6.2. SR 01-13-2003 - Laserfiche WebLink

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STS Subgrade Protection Guideline Care should be exercised to minimize disturbance and degradation of subgrade soils for foundations, slabs-on-grade, pavements and areas to be filled. 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. Disturbed or softened subgrade soils should be removed to a suitable undisturbed subgrade prior to fill or concrete placement. Wet subgrade conditions may result from precipitation, runoff and groundwater seepage through excavation walls and bottom. Precipitation risk can be minimized by scheduling construction for drier seasons. The subgrade should be sloped to drainage ditches and sumps to minimize water accumulations. Runoff from adjacent areas should be eliminated by use of berms and ditches to channel water away. Groundwater seepage may be minimized by use of dewatering systems such as wells and/or groundwater isolation systems such as cutoff walls or trenches. Dewatering wells and/or groundwater isolation systems are recommended where upward seepage is likely to cause the subgrade to loosen and become "quick" or where lateral seepage may erode the face soil or cause "piping" of fines from the soil matrix as exhibited by muddy or silt laden water. If moisture or disturbance sensitive subgrade soils and wet conditions are expected and construction of faci',ities bearing on the subgrade will not promptly protect the subgrade soils, then consideration should be given to protecting the subgrade by promptly placing appropriate combinations of a geotextile, a gravel base course and a lean concrete mud mat over the prepared and approved subgrade. Geotextiles should be considered for use to separate the subgrade and gravel where subgrade soils are at risk of migrating into the gravel base course. A suitably designed gravel base course should help surcharge the subgrade and act as a drainage layer for removing water accumulations. A lean concrete or flowable fill mud mat with a thickness of several inches or more may be placed directly on the subgrade if upward seepage does not exist. If base drainage is needed, a lean concrete or flowable fill mud mat may be placed over a gravel base course. A mud mat will help to isolate water, provide surcharge against loosening and will provide a stable surface which is resistant to disturbance from construction traffic. Sump and pump systems or dewatering wells should be used to remove any accumulating water or water pressure in the gravel base course. In any areas where unsuitable conditions develop despite protection measures, subgrade stabilization should be performed as described in a separate sheet entitled "STS Subgrade Stabilization Guideline".