures that are wholly or partially below ground level. The standard identifies three principal forms of waterproofing protection:

• Type A (Barrier Protection): Waterproofing barrier systems applied to the external or internal surfaces of the structure to resist water ingress.
• Type B (Structurally Integral Protection): Reinforced concrete, steel sheet piles or other structurally integral watertight construction methods designed to limit water penetration through the structure itself.
• Type C (Drained Protection): Drained cavity construction that manages and controls any water entering the structure.  Water collected within the drained cavity system is directed to a basement drainage system and conveyed to a suitable point of discharge, such as a gravity drainage connection, surface water drainage system, soakaway, or sump and pump arrangement, where appropriate (all subject to appropriate risk assessment).

BS 8102:2022 recommends that the choice of waterproofing strategy should be based on a thorough assessment of the site conditions, groundwater regime, intended use of the structure, and the required internal environmental grade.

A successful solution often incorporates a combination of waterproofing measures.  By adopting a coordinated approach to design, detailing, and installation, risks posed by potential water ingress can be fully understood and addressed, ensuring the structure performs as intended throughout its service life.

Combination Protection

As stated, there are a number of waterproofing options and systems available for below-ground structures. The selection should be based on the specific site characteristics, which can be assessed by suitably qualified experts adhering to BS 5930:2015 Code of practice for ground investigations and other site investigation protocols, and disseminated to the waterproofing designer in the form of Phase 1 and Phase 2 site investigation and monitoring reports which define and quantify actual risk.

It is important to note that most structural warranty providers require a minimum of two forms of waterproofing protection to be incorporated within the design of habitable or high-value below-ground spaces. This approach provides greater resilience against water ingress and eliminates the inevitable risk inherent with reliance on a single waterproofing system, because BS 8102:2022 states that no single waterproofing system can be expected to be defect free when factoring building site conditions, accidental damage, potential for less than adequate workmanship, practical difficulties associated with testing during construction etc.

The combined protection system should comprise a combination of not less than two of Type A (Barrier Protection), Type B (Structurally Integral Protection), and Type C (Drained Protection) in accordance with BS 8102:2022.   However, the two forms of protection selected should be of different waterproofing types and performance characteristics; simply providing two systems of the same type (e.g. two Type A barrier protection systems used in combination) is generally not considered to constitute combined protection. Examples of acceptable combinations include:

• Type A + Type B
• Type A + Type C
• Type B + Type C

In higher-risk situations, or where the consequences of water ingress are particularly severe, the use of all three waterproofing types may be appropriate to provide enhanced protection and long-term performance, and indeed, these are becoming more common.

Collaboration

Early collaboration between the architect, structural engineer, waterproofing designer, specialist manufacturer, main contractor, specialist installing contractor and other key stakeholders is essential to develop a robust and practical waterproofing strategy for any earth-retaining structure. This integrated approach helps identify potential risks at an early stage, improve buildability, and deliver a durable, integrated waterproofing solution.

Assumptions

It should always be assumed that groundwater, or water deriving from other sources has the potential to bear against an earth-retaining structure at some point during its design life.

Consequently, waterproofing systems should be designed and installed to the full height of the external ground level, or higher (depending upon surface water flood risk) irrespective of the groundwater conditions observed at the time of investigation or construction.

Particular attention should be given to the continuity of the waterproofing system at the interface with the damp-proof course (DPC), which is typically located a minimum of 150 mm above external finished ground level.  The waterproofing protection should be detailed to provide an uninterrupted barrier between the below-ground and above-ground elements of the structure.  This continuity of the below ground waterproofing system to min. 150mm above external ground level, or higher may be achieved in various ways, by continuity of Type B water resisting construction to above external ground level, by continuity of the internal Type C drained protection system to above external ground level, by installing continuous Type A barrier protection products within the wall cavity or by dressing the waterproofing externally to the appropriate height and terminating it with an appropriate flashing/cavity tray interface to maintain the integrity of the moisture protection system.

How to Coordinate Waterproofing Design

Effective waterproofing design requires early and ongoing coordination between all members of the project team. Waterproofing should not be considered in isolation; it must be integrated with the structural, architectural, geotechnical, and building services designs to ensure a comprehensive and robust solution.

The most successful waterproofing strategies are developed at the earliest stages of a project, allowing potential risks and interface issues to be identified and addressed before construction begins.

Key Principles for Coordinating Waterproofing Design

Structural elements must be designed with waterproofing requirements in mind. Particular attention should be given to:

• Construction and movement joints
• The integral water-resisting characteristics of various forms of construction
• Appropriate substrate preparation
• Potential for structural settlement, differential movement and consequential cracking
• Thermal contraction or expansion
• Wall and slab junctures and interfaces
• Service penetrations

The waterproofing designer and structural engineer should work together to ensure compatibility between the waterproofing strategy and the structural design.

Service penetrations are among the most common points of waterproofing failure. M&E designers should coordinate closely with the waterproofing team to:

• Minimise the numbers of service penetrations below ground
• Locate penetrations above ground level where possible
• Use appropriate sealing and detailing systems
• Ensure details comply with manufacturer recommendations

The waterproofing system must remain continuous throughout the structure. Particular attention should be given to:

• Wall-to-floor junctions
• Changes in level
• Lift pits
• Service entries
• Internal and external thresholds
• Connections to DPCs and other peripheral waterproofing and dampness control elements

Any interruption or discontinuity in the waterproofing protection can create a potential route for water ingress.

Where multiple waterproofing products or systems are used, compatibility must be assessed at the design stage and verified/checked on site.  Key products could include membranes, water stops, sealants, drainage components, coatings, and ancillary materials.  Coordination with manufacturers is often essential to ensure long-term performance.

Clear drawings, sequencing considerations, written specifications and installation guidance reduce the risk of site errors.

Regular communication between the designer, contractor, installer, and manufacturer is essential.  Site inspections and quality control reviews should be undertaken to ensure the waterproofing system is being installed in accordance with the design intent and manufacturer’s requirements.

The Importance of Collaboration

Successful waterproofing design is achieved through collaboration. By coordinating the waterproofing design with structural, architectural, geotechnical, and building services disciplines from the outset, potential defects can be designed out, installation risks reduced, and the long-term performance of the structure enhanced.

A well-coordinated waterproofing strategy not only protects the building from water ingress but also minimises future maintenance costs, disruptive remedial works, and the risk of structural deterioration.

Expert Support You Can Depend On

At Delta Membrane Systems Limited, we are committed to excellence through collaboration in design, specification, and manufacturing.  We provide comprehensive design support backed by Professional Indemnity (PI) Insurance, giving clients confidence that every waterproofing solution is developed with performance, compliance, and durability in mind.

Our waterproofing and protection systems are installed by a nationwide network of Delta Registered Installers, with all installations fully guaranteed to ensure long-term peace of mind.

Whatever the requirements of your project, Delta’s experienced Technical Team is on hand to provide the advice, support, and practical guidance you need, exactly when you need it. From initial design considerations and product selection through to installation support and remedial solutions, we work closely with clients to help achieve successful outcomes at every stage of the project.

For expert waterproofing advice and technical support, call 01992 523 523 or email info@deltamembranes.com.