1,900m² of Vibro Replacement stone column densification to a maximum treatment depth of 11.5m

The Elle is a new high-rise residential development located in downtown Richmond, BC. The new development includes 18 stories of above grade residential space and a surrounding 3 storey mixed-use development.

In general, the native soil conditions comprised the following:

• Surface layer of imported mineral fill with a consistent thickness in the range of 0.5m to 0.9m.
• Overbank sediments of soft to firm silt underlying the fill with a thickness in the range of 3.0m to 4.2m.
• Underlain by a compact to dense, fine to medium grained sand layer with interbedded silt layers extending to an approximate depth of 17m.

Seismic analysis indicated that the underlying soil layers were susceptible to liquefaction induced strain softening and subject to large cyclic strain deformation during the design earthquake. Therefore, soil densification was required to limit the seismically induced settlement to post earthquake serviceability requirements. Bottom Feed Vibro Replacement was selected as the most suitable ground improvement option for mitigation of the liquefaction risk.

Geopac was awarded the stone column installation work utilizing bottom-feed methods to a maximum depth of 11.5m. Beneath the tower foundations, additional ‘secondary’ stone columns were installed to 6.5m depth at centroid locations of the primary equilateral stone column grid to strengthen the upper silt layer.

The small and narrow downtown site was bounded busy roads, pedestrian walkways, overhead powerlines and a low rise structure immediately adjacent to the densification limits. Special safety procedures were developed and followed when operating bottom-feed Vibro equipment next to these boundary constraints.

Quality assurance CPT testing showed that the ground improvement methods satisfied the design performance requirements.