A3 Cockrow – Post-Tensioned Bridge Demolition

Demolition of a lightweight post-tensioned bridge over the A3 as part of a broader scheme of infrastructure improvements.

Client: National Highways

Principal Contractor: Balfour Beatty

Demolition Contractor: TRU Demolition (formerly Clarke Demolition Company)

Project Overview

The bridge was a lightweight post-tensioned bridge formed of a hollow box with post-tensioned tendons on either side.

The demolition of this bridge was part of a three-year and £317 million Wisley interchange improvement works around the A3/M3 junctions.

Due to the bridge’s proximity to Wisley footbridge, the bridge was demolished simultaneously to minimise the impact of the road possession.

Post-Tensioned Demolition Considerations

As this was a post-tensioned bridge, verifying that the tendons were grouted before selecting the demolition methodology was necessary. A PTSI report was used to verify that the tendons were grouted, avoiding the need for additional site investigation.

Demolition Methodology

Due to the structural form of the bridge, the demolition methodology was slightly different from the standard process of using a muncher to reduce the weight of the structure in the air.

Instead, some of the weight was taken out of the structure, and then the muncher was used to create a series of hinge points. We then used the machines onsite to collapse the span in a controlled manner onto the road protection. Larger piles of fill were placed in the planned spot on the deck to protect the road surface.

The reason for this change was that the position of the tendons made it difficult to predict when the structure would fail. So, the methodology was designed to control when and how it would fail to ensure it happened in a controlled and safe manner.