M4 Windsor Bridge trial holes

M4 junction 3 Windsor Bridge trial holes
Balfour Beatty Vinci JV
Nov 2017
Jul 2018

We carried out investigative works on the Windsor Bridge structure for Balfour Beatty Vinci joint venture and Highways England, as part of the M4 J3 – J12 smart motorway upgrade project.

The project consisted of a series of deep trial holes on the east and west bridge piers and abutments.


  • Management and installation of temporary works
  • Excavations 3m to 5m deep using a vacuum excavator
  • Surveys of the existing bridge piers in both the highways and rail environment
  • Street works and thoroughfare reinstatement
  • Track monitoring activities within the Network Rail boundary

Windsor Bridge trial holesThe trial holes confirmed the size of foundations beneath the existing ground level, and the location and profile of sheet piles and anchors near the abutments and piers next to the rail tracks. They also confirmed the location of SSE oil-filled cables.

We photographed each trial hole to record the complexity of the utility and structure interaction, and we recorded the setting out points of all utilities we found.

The trial holes were used to determine the location, depth and other key dimensions of all existing underbridge foundations where widening is proposed. They were located next to the existing M4 carriageway and the Network Rail track, so most work was carried out at night. The project required our expertise in motorways and mainline rail civil engineering.

VGC is also providing skilled trades and labour to BBV JV for the smart motorway project.

About the project

The M4 motorway carries around 130,000 vehicles per day between London and west England and Wales. Upgrading the section between junction 3 at Hayes and junction 12 at Theale to a smart motorway will add capacity through an additional traffic lane and more technology to smooth traffic flow.

As part of the project:

  • Eleven bridges carrying local traffic over the motorway will need to be replaced to give room for the extra lane.
  • Four motorway bridges over roads, railways and rivers will need to be widened, including the bridge over the Thames at Bray, and the railway line at Windsor near junction 6.
  • Two subways under the motorway will be lengthened.

Where possible, the new bridges will be built next to the existing ones, before the old ones are demolished. However, where there is not enough space for this, some bridges will be demolished first, then a new bridge built in the same place.

The M4 junctions 3 to 12 smart motorway scheme will be the longest smart motorway project in England to date (51km, 32 miles). Main works on  will start in autumn 2018 between junctions 8/9 and 10.

For more information see the Highways England website.