Grouting Beneath a Main Output Transformer at the Pickering Nuclear Generating Station

A grouting program has been carried out beneath a main output transformer at Ontario Power Generation (OPG) Pickering B Nuclear Generating Station (Pickering B NGS) Unit 8 Main Output Transformer (MOT 8). The objectives of the program were to fill voids and compact loose zones below the MOT foundation in order to reduce the risk of further unpredictable, and possibly large, settlements due to the collapse of voids. The construction period extended from April 5 and to May 24, 2006, during which 79 grout holes were drilled into the soil beneath the MOT. The program was divided into two phases: Primary, which addressed most of the voids and loose fill zones and Secondary, which filled remaining small voids and loose zones between primary treatment areas. The grout consisted of a mixture of cement, bentonite, water, and other admixtures. The total volume of grout introduced into the soils below the MOT was approximately 29.2 m³, representing approximately 3 per cent of the volume of granular fill below the treatment area. Approximately 93 per cent of this volume was introduced during the primary phase. The amount of grout introduced in the program is significant and is consistent with the theory that granular backfill has been lost from beneath the MOT through an ongoing process of erosion. It is estimated that about 100 mm of potential settlement was arrested by the grouting treatment, based on the total volume of grout injected and gross area of treatment. During construction, cumulative settlement of approximately 3.4 mm occurred on the south side of the MOT slab, with cumulative uplift of about 3.6 mm on the north side. Since the completion of construction, the settlement trend has been generally flat. Based on the measurements of grout volume injected, the significant difference in the performance of the ground between primary and secondary grouting, and the small amount of settlement that occurred during treatment, the grouting program successfully met its objectives.