6.2. SR 01-13-2003 - Laserfiche WebLink

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STS Subgrade Stabilization Guideline Subgrade stabilization may be required if zones of unsuitable soil are encountered upon excavating to the subgrade level or if subgrade degradation occurs from construction traffic, moisture accumulations, freeze-thaw cycles or other causes. Care should always be used to minimize disturbance and degradation of subgrade soils below foundations, slabs-on-grade, pavements and fill areas. Water should not be allowed to pond on the surface of exposed subgrade soils, as this could cause a softening of the subgrade, particularly when subjected to construction traffic. Detrimental groundwater seepage should not be allowed to soften or loosen the subgrade. Unsuitable subgrade soils that are encountered or subgrade soils that become disturbed or softened after exposure should be improved prior to concrete or new material placement. The unsuitable soils should either be properly compacted in place (if feasible based on material type, moisture content and thickness), or over- excavations should extend through the unsuitable soils to remove them to an underlying competent soil stratum. If improvement by over-excavating is performed, footing walls can be extended deeper and supported at the level where suitable soil is encountered. Alternatively, the over- excavations can be backfilled to the design level using either a suitable compacted structural fill material or a flowable cementitious fill. If the over-excavations are backfilled using structural soil fill, the over-excavations should extend a minimum of 1 foot horizontally from each edge of the footing for each foot of fill required below the footing base. The structural soil fill should be placed, compacted and tested in accordance with a separate document entitled STS Earthwork Guideline. Generally, a well-graded granular material is more suitable for stabilization work than cohesive soils. If an open-graded granular material is planned as the backfill and the new subgrade or surrounding soils contain zones of cohesionless fine sands or silts which may migrate into the open-graded backfill, then an appropriately designed geotextile should be utilized to separate the stabilization material from the subgrade and surrounding trench soils. Failure to provide such separation may cause lost ground from surrounding soils and detrimental settlements. Horizontal over-excavation is unnecessary if footing walls are extended to the lower suitable subgrade level or if flowable fill is used to backfill the over-excavated area. FIowable fill should have a sufficient Portland cement and/or fly ash content to achieve 28 day unconfined compressive strengths in the range of 50 to 200 pounds per square inch (psi).