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MOISTURE CONTROL OF FILL The contractor should be required to adjust the moisture content of the soil to within a narrow range near the optimum moisture content(as deflned by the applicable Proctor or AASHTO Test). In general,fill should be placed within about 2%of optimum.The need for moisture control is more critical as the percentage of fines increases.Naturally-occurring clayey sand or cohesive/semi-cohesive soil are often much wetter than the optimum. Placing and attempting to compact such soils to the specified density may be difficult,or not possible.Even if compacted to the specified density, � excessively wet soils may not be suitable as fioor slab or pavement subgrades due to pumping under applied load.This is especially true when wet cohesive/semi-cohesive soil is used as backfiil in utility trenches under streets.Excessively wet soil in thick fill sections may cause post-construction settlement beyond that estimated for fill placed at or near (±2%) the optimum moisture content. An exception to this would be low permeability soil placed as a pond liner or for a dam. Such soil should usualiy be placed at 2%to 4%above the opti,mum moisture content, to provide for a lower insitu permeability.Also, shrinking/ swelling soiis(expansive clay)should be placed at about 2%to 4%above optimum moisture to reduce the possibility of soil expansion. Clayey silt, silt, or very sifty fine sand should be placed excessively dry. Such soils can undergo post-construction consolidation upon being wetted,even if the specified density had been achieved.This is caused by the collapse of flocculant soil particle arrangement,and can result in settlement of buildings or slabs constructed over the soiL Proper control of fill soil moisture is the responsibility of the excavator. The excavator should evaluate the need for wetting or drying the soils,based either on the data in the soil report,or his own site testing. If the excavator is bringing in off-site fill,it is also his responsibility to evaluate the moisture content of the soil,and the need for wetting or drying. We recommend that this matter be addressed in the project specifications. CONSTRUCTION ON COMPACTED SOIL After the select fill has been placed, compacted,and tested, it must be maintained and protected in order to properly support structures. The suitability of compacted fill soil can be greatly diminished if it is altowed to freeze, become saturated while unconfined(such as in footing excavations or at the surface of slab/placement subgrade),or disturbed by construction equipment. The responsibility for protecting the soil,or for correcting any disturbance,should be clearly defined in the specifica- tions. Soils which become wet and soft after compaction testing do not necessarily reflect inaccurate field density tests. Especially with non-expansive cohesive/semi-cohesive soils, saturation when unconfined can severely reduce the shear sirength while the density remains adequate.The reduced shear strength can cause footings,floor slabs,or pavements to settle or fail under load. We strongly recommend that all pavement subgrade be test rolled (MN/DOT Specification 2111) immediately before paving to determine if the subgrade has not been protected and soft spots have developed. FLOOR SLAB SUBGRADE AND UTILITY TRENCHES This facet of constructfon presents special problems,especially if the slab subgrade is allowed to freeze.When the soil thaws, it undergoes a period of temporarily lower shear strength. Floor slabs should not be cast over soil in such a weakened or frozen condition (reference pertinent PCA and ACI publications). To do so can result in cracked and failing slabs. The time period to heat and thaw a building may place the construction schedule and/or costs in jeopardy.We strongly recommend that this matter be reviewed in pre-bid and pre-construction meetings. Backfilling of utility trenches in the floor slab subgrade can be difficult. If the soil is wet,compaction to the specified density may be dtfficult, or not possible. The narrowly cut trenches may preclude the use of proper compaction equipment. With the use of small equipment in confined areas, the contractor must place the soil In thin lifts (4 to 6 , inches), with the soil at the proper moisture content.This work is typically carried out by contractors other than the mass grading or earthwork contractor.We strongly recommend that the responsibility to carry out the compaction be clearly detailed in the applicable section of the specifications, and reviewed with the appropriate contractor and subcontractor. GME CONSULTANTS,INC. -