M621 Smools Lane Underpass, located approximately 4.4 km south of Leeds, is an in-situ reinforced concrete box structure constructed in 1972 to provide pedestrian access beneath the M621 motorway.
The structure comprises four independent concrete box units with cantilevered headwalls and wingwalls, with joints dowelled and sealed using water bars. As part of ongoing asset management by National Highways, the existing parapet systems, originally installed in 1972 and modified in 1981, required replacement to meet current standards.
Concrete Repairs Ltd (CRL), acting as Principal Contractor, was appointed to deliver enabling works and structural concrete repairs to facilitate the safe installation of compliant parapet and vehicle restraint systems.
Services Undertaken
- Replacement with new N2 W2 vehicle parapets
- Installation of P4 start terminal and P1 end terminals
- Installation of N2 W1 and N2 W2 restraint systems
- Controlled hydro-demolition of south parapet plinth and 1981 extension
- Construction of new in-situ reinforced concrete plinth
- Installation of reinforcement tied into existing structure and parapet post anchor assemblies
- Localised structural concrete repairs to north parapet coping
- Installation of hot rolled asphalt surface course to hard shoulder and new edge line and kerb
- Coordination with parapet and VRS installation teams
CRL adopted a structured and technically informed approach to deliver the works within a live motorway environment, where safety, coordination, and programme control were critical. Early-stage trial holes, commencing in September 2023, were undertaken to confirm the existing construction details and identify the presence and location of buried services. This information was essential in developing a safe and efficient methodology for the subsequent structural repairs and parapet replacement works.
The scheme required close interface management between structural concrete repairs, parapet installation, and vehicle restraint system upgrades. A phased breakout and repair sequence was implemented to maintain stability of the structure and ensure continuity of operations. Controlled hydro-demolition techniques were used to remove the existing south parapet plinth, including the 1981 extension across the deck and wingwalls. This method allowed precise removal of defective material while minimising damage to the surrounding concrete.
Following breakout, a new in-situ reinforced concrete plinth was constructed. Reinforcement was carefully installed and tied into the existing structure to ensure composite action and long-term durability. New parapet post anchor assemblies were incorporated into the plinth construction to meet current design and safety standards. Localised structural concrete repairs were also undertaken to the north parapet coping, addressing deterioration and preparing suitable substrates for new installations.
The works included the replacement of existing parapets with new N2 W2 vehicle parapet systems and the installation of compliant road restraint systems across all approaches and departures. Variations in embankment geometry and existing road sign constraints required the use of both N2 W1 and N2 W2 systems in specific locations. Additional works included the installation of a new hot rolled asphalt surface course to the hard shoulder, along with new edge lines and kerbing.
CRL’s expertise in concrete repairs and structural repairs UK ensured that all works were delivered efficiently and in full compliance with National Highways standards. Their ability to integrate complex repair methodologies with parapet and VRS installation highlights a strong capability in delivering coordinated infrastructure solutions within constrained environments.
The construction of a new reinforced concrete plinth, combined with targeted repairs to the parapet coping, provides a durable and structurally sound foundation for the new parapet and vehicle restraint systems. The use of controlled hydro-demolition ensures that only defective concrete was removed, preserving the integrity of the remaining structure and enhancing long-term performance.
By installing modern N2-rated parapets and restraint systems, the scheme significantly improves safety and compliance with current standards. The integration of reinforcement tied into the existing structure ensures effective load transfer and resistance to future deterioration. New asphalt surfacing and edge detailing further protect the deck and improve water management at the interface.
These measures reduce the need for ongoing maintenance while enhancing resilience and extending the service life of the structure. CRL’s methodical and technically robust approach supports long-term asset management objectives, ensuring that the underpass continues to perform effectively within a demanding motorway environment
