10.1. SR 01-22-2013 - Laserfiche WebLink

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STANDARD RECOMMENDATIONS FOR UTILITY 'FRENCH BACKFYLLING GENERAL Clayey and silty soils are often diff icuit to compact.as they may be naturally wet or may become wet due to ground water or surface/rain water during construction.Soils wilt need to be placed within a certain range of water{moisture)content to attain desired compaction levels. Moisture conditioning to within this range can be time consuming,labor intensive; and requires favorable weather. The degree ofcompaction and the soil type used for backfiII within open cut utility trenches depends on the function of the overlying land surface. Details are as follows: ROADWAYS Where trenches are located below roadways.we recommend using inorganic fill and compacting these soils per Mn/DOT Specification2105.3F] (Specified Density Method).This specification requires 100%of the Standard Proctordensity in the upper one meter subgrade zone,and 95% below this.Note that this specification includes moisture content range requirements which are important for proper subgrade stability. Where available soils are wet or of poor quality,it may be possible to use the"Quality Compaction Method"(Mn/DOT Specification 2105.3172) for soils below the upper one teeter subgrade zone if you can tolerate some subsidence. However,a liigh level of stability is still important within the upper subgrade zone and recommend that the"Specified Density Method" be used in this upper subgrade area. We caution that if backfill soils in the lower trench area are significantly unstable, it may be difficult or even impossible to property compact soils within the upper one meter subgrade zone.In this case.placing a geotextile fabric directly over the unstable soils can aid in offsettina the instability. STRUCTURAL AREAS If fill is placed beneath or within the significant zone of influence of a structure(typically a 1:1 lateral oversize zone),the soil type and minimum compaction level will need to be evaluated on an individual basis. Because trenches result in variable fill depths over a short lateral distance,higher than normal compaction levels and/or more favorable(sandy)soil fill types maybe needed. If this situation exists,it is important that special geotechnical engineering review be performed. NON-STRUCTURAL AREAS In grass/ditch areas,backfill soils should be placed in reasonable lift thicknesses and compacted to a minimum of90%of the Standard Proctor density(ASTM:D698)and/or per the Mn/DDT"Quality Compaction Method."If lower compaction levels are attained, more noticeable subsidence at the surface can occur. Steep or high slopes require special consideration. O1 REPO 19(02/01) AMERICAN ENGINEERING TESTING, INC.