250m² of Vibro Replacement stone column densification to a treatment depth of 19m

Catalyst Paper has proposed constructing a new Turbine Generator / Condenser at their Pulp Mill in Powell River, BC. The new facilities will be supported by a mat foundation over densified ground to a depth of 19 m. This phase of the project involved a ground improvement treatment area of approximately 250m² beneath the Condenser foundation.

The proposed site for the new Condenser is located within an operating industrial environment at the heart of the Powell River Pulp Mill. The site is surrounded by numerous existing mill structures, tanks, services and underground utilities. Test holes indicate the site below surface is underlain by about 8 to 10 m of granular fill overlying sand and gravel deposits. The fill comprises sand, gravel and cobbles which are generally compact, except for a 3 to 4 m thick surface layer which is dense to very dense. The underlying native sands and gravels are generally compact. Both the fill and native soils were found to contain cobbles and boulders. Initial soil conditions were not adequate to satisfy seismic design criteria in terms of foundation deformations.

The project demanded a ground improvement method capable of reaching the desired treatment depth of 19 m within site’s difficult soil conditions. After considering various pile and ground improvement options, it was decided ground improvement by Vibro Replacement stone columns was the most suitable foundation solution. Menard Canada’s German built V-23 electric Vibroflots are generally considered the most capable and effective Vibro equipment in the industry making them well suited to the challenging initial penetration conditions encountered at this site. The ground improvement program was developed to meet a stringent performance criteria as measured by the Becker Penetration Test (BPT). Stone columns were installed over a uniform triangular grid pattern covering the extent of the treatment area. While penetration did encounter refusal prior to the 19 m target depth at many locations, the average depth achieved was 13 m and the BPT testing within the treatment depths demonstrated the ground improvement program achieved the specified performance criteria with average BPT blow counts greater than 70 blows/ft.