Subsurface Geotechnical Assessment-2008

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BITUMINOUS PAVEMENT SUBGRADE PREPARATION AND DESIGN GENERAL Bituminous pavements are considered layered`flexible" systems. Dynamic wheel loads transmit high local stresses through the bituminous/base onto the subgrade. Because of this,the upper portion of the subgrade requires height strength/stability to reduce deflection and fatigue of the bituminous/base system. The wheel load intensity dissipates through the subgrade such that the high level of soil stability is usually not needed below about 2'to 4' (depending on the anticipated traffic and underlying soil conditions). This is the primary reason for specifying a higher level of compaction within the upper subgrade zone versus the lower portion. Moderate compaction is usually desired below the upper critical zone,primarily to avoid settlements/sags of the roadway. However,if the soils present below the upper 3' subgrade zone are unstable,attempts to properly compact the upper 3'zone to the 100%level may be difficult or not possible. Therefore, control of moisture just below the 3'level may be needed to provide a non-yielding base upon which to compact the upper subgrade soils. Long-term pavement performance is dependent on the soil subgrade drainage and frost characteristic. Poor to moderate draining soils tend to be susceptible to frost heave and subsequent weakening upon thaw. This condition can result in irregular frost movements and"popouts," as well as an accelerated softening of the subgrade. Frost problems become more pronounced when the subgrade is layered with soils of varying permeability. In this situation.the free-draining soils provide a pathway and reservoir for water infiltration which exaggerates the movements. The placement of a well drained sand subbase layer as the top of subgrade can minimize trapped water. smooth frost movements and significantly reduce subgrade softening. In wet, layered and/or poor drainage situations.the long-tenni performance gain should be significant. If a sand subbase is placed,we recommend it be a"Select granular Borrow"which meets Mn/DOT specification 3149.2B. PREPARATION Subgrade preparation should include stripping surficial vegetation and organic soils. Where the exposed soils are within the upper"critical" subgrade zone(generally 2 '/2 deep for"auto only" areas and 3'deep for"heavy duty" areas).they should be evaluated for stability. Excavation equipment may make such areas obvious due to deflection and rutting patterns. Final evaluation of soils within the critical subgrade zone should be conducted by test rolling with heavy rubber-tired construction equipment,such as a loaded dump truck. Soils which rut or deflect 1 or more under the test roll should be corrected by either subcutting and replacement; or by scarification, drying. and recompaction. Reworked soils and new fill should be compacted per the"Specified Density Method" outlined in Mn/DOT Specification 2105.3171. Subgrade preparation scheduling can be an important consideration. Fall and Spring seasons usually have unfavorable weather for soil drying. Stabilizing non-sand subgrades during these seasons may be difficult,and attempts often result in compromising the pavement quality. Where construction scheduling requires subgrade preparation during these times.the use of a sand subbase becomes even more beneficial for constructability reasons. SUBGRADE DRAINAGE If a sand subbase layer is used, it should be provided with a means of subsurface drainage to prevent water build- up. This can be in the form of draintile lines which tap into storm sewer systems,or outlets into ditches. Where sand subbase layers include sufficient sloping, and water can migrate to lower areas,draintile lines can be limited to finger drains at the catch basins. Even if a sand layer is not placed, strategically placed draintile lines can aid in improving pavement performance. This would be most important in areas where adjacent non-paved areas slope towards the pavement. Perimeter edge drains can aid in intercepting water which may infiltrate below the pavement.