Temporary Works

Vine Street, London, UK

Project Brief

LSY Engineering Consultants Ltd was appointed by Cadent Gas Ltd to undertake structural surveys and structural appraisal checks for an existing road bridge near Farringdon Station, London.

The project formed part of a gas network upgrade programme and involved the installation of two new medium-pressure cast iron (CI MP) pipes beneath the existing bridge structure. The works included abandoning an existing 48-inch cast iron main and replacing it with a 36-inch pipe incorporating a 630mm polyethylene (PE) medium-pressure gas main.

Our role included assessing the existing bridge structures, evaluating the impact of the proposed additional loads, and undertaking post-construction inspections to verify that the completed works had not adversely affected the structural integrity of the bridges.

Challenges

The project presented several logistical and engineering challenges due to its location within one of London’s busiest transport corridors.

The existing road bridge spanned two separate operational railway assets:

A Transport for London (TfL) Underground line.
A TfL Overground line.

Access to both structures was heavily restricted and could only be undertaken during scheduled night-time rail possessions. This required careful planning and close coordination with multiple stakeholders to ensure inspections could be completed safely within limited access windows.

Given the operational nature of the railway environment, survey activities needed to be meticulously planned and coordinated with rail operators, contractors, and project stakeholders. The limited access periods made it essential that all survey requirements, inspection methodology, and structural assessment objectives were clearly defined in advance.

Process

Prior to undertaking the site inspections, LSY carried out a detailed review of the available desktop information, including archive drawings, previous inspection reports, and existing structural records.

This information proved invaluable in developing an understanding of the bridge structures before accessing the site and enabled us to prepare targeted inspection plans for the available possession periods.

The desktop review confirmed that:

One bridge comprised a steel girder structure spanning the Underground line.
The adjacent bridge comprised a masonry arch structure spanning the Overground railway.
Historical records contained valuable information regarding member sizes, construction details, and material properties.

Using this information, LSY developed preliminary structural appraisal calculations to assess the effects of the proposed gas main replacement works and the associated additional loading imposed on the existing structures.

During the night-time inspections, our engineers undertook detailed visual surveys to verify the information obtained during the desktop study and assess the condition of the existing bridge elements.

The site inspections confirmed that the archive information accurately reflected the existing structures, allowing us to validate our assessment assumptions and complete the structural appraisal with confidence.

A comprehensive structural report was subsequently produced, providing observations, conclusions, and recommendations to Cadent prior to commencement of the construction works.

Following completion of the gas main installation, LSY carried out two further night-time inspections during planned rail possessions. These post-construction surveys were undertaken to verify that the completed works had not resulted in any adverse structural effects on either bridge or that both structures remained in satisfactory condition following the installation works.

Outcome

The project was successfully delivered within the planned construction programme and during the scheduled railway possession periods.

The structural assessments provided Cadent with confidence that the existing bridge structures could safely accommodate the revised gas main arrangement and associated loading requirements.

Post-construction inspections confirmed that the completed works had not adversely affected either the steel girder bridge or the masonry arch bridge, allowing the client to proceed with confidence that the upgraded infrastructure had been installed safely and successfully. The successful delivery of the project strengthened LSY’s relationship with Cadent Gas Ltd and led to our continued involvement in similar bridge, pipeline, and infrastructure assessment projects across the UK.

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