The project required a seismic upgrade of the bridge abutment to prevent settlement or collapse of the abutments in an earthquake. The scope of work involved the installation of 105 x 10-12″ grout columns and 12 seismic drains.
The solution was to densify the liquefiable ground below the abutments to prevent a catastrophic failure in the event of an earthquake.
Compaction grouting was selected by Klohn Crippen Berger Ltd, the Geotechnical Engineers, as the preferred method of densification. Compaction grouting is a ground improvement technique that improves the strength and/or stiffness of the ground by slow and controlled injection of a low mobility grout. The soil displaced by the grout mass expands and densifies the surrounding soil mass.
Southwest Contracting Ltd, drilled 105 injection boreholes in a sequenced array, around the toe of the abutment to a depth of 40’. A low mobility grout was introduced through drill pipes, at pressure, in a bottom to top method. When the grout bulb reached critical mass, the pipe was raised 300mm and the injection process repeated. This creates a lineal column of grout bulbs from the bottom of the borehole to the surface.
The soil between the grouted columns is compressed, creating a solid soil mass, which is less susceptible to liquifaction in an earthquake.
The project began in June of 2011 with a test section to determine if the column spacing and grout pressures significantly improved the surrounding soils, until September, when the last of the compaction grout columns was completed.
Seismic drains were installed at the toe of the abutment, between the abutment and the compaction grouted zone.