Consolidation of lumpy clay backfill over buried pipelines

Offshore pipelines are usually buried to protect the pipe from external loads. When trenching is achieved by jetting or ploughing, some clayey soils can be cut into distinct lumps and this lumpy soil is then used as the backfill material under which the pipe is buried. To counter the effects of upheaval buckling, the resistance of the soil to pipe uplift must be known. There is still uncertainty about the performance of lumpy backfill in this regard. A series of centrifuge tests were performed with such soils as backfill, utilising a specially designed pore-pressure measuring pipe, to determine the influence of lump size, lump shape and pullout rate on uplift resistance Backfill comprising larger lumps consolidates quicker than if the backfill lumps are smaller. It is also observed that backfill comprising larger lumps provides greater resistance to pipe uplift after consolidation.