Use of Native Soil in Place of Aggregate in Controlled Low Strength Material (CLSM)

Controlled Low Strength Material (CLSM) is a mixture of fine aggregate, cement, water, and additives such as flyash that is used primarily as bedding and pipe zone fill for pipelines as an alternative to compacted granular fill. The benefits of CLSM are widely recognized as it can provide more effective support to the underside of rigid and flexible pipe, resulting in better quality with less inspection while producing a non-settling fill. Additional benefits of CLSM include: the capability for placement in wet trench conditions, reduced need for trench dams, reduced excavation costs by allowing narrower trenches, and improved worker safety due to reduced exposure. CLSM can be made with commercial aggregate or native soils and can be batched either on-site or off-site. This paper presents four case histories in Northern California where native soils were used exclusively to create CLSM for pipeline backfill. Two projects included trench-side batching of the CLSM, and two projects included central on-site batching of the CLSM with transport to the site in concrete mix trucks. Either granular or fine-grained native soils were used in the CLSM, with one project using lean and fat clay soils almost exclusively. Primary soil constituents, batching methods, and resulting CLSM compressive strength data are presented and discussed for each project. Quality control considerations associated with the use of trench-side material are also discussed.