Installation of Underground Retaining Walls

The underground retaining walls that were constructed onsite during the project were essential for protecting surrounding structures and services during the remediation, as well as allowing the excavation of contaminated materials.

The walls:

  • Resisted the pressures of the earth, groundwater and surcharge (the vertical load from an excavator at the ground’s surface) during excavations;
  • Supported nearby sensitive structures by limiting ground movement during excavations in accordance with advice from a suitably qualified structural engineer; and
  • Supported loads from the footings of the environmental control enclosure along parts of the walls.

The retaining walls were constructed as close as possible to the site boundaries to allow as much contaminated material to be removed as possible. They are located in the western part of the site, adjacent to sections of the northern, south-eastern and western boundaries. They descend from the surface of the site to depths of up to 8 metres and will remain in place.

Specialised consultants designed the retaining walls based on geotechnical data, the planned excavation stages of the remediation, and the specified movement criteria of nearby sensitive structures. The retaining walls were constructed using a track-mounted drill rig which bored holes to the required depth. Once the rig’s auger was at the correct depth, concrete was pumped through its centre to the bottom of the hole while the auger was extracted, creating a concrete column. When the auger was fully withdrawn, a steel reinforcing cage was pushed into the wet concrete to make the column strong enough to resist pressure from the earth, groundwater and surcharge during excavations. The concrete columns, known as ‘piles’, were installed adjacent to each other to create the continuous retaining walls.

Ground movement was monitored during the installation of the retaining walls. A structural engineer specified the limits for ground movement including for railway infrastructure and assets, and for residences along the east side of Burren St.